What is meant by the surface area to volume ratio of an organism?

It is the ratio of the total area of its outer surface to the volume of its body, and it determines how much exchange surface is available to serve each unit of living tissue.

Give two consequences for a large organism of having a small surface area to volume ratio.

Its general body surface is too small to absorb all the oxygen its tissues need, and the innermost cells lie too far from that surface for diffusion to supply them quickly enough.

Name three adaptations that allow an animal to grow larger and more active despite a low surface area to volume ratio.

It can develop a specialised respiratory surface of very large area, a ventilating mechanism that keeps the diffusion gradient steep, and a circulatory system that carries the gases between that surface and the tissues.

Eduqas A-Level Biology A400QS

Biological Molecules

Topic overview Inorganic ions and the properties of water Monosaccharides and disaccharides Polysaccharides, storage and structure Lipids, triglycerides and phospholipids Amino acids and protein structure Condensation, hydrolysis and the food tests

Cell Structure and Organisation

Topic overview The nucleus and the endomembrane system Mitochondria, chloroplasts and other organelles Prokaryotic cells and viruses Cell theory, microscopy and measuring cells Comparing cells and the levels of organisation

Cell Membranes and Transport

Topic overview The fluid-mosaic model Factors affecting membrane permeability Diffusion and facilitated diffusion Osmosis and water potential Active transport and bulk transport Determining water potential in practice

Enzymes

Topic overview Enzymes as biological catalysts The active site and induced fit Temperature, pH and buffers Substrate and enzyme concentration Inhibition and immobilised enzymes

Nucleic Acids and Their Functions

Topic overview Nucleotides and the structure of ATP ATP as the energy currency of the cell The structure of DNA Comparing DNA and RNA Semi-conservative replication The genetic code Transcription Translation and the fate of the polypeptide Extracting DNA from living material

Importance of ATP

Topic overview ATP synthetase and the synthesis of ATP Chemiosmosis and the electrochemical gradient Proton pumps and the electron transport chain Membranes of mitochondria and chloroplasts compared Investigating dehydrogenase activity

Photosynthesis

Topic overview Chloroplast structure and the distribution of chloroplasts Photosynthetic pigments and chromatography Photosystems and light harvesting Non-cyclic and cyclic photophosphorylation The light-independent stage Limiting factors and the rate of photosynthesis Nitrogen, magnesium and plant growth

Respiration

Topic overview The need for respiration and glycolysis The link reaction and the Krebs cycle Reduced NAD, FAD and oxidative phosphorylation The energy budget of aerobic and anaerobic respiration Respiratory substrates and investigating respiration in yeast

Microbiology

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Population Size and Ecosystems

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Human Impact on the Environment

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Mitosis and Meiosis

Topic overview The cell cycle and interphase The stages of mitosis Cytokinesis and the significance of mitosis The stages of meiosis Comparing mitosis with meiosis, and seeing them on prepared slides

Sexual Reproduction in Humans

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Sexual Reproduction in Plants

Topic overview Flower structure and the contrast between wind and insect pollination The development of pollen grains and ovules Pollination and double fertilisation The formation and structure of seed and fruit Germination of Vicia faba and the action of gibberellin

Inheritance

Topic overview Alleles, monohybrid inheritance and codominance Dihybrid inheritance and linkage The chi squared test and testing genetic ratios Sex linkage, haemophilia and Duchenne muscular dystrophy Mutation, mutagens and epigenetics

Variation and Evolution

Topic overview Genetic and environmental sources of variation Investigating continuous variation and the t test Competition and selective agencies Gene pools, genetic drift and selection The Hardy-Weinberg principle Isolation, speciation and Darwin's theory of evolution

Application of Reproduction and Genetics

Topic overview The Human Genome Project, other sequenced genomes and genomics in healthcare Screening embryos, stem cells and the ethical issues Genetic fingerprinting, PCR and electrophoresis in forensic work Recombinant DNA and the production of human insulin Genetically modified crops and gene therapy

Adaptations for Gas Exchange

Topic overview Surface area to volume ratio and gas exchange in small organisms Specialised respiratory surfaces and the need for ventilation Gills and counter current flow in bony fish The human breathing system and its histology Ventilation and gas exchange in humans The insect tracheal system The angiosperm leaf, stomata and gas exchange

Adaptations for Transport

Topic overview Vascular systems compared across animal groups The mammalian heart and blood vessels The cardiac cycle and its control Blood, haemoglobin and dissociation curves The Bohr effect, chloride shift and tissue fluid The root and the uptake of water The endodermis and the entry to the xylem Xylem, transpiration and cohesion-tension The potometer and xerophytes and hydrophytes Phloem and translocation

Adaptations for Nutrition

Topic overview Modes of nutrition Saprotrophic nutrition Holozoic nutrition and the unicellular gut From a sac-like gut to a tube gut The human gut and the effect of diet Herbivores, carnivores and parasites

Homeostasis and the Kidney

Topic overview Homeostasis and feedback The structure of the kidney and the nephron Ultrafiltration and selective reabsorption Water regulation and antidiuretic hormone Excretory products, adaptation and kidney failure

The Nervous System

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Immunology and Disease: Disease and Its TransmissionImmunology

Topic overview The language of infection and disease The human body as a host Cholera and tuberculosis Smallpox, influenza and viral pathogenicity Malaria and its transmission

Immunology and Disease: AntibioticsImmunology

Topic overview Bacteriostatic and bactericidal antibiotics Broad and narrow spectrum antibiotics Penicillin and the bacterial cell wall Tetracycline and the Gram negative barrier The spread of antibiotic resistance

Immunology and Disease: The Immune ResponseImmunology

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Human Musculoskeletal Anatomy: Skeletal TissuesMusculoskeletal

Topic overview Cartilage and its three types The composition and functions of compact bone Bone remodelling and the Haversian system Rickets, osteomalacia, osteoporosis and brittle bone disease The structure of skeletal muscle The sliding filament theory Fast and slow twitch fibres, and energy for contraction

Human Musculoskeletal Anatomy: Structure and Function of the SkeletonMusculoskeletal

Topic overview The axial and appendicular skeleton The girdles and the bones of the limbs The vertebral column and its vertebrae Fractures and postural deformities The functions of the skeleton

Human Musculoskeletal Anatomy: JointsMusculoskeletal

Topic overview Types of joint in the human body The structure of a synovial joint Osteoarthritis and rheumatoid arthritis Joints as levers Antagonistic muscle action in the forelimb

Neurobiology and Behaviour: The BrainNeurobiology

Topic overview The structure of the human brain The functions of the cerebrum, cerebellum and medulla oblongata The hypothalamus and the control of the endocrine system The sympathetic and parasympathetic nervous systems Sensory and motor areas, the homunculus and language

Neurobiology and Behaviour: NeuroscienceNeurobiology

Topic overview Non-invasive study of the brain: CT and MRI Functional imaging: fMRI, PET and EEG Brain development and critical periods Neuroplasticity Gene expression, brain development and mental illness

Neurobiology and Behaviour: BehaviourNeurobiology

Topic overview Innate behaviour: escape reflexes, kineses and taxes Habituation and imprinting Classical and operant conditioning Primate societies and the costs and benefits of group living Insect castes, dominance hierarchies, territory and courtship

Adaptations for Gas Exchange

Surface area to volume ratio and gas exchange in small organisms

3 things to know
Component 3

Overview

Recall questions on why the ratio falls as size rises, worked ratios, why Amoeba needs no respiratory surface, how a flatworm's shape keeps diffusion adequate, and why the earthworm's moist skin works only for an animal of its size and activity.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

Calculate the surface area to volume ratio of a cube of side 2 cm.

Key term

How does the surface area to volume ratio change as an organism becomes larger?

Key term

Why does a high metabolic rate make the problem of gas exchange worse?

Questions from this subtopic

Each one with the model answer underneath.

1
What is meant by the surface area to volume ratio of an organism?
Model answer
It is the ratio of the total area of its outer surface to the volume of its body, and it determines how much exchange surface is available to serve each unit of living tissue.
2
Calculate the surface area to volume ratio of a cube of side 2 cm.
Model answer
The surface area is 6 × 2 × 2 = 24 cm² and the volume is 2 × 2 × 2 = 8 cm³, so the ratio is 24 : 8, which simplifies to 3 : 1.
3
How does the surface area to volume ratio change as an organism becomes larger?
Model answer
Volume increases with the cube of the length while surface area increases only with the square of it, so the ratio becomes smaller as the organism grows.
4
Give two consequences for a large organism of having a small surface area to volume ratio.
Model answer
Its general body surface is too small to absorb all the oxygen its tissues need, and the innermost cells lie too far from that surface for diffusion to supply them quickly enough.
5
Why does a high metabolic rate make the problem of gas exchange worse?
Model answer
A high metabolic rate means oxygen is used up and carbon dioxide produced much faster, so a far greater area of exchange surface is needed for each unit of tissue.
6
Name three adaptations that allow an animal to grow larger and more active despite a low surface area to volume ratio.
Model answer
It can develop a specialised respiratory surface of very large area, a ventilating mechanism that keeps the diffusion gradient steep, and a circulatory system that carries the gases between that surface and the tissues.

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