Module 1 - Cells

Diffusion: The net movement of molecules or ions in a gas or liquid from an area of high concentration to an area of low concentration to an area where they are less concentrated.

• Occurs passively i.e. no ATP is required by the cell to move substances.
• Substances move down a cells concentration gradient.
• Molecules such as oxygen and carbon dioxide diffuse across membranes as long as a concentration gradient exists. (As both molecules are uncharged, they diffuse through the phospholipid bilayer very easily).

Osmosis: The movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.

The movement of water into and out of cells is influenced by:

• The amount of water present in the cytoplasm and in the exterior environment.
• The concentration of solutes, such as ions and sugars, on either side of the cell surface membrane.
• The presence of aquaporins in membranes.
• (Aquaporins - are special, highly selective channel proteins for water which make cells very permeable to water. e.g. red blood cell)
• (In plants) the pressure exerted on cell contents by the cell wall, which is rigid and resists expansion ans thus the uptake of water.

Water potential - A measure of the ability of water molecules to move freely in solution. Measures the potential for a solution to lose water - water moves from a solution with high water potential to one of lower water potential. Water potential is decreased by the presence of solutes.

Facilitated diffusion: The passive movement of molecules across membranes down their concentration gradient, which is aided by transport (carrier) protein molecules, No metabolic energy is required.

Channel proteins:

• Are the span of the membrane.
• They form tunnels (or pores) through the bilayer for water-soluble molecules.
• Some channels are open all the time, others need to be triggered by cell signalling.
• The lining of he pore allows water and polar substances to pass through.

Carrier proteins:

• Also the span of the membrane.
• Change shape to help move molecules into or out of the cell.
• The molecules bind to the protein, which stimulates the protein to change its overall shape, so allowing the molecules to pass through the membrane.

Active transport: Movement of substances across membranes against their concentration gradient, requiring the use of energy in the for of ATP. Active transport usually involves the use of transport proteins.

• Carrier proteins use energy from ATP made in respiration to move substances against their concentration gradient.
• Root hair cells absorb nutrients, such as potassium ions, from the water in the soil. Active transport is also used to pump molecules and ions out of cells.

Endocytosis:

• The process of taking materials into a cell by surrounding them with part of the plasma membrane, which then pinches off to form a vesicle inside the cell.
• Some cells take up large molecule e.g. proteins or larger objects e.g. bacteria.

Exocytosis:

• The process of removing materials from the cell by fusing vesicles containing the material with the plasma membrane (cell surface membrane).

Animal cells

• Distilled water (higher water potential than cells)
• = Water moves into cells by osmosis.
• = Cells swell. Cells burst as there is no cell wall and the cell membrane is not strong enough to withstand the pressure. It is haemolysed (burst open)

• Dilute solution of salt or sugar (same water potential as cells)
• = No net movement of water into or out of cells.
• = Cells remain the same shape and volume.

• Concentrated solution of salt or sugar (lower water potential than cells)
• = Water moves out of cells by osmosis.
• = Cells decrease in volume and shrink. Becomes crenated (shrunken and wrinkled)

Plant cells

• Distilled water (higher water potential than cells)
• = Water moves into cells by osmosis.
• = Cells become turgid. Cell wall prevents any more water entering.

• Dilute solution of salt or sugar (same water potential as cells)
• = No net movement of water into or out of cells.
• = Cells remain the same shape and volume.

• Concentrated solution of salt or sugar (lower water potential than cells)
• = Water moves out of cells by osmosis.
• = Plasmolysis - vacuole shrinks, pulling cytoplasm and cell membrane away from cell wall. Cell becomes plasmolysed.