1. Aerobic & Anaerobic Respiration

In brief:Respiration is the chemical process by which energy is released from food. It can occur with oxygen (aerobic) or without (anaerobic).

Respiration is a chemical process in which energy is released from food (mainly glucose). The energy is stored as ATP. All living cells respire. Do not confuse with breathing!

Aerobic Respiration

Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP)

A two-stage process:

Stage	Location	Oxygen?	ATP
Stage 1 (Glycolysis)	Cytosol	Not needed	2 ATP
Stage 2 (Krebs + ETC)	Mitochondria	Required	36 ATP

Stage 1: Glucose (6C) → 2 Pyruvate (3C) + 2 ATP

Stage 2: Pyruvate → CO₂ + H₂O + 36 ATP (in mitochondria)

Anaerobic Respiration (Fermentation)

Occurs without oxygen. One-stage process in the cytosol. Produces much less energy.

Alcohol fermentation (yeast/plants): Glucose → Ethanol + CO₂ + 2 ATP

Lactic acid fermentation (animals/some bacteria): Glucose → Lactic acid + 2 ATP

Lactic acid in muscles causes cramp. Extra oxygen is needed to break it down - this is the 'oxygen debt'.

Uses of Fermentation

Brewing and winemaking (alcohol fermentation by yeast)
Bread-making (CO₂ makes bread rise)
Bioethanol production from crops - renewable fuel

Mitochondrion structure labelled — Mitochondrion - matrix (Krebs cycle) and cristae (electron transport chain)
Wikimedia Commons (public domain / CC)

Aerobic respiration stages — Aerobic respiration - glycolysis, Krebs cycle and electron transport chain
Wikimedia Commons (public domain / CC)

Key Points

1Respiration releases energy from food as ATP - all living cells respire
2Aerobic: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 38 ATP (total)
3Stage 1 (glycolysis): cytosol, no O₂ needed, glucose → 2 pyruvate + 2 ATP
4Stage 2: mitochondria, needs O₂, pyruvate → CO₂ + H₂O + 36 ATP
5Anaerobic: no oxygen - alcohol fermentation (yeast) or lactic acid (animals)
6Lactic acid causes muscle cramp; oxygen debt needed to remove it

Learning Outcomes

Outline the processes of anaerobic respiration, aerobic respiration and photosynthesis
Investigate the conditions necessary for fermentation, use primary and secondary data to support conclusions
Model the two-stage processes of photosynthesis and respiration; make particular reference to the role of transfer molecules
Recognise the significance of the internal structures of mitochondria and chloroplasts in facilitating the processes of photosynthesis and respiration

2. The Breathing System

In brief:The breathing system draws air into the lungs so oxygen can enter the blood and carbon dioxide can leave it. Breathing is not the same as respiration.

Breathing (ventilation) is the mechanical movement of air in and out of the lungs. Respiration is the chemical release of energy from glucose inside cells - the two work together but are different processes.

Pathway of Air

Nose/mouth → pharynx → larynx → trachea → bronchi → bronchioles → alveoli.

Upper tract (nose, pharynx, larynx): warms, moistens and filters incoming air. Cilia and mucus trap dust and microbes.
Trachea: kept open by C-shaped rings of cartilage.
Bronchi & bronchioles: branching tubes leading to the alveoli.
Alveoli: tiny thin-walled air sacs surrounded by capillaries where gas exchange occurs by diffusion.

Mechanism of Breathing

	Inhalation	Exhalation
Diaphragm	Contracts, flattens	Relaxes, domes up
External intercostals	Contract - ribs move up & out	Relax - ribs move down & in
Chest volume	Increases	Decreases
Pressure in lungs	Decreases below atmospheric	Increases above atmospheric
Air movement	Air rushes in	Air pushed out

Control of Breathing

Breathing rate is controlled by the medulla oblongata in the brain. It responds mainly to the concentration of carbon dioxide in the blood (detected as a drop in blood pH), not oxygen. Rising CO₂ → faster, deeper breathing to remove it.

Human respiratory tract labelled — The human breathing system - trachea, bronchi and lungs
Wikimedia Commons (public domain)

Alveolus and capillary gas exchange — Gas exchange at an alveolus - O₂ into blood, CO₂ out
Wikimedia Commons (public domain)

Key Points

1Breathing = ventilation of the lungs; respiration = ATP production in cells
2Air pathway: nose → trachea → bronchi → bronchioles → alveoli
3Alveoli have thin walls and dense capillaries for efficient diffusion
4Inhalation: diaphragm and external intercostals contract, chest volume rises, pressure falls
5Exhalation: muscles relax, chest volume falls, pressure rises, air out
6Medulla oblongata controls breathing rate by monitoring blood CO₂