Movement of Materials into and out of the Cells

Also called cell transport mechanisms These may be: •Passive (physical) processes •Active (physiological) processes

cell membrane is a selective barrier that controls which substances enter and leaves the cell.oxygen and nutrient molecules enter through this membrane,whereas carbon dioxide and other wastes leave through it. Movements of substances into and out of cells use physical(or passive) processes,such as diffusion, osmosis, facilitated diffusion, and filtration, and physiological (or active) processes, such as active transport, endocytosis,and exocytosis.

Do not require energy e.g.: n Simple diffusion n Facilitated diffusion n Osmosis n Filtration

nMovement of substances from regions of higher concentration to regions of lower concentration i.e. with or along a concentration gradient simple diffusion don't requie have membrane all atoms/molecules have kinetic energy

Diffusion(also called simple diffusion)is the tendency of atoms,molecules,and ions in a liquid or air solution to move from areas of higher concentration to ares of lower concentration, thus becoming more even distributed to more diffuse.

Diffusion occurs because atoms, molecules and ions are in constant motion.Each particle travels in a separate path along a straight line until it collides with another particle and bounces off. Then it moves in its new direction until it collides again and changes direction once move, this difference in concentrations is called a concentration gradient,and atoms,molecules,and ions are said to fifuse down a concentration gradient

Figure 3.20 sugar(a solute)put into a glass of water(a solvent)illustrates diffusion.The sugar at first remains in high concentration at the bottom of the glass.As the sugar molecules move,they may collide or miss each other.They are less likely to collide where there are fewer of them,so molecules gradually diffues from areas of higher concentration to areas of lower concentration(down the concentration gradient).and eventually become uniformly distributed in the water. A sugar cube placed in water slowly disappears as the suger molecules dissolve and teh diffuse from regions where they are more concentrated toward regions where they are less concentrated.Evnetaully,the sugar molcules are distributed evenly throughout.

Diffusion of a substance across a cell membrane can occur only if the cell membrane is permeable to that substanc is at a higher concentration on one side of the membrane or the other.

Figure 3.21 consider oxygen and carbon dioxide. Cell membrane are permeable to both.In the body,oxygen diffuses into cells and carbon dioxide diffuses out of cells,but equilibrium is never reached.Intracellular oxygenis always low because oxygen is constantly used up in metabolic reactions.Extracellular oxygen is maintained at the a high levle by homeostatic mechanisms in teh rspiratory and cardiovascular systems.In this way, a concentration gradient always allows oxygen to diffuse into the cell,.

nAll atoms/molecules have kinetic energy nsolids < liquids < gases nThis causes particles to collide and rebound from each other which distributes the particles

ne.g. the movement of Oxygen, carbon dioxide and lipid-soluble substances across cell membranes

•Diffusion across a membrane with the help of channels or carrier molecules

§Movement of water through a selectively permeable membrane from regions of higher concentration to regions of lower concentration nOsmosis occurs due to differences in concentration of solutions on either side of a selectively permeable membrane

nA solution = solute + solvent ni.e. in a NaCl solution, NaCl is the solute and water is the solvent nThe concentration of a solution is expressed in terms of the concentration of the solute. nin osmosis, water moves toward a higher concentration of solute

nOsmotic Pressure – the ability of osmosis to generate enough pressure to move a volume of water nOsmotic pressure increases as the concentration of nonpermeable solutes increases