IGCSE Biology: Human Biology — Nutrition, Transport, Respiration, Reproduction

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IGCSE Biology: Human Biology — Nutrition, Transport, Respiration, Reproduction

TL;DR

Your body breaks down food for energy (respiration), moves nutrients and oxygen around via blood (transport), and can create new humans through sexual reproduction. These four systems are interconnected: digestion extracts nutrients, circulation delivers them, cells burn them for ATP, and reproductive organs pass genes to the next generation.

1. The Mental Model

Think of your body as a city. Nutrition is the supply chain bringing raw materials (food) in and breaking them down. Transport is the road network carrying those materials and oxygen everywhere they're needed. Respiration is the power station in every cell, converting glucose into usable energy. Reproduction is the blueprint factory ensuring the species continues. None of these work alone—each depends on the others.

2. The Core Material

2.1 Nutrition: From Mouth to Absorption

Digestion happens in stages. Your teeth mechanically break food into smaller pieces (more surface area). Then enzymes chemically break bonds:
- Amylase (in saliva and pancreas) breaks down starch → maltose
- Pepsin (in stomach) breaks proteins → peptides
- Lipase (in pancreas) breaks fats → fatty acids and glycerol

The small intestine is where magic happens. Its lining has millions of villi—tiny finger-like projections that absorb glucose, amino acids, and fatty acids into the bloodstream. This is active transport for glucose (uses ATP) and diffusion for some fats. The liver then processes these nutrients: excess glucose becomes glycogen (stored in liver and muscles), amino acids are deaminated (nitrogen removed, used to make new proteins), and fatty acids are transported as lipoproteins.

Key point: Not all food can be absorbed. Indigestible fibre (cellulose) passes through—it's not wasted; it helps digestion by adding bulk and feeding your gut bacteria.

2.2 Transport: The Circulatory System

Your heart pumps blood in a double circuit:
1. Pulmonary circulation: Right side of heart → lungs (pick up O₂, drop CO₂) → back to left side
2. Systemic circulation: Left side → body (deliver O₂ and nutrients, collect CO₂ and waste) → back to right side

Blood vessels matter:
- Arteries: Thick, muscular walls, carry blood away from heart at high pressure. Pulse = artery expanding.
- Capillaries: One cell thick, allow exchange of gases and nutrients with tissues. Red blood cells squeeze through single-file.
- Veins: Thin, low pressure, carry blood back to heart. Have valves (prevent backflow).

Red blood cells carry oxygen bound to haemoglobin (iron-containing protein). In lungs, oxygen saturation is high (~98%); in respiring tissues, it drops (~70%) as cells grab the oxygen they need. This is why tissue that works hard (muscle during exercise) needs more blood—it has higher oxygen demand.

The heart itself is a four-chambered pump: right atrium and ventricle (low-pressure side, pulmonary), left atrium and ventricle (high-pressure side, systemic). Valves between chambers prevent backflow. The cardiac cycle has two phases: systole (contraction, ventricles pump), diastole (relaxation, atria fill).

graph TD
    A["Right Atrium<br/>(deoxygenated blood from body)"] -->|tricuspid valve| B["Right Ventricle"]
    B -->|pulmonary valve| C["Pulmonary Artery → Lungs"]
    C --> D["Oxygenated blood returns<br/>via Pulmonary Vein"]
    D --> E["Left Atrium"]
    E -->|mitral valve| F["Left Ventricle"]
    F -->|aortic valve| G["Aorta → Body tissues"]
    G -->|oxygen extracted,<br/>CO₂ picked up| H["Deoxygenated blood<br/>returns via Vena Cava"]
    H --> A

2.3 Respiration: Aerobic and Anaerobic

Aerobic respiration (with oxygen) is your main energy source:
$$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{energy (ATP)}$$

It happens in three stages:
1. Glycolysis (cytoplasm): Glucose → 2 pyruvate, net gain of 2 ATP
2. Krebs cycle (mitochondrial matrix): Pyruvate → CO₂, releases energy to electron carriers
3. Electron transport chain (inner mitochondrial membrane): Oxygen accepts electrons, ~32 ATP produced

When oxygen runs out (intense sprint, heavy lifting), your muscles switch to anaerobic respiration: glucose → lactate + small ATP gain (2 ATP per glucose, vs ~30 from aerobic). Lactate builds up, causing muscle burn and fatigue. Your liver later converts lactate back to glucose (Cori cycle).

Respiratory substrate matters: carbohydrates are fastest, fats yield more energy per gram but require more oxygen, proteins are used only in starvation.

2.4 Reproduction: Making the Next Generation

Human reproduction is sexual: male and female gametes (sperm and egg) fuse. This requires:

Male system: Testes produce sperm via spermatogenesis. One diploid cell (2n) undergoes meiosis → four haploid sperm (n). Sperm mature in the epididymis, travel through the vas deferens, and are ejaculated mixed with seminal fluid (nutrients + protection).

Female system: Ovaries produce one egg per month via oogenesis. A diploid cell (2n) undergoes meiosis → one large ovum (n) + three polar bodies (discarded). The egg is released at ovulation (day 14 of a 28-day cycle) and travels down the fallopian tube.

Fertilisation: If sperm meets egg in the fallopian tube, the haploid nuclei fuse (n + n = 2n). This zygote is now diploid—it has genes from both parents.

Implantation & pregnancy: The zygote divides repeatedly (mitosis) as it travels to the uterus. By day 7, it's a blastocyst and embeds in the uterine lining. The placenta develops, allowing nutrient and oxygen exchange between mother's blood and fetal blood (they don't mix directly). Pregnancy lasts ~40 weeks; the fetus develops organs, grows, and eventually is born.

Hormones regulate all of this:
- FSH (follicle-stimulating hormone): Triggers egg development in ovary
- LH (luteinising hormone): Triggers ovulation
- Oestrogen: Rebuilds uterine lining, provides negative feedback to stop FSH
- Progesterone: Maintains uterine lining during pregnancy

3. Worked Example

Scenario: A student runs 100 m at maximum speed. Trace what happens in their body.

During the sprint (0–10 seconds):
- Muscles demand ATP fast. Aerobic respiration ramps up instantly (glucose + oxygen → CO₂ + energy), but oxygen delivery lags.
- Glycolysis runs at top speed: glucose → 2 pyruvate → 2 lactate (anaerobic).
- Heart rate spikes to ~180 bpm; left ventricle contracts forcefully; arteries dilate in legs.
- Oxygen saturation in leg capillaries drops to ~50% as muscle cells grab every oxygen molecule.

30 seconds after finishing:
- Breathing rate stays high; the student is in "oxygen debt."
- Lactate in blood is ~10 mmol/L (20× resting level). Liver begins the Cori cycle: lactate → glucose.
- Heart rate slowly falls (parasympathetic nervous system taking over).

In the longer term:
- Glucose stores are depleted. The student feels hungry. Eating a meal triggers insulin release; glucose is absorbed and either used immediately or stored as muscle and liver glycogen.
- If the student does this sprint regularly, muscle mitochondria multiply (more aerobic capacity), and lactate threshold rises.

4. Examiner's Breakdown

4.1 What Examiners Actually Reward

1. Name the enzyme AND its substrate and product, e.g., "Amylase breaks down starch into maltose" — not just "enzymes break down food."
2. State whether transport is active or passive, e.g., "Glucose is absorbed by active transport, using ATP" — specify the mechanism.
3. Link respiration stages to location, e.g., "Glycolysis occurs in the cytoplasm and produces 2 ATP; the Krebs cycle occurs in the mitochondrial matrix" — show you know the cell geography.
4. Distinguish aerobic from anaerobic explicitly, e.g., "Anaerobic respiration produces lactate and only 2 ATP per glucose, whereas aerobic respiration requires oxygen and produces ~30 ATP."
5. Describe the cardiac cycle in the correct sequence, e.g., "Diastole: ventricles relax, atria fill; systole: atria contract, then ventricles contract and push blood out."
6. Explain meiosis produces haploid gametes, e.g., "Meiosis halves the chromosome number, so sperm and egg are n, ensuring the zygote is 2n after fertilisation."

4.2 Trapdoor Mistakes

4.3 Score-Boosting Comparisons

5. Now Try It

Exam-style question (25 minutes):

A 16-year-old girl trains for a 1500 m running race. Answer the following:

(a) Explain how the circulatory system ensures her leg muscles receive enough oxygen during the race. (4 marks)

(b) During the final 200 m sprint, her muscles switch to anaerobic respiration. Explain why this happens and what happens to lactate afterward. (4 marks)

(c) A month later, she has not had her period for 6 weeks. Explain how FSH, LH, and oestrogen regulate the menstrual cycle, and why a missed period might occur. (6 marks)

Model answer checklist for full marks:

(a) Must include: heart rate increases / cardiac output increases; blood vessels in legs dilate (vasodilation) to allow more blood flow; oxygen is released from haemoglobin at higher rate because muscles are respiring faster and lowering oxygen saturation. (Accept any two of these.)

(b) Must state: oxygen is limiting / insufficient oxygen for aerobic respiration / oxygen debt; anaerobic respiration produces lactate faster; lactate is transported to the liver where it is converted back to glucose via the Cori cycle; this allows muscle to continue working despite limited oxygen.

(c) Must include: FSH stimulates egg development in the ovary; LH triggers ovulation (day 14); oestrogen is released by developing follicle, rebuilds uterine lining, and provides negative feedback to FSH; progesterone maintains lining if fertilisation occurs. Missed period = likely pregnancy (zygote embeds, progesterone maintains lining, no menstruation).

Success looks like: You've named the hormones, their sources, and their effects; explained the mechanism of oxygen delivery under stress; and linked lactate metabolism to oxygen availability.

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