Movement through cell membranes Diffusion Example exchange of

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. • CO 2 is a waste product of metabolism, and thus is high inside cells; homeostasis maintains lower levels of CO 2 outside the cell. Concentration gradient always favors CO 2 to diffuse out of the cell. • Thus – never an equilibrium between CO 2 and O 2

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