Cell Membrane Structure and Function Prof. Dr. Turgut Ulutin
Why do we need the plasma membrane? Keeping the goods concentrated Keeping harmful materials out Transports substances in and out of the cell Communication with other cells chemical signals
Cell membrane functions include: Physical isolation Regulation of exchange with the environment Changes in ECF, ph, receptor recognition Structural support
The Cell Membrane The cell membrane is a phospholipid bilayer with proteins, lipids and carbohydrates.
Membrane Structure Primarily made up of lipids With lipids With carbohydrates 1. Lipids are the most abundant Membrane called phospholipid bilayer Outermost portions - hydrophilic Innermost layers - hydrophobic
Four Components of the Plasma Membrane Phospholipid Bilayer Cholesterol Proteins Glycocalyx
Membranes are Fluid Mosaics Phospholipids are the fluid part of the membrane Changes shape without breaking apart The proteins move through the phospholipids Think of it as icebergs moving through the ocean
Phospholipid Bilayer Phospholipids two fatty-acid chains and a polar phosphate group attached to glycerol Phospholipids in water -two layers, heads pointed out, and tails pointed in Outside of cell mostly water and inside of cell mostly water so polar heads point towards the watery extracellular fluid and watery cytoplasm inside the cell Passage through bilayer lipid center is a barrier to passage of large hydrophilic molecules, but it allows nonpolar, hydrophobic molecules to pass. Small molecules like water and uncharged, lipid soluble moleucles can pass freely
Cholesterol Lipid material Two Functions Acts as patching substance on the bilayer that prevents passage of some molecules (such as ions or simple sugars) Helps keep membrane stronger and flexible
Proteins Two types with many functions Integral - span entire membrane, popping out on both sides or extending partway into the membrane; bound to the hydrophobic interior Peripheral - lie on either side of the membrane, not bound to hydrophobic interior Function Structural support attach to cytoskeleton Recognition binding sites on some proteins identify the cell to other cells help immune system determine self from foreign cells Communication signals transferred through receptor proteins Transport molecules can pass
Glycocalyx sugar coat Sugar components protruding from lipids and proteins Binding sites for proteins in communication and recognition Lubricate cells Stick cells down sticky adhesion layer
Membranes serve a wide variety of functions 1. Barriers to separate cells from environment (plasma membrane) and one compartment from another (organelle membranes) 2. Transport Passive, active, and vesicular transport 3. Sites for enzyme systems, e.g. a) ATP synthase b) Cytochrome p450s 4. Conduct electrical signals 5. Recognize and respond to molecules or surfaces a) Signal mol. Receptors b) Immune response to antigens
Moving Materials In and Out: Diffusion and Gradients A. Random Movement and Diffusion Concentration - # of molecules in a given unit of volume Concentration gradient = difference between the highest and lowest concentration of a solute Example bike coasting down a hill Diffusion = movement of molecules from region of higher to lower concentration The greater the concentration gradient the faster the rate Diffusion will continue until concentration gradient is eliminated (as long as no other processes interfere)
Structure and Function of Organelles cell (plasma) membrane semi-permeable membrane which regulates the passage of substances into and out of the cell composition: protein molecules dispersed throughout a double layer of lipid (fluid mosaic)
Permeability The ease with which substances can cross the cell membrane Nothing passes through an impermeable barrier Anything can pass through a freely permeable barrier Cell membranes are selectively permeable Selective permeability is based on size, electrical charges, molecular shape, and lipid solubility. Transport of substances across the membrane can be Passive or Active Active transport requires energy to occur Passive transport does not require energy Diffusion, Osmosis and Active Transport are different types of movement
Factors affecting permeability of cell membrane to a particular substance: 1. molecular size of the substance 2. electrical charge on the atom or molecule of the substance 3. solubility of the substance in water 4. solubility of the substance in lipid
What determines the direction of the movement of molecules (diffusion) across the cell membrane? concentration of substances on each side of the membrane Diffusion is a form of passive transport. (no energy required)
Osmosis diffusion of water through a semipermeable membrane from an area where the water molecules are more concentrated to an area where the water molecules are less concentrated
Diffusion through Membranes Permeability verses selectively permeable Permeable Membrane permeable to both water and solute solute diffuses down its concentration gradient diffuses through the membrane becoming evenly distributed on both sides Selectively permeable water freely moves, but solute cannot more solute on one side of the membrane water moves both ways, but more flows to the greater concentration of solutes result more solution on one side of the membrane than the other Osmosis - net movement of water across a selectively permeable membrane from an area of lower solute concentration to higher solute concentration Importance of osmosis to membrane function in animals and plants Cells shrinking or bursting in animals Cells expanding and shrinking in plants (wilting and standing up)
Two Main Types of Transport across Cell Membranes Passive Transport no energy expended Simple diffusion - membrane is permeable to water, gases, and fat-soluble molecules, so can enter without energy output by cell Facilitated diffusion - membrane is impermeable to larger polar molecules, so to travel down the concentration gradient, need help from a transport protein Channel protein Carrier protein Osmosis Active Transport molecules passing across the membrane up their concentration gradient cannot use diffusion - must expend energy (ATP) Transport protein binds with ATP and the molecule being transported. The protein changes shape to move the ion across the membrane Ex. Sodium (Na ) Potassium (K ) pump Maintain high K inside the cell and high Na outside the cell Think of it as pumping out seawater from a leaking boat
Active transport requires energy A transport protein actively pumps a specific solute across a membrane against the solute s concentration gradient; i.e., away from where the solute is less concentrated. Membrane proteins use ATP as their energy source for active transport.