Movement of substances into and out of cells
Diffusion
Diffusion is the net movement of particles from an area of higher concentration to an area of lower
concentration
Once all the molecules are evenly spread out, there is no longer a concentration gradient. Only
when there is a net movement is there diffusion.
Particles are said to move down their concentration gradient. This happens because of the random
movement of particles and needs no energy from the cell. This is a passive process.
Diffusion in Cells
Only small molecules like amino acids, glucose, water and oxygen
can pass through the selectively permeable membranes. Big
molecules like starch and protein can’t pass.
Investigating diffusion
Diffusion in non-living systems may be investigated in different ways.
Visking tubing is an artificial selectively permeable membrane which can be used to model
diffusion across cell membranes.
Questions - page 60
1. In your own words, describe the terms net movement and diffusion
2. Is diffusion a passive or active process? Explain your answer.
3. a) Which of these molecules can diffuse through a cell membrane: starch or glucose?
b) Explain why some particles can diffuse through cell membranes but not others
Osmosis
Osmosis is the net movement of water molecules across a selectively permeable membrane from
a region of higher water concentration to a region of lower water concentration.
Osmosis is a passive process and is a result of the random movement of particles.
If there are more water molecules on one side than the other, there is a steady net flow of water
into the region with few water molecules.
Concentrations in solutions
● A low concentration of dissolved solutes means a high concentration of water molecules
● A high concentration of dissolved solutes means a low concentration of water molecules
Investigating osmosis
Diffusion
Diffusion is the net movement of particles from an area of higher concentration to an area of lower
concentration
Once all the molecules are evenly spread out, there is no longer a concentration gradient. Only
when there is a net movement is there diffusion.
Particles are said to move down their concentration gradient. This happens because of the random
movement of particles and needs no energy from the cell. This is a passive process.
Diffusion in Cells
Only small molecules like amino acids, glucose, water and oxygen
can pass through the selectively permeable membranes. Big
molecules like starch and protein can’t pass.
Investigating diffusion
Diffusion in non-living systems may be investigated in different ways.
Visking tubing is an artificial selectively permeable membrane which can be used to model
diffusion across cell membranes.
Questions - page 60
1. In your own words, describe the terms net movement and diffusion
2. Is diffusion a passive or active process? Explain your answer.
3. a) Which of these molecules can diffuse through a cell membrane: starch or glucose?
b) Explain why some particles can diffuse through cell membranes but not others
Osmosis
Osmosis is the net movement of water molecules across a selectively permeable membrane from
a region of higher water concentration to a region of lower water concentration.
Osmosis is a passive process and is a result of the random movement of particles.
If there are more water molecules on one side than the other, there is a steady net flow of water
into the region with few water molecules.
Concentrations in solutions
● A low concentration of dissolved solutes means a high concentration of water molecules
● A high concentration of dissolved solutes means a low concentration of water molecules
Investigating osmosis
