Lecture # 8: The Cell Surface (Part 2) (Chapter 3) Objectives: 1- Define a cell and distinguish...
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Transcript of Lecture # 8: The Cell Surface (Part 2) (Chapter 3) Objectives: 1- Define a cell and distinguish...
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 TRANSPORT ACTIVE TRANSPORTIt 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- ExocytosisReceptor-Mediated EndocytosisPinocytosisPhagocytosis
Membrane Transport
Filtration (driven by hydrostatic pressure)
1- FiltrationIt 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
A B
Osmosis:
OSMOSIS
Osmosis and Tonicity
Osmosis
Reverse osmosis
Side A Side BThe 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 osmosis Osmotic 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 BSolution B has a higher osmolarity than solution A
OSMOSIS
A BIf 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 waterwater
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 TransportIt 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 EndocytosisPinocytosis
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
LysosomeVesicle fusingwith membrane
Phagolysosome
1
2
3
45
6
7
A phagocytic cell encounters aparticle of foreign matter.
The cell surroundsthe particle with itspseudopods.
The particle is phagocytizedand contained in aphagosome.
The phagosome fuseswith a lysosome andbecomes a phagolysosome.
The indigestibleresidue is voided byexocytosis.
The phagolysosomefuses with theplasma membrane.
Enzymes from thelysosome digest theforeign matter.
Pinocytosis (cell drinking)
It is a form of endocytosis that brings into the cell a small volume of extracellular fluid (ECF)
Receptor-Mediated EndocytosisIt 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+)
ExocytosisIt is the movement of materials out of the cell through the fusion of a vesicle with the plasma membrane