Exchange at cell membranes

• Diffusion is the movement of particles from an area of high concentration to low concentration until equilibrium is reached.
• Facilitated diffusion occurs when substances move across the plasma membrane through specific transport proteins, such as ion channels or carrier proteins.
• The concentration gradient is the difference between the concentrations of molecules on either side of the membrane.
• Active transport requires energy (ATP) and moves molecules against their concentration gradients.
• Osmosis is the passive movement of water molecules across a selectively permeable membrane from an area of higher water potential to lower water potential.
• Passive transport does not require ATP and moves molecules down their concentration gradients.
• Carrier proteins bind with specific molecules and undergo conformational changes to facilitate passive transport.
• Water potential refers to the tendency of water to flow into or out of cells due to differences in solute concentration.
• Isotonic solutions have equal water potential inside and outside the cell, resulting in no net gain or loss of water by osmosis.
• Passive transport involves the exchange of substances without requiring metabolic energy from the cell.
• Simple diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration down their concentration gradient.
• Facilitated diffusion is the net movement of particles down their concentration gradient across a partially permeable cell membrane via carrier or channel proteins.
• Osmosis is the net movement of water from an area of higher water potential to an area of lower water potential across a partially permeable membrane.
• Water potential is a measure of the tendency of water molecules to move from one area to another area and describes the pressure created by these water molecules; the more dilute a solution, the higher (less negative) the water potential (Ѱ).
• The rate of diffusion can be increased by increasing the number of channel & carrier proteins, the surface area of the cell membrane, and/ or reducing the diffusion distance, and creating a steeper concentration gradient.
• Active transport is the movement of particles from an area of low concentration to an area of high concentration (against their concentration gradient) across a cell membrane, using ATP and carrier proteins.
• The cell membrane is the arrangement and any movement of phospholipids, proteins, glycoproteins and glycolipids in the fluid-mosaic model of membrane structure.
• Cholesterol may also be present in cell membranes where it restricts the movement of other molecules making up the membrane.
• Movement across membranes occurs by simple diffusion (involving limitations imposed by the nature of the phospholipid bilayer), facilitated diffusion (involving the roles of carrier proteins and channel proteins), osmosis (explained in terms of water potential), active transport (involving the role of carrier proteins and the importance of the hydrolysis of ATP), and co-transport (illustrated by the absorption of sodium ions and glucose by cells lining the mammalian ileum).
• Co-transport occurs when the transport of one substance is coupled with the transport of another substance across a membrane.
• Na+ ions are actively transported out of epithelial cells through a protein carrier molecule.
• 3 Na+ ions are transported out of the epithelial cells into the bloodstream in exchange for 2 K+ ions.
• The movement gives a higher concentration of Na+ ions in the lumen of the intestine rather than inside of the cell.
• Na+ ions move down the concentration gradient using a co-transport protein.
• Both Na+ and glucose can bind to the protein (the binding of one makes the other more effective).
• 2 Na+ and a glucose molecule must bind before they can be transported across the membrane.
• The glucose moves into the blood plasma using facilitated diffusion and a uniporter.
• The smaller the organism the larger surface area to volume