Cambridge IGCSE Biology: Coordination, Response & Homeostasis Study Guide
Mammalian Nervous System
Components of the Human Nervous System:
Central Nervous System (CNS): Comprises the brain and the spinal cord.
Peripheral Nervous System (PNS): Comprises all the nerves in the body.
Functions of the Nervous System:
Allows organisms to make sense of their surroundings and respond to them.
Coordinates and regulates body functions.
Nerve Impulses: Information is transmitted through the nervous system as nerve impulses, which are electrical signals passing along nerve cells called neurones.
Nerves: A bundle of neurones is classified as a nerve.
Types of Neurones
Three Main Types of Neurones:
Sensory Neurones: Carry electrical impulses from sense organs to the Central Nervous System (brain or spinal cord). They feature a long axon with a cell body branching off the middle of the axon.
Relay Neurones (Intermediate Neurones): Located entirely within the CNS; connect sensory neurones to motor neurones. They are short and have a small cell body at one end with many dendrites branching off it.
Motor Neurones: Carry electrical impulses from the CNS to effectors (muscles or glands). They have a long axon with a large cell body situated at one end, featuring long dendrites branching off the cell body.
Structural Adaptations of Neurones:
Axon (Long Fibre): Reduces time wasted transferring the electrical impulse across multiple cells.
Myelin Sheath: A fatty insulating layer surrounding the axon, interrupted by small uninsulated sections called nodes of Ranvier. This insulation causes the electrical impulse to jump from node to node rather than travelling continuously down the axon, drastically increasing transmission speed.
Dendrites: Branched extensions extending from the cell body that enable connection to multiple other neurones to receive impulses, forming an extensive communication network.
Functional Associations:
Sensory neurones connect sense organs (e.g., eye, nose, tastebuds) to the CNS.
Motor neurones result in responses, which frequently involve physical movement (similar to a moving motorbike).
The Reflex Arc
Voluntary Responses:
Actions involving conscious decisions made by the brain.
Initiates in the brain (e.g., consciously reaching out to pick up a cup of coffee).
Take longer to execute because potential consequences are processed before acting.
Involuntary (Reflex) Responses:
Automatic and rapid responses to a stimulus (e.g., touching a sharp or hot object).
Do not involve the conscious brain as the coordinator, meaning the action is completed before conscious awareness occurs.
Faster than voluntary responses, acting to minimise physical damage to the body.
Steps in a Reflex Arc Pathway (e.g., Stepping on a Sharp Pin):
Stimulus: A sharp pin exerts pressure on the skin.
Receptor: Pain, pressure, or touch receptors in the skin detect the stimulus.
Sensory Neurone: Generates and sends electrical impulses along its axon to the spinal cord (coordinator).
Relay Neurone: Located in the spinal cord; receives the impulse and passes it across synapses to a motor neurone.
Motor Neurone: Transmits the impulse away from the spinal cord to the designated effector.
Effector: A muscle in the leg receives the impulse.
Response: The muscle contracts, pulling the leg and foot up and away from the sharp object.
Impulse Transmission to Brain: An impulse is also transmitted up the spinal cord to the brain during a reflex, but the brain is not involved in coordinating the immediate response.
Reflex Pathway Order:
Core Properties of Reflex Actions:
Automatic
Fast
Protective
Synapses and Neurotransmitters
Definition: A synapse is a specialized junction where two neurones meet.
Role of Synapses:
Connect neurones to form an integrated network throughout the nervous system.
Direct nerve impulses along specific neural pathways (similar to railway points guiding trains to correct tracks).
Ensure directional flow of impulses (one-way transmission only).
Synaptic Structure and Transmission Mechanism:
Neurones never physically touch; they are separated by a small gap called the synaptic cleft (or synaptic gap).
Step 1: An electrical impulse arrives at the end of the presynaptic neurone (presynaptic terminal).
Step 2: The arrival of the impulse triggers vesicles containing chemical messengers called neurotransmitters to move towards and fuse with the presynaptic membrane.
Step 3: Vesicles release neurotransmitters into the synaptic cleft by exocytosis.
Step 4: Neurotransmitters diffuse across the synaptic cleft down a concentration gradient.
Step 5: Neurotransmitters bind to specific complementary neurotransmitter receptor molecules located on the postsynaptic membrane.
Step 6: Binding stimulates the postsynaptic neurone to generate a new electrical impulse that travels down its axon.
Step 7: Neurotransmitters are rapidly broken down by enzymes or recycled back into the presynaptic neurone to prevent continuous stimulation and uncoordinated repeated impulses.
Unidirectional Flow: Synapses guarantee that impulses travel strictly in one direction, preventing conflicting signals within the nervous system.
Pharmacological Site: Because synapses represent the only region where neural signals are chemical rather than electrical, they are the specific site where therapeutic and recreational drugs (e.g., heroin) exert their effects.
Sense Organs
Definition: Sense organs are groups of specialized receptor cells that detect changes in the environment (stimuli) and generate electrical impulses in response.
Sense Organs and Specific Stimuli:
Skin: Contains receptors sensitive to pressure, heat, cold (temperature), and pain. Responsible for touch and temperature perception.
Tongue: Contains receptors sensitive to chemical substances in food and drink. Responsible for taste.
Nose: Contains receptors sensitive to chemical substances in the air. Responsible for smell.
Ear: Contains receptors sensitive to sound waves and movement. Responsible for a) hearing and b) balance.
Eye: Contains receptor cells sensitive to light. Responsible for sight.
Signal Processing Flow: Receptors in sense organs generate electrical impulses Sensory neurones carry impulses to the CNS $ ightarrow$ Relay neurones process impulses and determine response $ ightarrow$ Motor neurones transmit impulses to effectors (muscles or glands) $ ightarrow$ Effectors execute the biological response.
Structure and Function of the Eye
Anatomical Components and Functions:
Cornea: Transparent, protective covering at the front of the eye that refracts (bends) incoming light rays into the eye.
Iris: Colored muscular ring that controls the diameter of the pupil to regulate the amount of light entering the eye.
Pupil: Central aperture in the iris through which light enters the interior of the eye.
Lens: Transparent, biconvex disc that changes shape to finely refract light onto the retina.
Retina: Inner lining of the eye containing light-sensitive receptor cells (rods and cones) that detect light intensity and colour.
Optic Nerve: Sensory nerve composed of neurones that transmit electrical impulses from retinal receptors directly to the brain.
Fovea: Specific central region of the retina with the highest concentration of cone cells, responsible for high-acuity, sharp, colored vision.
Ciliary Muscle: Ring of smooth muscle that alters the tension on suspensory ligaments to change lens shape during accommodation.
Suspensory Ligaments: Fibrous cords connecting the ciliary body to the lens, maintaining tension on the lens capsule.
Sclera: Tough, opaque white outer protective layer of the eyeball.
Conjunctiva: Thin, moist membrane covering the outer front surface of the sclera.
The Pupil Reflex
Purpose: An involuntary reflex action designed to protect the retina from light damage in high-light conditions and maximize light capture in low-light conditions.
Antagonistic Iris Muscles:
Circular Muscles: Arranged in concentric rings surrounding the pupil.
Radial Muscles: Radiate outwards from the pupil margin to the outer iris edge.
Antagonistic Action: When circular muscles contract, radial muscles relax; when radial muscles contract, circular muscles relax.
Pupil Reflex in Dim Light (Dark Environment):
Photoreceptors in the retina detect low light intensity.
Radial muscles contract and circular muscles relax.
The pupil dilates (diameter widens).
Allows more light to enter the eye to form a clear image.
Pupil Reflex in Bright Light (Bright Environment):
Photoreceptors in the retina detect excessive light intensity.
Radial muscles relax and circular muscles contract.
The pupil constricts (diameter narrows).
Allows less light to enter the eye, protecting retinal cells from photic damage.
Summary Table of Pupil Reflex Responses:
Dim Light: Radial Muscles = Contract | Circular Muscles = Relax | Pupil State = Dilated | Light Entered = More
Bright Light: Radial Muscles = Relax | Circular Muscles = Contract | Pupil State = Constricted | Light Entered = Less
Eye Accommodation
Definition: The process by which the human eye changes the shape of its lens to maintain a clear focus on objects at varying distances.
Focusing on Near Objects:
The ciliary muscles contract (reducing the diameter of the ciliary ring).
The suspensory ligaments slacken / loosen (tension on the lens is released).
The elastic lens becomes fatter / more rounded (convexity increases).
Light rays passing through are refracted more strongly to focus on the retina.
Focusing on Distant Objects:
The ciliary muscles relax (ciliary ring expands).
The suspensory ligaments tighten / stretch thin (pulling outwards on the lens).
The lens is pulled into a thinner shape (convexity decreases).
Light rays passing through are refracted less to focus on the retina.
Summary Table of Accommodation Mechanisms:
Near Objects: Ciliary Muscles = Contract | Suspensory Ligaments = Loosen / Slacken | Lens Shape = Fatter / Rounded | Refraction = Refracted More
Distant Objects: Ciliary Muscles = Relax | Suspensory Ligaments = Tighten / Stretched | Lens Shape = Thinner | Refraction = Refracted Less
Rod and Cone Cells
Rod Cells:
Sensitive to light at very low intensities.
Provide monochrome vision; essential for night vision.
Distributed widely across the entire retina, except at the blind spot.
Cone Cells:
Require high light intensity to function.
Three distinct types sensitive to different wavelengths of light (red, green, blue light), enabling full colour vision.
Highly concentrated in the fovea, providing maximum visual acuity and sharp, colored image formation.
Hormones in Humans
Definition: A hormone is a chemical substance produced by a gland and transported via the blood plasma to alter the activity of specific target organs.
Endocrine System: Composed of glands that secrete hormones directly into capillaries passing through them.
Target Specificity: Hormones travel throughout the systemic circulation but only affect target cells containing complementary surface or intracellular receptors.
Hormone Degradation: The liver breaks down and metabolizes excess hormones remaining in the bloodstream.
Major Human Endocrine Glands and Secretions:
Pituitary Gland: Situated at the base of the brain; often termed the 'master gland'.
Thyroid Gland: Located in the neck; produces thyroxine.
Pancreas: Located near the stomach; secretes insulin and glucagon to regulate blood glucose, as well as digestive enzymes (pancreatic amylase, lipase, trypsin).
Adrenal Glands: Situated above the kidneys; produce adrenaline.
Testes: Male reproductive glands; produce testosterone.
Ovaries: Female reproductive glands; produce oestrogen.
Key Hormones Summary Table:
Adrenaline: Secreted by Adrenal Glands. Readies the body for a 'fight or flight' response. Increases heart rate, breathing rate, and dilates pupils.
Insulin: Secreted by Pancreas. Lowers blood glucose levels. Causes excess glucose in the blood to be taken up by muscle and liver cells and converted into glycogen for storage.
Testosterone: Secreted by Testes. Main male sex hormone. Stimulates development of secondary sexual characteristics in males.
Oestrogen: Secreted by Ovaries. Main female sex hormone. Stimulates development of secondary sexual characteristics in females and controls the menstrual cycle.
Comparison of Nervous and Hormonal Control
Structural and Functional Differences:
System Components: Nervous System = Nerves (neurones), brain, spinal cord | Endocrine System = Glands
Type of Signal: Nervous System = Electrical impulses | Endocrine System = Chemical hormones
Transmission Speed: Nervous System = Very fast | Endocrine System = Slower
Duration of Effect: Nervous System = Short (lasts only while nerve impulses are transmitted) | Endocrine System = Longer (lasts until hormone is metabolized/broken down)
Detailed Physiological Effects of Adrenaline ('Fight or Flight'):
Purpose: Prepares the body to either flee from physical danger ('flight') or engage in combat ('fight').
Increases Blood Glucose Concentration: Stimulates the breakdown of stored liver glycogen into glucose to supply additional substrate for cellular respiration in muscles.
Increases Pulse and Breathing Rate: Accelerates heart rate and ventilation rate to rapidly deliver oxygen and glucose to active skeletal muscles while expelling carbon dioxide.
Diverts Blood Flow to Muscles: Constricts arterioles supplying non-essential body parts (such as the digestive tract/alimentary canal) while dilating arterioles supplying skeletal muscles.
Dilates Pupils: Expands pupil diameter to allow maximum light onto the retina, maximizing visual input to the brain.
Homeostasis and Blood Glucose Regulation
Definition of Homeostasis: The maintenance of a constant internal environment within set physical and chemical limits (including body temperature, blood pressure, water balance, and blood glucose concentration) to ensure optimal enzyme function and cellular activity.
Concept of Negative Feedback:
A continuous regulatory system where any change away from an ideal set point triggers a corrective mechanism that reverses the change back towards the set point.
When conditions return to normal, corrective mechanisms are automatically switched off.
Blood Glucose Regulation Mechanisms (Pancreatic Control):
High Blood Glucose Response (e.g., following a carbohydrate-rich meal):
Blood glucose concentration rises above set point.
Pancreas detects the increase and secretes insulin into the blood plasma.
Insulin stimulates liver and muscle cells to absorb excess glucose from the blood and convert it into the insoluble storage carbohydrate glycogen ().
Blood glucose concentration decreases back to normal set point.
Low Blood Glucose Response (e.g., during exercise or fasting):
Blood glucose concentration falls below set point.
Pancreas detects the decrease and secretes glucagon into the blood plasma.
Glucagon stimulates liver cells to convert stored insoluble glycogen back into soluble glucose and release it into the bloodstream ().
Blood glucose concentration increases back to normal set point.
Terminology Distinction:
Glucagon: The peptide hormone secreted by the pancreas.
Glycogen: The complex polysaccharide molecule used as glucose storage in animal tissues.
Renal Threshold Note: If blood glucose concentration rises to excessively high levels, the kidneys cannot reabsorb all the filtered glucose, causing excess glucose to be excreted in urine.
Type 1 Diabetes
Pathophysiology: An autoimmune condition in which the immune system destroys the insulin-secreting cells in the pancreas, rendering the organ incapable of producing insulin.
Consequence: Blood glucose levels cannot be regulated internally and rise to dangerously high concentrations.
Clinical Symptoms: Extreme thirst, frequent urination, severe weakness and fatigue, blurred vision, rapid weight loss, and potential loss of consciousness or diabetic coma in extreme cases.
Management and Treatment:
Subcutaneous insulin injections administered before meals, driving liver and muscle cells to store excess glucose as glycogen.
Continuous monitoring of blood glucose levels throughout the day.
Dietary management: Regulation of dietary carbohydrate intake to prevent severe hyperglycemia.
Regular physical exercise: Promotes direct uptake and utilisation of blood glucose by muscle cells for cellular respiration.
Homeostasis and Temperature Control
Optimal Temperature: The human body maintains an internal core body temperature of approximately , which is optimal for metabolic enzyme activity.
Consequence of Overheating: Temperatures substantially above denature metabolic enzymes, altering their active sites and causing cellular dysfunction.
Anatomical Structures in Skin Thermoregulation:
Thermoreceptors: Detected by thermoreceptors located in both the skin and the hypothalamus of the brain.
Sweat Glands & Pores: Produce and excrete sweat onto the epidermal surface.
Hair & Hair Erector Muscles: Control insulating air layers.
Arterioles & Capillaries: Control surface blood perfusion.
Shunt Vessels: Divert blood between deep arterioles and venules.
Fatty Tissue (Adipocytes): Layer beneath the dermis providing subcutaneous thermal insulation.
Thermoregulatory Responses in Hot Environments (Body Temperature Increases):
Increased Sweating: Sweat glands secrete liquid sweat onto the skin surface. As liquid sweat absorbs body heat energy and evaporates into water vapour (a physical change of state), heat is dissipated from the skin.
Hairs Lie Flat: Hair erector muscles relax, causing hairs to lie flat against the skin surface. This prevents trapping of still air, enabling air to circulate freely and maximizing heat loss via radiation.
Vasodilation: Smooth muscle in the walls of arterioles supplying skin surface capillaries relaxes, widening the vessel lumen. Blood flow through surface capillaries increases while flow through deeper shunt vessels decreases, accelerating thermal radiation from the blood into the environment.
Thermoregulatory Responses in Cold Environments (Body Temperature Decreases):
Sweating Ceases: Sweat glands stop producing sweat, eliminating evaporative cooling.
Hairs Erect: Hair erector muscles contract, pulling hair follicles upright. This traps a warm, motionless layer of air above the skin that acts as an insulator, significantly reducing thermal loss by radiation.
Shivering: Involuntary, rapid contraction of skeletal muscles. Muscle contractions require energy from increased respiration, which converts biochemical energy into thermal energy to warm internal tissues.
Vasoconstriction: Smooth muscle in the walls of arterioles supplying skin surface capillaries contracts, narrowing the lumen. Blood flow to surface capillaries is restricted, and blood is diverted through deeper shunt vessels, dramatically reducing radiant heat loss from the skin surface.
Plant Tropisms
Definition: A tropism is a plant growth response toward or away from a directional environmental stimulus.
Directionality:
Positive Response: Growth directed towards the stimulus source.
Negative Response: Growth directed away from the stimulus source.
Types of Tropisms Summary Table:
Gravitropism (Geotropism): Response to gravity. Shoots exhibit negative gravitropism (grow away from gravity/upwards). Roots exhibit positive gravitropism (grow towards gravity/downwards).
Phototropism: Response to light. Shoots exhibit positive phototropism (grow towards light source). Roots exhibit negative phototropism (grow away from light source).
Biological Importance:
Shoots: Must grow upwards away from gravity and towards light so that developing leaves can absorb maximum sunlight for photosynthesis.
Roots: Must grow downwards into the soil towards gravity to firmly anchor the plant and absorb soil moisture and essential mineral ions.
Experimental Investigation of Phototropism:
Set-up A (Unilateral Light): Plant exposed to directional light from one side in a light-proof box. Result: Shoots bend and grow towards the light source.
Set-up B (Clinostat with Unilateral Light): Plant placed on a slowly revolving clinostat exposed to unilateral light. Result: Revolving motion distributes directional light evenly across all sides of the shoots; shoots grow straight upwards without curving.
Set-up C (Complete Darkness): Plant placed in complete darkness. Result: Seedlings grow tall, slender, and yellowed (etiolated) as they rapidly elongate upwards searching for light.
Experimental Investigation of Gravitropism:
Damp cotton wool placed in petri dishes with germinating bean seedlings positioned horizontally, downwards, and upwards.
Stationary Set-up (Darkness): Petri dish held vertically on its edge in darkness for 2+ days. Result: All radicles (primary roots) curve and grow downwards (positive gravitropism); all plumules (primary shoots) curve and grow upwards (negative gravitropism).
Revolving Clinostat Set-up (Darkness): Petri dish attached to a revolving clinostat held on its edge in darkness. Result: Continuous rotational movement cancels the directional effect of gravity; radicles and plumules continue growing straight in whichever direction they were initially placed.
Auxins and Chemical Control of Tropisms
Role of Auxin: Plants coordinate directional growth responses chemically using a growth hormone named auxin.
Synthesis and Transport:
Auxin is produced primarily in the tips of growing stems and roots.
Auxin diffuses downwards from the shoot tip to regions of lower concentration below the tip.
Growth occurs in the meristem, the region directly behind the tip, where cells undergo cell elongation (existing cells expand in length, rather than dividing).
Mechanism of Positive Phototropism in Shoots:
When light shines uniformly around a shoot tip, auxin is distributed evenly throughout the tissue; cells elongate uniformly, causing straight upward growth.
When a shoot is exposed to unilateral light (light predominantly from one side), auxin synthesized at the tip diffuses away from the light source towards the shaded side.
The higher concentration of auxin on the shaded side stimulates increased cell elongation compared to the illuminated side.
Unequal elongation rates cause the shaded side to grow faster than the light side, forcing the shoot stem to bend directly towards the light source.
Experimental Proof of Auxin Action in Shoots:
Decapitated Tip (Tip Removed): No auxin is synthesized Stem growth ceases entirely.
Opaque Cap over Tip (No Light Detected at Tip): Auxin distribution remains uniform $ ightarrow$ Stem grows straight upwards without phototropic bending.
Unilateral Light on Intact Tip: Auxin accumulates on the shaded side $ ightarrow$ Stem exhibits positive phototropism, bending towards the light.
Mechanism of Gravitropism in Shoots vs. Roots:
Gravity causes auxin to settle and accumulate on the lower side of horizontally positioned plant organs.
In Plant Shoots: High auxin concentration promotes cell elongation. Accumulation on the lower side accelerates cell elongation on the bottom of the stem, bending the shoot upwards (negative gravitropism).
In Plant Roots: High auxin concentration inhibits cell elongation. Accumulation on the lower side reduces cell elongation on the bottom relative to the top side, bending the root downwards (positive gravitropism).