10 min readPathway sheet1-mark MCQ, a 3-mark flow chart of the pathways of glucose breakdown, and a 5-mark answer with the respiratory system diagram

Respiration and the human respiratory system

Respiration breaks down glucose to release energy stored as ATP. With oxygen it gives CO₂ and water; without oxygen it gives ethanol or lactic acid.

First step (all cells)Glucose (6-carbon) → pyruvate (3-carbon) + energy, in the cytoplasm

Aerobic stepIn mitochondria: pyruvate → CO₂ + H₂O + much energy

Energy currencyATP; breaking its end phosphate bond releases 30.5 kJ/mol

Gas exchange siteAlveoli of the lungs

Three pathways of breaking down glucose

Pathway	Where	What pyruvate becomes	Example
Absence of oxygen (anaerobic)	Cytoplasm of yeast	Ethanol + CO₂ + energy (fermentation)	Yeast making bread dough rise
Lack of oxygen	Cytoplasm of muscle cells	Lactic acid + energy	Muscle cramps after a fast race
Presence of oxygen (aerobic)	Mitochondria	CO₂ + H₂O + much more energy	Most cells of plants and animals

Aerobic vs anaerobic respiration

AerobicUses oxygen. Glucose first becomes pyruvate in the cytoplasm; the pyruvate is then broken down in the mitochondria. Glucose is broken down completely to CO₂ and water. Releases much more energy.

AnaerobicNo oxygen. Takes place in the cytoplasm. Breakdown is incomplete: ethanol and CO₂ in yeast, lactic acid in muscle. Releases less energy.

The energy released makes ATP from ADP and phosphate. Cells break ATP to power muscle contraction, nerve impulses and the making of proteins. Plants exchange gases through stomata: at night they mainly give out CO₂; in the day, CO₂ is used in photosynthesis and oxygen release is the main event.

Breathing and gas exchange in humans

1
Air enters through the nostrils. Fine hairs and mucus trap dust and germs.
2
Air passes through the pharynx (throat) into the trachea (windpipe). Rings of cartilage stop the trachea from collapsing when there is little air in it.
3
The trachea divides into two bronchi, which branch into smaller bronchioles in the lungs.
4★
Bronchioles end in tiny balloon-like alveoli, covered by a network of blood capillaries.
Spread out flat, the alveoli would cover about 80 m²
5
Breathing in: the ribs are lifted and the diaphragm flattens, so the chest cavity grows and air is sucked in.
The lungs always hold some air, so there is time for gas exchange
6★
In the alveoli, oxygen passes into the blood and CO₂ passes out into the air, to be breathed out.
7
Haemoglobin in red blood cells carries oxygen. CO₂ is more soluble in water, so it is carried mostly dissolved in the plasma.

★ key step

Activity: carbon dioxide is given out in respiration

1
Put some germinating seeds (moong or gram) in a conical flask.
2
Hang a small test tube of potassium hydroxide (KOH) solution inside the flask with a thread.
3
Close the flask with a cork carrying a bent glass tube. Dip the other end in a beaker of coloured water. Seal the cork with Vaseline and mark the water level in the tube.
4
Leave the set-up for one or two hours.
5
Observe: water rises in the bent tube.
6★
Reason: the seeds respire and give out CO₂. KOH absorbs the CO₂, so the air pressure in the flask falls and water is pushed up the tube.
Quick check: CO₂ turns lime water milky

★ key step

How to draw the human respiratory system

Draw the nasal passage and mouth cavity at the top.
Draw the pharynx leading down to the larynx (voice box) and a straight tube, the trachea. Show rings of cartilage as short curved lines across it.
Split the trachea into two bronchi, one entering each lung.
In one lung, draw branching bronchioles ending in grape-like alveoli; add an enlarged circle showing an alveolar sac wrapped in blood capillaries.
Draw the ribs around the lungs and the dome-shaped diaphragm below them. Label every part.

In real life

After a fast race on sports day your legs may ache: that is lactic acid from respiration without enough oxygen. The same kind of pathway in yeast gives off the CO₂ that makes bread and pav dough rise.

Exam angle

3 marks: “Show by a flow chart how glucose is broken down in yeast, in muscle cells short of oxygen, and in mitochondria.” Glucose → pyruvate + energy (cytoplasm). Then: yeast, no oxygen → ethanol + CO₂ + energy; muscle, lack of oxygen → lactic acid + energy; mitochondria, oxygen → CO₂ + H₂O + energy.

2 marks: “Why do aquatic animals breathe much faster than land animals?” Much less oxygen is dissolved in water than is present in air. So aquatic animals must move water over their gills faster to get enough oxygen.

5 marks: “Describe the human respiratory system with a labelled diagram, and explain how gases are exchanged in the lungs.” Path of air from nostrils to alveoli; role of mucus, hairs and cartilage rings; breathing in by ribs and diaphragm; diffusion of O₂ into blood and CO₂ out; haemoglobin; labelled diagram.

Remember it

No oxygen: yeast makes Ethanol, muscle makes Lactic acid. With oxygen: mitochondria make CO₂ and Water.