Gas exchange and Surface area

• The smaller the surface area to volume ratio, the higher the metabolic rate
• The larger the organism, the lower the surface area to volume ratio
• Changes that increase surface area e.g. folding; body parts become larger e.g. elephant’s ears; elongating shape; developing a specialised gas exchange surface can help a large organism compensate for its small surface area to volume ratio
• Multicellular organisms require specialised gas exchange surfaces 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
• Three 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 due to their 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
• Process of gas exchange in insects:

  • Gases move in and out of the tracheae through the spiracles
  • A diffusion gradient allows oxygen to diffuse into the body tissue while waste CO2 diffuses out
  • Contraction of muscles in the tracheae allows mass movement of air in and out
• Fish can't use their bodies as an exchange surface due to their 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)
• Process of gas exchange in fish:

  • The fish opens its mouth to enable water to flow in, then closes its mouth to increase pressure
  • The water passes over the lamellae, and the oxygen diffuses into the bloodstream
  • Waste carbon dioxide diffuses into the water and flows back out of the gills
• Countercurrent exchange system maximises oxygen absorbed by the fish by maintaining a steep concentration gradient, as water is always next to blood of a lower oxygen concentration, and keeps the rate of diffusion constant along the whole length of the 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
• Plants limit their water loss while still allowing gases to be exchanged by regulating stomata with guard cells, allowing 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 in the mammalian gaseous exchange system: 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 in the mammalian gaseous exchange system:

  • 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 in the mammalian gaseous exchange system:

  • Supported by rings of cartilage and lined by ciliated epithelium cells
  • Narrower than the trachea and there are two of them, one for each lung
  • Allow passage of air into the bronchioles
• Bronchioles in the mammalian gaseous exchange system:

  • 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 in the mammalian gaseous exchange system:

  • 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
• Process of inspiration:

  • External intercostal muscles contract (while internal relax), pulling the ribs up and out
  • Diaphragm contracts and flattens
  • Volume of the thorax increases
  • Air pressure outside the lungs is therefore higher than the air pressure inside, so air moves in to rebalance
• Process of expiration:

  • External intercostal muscles relax (while internal contract), bringing the ribs down and in
  • Diaphragm relaxes and domes upwards
  • Volume of the thorax decreases
  • Air pressure inside the lungs is therefore higher than the air pressure outside, so air moves out to rebalance
• Tidal volume is the volume of air we breathe in and out during each breath at rest
• Breathing rate is the number of breaths we take per minute
• Pulmonary ventilation rate can be calculated by 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 is the hydrolysis of large, insoluble molecules into smaller molecules that can be absorbed across cell membranes
• Enzymes involved in carbohydrate digestion and where they are found:

  • Amylase in the mouth
  • Maltase, sucrase, lactase in the membrane of the small intestine
• Substrates and products of carbohydrate digestive enzymes:

  • Amylase: starch into smaller polysaccharides
  • Maltase: maltose into 2 x glucose
  • Sucrase: sucrose into glucose and fructose
  • Lactase: lactose into glucose and galactose
• Lipids are digested in the small intestine
• Before lipids can be digested, they must be emulsified by bile salts produced by the liver to break down large fat molecules into smaller, soluble molecules called micelles, increasing surface area
• Lipids are digested by lipase, which hydrolyses the ester bond between the monoglycerides and fatty acids
• Enzymes involved in protein digestion and their roles:

  • 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
• Certain molecules are absorbed into the ileum despite a negative concentration gradient through co-transport
• Molecules that require co-transport are amino acids and monosaccharides
• Sodium ions (Na+) are actively transported out of the cell into the lumen, creating a diffusion gradient, allowing nutrients to be taken up into the cells along with Na+ ions in co-transport
• Fatty acids and monoglycerides do not require co-transport because they are nonpolar molecules that can easily diffuse across the membrane of the epithelial cells
• Structure of 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
• Insects have a hard waterproof exoskeleton made of a polysaccharide called chitin
• Chitin is the same material found in the cell walls of fungal cells