1. Cell Biology Lecture 4

2. Plasma Membrane Transport Permeable Slightly permeable Impermeable

3. 1. Simple Diffusion In simple diffusion, small noncharged molecules or lipid soluble molecules pass between the phospholipids to enter or leave the cell, moving from areas of high concentration to areas of low concentration (they move down their concentration gradient).

4. Small hydrophobic molecules cross membrane by simple diffusion Gases (O2, CO2), lipids and small uncharged molecules (urea, ethanol) are transported by simple simple diffusion. They do not need transport proteins. No metabolic energy is consumed because the movement is from a high to a low concentration gradient.

5. 2. Osmosis Osmosis is a type of simple diffusion in which water molecules diffuse through a selectively permeable membrane from areas of high water concentration to areas of lower water concentration. (Note that the more particles dissolved in a solution, the less water there is in it, so osmosis is sometimes described as the diffusion of water from areas of low solute concentration to areas of high solute concentration).

6. Hypertonic and hypotonic solutions A hypertonic solution has more solutes and less water than a hypotonic solution. For example the solution inside the cell is hypertonic to the solution outside the cell. During osmosis, water moves from the hypotonic solution (more water, less solutes) to the hypertonic solution (less water, more solutes).

7. Which of the following is hypertonic solution?

8. Membrane proteins mediate transport of most molecules and all ions across biomembranes Few molecules (gases, water, ethanol, urea) can pass through pure phospholipid bilayer. Transport of most molecules require the help of transport proteins. All transport proteins are transmembrane proteins. Allow the movement of hydrophilic substance without their coming into contact with the hydrophobic interior of the membrane

9. 3. Facilitative Diffusion In facilitated diffusion, substances move into or out of cells down their concentration gradient through protein channels in the cell membrane. Simple diffusion and facilitated diffusion are similar in that both involve movement down the concentration gradient. The difference is how the substance gets through the cell membrane. In simple diffusion, the substance passes between the phospholipids; in facilitated diffusion there are a specialized membrane channels. Charged or polar molecules that cannot fit between the phospholipids generally enter and leave cells through facilitated diffusion.

10. homeostasis maintains constant conditions in a living organism. Membrane control is essential for this. Passive transport This is movement across the membrane without any energy input. There are 3 forms: diffusion, facilitated diffusion and osmosis Diffusion: - particles are in constant motion (Brownian or heat-kinetic motion) -the overall effect of this random motion over time is diffusion (or the movement of substances from an area of high concentration to an area of lower concentration, thus resulting in equal concentration in both areas) -for this to occur oacross the membrane, the membrane must be permeable to the substances (apolar particles and small molecules like H2O, O2 and CO2)

11. Active Transport transport of a substance against the concentration gradient requires energy must use transport proteins and ATP a substance binds the transport protein and energy is released from ATP to allow the protein to change shape or move and release the substance on the other side of the membrane. Example: Na+/K+ pump - this is especially important in nerve and muscle cell

12. Types of transport proteins

13. Uniport transport of Glucose and Water ‘’The protein-mediated transport of small hydrophillic molecules across a membrane’’ Most animals require glucose as a source of energy. They employ a glucose uniporter to take up glucose from the blood or extra cellular fluid down its conc. Gradient.

14. Features of Uniport transport The rate of diffusion is far higher than simple diffusion through a pure phospholipids. Transport occurs via limited number of uniporter molecules and not throughout the phospholipid bilayer. Transport is specific.

15. GLUT1 Glucose transporter found in the plasma membrane of erythrocytes. 12 GLUTS are encoded by human genomes. Glucose conc. is high in the extracellular medium than in cell. Like other uniporters, GLUT1 alternates between 2 confirmational states.

16. Osmotic pressure causes water to move across membranes Osmotic pressure is directly proportional to the difference in the conc. of the total number of solute molecules on each side of the memberane. Aquaporins are a family of membrane proteins that allow water and a few other small uncharged molecules (glycerol) to cross biomembrane, 11 aquaporins are known in humans

17. ATP-powered Pumps and Intracellular Ionic Environment They use the energy released by hydrolysis of ATP. Have one or more binding sites for ATP located on subunits of the protein facing the cytosol. These proteins do not hydrolyse ATP into ADP and Pi unless ions or other molecules are simultaneously transported.

18. Classes of pumps