CHAPTER 8 Membrane Structure and Function Membrane Structure
CHAPTER 8 Membrane Structure and Function
Membrane Structure Plasma Membrane – selectively permeable membrane that allows some substances to diffuse across. Amphipathic – condition where a molecule has hydrophilic and hydrophobic regions. Phospholipids allow for the hydrophilic head due to a polar phosphate group and hydrophobic fatty acid tails (nonpolar hydrocarbon chains).
Membrane Structure (Continued) Fluid Mosaic Model – lipid bilayer involving the lateral movement of lipids, proteins, and carbohydrates (oligosaccharides). Most lipids and proteins drift laterally within the membrane. Molecules rarely flip within the membrane due to negative interactions of hydrophobic and hydrophilic regions. Unsaturated hydrocarbon tails enhance fluidity because of the non-kinking of single carbon to carbon bonds. Cholesterol inhibits fluidity at warm temperatures and limits packing of lipids at lower temperatures.
Membrane Structure (Continued) Cell-Cell Recognition – the ability to recognize a cell that another cell encounters by looking at extracellular membrane carbohydrates such as: Oligosaccharides Glycolipids Glycoproteins
Transport Across Membranes Permeability of the lipid bilayer Nonpolar molecules- dissolve in membranes; move across with ease (hydrocarbons, oxygen, carbon dioxide) Polar molecules- small polar molecules pass through membrane lipids with ease (water, ethanol) but large molecules such as glucose will have difficulty passing Transport Proteins- integral membrane proteins that transport specific molecules or ions across biological membranes May be a channel protein May physically move molecules using ATP Are specific to molecules they translocate
Transport Across Membranes (Continued) Passive Transport (diffusion) – net movement of a substance down a concentration gradient (graded concentration change over a distance) over a biological membrane Continues until a dynamic equilibrium is reached Osmosis – passive transport of water that results in movement of water due to the total solute concentration. Hypertonic Solution – greater solute concentration Hypotonic Solution – lower solute concentration Isotonic Solution – equal solute concentration Water is moving at the same rate in both directions.
Transport Across Membranes (Continued) Balance of water uptake and loss Water balance of cells without walls (animal, protists) Hypertonic Solution – cell will crenate (shrivel) Hypotonic Solution – cell will lyse (burst) Osmoregulation – organisms may pump water out of vacuoles when present in a hypotonic solution or pump slats out when present in a hypertonic solution. Water balance of cells with walls Hypotonic Solution – cell will become turgid (hard) Ideal state for plants Isotonic Solution – cell will become flaccid (limp) Hypertonic Solution – cell will plasmolyze (pulling away of the cell wall from the plasma membrane)
Transport Across Membranes (Continued) Facilitated Diffusion – diffusion of solutes across the membrane with the aid of transport proteins. Solute is transported down its concentration gradient Transport proteins are specific for the solutes they transport having binding sites analogous to an enzymes active site Transport proteins change conformations to deposit solute on the other side of the membrane
Transport Across Membranes (Continued) Active Transport – energy-requiring process during which a transport protein pumps a molecule across a membrane against its concentration gradient Example Sodium-Potassium Pump Protein oscillates between 2 conformations High affinity for NA+ towards the cytoplasm High affinity for K+ towards the cell’s exterior ATP helps power the movement Translocates solutes across their gradient
Transportation of Large Molecules Exocytosis Endocytosis Exports macromolecules from a cell by fusion of vesicles with the plasma membrane Vesicle usually budded from ER or Golgi then migrates to PM Process of importing macromolecules into cell by forming vesicles derived from the plasma membrane Vesicle forms from a localized region of the PM; is pinched off into the cytoplasm Used by cell to incorporate extracellular substances Used by secretory cells to export products
Three Types of Endocytosis Phagocytosis – cell eating; pinches off food vacuole that will attach to a lysosyme containing hydrolytic enzymes Pinocytosis – cell drinking; droplets are formed into vesicles and the solute in the droplet is extracted Receptor-Mediated Endocytosis – importing of molecules from another cell
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