Future Scientists

Revision note 2.1

Biological Organisation and Exchange

Multicellular organisms organise specialised cells into working systems. As organisms become larger, exchange surfaces and transport systems keep every cell supplied.

CoreCalculationExtended

What you need to know

  • Order cell, tissue, organ, organ system and organism.
  • Explain why large multicellular organisms need exchange and transport systems.
  • Calculate surface area, volume and surface-area-to-volume ratio.
  • Relate the features of an exchange surface to a high rate of diffusion.
  • Apply Fick's law qualitatively and, where required, quantitatively.

Levels of biological organisation

CellA basic living unit, such as a muscle cell.
TissueSimilar cells working together, such as muscle tissue.
OrganDifferent tissues working together, such as the heart.
Organ systemOrgans cooperating, such as the circulatory system.
OrganismThe complete living individual.

An organ is not made from only one cell type. The heart, for example, contains cardiac muscle tissue, nervous tissue, connective tissue and blood vessels. Together they allow the organ to contract rhythmically and pump blood.

Digestive system

Organs break large food molecules into small soluble molecules and absorb them.

Breathing system

Ventilates the lungs and exchanges oxygen and carbon dioxide.

Circulatory system

Transports gases, nutrients, hormones, heat and waste materials.

Why larger organisms need specialised exchange

A very small organism has a short diffusion distance and a large surface area compared with its volume. Diffusion across its outer surface may meet every cell's needs. In a large multicellular organism, many cells lie far from the body surface and the volume requiring supplies grows faster than the surface available for exchange.

Metabolically active cells require oxygen and nutrients and release carbon dioxide and other wastes. Specialised exchange surfaces move substances between the body and its environment. Mass-transport systems then carry those substances between the exchange surface and cells.

Exchange surface

A specialised boundary across which substances move between an organism and its environment, such as an alveolus or the lining of the small intestine.

Misconception AlertLarge organisms have a smaller total surface area than small organisms.Select to reveal the correctionSelect to hide the correction
Correct understanding

Large organisms generally have a greater total surface area but a smaller surface-area-to-volume ratio.

Surface-area-to-volume ratio

surface-area-to-volume ratio = surface area ÷ volumeWrite the final comparison as a ratio to 1 where possible.

For a cube of side length l, surface area = 6l2 and volume = l3.

Cube sideSurface areaVolumeSA:V
1 cm6 cm21 cm36:1
2 cm24 cm28 cm33:1
4 cm96 cm264 cm31.5:1

As linear size increases, SA:V decreases. Exchange through the surface becomes less able to meet the needs of the whole volume.

Features of efficient exchange surfaces

  • Large surface area: more membrane is available for particles to cross.
  • Thin barrier: a short diffusion distance increases the rate.
  • Steep concentration gradient: a large difference in concentration drives faster net diffusion.
  • Good blood supply: carries absorbed substances away and delivers substances for removal.
  • Ventilation: refreshes air or water at a gas-exchange surface.

Alveoli, villi and capillary networks use different combinations of these adaptations. Their structures are not identical, but each maintains rapid exchange.

rate of diffusion ∝ (surface area × concentration difference) ÷ membrane thicknessThis GCSE form of Fick's law shows how each factor affects rate.
Exam tip

Do not simply list an adaptation. Link it to its consequence: “The alveolar wall is one cell thick, giving a short diffusion distance, so oxygen diffuses rapidly.”

Quick retrieval check

1. What is a tissue?
A group of similar cells working together to perform a function.
2. Why is diffusion across the outer surface insufficient for a large organism?
It has a low surface-area-to-volume ratio and many cells are a long distance from the surface.
3. Calculate the SA:V of a cube with side 3 cm.
Surface area = 6 × 3² = 54 cm². Volume = 3³ = 27 cm³. SA:V = 54:27 = 2:1.
4. How does a blood supply maintain a concentration gradient?
It continually brings one substance to the surface or carries an exchanged substance away.

Exam connection

Question

A flatworm is thin and has no specialised breathing organs. Explain how its shape makes gas exchange possible.

Show the mark points
  • The thin shape gives a large surface-area-to-volume ratio.
  • Every cell is close to the body surface, so diffusion distance is short.
  • Oxygen can diffuse in and carbon dioxide can diffuse out rapidly enough to meet demand.