Gas Exchange, Cell Membranes & Transport

• Properties of Gas Exchange Surfaces


  • Surface area to volume ratio
  • Diffusion pathway
  • Concentration gradient
• Surface area to volume ratio (SA:V ratio)

  The surface area of an organism in relation to its volume
• 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 allows for maximum absorption of nutrients and gases and removal of waste products
  • The small volume within the cell means the diffusion distance to all organelles is short
• Large multicellular organisms


  • Have evolved adaptations to facilitate the exchange of substances with their environment
  • The gas exchange systems are adapted to increase the surface area available for the exchange of gases
• Adaptations to increase surface area

  • Alveoli increase the surface area of mammalian lungs
  • Fish gills have structures called lamellae which provide a very large surface area
  • Leaves have a spongy mesophyll layer within which a large area of leaf cell surface is exposed to the air
• Diffusion pathway


  The diffusion distance across an exchange surface is very short
• Concentration gradient


  The difference in concentration of the exchange substances on either side of the exchange surface
• A greater difference in concentration means a greater rate of diffusion as the gas molecules move across the exchange surface
• The continued movement of exchange substances away from the exchange surface mean that a concentration gradient is maintained
• Fick's Law of Diffusion


  Relates the rate of diffusion to the concentration gradient, the diffusion distance and the surface area
• Fick's Law of Diffusion


  1. Rate = P x A x ((C1 - C2)÷ T)

  2. P = Permeability constant

  3. A = Surface area

  4. C1 - C2 = Difference in concentration

  5. T = Thickness of exchange surface
• The lungs of air-breathing animals provide an ideal exchange surface for the diffusion of gases
• Mammalian lungs


  • Located in the thorax
  • Enable efficient gas exchange
• Structures of the mammalian lung


  • Trachea
  • Bronchi
  • Bronchioles
  • Alveoli

  
• Trachea


  • Tube that allows air to travel to the lungs
  • Contains C-shaped rings of cartilage
  • Lined with mucus and cilia

  
• Bronchi


  • Similar structure to trachea but thinner walls and smaller diameter
  • Cartilage rings are full circles
• Bronchioles


  • Narrow, self-supporting tubes with thin walls
  • Larger ones have elastic fibres and smooth muscle
  • Smaller ones have no smooth muscle but elastic fibres
• Alveoli


  • Groups located at ends of bronchioles
  • Alveolar wall is a single layer of flattened epithelium
  • Surrounded by extensive capillary network
  • Lined with moisture to facilitate gas diffusion
• Membranes are vital structures found in all cells
• Cell surface membrane


  Creates an enclosed space separating the internal cell environment from the external environment
• Intracellular membranes


  Form compartments within the cell such as the nucleus, mitochondria, and endoplasmic reticulum
• Membranes


  • Control the exchange of substances from one side to the other
  • Act as an interface for communication
• Phospholipids


  • Consist of a glycerol molecule, a phosphate group, and two fatty acid tails
  • Phosphate head is polar (hydrophilic)
  • Lipid tails are non-polar (hydrophobic)
• Phospholipid bilayer


  • Forms the basic structure of the cell membrane
• Phospholipids


  Molecules that form the basic structure of cell membranes
• Representations of phospholipids


  • Simple representation of phosphate head and lipid tails
  • Chemical structure showing glycerol, phosphate group, and ester bonds
  • Diagrammatic representation of chemical structure
• Phospholipid monolayer


  Phospholipids spread over surface of water with hydrophilic phosphate heads in water and hydrophobic fatty acid tails sticking up away from water
• Phospholipid bilayer


  Two-layered structures that form in sheets
• Phospholipid bilayers form the basic structure of the cell membrane
• Components of cell membranes


  • Phospholipid bilayers
  • Proteins
  • Cholesterol
• Intrinsic (integral) proteins


  Embedded in the membrane with their precise arrangement determined by their hydrophilic and hydrophobic regions
• Extrinsic (peripheral) proteins


  Found on the outer or inner surface of the membrane
• Cholesterol


  • Regulates membrane fluidity by increasing fluidity at low temperatures and stabilising the membrane at higher temperatures
  • Increases the mechanical strength and stability of membranes
• Glycolipids and glycoproteins


  Present on the surface of the cell, aid cell-to-cell communication
• Fluid mosaic model


  Describes the scattered pattern and fluid movement of components within the phospholipid bilayer
• The cell membrane is partially permeable
• Small, non-polar molecules


  Can pass through the gaps between the phospholipids
• Large, polar molecules


  Must pass through specialised membrane proteins called channel proteins and carrier proteins
• The distribution of proteins within the membrane gives a mosaic appearance