Exchange and Transport

• Surface area to volume ratio

  Relationship between an organism's size and its surface area to volume ratio
• The larger the organism

  The lower the surface area to volume ratio
• The smaller the surface area to volume ratio

  The higher the metabolic rate
• How a large organism might adapt to compensate for its small surface area to volume ratio

  1. Changes that increase surface area e.g. folding

  2. Body parts become larger e.g. elephant's ears

  3. Elongating shape

  4. Developing a specialised gas exchange surface
• Specialised gas exchange surfaces

  Needed by multicellular organisms because their smaller surface area to volume ratio means the distance that needs to be crossed is larger and substances cannot easily enter the cells as in a single-celled organism
• Features of an efficient gas exchange surface

  • Large surface area, e.g. folded membranes in mitochondria
  • Thin/short distance, e.g. wall of capillaries
  • Steep concentration gradient, maintained by blood supply or ventilation, e.g. alveoli
• Insects can't use their bodies as an exchange surface because they have a waterproof chitin exoskeleton and a small surface area to volume ratio in order to conserve water
• Main features of an insect's gas transport system

  • Spiracles= holes on the body's surface which may be opened or closed by a valve for gas or water exchange
  • Tracheae= large tubes extending through all body tissues, supported by rings to prevent collapse
  • Tracheoles= smaller branches dividing off the tracheae
• Gas exchange in insects

  1. Gases move in and out of the tracheae through the spiracles

  2. A diffusion gradient allows oxygen to diffuse into the body tissue while waste CO2 diffuses out

  3. Contraction of muscles in the tracheae allows mass movement of air in and out
• Fish can't use their bodies as an exchange surface because they have a waterproof, impermeable outer membrane and a small surface area to volume ratio
• Main features of a fish's gas transport system

  • Gills= located within the body, supported by arches, along which are multiple projections of gill filaments, which are stacked up in piles
  • Lamellae= at right angles to the gill filaments, give an increased surface area. Blood and water flow across them in opposite directions (countercurrent exchange system)
• Gas exchange in fish

  1. The fish opens its mouth to enable water to flow in, then closes its mouth to increase pressure

  2. The water passes over the lamellae, and the oxygen diffuses into the bloodstream

  3. Waste carbon dioxide diffuses into the water and flows back out of the gills
• Countercurrent exchange system


  Maintains a steep concentration gradient, as water is always next to blood of a lower oxygen concentration. Keeps rate of diffusion constant along whole length of gill enabling 80% of available oxygen to be absorbed
• Adaptations of a leaf that allow efficient gas exchange

  • Thin and flat to provide short diffusion pathway and large surface area to volume ratio
  • Many minute pores in the underside of the leaf (stomata) allow gases to easily enter
  • Air spaces in the mesophyll allow gases to move around the leaf, facilitating photosynthesis
• How plants limit water loss while allowing gas exchange

  Stomata regulated by guard cells which allows them to open and close as needed. Most stay closed to prevent water loss while some open to let oxygen in
• Pathway taken by air as it enters the mammalian gaseous exchange system


  Nasal cavity → trachea → bronchi → bronchioles → alveoli
• Function of the nasal cavity


  A good blood supply warms and moistens the air entering the lungs. Goblet cells in the membrane secrete mucus which traps dust and bacteria
• Trachea and its function

  Wide tube supported by C-shaped cartilage to keep the air passage open during pressure changes. Lined by ciliated epithelium cells which move mucus towards the throat to be swallowed, preventing lung infections. Carries air to the bronchi
• Bronchi and their function

  Like the trachea they are supported by rings of cartilage and are lined by ciliated epithelium cells. However they are narrower and there are two of them, one for each lung. Allow passage of air into the bronchioles
• Bronchioles and their function

  Narrower than the bronchi. Do not need to be kept open by cartilage, therefore mostly have only muscle and elastic fibres so that they can contract and relax easily during ventilation. Allow passage of air into the alveoli
• Alveoli and their function

  Mini air sacs, lined with epithelium cells, site of gas exchange. Walls only one cell thick, covered with a network of capillaries, 300 million in each lung, all of which facilitates gas diffusion
• Inspiration and changes in the thorax

  1. External intercostal muscles contract (while internal relax), pulling the ribs up and out

  2. Diaphragm contracts and flattens

  3. Volume of the thorax increases

  4. Air pressure outside the lungs is therefore higher than the air pressure inside, so air moves in to rebalance
• Expiration and changes in the thorax

  1. External intercostal muscles relax (while internal contract), bringing the ribs down and in

  2. Diaphragm relaxes and domes upwards

  3. Volume of the thorax decreases

  4. Air pressure inside the lungs is therefore higher than the air pressure outside, so air moves out to rebalance
• Tidal volume


  The volume of air we breathe in and out during each breath at rest
• Breathing rate


  The number of breaths we take per minute
• Calculating pulmonary ventilation rate

  Tidal volume x breathing rate. These can be measured using a spirometer, a device which records volume changes onto a graph as a person breathes
• Digestion


  The hydrolysis of large, insoluble molecules into smaller molecules that can be absorbed across cell membranes
• Carbohydrate digestive enzymes


  • Amylase in mouth
  • Maltase, sucrase, lactase in membrane of small intestine
• Carbohydrate digestion


  1. Amylase → starch into smaller polysaccharides

  2. Maltase → maltose into 2 x glucose

  3. Sucrase → sucrose into glucose and fructose

  4. Lactase → lactose into glucose and galactose
• Lipid digestion


  Occurs in the small intestine
• Lipid digestion


  1. Bile salts emulsify lipids

  2. Lipase hydrolyses the ester bond between monoglycerides and fatty acids
• Protein digestive enzymes


  • Endopeptidases break between specific amino acids in the middle of a polypeptide
  • Exopeptidases break between specific amino acids at the end of a polypeptide
  • Dipeptidases break dipeptides into amino acids
• Co-transport


  Mechanism by which certain molecules are absorbed into the ileum despite a negative concentration gradient
• Molecules requiring co-transport


  • Amino acids
  • Monosaccharides
• Co-transport mechanism


  1. Sodium ions (Na+) are actively transported out of the cell into the lumen, creating a diffusion gradient

  2. Nutrients are then taken up into the cells along with Na+ ions
• Fatty acids and monoglycerides

  Do not require co-transport as they are nonpolar and can easily diffuse across the membrane of the epithelial cells
• Haemoglobin


  Globular, water soluble. Consists of four polypeptide chains, each carrying a haem group (quaternary structure)
• Role of haemoglobin


  Present in red blood cells. Oxygen molecules bind to the haem groups and are carried around the body to where they are needed in respiring tissues
• Factors affecting oxygen-haemoglobin binding

  • Partial pressure/concentration of oxygen
  • Partial pressure/concentration of carbon dioxide
  • Saturation of haemoglobin with oxygen
• Partial pressure of oxygen increases


  Affinity of haemoglobin for oxygen increases, so oxygen binds tightly to haemoglobin. When partial pressure is low, oxygen is released from haemoglobin