Definitions

• Diffusion is the net movement of molecules/ions from a region of high concentration to a region of low concentration until an equilibrium is reached
• Facilitated diffusion is a type of diffusion involving a protein molecule. It is a passive process and there are two types of protein involved: carrier proteins and channel proteins
• Carrier proteins have binding sites for specific molecules The carrier protein can change shape many times per second.
• Channel proteins form pores in the membrane to allow molecules through. They can be shaped to allow only a specific molecule or ion to pass
• Passive transport – a way of transporting molecules that relies on kinetic energy of the molecules and does not require metabolic energy. e.g, diffusion, osmosis
• Active transport – the movement of molecules/ions from a region of low concentration to a region of high concentration (against the concentration gradient).This requires the presence of specific carrier proteins within the membrane, and requires the use of energy in the form of ATP.
• Endocytosis – the bulk transport of large particles or fluids through the cell surface membrane. The membrane surrounds the particles concerned, and a vesicle is formed which pinches off and moves into the cytoplasm. “Phagocytosis” is generally used where solid particles are involved, while “pinocytosis” refers to the cell taking in small droplets of fluid.
• Exocytosis – the bulk transport of materials out of a cell. The membrane surrounding the proteins fuses with the cell surface membrane, and the contents of the vesicle are released outside the cell.
• Osmosis – the net movement of water molecules from a solution of higher water potential to a solution of lower water potential across a partially-permeable membrane.
• Water potential – a measure of the ability of water molecules to move freely in solution – water moves from a solution with higher water potential to a solution of lower water potential. Water potential is decreased by the presence of solutes.
• Isotonic – solutions that have the same water potential as each other.
• In a hypotonic solution, water molecules will move into the cell by osmosis.
• In a hypertonic solution, water molecules will move out of the cell by osmosis.
• Turgid –A cell that is full of water as a result of entry of water molecules by osmosis. The pressure of the cell wall prevents more water entering, the cell is turgid.
• Plasmolysis – detachment of the plasma membrane. The cytoplasm shrinks when a plant cell loses water by osmosis.
• Crenation – When animal cells have been immersed in a solution of lower water potential and have lost water by osmosis – they become shrivelled.