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

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Transport in Plants

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Communicable Diseases, Disease Prevention and the Immune System

Topic overview Pathogens and how disease is transmitted Plant defences and non-specific animal defences Phagocytes and phagocytosis The specific immune response Antibodies Types of immunity Vaccination Medicines and antibiotic resistance

Biodiversity

Topic overview The three levels of biodiversity Sampling methods and equipment Simpson's Index of Diversity Measuring genetic biodiversity Threats to biodiversity and reasons to maintain it Conservation: in situ and ex situ

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Communication and Homeostasis

Topic overview Cell signalling and communication systems Homeostasis, negative and positive feedback Thermoregulation in endotherms and ectotherms

Excretion as an Example of Homeostatic Control

Topic overview Excretion and nitrogenous waste The liver: structure and function The structure of the kidney The nephron Ultrafiltration Selective reabsorption and the loop of Henle Osmoregulation and ADH Kidney failure, dialysis and transplant Urine testing and monoclonal antibodies

Neuronal Communication

Topic overview Sensory receptors and the Pacinian corpuscle Types of neurone and myelination Resting potential and the action potential Transmission across a cholinergic synapse Summation and inhibitory synapses

Hormonal Communication

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Plant and Animal Responses

Topic overview Tropisms and plant responses to stress and herbivory Plant hormones and their commercial uses The nervous system and the brain Reflex actions and the knee jerk reflex Fight or flight and the control of heart rate The structure of skeletal muscle The neuromuscular junction and the sliding filament model

Photosynthesis

Topic overview Photosynthesis and chloroplast structure Photosynthetic pigments and chromatography The light-dependent stage The Calvin cycle Limiting factors

Respiration

Topic overview Respiration, ATP and the mitochondrion Glycolysis The link reaction and the Krebs cycle Oxidative phosphorylation and chemiosmosis Anaerobic respiration Respiratory substrates and the respiratory quotient

Cellular Control

Topic overview Gene mutations and their effects The lac operon Transcription factors and post-transcriptional control Homeobox and Hox genes Mitosis and apoptosis Oncogenes, tumour suppressor genes and cancer

Patterns of Inheritance

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Manipulating Genomes

Topic overview DNA sequencing and bioinformatics DNA profiling, PCR and gel electrophoresis Genetic engineering: restriction enzymes and vectors Genetic modification, its ethics, and gene therapy

Cloning and Biotechnology

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Ecosystems

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Populations and Sustainability

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Practical Skills and Mathematical Requirements

Topic overview Planning investigations and choosing apparatus Recording and presenting data Gradients, intercepts and logarithmic scales Averages, spread and statistical tests Evaluating experiments and uncertainty Estimates, symbols and biological calculations

Exchange Surfaces

Surface area to volume and the features of an exchange surface

2 things to know
Components 01 and 03

Overview

Why diffusion alone fails as an organism grows, the surface area to volume calculation, and the large area, thin barrier and steep gradient an exchange surface needs.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What makes an exchange surface efficient?

Key term

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

Questions from this subtopic

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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