adaptions of gas exchange

• surface area to volume ratio

  • Surface area and volume are both very important factors in the exchange of materials in organisms
  • The surface area refers to the total area of the organism that is exposed to the external environment
  • The volume refers to the total internal volume of the organism (total amount of space inside the organism)
  • As the surface area and volume of an organism increase (and therefore the overall ‘size’ of the organism increases), the surface area : volume ratio decreases
  • This is because volume increases much more rapidly than surface area as size increases
• Single-celled organisms

  Have a high surface area to volume ratio which allows for the exchange of substances to occur via simple diffusion
• Single-celled organisms


  • Large surface area allows for maximum absorption of nutrients and gases and secretion of waste products
  • Small volume means the diffusion distance to all organelles is short
• As organisms increase in size

  Their surface area to volume ratio decreases
• Large multicellular animals and plants

  • Have evolved adaptations to facilitate the exchange of substances between their environment
  • Have a large variety of specialised cells, tissues, organs, and systems
• Specialised systems for gas exchange

  • Gas exchange system
  • Circulatory system
  • Lymphatic system
  • Urinary system
  • Xylem and phloem
• Organisms require ATP for survival, which is produced through aerobic respiration requiring oxygen
• Carbon dioxide is a toxic waste product of aerobic respiration
• Accumulation of carbon dioxide alters the pH in cells/tissues
• Diffusion is a viable transport mechanism for single-celled organisms but not for larger multicellular organisms
• The time taken for oxygen to diffuse from the cell-surface membrane to the tissues in larger multicellular organisms would be too long
• Metabolic rate


  The amount of energy expended by an organism within a given period of time
• Basal metabolic rate (BMR)


  The metabolic rate of an organism when at rest
• The BMR is significantly lower than when an organism is actively moving
• During periods of rest, the body of an organism only requires energy for the functioning of vital organs such as the lungs, heart and brain
• Methods for measuring/estimating metabolic rate


  • Oxygen consumption
  • Carbon dioxide production
  • Heat production
  • Body Mass
• The greater the mass of an organism, the higher the metabolic rate
• Although metabolic rate increases with body mass, the BMR per unit of body mass is higher in smaller animals than in larger animals
• Smaller animals

  Have a greater SA:V ratio, so they lose more heat, meaning they have to use up more energy to maintain their body temperature
• Apparatus for investigating metabolic rates in organisms

  • Respirometers
  • Oxygen/carbon dioxide probes
  • Calorimeters
• gas exchange across single-celled organisms

  substances diffuse quickly in and out of cells across cell-surface membrane, diffusion quick since has a short diffusion pathway
• Insects

  • All possess a rigid exoskeleton with a waxy coating that is impermeable to gases
  • Have evolved a breathing system that delivers oxygen directly to all the organs and tissues of their bodies
• Spiracle


  An opening in the exoskeleton of an insect which has valves
• Spiracle function

  1. Allows air to enter the insect and flow into the system of tracheae

  2. Most of the time, closed to reduce water loss
• Tracheae

  • Tubes within the insect breathing system which lead to tracheoles (narrower tubes)
  • Walls have reinforcement to keep them open as air pressure fluctuates
• Gas exchange in insects

  1. Tracheoles run between cells and into muscle fibres, the site of gas exchange

  2. Concentration gradient created as oxygen is used by respiring tissues allowing more to move in through spiracles by diffusion

  3. Carbon dioxide produced by respiring tissues moves out through spiracles down a concentration gradient
• Rapid oxygen supply for flying insects

  1. Closing spiracles

  2. Using muscles to create a pumping movement for ventilation
• Effect of lactate production during flight

  1. Lowers the water potential of muscle cells

  2. Water found at the narrow ends of the tracheoles is drawn into the respiring muscle by osmosis

  3. Allows gases to diffuse across more quickly
• For smaller insects, this system provides sufficient oxygen via diffusion
• Oxygen dissolves less readily in water
• A given volume of air contains 30 times more oxygen than the same volume of water
• Gills of fish


  Structure adapted to directly extract oxygen from water
• Structure of fish gills in bony fish

  • Series of gills on each side of the head
  • Each gill arch is attached to two stacks of filaments
  • On the surface of each filament, there are rows of lamellae
  • The lamellae surface consists of a single layer of flattened cells that cover a vast network of capillaries
• Mechanism of fish gills

  1. Capillary system within the lamellae ensures that the blood flow is in the opposite direction to the flow of water - it is a counter-current system

  2. The counter-current system ensures the concentration gradient is maintained along the whole length of the capillary

  3. The water with the lowest oxygen concentration is found adjacent to the most deoxygenated blood
• In order to carry out photosynthesis, plants must have an adequate supply of carbon dioxide
• Leaf adaptations

  • Waterproof cuticle
  • Upper epidermis
  • Palisade mesophyll layer
  • Spongy mesophyll layer
  • Stomata
  • Guard cells
  • Lower epidermis
• Waterproof cuticle

  A waxy layer on the leaf surface that prevents water loss
• Upper epidermis


  A layer of tightly packed cells that forms the outermost layer of the leaf
• Palisade mesophyll layer

  A layer of elongated cells containing chloroplasts that are responsible for most of the leaf's photosynthesis
• Spongy mesophyll layer


  A layer of cells that contains an extensive network of air spaces, allowing carbon dioxide to rapidly diffuse into cells