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 substance2. concentration gradient exists such that the substance is at a higher concentration either outside or inside the cell.

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

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