Movement through cell membranes

Diffusion Example: exchange of oxygen and carbon dioxide in lungs Molecules or ions moving from areas of higher concentration to lower concentration Difference in concentration is the concentration gradient Concentrations are equal = equilibrium

Diffusion in & out of a cell 1. the cell membrane is permeable to that substance 2. concentration gradient exists such that the substance is at a higher concentration either outside or inside the cell.

Diffusion in & out of a cell example Intracellular Oxygen is always low b/c it is constantly used during metabolism; extracellular oxygen is high due to homeostatic mechanisms. Concentration gradient always favors oxygen diffusing into the cell. CO2 is a waste product of metabolism, and thus is high inside cells; homeostasis maintains lower levels of CO2 outside the cell. Concentration gradient always favors CO2 to diffuse out of the cell. Thus – never an equilibrium between CO2 and O2

Facilitated Diffusion Substances not able to pass through the lipid bilayer need help from membrane proteins Examples: glucose and amino acids Glucose needs insulin to help it diffuse through membranes of certain cells.

Example: distilled water entering the cell Water molecules diffuse from higher water concentrations to lower water concentrations In solutions: higher concentration of solute, lower concentration of water; lower concentration of solute, higher concentration of water.

Greater the concentration of solute (proteins), lower water concentration, greater osmotic pressure. Lower the concentration of solute (proteins), higher water concentration, lower osmotic pressure

Osmotic Pressure When intracellular and extracellular fluids and pressure are the same = isotonic When extracellular fluid has greater pressure than intracellular – water leaves cell = hypertonic When intracellular fluid has greater pressure than extracellular – water enters cell = hypotonic

Filtration Forced movement of molecules through membranes Separate solids from water Tissue fluid forms when water and small dissolved substances are forced out through thin porous walls of blood capillaries, but larger particles are left Force of this movement comes from blood pressure, generated by a higher pressure inside the heart, less pressure outside the heart.

Active Transport Similar to facilitated diffusion Differs in that particles are moving from areas of low concentration to high concentration Carrier proteins also called pumps Examples: sugars, amino acids; sodium, potassium, calcium, and hydrogen ions Also absorb nutrients into cells of the intestinal walls

Endocytosis/Exocytosis

Endocytosis Molecules or other particles that are too large to enter a cell by diffusion, facilitated diffusion or active transport. Three types –Phagocytosis: cell eating, takes in solids, ie. Bacteria –Pinocytosis: cell drinking, takes in tiny droplets, ie. Water –Receptor-mediated endocytosis: moves specific particles into the cell, particle binds to a receptor site protein.

Exocytosis Reverse process of endocytosis Cell secretes a substance stored in vesicle ie. Neurons – release neurotransmitters that signal nerves, muscles, or glands