Transcript File
Homeostasis: Movement Across the Cell Membrane Students know cells are enclosed within semipermeable membranes that regulate their interaction with their surroundings. A. Cell Membrane Cell Membranes surround ALL cells! Controls what molecules enter and leave the cell - food & oxygen molecules must enter the cell - waste products need to be released SEMIPERMEABLE – it allows some things through but not others Structure of Cell Membrane Phospholipid bilayer Polar heads “love” water (hydrophilic) Nonpolar tails “fear” water (hydrophobic) C. Fluid Mosaic Model Describes the various molecules found in the cell membrane Example: -Proteins: forms channels and pumps to move materials across membranes - Carbohydrates: act like identification cards so cells can identify one another D. Transport Across a Membrane Molecules can enter/leave the cell in 2 ways: 1. Passive Transport * Simple Diffusion * Carrier-Facilitated (“helped”) Diffusion * Osmosis 2. Active transport * Protein Pump * Endocytosis * Exocytosis Passive Transport Energy is not required for movement across the membrane to occur Molecules move from area of high concentration to area of low concentration Example: Like riding a bike downhill Two factors determine if passive transport takes place, equilibrium and permeability - Equilibrium: when the concentrations on both sides of the membrane are equal membrane - Permeability: ability of a molecule to diffuse (move) across a membrane * Impermeable: molecules that cannot pass across a membrane * semi-permeable/selective permeability: Some molecules can pass across the membrane while other molecules cannot A. Diffusion Molecules moving from area of high concentration to area of low concentration No energy is required Occurs because molecules constantly move and collide with each other *Click link Cell membrane higher conc. lower conc. B. Carrier-Facilitated Diffusion No energy required Molecules “helped” across by carrier proteins from high to low concentration Video (click here) C. Osmosis Osmosis is a special type of diffusion The diffusion of water molecules through a selectively permeable membrane *Click pic ISOTONIC Concentration of solutes (dissolved substance) is the same in and out of the cell Equal amount of water leaving and entering cell HYPERTONIC Solution has a higher solute concentration than the inside of cell More water leaves cell, causing cell to shrink HYPOTONIC Solution has a lower solute concentration than the inside of cell More water enters cell, causing cell to expand (get big) *Click pic Active TransPort Energy required for molecules to pass across ◦ ATP – the “battery” of the cell ◦ Breaking a bond in ATP releases energy ◦ Can “pump” molecules from low to high concentration (against concentration gradient) Example: Like riding a bike uphill ATP ATP A. Protein Pump ◦ Protein Pumps: pulls molecule through ◦ ATP Energy is required *Click Pic B. Endocytosis Endocytosis: cell membrane engulfs the larger molecule bring it into the cell Two types: ◦ Phagocytosis: cytoplasm surrounds a molecule and packages it in a food vacuole ◦ Pinocytosis: cell membrane forms pockets filled with liquid and pinch off to form vacuoles in a cell C. Exocytosis Exocytosis: The membrane of the vacuole surrounding the material fuses with the cell membrane, releasing the material outside the cell *Click Pic