Cellular Transport - Diffusion & Osmosis (OCR Gateway GCSE Biology: Combined Science)

Transport across cell membranes

• In order for any organism to function properly, it needs to move substances in and out of cells
  • Exchange of substances occurs across the cell membrane
• There are three transport processes that living organisms use for exchange: diffusion, osmosis and active transport

Diffusion is the movement of particles from higher to lower concentration

• Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement of particles from an area of higher concentration to an area of lower concentration
• This means:
  • Particles in a solution or a gas are always moving about randomly
  • If there happen to be more particles in one area/region, then there tends to be an overall (net) movement of particles from this region of higher concentration to a region where there are fewer particles (an area of lower concentration)

Diffusion of perfume particles through a room is one of the simplest examples of diffusion

• Diffusion is an entirely passive process; the movement of particles in a fluid is dependent on how much kinetic energy they have
  • The higher the temperature, the more kinetic energy particles have and therefore the faster they can move by diffusion 

Diffusion across cell membranes

• All cells are surrounded by a cell membrane which separates the inside of the cell from its outside environment
• Substances have to be able to diffuse through the cell membrane to enter or exit the cell unaided (there are other ways for substances to get in)
• Not all substances are able to diffuse across the cell membrane which is why it is described as being partially permeable; some substances are able to enter or leave the cell whilst others aren’t
  • For example, oxygen and carbon dioxide can diffuse across the cell membrane during gas exchange whereas complex carbohydrates like starch cannot (it’s too big)

The cell membrane acts as a barrier which substances need to move across to get into or out of a cell

Examples of diffusion in living organisms

• You will need to learn examples of substances that organisms obtain by diffusion

Examples of Diffusion in Living Organisms Table

• The movement of water molecules into and out of cells occurs by osmosis
• Osmosis is the net movement of water molecules across a partially-permeable membrane from a region of higher water concentration to a region of lower water concentration
• Like, diffusion, osmosis is a form of passive transport (does not require energy) but it only applies to water
• Water can move in and out of cells by osmosis and will move down its concentration gradient

Osmosis and the partially permeable membrane

Osmosis in cells

Water potential indicates how concentrated a solution is

• It can get a little confusing to talk about the concentration of water
• Instead, we can talk about osmosis in terms of water potential
  • Water potential is the potential (likelihood) of water molecules to diffuse out of or into a solution
• A concentrated solution (of sugar) has a low concentration of water molecules and a low water potential
  • It is highly likely for water molecules to move into the concentrated sugar solution
• A dilute solution (of sugar) has a high concentration of water molecules and a high water potential
  
  • It is less likely for water molecules to move into the dilute sugar solution
• It helps to remember that pure water has the highest water potential of any solution

How osmosis works

Osmosis in animal cells

• Animal cells lose and gain water as a result of osmosis
• As animal cells do not have a supporting cell wall, the results of osmosis can be severe
• If an animal cell is placed into a strong sugar solution (with a lower water potential than the cell), it will lose water by osmosis and become crenated (shrivelled up)
• If an animal cell is placed into distilled water (with a higher water potential than the cell), it will gain water by osmosis as it has no cell wall to create turgor pressure
• It will continue to gain water until the cell membrane is stretched too far and it bursts

Effect of osmosis on animal cells

Osmosis in plant cells

• Plant cells lose or gain water as a result of osmosis
• Unlike animal cells, plant cells have a supporting cell wall so are protected from cell lysis (bursting of the cell)
• If a plant cell is placed into a strong sugar solution (with a lower water potential than the cell), it will lose water by osmosis
  • The vacuole gets smaller and the cell membrane shrivels away from the cell wall
  • It becomes flaccid (shrivelled up)
• If a plant cell is placed into distilled water (with a higher water potential than the cell), it will gain water by osmosis
  • The vacuole gets bigger, pushing the cell membrane against the cell wall
  • The plant cell is described as being turgid or as containing a high turgor pressure (the pressure of the cytoplasm pushing against the cell wall)

The effect of osmosis on plant cells. Hypertonic solutions contain less water, hypotonic solutions contain more water (you don't need to remember these terms!)

• Water entering the cell by osmosis makes the cell rigid and firm
• This is important for plants as the effect of all the cells in a plant being firm is to provide support and strength for the plant - making the plant stand upright with its leaves held out to catch sunlight
• If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant wilts