Movement through cell membranes

1. Movement through cell membranes

2. 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

3. 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.

4. Diffusion in & out of a cellexample • 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

5. 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.

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

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

10. 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

12. 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.

13. 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

14. Endocytosis/Exocytosis

15. 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.

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