Cell Membranes and Cell Transport

1. Cell Membranes and Cell Transport Cell (plasma) membranes perform three general functions: Isolate the cell’s contents from the external environment Regulate the exchange of essential substances btw cell and environment Communicate with other cells

2. Fluid Mosaic Model – developed by S.J. Singer and G.L. Nicholson in 1972 • membranes are made up of a double layer of phospholipids with proteins floating within the phospholipids • phospholipids are responsible for the isolating function of membranes • proteins are responsible for regulating exchange of substances and communication with environment • overall structure is constantly changing

4. Fluid nature of the plasma membrane

5. Phospholipids consist of a polar, hydrophilic head and a pair of nonpolar, hydrophobic tails • phospholipids spontaneously arrange themselves into a bilayer

6. Membrane Proteins – two major classes: • integral proteins – firmly bound to membrane and part extends out of the cell or into the cytoplasm • transmembrane proteins – integral proteins that extend completely through the membrane • peripheral proteins – not embedded in the lipid bilayer, located on inner and outer surfaces of the membrane

7. Membrane proteins have different functions divided into four major categories: • Transport proteins – regulate the movement of water-soluble molecules through the membrane • channel proteins – form pores or channels that allow small molecules or ions to pass • carrier proteins – have binding sites that attach to and move specific molecules across the membrane

8. Receptor proteins – trigger cellular responses when specific molecules in the extracellular fluid bind to them (hormones) • Recognition proteins – serve as identification tags • Enzymes – some enzymes are built into the membrane (ex. Enzymes for cellular respiration)

9. Transport across the membrane • Cell (plasma) membranes are selectively permeable– they allow some molecules to pass through but prevent others from passing • Passive transport – substances move in or out of cells down concentration gradients (movement from regions of high concentration to low) – does not require energy • concentration gradient – a difference in concentration between one region and another • Diffusion – net movement of molecules down a concentration gradient from regions of high concentration to low – ex. water, O2, CO2 and lipid-soluble molecules

10. Facilitated diffusion – movement across the membrane down the concentration gradient with the help of a channel or carrier protein (carrier proteins bind to specific molecules)

11. Osmosis – the diffusion of water from regions of high water concentration to low – water moves freely across membranes • pure water has the highest water concentration • any substance added to pure water displaces some of the water molecules; resulting solution will have a lower water content than pure water • Osmotic concentration – total solute concentration of a solution • as the solution’s osmotic concentration increases, the dissolved substances may form weak bonds with some water molecules making these water molecules unavailable to diffuse across the membrane • Water tends to move from the solution with the lowest osmotic concentration to the solution with the highest osmotic concentration

12. Osmotic pressure – measure of the tendency for a solution to take up water when separated from pure water by a selectively permeable membrane • osmotic pressure of pure water is zero • osmotic pressure of a solution is proportional to its osmotic concentration (the greater the solute concentration, the greater the osmotic pressure) • pressure corresponds to the hydrostatic pressure (fluid pressure) exerted by the solute particles – the more solutes, the more pressure must be exerted to prevent water from diffusing in

13. Water potential – a measure of the tendency of a solution to take up or lose water • Pure water has an water potential of zero • As solutes are added to the solution, the water potential becomes negative (always measured in negative numbers) • The more negative the number (lower water potential, therefore more concentrated) the more the solution will tend to take up water not lose water • net movement of water will be from the solution with the higher water potential (hypotonic) to the solution with the lower water potential (hypertonic) • Important mechanism in the movement of water in plants

14. Isotonic, hypertonic, and hypotonic solutions • all cells contain dissolved salts, proteins, sugars, and other substances – the flow of water across the membrane depends on the concentration of water in the liquid that bathes the cells • Isotonic solutions – when a cell is placed in a fluid with exactly the same water concentration – total concentration of dissolved particles is equal on both sides of the membrane – ex. blood plasma and body fluids are isotonic to our cells • Hypertonic solutions – solutions that have a higher concentration of dissolved particles than the cell’s cytoplasm – water moves out of the cell • Hypotonic solutions – solutions that have a lower concentration of dissolved particles than the cell’s cytoplasm – water moves in to the cell

16. Active transport – movement of materials up or against the concentration gradient – requires the use of energy (ATP) • active transport proteins are called pumps • substances are “pumped” from regions of low concentration to high

17. Exocytosis and Endocytosis (Bulk Transport) – movement of materials that are too large to cross the membrane • Endocytosis – membrane engulfs a fluid droplet or particle and pinches off a membranous sac called a vesicle – three types of endocytosis: • pinocytosis – moves liquids into the cell • receptor-mediated endocytosis – specific molecules combine with receptor proteins in the membrane that lie in coated pits (depressed regions on membrane) – binding causes coated pit to deepen and eventually pinch off into the cytoplasm as a coated vesicle – ex. uptake of cholesterol • phagocytosis – used to ingest large solid particles (such as an amoeba taking in food) • Exocytosis is essentially the reverse of endocytosis