Organisation

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1. The Digestive System

1.1 Overview

The digestive system breaks down large, insoluble food molecules into small, soluble molecules that Can be absorbed into the blood. This is not a single process but a coordinated sequence of Mechanical and chemical events, each occurring in a specific region of the gut where conditions are Optimised for that step.

Organs of the digestive system:

Mouth $\to$ Oesophagus $\to$ Stomach $\to$ Small intestine $\to$ Large intestine $\to$ Rectum $\to$ Anus

Accessory organs: Liver, pancreas, salivary glands. These are called “accessory” because food does Not pass through them directly; instead, they produce substances (bile, enzymes) that are delivered To the digestive tract via ducts.

Why digestion is necessary. The food you eat contains macromolecules (starch, proteins, lipids) That are too large and insoluble to cross cell membranes. Even if they could cross, your body cannot Use them directly — it needs the subunits: glucose from starch, amino acids from proteins, fatty Acids and glycerol from lipids. Digestion is the process of breaking these large molecules down into Their absorbable subunits.

1.2 Enzymes

Enzymes are biological catalysts — they speed up chemical reactions without being used up. They Are proteins, and each enzyme catalyses a specific reaction. The specificity of enzymes arises from The unique three-dimensional shape of their active site, which is complementary to the shape of Their substrate.

The lock and key model: The substrate (reactant) fits into the active site of the enzyme, like a Key fitting into a lock. Only the correct substrate can fit each enzyme. An alternative and more Widely accepted model is the induced fit model, in which the active site changes shape slightly When the substrate binds, improving the fit and making the reaction more efficient.

Why enzymes are essential for digestion. Without enzymes, digestion would be far too slow to Provide the body with the nutrients it needs. At body temperature, the chemical reactions of Digestion would take days or weeks without catalysis. Enzymes speed up these reactions by a factor Of millions, allowing digestion to occur within hours.

Factors affecting enzyme activity:

Factor	Effect
Temperature	Rate increases up to the optimum (about 37 $^{\circ}$ C for human enzymes), then decreases sharply as the enzyme denatures
pH	Each enzyme has an optimum pH; deviations reduce activity
Substrate concentration	Rate increases with concentration, then plateaus (all active sites occupied)

Denaturation: At high temperatures or extreme pH, the active site changes shape and the Substrate can no longer fit. This is because the weak bonds (hydrogen bonds, ionic bonds) that Maintain the enzyme’s tertiary structure are broken. Denaturation is irreversible: once an Enzyme has been denatured, it cannot recover its function.

Worked Example: Interpreting an enzyme rate graph.

A student investigates the effect of temperature on amylase activity. The results are:

Temperature ( $^{\circ}$ C)	Time for starch to disappear (s)	Rate ( $1/\mathrm{time$ ) ( $\times 10^{-3}$ s $^{-1}$ )
10	300	3.3
20	120	8.3
30	60	16.7
37	40	25.0
45	55	18.2
55	120	8.3
60	No reaction	0

Step-by-step analysis:

From 10 $^{\circ}$ C to 37 $^{\circ}$ C, the rate increases. This is because increasing temperature increases the kinetic energy of the enzyme and substrate molecules, so they collide more frequently with the active site.
The maximum rate occurs at 37 $^{\circ}$ C (the optimum temperature). At this temperature, the enzyme is working at its maximum rate.
Above 37 $^{\circ}$ C, the rate decreases. At 60 $^{\circ}$ C, there is no reaction at all. This is because the enzyme has denatured: the high temperature has broken the bonds maintaining the active site shape, so the substrate can no longer bind.

1.3 Digestive Enzymes

Enzyme	Produced in	Substrate	Product	Optimum pH
Amylase	Salivary glands, pancreas, small intestine	Starch	Maltose	Neutral/alkaline
Protease	Stomach, pancreas, small intestine	Proteins	Amino acids	Acidic (stomach), alkaline (intestine)
Lipase	Pancreas, small intestine	Lipids	Glycerol and fatty acids	Alkaline
Maltase	Small intestine	Maltose	Glucose	Alkaline

Why different enzymes work at different pH. The stomach produces hydrochloric acid, creating a PH of approximately 2. This is optimal for pepsin (a protease), which begins protein digestion. The Small intestine has an alkaline pH (approximately 8) because bile neutralises the acid arriving from The stomach. This alkaline environment is optimal for pancreatic enzymes (amylase, trypsin, lipase). This compartmentalisation means that each enzyme works in the conditions best suited to it, Maximising the overall efficiency of digestion.

Summary table: enzyme sites and conditions.

Region	pH	Enzymes present
Mouth	~7	Salivary amylase
Stomach	~2	Pepsin (protease)
Small intestine	~8	Pancreatic amylase, trypsin, lipase
Small intestine	~8	Maltase, other brush-border enzymes

1.4 Bile

Bile is produced in the liver, stored in the gall bladder, and released into the small Intestine through the bile duct.

Functions of bile:

Emulsifies fats: Breaks large fat droplets into smaller droplets, increasing surface area for lipase action. Bile does not chemically digest fats; it is a physical process. Think of it like washing-up liquid: it does not dissolve grease, but it breaks it into smaller droplets that are easier to clean.
Neutralises stomach acid: Provides alkaline conditions in the small intestine (optimum pH for pancreatic enzymes). Without bile, the acid from the stomach would denature pancreatic enzymes.

Bile vs. Lipase — a common confusion. Bile is not an enzyme. It does not break chemical bonds. It only physically breaks large fat droplets into smaller ones (emulsification). Lipase is the Enzyme that chemically digests fats by breaking ester bonds in triglycerides. Bile prepares the fats For lipase by increasing the surface area available for the enzyme to act upon.

1.5 Absorption

The small intestine is adapted for absorption:

Villi: Finger-like projections that increase surface area. Each villus is covered in even smaller projections called microvilli, further increasing surface area.
Thin walls: One cell thick, short diffusion distance.
Dense capillary network: Maintains steep concentration gradient by carrying absorbed nutrients away from the intestine.
Lacteal: A small lymph vessel in the centre of each villus that absorbs fatty acids and glycerol (products of fat digestion).

Large intestine: Absorbs water from remaining undigested material. The remaining waste forms Faeces. If the large intestine does not absorb enough water, the result is diarrhoea. If it absorbs Too much, the result is constipation.

Worked Example: Why villi are so effective.

A student calculates the total surface area of the small intestine. Without villi, the surface area Is approximately 0.5 m $^2$ . With villi, the surface area increases to approximately 10 m $^2$ . With Microvilli on top of the villi, the total absorptive surface area reaches approximately 200 m $^2$ .

The rate of diffusion is directly proportional to surface area (Fick’s law). So, by increasing the Surface area by a factor of 400 (from 0.5 to 200 m $^2$ ), the small intestine can absorb nutrients 400 times faster than a simple tube without villi. This is why the small intestine is so effective At absorbing digested food.

1.6 Required Practical: Enzyme Activity

Method:

Place a drop of iodine solution into each well of a spotting tile.
Add amylase solution to starch solution in a test tube.
At regular intervals (e.g. Every 30 seconds), remove a sample and add it to the iodine.
Record the time taken for the iodine to stop turning blue-black (starch is fully digested).
Repeat at different temperatures or pH values.
Plot a graph of rate (1/time) against temperature or pH.

Controls: Use a control tube containing starch solution without amylase to confirm that the Colour change is due to enzyme activity, not spontaneous breakdown.

Variables:

Independent variable: temperature or pH
Dependent variable: time taken for starch to be digested (rate = 1/time)
Control variables: volume and concentration of amylase and starch solutions, volume of iodine

Ensuring reliability and validity:

Reliability: Repeat the experiment at least three times at each temperature/pH and calculate a mean rate. If the repeats are consistent, the results are reliable.
Validity: Ensure that only the independent variable changes. Use the same volume and concentration of amylase and starch each time. Use a water bath to control temperature accurately. Use a buffer solution to control pH.

2. The Circulatory System

2.1 Blood Vessels

Vessel	Structure	Function
Artery	Thick, elastic walls; narrow lumen	Carry blood away from the heart at high pressure
Vein	Thin walls; wide lumen; valves	Carry blood back to the heart at low pressure
Capillary	One cell thick walls; very narrow	Exchange of materials between blood and cells

Understanding the structural differences. Arteries need thick, elastic walls because they carry Blood at high pressure generated by the heart’s contraction. The elasticity allows them to expand And recoil, smoothing out the pulsatile flow. Veins carry blood at low pressure and need valves to Prevent backflow (since there is not enough pressure to keep blood moving on its own). Capillaries Are only one cell thick to allow rapid diffusion of oxygen, nutrients, and waste products between Blood and tissues.

Detailed comparison of artery and vein structure.

Feature	Artery	Vein
Wall thickness	Thick (muscle and elastic tissue)	Thin
Lumen	Narrow (maintains high pressure)	Wide (reduces resistance to flow)
Valves	Absent (pressure keeps blood flowing)	Present (prevent backflow)
Blood pressure	High (generated by heart contraction)	Low
Blood flow speed	Fast	Slow
Direction of flow	Away from the heart	Towards the heart
Oxygenation	oxygenated (except pulmonary)	deoxygenated (except pulmonary)

2.2 The Heart

The heart is a double pump:

Right side: Pumps deoxygenated blood to the lungs (pulmonary circulation).

Left side: Pumps oxygenated blood to the body (systemic circulation). The left ventricle has Thicker walls than the right because it must pump blood further (to all body organs, not just the Nearby lungs).

Blood flow through the heart:

Vena cava $\to$ Right atrium $\to$ Tricuspid valve $\to$ Right ventricle $\to$ Semilunar valve $\to$ Pulmonary artery $\to$ Lungs $\to$ Pulmonary vein $\to$ Left atrium $\to$ Bicuspid valve $\to$ Left Ventricle $\to$ Semilunar valve $\to$ Aorta $\to$ Body

Key points:

Valves prevent backflow of blood. The bicuspid (mitral) and tricuspid valves are between the atria and ventricles; semilunar valves are in the aorta and pulmonary artery.
The left ventricle wall is thicker than the right (pumps blood further and against higher resistance in the systemic circulation).
Coronary arteries supply the heart muscle with oxygen and glucose. Blockage of these arteries causes coronary heart disease.
The cardiac cycle is controlled by electrical impulses from the SAN (sinoatrial node, the pacemaker), which is located in the wall of the right atrium.

Worked Example: Understanding blood flow through the heart.

A student is asked to trace the path of a red blood cell from the vena cava to the aorta.

Step-by-step:

The red blood cell enters the right atrium via the vena cava. At this point, the blood is deoxygenated (has given up its oxygen to body tissues).
The tricuspid valve opens, and the cell passes into the right ventricle.
The right ventricle contracts, the tricuspid valve closes (preventing backflow), and the semilunar valve in the pulmonary artery opens. The cell is pumped to the lungs.
In the lungs, the cell picks up oxygen and releases carbon dioxide. It is now oxygenated.
The oxygenated blood returns to the heart via the pulmonary vein, entering the left atrium.
The bicuspid valve opens, and the cell passes into the left ventricle.
The left ventricle contracts (its thick muscular wall generates high pressure), the bicuspid valve closes, and the semilunar valve in the aorta opens. The cell is pumped to the body.

2.3 Blood Components

Component	Function
Red blood cells	Transport oxygen (contain haemoglobin, which binds to O $_2$ )
White blood cells	Defend against disease (phagocytes engulf pathogens; lymphocytes produce antibodies)
Platelets	Cell fragments that help blood clot at wounds
Plasma	Liquid that carries dissolved substances (glucose, amino acids, urea, hormones, antibodies)

Haemoglobin and oxygen transport. Haemoglobin is a protein in red blood cells that binds Reversibly to oxygen. In the lungs (high oxygen concentration), haemoglobin binds oxygen to form Oxyhaemoglobin. In the tissues (low oxygen concentration), oxyhaemoglobin releases oxygen. This Reversible binding is critical: it allows the same molecule to pick up oxygen where it is abundant And release it where it is needed.

Phagocytes and lymphocytes. Phagocytes (a type of white blood cell) defend the body by engulfing And digesting pathogens in a process called phagocytosis. Lymphocytes (another type of white blood Cell) produce antibodies — specific proteins that bind to and neutralise specific pathogens. Each Lymphocyte produces one type of antibody that is specific to one type of pathogen. This specificity Is the basis of the immune response.

Worked Example: Adaptations of red blood cells.

Red blood cells are highly specialised for oxygen transport. Their adaptations include:

Biconcave disc shape: Increases the surface area to volume ratio, allowing more oxygen to diffuse across the cell membrane in a given time. It also makes the cell flexible, allowing it to squeeze through narrow capillaries.
No nucleus: This creates more space inside the cell for haemoglobin molecules, increasing the cell’s oxygen-carrying capacity.
Contains haemoglobin: A red pigment that binds reversibly to oxygen, forming oxyhaemoglobin in the lungs and releasing oxygen in the tissues.
Flexible membrane: Allows the cell to deform as it passes through capillaries that are narrower than the cell itself.

2.4 Coronary Heart Disease

Coronary heart disease (CHD) occurs when the coronary arteries become narrowed by a build-up of Fatty deposits (atheroma/plaque) in a process called atherosclerosis.

Treatments:

Stents: Wire mesh tubes inserted into arteries to keep them open. Mechanical solution; does not address the underlying cause.
Statins: Drugs that reduce blood cholesterol levels, slowing the build-up of plaque. Must be taken long-term.
Bypass surgery: Using a blood vessel from elsewhere (e.g., leg vein) to create a new route around the blocked artery. Major surgery with significant recovery time.

Risk factors: High saturated fat diet, smoking, high blood pressure, obesity, lack of exercise, Genetic factors, stress.

Comparison of CHD treatments.

Treatment	How it works	Advantages	Disadvantages
Stents	Physically widens the narrowed artery	Relatively quick procedure	Does not address underlying cause
Statins	Reduces blood cholesterol, slowing plaque build-up	Reduces risk of further blockages	Must be taken for life; side effects
Bypass surgery	Creates a new route around the blockage	Effective for severe blockages	Major surgery; long recovery time

3. The Respiratory System

3.1 Structure

The respiratory system consists of the trachea, bronchi, bronchioles, and alveoli in the lungs. The Trachea is kept open by C-shaped rings of cartilage (the open part of the C allows the oesophagus to Expand when food is swallowed).

3.2 Gas Exchange

Gas exchange occurs in the alveoli (air sacs in the lungs). Oxygen diffuses from the alveoli Into the blood; carbon dioxide diffuses from the blood into the alveoli.

Adaptations of alveoli for gas exchange:

Large surface area: Millions of tiny alveoli provide an enormous total surface area (approximately 70 m $^2$ ).
Thin walls: One cell thick, short diffusion distance.
Moist surface: Gases dissolve in the thin layer of moisture before diffusing.
Dense capillary network: Maintains steep concentration gradient.
Ventilation: Breathing constantly refreshes the air in the alveoli.

Worked Example: Applying Fick’s Law to gas exchange.

Fick’s law states:

$\mathrm{Rate of diffusion \propto \frac{\mathrm{Surface area \times \mathrm{Concentration difference}{\mathrm{Diffusion distance}$

The alveoli are adapted to maximise the rate of diffusion by:

Large surface area: Millions of alveoli provide approximately 70 m $^2$ of surface area. Increasing surface area directly increases the rate of diffusion.
Steep concentration gradient: Blood flow and ventilation constantly refresh the air in the alveoli and carry blood away. Oxygen in the alveoli is maintained at a high concentration (about 21% in inhaled air, lower in exhaled air), while oxygen in the blood arriving at the lungs is low (depleted after passing through body tissues). This steep gradient maximises the rate of diffusion.
Short diffusion distance: The alveolar wall and capillary wall are each only one cell thick (combined thickness of about 1 micrometre). This short distance maximises the rate of diffusion.

3.3 Ventilation

Breathing in (inhalation):

Intercostal muscles and diaphragm contract
Rib cage moves up and out; diaphragm flattens
Volume of thorax increases
Pressure decreases below atmospheric pressure
Air rushes in

Breathing out (exhalation):

Intercostal muscles and diaphragm relax
Rib cage moves down and in; diaphragm curves up
Volume of thorax decreases
Pressure increases above atmospheric pressure
Air is forced out

Worked Example: Boyle’s law and ventilation.

Boyle’s law states that at constant temperature, pressure is inversely proportional to volume: $P_1 V_1 = P_2 V_2$ .

During inhalation, the volume of the thorax increases (from about 3 litres to about 5 litres in an Adult at rest). This increase in volume causes the pressure inside the lungs to decrease below Atmospheric pressure (about 101 kPa). Air flows from the higher-pressure atmosphere into the Lower-pressure lungs.

During exhalation, the volume of the thorax decreases, causing the pressure inside the lungs to Increase above atmospheric pressure. Air flows from the higher-pressure lungs to the lower-pressure Atmosphere.

4. The Nervous System

4.1 Structure

The nervous system allows the body to detect and respond to changes in the environment. It has two Main parts:

Central nervous system (CNS): Brain and spinal cord.
Peripheral nervous system (PNS): Sensory neurons, relay neurons, and motor neurons.

4.2 Neurones

Type	Function
Sensory neurone	Carries impulses from receptors to the CNS
Relay neurone	Connects sensory and motor neurones in the CNS
Motor neurone	Carries impulses from the CNS to effectors

Structure of a motor neurone:

The cell body contains the nucleus and is located in the CNS or in a ganglion.
A long axon carries the electrical impulse from the cell body to the effector.
The axon is insulated by a myelin sheath, which speeds up transmission by allowing the impulse to jump between gaps (nodes of Ranvier) in a process called saltatory conduction.
Dendrites at the cell body receive signals from other neurones.
Synaptic terminals at the end of the axon release neurotransmitters.

4.3 Synapses

A synapse is the junction between two neurones. When an electrical impulse reaches the end of a Neurone, it triggers the release of neurotransmitters into the synaptic cleft. These chemicals Diffuse across the gap and bind to receptors on the next neurone, triggering a new impulse.

Why synapses are important:

They ensure impulses travel in one direction only (receptors are only on the postsynaptic membrane).
They allow one neurone to connect to many others, creating complex neural networks.
They can amplify or dampen signals, providing a mechanism for modulating responses.

Step-by-step process of synaptic transmission:

An electrical impulse (action potential) arrives at the presynaptic terminal.
The impulse causes synaptic vesicles to fuse with the presynaptic membrane.
Neurotransmitter molecules are released into the synaptic cleft.
The neurotransmitter diffuses across the synaptic cleft.
The neurotransmitter binds to specific receptor proteins on the postsynaptic membrane.
This binding triggers a new electrical impulse in the postsynaptic neurone.
The neurotransmitter is broken down by enzymes in the synaptic cleft (e.g., acetylcholinesterase breaks down acetylcholine) or taken back up by the presynaptic neurone (reuptake), preventing continuous stimulation.

4.4 Reflex Arc

A reflex arc is the pathway taken by nerve impulses in an automatic, rapid response (reflex):

Stimulus $\to$ Receptor $\to$ Sensory neurone $\to$ Relay neurone $\to$ Motor neurone $\to$ Effector $\to$ Response

Reflexes are fast, automatic, and protective. They do not involve conscious thought.

Worked Example: The reflex arc for touching a hot object.

Stimulus: Heat from the hot object.
Receptor: Temperature receptors in the skin detect the heat and generate an electrical impulse.
Sensory neurone: Carries the impulse to the spinal cord.
Relay neurone: Connects the sensory neurone to the motor neurone in the CNS.
Motor neurone: Carries the impulse from the CNS to the effector (muscle in the arm).
Effector: The biceps muscle in the arm contracts.
Response: The hand is pulled away from the hot object.

This response is automatic and does not involve the brain, making it very fast. This speed is Important because it minimises tissue damage.

4.5 The Brain

Part	Function
Cerebral cortex	Intelligence, memory, language, consciousness
Cerebellum	Coordination and balance
Medulla	Unconscious processes (breathing, heart rate)
Hypothalamus	Temperature regulation and hormone production

Studying the brain. The brain is difficult to study because it is protected by the skull, it is Extremely complex, and it is unethical to carry out experiments on living human brains. Methods of Studying the brain include:

MRI scans: Produce detailed images of brain structure. Can identify tumours, damage, and structural abnormalities.
fMRI scans: Show which parts of the brain are active by detecting changes in blood flow.
Studying patients with brain damage: If a specific area of the brain is damaged and the patient loses a specific function, this provides evidence for the function of that area.
Electrical stimulation: Stimulating specific areas of the brain with electrodes and observing the effects.

4.6 The Eye

Structure	Function
Cornea	Refracts light; protective outer layer
Iris	Controls the size of the pupil (amount of light)
Pupil	Hole that allows light to enter
Lens	Focuses light onto the retina
Retina	Contains light-sensitive cells (rods and cones)
Optic nerve	Carries impulses to the brain
Ciliary muscles	Change the shape of the lens (accommodation)

Accommodation: The process by which the eye focuses on near and distant objects:

Distant objects: Ciliary muscles relax, lens becomes thin (less refraction).
Near objects: Ciliary muscles contract, lens becomes thick and more curved (more refraction).

Eye defects:

Short-sightedness (myopia): Image focuses in front of the retina; corrected with a concave lens.
Long-sightedness (hyperopia): Image focuses behind the retina; corrected with a convex lens.

5. Non-Communicable Diseases

5.1 Types

Non-communicable diseases cannot be transmitted between people.

Cardiovascular disease (CHD, strokes)
Cancer
Diabetes (type 1 and type 2)
Respiratory diseases (asthma, COPD)

5.2 Risk Factors

Risk Factor	Associated Diseases
Smoking	Lung cancer, CHD, COPD
Obesity	Type 2 diabetes, CHD, some cancers
Alcohol	Liver disease, some cancers
UV exposure	Skin cancer
Ionising radiation	Various cancers
Poor diet	CHD, type 2 diabetes

5.3 Diabetes

Type 1 diabetes: The pancreas produces little or no insulin. Diagnosed in childhood. Autoimmune condition. Treated with insulin injections.

Type 2 diabetes: Body cells become resistant to insulin. Linked to obesity and lifestyle. Treated with diet, exercise, and medication.

Worked Example: Why insulin must be injected, not taken orally.

Insulin is a protein hormone. If taken orally, it would be digested by proteases in the stomach and Small intestine, breaking it down into amino acids. These amino acids would not function as insulin. Therefore, insulin must be injected directly into the bloodstream, bypassing the digestive system, So that it reaches the target cells intact and can bind to its receptors.

Detailed comparison of type 1 and type 2 diabetes.

Feature	Type 1 Diabetes	Type 2 Diabetes
Cause	Autoimmune destruction of beta cells	Insulin resistance; reduced sensitivity
Age of onset	childhood	adulthood (but increasingly younger)
Insulin production	None (beta cells destroyed)	Reduced or cells become resistant
Treatment	Insulin injections	Diet, exercise, medication
Risk factors	Genetic predisposition	Obesity, lack of exercise, poor diet
Prevalence	About 10% of diabetes cases	About 90% of diabetes cases

6. Higher Tier: Homeostasis and Negative Feedback

6.1 Negative Feedback

Homeostasis is the maintenance of a constant internal environment. Negative feedback is the Mechanism by which the body restores conditions to their optimum level when they deviate.

General pattern:

A change is detected by a receptor.
Information is sent to a coordinator (brain or endocrine gland).
The coordinator sends a signal to an effector.
The effector brings about a response that reverses the change.
The level returns to normal.

Example: Thermoregulation. When the body temperature rises above 37 $^{\circ}$ C, thermoreceptors In the skin and hypothalamus detect the change. The hypothalamus sends nerve impulses to effectors (sweat glands, blood vessels in the skin) that bring about cooling responses (sweating, Vasodilation). When the temperature returns to normal, the response stops.

Detailed mechanism of thermoregulation.

When body temperature rises (too hot):

Thermoreceptors in the skin and hypothalamus detect the increase.
The hypothalamus sends nerve impulses to:

Sweat glands: Increased sweat production. Water in sweat evaporates from the skin surface, removing heat energy (latent heat of vaporisation).
Blood vessels in the skin: Vasodilation occurs. The blood vessels dilate (widen), increasing blood flow near the skin surface so more heat can be radiated away.

Body temperature decreases back to 37 $^{\circ}$ C.

When body temperature falls (too cold):

Thermoreceptors detect the decrease.
The hypothalamus sends nerve impulses to:

Blood vessels in the skin: Vasoconstriction occurs. Blood vessels narrow, reducing blood flow near the skin surface to reduce heat loss.
Muscles: Shivering occurs. Rapid, involuntary muscle contractions generate heat as a by-product of respiration.
Body hairs: Erector pili muscles contract, causing body hairs to stand up. This traps a layer of insulating air next to the skin (more effective in furry animals than in humans).

Body temperature increases back to 37 $^{\circ}$ C.

6.2 The Endocrine System

Feature	Nervous system	Endocrine system
Signal	Electrical impulses	Chemical (hormones)
Speed	Very fast	Slower
Duration	Short-lived	Longer-lasting
Target	Very specific	Widespread (cells with receptors)

Worked Example: Comparing nervous and endocrine responses.

Consider two situations:

You touch a hot object. This triggers a nervous response (reflex arc). The impulse travels along neurones at high speed, reaching the effector in milliseconds. The response (pulling your hand away) is fast and short-lived.
You eat a large meal. This triggers an endocrine response. The pancreas detects high blood glucose and releases insulin into the bloodstream. Insulin travels more slowly than a nerve impulse (seconds to minutes), but its effects are longer-lasting (hours), as it causes cells throughout the body to take up glucose.

Key point: The nervous system is for fast, short-lived, targeted responses. The endocrine system Is for slower, longer-lasting, widespread responses.

Common Pitfalls

Confusing arteries and veins. Arteries carry blood away from the heart ( oxygenated); veins carry blood towards the heart ( deoxygenated). Exception: pulmonary arteries carry deoxygenated blood.
Misunderstanding the lock and key model. High temperatures denature the enzyme by changing the active site shape, not by “melting” the enzyme.
Confusing the left and right sides of the heart. The right side pumps to the lungs; the left side pumps to the body.
Forgetting the adaptations of the alveoli. A complete answer must mention surface area, thin walls, moisture, and good blood supply.
Writing the reflex arc in the wrong order. Receptor $\to$ sensory $\to$ relay $\to$ motor $\to$ effector.
Confusing type 1 and type 2 diabetes. Type 1 = no insulin (autoimmune, injection); type 2 = insulin resistance (lifestyle-related).
Stating that bile digests fat. Bile emulsifies fat (physical breakdown); lipase digests fat.
Forgetting that the pulmonary artery carries deoxygenated blood. A common exam trap.
Writing “energy” in the word equation for respiration. Energy is not a substance. Only list chemical substances (glucose, oxygen, carbon dioxide, water).
Confusing osmosis and diffusion. Osmosis is the specific movement of water across a selectively permeable membrane. Diffusion is the movement of any substance from high to low concentration.
Forgetting the control variables in the enzyme practical. The volume and concentration of amylase and starch, the volume of iodine, and the pH (when testing temperature) must all be kept constant.

Practice Questions

Explain how the small intestine is adapted for absorption of digested food molecules.
Describe the pathway of blood through the heart, naming the chambers, valves, and major blood vessels.
Explain the lock and key model of enzyme action and why high temperatures cause enzymes to denature.
Describe the mechanism of ventilation (breathing in and breathing out).
Draw and label a reflex arc for the response to touching a hot object.
Explain how the structure of an artery differs from that of a vein and relate this to their functions.
Describe the role of bile in digestion and explain why it is important.
Explain how the eye focuses on near and distant objects (accommodation).
Compare type 1 and type 2 diabetes, including causes and treatments.
Explain the effect of increasing temperature on enzyme activity, using a graph to illustrate your answer.
(Higher Tier) Explain the mechanism of negative feedback in thermoregulation.
(Higher Tier) Compare the nervous system and the endocrine system.
Explain why capillaries are adapted for efficient exchange of materials between the blood and body cells.
A person has a resting heart rate of 72 beats per minute and a stroke volume of 70 mL. Calculate their cardiac output in litres per minute.
Explain how the structure of a red blood cell is adapted for its function of transporting oxygen.
Describe the structure of a synapse and explain how nerve impulses are transmitted across it.
Explain why enzymes in the stomach have a different optimum pH from enzymes in the small intestine.
A patient is diagnosed with coronary heart disease. Describe two treatments for this condition and explain how each works.
Explain the difference between a receptor and an effector, giving one example of each.
Describe the role of the liver in the digestive system and explain why bile is important even though it does not contain any digestive enzymes.
Explain why a person with type 1 diabetes must inject insulin rather than take it orally.
A student investigates the effect of pH on the activity of pepsin. Describe the method they would use, the results they would expect, and explain the shape of the graph.
Describe and explain what happens during an asthma attack, including the effects on the airways and gas exchange.
Explain how vaccination provides protection against disease, referring to the role of lymphocytes and memory cells.
Evaluate the claim that “most cases of type 2 diabetes could be prevented through lifestyle changes.” Support your argument with scientific evidence.
Explain why the left ventricle has a thicker muscular wall than the right ventricle.
Describe the process of blood clotting, naming the key substances involved.
A student sets up an experiment to investigate the effect of temperature on amylase activity. The student forgets to include a control. Explain why this is a problem and describe what the control should be.
Explain how the structure of the trachea is related to its function, including the role of the C-shaped cartilage rings and the ciliated epithelium.
(Higher Tier) A person has been drinking alcohol. Explain the sequence of events that occurs in their liver to detoxify the alcohol, and explain why excessive alcohol consumption can lead to liver disease.

Practice Problems

Question 1: Enzyme experiment analysis

A student tests the effect of temperature on the rate of catalase activity using hydrogen peroxide and potato tissue. The results show that the rate increases from $10^\circ\mathrm{C$ to $40^\circ\mathrm{C$ and then decreases rapidly. Explain the shape of the graph.

Answer

From $10^\circ\mathrm{C$ to $40^\circ\mathrm{C$ : increasing temperature increases the kinetic energy of enzyme and substrate molecules, leading to more frequent successful collisions and a higher rate of reaction.

Above $40^\circ\mathrm{C$ : the enzyme denatures because the high temperature breaks the bonds maintaining its tertiary structure. The active site changes shape and can no longer bind the substrate, so the rate drops rapidly. The optimum temperature is approximately $40^\circ\mathrm{C$ .

Question 2: Digestive system

Explain why the small intestine is well adapted for absorbing digested food molecules into the blood.

Answer

The small intestine has a large surface area due to: (1) its length (several metres), (2) villi (finger-like projections), and (3) microvilli on the villi surface cells. It has a single layer of epithelial cells, providing a short diffusion distance. It has a dense network of blood capillaries to carry away absorbed molecules and maintain a steep concentration gradient. It also has lacteals for absorbing fatty acids and glycerol into the lymphatic system.

Question 3: Blood components

Describe the functions of red blood cells, white blood cells, and platelets in the blood.

Answer

Red blood cells: transport oxygen (bound to haemoglobin) from the lungs to body tissues. Their biconcave disc shape increases surface area for gas exchange. They have no nucleus, maximising space for haemoglobin.

White blood cells (phagocytes): defend against pathogens by phagocytosis (engulfing and digesting pathogens). Lymphocytes produce antibodies for specific immune defence.

Platelets: cell fragments involved in blood clotting. They accumulate at wound sites and trigger the cascade of reactions that produces fibrin, forming a mesh that traps red blood cells and seals the wound.

Question 4: Coronary heart disease

Explain how a blockage in a coronary artery can lead to a heart attack, and describe two lifestyle factors that increase the risk.

Answer

A blockage (caused by a thrombus/blood clot or atherosclerosis — fatty deposits) in a coronary artery reduces or stops blood flow to part of the heart muscle. The cardiac muscle is deprived of oxygen and glucose (ischaemia), so it cannot respire aerobically. The muscle cells may be damaged or die (myocardial infarction), causing a heart attack.

Risk factors: high-fat diet (leading to cholesterol deposits in arteries), smoking (damages artery walls and increases blood pressure), lack of exercise, obesity, high blood pressure, stress.

Question 5: Nervous system reflex arc

Describe the pathway of a reflex arc when a person touches a hot object. Explain why reflex actions are important for survival.

Answer

Pathway: stimulus (heat) $\to$ receptor (temperature receptor in skin) $\to$ sensory neuron $\to$ relay neuron (in spinal cord) $\to$ motor neuron $\to$ effector (muscle in arm) $\to$ response (hand is pulled away).

Reflexes are important because they are fast (the pathway is short, bypassing the brain) and automatic, providing immediate protection from harmful stimuli before the brain has time to process the information. This prevents tissue damage.

Worked Examples

Example 1:

A typical exam question on Organisation requires you to apply your knowledge to an unfamiliar context. Read the question carefully, identify the key concept being tested, and structure your answer using the appropriate terminology.

Example 2:

Multi-step problems in Organisation often combine two or more concepts. Break the problem down: identify what you need to find, recall the relevant formula or principle, substitute values, and state your answer with correct units or formatting.

Summary

This topic covers the biological principles of organisation, including key concepts, experimental evidence, and real-world applications.

Key concepts include:

key biological principles and concepts
experimental methods and data analysis
applications of biology in medicine and industry
ethical considerations in biological research
the relationship between structure and function

Success requires the ability to recall specific factual content, apply knowledge to novel scenarios, and evaluate experimental evidence critically.

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