Membrane Structure and Function Chapter 5

Membrane Structure and Function 5.1

Membrane Structure and Function Plasma membrane- phospholipid bilayer. Creates separation between the cell and the external environment as well as compartments within the cell itself. . Highly similar to each other, easy to fuse.

Fluid-Mosaic Model Flexible Lipid content responsible for fluidity. Like Olive Oil Cholesterol prevents it from becoming too fluid Number and types of proteins may vary Both side of the plasma membrane are not identical Carbon chains on the outer surface

Glycoproteins and Glycolipids Glycoproteins- attached to proteins Glycolipids- attached to lipids Both give the cell its “fingerprint” Plays a role in cellular identification Transplant rejection

Protein Functions Channel Protein- move substance from one side to the other Carrier Protein- receives substance, changes its shape thus moving substance across membrane Cell Recognition Protein- glycoproteins that help cell recognize when its being invaded

a. b. c. d. e. f.

Protein Functions Receptor Protein- allow only specific molecule to bind to it. No passing of a substance. Something binds and cell responds. “Message” Enzymatic Protein- carry out metabolic reactions Junction Protein- binds to cells together so signal molecule can be passed cell-to-cell

a. b. c. d. e. f.

Permeability Selectively permeable- only allows certain things Hydrophobic substances can diffuse freely across because they can get through the middle. Concentration gradient- area of concentration Things that do NOT use energy go from high to low concentration or down their concentration gradient Water should NOT be able to freely pass through because it is polar BUT cells have channel proteins (aquaporins) that make it go a little faster.

Bulk Transport Exocytosis- exit cell through a vesicle Endocytosis- enter cell through a vesicle

Passive Transport 5.2

Diffusion Movement across from high to low concentration or down its concentration gradient Solution Solvent (liquid) Solute (Solid) Example: Making sweet tea. Sugar diffuses through tea, going down its concentration gradient **Gases freely diffuse because small and nonpolar Oxygen/ carbon dioxide exchange

Osmosis Diffusion of water across a selectively permeable membrane from high to low concentration Isotonic-same, inside the cell and outside the cell content is equal Hypotonic-cell swells, intake of water Hypertonic- cell shrinks, water rushes out Meat preservation, bacon, jerky

Osmosis in Animal and Plant Cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Animal cells plasma membrane nucleus In an isotonic solution, there is no net movement of water. In a hypotonic solution, water mainly enters the cell, which may burst (lysis). In a hypertonic solution, water mainly leaves the cell, which shrivels (crenation). Plant cells cell wall nucleus central vacuole plasma membrane chloroplast In an isotonic solution, there is no net movement of water. In a hypotonic solution, vacuoles fill with water, turgor pressure develops, and chloroplasts are seen next to the cell wall. In a hypertonic solution, vacuoles lose water, the cytoplasm shrinks (plasmolysis), and chloroplasts are seen in the center of the cell.

Facilitated Diffusion Rapid transport by carrier protein No energy needed STILL diffusion Usually uses carrier protein to help or facilitate across membrane

Active Transport 5.3

Active Transport Moves molecules against their concentration gradient. Requires energy (ATP) From low to high concentration Carrier proteins also needed. When used in active transport they are called PUMPS Ex: iodine collects in cells of thyroid

Sodium-Potassium Pump 3 sodium (Na+) ions bind A Phosphate from ATP is added to carrier protein It changes shape moving 3 sodium ions outside the cell Now, the protein can pick up 2 potassium ions Phosphate from earlier leaves Changes back to original shape moving potassium ions inside the cell

Exocytosis A vesicle fuses with the plasma membrane a secretion occurs. Hormones, neurotransmitters, and digestive enzymes are transported through exocytosis The vesicle becomes part of the plasma membrane

Endocytosis Cells take in substances by forming vesicles around the material The membrane pinches off to form a vesicle inside the cell 3 types Phagocytosis-large particles or another cell. Pinocytosis- engulfs a liquid or tiny particles Receptor-mediated Endocytosis-form of pinocytosis, uses receptors to recognize molecules and take them into the cell. Called a coated pit

Three Methods of Endocytosis

Modification of Cell Surfaces 5.4

Modifications of Cell Surfaces Cell Surfaces in Animals Junctions Between Cells Adhesion Junctions – Intercellular filaments between cells Desmosomes – internal cytoplasmic plaques Tight junctions – form impermeable barriers Gap Junctions Plasma membrane channels are joined (allows communication) Important in heart muscle and smooth muscle

Junctions Between Cells of the Intestinal Wall  Adhesion  Tight  Gap

Modifications of Cell Surfaces Plant Cell Walls Plants have a freely permeable cell wall, with cellulose as the main component. Plasmodesmata penetrate the cell wall Each contains a strand of cytoplasm Allow passage of material between cells

Plasmodesmata