Unit 1 Biology - 2

• Gas exchange surfaces


  Surfaces over which gas exchange occurs by diffusion
• Properties of gas exchange surfaces

  • Large surface area to volume ratio
  • Short diffusion pathway
  • Steep concentration gradient
• All organisms need to exchange gases with their environment
• 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
• Diffusion pathway


  The distance across an exchange surface
• Concentration gradient


  The difference in concentration of the exchange substances on either side of the exchange surface
• Fick's Law of Diffusion


  Relates the rate of diffusion to the concentration gradient, the diffusion distance and the surface area
• The lungs of air-breathing animals provide an ideal exchange surface for the diffusion of gases
• Parts of the lung

  • Trachea
  • Bronchi
  • Bronchioles
  • Alveoli
• Trachea


  • Contains C-shaped rings of cartilage
  • Lined with mucus and cilia
• Bronchi


  • Have thinner walls and smaller diameter than trachea
  • Cartilage rings are full circles
• Bronchioles


  • Narrow, self-supporting tubes with thin walls
  • Larger ones have elastic fibres and smooth muscle
  • Smaller ones lack smooth muscle but have elastic fibres
• Alveoli


  • Thin, permeable squamous epithelial wall
  • Surrounded by extensive capillary network
  • Lined with moisture to facilitate gas diffusion
• Membranes are vital structures found in all cells
• Phospholipid


  Molecule consisting of glycerol, phosphate group, and two fatty acid tails
• Phospholipid structure


  • Polar phosphate head (hydrophilic)
  • Non-polar fatty acid tails (hydrophobic)
• Phospholipids can form monolayers and bilayers
• Phospholipid bilayer


  The basic structure of the cell membrane
• Components of cell membranes

  • Phospholipid bilayer
  • Proteins (intrinsic and extrinsic)
• Phospholipids


  Molecules that make up the cell membrane, consisting of a glycerol molecule, two fatty acid tails, and a phosphate group
• Phospholipid bilayer


  • Phospholipids form a double layer with the hydrophilic phosphate heads facing the water and the hydrophobic fatty acid tails facing inwards
• Formation of phospholipid monolayer and bilayer


  1. If phospholipids are spread over the surface of water, they form a single layer with the hydrophilic phosphate heads in the water and the hydrophobic fatty acid tails sticking up away from the water (phospholipid monolayer)

  2. Alternatively, two-layered structures may form in sheets; these are called phospholipid bilayers
• Cell membrane


  Phospholipid bilayers form the basic structure of the cell membrane
• Components of cell membranes

  • Phospholipids
  • Proteins
  • Cholesterol
  • Glycolipids
  • Glycoproteins
• Intrinsic proteins


  • Embedded in the membrane with their precise arrangement determined by their hydrophilic and hydrophobic regions
• Cholesterol in cell membranes

  Increases fluidity at low temperatures, stabilises the membrane at higher temperatures
• Glycolipids and glycoproteins


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


  Describes the scattered pattern and fluid nature of the 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 the proteins within the membrane gives a mosaic appearance
• The structure of proteins determines their position in the membrane
• Models of cell membrane structure have evolved over time as new evidence and technology became available
• Investigating the effect of temperature on membrane permeability using beetroot

  1. Cut equal-sized beetroot pieces

  2. Rinse the beetroot pieces

  3. Add the beetroot pieces to test tubes with water

  4. Put each test tube in a water bath at a different temperature

  5. Remove the beetroot pieces, leaving just the coloured liquid

  6. Use a colorimeter to measure the absorbance of the coloured liquid
• As temperature increases


  Membrane permeability also increases
• Both high and very low temperatures can increase membrane permeability
• Limitations of the investigation include differences in cuvette thickness, slight variations in beetroot piece size and shape, and uneven distribution of pigment within the beetroot
• As alcohol concentration increases

  The permeability of the membrane increases