M3:S1 Exchange and transport

• Exchanging things with the environment is pretty easy if you're a single-celled organism, but if you're multicellular it all gets a bit more complicated... and it's all down to this 'surface area to volume ratio' malarkey
• Every organism, whatever its size, needs to exchange things with its environment
• Things organisms need to exchange with their environment

  • Cells need to take in oxygen and glucose for aerobic respiration and other metabolic reactions
  • Cells need to excrete waste products like carbon dioxide and urea
• Surface area to volume ratio (SA:V)


  How easy the exchange of substances is depends on this
• Smaller animals have higher surface area: volume ratios
• Surface area to volume ratio

  Calculated by dividing the surface area by the volume
• In single-celled organisms, substances can diffuse directly into or out of the cell across the cell surface membrane. The diffusion rate is quick because of the small distances the substances have to travel
• Reasons why diffusion across the outer membrane is too slow in multicellular animals


  • Some cells are deep within the body-there's a big distance between them and the outside environment
  • Larger animals have a low surface area to volume ratio- it's difficult to exchange enough substances to supply a large volume of animal through a relatively small outer surface
  • Multicellular organisms have a higher metabolic rate than single-celled organisms, so they use up oxygen and glucose faster
• Multicellular animals need specialised exchange surfaces like the alveoli in the lungs
• Exchange surfaces


  • They have a large surface area
  • They're thin
  • They have a good blood supply and/or ventilation
• Root hair cells


  • The cells on plant roots grow into long 'hairs' which stick out into the soil, giving the roots a large surface area
• Alveoli


  • The alveoli are the gas exchange surface in the lungs, made from a single layer of thin, flat cells called the alveolar epithelium
  • The alveoli are surrounded by a large capillary network, giving each alveolus its own blood supply
  • The lungs are also ventilated so the air in each alveolus is constantly replaced
• Fish gills


  • The gills are the gas exchange surface in fish, containing a large network of capillaries and being well-ventilated with fresh water constantly passing over them
• In mammals the lungs are exchange organs
• How air moves in and out of the mammalian lungs

  1. Air enters the trachea, then splits into two bronchi, which branch into smaller bronchioles, which end in alveoli

  2. The ribcage, intercostal muscles and diaphragm all work together to move air in and out
• Structures in the mammalian gaseous exchange system and their functions

  • Goblet cells secrete mucus to trap microorganisms and dust
  • Cilia beat the mucus upwards away from the alveoli
  • Elastic fibres in the airways help with breathing out
  • Smooth muscle in the airways controls their diameter
  • Cartilage in the trachea and bronchi provides support
• The different parts of the mammalian gaseous exchange system are found in different places
• Ventilation


  Breathing in (inspiration) and breathing out (expiration)
• How ventilation works in mammals

  1. Inspiration: External intercostal and diaphragm muscles contract, causing ribcage to move up and out and diaphragm to flatten, decreasing thorax pressure so air flows in

  2. Expiration: External intercostal and diaphragm muscles relax, ribcage moves down and in, thorax pressure increases so air flows out
• Tidal volume


  The volume of air in each normal breath
• Vital capacity


  The maximum volume of air that can be breathed in or out
• Breathing rate


  The number of breaths per minute
• Expiration


  1. External intercostal and diaphragm muscles relax

  2. Ribcage moves downwards and inwards

  3. Diaphragm becomes curved again

  4. Thorax volume decreases

  5. Air pressure increases

  6. Air is forced out of the lungs
• Forced expiration requires the internal intercostal muscles to contract, to pull the ribcage down and in
• Normal expiration is a passive process that doesn't require energy
• Tidal volume (TV)


  The volume of air in each breath - usually about 0.4 dm³
• Breathing rate


  How many breaths are taken - usually in a minute
• Oxygen consumption or oxygen uptake


  The rate at which an organism uses up oxygen (e.g. the number of dm³ used per minute)
• Residual air can't be expelled
• Spirometer


  • It has an oxygen-filled chamber with a movable lid
  • The person breathes through a tube connected to the oxygen chamber
  • As the person breathes in and out, the lid of the chamber moves up and down
  • These movements can be recorded by a pen attached to the lid of the chamber or picked up by a motion sensor
• The soda lime in the tube the subject breathes into absorbs carbon dioxide
• The total volume of gas in the chamber decreases over time because the air that's breathed out is a mixture of oxygen and carbon dioxide, and the carbon dioxide is absorbed by the soda lime
• Analysing spirometer data


  1. Determine breathing rate

  2. Determine tidal volume

  3. Determine vital capacity

  4. Determine oxygen consumption
• Counter-current system


  Blood flows through the gill plates in one direction and water flows over in the opposite direction, maintaining a large concentration gradient between the water and the blood
• Ventilation in bony fish

  1. Fish opens mouth, lowering floor of buccal cavity to decrease pressure and suck in water

  2. Fish closes mouth, raising floor of buccal cavity to increase pressure and force water out across gill filaments

  3. Operculum opens to allow water to leave the gills
• Insects have microscopic air-filled pipes called tracheae which they use for gas exchange
• Gas exchange in insects

  1. Air moves into the tracheae through pores on the insect's surface called spiracles

  2. Oxygen travels down the concentration gradient towards the cells

  3. Carbon dioxide from the cells moves down its own concentration gradient towards the spiracles to be released into the atmosphere

  4. The tracheae branch off into smaller tracheoles which have thin, permeable walls and go to individual cells

  5. Insects use rhythmic abdominal movements to change the volume of their bodies and move air in and out of the spiracles
• The tracheae appear silver or grey because they are filled with air
• Chitin rings in insect tracheae

  Provide structural support, like the rings of cartilage in a human trachea
• Exchanging things with the environment is pretty easy if you're a single-celled organism, but if you're multicellular it all gets a bit more complicated... and it's all down to this 'surface area to volume ratio' malarkey