BIOL 122 1/30 Membrane Intro

Importance of the Plasma Membrane
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Importance of the Plasma Membrane
Selectively permeable: some molecules cross more easily than others. Allows transport in both directions and communication between cells. Malfunctioning transport leads to disease.
There are numerous membranes in the ___
cell
Components of the plasma membrane
phospholipids, cholesterol, proteins, carbohydrates
Image: Components of the plasma membrane
What are membranes composed of?
A lipid bilayer
Phospholipids
A lipid consisting of a glycerol bound to two fatty acids and a phosphate group.
Amphipathic molecule (hydrophobic and hydrophilic ends)
Image: Phospholipids
Membrane fluidity
Bilayer is a stable structure - forced by hydrogen bonding with water. Relative position of phospholipid molecule in a layer vary.
Fluid mosaic model
- biological membranes are described as this
- refers to the phospholipid bilayer and the embedded protein components which allow easy lateral movements.
Image: Fluid mosaic model
Organisms may change the type of phospholipids to
accommodate the weather,
winter wheat: increases the % of unsaturated fatty acids tails in the fall
Cholesterol
helps maintain flexibility and fluidity of membrane
- present in the plasma membrane of animal cells (prevents passage of some small molecules and adds fluidity)
Cholesterol: warm temps
restrains movement of phospholipids
Cholesterol: cool tempsmaintains fluidity by preventing tight packingMembrane proteinsintegral and peripheralFunctions of the membrane proteinsAnchoring membrane to and within the membranePeripheral proteins - may be attached to the head of the phospholipids Intergral proteins (transmembrane) have a nonpolar regionChannel proteinPore proteinsB sheets from a barrel structureNetworking of supporting fibersProtein fibers reinforce membrane shape, links membrane to cytoskeleton, and anchors proteins in cell membrane.GlycocalyxThe external surface of a plasma membrane that is important for cell-to-cell communication. Composed of sugars protruding from lipids and proteins. Serves as binding sites for proteins in communication and recognition. Lubricate cells, stick cell downCell junctions for plantsplasmodesmata Channels- lined by membrane that connect adjacent plant cells water and small solutes (some proteins and RNA) can pass from cell to cell cell to cellCell junctions in animal cellstight junctions, desmosomes, gap junctionsTight junctionsMembranes of neighboring cells are pressed together, preventing leakage of extracellular fluid. Materials must enter the cells (by diffusion or active transport) to pass through the tissue. Skin cells are joined-Makes the skin water tight Intestinal epitheliumDesmosomes(anchoring junctions) fasten cells together into strong sheetsGap junctions(communicating junctions) provide cytoplasmic channels between adjacent cells electrically excitable cells (example cardiac and smooth muscle) communicate by gap junctions. Direct passage of ions synchronizes the contractions of neighboring cells. May allow intracellular signaling molecules (cAMP and Ca2+) to pass, coordinating the responses of cells. Connexins in the plasma membrane of one cell aligns with an adjacent cell, open channel between the cytoplasms. The plasma membranes of the two cells are separated by a gap.Movement across the membraneA steady traffic of small molecules and ions moves across the plasma membrane in both directions. Sugars, amino acids, and other nutrients enter a muscle cell and metabolic waste products leave. Cell absorbs O2and expels CO2. A cell regulates concentrations of inorganic ions, like Na+, K+, Ca2+, and Cl-, by shuttling them across the membrane.Membrane permeabilityThe phospholipid is semi permeable - depends on the interaction of that molecule with the hydrophobic core of the membrane - hydrophobic molecules, like hydrocarbons, can dissolve in the lipid bilayer and cross easily - ions and polar molecules pass through with difficulty Depending on the molecule, it was move by passive and active transportTwo types of transport across membranepassive and activePassive transportDoes not require input of energy (ATP) from cell, but relies on molecules moving down concentration gradients. diffusion—Membrane is permeable to water, gases, and fat-soluble molecules.• Facilitated diffusion—Larger polar molecules cross with the help from a membrane channel (transport protein).Active transportMolecules have to pass across the membrane up their concentration gradient, must expend energy (ATP).