Biology Unit 3

B3.1  MOVEMENT OF MOLECULES IN AND OUT OF CELLS

Summary

The cells, tissues and organs in plants and animals are adapted to take up and get rid of dissolved substances.  Different conditions can affect the rate of transfer. Sometimes energy is needed for transfer to take place.

Dissolved substances

Dissolved substances move by diffusion and by active transport.

Diffusion (covered in more detail in unit 2):

·         The movement of particles in a liquid or gas from an area where they are in higher concentration to an area where they are in lower concentration.

·         The greater the difference in concentration, the faster the rate of diffusion.

·         2 examples of diffusion through the cell membrane:

o   Oxygen required for respiration diffuses into cells.

o   Carbon dioxide produced by respiration diffuses out of cells.

Osmosis:

·         Water often moves across boundaries by osmosis.

·         Osmosis is the diffusion of water from a dilute to a more concentrated solution through a partially permeable membrane that allows the passage of water molecules.

·         Differences in the concentrations of the solutions inside and outside a cell cause water to move into or out of the cell by osmosis.

If there is a higher solute concentration on one side of a membrane, water will move in that direction.

Osmosis in animal cells:

if animal cells are placed in a solution that has a higher solute concentration than the cytoplasm, then water will leave the cell by osmosis, until it shrinks and dies.

 If animal cells are placed in a solution that has a lower solute concentration than the cytoplasm, then water will enter the cell by osmosis until it bursts.

 This is why it is vital that we maintain the concentration of our body fluids at an equal solute concentration to our cells’ cytoplasm.

Osmosis in plant cells:

 If plant cells are placed in a solution that has a higher solute concentration than the cytoplasm, then water will leave the cell by osmosis, and the cell membrane separates from the cell wall.  This will cause a plant to wilt.

If plant cells are placed in a solution that has a lower solute concentration than the cytoplasm, then water will enter the cell by osmosis until it is fully turgid, and the cell wall prevents any more water entering.  This is important in enabling plants to remain upright.

  Sports Drinks

·         Most soft drinks contain water, sugar and ions.

·         Sports drinks contain sugars to replace the sugar used in energy release during the activity. 

·         They also contain water and ions to replace the water and ions lost during sweating.

·         If water and ions are not replaced, the ion / water balance of the body is disturbed and the cells do not work as efficiently.

Energy drinks

·         Contain the same concentration of ions as…