What you'll learn
Respiration and gas exchange are fundamental processes tested extensively in CIE IGCSE Biology papers. This topic covers aerobic and anaerobic respiration at cellular level, the structure and function of gas exchange surfaces in humans and other organisms, and the mechanics of breathing. Understanding the distinction between respiration (the chemical process releasing energy) and gas exchange (the physical movement of gases) is essential for exam success.
Key terms and definitions
Aerobic respiration — the chemical reaction in cells that uses oxygen to break down glucose and release energy, producing carbon dioxide and water as waste products.
Anaerobic respiration — the chemical reaction in cells that breaks down glucose without oxygen, releasing less energy and producing lactic acid (in animals) or ethanol and carbon dioxide (in yeast and plants).
Gas exchange — the diffusion of oxygen and carbon dioxide between an organism and its environment across a gas exchange surface.
Alveoli — tiny air sacs in the lungs with thin walls and rich blood supply, providing a large surface area for gas exchange.
Ventilation — the movement of air into and out of the lungs through breathing movements (inhalation and exhalation).
Tidal volume — the volume of air that moves in or out of the lungs with each breath during normal breathing.
Vital capacity — the maximum volume of air that can be breathed out after the deepest possible breath in.
Diffusion gradient — the difference in concentration of a substance between two areas, which causes net movement from high to low concentration.
Core concepts
The chemistry of respiration
Aerobic respiration occurs in mitochondria and requires oxygen. The word equation is:
glucose + oxygen → carbon dioxide + water (+ energy)
The balanced chemical equation is:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
This reaction releases approximately 2880 kJ per mole of glucose, making it highly efficient for ATP production. CIE IGCSE Biology exams frequently ask students to write both word and balanced chemical equations.
Anaerobic respiration in animals occurs when oxygen supply is insufficient, particularly during intense exercise:
glucose → lactic acid (+ energy)
This releases much less energy than aerobic respiration (approximately 150 kJ per mole). Lactic acid builds up in muscles, causing fatigue and pain. After exercise, the oxygen debt must be repaid through continued rapid breathing to oxidise the accumulated lactic acid in the liver.
Anaerobic respiration in yeast and plants produces different products:
glucose → ethanol + carbon dioxide (+ energy)
This process, called fermentation, is exploited in bread-making (carbon dioxide makes dough rise) and brewing (ethanol production). Exam questions often compare these different types of respiration, requiring students to identify contexts and products.
Uses of energy from respiration
Energy released from respiration is used for:
- Muscle contraction — enabling movement in animals
- Protein synthesis — building proteins from amino acids for growth and repair
- Cell division — providing energy for mitosis and meiosis
- Active transport — moving substances against concentration gradients across cell membranes
- Maintaining constant body temperature — in mammals and birds (endotherms)
- Nerve impulse transmission — enabling communication in the nervous system
Questions requiring students to state uses of energy appear regularly, typically worth 2-3 marks.
Gas exchange surfaces: key adaptations
Efficient gas exchange surfaces share common features that maximise diffusion rates:
- Large surface area — increases the total area available for diffusion
- Thin walls — provides a short diffusion distance (often one cell thick)
- Good blood supply — maintains concentration gradients by bringing oxygen-poor blood and removing oxygen-rich blood
- Ventilation mechanism — maintains concentration gradients on the other side of the exchange surface
These adaptations apply to alveoli in human lungs, gills in fish, and leaves in plants.
Human gas exchange system structure
The respiratory system includes:
Nasal cavity — air is warmed, moistened, and filtered. Mucus traps dust and pathogens; cilia move mucus to the throat.
Trachea — the windpipe, supported by C-shaped cartilage rings that prevent collapse during inhalation. Lined with ciliated epithelial cells and mucus-secreting goblet cells.
Bronchi (singular: bronchus) — two tubes branching from the trachea into each lung, also containing cartilage rings.
Bronchioles — smaller airways branching from bronchi, with smooth muscle in walls that can contract or relax to control airflow.
Alveoli — microscopic air sacs (approximately 300 million in human lungs) where gas exchange occurs. Each alveolus is surrounded by a network of capillaries. The alveolar wall is one cell thick (squamous epithelium), and the capillary wall is also one cell thick, creating a diffusion distance of just two cells.
Mechanics of breathing (ventilation)
Inhalation (breathing in) is an active process:
- External intercostal muscles contract, moving ribs upward and outward
- Diaphragm muscles contract, causing it to flatten and move downward
- These movements increase the volume of the thorax (chest cavity)
- Increased volume decreases pressure in the lungs (below atmospheric pressure)
- Air moves down the pressure gradient from outside into the lungs
Exhalation (breathing out) is normally passive:
- External intercostal muscles relax, ribs move downward and inward
- Diaphragm muscles relax, it returns to its domed shape
- These movements decrease the volume of the thorax
- Decreased volume increases pressure in the lungs (above atmospheric pressure)
- Air moves down the pressure gradient from lungs to outside
The pleural membranes surround each lung and line the thorax, with a thin layer of pleural fluid between them reducing friction during breathing movements.
Gas exchange in alveoli
Oxygen diffuses from alveolar air into blood because:
- Concentration (partial pressure) of oxygen is higher in alveolar air than in deoxygenated blood arriving from the body
- The thin alveolar and capillary walls provide a short diffusion distance
- The large surface area of millions of alveoli allows rapid diffusion
- Constant blood flow maintains the concentration gradient
Carbon dioxide diffuses from blood into alveolar air because:
- Concentration of carbon dioxide is higher in blood arriving from respiring tissues than in alveolar air
- Ventilation constantly removes carbon dioxide-rich air and brings in fresh air, maintaining the gradient
Blood arriving at alveoli contains approximately 16% oxygen and 5% carbon dioxide. Blood leaving alveoli contains approximately 20% oxygen and 4% carbon dioxide. These percentage changes are commonly tested in data analysis questions.
Effects of physical activity on breathing
During exercise, muscles respire faster, requiring more oxygen and producing more carbon dioxide. The body responds by:
- Increasing breathing rate (number of breaths per minute)
- Increasing depth of breathing (tidal volume)
- Increasing heart rate to deliver oxygen faster to muscles
These changes are detected and controlled by the brain (medulla) in response to increased carbon dioxide levels in blood. Students must explain why these changes occur, not just state that they happen.
Gas exchange in other organisms
Fish gills consist of gill filaments with many gill lamellae (plate-like structures) providing large surface area. Water flows over gills in the opposite direction to blood flow (countercurrent flow), maintaining a concentration gradient along the entire length of the gill lamella for maximum oxygen uptake.
Insects use a tracheal system — air-filled tubes (tracheae) branching into smaller tracheoles that deliver oxygen directly to respiring tissues. Gases move by diffusion; larger insects can ventilate by muscular movements. Spiracles (openings along the body) can close to reduce water loss.
Plant leaves exchange gases through stomata (pores, mostly on the lower surface). Guard cells control stomatal opening. The spongy mesophyll layer has air spaces providing a large internal surface area for gas exchange.
Worked examples
Example 1: Comparing respiration types
Question: Complete the table comparing aerobic and anaerobic respiration in humans. [3 marks]
| Feature | Aerobic respiration | Anaerobic respiration |
|---|---|---|
| Oxygen required? | ||
| Products in humans | ||
| Relative amount of energy released |
Answer:
| Feature | Aerobic respiration | Anaerobic respiration |
|---|---|---|
| Oxygen required? | Yes [1] | No [1] |
| Products in humans | Carbon dioxide and water | Lactic acid [1] |
| Relative amount of energy released | Large/more | Small/less |
Examiner note: One mark for each correct pair. "More" and "less" are acceptable for the energy row, but the products must be precise — just writing "CO₂" or "lactate" loses marks if the question specifies humans and full names.
Example 2: Explaining alveolar adaptations
Question: Explain how alveoli are adapted for efficient gas exchange. [4 marks]
Mark scheme answer:
- Large surface area / millions of alveoli [1] — increases rate of diffusion
- Thin walls / one cell thick [1] — short diffusion distance / fast diffusion
- Good/rich blood supply [1] — maintains concentration/diffusion gradient
- Moist surface [1] — allows gases to dissolve
Examiner note: Each feature must be linked to how it improves gas exchange. Simply listing "thin walls, blood supply" without explanation scores zero. The question tests understanding, not just recall.
Example 3: Breathing mechanics
Question: Describe what happens to the ribs, intercostal muscles and diaphragm during inhalation. [3 marks]
Answer:
- External intercostal muscles contract [1]
- Ribs move up and out [1]
- Diaphragm contracts and flattens / moves down [1]
Examiner note: The question asks for three specific structures. Stating "volume increases" or "pressure decreases" doesn't answer what was asked. Read the question carefully and answer precisely what is requested.
Common mistakes and how to avoid them
Mistake: Confusing respiration with breathing/gas exchange. Writing "we respire to get oxygen" or "plants respire during the day and photosynthesize at night." Correction: Respiration is the chemical reaction in cells that releases energy from glucose. It occurs constantly in all living cells. Gas exchange and breathing are the physical processes of moving gases in and out. Plants respire 24 hours a day.
Mistake: Stating that aerobic respiration "produces energy" rather than "releases energy." Correction: Energy cannot be created or destroyed, only converted between forms. Respiration releases energy stored in chemical bonds in glucose. Use precise terminology: "releases energy" not "makes energy" or "produces energy."
Mistake: Giving the product of anaerobic respiration in humans as ethanol and carbon dioxide (that's yeast/plants) or just carbon dioxide. Correction: In humans and other animals, anaerobic respiration produces only lactic acid. No carbon dioxide is produced. Learn the different products: animals = lactic acid; yeast/plants = ethanol + carbon dioxide.
Mistake: Describing alveoli adaptations without explaining how each feature helps gas exchange. Correction: Link each structural feature to its function using "because" or "so that." Example: "Thin walls so short diffusion distance" not just "thin walls."
Mistake: Stating the diaphragm "moves up" during inhalation or that it's a muscle rather than having muscles in it. Correction: The diaphragm is a sheet of muscle and connective tissue. During inhalation, the diaphragm muscles contract, causing it to flatten and move downward. During exhalation, these muscles relax and it returns to a domed shape moving upward.
Mistake: Writing that oxygen and carbon dioxide are "exchanged" in the blood or that oxygen is "picked up" without mentioning diffusion. Correction: Gases move by diffusion down concentration gradients. Use the term "diffusion" explicitly in answers about gas movement. Oxygen diffuses from alveolar air into blood; carbon dioxide diffuses from blood into alveolar air.
Exam technique for Respiration and Gas Exchange
Command words matter: "State" requires a brief answer without explanation (1 mark each point). "Explain" requires a reason or mechanism (typically 2 marks: feature + how it helps). "Describe" requires an account of what happens in sequence. "Compare" requires stating similarities AND differences.
Equation questions: Be prepared to write both word equations AND balanced chemical equations for aerobic respiration. For balanced equations, count atoms on both sides to check. Word equations must use full names: "glucose + oxygen → carbon dioxide + water" not "sugar + O₂ → CO₂ + H₂O."
Graph and data interpretation: Questions may present spirometer traces showing tidal volume and breathing rate, or bar charts comparing oxygen uptake during rest/exercise. Extract data carefully, quote figures with units, and calculate changes when asked. Show working for calculations.
Extended response questions: A 6-mark question might ask you to "explain how the structure of alveoli and the process of ventilation enable efficient gas exchange." Structure your answer logically: introduce alveoli features, link each to function, then explain how breathing maintains gradients. Use appropriate technical vocabulary consistently.
Quick revision summary
Aerobic respiration uses oxygen to break down glucose, releasing large amounts of energy and producing carbon dioxide and water. Anaerobic respiration occurs without oxygen, producing lactic acid in animals (or ethanol and carbon dioxide in yeast), releasing less energy. Gas exchange in human lungs occurs in alveoli, which have large surface area, thin walls, rich blood supply and moist surfaces. Inhalation occurs when intercostal muscles and diaphragm contract, increasing thorax volume and decreasing pressure. Oxygen and carbon dioxide move by diffusion down concentration gradients maintained by blood flow and ventilation.