Lecture # 8: The Cell Surface (Part 2) (Chapter 3) Objectives: 1- Define a cell and distinguish between cytosol and cytoplasm. 2- Explain the structure and molecular organization of the cell membrane and list factors determining its selective permeability. 4- Explain and illustrate the following mechanisms of cell transport: diffusion, osmosis, facilitated diffusion, active transport endocytosis, and exocytosis. 3- Explain osmotic pressure and how isotonic, hypertonic, and hypotonic solutions affect cells.

PASSIVE TRANSPORTACTIVE TRANSPORT It moves ions or molecules across the plasma membrane with no expenditure of energy by the cell (down gradient) It requires that the cell expends energy, generally in the form of ATP (up gradient) Simple Diffusion (down gradient) Osmosis (a special case of simple diffusion) Facilitated Diffusion (down gradient, carrier mediated) The Sodium-Potassium Pump Secondary Active Transport Vesicular Transport 1- Endocytosis 2- Exocytosis Receptor-Mediated Endocytosis Pinocytosis Phagocytosis Membrane Transport Filtration (driven by hydrostatic pressure)

1- Filtration It is the process in which particles are driven through a selectively permeable membrane by hydrostatic pressure (force exerted on a membrane by water) 1- Filtration of nutrients through gaps in blood capillary walls into tissue fluids Examples: Blood pressure in capillary forces water and small solutes such as salts through narrow clefts between capillary cells. 2- Filtration of wastes from the blood in the kidneys while holding back blood cells and proteins

It is the passive net movement of ions or molecules from the region of higher concentration to the region of lower concentration. It is the difference between the high and low concentration areas Lower concentration Higher concentration Concentration Gradient: The tendency of ions and molecules in a concentration gradients is to move from the region of higher concen- tration to the region of lower concentration Diffusion: Diffusion tends to eliminate the concentration gradient Brownian Movement: It is the irregular movement of particles suspended in a liquid or gas caused by the kinetic energy of these particles

Concentration Gradient: The natural tendency of a solute is to move down its concentration gradient, from higher concentration to lower concentration. It is the difference between the high and low concentration areas of a solute within a given medium 5- Membrane permeability :  permeability,  rate Factors affecting diffusion rate through a membrane 3- Temperature:  temp.,  motion of particles 1- Steepness of concentrated gradient:  difference,  rate 2- Molecular weight: larger molecules move slower 4- Membrane surface area:  area,  rate The random molecular motion of particles (Brownian movement) provides the necessary energy

Down gradient Larger molecules move slower Diffusion in a Concentration Gradient Substance A is larger (higher molecular weight) than substance B A B

Impermeable Membrane It is a membrane through which none substance can pass

It is a membrane through which any substance can pass Freely Permeable membrane

Selectively Permeable Membrane It is a membrane that permits the free passage of some materials and restricts the passage of other The plasma membrane works as a selectively permeable membrane It allows some things through, such as nutrients and wastes, but usually prevents other things as proteins and phosphates, from entering or leaving the cell.

It is the passive net diffusion of solvent (water), across a selectively permeable membrane, toward the solution that has the higher concentration of solutes AB Osmosis: OSMOSIS Osmosis and Tonicity

Osmosis Reverse osmosis Side ASide B The hydrostatic pressure causes a filtration of water from A to B (reverse osmosis), which produces that net osmosis slows down and stop At this point, an equilibrium would exist It is the hydrostatic pressure required to stop osmosisOsmotic Pressure: The higher the concentration of nonpermeating solute in A, the greater the osmotic pressure Hydrostatic pressure

Osmolarity and Tonicity It is the total solute concentration (osmole/L) in an aqueous solution Solution A has a lower osmolarity than solution B Solution B has a higher osmolarity than solution A OSMOSIS A B If two solutions of different osmolarity are separated by a selectively permeable membrane, the solvent (water) will pass the membrane until equilibrium is attained and both solution contain equal concentration of solutes One osmole:It is one mole of dissolve particles Osmolarity:

It is the ability of a solution to affect fluid volume and pressure in a cell Isotonic solution It is a solution with the same concentration of non- penetrating solutes as that found in the cells. (Intravenous solutions are isotonic solutions). Hypertonic solution It is a solution with a higher concentration non-penetrating solutes than that found in the cells. Hypotonic solution It is a solution with a lower concentration of non-penetrating solutes than that found in the cells. water Tonicity:

Osmotic Flow across a Plasma Membrane

Effects of Tonicity on RBCs Hypotonic, isotonic and hypertonic solutions affect the fluid volume of a red blood cell. Notice the crenated and swollen cells. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. (a) Hypotonic(b) Isotonic(c) Hypertonic © Dr. David M. Phillips/Visuals Unlimited Swollen cell Normal cell Crenated cell

Carrier-Mediated Transport It is a process that employs transport proteins (carriers) in the plasma membrane that carry solutes from one side of the membrane to the other. Carrier-Mediated Transport: 1- Facilitated Diffusion: It is the carried-mediated transport of a solute through the membrane down its concentration gradient. It is a passive transport process; that is, does not consume ATP (energy) 2- Active Transport: It is the carried-mediated transport of a solute through the membrane up its concentration gradient, using energy provided by the ATP. Carriers: 1- Uniport: It carries one solute at a time 2- Symport: It carries two or more solutes through the membrane simultaneously in the same direction (cotransport) 3- Antiport: It carries two or more solutes through the membrane in opposite directions (countertransport)

1- Facilitated Diffusion: It is the carried-mediated transport of a solute through the membrane down its concentration gradient. It is a passive transport process; that is, does not consume ATP (energy) Uniport: It carries one solute at a time

2- Active Transport: It is the carried-mediated transport of a solute through the membrane up its concentration gradient, using energy provided by the ATP. Sodium-Potassium Pump 2- Each pump cycle consumes one ATP and exchanges three Na+ for two K+ against (up) the gradient 1- Na + and K + are constantly leaking through membrane down the gradient 3- The pump keeps the K+ concentration higher and the Na+ concentration lower with in the cell than in the extracellular fluid (ECF) 4- Half of daily calories is used for the Na+ - K+ pump

Vesicular Transport It is when materials move into or out of the cell in vesicles, small membranous sacs that form at, or fuse with, the plasma membrane 1- Endocytosis 2- Exocytosis Receptor-Mediated Endocytosis Pinocytosis Phagocytosis

Phagocytosis (cell eating) It is a process of bringing large materials (bacteria, dust, cellular debris) into the cell by means of pseudopodia that surround the material

Phagocytosis or “Cell-Eating” Keeps tissues free of debris and infectious microorganisms. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Particle Pseudopod Nucleus Residue Phagosome Lysosome Vesicle fusing with membrane Phagolysosome A phagocytic cell encounters a particle of foreign matter. The cell surrounds the particle with its pseudopods. The particle is phagocytized and contained in a phagosome. The phagosome fuses with a lysosome and becomes a phagolysosome. The indigestible residue is voided by exocytosis. The phagolysosome fuses with the plasma membrane. Enzymes from the lysosome digest the foreign matter.

Pinocytosis (cell drinking) fluid (ECF) It is a form of endocytosis that brings into the cell a small volume of extracellular fluid (ECF)

Receptor-Mediated Endocytosis It involves the formation of vesicles at the surface of the plasma membrane, which contain specific receptors (ligand receptors) for the substance that will be interiorized (ligand) Lipoproteins (cholesterol), iron ions (Fe 2+ )

Exocytosis It is the movement of materials out of the cell through the fusion of a vesicle with the plasma membrane