Cellular Transport

Lesson Objectives Explain the processes of diffusion, facilitated diffusion, and active transport Predict the effect of a hypotonic, hypertonic, or isotonic solution on a cell Discuss how large particles enter and exit cells

Main Idea Cellular transport moves substances within the cell and moves substances into and out of the cell.

Diffusion Molecules and ions dissolved in water are in constant motion, moving randomly. The net movement of particles (caused by the random motion) from an area of high concentration (many particles) to low concentration (few particles) is called diffusion. Net movement will occur until the concentration in all regions are the same, which is dynamic equilibrium.

Diffusion The amount of substance in a particular area is called concentration. There must be a concentration gradient, a difference between concentrations across a membrane, for cellular transport to occur.

Diffusion All types of diffusion are passive transport mechanisms because no energy is needed. Three main factors that affect the rate of diffusion: –Concentration –Temperature –Pressure

Facilitated Diffusion Many molecules needed by cells are polar and cannot pass through the plasma membrane, so an alternate route is needed. Channels are made out of proteins that form a polar interior that is “friendly” to the molecules. Carrier proteins help substances diffuse by changing shape as the diffusion process occurs to help move the particle through the membrane.

Osmosis Water is a universal solvent. The molecules dissolved into the solvent are called solutes. The net movement of water across a membrane by diffusion is called osmosis. 3 types of solution: –Isotonic –Hypertonic –hypotonic

Osmosis When a cell is in a solution that has the same concentration of water and solutes (ex. Ions, sugars, proteins, etc.) as its cytoplasm, the cell is said to be in an isotonic solution. The cell is in equilibrium with the solution.

Osmosis If a cell is in a solution that has a lower concentration of solute, the cell is in a hypotonic solution. There is more water on the outside of the cell than the inside, so water moves into the cell. Causes swelling as the vacuoles in the cell fill with water

Osmosis When a cell is placed in a hypertonic solution, the concentration of the solute outside the cell is higher than inside. Net movement of water is out of the cell. Vacuoles are emptied of water, causing cells to shrink.

Osmosis

Active Transport Sometimes substances must move from an area of low concentration to high concentration. The movement of substances across the plasma membrane against a concentration gradient is called active transport. Requires energy! Helps maintain homeostasis

Active Transport Occurs with the aid of carrier proteins, called pumps Endocytosis is a process by which a cell surrounds and takes in material from its environment. –Does not pass directly through the membrane –Engulfed –Phagocytosis – eating of the particle –Pinocytosis – drinking of the substances Exocytosis is the expulsion or secretion of materials from a cell. –used to expel wastes and secrete hormones

Active Transport

Sodium Potassium Pump 1. Protein in the membrane binds intracellular sodium ions. 2. ATP attaches to protein with bound sodium ions. 3. The breakdown of ATP causes shape change in the protein, allowing sodium ions to leave. 4. Extracellular potassium ions bind to exposed sites.

Sodium Potassium Pump 5. Binding of potassium causes release of phosphate from protein. 6. Phosphate release changes protein back to its original shape, and potassium ions move into the cell.

Sodium Potassium Pump

Coupled Transport Many molecules are cotransported into cells up their concentration gradients by coupling their movement to that of sodium ions or protons moving down their concentration gradients.