STD 10 Biology
๐ Malayalam
Chapter 1: Genetics of Life
This chapter introduces the fundamental concepts of genetics, from the basic structure of DNA to heredity, variations, and the historical development of the field.
• Gene Editing: A methodology developed in the field of chemistry for bringing desirable changes in genes within DNA.
◦ CRISPR-Cas 9: A specific gene editing technology for which Emmanuelle Charpentier and Jennifer A Doudna shared the Nobel Prize in Chemistry in 2020. This discovery is anticipated to lead to revolutionary advancements in genetic disease therapy, cancer treatment, and the development of pest- and disease-resistant crops.
• DNA (Deoxyribonucleic acid): A nucleic acid, a deeper understanding of which paved the way for gene editing.
◦ Discovery of DNA Structure: The double helical model of DNA was presented in 1953 by James Watson and Francis Crick, based on X-ray diffraction studies by Rosalind Franklin and Maurice Wilkins. The crucial information came from 'Photo 51', an X-ray diffraction image by Rosalind Franklin. Watson, Crick, and Wilkins received the Nobel Prize in Medicine in 1962 for their contributions to this discovery.
◦ Location of DNA: DNA is located within chromosomes, which are found in the cell nucleus.
◦ Size of DNA: The DNA in each chromosome is about 2 inches (5 cm) long. If the DNA from all 46 chromosomes of a human cell were joined, it would be around 6 feet (2m) in length. The total DNA from all trillions of human cells would be approximately 67 billion miles long, enough to wrap around the Earth over two million times.
◦ Structure of DNA:
▪ Nucleotide: The basic building block of DNA. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogen base.
▪ Nitrogen Bases: There are four types: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). These are nitrogen-containing alkaline compounds.
▪ Strands: DNA has two strands. These strands are composed of sugar and phosphate molecules.
▪ Rungs: The "rungs" of the DNA ladder are formed by the pairing of nitrogen bases. Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C).
• Chromosome: The primary components of a chromosome are DNA and histone proteins.
◦ Histone Octamer: Eight histone proteins join together to form a histone octamer.
◦ Nucleosome: DNA strands wind around a histone octamer to form a nucleosome.
◦ Formation of Chromosome: Chromosomes are formed by the packing and coiling of numerous nucleosomes and the recoiling of chains of nucleosomes.
◦ Chromatid: Parts of a chromosome connected by a centromere.
• Human Chromosomes: Humans possess a specific number of chromosomes.
◦ Somatic Chromosomes: These control physical characteristics. Humans have twenty-two pairs of somatic chromosomes. A pair of identical chromosomes forms a homologous chromosome, with one inherited from the mother and the other from the father.
◦ Sex Chromosomes: These determine sex and are of two types: X chromosome and Y chromosome. The Y chromosome is smaller than the X chromosome. The SRY gene on the Y chromosome is responsible for testis development in the embryo.
◦ Genetic Constitution:
▪ Normal female: 44 + XX.
▪ Normal male: 44 + XY.
▪ Variant genetic constitutions can influence physical and mental development. Examples include:
• 44 + X0: Females with only one X chromosome, leading to Turner syndrome. They may have female sex organs but not develop female sexual characteristics at adolescence.
• 44 + XXX: Females with three X chromosomes, leading to Triple-X syndrome.
• 44 + XXY: Males with two X chromosomes and one Y chromosome, leading to Klinefelter syndrome. They may have male sex organs but exhibit female characteristics.
• 44 + XYY: Males with one X chromosome and two Y chromosomes, leading to XYY syndrome.
• Gene: A specific sequence of nucleotides in DNA. Genes provide instructions for protein synthesis, which is responsible for characteristic features and controlling metabolic activities.
• RNA (Ribonucleic acid): Another type of nucleic acid crucial for protein synthesis.
◦ Structure of RNA: Made up of nucleotides, each containing a ribose sugar, a phosphate group, and nitrogenous bases.
◦ Nitrogen Bases in RNA: Adenine (A), Guanine (G), Uracil (U), and Cytosine (C). (Note: Uracil replaces Thymine found in DNA).
◦ Strands: Most RNAs have a single strand.
• Protein Synthesis: The process by which proteins are synthesised according to the instructions of genes.
◦ Stages of Protein Synthesis:
▪ 1. Transcription: mRNA (messenger RNA) is formed from a specific nucleotide sequence (gene) in DNA with the help of various enzymes. mRNA carries messages for protein synthesis. This process occurs in the nucleus.
▪ 2. Translation: tRNAs (transfer RNA) carry specific amino acids to the ribosome based on the mRNA message. rRNAs (ribosomal RNA), part of ribosomes, combine amino acids to make protein. This process occurs in the cytoplasm (specifically, at the ribosome).
▪ rRNA (Ribosomal RNA): The primary component of the ribosome, helping in the formation of bonds between amino acids.
• Heredity and Variation:
◦ Heredity: The transmission of characteristics from parents to their offspring.
◦ Variations: Characters expressed in offspring that differ from their parents. Genes inherited from parents are responsible for both heredity and variations.
• Genetics: The branch of science dealing with genes, heredity, and variation.
• Gregor Johann Mendel (1822-1884): Known as the Father of Genetics.
◦ Experiments: Conducted hybridisation experiments on pea plants (Pisum sativum), focusing on seven specific characters like flower colour and seed shape. He studied Physics, Mathematics, and Natural sciences, learning statistical methods for scientific data analysis.
◦ Factors: Mendel hypothesised that a pair of factors controls each character. These factors are now known to be genes.
◦ Laws of Inheritance: Mendel's conclusions are known as the Laws of Inheritance, providing the fundamental genetic framework for understanding heredity and variation. His findings were initially ignored but were recognised posthumously in 1900 by botanists Hugo de Vries, Carl Correns, and Erich von Tschermak.
• Mendel's Experiments (Monohybrid Cross): Hybridisation experiments considering a single pair of contrasting traits.
◦ Example (Height): Crossed tall and dwarf parental plants.
▪ First Generation (F1): All offspring were tall, indicating that tallness is the dominant trait and dwarfness is the recessive trait.
▪ Self-pollination of F1: Self-pollinating F1 plants produced a Second Generation (F2) with both tall and dwarf plants.
▪ F2 Ratio: The ratio of dominant to recessive traits in the F2 offspring is approximately 3:1 (e.g., 787 Tall: 277 Dwarf).
◦ Inferences from Monohybrid Cross:
▪ A trait is controlled by two factors (alleles).
▪ In hybridisation of contrasting traits, only one (dominant trait) is expressed in F1, while the other (recessive trait) remains hidden but reappears in F2.
▪ When gametes are formed, the factors (alleles) for a trait separate without mixing.
• Key Genetic Terms:
◦ Alleles: Different forms of a gene that determine a character. A gene usually has two alleles. (e.g., T for tall, t for dwarf).
◦ Phenotype: The observable characteristics of an organism.
◦ Genotype: The genetic constitution responsible for the observable characteristics.
• Mendel's Experiments (Dihybrid Cross): Observation of the inheritance of two pairs of contrasting traits simultaneously.
◦ Example (Height and Seed Shape): Crossed tall and round-seeded plants with dwarf and wrinkled-seeded plants.
◦ Mendel's Postulate (Principle of Independent Assortment): When two or more different traits are combined, each trait is inherited independently to the next generation without mixing. A pair of alleles in an organism does not influence the separation of another pair of alleles.
• Non-Mendelian Inheritance: Limitations of Mendel's laws due to complex interactions among genes, environment, and other factors.
◦ Incomplete Dominance: A dominant allele cannot fully hide the allele of the recessive trait. (e.g., Red x White four o'clock plants produce pink offspring).
◦ Co-dominance: Both alleles exhibit their traits at the same time. (e.g., Roan coat pattern in cattle/horses).
◦ Multiple Allelism: The gene that determines a character has more than two alleles. (e.g., ABO blood group in humans, determined by IA, IB, and i alleles).
◦ Polygenic Inheritance: More than one gene controls a characteristic. (e.g., Difference in human skin colour, caused by the action of multiple genes influencing melanin production).
• Genetic Processes Responsible for Variations:
◦ Crossing over: Occurs during the first phase of meiosis (cell division for gamete formation).
▪ Process: Homologous chromosomes pair up. At points of contact called chiasma, chromatids break, and segments are exchanged.
▪ Result: This exchange causes recombination of alleles, leading to new traits in the offspring.
◦ Mutation: A sudden heritable change in the genetic constitution of an organism.
▪ Causes: Errors during DNA replication, exposure to certain chemicals, radiations, etc..
▪ Importance: Causes changes in genes that are transferred through generations, leading to variations. Plays a crucial role in evolution.
◦ Recombination of alleles during fertilisation.
◦ Genetic recombination and gene flow (mentioned later in Neo-Darwinism).
• Genetics - Career Opportunities: A broad field including molecular genetics, population genetics, medical genetics, cytogenetics, behavioural genetics, and genomics. Offers opportunities in healthcare, research, education, pharmaceuticals, agriculture, and other industries. Examples include genetic counselling, genomics, medical genetics, and forensic science.
Chapter 2: Paths of Evolution
This chapter delves into the concept of evolution, exploring historical theories, evidences, and the mechanisms that drive the diversity of life.
• Antibiotic Resistance: The ability of bacteria to resist the effects of antibiotics.
◦ Case Study (Tuberculosis): A mutation in a specific gene made tuberculosis bacteria resistant to antibiotics, allowing them to multiply and cause disease even in the presence of drugs.
◦ Superbugs: Bacteria resistant to antibiotics that multiply and cause diseases. This resistance, caused by mutations, can be transmitted to the next generation. Further mutations can lead to multi-drug-resistant strains.
• Evolution: The process by which biodiversity formed from primitive cells.
• Lamarckism (Theory of Inheritance of Acquired Characters): Initiated by French biologist Jean Baptiste Lamarck (1744-1829).
◦ Core Idea: Organisms adapt to changes in their environment, and acquired characters (changes developed during an organism's lifetime due to use or disuse) are transmitted through generations.
◦ Example (Giraffes): Giraffes stretched their necks to reach higher leaves due to food scarcity, leading to longer necks. These longer necks were then transmitted to offspring, resulting in giraffes with longer necks over generations. (Note: Later proved that acquired characters are not inherited genetically).
• Darwinism (The Theory of Natural Selection): The foundation of modern evolutionary perspectives, proposed by English naturalist Charles Darwin (1809-1882).
◦ Influences: Influenced by Thomas Malthus's (1766-1834) ideas on population growth and resource limitation. He also noted the evolutionary studies of Alfred Russel Wallace (1823-1913).
◦ Key Work: Elaborated his ideas in the book 'On the Origin of Species' (1859).
◦ Observations (Galapagos Finches): Darwin observed the diversity in the beaks of finches in the Galapagos Islands. Different finch species (e.g., ground, cactus, tree finches) had varied beak shapes and sizes, suited to their specific food sources.
◦ Postulates of Natural Selection:
▪ Over production: Organisms produce more offspring than the environment can support.
▪ Variations: Organisms show differences in features like size, immunity, and seed production. These variations can be favourable or harmful.
▪ Struggle for existence: Limited resources (food, shelter, mates) lead to competition among organisms.
▪ Survival of the fittest: Organisms with favourable variations survive in the struggle for existence, reproduce more effectively, and create new generations.
▪ Natural selection: Favourable variations are passed on to the next generations. Over time, accumulated variations lead to the creation of new species that cannot reproduce within the original species.
• Neo Darwinism: An updated version of Darwin's theory that incorporates modern genetic understanding.
◦ Limitations of original Darwinism: Darwin had no idea about the genetic basis of variations and inheritance.
◦ Integration: With the discoveries of Gregor Mendel and concepts of chromosomes and genes, it was recognised that the causes of variations leading to evolution were genetic changes, genetic recombination during sexual reproduction, and gene flow.
◦ Support: Further studies from population genetics, palaeontology, environmental science, etc., added evidence and rationalised Darwinism into Neo Darwinism.
• Evolutionary Clinical Medicine: Makes use of evolutionary ideas in healthcare, studying how bacteria or viruses evolve drug resistance to create new or improved treatments.
• Personalised Medicine: Designed by considering an individual's genes and family genetic history.
• Evolutionary Tree: Represents how each species is formed and their relationships.
◦ Speciation: The process in which new species arise from a common ancestor.
◦ Last Universal Common Ancestor (LUCA): Thought to be the common ancestor from which all species have descended.
◦ Most Recent Common Ancestor (MRCA): A common ancestor shared by different species.
◦ Process of Speciation: If members of a population are isolated by ecological or other factors (mutation, natural selection, genetic recombination), variations accumulate over time. When members become unable to reproduce mutually, they evolve into different species.
• Evidences of Evolution:
◦ Molecular Biology: Compares nucleotide sequences in DNA and amino acid sequences in proteins to find evolutionary relationships. (e.g., Chimpanzees have 0 difference in beta chain amino acids compared to humans, while gorillas have 1, and rats have 31, suggesting closer relationship).
◦ Comparative Anatomy: Similarities in the internal structure of different organisms validate evolution. (e.g., Forelimbs of humans, cats, whales, and bats have similar bone structures despite differing external appearance and function).
◦ Fossil Evidences: Remains or traces of ancient organisms.
▪ Show gradual processes (e.g., horse evolution with shorter legs in ancestors).
▪ Reveal evolutionary relationships through connecting links (e.g., Archaeopteryx, possessing features of both reptiles and birds).
▪ Prove the extinction of past species (e.g., Dinosaurs, Mammoths).
• Evolution in Viruses: Viruses evolve rapidly, especially RNA viruses, because RNA is more prone to mutations than DNA. Mutations allow viruses to overcome host immunity, resist drugs, and proliferate effectively, leading to new variants (e.g., Delta and Omicron COVID variants).
• Human Evolution: A prolonged process spanning lakhs of years.
◦ Primates: The common ancestors of mammals like monkeys, apes, and humans.
▪ Common Characteristics: Opposable thumb, binocular vision.
▪ Categories: Anthropoidea (includes humans, monkeys, apes like gibbon, orangutan, gorilla, chimpanzee) and Cercopithecoidea (monkeys).
▪ Hominoidea: A category including apes and humans.
◦ Members in the Human Evolutionary Path:
▪ Sahelanthropus tchadensis: First link, fossils from Tchad, Africa. Cranial capacity: 350 cm3. Small sized brain, having tail.
▪ Australopithecus: Almost complete fossils from Africa, skeletal structure confirms bipedalism (walking on two legs). Cranial capacity: 450 cm3.
▪ Homo habilis: Fossils from Africa, large skull, made stone tools, lived in small groups, began hunting. Cranial capacity: 600 cm3.
▪ Homo erectus: Fossils from Africa, Asia, Europe. Able to walk upright on two legs, large forehead, omnivores, used excellent stone weapons for hunting. Cranial capacity: 900 cm3.
▪ Homo neanderthalensis: Contemporaries of modern man, fossils from Germany. Small, sloping forehead, thick eyebrows, buried dead bodies. Cranial capacity: 1450 cm3. Brain structure suitable for vision and body control.
▪ Homo sapiens: Modern man. Acquired technology, agricultural methods, domesticated animals, built cities, culturally most evolved. Cranial capacity: 1350 cm3. Brain helpful for social interaction and complex thinking.
◦ Brain Development: Major trend in human evolution. Brain size nearly tripled over two million years, leading to complex social behaviour, tool making, language use, higher cognitive functions, adaptation to changing environments, culture development, and advanced technologies.
• Human Nervous System: Plays a major role in controlling and coordinating vital body functions. Consists of the brain, spinal cord, nerves, and receptors.
◦ Neurons (Nerve cells): Basic building blocks of the nervous system. Specialised cells that receive stimuli and form messages.
▪ Structure:
• Cell body (Cyton): Centre of the neuron, containing cell membrane, cytoplasm, nucleus, and organelles.
• Dendrons: Fine fibres arising from the cell body.
• Dendrites: Branches of dendrons, receive messages from adjacent neurons and transmit them to the cyton.
• Axon: Longest fibre from the cell body, transmits impulses from the cell body to axonites.
• Axonites: Branches of the axon.
• Synaptic knob: Knob-like structure at the tip of the axonite, contains neurotransmitters (e.g., Acetylcholine) to transfer chemical messages to adjacent neurons.
◦ Neuroglial cells: More than half of the brain and spinal cord are neuroglial cells. They cannot receive stimuli or transmit messages but perform functions like providing nutrition to neurons, eliminating wastes, and acting as defence cells. Examples include Ependymal cells, Oligodendrocytes, Microglial cells, Schwann cells, Astrocytes.
◦ Myelin Sheath: A shiny white fat layer covering the axon in some neurons.
▪ Functions: Increases message transmission speed (insulator), provides nourishment, protects axon from injury.
▪ Production: In brain/spinal cord by oligodendrocytes; in nerves by Schwann cells.
▪ White Matter: Part of brain/spinal cord with abundant myelinated neurons.
▪ Grey Matter: Part where cell bodies and unmyelinated neurons are seen.
◦ Protection of Brain and Spinal Cord:
▪ Meninges: Three-layered membrane covering the brain and spinal cord.
▪ Cerebrospinal Fluid: Fluid between inner meningeal membranes, in brain cavities, and spinal cord's central canal.
• Functions: Provides oxygen and nutrients to tissues, eliminates wastes, regulates pressure, protects from external injuries. Ependymal cells play a role in its formation.
• Parts of the Nervous System:
◦ Central Nervous System (CNS): Includes the Brain and Spinal Cord.
◦ Peripheral Nervous System (PNS): Includes 12 pairs of cranial nerves and 31 pairs of spinal nerves that connect the CNS to organs, also includes receptors and nerve ganglia.
▪ Ganglia: Spherical parts where cytons of a group of neurons are covered by a membrane.
• Parts and Functions of the Brain:
◦ Cerebrum: Largest part. Outer part is cortex (grey matter), inner part is medulla (white matter). Involved in problem solving, planning, voluntary movements, memory, intelligence, thinking, imagination, and sensory experiences.
◦ Cerebellum: Second largest part, behind and below the cerebrum. Helps maintain body equilibrium by coordinating muscular activities.
◦ Thalamus: Inner part of the brain. Acts as a relay station for messages to and from the cerebrum. Painkillers act on this part.
◦ Hypothalamus: Helps maintain homeostasis by regulating body temperature, hunger, thirst, and emotions.
◦ Brain Stem: Includes Midbrain, Pons, and Medulla Oblongata.
▪ Midbrain: Initial assessment of vision and hearing messages; role in eye and eyebrow movement.
▪ Pons: Coordinates muscular activities of the eye and face; regulates ventilation rate.
▪ Medulla Oblongata: Controls involuntary activities like heartbeat, ventilation, vomiting, cough, sneezing.
• Spinal Cord: Continuation of medulla oblongata.
◦ Structure: Has a central canal (filled with cerebrospinal fluid), grey matter, and white matter.
◦ Roots: Dorsal root transmits messages from the body to the spinal cord; Ventral root transmits instructions from the spinal cord to the body.
◦ Function: Transmits messages between body parts and the brain, and instructions from the brain to body parts.
• Nerve Impulses: Messages transmitted through neurons.
◦ Mechanism: Neurons have an electric charge (inner side negative, outer side positive when not stimulated). When stimulated, positive ions enter, causing a temporary charge variation (action potential) which travels as a nerve impulse.
• Synapse: The part where an impulse is transferred from one neuron to another.
◦ Parts:
▪ Synaptic knob: Tip of the axon, contains vesicles with neurotransmitters.
▪ Synaptic cleft: Small gap between neurons.
▪ Post Synaptic membrane: Tip of dendrites, contains receptors for neurotransmitters.
◦ Transmission: Synaptic knob secretes neurotransmitters into the synaptic cleft, which bind to receptors on the post-synaptic membrane, stimulating the next neuron.
◦ Role: Transmits impulses in only one direction and increases impulse speed.
• Neocortex: The cerebral cortex of the mammalian brain, modified into a complex six-layered structure, highly developed in humans.
◦ Function: Contains billions of neurons and trillions of synapses, enabling advanced mental processes like thinking, decision-making, learning, and recalling.
• Types of Neurons:
◦ Sensory Neuron: Carries impulses from receptors to the central nervous system.
◦ Motor Neuron: Carries instructions from the central nervous system to organs (muscles/glands).
◦ Inter Neuron: Connects sensory and motor neurons.
• Nerves: Made up of axons of a group of neurons, covered with fat and connective tissue.
◦ Sensory Nerve: Contains sensory neurons.
◦ Motor Nerve: Contains motor neurons.
◦ Mixed Nerve: Contains both sensory and motor neurons.
• Autonomous Nervous System (ANS): A part of the peripheral nervous system that regulates body activities involuntarily.
◦ Sympathetic Nervous System: Equips the body for emergency situations (e.g., dilates pupils, decreases saliva, increases heartbeat, expands bronchioles, slows digestion, increases hormone production, slows peristalsis, retains urine).
◦ Parasympathetic Nervous System: Prepares the body for relaxation and routine functions like digestion (e.g., constricts pupils, increases saliva, decreases heartbeat, constricts bronchioles, stimulates digestion, increases peristalsis, empties urine).
• Spontaneous Responses (Reflex actions): Reactions that occur spontaneously and involuntarily in response to stimuli. Can originate from the spinal cord or brain.
◦ Reflex Arc: The pathway through which impulses are transmitted in a reflex action.
▪ Parts: Receptor, Sensory neuron, Inter neuron (in spinal cord/brain), Motor neuron, Effector (muscle/gland).
• Protection of the Nervous System: Includes using helmets/seat belts, taking precautions during play, avoiding stagnant water, using safety equipment in risky jobs, avoiding smoking/alcohol/drug abuse, exercising, and getting adequate sleep (8-10 hours/day for brain function, memory, emotional development).
• Evolution of Nervous System: From simple neural networks (e.g., Hydra with no control center) to nerve ganglia (e.g., Planaria with a pair in head) to developed brains (e.g., Insects with a clear brain and segmented ganglia), culminating in the complex human nervous system with a highly developed neocortex, enabling cognitive and technological progress.
Chapter 3: Behind Sensations
This chapter explores how organisms perceive internal and external stimuli through sense organs and how the nervous system processes these sensations to enable responses.
• Stimuli: Circumstances that lead to responses in living beings.
◦ External Stimuli: Changes in the surroundings (e.g., light, sound, temperature).
◦ Internal Stimuli: Changes within the body (e.g., hunger, body temperature increase).
• Responses: Reactions to stimuli, involving biological and chemical processes.
• Receptors (Sensory Receptors): Specialised cells or nerve endings that recognise stimuli.
◦ Electrical Impulses: Produced in receptors in response to stimuli, known as receptor potential. High concentration of receptor potential forms action potential in associated neurons, which travels as nerve impulses.
• Senses:
◦ General Senses: Detected by receptors in skin, muscles, joints, internal organs, and blood vessels (e.g., touch, pain, heat, pressure).
◦ Special Senses: Detected by receptors concentrated in specific organs (e.g., vision, hearing, taste, smell).
• Eye (Vision): An important sense organ for perceiving the external world.
◦ Protective Parts:
▪ Eyelid and Eyelashes: Protect the eye.
▪ Conjunctiva: Membrane covering the anterior eye (except cornea), keeping it moist, lubricated, and preventing dust/germs.
▪ Lacrimal Glands: Produce tears, which keep the eye moist, provide nutrients, eliminate waste, and contain lysozyme for infection protection.
▪ Eye Muscles: Control eye movement and position.
▪ Eye Socket: Provides protection.
◦ Layers of the Eye:
▪ Sclera (Outer Layer): Provides firmness and protection.
• Cornea: Transparent anterior part of the sclera, allows light to enter.
▪ Choroid (Middle Layer): Provides oxygen and nutrients to the retina, regulates temperature.
• Ciliary Muscles: Adjust the curvature of the lens.
• Iris: Part behind the cornea, containing melanin pigment. Regulates the amount of light entering the eye.
• Pupil: Aperture at the centre of the iris. Its size is regulated by radial muscles (contracts in dim light, dilates pupil) and circular muscles (contracts in intense light, constricts pupil).
▪ Retina (Inner Layer): Contains photoreceptor cells where the image is formed.
• Photoreceptor Cells:
◦ Rod cells: Cylindrical, about 9 crores in number. Recognise objects in dim light and in shades of black and white. Contain rhodopsin pigment.
◦ Cone cells: Cone-shaped, about 45 lakhs in number. Provide vision in intense light and help recognise colours. Contain photopsin pigment.
◦ Pigments (Rhodopsin & Photopsin): Both contain opsin (a protein) and retinal (formed from Vitamin A). Variations in retinal structure differentiate the pigments.
• Bipolar Cell Layer: Transmits impulses from photoreceptors to ganglion cells.
• Ganglion Cell Layer: Transmits impulses from bipolar cells to the optic nerve.
• Blind Spot: Area where the optic nerve originates; has no photoreceptor cells, thus no vision.
• Yellow Spot (Macula): Middle of the retina, where cone cells are abundant, providing sharp vision.
• Eye Fluids (Humors):
◦ Aqueous Chamber: Between cornea and lens. Contains watery aqueous humor. Provides oxygen and nutrients to lens and cornea, regulates pressure.
◦ Vitreous Chamber: Between lens and retina. Contains transparent, jelly-like vitreous humor, which maintains the eyeball's shape.
• Lens: Convex lens, forms a small, real, and inverted image on the retina. Attached to ciliary muscles by ligaments.
◦ Power of Accommodation: The eye's ability to focus images of near and distant objects accurately on the retina by changing the curvature of the lens through ciliary muscle action.
• Photoreceptors and Vision Pathway:
◦ Glutamate: Primary neurotransmitter in photoreceptors.
◦ In the Dark: Photoreceptors continuously produce glutamate. On bipolar cells (sense light) are inactivated; off bipolar cells (sense darkness) are activated and send impulses to the brain via the optic nerve, creating a sense of darkness.
◦ In Light: Photoreceptors do not produce glutamate. On bipolar cells become active; off bipolar cells inactive. On bipolar cells send impulses to the brain via the optic nerve, creating a sense of vision.
• Colour Vision: Made possible by three types of cone cells sensitive to primary colours:
◦ S-cones: Sensitive to short wavelengths (blue light).
◦ M-cones: Sensitive to medium wavelengths (green light).
◦ L-cones: Sensitive to longer wavelengths (red light).
◦ Colour perception occurs when these cones are stimulated in varying proportions.
◦ Colour Blindness: Genes for green and red cone pigments are on the X chromosome; blue cone pigment gene on chromosome 7. This explains why men are more affected (X-linked inheritance).
• Binocular Fusion: The brain compares and combines two slightly different images from each eye (received due to different angles of light) to form a single, 3D vision. Enables understanding of object distance and depth perception.
• Eye Diseases/Disorders:
◦ Short-sightedness: Enlarged eyeball. Corrected with concave lenses.
◦ Long-sightedness: Eyeball too short or lens too flat (implied opposite of short-sightedness). Corrected with convex lenses.
◦ Astigmatism: Irregular curvature of cornea or lens. Corrected with cylindrical lenses.
◦ Cataract: Eye lens becomes opaque. Treated with surgery.
◦ Glaucoma: Failure in reabsorption of aqueous humor, increasing pressure and damaging optic nerve.
◦ Conjunctivitis: Infection in the conjunctiva.
◦ Diabetic Retinopathy: Due to uncontrolled diabetes.
◦ Night Blindness: Deficiency of Vitamin A.
◦ Xerophthalmia: Prolonged Vitamin A deficiency, causing cornea to become opaque.
• Eye Care: Frequent washing with clean water, eating Vitamin A rich food, reducing screen time (especially for children), and regular eye tests are important.
• Eye Donation: A noble act where corneas are surgically transplanted to restore vision for those with corneal damage.
• Ophthalmology: Specialised branch of medicine dealing with diagnosis, treatment, and prevention of eye diseases. Ophthalmologists are medical doctors providing comprehensive eye care, including surgery. Other roles include ophthalmic assistants, optical dispensary, and specialists in retina, paediatrics, ocular oncology, vision therapy, imaging, and AI diagnostics.
• Snellen Chart: Commonly used chart to test visual acuity, consisting of letters/symbols decreasing in size.
• Hearing: Combined experience of the ears and the brain, also crucial for maintaining body balance.
◦ Parts of the Ear:
▪ Outer Ear:
• Pinna: Directs sound waves into the auditory canal, helps identify sound direction, protects canal from foreign particles.
• Auditory Canal: Directs sound waves to the tympanum, hair, earwax, and sebum protect from dust/germs, earwax has disinfectant properties.
▪ Middle Ear:
• Tympanum (Eardrum): Vibrates with sound waves.
• Ear Ossicles: Three bones (Malleus, Incus, Stapes) arranged in connection with the tympanum, amplifying vibrations.
• Eustachian Canal: Connects middle ear to pharynx, balances pressure on both sides of tympanum, facilitates mucus flow.
▪ Inner Ear:
• Oval Window: Membrane covering the opening to the cochlea's upper chamber.
• Cochlea: Snail-shaped structure with three chambers.
◦ Upper and Lower Chambers: Filled with perilymph fluid.
◦ Middle Chamber: Filled with endolymph fluid.
◦ Basilar Membrane: Between middle and lower chambers, houses the Organ of Corti.
◦ Organ of Corti: Contains auditory receptors (hair cells). Vibrations reaching these hair cells generate impulses.
• Auditory Nerve: Transmits impulses from the Organ of Corti to the brain, resulting in the sense of hearing.
• Body Balance: Maintained by the vestibular system in the inner ear.
◦ Parts: Three semicircular canals (arranged perpendicularly), vestibule (containing utricle and saccule), and hair cells.
◦ Mechanism: Endolymph in canals moves with head's rotational movement, stimulating hair cells. Linear head movement stimulates hair cells in utricle and saccule. Impulses reach the vestibular nerve to the brain (specifically cerebellum), which, with input from eyes and muscles, maintains balance.
• Hearing Impairments: Can be caused by various reasons, with remedies including surgery or hearing aids. Noise pollution (above 80 decibels) is a significant risk, with prolonged exposure to loud noise (above 85 decibels) causing permanent damage.
• Olfaction (Smell):
◦ Mechanism: Olfactory molecules enter the nasal cavity during breathing, dissolve in mucus produced by the mucus membrane. Millions of olfactory receptor neurons in the mucus membrane are stimulated by these particles. Receptors generate impulses, which travel through the olfactory nerve to the brain, enabling the sense of smell.
• Taste:
◦ Taste Buds: Crucial for perceiving food flavours. Located on papillae on the tongue.
◦ Chemoreceptors: Each taste bud has about 100 chemoreceptors. Microvilli from chemoreceptors reach minute pores in papillae.
◦ Mechanism: Substances responsible for taste dissolve in saliva and stimulate chemoreceptors. Impulses travel through nerves to the brain, creating the sense of taste.
◦ Main Tastes: Sweet, sour, salty, pungent, bitter, and umami.
• Skin (Touch, Pain, Temperature, Pressure, Vibration): Contains various receptors.
◦ Receptors in the Skin:
▪ Independent nerve endings: Pain, temperature fluctuations.
▪ Merkel disc: Touch, pressure, hair movement.
▪ Meissner corpuscles: Shape, quantity, and structure of objects.
▪ Krause end bulbs: Cold, touch.
▪ Rufini end organ: Intense touch, pressure, heat.
▪ Root hair plexus: Movement of hair.
▪ Pacinian corpuscles: Vibration, high-frequency touch.
• Pain (Nociception): A response that alerts the body to dangers like injury, wound, or infection.
◦ Nocireceptors: Specialised nerve endings found throughout the skin, muscles, and internal organs that identify pain.
◦ Nociceptive Pain: Pain experienced from direct stimuli (e.g., touching a hot vessel).
◦ Other Causes: Infection or tissue damage producing molecules like cytokines and chemokines; neuropathy (peripheral nerve damage).
• Sensory Diversity in Organisms: Different organisms have unique sensory adaptations for survival.
◦ Amoeba/Bacteria: Detect chemicals in surroundings.
◦ Euglena: Has an eyespot (stigma) to detect light.
◦ Insects: Compound eye (made of Ommatidia), antenna for smell and touch.
◦ Bat: Ears and echo location for hunting and travel.
◦ Snake: Jacobson’s organ to detect smell.
◦ Hawk: Eyes with high vision, long-distance vision, and UV detection.
◦ Dog: Highly sensitive olfactory receptors (300 million).
◦ Elephant: Largest number of olfactory genes (2000), mainly depends on smell for food, can detect water from 19 km, sense ground vibration.
