2.2.2Scaling Up

Transport Into and Out of Organisms — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions

5 min read 9 parts Lesson 9 of 16

📚 OCR Gateway GCSE Combined Science A J250 - Biology Hub / Topical Revision

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Part 1 of 7Gas exchange in the human lungs

Air travels down the trachea, into the bronchi and then bronchioles to the alveoli.

Gas exchange in the human lungs

Oxygen diffuses from the air in the alveoli, where its concentration is high, into the blood in the surrounding capillaries, where it is low.

Carbon dioxide diffuses the opposite way and is breathed out.

Ventilation (breathing in and out) keeps the concentration of oxygen in the alveoli high and the carbon dioxide low,

and blood flow removes oxygen quickly,
so the concentration gradient stays steep.

Adaptationshuge number of alveoli (large area), walls one cell thick, moist lining so gases dissolve, and a dense network of capillaries.

Part 2 of 7Absorption in the small intestine

After digestion, soluble molecules such as glucose, amino acids, fatty acids and glycerol are absorbed into the blood across the wall of the small intestine.

The inner surface is covered with villi, and each epithelial cell has microvilli, which greatly increases surface area.

The villus wall is one cell thick, so the diffusion distance is short.

Each villus has a capillary network that takes away glucose and amino acids, and a lacteal that takes up fatty acids and glycerol.

Blood flow keeps the concentration gradient steep.

When the gut concentration is lower than the blood, cells use active transport, powered by many mitochondria, to take in glucose.

Part 3 of 7Gills in fish

Fish take in oxygen dissolved in water.

Water flows over gills made of many thin filaments; each filament carries many lamellae, which give a very large surface area.

The lamellae are thin and have a capillary network.

Blood flows through the lamellae in the opposite direction to the water (counter-current flow),

so blood always meets water with a higher oxygen concentration, and the gradient is maintained along the whole length.

Out of water the gills collapse and stick together, so the surface area falls and the fish suffocates.

Part 4 of 7Leaves and roots

Leaves carbon dioxide diffuses in through the stomata and into the air spaces of the spongy mesophyll, where it dissolves in the film of water on cell walls and diffuses into the cells.

Leaves and roots

Oxygen and water vapour leave the same way.

The flat shape, thin structure and many air spaces give a large surface area and short distance.

Guard cells open and close the stomata to control water loss.

Roots

root hair cells have long thin extensions that give a large surface area for absorbing water by osmosis and mineral ions by active transport.
They are close to the xylem in the root.

Part 5 of 7Comparing surfaces

Exchange surface	Substances	Key adaptations
Alveoli	O₂ in, CO₂ out	Many alveoli, thin walls, capillaries, ventilation
Villi	Digested food	Folding, microvilli, capillaries, lacteal
Gill lamellae	O₂ and CO₂ in water	Thin plates, counter-current blood flow
Leaf mesophyll	CO₂, O₂, water vapour	Air spaces, stomata, thin shape
Root hairs	Water, mineral ions	Extended hair, many mitochondria

Part 6 of 7Describing and explaining a gradient

Questions on exchange surfaces use the same structure of answer: name the substance, state the direction of movement and the process, state the concentration difference, then explain how the adaptation maintains or exploits it.

Describing and explaining a gradient

For example, in the alveolus: oxygen moves from the alveolar air to the blood by diffusion; its concentration is higher in the air because breathing brings in fresh air; the capillary wall and alveolus wall are each one cell thick;

and the blood keeps flowing so that oxygenated blood is carried away and replaced by blood of lower oxygen concentration.

In the exam, avoid vague phrases such as "so that it is easier"; instead, give the physical reason,

such as a shorter diffusion distance or a steeper gradient.

Exercise and disease change these conditions. During exercise, breathing becomes deeper and faster and the heart pumps faster, so the gradient is maintained even though the muscles use oxygen more quickly. In diseases such as emphysema the alveolar walls break down, reducing the surface area and making gas exchange less effective, so the patient feels breathless.

Remember that the same principles apply to every surface: a large area, a thin barrier, a steep gradient maintained by transport or movement of the medium, and in some cases active transport when diffusion alone cannot move the substance in the needed direction.

Part 7 of 7Exam-style questions

Describe how the structure of a villus helps absorption of glucose. [3]3 marks
Explain why a fish dies quickly when taken out of water. [3]3 marks
Explain how carbon dioxide enters a leaf for photosynthesis. [2]2 marks

Key terms

Term	Meaning
Ventilation	Movement of air in and out of the lungs.
Villi	Finger-like projections lining the small intestine.
Lacteal	A lymph vessel in a villus that absorbs fatty acids and glycerol.
Lamellae	Thin plates on gill filaments where gas exchange happens.
Counter-current flow	Water and blood flowing in opposite directions to keep a diffusion gradient along the whole surface.
Stomata	Pores in the leaf surface for gas exchange.
Root hair cell	Plant cell specialised to absorb water and mineral ions.

Avoid these mistakes

Saying that oxygen is "pumped" into the blood; it diffuses down a concentration gradient.

Forgetting the role of ventilation or blood flow in maintaining the gradient.

Saying that lacteals absorb glucose; they absorb fats (fatty acids and glycerol).

Describing counter-current flow without saying that the flows run in opposite directions.

Writing that plant roots absorb oxygen for photosynthesis; roots absorb water and ions.

Last updated: 20 October 2026