SSLC: SCERT Biology
๐ Malayalam
Chapter 1: Genetics of Life
Questions within the Text
• How does such a larger DNA fit into the chromosomes of tiny cells? The DNA in each chromosome is about 2 inches (5 cm) long. To fit into tiny cells, DNA and histone proteins are the primary components of a chromosome. Eight histone proteins join together to form a histone octamer. DNA strands wind around this octamer to form a structure called a nucleosome. The chromosomes are formed by packing and coiling numerous nucleosomes and recoiling the chains of nucleosomes.
• How does normal sugar differ from a sugar molecule in DNA? The source states that DNA's basic building block, the nucleotide, is composed of a deoxyribose sugar, a phosphate group, and a nitrogen base. Normal sugar (like glucose or sucrose) is not specified, but the key difference for DNA is the deoxyribose sugar.
• What is the relationship between chromatin reticulum and chromosomes? The provided source discusses chromosomes being formed by the packing and coiling of nucleosomes, which are structures where DNA strands wind around histone octamers. However, the direct relationship between "chromatin reticulum" and chromosomes is not explicitly detailed in the provided text. Chromatin is mentioned in the context of DNA and histone proteins forming chromosomes.
• Why are chromosomes seen in pairs? Chromosomes are seen in pairs because a pair of identical chromosomes together form a homologous chromosome. One of these is inherited from the mother and the other from the father.
• Are there multiple organisms with the same number of chromosomes? The source prompts this question but does not provide examples of multiple organisms with the same chromosome number. It only states that "Each species possess a specific number of chromosomes".
• The Y chromosome of the father is important in sex determination. Why? The Y chromosome is important in sex determination because the SRY gene on the Y chromosome is responsible for the development of testis in the embryo.
• Is dominant character always a phenotype? The source states that phenotype refers to the observable characteristics of an organism. A dominant character is a trait that appears in the first generation of a hybridisation experiment. Therefore, a dominant character, when expressed, is a phenotype. The question, however, might be hinting at the distinction between genotype and phenotype and whether a dominant allele always leads to a dominant phenotype if a recessive allele is also present, which it does if it's fully dominant. The source implies this through Mendel's findings that only one contrasting trait (the dominant one) is expressed in the F1 generation.
• Why was a plant with intermediate height not formed by the combination of tall and dwarf? This relates to Mendel's inferences from the monohybrid cross, which state that "When a pair of contrasting traits is subjected to hybridisation, only one of the contrasting traits is expressed in the offspring of the first generation and the other remains hidden". The trait that appears is called the dominant trait, and the hidden one is recessive. This implies that there isn't a blending of traits into an intermediate form, but rather one trait (tallness, in this case) completely masks the other. This contrasts with incomplete dominance, where an intermediate phenotype is formed.
• Hasn’t the character that is not expressed in the first generation appeared in the second generation? How would that be? Yes, the character that is not expressed in the first generation (the recessive trait, e.g., dwarfness) reappears in the second generation. This occurs because, according to Mendel's inferences, "When gametes are formed, the factors that determine trait gets separated without mixing". Even though the recessive allele (t) is hidden in the F1 generation (Tt genotype), it segregates into gametes independently. During self-pollination of the F1 generation, these recessive alleles can combine again (tt genotype) to express the recessive phenotype in the F2 generation. The ratio of dominant to recessive traits in the F2 generation is approximately 3:1.
• Characters that are not found in the parent plants are found in the second generation. Why? This question relates to Non-Mendelian Inheritance. While Mendel's laws explain many traits, they don't explain all diversity. In situations like incomplete dominance (e.g., pink flowers from red and white parents), a dominant allele cannot fully hide the allele of the recessive trait, leading to an intermediate character not present in the pure parental forms. In co-dominance, both alleles exhibit their traits at the same time (e.g., roan coat pattern), which might also appear as a "new" character compared to individual pure parents. Also, processes like crossing over and mutation are responsible for variations and the appearance of new traits in offspring that their parents don't have.
• Didn’t characters often exhibit more than two traits? What could be the reason for this? Characters can exhibit more than two traits due to phenomena like multiple allelism and polygenic inheritance.
◦ In multiple allelism, the gene that determines a character has more than two alleles (e.g., the ABO blood group in humans has three alleles: IA, IB, and i).
◦ In polygenic inheritance, more than one gene controls the character (e.g., skin colour, where the action of multiple genes causes variation in melanin production).
• How do offspring have characters that their parents don’t have? Offspring can have characters that their parents don't have due to key genetic processes responsible for variations. These include:
◦ Crossing over: During meiosis (cell division for gamete formation), homologous chromosomes exchange broken segments at the chiasma, causing a recombination of alleles, which leads to the appearance of new traits.
◦ Mutation: This is a sudden heritable change in the genetic constitution of an organism. Mutations can be caused by errors during DNA replication, exposure to certain chemicals, or radiations. Changes in genes due to mutation are transferred through generations, leading to variations in characters.
"Let us Assess" Questions
1. Are basic building blocks of DNA and RNA the same? Explain. No, the basic building blocks of DNA and RNA are not entirely the same.
◦ Both DNA and RNA are made up of nucleotides.
◦ Each nucleotide in DNA is composed of a deoxyribose sugar, a phosphate group, and a nitrogen base. The nitrogen bases in DNA are Adenine (A), Guanine (G), Cytosine (C), and Thymine (T).
◦ Each nucleotide in RNA contains a ribose sugar, a phosphate group, and a nitrogenous base. The nitrogen bases in RNA are Adenine (A), Guanine (G), Uracil (U), and Cytosine (C).
◦ Therefore, they differ in their sugar (deoxyribose vs. ribose) and one of their nitrogen bases (Thymine in DNA vs. Uracil in RNA) [12, 22, 23, Table 1.2].
2. Analyse the statements and choose the appropriate one.
◦ i. F1 has similarity with both the parents.
◦ ii F1 has no similarity with any of the parents’ character intermediate to them.
◦ iii F1 has similarity with one of the parents
◦ a) i - Dominance, ii - Incomplete dominance, iii - Co-dominance
◦ b) i - Incomplete dominance, ii - Dominance, iii - Co-dominance
◦ c) i - Co-dominance, ii - Incomplete dominance, iii - Dominance
◦ d) i - Dominance, ii - Co-dominance, iii - Incomplete dominance
3. The correct option is (c) i - Co-dominance, ii - Incomplete dominance, iii - Dominance.
◦ i. F1 has similarity with both the parents: This describes Co-dominance, where both alleles exhibit their traits at the same time, such as the roan coat pattern in cattle and horses.
◦ ii. F1 has no similarity with any of the parents’ character intermediate to them: This describes Incomplete Dominance, where the dominant allele cannot fully hide the recessive allele, resulting in an intermediate phenotype (e.g., pink flowers from red and white parents).
◦ iii. F1 has similarity with one of the parents: This describes Dominance, where only one of the contrasting traits (the dominant one) is expressed in the first generation, resembling one parent (e.g., tall pea plants from a tall and a dwarf cross).
4. Which of the following is contributed by organisms that reproduce sexually, to their offspring?
◦ a) All genes
◦ b) Half of their genes
◦ c) One fourth of their genes
◦ d) Double the number of genes
5. The correct answer is (b) Half of their genes. While not explicitly stated with the term "half of their genes", the source implies this by explaining that one homologous chromosome is inherited from the mother and the other from the father. When gametes are formed, the factors (genes) for a trait get separated. Since offspring receive one set of chromosomes (and thus genes) from each parent through gametes, they receive half of their genes from each parent.
6. A tall pea plant with purple flowers (dominant character) is crossed with a dwarf plant with white flowers.
◦ a) Illustrate the dihybrid cross of these and write the F2 ratio.
▪ Illustration: As an AI, I cannot create a visual illustration. However, I can describe the process.
• Let T = Tall, t = dwarf; P = Purple, p = white.
• Parental plants (P): TTPP (Tall, Purple) x ttpp (Dwarf, white) [36, Illustration 1.11, assuming purebreds].
• Gametes: TP from TTPP; tp from ttpp [37, Illustration 1.11].
• First generation (F1): All plants will be TtPp (Tall, Purple) because Tall and Purple are dominant traits [37, Illustration 1.11].
• Self-pollination of F1 (TtPp x TtPp) [37, Illustration 1.11].
• Gametes from F1: TP, Tp, tP, tp (produced in equal proportion due to independent assortment) [37, Illustration 1.11].
• Second generation (F2): The phenotypes will appear in a ratio.
▪ F2 ratio: For a dihybrid cross where genes assort independently, the phenotypic ratio in the F2 generation is 9:3:3:1 [37, Illustration 1.11 implies a 9:3:3:1 ratio for Tall & Round, Tall & Wrinkled, Dwarf & Round, Dwarf & Wrinkled phenotypes when considering the example dihybrid cross for height and seed shape]. Applying this to height and flower colour:
• 9 Tall, Purple
• 3 Tall, white
• 3 Dwarf, Purple
• 1 Dwarf, white
◦ b) Did characters that differ from the parents appear in the F2 generation? Why? Yes, characters that differ from the original purebred parents (Tall & White and Dwarf & Purple) appear in the F2 generation. This occurs due to independent assortment of traits. Mendel's dihybrid cross inference states that "When two or more different traits are combined, each trait is inherited independently to the next generation without mixing each other". This means the alleles for height (T/t) segregate independently from the alleles for flower colour (P/p), leading to new combinations of traits in the F2 generation that were not present in the original pure parental lines (Tall, Purple and Dwarf, white). For example, a Tall plant with white flowers (T_pp) or a Dwarf plant with purple flowers (ttP_) can appear in the F2 generation.
◦ c) If both the genes are not assorting independently, how does it affect the F2 ratio? If both genes are not assorting independently (i.e., they are linked), it would alter the F2 ratio from the Mendelian 9:3:3:1 dihybrid ratio. The source specifically states Mendel's inference that "When two or more different traits are combined, each trait is inherited independently to the next generation without mixing each other". If they do not assort independently, this fundamental principle of Mendelian genetics is violated, leading to different proportions of phenotypes in the F2 generation, often with the parental combinations being more frequent. The exact ratio would depend on the degree of linkage.
7. How do dominance, co-dominance and incomplete dominance differ from one another? These three modes of inheritance differ in how alleles interact to express a phenotype:
◦ Dominance: When a pair of contrasting traits is hybridised, only one trait (the dominant one) is expressed in the first generation offspring, while the other (recessive) remains hidden but reappears in the second generation. The dominant allele completely masks the expression of the recessive allele when both are present. Example: Tall pea plants (TT or Tt) are phenotypically the same.
◦ Incomplete Dominance: A dominant allele cannot fully hide the allele of the recessive trait, resulting in an intermediate phenotype in the heterozygote. Neither allele is completely dominant over the other. Example: A red-flowered four o'clock plant hybridised with a white-flowered plant produces pink flowers.
◦ Co-dominance: Both alleles exhibit their traits at the same time in the heterozygote. Neither allele masks the other, and both are expressed simultaneously and distinctively. Example: Roan coat pattern in some cattle and horses, where both red and white hairs are present.
8. Different phenotypic ratios are obtained in monohybrid and dihybrid cross. Why? What does it indicate about the inheritance of characters?
◦ Different phenotypic ratios are obtained because a monohybrid cross involves the inheritance of a single pair of contrasting traits, while a dihybrid cross involves the inheritance of two pairs of contrasting traits.
▪ In a monohybrid cross (e.g., height), the F2 phenotypic ratio is typically 3:1 (3 Tall : 1 Dwarf). This indicates that a single gene with dominant and recessive alleles controls the character, and these alleles segregate during gamete formation.
▪ In a dihybrid cross (e.g., height and seed shape), the F2 phenotypic ratio is typically 9:3:3:1. This indicates that two different genes, each controlling a distinct character, are involved, and they assort independently during gamete formation, leading to new combinations of traits.
◦ What it indicates about the inheritance of characters: These different ratios indicate fundamental principles of inheritance:
▪ Law of Segregation (from monohybrid cross): Factors (alleles) for a trait separate without mixing during gamete formation.
▪ Law of Independent Assortment (from dihybrid cross): When two or more different traits are combined, each trait is inherited independently of the others to the next generation.
9. Even though a gene responsible for certain characters has more than two alleles, why does that particular gene have only two alleles in an individual? A gene may have multiple alleles (more than two forms of the gene) within a population (e.g., ABO blood group has IA, IB, and i alleles). However, an individual organism is diploid, meaning it inherits one set of chromosomes (and thus one allele for each gene) from each parent. Therefore, despite the existence of multiple alleles in the species, any single individual can only possess two alleles for that gene – one on each of the homologous chromosomes.
10. Although the DNA possesses all genetic information for protein synthesis, RNA is also required for protein synthesis. Why? While DNA contains the complete genetic instructions (genes) for protein synthesis, RNA is required because it plays crucial roles in the process, acting as intermediaries and machinery.
◦ mRNA (messenger RNA) is formed from a specific nucleotide sequence (gene) in DNA during transcription. The mRNA carries the genetic message for protein synthesis from the nucleus to the ribosomes in the cytoplasm.
◦ tRNAs (transfer RNA) carry specific amino acids to the ribosome, aligning them based on the message in the mRNA.
◦ rRNAs (ribosomal RNA) are components of ribosomes and combine amino acids to make proteins. Therefore, RNA molecules are essential for interpreting and translating the genetic information from DNA into proteins.
11. How do co-dominance and multiple allelism function in the determination of blood group in the ABO blood grouping in human beings? Explain. The ABO blood group system in humans demonstrates both co-dominance and multiple allelism.
◦ Multiple Allelism: The gene that determines blood group has more than two alleles. Specifically, there are three alleles: Iแดฌ, Iแดฎ, and i. An individual, however, only possesses two of these alleles.
◦ Co-dominance: The Iแดฌ and Iแดฎ alleles are co-dominant with respect to each other. This means that if an individual inherits both the Iแดฌ allele from one parent and the Iแดฎ allele from the other (genotype IแดฌIแดฎ), both alleles are fully expressed, resulting in the AB blood group, where both A and B antigens are present on the red blood cells.
◦ The 'i' allele is recessive to both Iแดฌ and Iแดฎ. For example, an individual with genotype Iแดฌi will have blood group A, and an individual with genotype Iแดฎi will have blood group B. Only individuals with the genotype 'ii' will have blood group O.
12. All ova formed in females contain one type of sex determining chromosome. Why? Human females have a genetic constitution of 44 + XX chromosomes. This means they have two X chromosomes as their sex chromosomes. During meiosis, when ova (egg cells) are formed, the two X chromosomes segregate, and each ovum receives only one X chromosome. Therefore, all ova formed in females contain only one type of sex-determining chromosome, which is the X chromosome.
Chapter 2: Paths of Evolution
Questions within the Text
• What was the treatment given initially to cure the disease? What was its result? Initially, the young man was given several antibiotics for six weeks and then a specific antibiotic for 33 weeks. The result was that the tuberculosis was confirmed to be cured after 10 months of treatment, and the treatment was completed.
• Why didn’t the disease get cured even after giving treatment for the second time? The disease did not get cured the second time because antibiotic-resistant tuberculosis bacteria were activated again, leading to a relapse. Despite being treated with various antibiotics as done previously, the patient died. It was found that a mutation in a specific gene made the bacteria resistant to antibiotics, allowing the mutated bacteria to multiply even in the presence of antibiotics and cause the disease.
• How did the tuberculosis bacteria acquire resistance to antibiotics? The tuberculosis bacteria acquired resistance to antibiotics due to a mutation in a specific gene. This mutation enabled the bacteria to multiply even when antibiotics were present.
• What will happen if this bacteria transmit this ability to their next generations? If these antibiotic-resistant bacteria transmit this ability to their next generations, the proportion of resistant bacteria will increase over time. This can lead to the formation of multi-drug-resistant strains, known as 'superbugs', which are resistant to common antibiotics and pose a major concern in healthcare.
• Does the formation of such bacteria raise challenges in the field of medicine? Analyse the news given below and draft your opinion. Yes, the formation of such bacteria raises significant challenges in the field of medicine. The news report states that doctors, public health professionals, and scientists warn that some of the most trusted antibiotics are ineffective against these 'superbugs'. This means common infections become much harder to treat, leading to higher mortality rates and prolonged illnesses. It necessitates the development of new treatments and methods for fighting infections, making healthcare more complex and expensive.
• What would be the circumstances that might lead a new plant species to evolve from this plant after millions of years? Prepare notes based on the theory of natural selection. Based on Darwin's theory of Natural Selection, the evolution of a new plant species from one that produces hundreds of seeds but only allows a few to mature would involve the following circumstances over millions of years:
1. Overproduction: The plant produces hundreds of seeds, far more offspring than the environment can support.
2. Variations: Within these hundreds of seeds, there would be natural differences or variations among the offspring (e.g., in size, immunity to specific pathogens, efficiency in nutrient uptake, or seed production rate). Some variations could be favourable, others harmful.
3. Struggle for existence: Since only a few seeds attain complete maturity, there is intense competition for limited resources like sunlight, water, nutrients, and space. This competition constitutes the "struggle for existence".
4. Survival of the fittest: Plants with favourable variations (e.g., greater drought resistance, better pest resistance, more efficient photosynthesis, or faster growth rate) would be better adapted to the prevailing environmental conditions. These individuals would be more likely to survive the struggle for existence.
5. Natural Selection: The environment "selects" these individuals with favourable variations, allowing them to reproduce more effectively and produce more offspring.
6. Inheritance of Favourable Variations: These favourable variations are passed on to the next generations.
7. Accumulation of Variations and Speciation: Over millions of years, as these favourable variations accumulate and new ones arise (e.g., through genetic changes like mutations and recombination), the descendants may become so genetically different from the original plant species that they are no longer able to reproduce with the original species. At this point, they would have evolved into a new species, a process called speciation.
• Was there any limitation to Darwin's theory related to the origin of species? Did these limitations pose any challenge to Darwinism? Yes, Darwin's theory of evolution had limitations. A primary criticism was that Charles Darwin had no idea about the genetic basis of variations and inheritance. He observed variations but could not explain how they arose or were passed down. This indeed posed a challenge because without a mechanism for heredity, the theory lacked a complete explanation for how favourable traits could be reliably transmitted and accumulated over generations. However, this challenge was later addressed by Neo Darwinism, which integrated Mendel's discoveries and concepts of chromosomes and genes. It was recognised that genetic changes, genetic recombination during sexual reproduction, and gene flow were the causes of variations leading to evolution.
• Compare LUCA and MRCA?
◦ LUCA (Last Universal Common Ancestor): It is thought that all species have descended from a Last Universal Common Ancestor. LUCA represents the earliest common ancestor of all life forms currently known.
◦ MRCA (Most Recent Common Ancestor): Different species might have a Most Recent Common Ancestor. MRCA refers to the most recent individual from which a given set of organisms (e.g., chimpanzees and humans) are descended. While LUCA is the ancestor of all life, an MRCA is specific to a smaller group of related species. For example, Archaea and Eukarya share a common ancestor that Bacteria do not, making that ancestor the MRCA for Archaea and Eukarya, but not the LUCA for all three.
• Which is the category of organism that is nearest to fungi? Why? Based on Illustration 2.4, the category of organism that is nearest to fungi is Animalia [69, Illustration 2.4]. This is indicated by their position on the evolutionary tree, where Protista, plants, fungi, and animals originate from a common ancestor, but the branching shows fungi and animals sharing a more recent common ancestor than either does with plants or Protista [69, Illustration 2.4].
• What are the circumstances that lead to the formation of variations in organisms? The circumstances that lead to the formation of variations in organisms include:
◦ Mutation: Sudden heritable changes in the genetic constitution, caused by errors during DNA replication, exposure to chemicals, radiations, etc..
◦ Genetic recombination during sexual reproduction: The process of combining genetic material from two parents. This is significantly enhanced by crossing over, where homologous chromosomes exchange segments, leading to a recombination of alleles and new traits.
◦ Gene flow: The movement of genes between populations.
◦ Ecological or other factors: These can isolate populations, allowing variations to accumulate over time.
• How do these circumstances lead to the formation of species? These circumstances lead to the formation of species through a process called speciation.
◦ Variations accumulate over time: If members of a population become isolated from each other by ecological or other factors (which can involve mutation, natural selection, genetic recombination, etc.), several variations might accumulate in these separated groups.
◦ Reproductive Isolation: As these variations accumulate, the isolated groups diverge genetically. Eventually, when the members of a species become unable to reproduce new offspring mutually, they evolve into different species. This reproductive isolation marks the formation of new species from a common ancestor.
• Which organism has the most evolutionary relationship with humans? Why? Based on Table 2.1, the Chimpanzee has the most evolutionary relationship with humans. The reason is that the chimpanzee shows 0 difference in the amino acids of the beta chain of haemoglobin as compared to humans, indicating a very close genetic and evolutionary relationship [71, Table 2.1].
• Which organism has a distant evolutionary relationship with humans? Why? Based on Table 2.1, the Rat has a distant evolutionary relationship with humans. The reason is that the rat shows the largest difference (31 amino acids) in the beta chain of haemoglobin as compared to humans among the given organisms, indicating a more distant evolutionary divergence [71, Table 2.1].
• How does molecular biology help to find out the evolutionary relationship among organisms? Molecular biology helps to find out evolutionary relationships among organisms by comparing the sequence of nucleotides in the DNA and the sequence of amino acids in proteins in one organism with those of other organisms. The greater the similarity in these molecular sequences, the closer their evolutionary relationship. For example, the number of amino acid differences in the beta chain of haemoglobin can indicate evolutionary distance.
• Did humans evolve from monkeys? No, the source states that humans did not evolve directly from monkeys. Instead, the common ancestors of mammals such as monkeys, apes, and humans belonged to the group of primates. The human evolutionary tree (Illustration 2.8) shows that monkeys, gibbons, orangutans, gorillas, chimpanzees, and humans all belong to the broader group Anthropoidea, and humans and apes (gibbon, orangutan, gorilla, chimpanzee) belong to Hominoidea, sharing a common ancestor but not one evolving directly from the other.
• Find out the closest organism to humans in the evolutionary point of view. Identify and write down the reason. The closest organism to humans in the evolutionary point of view is the Chimpanzee [77, Illustration 2.8]. The reason is supported by molecular biology evidence, which shows 0 difference in the amino acids of the beta chain of haemoglobin between humans and chimpanzees [71, Table 2.1]. This indicates a very close genetic relationship and a shared recent common ancestor.
• What is the evolutionary trend in the development of human cranial capacity? The evolutionary trend in the development of human cranial capacity shows a significant increase over time [79, Table 2.2].
◦ Sahelanthropus tchadensis: 350 cm³
◦ Astralopithecus: 450 cm³
◦ Homo habilis: 600 cm³
◦ Homo erectus: 900 cm³
◦ Homo neanderthalensis: 1450 cm³
◦ Homo sapiens: 1350 cm³ The size of the brain nearly tripled over the course of two million years.
• What influence does brain development have in human evolution? Discuss and draw conclusions based on the information given below. Brain development has had a profound influence on human evolution. The significant increase in brain capacity over millions of years (nearly tripling in two million years) has led to:
◦ Complex social behaviour: Enabled more intricate interactions within groups.
◦ Tool making: Facilitated the creation and use of sophisticated tools.
◦ Language use: Developed the capacity for complex communication.
◦ Higher-level cognitive functions: Allowed for advanced thinking, problem-solving, planning, and imagination.
◦ Adaptation to changing environments: Enhanced human ability to respond to and manipulate their surroundings.
◦ Development of culture: Supported the growth of complex cultural practices.
◦ Use of advanced technologies: Provided the cognitive foundation for technological progress. This development laid the foundation for the transition from 'small-brained' ancestors to Homo sapiens, making humans distinct through the presence of a developed neocortex and resultant language, intelligence, and creativity.
• Are there only nerve cells in the nervous system? No, there are not only nerve cells (neurons) in the nervous system. More than half of the brain and spinal cord are composed of neuroglial cells. These cells, unlike neurons, have the ability to divide but cannot receive stimuli or transmit messages. They perform various supportive functions.
• How do smoking, consumption of alcohol and drug abuse affect the health of brain? The source explicitly lists these as habits to avoid for the protection of the nervous system. It implies they are harmful, but does not detail the specific mechanisms or effects of smoking, alcohol consumption, or drug abuse on brain health.
• Are synapses found anywhere else other than the junctions neurons meet? The source describes synapses as "the part where an impulse is transferred from one neuron to another" and discusses their role in the neocortex for mental processes like thinking, decision-making, learning, and recalling. It does not explicitly mention synapses being found elsewhere (e.g., between neurons and muscle cells, which are neuromuscular junctions, a type of synapse). Therefore, based only on this source, it focuses solely on neuron-to-neuron junctions.
• Are there any organisms with brain size larger than those of humans? The source mentions in "Let us Assess" question 2 that dolphins have a larger brain than that of humans.
"Let us Assess" Questions
1. Two illustrations related to human evolution are given below. Based on the theory of natural selection, find out the correct one and explain the reason. As the illustrations (A and B) are not provided in the source text, I cannot identify the correct one or explain the reason based on the visual information.
2. Although dolphins have a larger brain than that of humans, the level of thinking, language skills and social relationships are higher in humans. Evaluate this statement based on the following indicators.
◦ Difference in the structure of the brain: While dolphins may have a larger brain size, the human brain (specifically Homo sapiens) has undergone significant evolutionary development that makes it unique. The human neocortex, the cerebral cortex of the brain, is a complex structure with a six-layered neocortex that is more developed in humans as compared to other mammals. It contains approximately 16 billion neurons, with an average of 7000 synapses created with each other. This complex structure of the neocortex enables advanced mental processes such as thinking, decision-making, learning, and recalling, which are crucial for higher-level cognitive functions and sophisticated language and social skills. Neanderthal man, with a slightly larger cranial capacity, had a brain structure more suitable for vision and body control, whereas the modern human brain is specifically helpful for social interaction and complex thinking.
◦ Influence of natural selection: The development of the complex human brain, including the neocortex, played a crucial role in human dominance over nature. This highly developed nervous system, shaped by natural selection, enabled unique cognitive and technological progress. Over time, favourable variations (such as mutations leading to increased brain size and complexity, especially in areas like the neocortex) were naturally selected, leading to individuals with superior cognitive abilities surviving and reproducing more effectively. This continuous selective pressure resulted in the evolution of the human brain's specific structure that facilitates advanced thinking, language, and social relationships.
3. Redraw the figure of the neuron and label the following parts by writing their names. As an AI, I cannot redraw figures. However, I can describe the labelling based on Illustration 2.9 and the accompanying text:
◦ (a) The part of the neuron that receives impulses from the adjacent neuron: These are the Dendrites, which are branches of the dendrons, fine fibres arising from the cell body. Messages received by dendrites from adjacent neurons are transmitted to the cyton.
◦ (b) Part that contains neurotransmitter: This is the Synaptic knob, the knob-like structure at the tip of the axonite. It contains neurotransmitters (e.g., Acetylcholine) to transfer chemical messages to adjacent neurons.
◦ (c) Part that acts as insulator: This is the Myelin sheath, which covers the axon in some neurons. It is made of shiny white fat and increases the speed of message transmission by acting as an insulator.
4. Darwin failed to explain the reasons of variation even though he argued that variations occur continuously in organisms. Evaluate this statement based on the findings of Neo Darwinism. The statement is correct. Charles Darwin's original 'Theory of Natural Selection' was limited because he had no idea about the genetic basis of variations and inheritance. He observed that variations existed and were crucial for natural selection, but he could not explain how these variations arose or how they were passed from one generation to the next. Neo Darwinism addresses this limitation by integrating later scientific discoveries. With the findings of Gregor Mendel and the subsequent understanding of chromosomes and genes, it was recognised that the causes of variations that drive evolution are genetic changes (mutations), genetic recombination during sexual reproduction, and gene flow. Neo Darwinism thus rationalises Darwin's theory by providing the essential genetic framework that was missing from his original work.
5. Observe the illustration and answer the questions. As the illustration (A and B showing Central Nervous System and Different parts of the body) is not provided in the source text, I cannot answer questions (a) and (b) based on visual information.
6. A table that includes the parts of spinal cord and their functions is given below. Arrange column B appropriately in accordance with column A of the table.
7. Correct arrangement:
◦ Central canal: The part which is filled with cerebrospinal fluid. Cerebrospinal fluid provides oxygen and nutrients to tissues, eliminates wastes, regulates pressure, and protects from external injuries. So, "Fluid present here nourishes the spinal cord" is appropriate.
◦ White matter: The part where myelinated neurons are more abundant. So, "Neurons with myelin sheath are numerous" is appropriate.
◦ Dorsal root: Transmits messages from different parts of the body to the spinal cord. So, "Transmits impulses to the spinal cord" is appropriate.
◦ Grey matter: The part where the cell bodies and parts of neurons without a myelin sheath are seen. So, "Cell body of neurons are numerous" is appropriate.
◦ The function "Transmits impulses to different parts of the body" is associated with the Ventral root, which transmits instructions from the spinal cord to different parts of the body. This part is missing in column A.
8. Based on the information obtained from fossils, some human ancestors and their characteristics are given below. Find out the one which is arranged correctly from the given answers.
◦ (A) Homo habilis (i) buried dead bodies
◦ (B) Homo neanderthalensis (ii) able to stand upright on two legs
◦ (C) Australopithecus (iii) made tools with stones using their hands
◦ (D) Homo erectus (iv) The skeletal structure confirms bipedalism
◦ (a) A : i, B : ii, C : iii, D : iv
◦ (b) A : iii, B : i, C : iv, D : ii
◦ (c) A : iii, B : iv, C : ii, D : i
◦ (d) A : iv, B : i, C : iii, D : ii
9. Let's match them based on the source:
◦ (A) Homo habilis: Made tools with stones using hands. So, A : iii.
◦ (B) Homo neanderthalensis: They buried dead bodies. So, B : i.
◦ (C) Australopithecus: The skeletal structure confirms bipedalism. So, C : iv.
◦ (D) Homo erectus: They were able to walk upright on two legs. So, D : ii.
10. The correct arrangement is (b) A : iii, B : i, C : iv, D : ii.
11. Examine the information mentioned in the boxes labelled as P, Q, R, S. Identify the part of the brain associated with them and choose the correct answer.
◦ P: Maintaining Homeostasis
◦ Q: Relay station of impulses
◦ R: Provide Sensory experiences
◦ S: Controls the rate of ventilation
◦ (a) P – Medulla oblongata Q – Pons R – Hypothalamus S – Thalamus
◦ (b) P – Pons Q – Hypothalamus R – Medulla oblongata S – Cerebrum
◦ (c) P – Hypothalamus Q – Cerebrum R – Thalamus S – Pons
◦ (d) P – Thalamus Q – Cerebrum R – Hypothalamus S – Medulla oblongata
12. Let's match based on Illustration 2.12:
◦ P – Maintaining Homeostasis: This is a function of the Hypothalamus (regulates body temperature, hunger, thirst, and emotions).
◦ Q – Relay station of impulses: This is a function of the Thalamus (acts as the relay station of messages to and from the cerebrum).
◦ R – Provide Sensory experiences: This is a function of the Cerebrum (provides various sensory experiences).
◦ S – Controls the rate of ventilation: This is a function of the Pons (regulates the rate of ventilation).
13. Therefore, the correct choice should be (P-Hypothalamus, Q-Thalamus, R-Cerebrum, S-Pons). Looking at the options provided, none perfectly match the comprehensive description for R (sensory experiences, which is Cerebrum). However, option (c) has P - Hypothalamus, Q - Cerebrum, R - Thalamus, S - Pons. Let's re-evaluate. The illustration directly links "Initial assessment of messages regarding vision and hearing" to Mid brain and "Regulates the rate of ventilation" to Pons. "Relay station of messages to and from the cerebrum" to Thalamus. "Helps in maintaining homeostasis by regulating body temperature, hunger, thirst and emotions" to Hypothalamus. "Provides various sensory experiences" to Cerebrum.
14. Let's re-examine the options carefully against the direct text:
◦ P: Maintaining Homeostasis -> Hypothalamus
◦ Q: Relay station of impulses -> Thalamus
◦ R: Provide Sensory experiences -> Cerebrum
◦ S: Controls the rate of ventilation -> Pons
15. Now let's check the given options.
◦ (a) P – Medulla oblongata (Incorrect for Homeostasis)
◦ (b) P – Pons (Incorrect for Homeostasis)
◦ (c) P – Hypothalamus (Correct) Q – Cerebrum (Incorrect for Relay Station) R – Thalamus (Incorrect for Sensory experiences) S – Pons (Correct)
◦ (d) P – Thalamus (Incorrect for Homeostasis)
16. There appears to be an inconsistency in the provided multiple-choice options relative to the text's detailed functions for Q and R. However, if forced to choose the "best fit" among flawed options, let's consider common associations:
◦ Homeostasis is strongly Hypothalamus.
◦ Relay station is strongly Thalamus.
◦ Sensory experiences are broadly Cerebrum, but Thalamus is the gateway.
◦ Rate of ventilation is Pons.
17. Let's assume there's a typo in the options and try to deduce the intended match. If Q and R are swapped in option (c), it might be closer. Given the explicit descriptions:
◦ P (Homeostasis) = Hypothalamus
◦ Q (Relay station) = Thalamus
◦ R (Sensory experiences) = Cerebrum
◦ S (Ventilation rate) = Pons
18. None of the options perfectly align with all four. However, let's look for the option that gets most or the key ones correct. Option (c) has P–Hypothalamus and S–Pons correct. The issue is with Q and R. If Q was Thalamus and R was Cerebrum, that would be ideal. Since Q is given as Cerebrum and R as Thalamus in option (c), this option is problematic for Q and R. Let me double-check the image's text: Q: Relay station of impulses -> Thalamus R: Provide Sensory experiences -> Cerebrum
19. Given the options, and assuming a possible error in the options provided in the source itself, I must point out the discrepancy. However, based on the closest matches for P and S which are usually very specific: P (Maintaining Homeostasis) is Hypothalamus. S (Controls the rate of ventilation) is Pons. Option (c) correctly identifies P as Hypothalamus and S as Pons. The assignments for Q and R in option (c) are incorrect based on the text. Due to the ambiguity and potential error in the multiple-choice question's options, I cannot definitively select a fully correct option. I will highlight the correct individual assignments.
20. Complete the table by including the following in appropriate columns.
21. Completed table based on the source:
Chapter 3: Behind Sensations
Questions within the Text
• The tongue can detect taste, but the ears can’t. What could be the reason? The reason is the specialisation and distribution of sensory receptors. The source explains that stimuli are recognised by specialised cells or nerve endings called Sensory Receptors. Senses are divided into general senses (like touch, pain, heat) and special senses (like vision, hearing, taste, smell). Special senses are recognised by receptors concentrated only in certain organs. The tongue has chemoreceptors in taste buds specifically designed to detect chemical stimuli related to taste. The ears, on the other hand, contain structures like the Organ of Corti with auditory receptors designed to detect sound vibrations. Thus, organs have specific receptors for specific stimuli.
• In which situations are the size of the pupil changed? How does it become possible? The size of the pupil changes in situations of dim light and intense light. It is regulated to allow clear vision in dim light and to prevent damage to the retina in intense light. This regulation is made possible by the radial muscles and circular muscles found in the iris [130, 131, Illustration 3.3]:
◦ In dim light, the radial muscles contract, which increases the size of the pupil [131, Illustration 3.3]. This allows more light (up to 16 times more than normal) to enter the retina.
◦ In intense light, the circular muscles contract, which decreases (constricts) the size of the pupil [131, Illustration 3.3]. This reduces the amount of light entering the eye, protecting the retina.
• What is the importance of this arrangement? (lens, ciliary muscles, ligaments) The arrangement where the lens is attached to the ciliary muscles through ligaments is crucial for the power of accommodation. This arrangement allows the eye to focus images of both near and distant objects accurately on the retina. It achieves this by changing the curvature of the lens through the actions of the ciliary muscles.
◦ When viewing a distant object, ciliary muscles are relaxed, and ligaments are stretched, making the lens flatter [133, Illustration 3.4].
◦ When viewing a near object, ciliary muscles contract, relaxing the tension on the ligaments, which allows the elastic lens to become more convex (curved) [133, Illustration 3.4].
• Can we see the object as soon as the image is formed on the retina? No, we cannot see the object as soon as the image is formed on the retina. The formation of the image on the retina is just the initial step. For vision to occur, the photoreceptor cells (rod and cone cells) in the retina must be stimulated, generating impulses. These impulses are then transmitted through bipolar cell layers and ganglion cell layers to the optic nerve, which carries them to the visual centre of the brain, where the sense of vision is created. The brain interprets these impulses to form the perception of the object.
• What is the importance of pigments in photoreceptors? Pigments in photoreceptors are essential for sensing light and initiating the process of vision.
◦ Rhodopsin in rod cells enables vision in dim light and in shades of black and white.
◦ Photopsin in cone cells provides vision in intense light and helps in recognising colours. These pigments (both containing opsin and retinal, formed from Vitamin A) undergo changes in the presence or absence of light. In the presence of light, photoreceptors stop producing glutamate, which activates 'on bipolar cells' that send impulses to the brain, creating vision. In the dark, continuous glutamate production by photoreceptors activates 'off bipolar cells' that send impulses, creating a sense of darkness. Therefore, the pigments are crucial for the transduction of light stimuli into neural signals the brain can interpret.
• Is retina necessary to see black colour? The source indicates that the retina, specifically the photoreceptors, are involved in perceiving light and its absence. When there is an absence of light, photoreceptors continuously produce glutamate, activating 'off bipolar cells' which send impulses to the brain, creating a sense of darkness. Black is the absence of light. Therefore, the retina and its photoreceptors are necessary for the perception of black (or darkness), as they generate the signals related to the absence of light that the brain interprets.
• Can you find out the reason for colour blindness? The source states that colour vision is made possible by three types of cone cells (S-cones for blue, M-cones for green, L-cones for red), which get stimulated in varying proportions by coloured light. It also mentions that the genes responsible for the production of pigments in cone cells sensitive to green and red are found on the X chromosome, while the gene for blue cone pigment is on chromosome 7. Therefore, the reason for colour blindness (particularly red-green colour blindness) is often a defect or absence in the genes responsible for producing the specific pigments in the cone cells, especially those on the X chromosome. Since these genes are X-linked, it also explains why men are more affected (they have only one X chromosome, so a single defective gene on it will cause the condition, whereas women have two X chromosomes and usually need both to be defective for full expression).
• Why don’t we see objects as two, even though images are formed in both eyes? We don't see objects as two because of a process called binocular fusion. Each eye receives slightly different images on its retina because they receive light from different angles. These two slightly different images are sent to the visual cortex in the brain through the optic nerve. The brain then compares these images and combines them (fuses them) into a single, coherent perception. This process also helps to determine depth and how distant or near objects are, providing 3D vision.
• What do the numerator and denominator of the fraction in an eye test result using Snellen chart indicate? The source states that in a Snellen chart test, the person stands twenty feet away and reads the smallest line clearly seen. The results are recorded as a fraction. While the source doesn't explicitly define numerator and denominator, in standard Snellen notation:
◦ The numerator (top number) represents the standard testing distance (e.g., 20 feet in the US system, or 6 metres in the metric system) at which the test is conducted.
◦ The denominator (bottom number) represents the distance at which a person with normal vision could read the line that the test subject could read at the standard testing distance. For example, 20/20 vision means a person can see at 20 feet what a person with normal vision can also see at 20 feet. 20/40 vision means a person can see at 20 feet what a person with normal vision could see at 40 feet (indicating poorer vision).
• A big tree happens to get uprooted in the forest. Was there a sound if no one was there to hear it? The source provides information that sound travels in the form of waves and can be transmitted only through a medium. It also explains that hearing is a "combined experience of the ears and the brain". While the physical event of the tree falling would generate sound waves (vibrations traveling through the air), if no auditory receptors (ears) and no brain were present to interpret these waves into the sensation of "sound," then, from the perspective of conscious perception, there would not be a "sound" experience. The physical phenomenon occurs, but the subjective sensory experience of "sound" requires a receiver and interpreter.
"Let us Assess" Questions
1. Which of the following statements is correct?
◦ a. Each taste bud has different chemoreceptors.
◦ b. Opsin in rhodopsin contains retinal which is formed from vitamin A.
◦ c. Photoreceptors are formed from optic nerve.
◦ d. Both rhodopsin and photopsin contain retinal.
2. Let's evaluate each statement based on the source:
◦ a. Each taste bud has different chemoreceptors. The source states "A taste bud will have about 100 chemoreceptors" and mentions main tastes (sweet, sour, salty, etc.). It doesn't explicitly say different types of chemoreceptors within one bud, but rather that each chemoreceptor detects substances dissolved in saliva. This statement is ambiguous or potentially incorrect as receptors are specific for certain tastes.
◦ b. Opsin in rhodopsin contains retinal which is formed from vitamin A. The source states that "The components of both pigments include a protein namely opsin and retinal, formed from Vitamin A". So, it's not opsin in rhodopsin containing retinal, but rather rhodopsin composed of opsin and retinal. This wording makes the statement technically inaccurate.
◦ c. Photoreceptors are formed from optic nerve. Photoreceptors (rod cells and cone cells) are in the retina. The optic nerve transmits impulses from the retina (specifically from ganglion cells which are stimulated by bipolar cells, which are stimulated by photoreceptors) to the brain. Photoreceptors are not formed from the optic nerve. This is incorrect.
◦ d. Both rhodopsin and photopsin contain retinal. The source clearly states: "The components of both pigments include a protein namely opsin and retinal, formed from Vitamin A. However, the chemical structure of retinal is different in rhodopsin and photopsin". This statement is correct.
3. The correct statement is (d) Both rhodopsin and photopsin contain retinal.
4. Recreate the illustration as given below by including any sense organ. As an AI, I cannot recreate illustrations. However, I can describe the flow for a sense organ, e.g., the eye: Stimulus (Light rays from object) → Receptors (Photoreceptors in retina) → Stimulate (Photoreceptors stimulated by light, generating impulses) → Impulse (Action potential/nerve impulses generated) → Nerve Centre in the Brain (Visual centre of the brain via optic nerve) → Sensation (Sense of vision/seeing the object).
5. Listen to the conversation between two children. Evaluate it and record your opinion.
◦ Child 1 - We should be proud of being born as humans. Humans have the most developed and efficient brain and sense organs.
◦ Child 2 - It is enough to talk about the brain, as the sense organs are its continuation.
6. Evaluation:
◦ Child 1's statement is largely supported by the source. The human brain, particularly the neocortex, is highly developed, enabling complex cognitive and social functions, language, and technology. While other organisms may have specialized senses superior to humans in certain aspects (e.g., dog's smell, bat's hearing), the human brain's ability to interpret and integrate diverse sensory information for complex thought and action is unique. The source highlights the "remarkable transition from simple nervous structures to highly complicated nervous systems" and the "crucial role in human dominance over nature" due to the complex brain. So, pride in the human brain's development is justified by the text.
◦ Child 2's statement is an oversimplification and partially incorrect. While the sense organs do send information to the brain, and the brain is essential for interpreting this information and forming sensations, the sense organs are not merely "continuations" of the brain. They are distinct, specialized structures with their own unique receptors and initial processing capabilities. The source emphasizes that responses and sensations occur due to the "combined action" of sense organs and the brain. The information from the external environment is perceived through sense organs, and then it reaches the central nervous system for analysis and response. Without the specialized receptors in sense organs (e.g., photoreceptors in the eye, chemoreceptors in the tongue), the brain would not receive the necessary stimuli to form sensations. Therefore, both the sense organs and the brain are indispensable and work synergistically.
7. Analyse the information given in the columns A,B,C. If inter connected information is present in three columns. Rearrange them suitably.
8. Rearranged interconnected information:
◦ Cochlea (A) -> Perilymph (B) -> Organ of Corti (C) OR Auditory receptors (C)
▪ Explanation: The cochlea contains fluid-filled chambers (upper and lower chambers with perilymph, middle chamber with endolymph). The Organ of Corti, which contains auditory receptors, is situated in the basilar membrane within the cochlea.
◦ Tympanum (A) -> Ear ossicles (B) -> Oval window (C)
▪ Explanation: Sound vibrations make the tympanum vibrate, which then transmits these vibrations through the ear ossicles (malleus, incus, stapes) to the oval window.
◦ Vestibule (A) -> Endolymph (B) -> Nerve fibres (C) (leading to vestibular nerve)
▪ Explanation: The vestibule (utricle and saccule) contains hair cells and endolymph; linear head movements generate impulses in these hair cells which reach the brain through the vestibular nerve.
◦ Eustachian Tube (A) -> Pharynx (B) -> (Balances pressure) (C - implied function)
▪ Explanation: The Eustachian canal connects the middle ear to the pharynx and helps to balance pressure on both sides of the tympanum.
◦ Outer ear (A) -> Auditory canal (B - implied from description) -> (Directs sound waves) (C - implied function)
▪ Explanation: The pinna (part of the outer ear) directs sound waves into the auditory canal.
9. (Note: Some connections might be indirect, and not every item from column B and C will perfectly align in a three-column chain with A, as they are general lists. The aim is to find strong direct functional/structural relationships. The Cerebellum and Hypothalamus are parts of the brain related to balance and homeostasis respectively, not directly parts of the ear's structure for sound/balance transmission, though the brain is the final receiver.)
10. Correct the mistake, if any.
◦ Impulses are formed in the retina due to the dissociation of rod cells.
11. Mistake: The mistake is the word "dissociation". Correction: Impulses are formed in the retina due to the stimulation (or excitation) of rod cells (and cone cells) by light. The pigments (like rhodopsin in rod cells) undergo changes in the presence of light, leading to a cascade of events that stimulate the photoreceptors to stop producing glutamate, which in turn leads to the formation of impulses.
12. Analyse the given word pairs below, and find the relationship between them.
◦ Retinal- Night blindness: Retinal is a component of rhodopsin, the pigment in rod cells responsible for vision in dim light. Deficiency of Vitamin A, from which retinal is formed, causes Night blindness. So, a lack of retinal (due to Vitamin A deficiency) leads to night blindness.
◦ Umami- Taste buds: Umami is one of the main tastes we recognise. Taste buds, located on the tongue, contain chemoreceptors that are responsible for the sense of taste, including umami.
◦ Cone cells- Colour Blindness: Cone cells are the photoreceptors responsible for vision in intense light and recognising colours. Colour blindness results from defects in the genes responsible for the production of pigments in cone cells, particularly those sensitive to red and green light.
13. Classify the given activities based on the indicators.
▪ The image of your friend formed in the retina
▪ Looking at the friend and smiling
▪ The impulses of the image formed on the retina is sent to the brain.
▪ Photoreceptors are stimulated
14. Indicators:
▪ Activity/activities related to sensory nerve fibres
▪ Activity/activities related to motor nerve fibres
▪ The activities not related to the above ones
15. Classification:
◦ Activity/activities related to sensory nerve fibres:
▪ The impulses of the image formed on the retina is sent to the brain. (Sensory neurons transmit impulses from receptors to the central nervous system).
▪ Photoreceptors are stimulated. (This is the initial step leading to impulse formation in sensory receptors).
◦ Activity/activities related to motor nerve fibres:
▪ Looking at the friend and smiling. (This involves voluntary muscular activities controlled by instructions from the brain transmitted via motor neurons).
◦ The activities not related to the above ones:
▪ The image of your friend formed in the retina. (This is a physical process of light focusing on a surface, not a neural activity itself, though it's a prerequisite for neural activity).
16. Which part of the eye helps in focusing the light towards the retina
◦ a) Cornea
◦ b) Pupil
◦ c) Lens
◦ d) Iris
17. The correct answer is (c) Lens. The source states that the convex lens forms a small, real and inverted image of the object on the retina, indicating its role in focusing light. The cornea allows light to enter. The pupil regulates the amount of light. The iris regulates the size of the pupil. While cornea also plays a role in bending light, the primary part highlighted for focusing an image on the retina is the lens, especially through accommodation.
18. What is the main function of a neuron?
◦ a) Protects organs
◦ b) Exchange electric signals
◦ c) Filters blood
◦ d) Produce retinalin
19. The correct answer is (b) Exchange electric signals. Neurons (nerve cells) are "specialised cells which are capable of receiving stimuli from the surroundings and to form suitable messages". These messages are transmitted as nerve impulses, which are electrical impulses. The exchange of these impulses (electric signals) enables communication within the nervous system.
20. How does the arrangement of rod cells and cone cells in the retina affect our ability to see in different conditions of light? Why does this adaptation become beneficial evolutionary?
◦ Effect on seeing in different light conditions: The retina contains two types of photoreceptor cells: rod cells and cone cells, which are arranged to facilitate vision across a range of light intensities.
▪ Rod cells: There are about 9 crores (90 million) rod cells, making them very numerous. They contain the pigment rhodopsin and are responsible for recognising objects in dim light (scotopic vision) and for perceiving shades of black and white. Their high number and sensitivity allow us to see in low-light conditions.
▪ Cone cells: There are about 45 lakhs (4.5 million) cone cells. They contain the pigment photopsin and provide vision in intense light (photopic vision) and are responsible for recognising colours. There are three types of cone cells sensitive to different primary colours (blue, green, red). The yellow spot (macula) in the middle of the retina has an abundance of cone cells, providing sharp, detailed, and colour vision.
◦ Evolutionary benefit: This dual system (rods for dim light, cones for bright light/colour) is highly beneficial evolutionarily because it provides organisms with versatile vision.
▪ Enhanced survival in diverse environments: It allows for effective navigation and foraging in varying light conditions (e.g., dawn, dusk, moonlight for rods; daytime for cones).
▪ Improved foraging and predator avoidance: Colour vision (cones) can help in identifying ripe fruits, camouflaged prey, or dangerous predators. Black and white vision (rods) is crucial for movement and detecting shapes in low light where colour is not discernible.
▪ Overall adaptability: This adaptation means humans (and other organisms with similar retinal structures) are not limited to functioning only in specific light environments, increasing their chances of survival and reproduction across different habitats and times of day.
21. How does the structure of the ear enable it to convert the sound waves into signals that the brain can interpret? How do damages in different parts of the ear affect hearing and balance?
◦ How the ear converts sound waves into signals: The ear's structure is a complex system designed to capture, amplify, and transduce sound vibrations into electrical impulses for the brain.
1. Outer Ear: The pinna (external ear) directs sound waves into the auditory canal, which channels them towards the eardrum.
2. Middle Ear: Sound waves cause the tympanum (eardrum) to vibrate. These vibrations are then transferred and amplified by a chain of three small bones, the ear ossicles (malleus, incus, stapes), which are connected to the tympanum. The vibrations from the stapes are directed to the oval window. The Eustachian canal helps to balance the air pressure on both sides of the tympanum, ensuring optimal vibration.
3. Inner Ear (Cochlea for hearing): Vibrations from the oval window are transmitted into the fluid-filled cochlea. The cochlea has three chambers, with the upper and lower chambers filled with perilymph, and the middle chamber with endolymph. The vibrations in these fluids cause movements of the basilar membrane, where the Organ of Corti is located. The Organ of Corti contains auditory receptors (hair cells). Movement of the fluid and basilar membrane stimulates these hair cells, which then generate electrical impulses. These impulses are carried by the auditory nerve to the brain, which interprets them as the sense of hearing.
◦ How damages affect hearing and balance:
▪ Damages to Hearing (Auditory System):
• Outer/Middle Ear: Damage to the tympanum (e.g., perforation) or ear ossicles (e.g., stiffening, dislocation) can impair the efficient transmission and amplification of sound vibrations to the inner ear, leading to conductive hearing loss.
• Inner Ear (Cochlea): Damage to the hair cells in the Organ of Corti (e.g., from loud noise exposure >85 decibels, or certain diseases) prevents the generation of proper impulses. Damage to the auditory nerve can also prevent signals from reaching the brain. These types of damage can lead to sensorineural hearing loss, which can be permanent.
▪ Damages to Balance (Vestibular System):
• Inner Ear (Semicircular Canals and Vestibule): The ear also plays a major role in maintaining body balance. The vestibular system, including three semicircular canals, utricle, saccule, and hair cells, is responsible for this. The endolymph in the semicircular canals moves with rotational head movements, stimulating hair cells. Linear head movements stimulate hair cells in the utricle and saccule.
• Damage or dysfunction of these structures (e.g., inner ear infections, Meniere's disease, head injuries) can lead to impaired balance, causing symptoms like dizziness, vertigo, and spatial disorientation. When impulses from these hair cells do not reach the brain (specifically the part responsible for balance, which is the cerebellum) correctly via the vestibular nerve, the body's equilibrium is compromised.
