OxfordAQA IGCSE Biology: Bioenergetics — Revision Notes

Condensed for the final weeks. For the full explanation, use the Bioenergetics study guide.

Photosynthesis vs respiration

	Photosynthesis (3.2.1)	Respiration (3.2.6)
Equation	6CO2 + 6H2O → C6H12O6 + 6O2	glucose + oxygen → CO2 + water
Requires light?	Yes	No (continuous)
Occurs in	Chlorophyll-containing cells only	Every living cell
Location	Chloroplasts	Mitochondria

Limiting factors for photosynthesis: light, temperature, CO2 — identify which one is in short supply in the specific scenario given, not all three by default.

Exchange and transport in plants (3.2.2)

Stomata/guard cells control gas exchange and water loss. Xylem carries water (roots → leaves). Phloem carries dissolved sugars (translocation, leaves → rest of plant, either direction depending on need). Increased surface area via root hairs and flattened leaf shape.

The human energy chain

Breathing (in oxygen) → Circulation (double circulatory system, 4 chambers, transports oxygen and glucose) → Respiration (energy released in mitochondria). Trace a single oxygen or glucose molecule through this chain when revising — it links the sub-topics far better than revising each system alone.

Digestion (3.2.4)

Enzymes break down insoluble starch, protein and fat into soluble substances for absorption. Amylase — starch. Protease — protein. Lipase — fat. Enzyme activity depends on temperature and pH, which affect enzyme shape and function.

Worked example: limiting factors

A greenhouse crop grows slower than expected on a bright but cold winter morning.

Light:       plentiful -- not limiting
Temperature: low -- likely limiting (slows enzyme-controlled reactions)
CO2:         assume adequate -- not limiting
Conclusion:  temperature is limiting; heating the greenhouse would be
             more cost-effective than adding supplementary lighting

Circulation and blood (3.2.3)

The heart is an organ that pumps blood around the body in a double circulatory system, and it has four chambers. Arteries carry blood away from the heart (thick, muscular, elastic walls to withstand high pressure); veins carry blood back to the heart (thinner walls, valves to prevent backflow at low pressure); capillaries are the site of exchange (one cell thick, large total surface area). Blood consists of plasma (transports dissolved substances), red blood cells (carry oxygen via haemoglobin), white blood cells (defence) and platelets (clotting).

Content marked B in this sub-topic includes the pacemaker and artificial pacemakers, coronary heart disease and stents, faulty heart valves and their replacement, artificial hearts, and blood groups/transplant rejection. The B marker means this content is examined in Biology only and is not shared with the co-teachable Combined Science qualification – the specification is linear and untiered, so it remains compulsory alongside the structure and function of the heart, blood vessels and blood components.

Breathing mechanics (3.2.5)

Breathing in and out is driven by the intercostal muscles and diaphragm changing the volume (and therefore pressure) of the chest cavity. Alveoli are adapted for efficient gas exchange through a large surface area and rich blood supply, alongside a thin exchange surface and moist lining. Mechanical ventilation for patients whose spontaneous breathing has stopped is marked B (biology-only), but remains compulsory content.

Worked example: tracing oxygen through the energy chain

A sprinter’s leg muscles need a rapid increase in energy supply during a 100m race.

Step 1 (breathing):    intercostal muscles and diaphragm increase
                        breathing rate, drawing more oxygen into
                        the alveoli
Step 2 (circulation):  increased heart rate speeds oxygen and
                        glucose delivery via the double circulatory
                        system to the leg muscle cells
Step 3 (respiration):  mitochondria in the muscle cells use the
                        extra oxygen and glucose supplied to release
                        energy faster via aerobic respiration

This three-step chain – breathing supplies, circulation transports, respiration releases – is the connected-systems answer this topic is built to test, rather than describing each system as an isolated fact set.

Key terms

Photosynthesis — the process converting CO2 and water into glucose and oxygen using light energy, in chlorophyll-containing cells. Respiration — the process releasing energy from glucose and oxygen, producing CO2 and water, in every living cell. Translocation — movement of dissolved sugars through phloem. Transpiration — loss of water vapour from a plant’s leaves, mostly through the stomata; the resulting movement of water from the roots through the xylem and out of the leaves is called the transpiration stream. Enzyme — a biological catalyst whose activity is affected by temperature and pH.

Common mistakes

• Writing the respiration equation when asked about photosynthesis, or vice versa.
• Describing xylem/phloem without stating direction of flow.
• Naming an enzyme without stating what it breaks down.
• Treating B-marked content (heart pacemakers, stents, valve replacement, ventilators) as optional – it is compulsory for every candidate, since the qualification is linear and untiered.

Quick self-test

1. Give the word equation for photosynthesis.
2. Which organelle is the site of aerobic respiration?
3. Name the three digestive enzymes and what each breaks down.
4. What does xylem transport, and in which direction?
5. In the greenhouse scenario above, which factor is limiting growth?
6. Name the four components of blood and state one function of each.

Answers: 1. Carbon dioxide + water → glucose + oxygen (light required). 2. Mitochondria. 3. Amylase (starch), protease (protein), lipase (fat). 4. Water, from roots to leaves. 5. Temperature. 6. Plasma (transports dissolved substances), red blood cells (carry oxygen via haemoglobin), white blood cells (defence against pathogens), platelets (blood clotting).

How this connects forward

Topic 1 (Organisation) covers cells, tissues and organs in general; Bioenergetics applies that structure to the specific systems responsible for capturing and using energy. Several sub-topics later in the specification, including infection and response, assume the digestion, circulation and gas-exchange content covered here, so a solid grasp of this topic’s connected systems pays off well beyond Topic 2 itself.

Official syllabus

OxfordAQA International GCSE Biology (9201) specification, Version 4.3 — oxfordaqaexams.org.uk/9201.