exchange of substances

• Surface area to volume ratio

  Affects the rate of transfer of substances between an organism and its environment
• Surface area to volume ratio

  • Smaller animals have a higher surface area: volume ratio than larger animals
  • In single-celled organisms, nutrients can directly diffuse through the cell wall into the organism
  • Multicellular organisms have to use transport systems to move nutrients around the body because diffusion alone would be too slow
  • Smaller animals in colder climates tend to have a compact shape to reduce their surface area to volume ratio, while in warmer climates they have adaptations to maximise surface area to volume
  • Smaller animals tend to have a higher metabolic rate so they end up losing more heat than larger animals
• Gas exchange in single-celled organisms

  Occurs across the cell surface membranes directly into the cytoplasm of the cell
• Gas exchange in insects

  Occurs through openings called spiracles that lead to air-filled tubes called trachea
• Gas exchange in insects

  • The trachea branch off into smaller tracheoles that extend into the body systems of the insect
  • Gases move in and out by diffusion down a concentration gradient, mass transport via rhythmic abdominal movements, and the ends of the tracheoles being filled with water which makes respiration easier during exercise
• Gas exchange in fish

  Occurs in the gills
• Gills


  • Made up of gill filaments that are thin plates covered in small structures called lamellae to increase surface area
  • Have many small capillaries and a very thin cell membrane so there is a small distance for diffusion
  • Water follows across the gills in the opposite direction to blood so that the diffusion gradient is maintained - known as a counter-current mechanism

  
• Gas exchange in dicotyledonous plants

  Occurs through pores called stomata to the gas exchange surface which is the surface of mesophyll cells
• Gas exchange in plants

  • Mesophyll cells have a very high surface area so they are adapted for gas exchange
  • The stomata can open and close to control the exchange of gases
  • Plants have adapted to control water loss by having the guard cells around the stomata become turgid to keep them open when there is enough water, and flaccid to close them and conserve water when there is not enough
• Xerophytes


  Plants adapted to survive in particularly dry conditions
• Xerophytes


  • Have a thick cuticle so less water can escape
  • The leaves may roll up so the stomata on the lower epidermis are not exposed, reducing water loss
  • May have hairs to trap a layer of moist air near the leaves
  • The stomata may be in pits and grooves
  • The leaves have a small surface
• Mesophyll cells


  Have a very high surface area so they are adapted for gas exchange
• Stomata


  Can open and close to control the exchange of gases
• Plants have adapted to control water loss
• Gas exchange


  Leads to water loss so there is a trade-off between the two
• When plants have enough water

  The guard cells around the stomata are turgid and this keeps the stomata open for gas exchange
• When plants do not have enough water


  The guard cells become flaccid and this closes the stomata to conserve water
• Xerophytes


  • They have a thick cuticle so less water can escape
  • The leaves may roll up so the stomata on the lower epidermis on the leaf are not exposed to the outside, reducing the water potential gradient and water loss
  • The leaves may have hairs to trap a layer of moist air near the surface of the leaves to reduce the water potential gradient
  • The stomata may be in pits and grooves
  • The leaves have a small surface area to volume ratio
• Both insects and plants have to compromise between efficient gas exchange and the limitation of water loss
• Efficient gas exchange


  Would mean that a lot of water is lost
• Minimising the amount of water lost


  Would result in very inefficient gas exchange
• Gas exchange in humans

  Takes place in the lungs
• Lungs


  • Made up of lots of small air chambers that increase surface area
  • The trachea (windpipe) branches off into two bronchi, which split into many small bronchioles, that end in tiny air sacs called alveoli
  • The alveolar epithelium is the site of gas exchange in humans
  • There are millions of alveoli so there is a huge surface area for gas exchange
  • The alveolar wall is only one cell thick so there is a very short diffusion distance
  • The alveoli are covered in capillaries
• Ventilation (breathing)


  1. Inspiration (breathing in)

  2. Expiration (breathing out)

  
• During inspiration


  1. The external intercostal muscles contract, while the internal intercostal muscles relax which pulls the ribs upwards and outwards

  2. The diaphragm contracts, causing it to flatten

  3. The volume of the thorax increases and the pressure in the lungs decreases so air is sucked into the lungs
• During expiration


  1. The external intercostal muscles and the diaphragm relax

  2. The volume of the thorax decreases, increasing the pressure and this causes air to be forced out
• Tidal volume


  The volume of air in each breath (0.4 - 0.5 dm3 for healthy adults)
• Ventilation rate


  Number of breaths per minute (usually 15)
• Forced expiratory volume (FEV)


  The volume of air that can be expelled in 1 second
• Forced vital capacity (FVC)


  Maximum volume of air that can be breathed out after a deep breath in

  
• Tuberculosis (TB) results in the formation of small lumps in the lungs called 'tubercles' which damages the gas exchange surface so tidal volume is reduced
• Fibrosis is when scar tissue forms in the lungs which can be due to an infection or exposure to substances like asbestos
• Scar tissue in fibrosis cannot expand as much as normal tissue so tidal volume and FCV decrease
• Asthma attacks cause the airways to become narrow so breathing becomes difficult, reducing FEV and leading to wheezing and shortness of breath
• Emphysema occurs as a result of smoking or exposure to air pollution, where foreign particles become trapped in the alveoli causing inflammation and damage to the elastic walls of the lungs
• Research in the 1950s-60s showed a link between smoking and several types of cancer, resulting in health warnings being printed on cigarette packets
• Many studies have documented the link between air pollution and various diseases, resulting in upper limits being placed on the amount of pollution that can be emitted and taxes on cars that pollute
• Digestion


  Involves large biological molecules being hydrolysed into smaller molecules that can be absorbed across cell membranes
• Carbohydrate digestion


  1. Amylases (breaks down starch into maltose) in the mouth and small intestines

  2. Membrane-bound disaccharides attached to the cell membrane of epithelial cells in the small intestine (breaks disaccharides into monosaccharides)
• Lipid digestion


  1. Lipase and bile salts in the small intestine

  2. Bile salts break up lipids into small droplets called micelles in a process of emulsification

  3. Lipase breaks down the micelles into fatty acids and glycerol