Membrane Structure & Function Chapter 7

YOU MUST KNOW Why membranes are selectively permeable The role of phospholipids, proteins, and carbohydrates in membranes How water will move if a cell is placed in an isotonic, hypertonic, or hypotonic solution and be able to predict the effect of different environments on the organism How electrochemical gradients and proton gradients are formed and function in cells

Cell Membrane Structure – Fluid Mosaic Model

Phospholipids Modified triglyceride Polar, hydrophilic heads Non-polar, hydrophobic tails

Embedded Proteins Integral Proteins – span the entire membrane Used for transport through membrane or as receptors for chemical messengers Peripheral Proteins – on one side or the other Often there temporarily Often associate with integral proteins (gates) Can move fluidly along the membrane to transport molecules

Carbohydrates Cell-to-cell recognition Immune function (recognize self/non-self) Important in transfusions and transplants Cell differentiation

Selective Permeability Small, non-polar molecules (CO2, O2) can cross the phospholipids easily Ions and polar molecules cannot cross the hydrophobic region so must travel through membrane proteins Aquaporins – Protein channels for water Anatomy of a protein channel

Passive Transport Requires no energy Does require a concentration gradient – difference in concentration in 2 adjacent areas Molecules move constantly, randomly, and away from each other, down or with the concentration gradient

Diffusion Can happen anywhere (through air, water, across a cell membrane) Small, hydrophobic molecules can cross the phospholipid bilayer by this process Carbon dioxide, oxygen, small hydrocarbons End result is the elimination of the concentration gradient

Facilitated Diffusion Because ions & polar molecules cannot pass through the hydrophobic region of the phospholipid bilayer, they require integral membrane protein channels to pass from high concentration to low concentration Protein channels – always “open” Carrier proteins – interaction with molecule that fits causes conformational change which passes them across membrane ***Remember – proteins are specific! So these channels and carriers are specific, too- each one is for a specific substance to pass through

Osmosis Diffusion of water across a selectively permeable membrane A cell has 3 possible water relationships with its surroundings Remember, we always talk in terms of NET movement of water!

Active Transport Movement of molecules from an area of low concentration to an area of high concentration (against the concentration gradient) Uses energy (usually in the form of ATP) Also requires a protein pump Creates a concentration gradient instead of eliminating it

Membrane Potential The differences in charge between the inside and outside of a cell A relative voltage created by the concentration of ions inside and outside the cell Resting potential (normal conditions) – the inside of the cell is negatively charged in comparison to the exterior

Electrochemical gradients A combination of forces acting on an ion Chemical force which is created by the concentration gradient Voltage gradient created by the membrane potential Because the inside of the cell is negatively charged, positively charged ions outside the cell are attracted to the inside of the cell

Cotransport is the name of a process in which two substances are simultaneously transported across a membrane by one protein, or protein complex. One substance uses the concentration gradient of the other to move against its gradient. Taking your ugly friend into the club

Endocytosis Exocytosis