Chapter 5-Membrane Structure and Function


5.1 Membrane Models

• lipid-soluble molecules enter cells more rapidly than water-soluble molecules
• sandwich/unit membrane model: the outer dark layer of the membrane contains protein plus the hydrophilic heads of the phospholipids, and the interior is the hydrophobic tails of these molecules
• fluid-mosaic model-introducted in 1972 by S. Singer and G. Nicolson proposing that the membrane is a fluid phospholipid bilayer in which protein molecules are either partially or wholly embedded. The proteins are scattered throughout the membrane in an irregular pattern that can vary from membrane to membrane

5.2 Plasma Membrane Structure and Function

• phosopholipids-molecules that form the bilayer of cell membranes, with polar hydrophilic heads bonded to two nonpolar hydrophobic tails
• the hydrophilic (polar) heads face the intracellular and extracellular fluids
• the hydrophobic (nonpolar) tails face each other
• glycolipids-lipid in plasma membranes that bear carbohydrate chains attached to hydrophobic tails
• cholesterol-lipid found in animal plasma membranes; related steroids are found in plant plasma membranes
• cholesterol reduces permeability of the membrane to most biological molecules
• peripheral proteins occur either on the outside of inside surface of the membrane or the inside surface; some are anchored to the membrane by covalent bonding, others by noncovalent interactions that can be disrupted by shaking or changing pH
• integral proteins are within the membrane; hydrophobic regions are embedded within the membrane and hydrophilic regions project from both surfaces of the bilayer
• many integral proteins are glycoproteins
• glycoproteins-proteins in plasma membranes that bear carbohydrate chains that project externally, "sugar-coated"
• plasma membrane is asymmetrical
• carbohydrate chains occur only on outside surface and cytoskeletal filaments attach to proteins only on inside surface
• at body temperature, phospholipid bilayer of plasma membrane has olive oil consistency
• the greater the concentration of unsaturated fatty acid residues, the more fluid is the bilayer
• in each monolayer, hydrocarbon tails wiggle, and entire phospholipid molecule can move sideways at a rage averaging about 2 m per second
• proteins are generally free to drift laterally in the fluid lipid bilayer
• fluidity of membrane is needed for the functioning of some proteins
• enzymes become inactive when the membrane solidies
• plasma mebranes of various cells and their organelles each have their own unique collections of proteins
• channel proteins-proteins through which a substance can simply move across the membrane
• carrier proteins-proteins that combine with a substance and help it to move across the membrane
• receptor proteins-proteins with shapes that allow specific molecules to bind to them
• binding molecules can cause proteins to change shape and bring about a cellular response
• enzymatic proteins-proteins that carry out metabolic reactions directly
• peripheral proteins stabilize and shape the plasma membrane
• carbohydrate chains of glycolipids and glycoproteins serve as the "fingerprints" of the cell
• cell-cell recognition
  

5.3 Permeability of the Plasma Membrane

• differentially-selectively
• permeable-can move across the membrane
• macromolecules cannot diffuse across the membrane because they are too large
• ions and charged molecules cannot cross the membrane because they are unable to enter the hydrophobic phase of the lipid bilayer
• noncharged, lipid-soluble molecules such as alcohols and oxygen can cross the membrane with ease by slipping between the hydrophilic heads of the phospholipids and pass through the hydrophobic tails of the membrane
• concentration gradient-gradual decrease in concentration over distance
• oxygen is more concentrated outside because a cell uses oxygen during cellular respiration
• carbon dioxide is more concentrated inside because it is produced when a cell carries out cellular respiration
• macromolecules can cross through vesicle formation
• ions and molecules (amino acids and sugars) cross through transport proteins
• carrier proteins combine with an ion/molecule and then transport it
• channel proteins form a channel that allows ions/charged molecules to pass through
• passive transport does not use chemical energy
• diffusion
• facilitated transport
• active transport requires chemical energy
• active transport
• endocytosis
• exocytosis
• diffusion-movement of molecules from a higher to a lower concentration until equilibrium is achieved and they are distributed equally
• diffusion is a physical process that can be observed with any type of molecule
• solution-fluid (solvent) that contains a dissolved solid (solute)
• solvent-liquid portion of a solution that dissolved the solute
• solute-substance dissolved in a solvent, forming a solution
• gases can diffuse through lipid bilayer (oxygen and carbon dioxide)
• after inhaling, the concentration of oxygen in alveoli is higher than that in blood, so the oxygen diffuses into the blood
• osmosis-diffusion of water into and out of cells
• a thistle tube containing a 10% sugar solution is covered at one end by a differentially permeable membrane and placed in a beaker containing a 5% sugar solution
1. a differentially permeable membrane separates 2 solutions, does not permit passage of solute
2. beaker has more water (lower percentage of solute) and thistle tube has less water (higher percentage of solute) per volume
3. membrane permit passage of water, there is net movement of water from the beaker to the inside of thistle tube
4. concentration of solute in thistle tube is less than 10% because there is now less solute per volume and the concentration of solute in the beaker is greater than 5% because there is now more solute per volume
• osmotic pressure-pressure that develops in a system due to osmosis
• the greater the possible osmotic pressure, the more likely water will diffuse in that direction
• due to osmotic pressure, water is absorbed from the human large intestine, retained by the kidneys, and taken up by capillaries from tissue fluid
• tonicity-degree to which a solution's concentration of solute versus water causes water to move into or out of cells
• isotonic solutions-solute concentration is same on both sides of membrane, no net gain or loss of water
• hypotonic solutions-solutions that cuse cells to swell or burst due to an inake of water, lower of percentage of solute
  
• any concentration of a salt solution lower than 0.9% is hypotonic to red blood cells
• lysis refers to disrupted cells
• hemolysis is disrupted red blood cells
• turgor pressure-swelling of a plant cell in a hypotonic solution
• hypertonic solutions-solutions causing cells to shrink or shrivel due to a loss of water, solution with a higher percentage of solute
• crenation-shrunken red blood cells
• plasmolysis-shrinking of cytoplasm due to osmosis
• carrier proteins-protein that combines with and transports a molecule or ion across the plasma membrane, required for faciliated transport and active transport
• facilitated transport-passage of molecules such as glucose and amino acids across plasma membrane even though they are not lipid-soluble
• active transport-use of a plasma membrane carrier protein to move a molecule/ion from a region of lower to higher concentration, opposing equilibrium and requiring energy ; molecules or ions move through the plasma membrane, accumulating on either side
• sodium-potassium pump-carrier protein in the plasma membrane that moves sodium ions iout of and potassium into animal cells, important in nerve and muscle cells
• passage of salt (NaCl) across plasma membrane is important
• Cl^- crosses plasma membrane because it is attracted by Na⁺
• sodium ions are pumped across and chloride follows
• exocytosis-vesicles formed by the Golgi apparatus carrying a specific molecules fuse with the plasma membrane as secretion occurs
• insulin
• during cell growth, exocytosis is used as a means to enlarge the plasma membrane, whether or not secretion is taking place
• endocytosis-cells take in substances by vesicle formation, a portion of the plasma membrane invaginates to envelop the substance and then the membrane pinches off to form an intracellular vesicle
• phagocytosis-when the material taken in by endocytosis is large (e.g. food particle, another cell)
• phagocytosis is common in unicellular organisms like amoebas and in amoeboid cells like macrophages
• pinocytosis-vesicles form around a liquid or very small particles (blood cells, cells lining kidney tubules or intestinal walls, plant root cells)
• receptor-mediated endocytosis-form of pinocytosis that is specific because it involves use of a receptor protein shaped in such a way that a specific substace (ligand) can bind to it (vitamins, peptide hormones, lipoproteins)
• receptors gather at a coated pit (layer of fibrous protein on the cytoplasmic side). Once the vesicle is formed, the fibrous coat is released and the vesicle appears uncoated
• coated pits are also involved in the transfer and exchange of substances between cells
• receptor-mediated endocytosis is important, ex: in familial hypercholesterolemia, cholesterol is transported in blood by a complex of lipids and proteins called low-density lipoprotein (LDL). Individuals with familial hypercholesterolemia inherit a gene causing them to have a reduced number and/or defective receptors for LDL in their plasma membranes. Instead of cholesterol enterng cells, it accumulated in aterial blood vessel walls, leading to high blood pressure, occluded arteries, and heart attacks

5.4 Modification of Cell Surfaces

• cell wall-structure that surrounds a plant, protistan, fungal, or bacterial cell and maintains the cell's shape and rigidity
• primary cell wall contains cellulose fibrils in which microfibrils are held together by noncellulose substances
• pectins allow the wall to stretch when the cell is growing, noncellulose polysaccharides harden the wall when the cell is mature
• pectins are abundant in the middle lamella (layer of adhesive substances that holds cells together)
• some cells in woody plants have secondary walls, which have greater quantities of cellulose fibrils and layers of cellulose fibrils are laid down at right angles to one another
• lignin is a common ingredient in secondary cell walls
• plasmodesmata-numerous narrow membrane-lined channels passing through the cell wall connecting the cytoplasm of neighboring cells
• an extracellular matrix is a meshwork of insoluble proteins with carbohydrate chains (glycoproteins) that are produced and secreted by animal cells, influences development, migration, shape, and function of cells
• collagen (strength) and elastin fibers (resistance)
• fibronects and luminins are two adhesive proteins that play a dynamic role in influencing the behavior of cells, directing cell migration during development, necessary for production of milk by mammary gland cells, bind to receptors in the plasma membrane and permit communication between the extracellular matrix and cytoplasm
• proteoglycans are glycoproteins whose carbohydrate chains contain amino sugars, providing a rigid packing gel that joins various proteins in the matrix, regulating the activity of signaling sequences that bind to receptors in the plasma protein
• adhesion junctions-internal cytoplasmic plaques
• tight junctions-where plasma membrane proteins actually attach to each other, producing a zipperlike fastening
• gap junction-allows cells to communicate