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Transcript
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.