B2

In all cells: 

• nucleus (contains genetic material, which controls the activities of the cell)
• cytoplasm (most chemical processes take place here, controlled by enzymes)
• cell membrane (controls the movement of substances into and out of the cell)
• mitochondria (site of respiration)
• ribosomes (site of protein synthesis)

In plant cells, also:

• cell wall (strengthens the cell)
• chloroplasts (contain chlorophyll, which absorbs light energy for photosynthesis)
• vacuole (filled with cell sap to make the cell turgid - swollen with water)

Diffusion is the movement of particles (molecules or ions) from an area of low concentration to an area of high concentration.

Diffusion happens when the particles are free to move, e.g. in gases/ particles dissolved in solutions.

Particles diffuse down a concentration gradient, from an area of high concentration to an area of low concentration. The greater the difference between the high and low levels (concentration gradient), the faster they spread. Particles continue to move from a high to a low concentration while there is a concentration gradient. When there is no longer a concentration gradient, the 

Osmosis is the movement of water from a high concentration to a low concentration through a partially permeable membrane (lets some substances pass through them, but not others). Each cell wall is a partially permeable membrane. In osmosis, the chemical that is let through the cell wall is water. This puts more water into the low concentration solution, making it more dilute and causing the level of liquid to rise. There is also some movement of water in the opposite direction due to random movement of molecules.

Eventually, the concentration on either side of the membrane is the same, and there is an equal flow of water in both directions. It has reached equilibrium.

Plants gain water by osmosis through their root cells. Water moves into the plant cells by osmosis, this is important to make them turgid so the plant holds itself up.