Movement of substances

What is diffusion?

• The random net movement of particles from an area of high concentration to an area of low concentration

What is active transport?

• Active transport is the movement of particles against the concentration gradient using energy released during respiration

What is 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

What are the four factors affecting the movement of substances?

• Surface area to volume ratio

• Distance

• Temperature

• Concentration gradient

How does surface area affect the movement of substances?

• The rate of diffusion, active transport and osmosis is higher in cells with a larger surface area to volume ratio

How does distance affect the movement of substances?

• If substances only have a short distance to move, then they’ll move in and out of cells faster e.g cell membranes are very thin

How does temperature affect the movement of substances?

• As the particles in a substance get warmer they have more energy so they move faster

• This means as temperature increases, substances move in and out of cells faster

How does concentration gradient affect the movement of substances?

• Substances move in and out of a cell faster if there’s a big difference in concentration between the inside and outside of the cell

• If there are lots more particles on one side, there are more there to move across

• This only increases the rate of diffusion and osmosis

Why can simple unicellular organisms rely on diffusion for movement of substances?

• In unicellular organisms, these substances can diffuse directly into and out of the cell across the cell membrane. The diffusion rate is quick because of the short distances substances have to travel

Why is a transport system needed in a multicellular organism?

• In multicellular organisms like animals and plants direct diffusion from the outer surface would be too slow, that’s because these substances would have to travel large distances to reach every single cell

• So multicellular organisms need transport systems to move substances to and from individual cells quickly