Cell physiology

• Cells that carry out a lot of active transport have large numbers of mitochondria. The mitochondria supply the ATP (energy) needed for active transport.
• Cytosis


  The bulk transport of substances into or out of the cell through the cell surface membrane
• Endocytosis


  1. The movement of substances into the cell

  2. The cell surface membrane invaginates and encloses material from outside the cell to form a membrane-bound vesicle

  3. The vesicle pinches off on the inside of the cell surface membrane
• Types of endocytosis


  • Phagocytosis (engulfing solid material like bacteria)
  • Pinocytosis (cell drinking, transport of fluid into the cell)
• Exocytosis


  1. The movement of substances out of the cell

  2. Secretory vesicles (possibly budded off from the Golgi apparatus) move to and fuse with the cell surface membrane

  3. The contents of the vesicle are then released outside the cell
• Exocytosis is important in the secretion of many proteins from cells, including digestive enzymes and many hormones
• Osmosis


  The net movement of water from a high water concentration (dilute solution) to a lower water concentration (more concentrated solution) across a selectively (differentially) permeable membrane
• Hypotonic, hypertonic, isotonic


  • Hypotonic - weaker solution
  • Hypertonic - stronger solution
  • Isotonic - equal concentration, no osmosis
• Water potential


  The tendency of a solution to take in water by osmosis from pure water across a selectively permeable membrane. Measured in kilopascals (kPa). Pure water has a water potential of 0 kPa.
• The water potential of a solution is reduced by the presence of solutes, as some of the water molecules form hydration shells around the solute particles and are not as free to move.
• A solution always has a negative water potential - it will always have less free water molecules than pure water.
• Water potential


  The tendency of a solution to take in water by osmosis from pure water across a selectively permeable membrane. Measured in kilopascals (kPa).
• Pure water has a water potential of zero (0 kPa), ie it is unable to take in any more water by osmosis.
• Water potential


  An indication of the free energy of the water molecules. In pure water, all the water molecules are free. In solutions, some water molecules form hydration shells around solutes, reducing the number of free water molecules.
• The more concentrated a solution is


  The more negative its water potential, as more water is bound up in hydration shells and less is free to move.
• Osmosis


  The net movement of water through a selectively (differentially) permeable membrane, from a solution of less negative water potential to a solution of more negative water potential.
• Solute potential


  The potential of a solution to take in water. May or may not be the same as the water potential, as it is affected by other factors like space available within a cell.
• Pressure potential


  The effect of pressure on the solution. A turgid plant cell will exert considerable pressure on its cell wall, influencing its ability to take in or lose water by osmosis.
• Water potential (Ψ)


  Solute potential (Ψs) + Pressure potential (Ψp)
• Changes in water potential, solute potential and pressure potential as a plant cell takes in or loses water by osmosis


  As the cell takes in water, solute potential decreases and pressure potential increases. At full turgor, water potential is zero as no further water can enter.
• Plant cells rely on turgor for support. Herbaceous (non-woody) plants are almost totally reliant on turgor pressure.
• If a plant cell loses too much water by osmosis, its vacuole can shrink to the extent that the cell membrane (or protoplast) can pull away from the cell wall (except at points where adjacent protoplasts are joined via plasmodesmata). This is described as plasmolysis and the cell is plasmolysed.
• In nature, plasmolysis seldom occurs, which is just as well as plant cells are unlikely to survive if they have become plasmolysed.
• Animal cells do not have a cell wall and therefore there is nothing to stop the expansion of the cell membrane until it bursts (lysis). Lysis will take place if animal cells, for example, red blood cells, are placed in hypotonic solutions.
• If animal cells are placed in a hypertonic solution, the cells lose water by osmosis, shrink and shrivel up (crenation).
• In healthy animals, the blood and tissue fluid are kept at the correct water potential to ensure that neither lysis nor crenation take place.
• Measuring the average water potential of cells in a plant tissue


  Sections of plant tissue are placed in a range of concentrations of a solution. When the solute potential of the external solution is equal to the water potential of the plant tissue, there will be no change in mass of the plant tissue.
• Facilitated diffusion

  The type of diffusion involved when the diffusion process is supported or 'facilitated' by proteins
• Proteins involved in facilitated diffusion

  • Carrier proteins
  • (Ion) channel proteins
• Carrier proteins

  • Take in the diffusing molecule, change shape, release the molecule on the other side of the membrane
  • Have binding sites that match specific molecules and assist the movement of these molecules across the membrane
• (Ion) channel proteins

  • Formed by proteins with a central pore that enables charged particles (ions) to pass through
  • Some are permanently open, others are gated and can open and close to allow control of ion movement
• The rate of facilitated diffusion is dependent on the number of carrier or channel proteins in the membrane
• Both diffusion and facilitated diffusion only transport molecules down the concentration gradient and neither involves the expenditure of metabolic energy as they are passive processes
• Active transport

  Involves protein carrier molecules (sometimes called pumps) to move substances across the membrane against the concentration gradient, requiring metabolic energy in the form of ATP
• There are two key differences between active transport and facilitated diffusion: 1) substances are moved against the concentration gradient, 2) metabolic energy in the form of ATP is required
• Non-polar molecules such as lipid soluble oxygen and carbon dioxide can pass through the membrane unaided (by simple diffusion)
• Very small molecules, such as water, can also pass through the membrane by simple diffusion due to their very small size
• Water soluble substances generally are unable to pass through the membrane by simple diffusion due to the hydrophobic nature of the centre of the phospholipid bilayer
• Diffusion

  The net movement of a substance from where it is in a higher concentration to where it is in a lower concentration
• Diffusion across membranes is affected by factors including: concentration gradient, size of the molecule, temperature, thickness of the exchange surface, and surface area of the membrane