Membrane Structure and Function

Plasma Membrane Flexible boundary separating living cell from nonliving surroundings Selectively permeable = choosy about what enters and exits Controls traffic into and out of cell

Structure Phospholipid Bilayer Hydrophilic heads are facing watery environment (extracellular fluid and cytoplasm) Hydrophobic tails are inside bilayer Fluid Mosaic Model Proteins embedded in bilayer of phospholipid molecules Freeze Fracture evidence supports Freeze, cut, see that proteins are embedded

Fluidity Membranes are held together by weak hydrophobic interactions Allows movement of phospholipids Can move laterally (side to side) or flip (rare) Need to be fluid to work Unsaturated tails prevent tight packing Cholesterol restrains phospholipid movement in warmer temps., and prevents close packing at lower temps.

Proteins Integral proteins Transmembrane (span entire membrane) Peripheral proteins Not embedded, just attatched to either ECM or cytoskeleton

8 Six Major Functions of Membrane Proteins Figure 7.9 Transport. (left) A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. (right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane. Enzymatic activity. A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway. Signal transduction. A membrane protein may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signal) may cause a conformational change in the protein (receptor) that relays the message to the inside of the cell. (a) (b) (c) ATP Enzymes Signal Receptor

9 Cell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells. Intercellular joining. Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions Attachment to the cytoskeleton and extracellular matrix (ECM). Microfilaments or other elements of the cytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes (d) (e) (f) Glyco- protein Six Major Functions of Membrane Proteins

Synthesis of Membranes Made in ER Transport vesicle to Golgi Modified and packaged in Golgi Secretory vesicle to cell membrane

Cell-to-Cell Recognition Helps cell recognize if a cell is alike or different Important for immune response Carbohydrates act as markers on cell membrane Oligosaccharides (<15 monomers) Can be found attached to proteins or lipids Glycoprotein = protein + oligosaccharide Glycolipid = lipid + oligosaccharide

Moving through Membranes Nonpolar (hydrophobic) molecules Cross easily Hydrocarbons, Gases Polar (hydrophilic) molecules Small can pass (water, ethanol) Large cannot (glucose) Ions have a hard time (Na +, H + ) Can use transport proteins

Passive Transport No energy required - ∆G Moves down concentration gradient Difference in solute concentration Net movement: High  Low Concentration Through Membrane = Diffusion Through Transport Protein = Facilitated Diffusion Water through Membrane = Osmosis

Osmosis Isotonic Solution = no net movement, equal solute concentration on both sides of membrane, equilibrium, water moves equally in and out. Hypotonic Solution = solution has LESS solute concentration than inside the cell, net movement into cell, cell swells, animal cells can burst (lyse). Hypertonic Solution = solution has MORE solute concentration than inside the cell, net movement out of cell, cell shrinks.

15 How Will Water Move Across Semi- Permeable Membrane? Solution A has 100 molecules of glucose per ml Solution B has 100 molecules of fructose per ml How will the water molecules move? How will the water molecules move? no net movement of water There will be no net movement of water since the concentration of solute in each solution is equal

16 How Will Water Move Across Semi-Permeable Membrane? Solution A has 100 molecules of glucose per ml Solution B has 75 molecules of fructose per ml How will the water molecules move? How will the water molecules move? There will be a net movement of water from Solution B to Solution A until both solutions have equal concentrations of solute

17 How Will Water Move Across Semi-Permeable Membrane? Solution A has 100 molecules of glucose per ml Solution B has 100 molecules of NaCl per ml How will the water molecules move? How will the water molecules move? Each molecule of NaCl will dissociate to form a Na+ ion and a Cl- ion, making the final concentration of solutes 200 molecules per mil. Therefore, there will be a net movement of water from Solution A to Solution B until both solutions have equal concentrations of solute

Cell (compared to beaker)  hypertonic or hypotonic Beaker (compared to cell)  hypertonic or hypotonic Which way does the water flow?  in or out of cell.05 M.03 M Do you understand Osmosis…

Differences in Animals + Plants

Adaptations Contractile Vacuole in paramecium Salt pumps in bony marine fish Full Empty

Facilitated Diffusion Specific to solute Max rate occurs at saturation of solute Can be inhibited by molecules that resemble solute Channel Proteins – no shape change Carrier Proteins – shape change Gated Channels – open only in response to stimuli

Channel Carrier Gated EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM

Active Transport Energy requiring Transport protein pumps a molecule AGAINST the concentration gradient Net movement: Low  High + ∆G Important to maintain ion gradients (Na, K, Cl, Ca)

Sodium-Potassium Pump Na + binding sites towards cytoplasm K + binding sites towards exterior ATP becomes ADP and phosphorylates protein Causes shape change from Na + receptive to K + receptive Solutes travel across membrane 3 Na + out of cell for every 2 K + into cell Keeps inside of cell negative compared to outside Major pump in animal cells (esp. neurons!)

Proton Pump Major pump in plants, bacteria, and fungi Found in Mitochondria and Chloroplasts

Cotransport 1 ATP driven pump transports 1 solute and indirectly drives the transport of other solutes against their concentration gradient

Large Molecules Endocytosis = importing large macromolecules by forming vesicles from plasma membrane Phagocytosis: endocytosis of solid particles Ex: Ameoba’s pseudopodia Pinocytosis: endocytosis of fluid droplets Receptor Mediated Endcytosis: a ligand (molecule that bonds to a receptor) binds to initiate endocytosis Cholesterol

In blood, cholesterol is bound to lipid and protein complexes called low-density lipoproteins (LDL). The LDLs bind to LDL receptors on cell membrane to initiate endocytosis of cholesterol. Defective LDL receptors mean build up of cholesterol in blood.

Large Molecules Exocytosis = exporting macromolecules from a cell by fusion of vesicles with the cell membrane Vesicles from ER or Golgi Ex: Insulin from pancreatic cells