molecules in motion
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Molecules in Motion. Year 11 biology. In sciences, a molecule is a group of atoms in a definite arrangement held together by chemical bonds.

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molecules in motion notes
In sciences, a molecule is a group of atoms in a definite arrangement held together by chemical bonds.

Molecules move into cells in different ways depending on the size of the molecules, the permeability of a membrane and the concentration of those molecules in the intracellular and extra cellular environment.

Molecules in motion notes
  • In a gas or liquid molecules move freely, bumping and bouncing into one another
  • This is random movement
  • Which results in an even spread of molecules throughout the space they occupy
  • In a liquid the molecules move from a region of high concentration to a region of low concentration, in an attempt to balance the levels or reach-equilibrium.
  • The difference between these two regions is referred to as a the concentration gradient or diffusion gradient
  • E.g.
    • Oxygen moving into cells and carbon dioxide moving out
      • Needs the molecules to be solution
      • Membrane to be moist and permeable to them
Diffusion is slow –
  • It can be sped up in the following ways:
    • Steeper gradient
    • Stirring the molecules
    • Heating
      • Size is important
      • Small ones (oxygen) diffuse quicker than large ones (glucose)
  • Diffusion is related to the surface area to volume ratio
    • Quicker the greater the surface area is to volume
  • Key Questions page 67
Movement of water across a selectively permeable membrane, to balance the levels of solutes.
    • Water actually moves from a region of low concentration of solutes to a region of high concentration of solutes
  • It is still diffusion but only of water
    • This happens when the membrane is permeable to water but not to larger molecules like sugar
  • Refer to diagram on page 65 fig. 4.13
    • Copy if needed
osmosis contd
Note: Osmosis in animal cells, especially blood cells could be fatal, as the water would cause the cell to burst
  • Water entering a red blood cell is endo-osmosis.
  • If red blood cells are put in salt water the water leaves the cell to balance the solutes and the cell shrivels and dies – exo-osmosis
  • This means that the fluid part of our body blood cells, the plasma, must be in balance to prevent osmosis and disaster.
Osmosis contd:
osmosis in plant cells
Plants have cellulose in their cells walls
  • They also have a vacuole inside which contains salts in solution
  • Membrane of the vacuole is semi permeable
  • Cell wall is fully permeable to salts and water
  • If a plant cell is in water it swells, but not bursts because the cell wall is tough:
    • Eventually it gets full and firm and is said to be turgid which keeps the plant firm and once here, the osmosis stops
  • If a plant cell is placed into a high solute solution, water is taken out of the vacuole and the cell goes flaccid eventually, cytoplasm pulls away from the cellulose – plasmolysis
Osmosis in plant cells
active transport
This when the cells need molecules of substance which are not in a higher concentration outside the cell, but the cell needs more of them anyway.
  • Therefore it moves molecules from a region of low concentration to one of high concentration – against the concentration gradient.
  • It needs energy for this to happen supplied by the process of cellular respiration, which occurs in the mitochondria

PAGE 66 NOTE Endocytosis and Exocytosis

Active transport