Transcript File
Chapter 7: Membrane Structure & Function • Early models of the plasma membrane were deduced from indirect evidence. 1. Charles Overton (1895): • Observation: Lipid and lipid soluble materials enter cells most rapidly • Deduction: Membranes are made of lipids 2. Irving Langmuir (1917): • Observation: Amphipathic phospholipids will form an artificial membrane on the surface of water • Deduction: phospholipids can form membranes. 3. Gorter and Grendel (1925): • Observation: Phospholipid content of membranes isolated from red blood cells is just enough to cover cells with two layers. • Deduction: Cell membranes are phospholipid bilayers • Observation: Membranes isolated from red blood cells contain proteins as well as lipids. • Deduction: There is protein in biological membranes 4. Hugh Davson and James Daneilli (1935) The cell membrane is a phospholipid bilayer sandwiched between two layers of globular protein The membrane is approximately 8 nm thick 5. In 1972, S.J. Singer and G.L. Nicolson proposed the fluid mosaic model Proteins are individually embedded in the phospholipid bilayer (not a solid surface coat) Hydrophilic portions of both proteins and phospholipids are maximally exposed to water Hydrophobic portions of proteins are phospholipids are in the nonaqueous environment inside the bilayer. The membrane is a mosaic of proteins bobbing in a fluid bilayer of phospholipids. a membrane is a fluid mosaic of lipids, proteins, and carbohydrates. Membranes are held together by hydrophobic interactions and most membrane lipids and some proteins can drift laterally within the membrane. Membranes must be fluid to work properly. Solidification may result in permeability changes and enzyme deactivation. Unsaturated hydrocarbon tails enhance membrane fluidity –why? Cholesterol, found in plasma membranes of animal eukaryotes, modulates membrane fluidity by making the membrane: • less fluid at warmer temperatures (e.g. 37C body temp) by restraining phospholipids movement • more fluid at lower temperatures by preventing close packing of phospholipids. Cells may alter membrane lipid concentration in response to changes in temperature. Many cold tolerant plants (e.g. winter wheat) increase the unsaturated phospholipids concentration in the autumn to maintain membrane fluidity The proteins found in the phospholipid bilayer vary in both structure and function • Integral proteins are embedded in the membrane – Unilateral – reaching only partway across the membrane – Transmembrane – exposed on both sides of the membrane. • Peripheral proteins are not embedded, but are attached to the membrane’s surface (cytoplasmic side ) Protein functions 1. Cell to cell recognition 2. enzymatic activity 3- signal transduction (hormones) 5- attachment to ECM 4- intercellular joining 6-Cell Transport: Selective permeability – property of biological membrane which allows some substances to cross more easily than others. depends on: • Phospholipid solubility characteristics • presence of specific integral transport proteins Solubility characteristics: (from artificial membrane only) Nonpolar and hydrophobic molecules dissolve in the membrane and cross it with ease (e.g. hydrocarbons and oxygen) Polar (hydrophilic) molecules are dependent on size and charge Small molecules (e.g. H2O, CO2) may be small enough to pass between membrane lipids Larger molecules (e.g. glucose) will not easily pass IONS- even small ones (e.g. Na+, H+) have difficulty penetrating the hydrophobic layer. Water, CO2, and nonpolar molecules rapidly pass through the plasma membrane as they do an artificial membrane. Transport proteins integral transmembrane proteins that transport specific molecules or ions across biological membranes. Are highly specific for the substance they translocate May provide a hydrophilic tunnel May bind to a substance and physically move it across Passive Transport diffusion of a substance across a biological membrane • does not require the cell to expend energy. • Driven by concentration gradient Rate is regulated by the permeability of the membrane Water diffuses freely across most cell membranes Osmosis – diffusion of water across a selectively permeable membrane Water diffuses down its concentration gradient. • Hypertonic solution – a solution with a greater solute concentration than that inside the cell • Hypotonic solution – a solution with a lower solute concentration than that inside the cell. • Isotonic solution – a solution with an equal solute concentration compared to that inside the cell. • U-tube If two isosmotic solutions are separated by a selectively permeable membrane, water molecules diffuse across the membrane in both directions at an equal rate. However, there is no NET movement of water. • Water potential – measure of the tendency for a solution to take up water by a selectively permeable membrane. Water potential of pure water is zero. - Solutes lower the water potential (ex -2) - Water flows from HIGH to LOW (water potential) - Water flows from hypertonic to hypertonic Water Balance of Cells Without Walls In a hypertonic environment, an animal cell will lose water by osmosis and shrivel. In a hypotonic environment, an animal cell will gain water by osmosis, swell, and perhaps lyse (burst). Organisms without cell walls prevent excessive loss or uptake of water by: Living in an isotonic environment Osmoregulating Water Balance of Cells With Walls prokaryotes, some protists, fungi, and plants • In a hypertonic environment, walled cells will lose water by osmosis and will plasmolyze, which is usually lethal. Plasmolysis = plasma membrane pulls away from the cell wall as the cell loses water to a hypertonic environment. In a hypotonic environment, water moves by osmosis into the plant cell, causing it to swell until internal pressure against the cell wall equals the osmotic pressure of the cytoplasm. Creates turgid cells = ideal for support Turgidity is the firmness or tension found in walled cells. In an isotonic environment, there is no net movement of water into or out of the cell and the plant cells become flaccid (limp). Loss of structural support from decreased turgid pressure causes plants to wilt. Facilitated diffusion – diffusion of solutes across a membrane with the help of transport proteins. A passive transport down the concentration gradient Helps the diffusion of many polar molecules and ions which are impeded by the membrane’s phospholipid bilayer. Transport proteins share some properties of enzymes: • are specific for the solutes they transport. - analogous to an enzyme’s active site. • can be saturated with solute = maximum transport rate occurring • can be inhibited by molecules that resemble the normal solute ( resembles competitive inhibition in enzymes). • However, they do not usually catalyze chemical reactions. One Model for Facilitated Diffusion: Transport proteins forms a channel through which water molecules or specific solutes can pass. (Aquaporins are water channels) • Some proteins are gated channels and only open in response to electrical or chemical stimuli. (neurotransmitters and neurons) Second Model for Facilitated Diffusion: Transport protein most likely remains in place in the membrane and translocates solute by alternating between two conformations. In one conformation, the transport protein binds the solute; in the second conformation, it deposits the solute on the other side of the membrane. The protein can transport in either direction, with the net movement being down the concentration gradient Active Transport • ATP requiring process during which a transport protein pumps a molecule across a membrane against its concentration gradient. Helps cells maintain steep ionic gradients across the cell membrane (e.g. Na+, Cl-) Sodium-potassium pump: (example of active transport) – Na+ binding sites on the cytoplasm side and K+ binding sites on the cell exterior • ATP phosphorylates the transport protein and powers the conformational change from Na+ receptive to K+ receptive • As the transport protein changes conformation, it translocates bound solutes across the membrane • three Na+ ions out of the cell for every two K+ pumped into the cell creates a polarized membrane Electrochemical gradient – diffusion gradient resulting from the combined effects of membrane potential and concentration gradient Ions always diffuse down their electrochemical gradients Uncharged solutes diffuse down concentration gradients only Electrogenic pump – a transport protein that generates voltage across a membrane • Na+/K+ pump is the major electrogenic pump in animals * A proton pump is the major electrogenic pump in plants, bacteria, and fungi. Also, mitochondria and chloroplasts use proton pumps for syntheis of ATP electrogenic pumps are sources of potential energy Exocytosis and Endocytosis Exocytosis – process of exporting macromolecules from a cell by fusion of vesicles with the plasma membrane Vesicle usually buds from the ER or Golgi and migrates to the plasma membrane. Used by secretory cells of pancreas (insulin) or neurons (neurotransmitters) Endocytosis – process of importing macromolecules into a cell by forming vesicles derived from the plasma membrane Vesicle forms from a localized region of the plasma membrane that sinks inward (pinches off) three types of endocytosis: Phagocytosis – endocytosis of solid particles amoeba engulfs particle with pseudopodia and forms a food vacuole. WBC’s do this too Pinocytosis – endocytosis of fluid droplets taken in as small vesicles. Receptor-mediated endocytosis – process of importing specific macromolecules into the cells in response to the binding of specific ligands to receptors on the cell’s surface Ligand is a generic term for a molecule that binds to a receptor site of another molecule A very discriminating process Stages of Receptor-Mediated Endocytosis: Extracellular ligand binds to receptors in a coated pit Causes inward budding of the coated pit Forms a coated vesicle Ingested material is released from the vesicle Protein receptors are recycled to the plasma membrane enables cells to acquire bulk quantities of specific substances, even if they are in low concentration in extracellular fluid. In the blood, cholesterol is bound to lowdensity lipoproteins (LDLs) These LDLs bind to LDL receptors on cell membranes defective LDL receptors can lead to atherosclerosis.