Topic 2 - Genes and Health

• Aerobic respiration requires oxygen and produces carbon dioxide as a waste product.
• Photosynthesis requires carbon dioxide and produces oxygen as a waste product.
• The process of gas exchange occurs by diffusion.
• The surface over which gas exchange takes place is known as the exchange surface.
• The surface area of an organism refers to the total area of the organism that is exposed to the external environment.
• The volume of an organism refers to the total internal volume of the organism.
• As the overall size of the organism increases, the surface area becomes smaller in comparison to the organism's volume and the organism's surface area:volume ratio decreases.
• Single-celled organisms have a high SA:V ratio which allows the exchange of substances to occur by simple diffusion.
• The large surface area in single-celled organisms allows for maximum absorption of nutrients and gases and removal of waste products
• The small volume within the cell means that the diffusion distance to all organelles is short
• As organisms increase in size, their SA:V ratio decreases
• Large multicellular organisms have evolved adaptations to facilitate the exchange substances with their environment
• Examples of adapted gas exchange systems: Fish gills, alveoli, spongy mesophyll layer
• Alveoli increase the surface area of mammalian lungs, allowing for a greater exchange of gases
• Fish gills have structures called lamellae which provide a very large surface area for gas exchange
• Leaves have a spongy mesophyll layer within which as large area of leaf cell surface is exposed to the air.
• The diffusion pathway (distance) across an exchange surface is very short
• The difference in concentration of the exchange substances on either side of the exchange surface is called the concentration gradient.
• A greater difference in concentration means a greater rate of diffusion.
• The continued movement of exchange substances away from the exchange surface means that a concentration gradient is maintained.
• In order to maintain the concentration gradient, the alveoli have a good blood supply through the capillaries - this constantly removes oxygen and supplies carbon dioxide
• The ventilation system in mammals ensure constant inhalation and exhalation which supplies oxygen and removes carbon dioxide from the alveoli side of the exchange surface.
• Fick's Law relates the rate of diffusion to the concentration gradient, the diffusion distance and the surface area.
• Fick's Law of Diffusion

  rate of diffusion ∝ (surface area × concentration difference) ÷ thickness of membrane
• Fick's Law - Calculate the rate of diffusion


  Rate = P x A x ((C1 - C2)÷T)

  • Where:
  • P = A permeability constant that is a quantitative measure of the rate at which a particular molecule can cross a particular membrane.
  • A = surface area
  • C1-C2 = the difference in concentration between two areas
  • T = thickness of the exchange surface
• The role of the lungs is to maximise gas exchange while minimising the loss of water across the exchange surface.
• The trachea is the tube that allows air to travel to the lungs. It contains c-shaped rings of cartilage that ensure that the tube remains open at all times and does not collapse.
• The rings of cartilage in the trachea prevent any friction from rubbing with the oesophagus, as well as providing increased flexibility when food is being swallowed.
• There is a layer of mucus covering the lining of the trachea which helps to trap dust and pathogens, preventing them from entering the lungs and causing infection.
• Tiny hairs called cilia are found in the lining of the airways where they waft mucus towards the top of the trachea, removing any trapped particles and pathogens from the airways.
• Bronchi have a similar structure to the trachea but they have thinner walls and a smaller diameter, the cartilage rings are full circles rather than c-shaped.
• Bronchioles are narrow, self-supporting tubes with thin walls that branch off the bronchi.
• The larger bronchioles possess elastic fibres and smooth muscle that enable adjustment of the size of the airway to increase or decrease airflow.
• The smallest bronchioles do not have any smooth muscle but they do have elastic fibres.
• The bronchioles vary in size. They get smaller as they get closer to the alveoli.
• Groups of alveoli are located at the ends of bronchioles.
• The alveolar wall consists of a single layer of flattened, or squamous, epithelium.
• The squamous epithelium is very thin and permeable for the easy diffusion of gases.
• Alveoli are surrounded by an extensive network of capillaries that carry oxygenated blood to the alveoli.
• A layer of moisture lines the alveoli, facilitating the diffusion of gases. Oxygen and carbon dioxide are able to dissolve in the layer of moisture, so exchange occurs in solution rather than with the air inside the alveoli.