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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

Molecules in Motion

Year 11 biology

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


  • Diffusion: arrangement held together by chemical bonds.

  • 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 – arrangement held together by chemical bonds.

  • 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 be fatal, as the water would cause the cell to burst

  • 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