Lecture Outline - Chapter 4
4.1. Plasma Membrane Structure and Function
4.2. How Molecules Cross the Plasma Membrane (Table 4.1) (p. 70)
- Introduction: plasma membrane is boundary of the cell that regulates the passage of molecules into and out of the cell, similar to a castle wall.
- Plasma membrane is a phospholipid bilayer (fluid consistency) with protein molecules embedded within but scattered (mosaic) throughout the lipid; therefore this is called the fluid-mosaic model. (Fig. 4.1)
- Each phospholipid molecule has a polar (hydrophilic) head that is attracted to polar water molecules and two nonpolar (hydrophobic) tails that face away from water and toward tails of other layer.
- Glycolipids are similar to phospholipids except polar head has carbohydrate molecules attached.
- Cholesterol is animal lipid that reduces permeability of membrane.
- Glycoproteins are integral proteins embedded in the membrane with carbohydrate chains that project externally; they are unique for cell identity.
- Proteins extending into cell from inside layer serve as links to cytoskeletal filaments.
- Plasma membrane is a fluid bilayer with consistency of olive oil.
- A mosaic of different proteins (Fig. 4.3) is embedded in this layer; red blood cells have over 50.
- Channel proteins allow molecules or ions to move across membrane.
- Carrier proteins combine with a particular substance to move it across membrane.
- Receptor proteins bind to a specific molecule (hormone) due to its shape.
- Enzymatic proteins carry out specific metabolic reactions.
- Different cells and tissues in body are recognized by their different surface carbohydrate chains; individuals in same species differ even more.
- Foreign tissues and transplanted organs are recognized by the immune system because of these surface differences.
4.3. Diffusion and Osmosis (p. 70)
- Ability of molecules to enter or leave the cell is regulated by the plasma membrane.
- Small noncharged molecules such as water have no difficulty passing through a lipid bilayer; the membrane is permeable to water.
- Because passage of many molecules through membrane is restricted, it is semipermeable.
- Large molecules cannot fit through membrane; charged ions often cannot pass through the hydrophobic part of the bilayer.
- The smaller and more oil-soluble a molecule, the easier it passes the lipid membrane.
- Channel and carrier proteins assist ions and polar molecules across.
- Passive transport does not require energy; active transport requires cell energy.
4.4. Transport by Protein Carriers (p. 74)
- Diffusion is a physical process where molecules move down a concentration gradient from higher to lower concentration until equally distributed.
- Diffusion applies to any type of molecule; for example, dye crystals are a solute that diffuse in water, the solvent. (Fig. 4.4)
- Lipid-soluble molecules (alcohols) pass easily through the lipid membrane.
- Gases such as oxygen and carbon dioxide diffuse easily through alveoli, capillaries, and blood plasma membranes. (Fig. 4.5)
- Implanted timed-release capsules rely on diffusion to distribute medication.
- Water probably passes through protein channels in the plasma membrane.
- Osmosis is a net movement of water molecules from a region of greater concentration to a region of lesser concentration across a differentially permeable membrane. For example, (Fig. 4.6) a thistle tube with a differentially permeable membrane separates different concentrations of sugar and water. As water enters the thistle tube, the hydrostatic pressure builds up and the net movement of water ceases. The hydrostatic pressure is equivalent to osmotic pressure of the solution inside the thistle tube.
- Due to osmosis, water is absorbed from human large intestine and taken up by blood.
- Tonicity refers to the strength of a solution in relationship to osmosis.
- a. Isotonic solutions hold cells stable since solute concentration is same on both sides of the membrane; there is no net gain or loss of water and cells neither swell nor shrink. For example, red blood cells are isotonic in 0.9% NaCl.
- b. Hypotonic solutions cause cells to swell; the solution has a lower percentage of solute (more water) than in the cell. This causes the cell to swell and it may burst (lysis) if it is an animal cell due to the intake of water. For example, for red blood cells, any solution less than 0.9% NaCl will causes hemolysis. A hypotonic solution brings about turgor pressure due to swelling of the cell. Plant cells do not burst in a hypotonic solution due to the rigid cell wall. Turgor pressure is important in plants to maintain their erect position.
- c. Hypertonic solutions allow cells to shrivel; the solution has a higher percentage of solute (less water) outside the cell, causing the cell to shrink due to net movement of water outward. For example, any solution with a concentration higher than 0.9% causes RBCs to shrink (termed crenated). Plasmolysis refers to the shrinking of the cytoplasm due to osmosis.
4.5. Endocytosis and Exocytosis (Fig. 4.11)
- Carrier proteins are specific; each can combine with only a certain type of molecule, which is transported through the membrane, probably by undergoing a change in shape. (Table 4.1)
- Facilitated transport occurs when certain sugars and amino acid molecules are transported across the membrane by a carrier protein down a concentration gradient at a faster rate than simple diffusion; this does not need energy. (Fig. 4.8)
- Active transport transports a molecule across a membrane against the concentration gradient from a lesser to a greater concentration, and it requires energy (ATP); for example: absorption of sugar from intestine, sodium in kidney tubules, or the sodium- potassium pump (Fig. 4.9) in nerve and muscle cells. Protein carriers change shape which allows it to alternately combine with sodium or potassium ions. In cystic fibrosis, the chloride channels malfunction after sodium ions are pumped out.
- Large molecules including polypeptides, polysaccharides, and polynucleotides are too large to be transported by protein carriers; must be enclosed in vesicles for transport.
- Endocytosis occurs when the plasma membrane forms a vesicle around a substance to be taken into the cell; requires energy.
- a. Phagocytosis occurs when large sized material is taken into the cell by forming a vesicle; for example, macrophages engulf bacteria and worn-out red blood cells.
- b. Pinocytosis occurs when liquids or small molecules such as proteins are taken into cell by forming vesicle; requires use of electron microscope to observe.
- c. Exocytosis is reverse of endocytosis where intracellular vesicles formed by Golgi apparatus transports cell products out of the cell when vesicle fuses with plasma membrane; exocytosis is required for secretion.
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