3. Exchange of Substances

• 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
• Changes that increase surface area can help large organisms compensate for their small surface area to volume ratio, e.g. folding, enlarging body parts like elephant's ears, elongating shape, developing a specialised gas exchange surface
• Multicellular organisms require specialised gas exchange surfaces because their smaller surface area to volume ratio means substances cannot easily enter the cells as in single-celled organisms
• 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 small surface area to volume ratio
• Main features of an insect's gas transport system:

  • Spiracles: holes on the body's surface that 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
  • Oxygen diffuses into 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 small surface area to volume ratio
• Main features of a fish's gas transport system:

  • Gills: located within the body, supported by arches, with multiple projections of gill filaments
  • Lamellae: at right angles to the gill filaments, increasing surface area, blood and water flow across them in opposite directions (countercurrent exchange system)
• Process of gas exchange in fish:

  • Fish opens its mouth to enable water to flow in, then closes its mouth to increase pressure
  • Oxygen diffuses into the bloodstream through the lamellae
  • Waste carbon dioxide diffuses into the water and flows back out of the gills
• Countercurrent exchange system maximises oxygen absorbed by fish by maintaining a steep concentration gradient and enabling 80% of available oxygen to be absorbed
• Adaptations of a leaf for efficient gas exchange:

  • Thin and flat for short diffusion pathway and large surface area to volume ratio
  • Many minute pores in the underside (stomata) for easy gas entry
  • Air spaces in the mesophyll for gas movement facilitating photosynthesis
• Plants limit water loss while allowing gas exchange by regulating stomata with guard cells, opening and closing as needed
• Pathway of air in the mammalian gaseous exchange system:

  Nasal cavity → trachea → bronchi → bronchioles → alveoli
• Function of the nasal cavity in the mammalian gaseous exchange system:

  • Warms and moistens air entering the lungs
  • Goblet cells secrete mucus to trap dust and bacteria
• Trachea in the mammalian gaseous exchange system:

  • Wide tube supported by C-shaped cartilage
  • Lined by ciliated epithelium cells moving mucus to prevent 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 trachea, two of them, one for each lung
  • Allow air passage into the bronchioles
• Bronchioles in the mammalian gaseous exchange system:

  • Narrower than bronchi, mostly have muscle and elastic fibres
  • Contract and relax easily during ventilation
  • Allow air passage into the alveoli
• Alveoli in the mammalian gaseous exchange system:

  • Mini air sacs lined with epithelium cells for gas exchange
  • Walls one cell thick, covered with capillaries facilitating gas diffusion
• Process of inspiration:

  • External intercostal muscles contract, pulling ribs up and out
  • Diaphragm contracts and flattens
  • Thorax volume increases, air moves in to rebalance pressure
• Process of expiration:

  • External intercostal muscles relax, bringing ribs down and in
  • Diaphragm relaxes and domes upwards
  • Thorax volume decreases, air moves out to rebalance pressure
• Tidal volume is the volume of air breathed in and out during each breath at rest
• Breathing rate is the number of breaths taken per minute
• Pulmonary ventilation rate is calculated by multiplying tidal volume by breathing rate, measured using a spirometer
• Digestion is the hydrolysis of large, insoluble molecules into smaller molecules that can be absorbed across cell membranes
• Enzymes involved in carbohydrate digestion and their locations:

  • 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
• 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 affects oxygen-haemoglobin binding:

  • As partial pressure of oxygen increases, the affinity of haemoglobin for oxygen also increases
  • When partial pressure is low, oxygen is released from haemoglobin