Why does diffusion alone become insufficient to meet metabolic demands as organisms increase in size?

As organisms increase in size, their surface area to volume ratio decreases, meaning the surface area available for exchange becomes too small relative to the volume of metabolically active tissue requiring oxygen and nutrients.

State the formula for calculating surface area to volume ratio.

SA:V ratio = surface area ÷ volume

Explain why single-celled organisms do not require specialised exchange surfaces.

Single-celled organisms have a sufficiently high surface area to volume ratio that diffusion across their cell surface membrane is adequate to meet all their metabolic needs.

Why do multicellular organisms require specialised exchange surfaces?

Their lower surface area to volume ratio combined with higher metabolic activity means simple diffusion across the body surface is inadequate to supply sufficient oxygen and remove waste products.

How does metabolic activity affect the requirements for gas exchange systems?

More metabolically active organisms have greater oxygen demands and produce more carbon dioxide, requiring more efficient exchange systems to meet these increased needs.

OCR A-Level Biology H420

Cell Structure

Topic overview Microscopy, slide preparation and staining Biological drawings, magnification and resolution Eukaryotic cell ultrastructure The secretory pathway and the cytoskeleton Prokaryotic and eukaryotic cells compared

Biological Molecules

Topic overview The properties of water Monomers, polymers and the elements of life Monosaccharides and disaccharides Polysaccharides and their functions Lipids Amino acids and protein structure Globular and fibrous proteins Inorganic ions The biochemical tests Colorimetry and chromatography

Nucleotides and Nucleic Acids

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Enzymes

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Biological Membranes

Topic overview Membrane structure and the fluid mosaic model Factors affecting membrane permeability Diffusion, active transport, endocytosis and exocytosis Osmosis and water potential

Cell Division, Cell Diversity and Cellular Organisation

Topic overview The cell cycle Mitosis and observing it in root tips Meiosis Specialised cells Tissues, organs and organ systems Stem cells and differentiation

Exchange Surfaces

Topic overview Surface area to volume and the features of an exchange surface The mammalian gas exchange system Ventilation and lung volumes Gas exchange in fish and insects Dissecting and examining gas exchange tissue

Transport in Animals

Topic overview Why animals need transport systems Arteries, veins and capillaries Tissue fluid and lymph The structure of the mammalian heart The cardiac cycle and control of the heartbeat The electrocardiogram Transporting oxygen and carbon dioxide Oxygen dissociation curves and the Bohr effect

Transport in Plants

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Communicable Diseases, Disease Prevention and the Immune System

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Biodiversity

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Respiration

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Cellular Control

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Exchange Surfaces

5 subtopics
11 things to know
Components 01 and 03

What Exchange Surfaces covers

Exchange surfaces is the first section of Module 3, examined on Biological Processes. It covers why surface area to volume ratio forces large organisms to develop exchange surfaces, the mammalian airways and how ventilation works, lung volumes measured on a spirometer, and gas exchange in fish and insects.

Where it is assessed

Components 01 and 03

Module 3 is examined on Biological Processes, then the unified paper.

Check the OCR specification ↗

The 5 subtopics

Each one has its own page.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What makes an exchange surface efficient?

Process

Explain mammalian ventilation.

Process

Explain gas exchange in fish gills.

Key term

Define tidal volume.

Key term

Define vital capacity.

Key term

What are spiracles in insects?

Key term

Why is a thin barrier important for an efficient exchange surface?

Key term

What is the function of cartilage in the trachea and bronchi?

Key term

What are gill lamellae and what is their function?

Process

How does the insect tracheal system respond to increased metabolic activity?

Key term

Which tissues are identified in a prepared slide of the trachea?

Take Exchange Surfaces further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

Ventilation vs Gas exchange
Ventilation: the movement of air into and out of the lungs. Gas exchange: the diffusion of gases across the alveolar surface.
Tidal volume vs Vital capacity
Tidal volume: the volume of air moved in or out in one normal breath. Vital capacity: the greatest volume that can be exhaled after the deepest inhalation.
Counter-current flow vs Parallel flow
Counter-current flow: blood and water moving in opposite directions to maintain a gradient along the gill. Parallel flow: blood and water moving in the same direction, so the gradient equalises.

Questions from this topic

Each one with the model answer underneath.

1
Why does diffusion alone become insufficient to meet metabolic demands as organisms increase in size?
Model answer
As organisms increase in size, their surface area to volume ratio decreases, meaning the surface area available for exchange becomes too small relative to the volume of metabolically active tissue requiring oxygen and nutrients.
2
State the formula for calculating surface area to volume ratio.
Model answer
SA:V ratio = surface area ÷ volume
3
Calculate the SA:V ratio when surface area = 24 arbitrary units, volume = 18 arbitrary units.
Model answer
SA:V ratio = 24 ÷ 18 = 1.33.
4
Explain why single-celled organisms do not require specialised exchange surfaces.
Model answer
Single-celled organisms have a sufficiently high surface area to volume ratio that diffusion across their cell surface membrane is adequate to meet all their metabolic needs.
5
Why do multicellular organisms require specialised exchange surfaces?
Model answer
Their lower surface area to volume ratio combined with higher metabolic activity means simple diffusion across the body surface is inadequate to supply sufficient oxygen and remove waste products.
6
How does metabolic activity affect the requirements for gas exchange systems?
Model answer
More metabolically active organisms have greater oxygen demands and produce more carbon dioxide, requiring more efficient exchange systems to meet these increased needs.

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