Biochem Chapter 8
Fluid mosaic model. The cell (plasma) membrane is often described as a semipermeable phospholipid bilayer. This phrase alone describes both the function and structure of the cell membrane: as a semipermeable barrier, it chooses which particles can enter and leave the cell at any point in time. This selectivity is mediated not only by the various channels and carriers that poke holes in the membrane, but also by the membrane itself. Composed primarily of two layers of phospholipids, the cell membrane permits fat-soluble compounds to cross easily, while larger and water-soluble compounds must seek alternative entry. The cell membrane, the theory that underlies the structure and function of the cell membrane is referred to as the fluid mosaic model. What is this model?
Accounts for the presence of lipids, proteins and carbohydrates in a dynamic, semisolid plasma membrane that surrounds the cells.
What does the concentration gradient help determine?
Appropriate membrane transport mechanism in cells.
Cholesterol and Steroids Cholesterol is associated with a number of negative health effects and receives a lot of negative press; however, it is also a very important molecule in our cells. Cholesterol not only imparts fluidity to membranes, but it is also necessary in the synthesis of all steroids, which are derived from cholesterol. The structure of cholesterol is similar to that of phospholipids in that cholesterol contains both a hydrophilic and hydrophobic region. Membrane stability is derived from interactions with both the hydrophilic and hydrophobic regions that make up the phospholipid bilayer. While cholesterol stabilizes adjacent phospholipids, it also occupies space between them. This prevents the formation of crystal structures in the membrane, increasing fluidity. By mass, cholesterol composes about 20 percent of the cell membrane; by mole fraction, it makes up about half. This large ratio of cholesterol to phospholipid ensures that the membrane remains fluid. Cholesterol?
CHolesterol Present in large amounts Contributes to membrane fluidity and stability.
CARBOHYDRATES Carbohydrates are generally attached to protein molecules on the extracellular surface of cells. Because carbohydrates are generally hydrophilic, interactions between glycoproteins and water can form a coat around the cell, as shown in Figure 8.5. In addition, carbohydrates can act as signaling and recognition molecules. For example, blood group (ABO) antigens on red blood cells differ only in their carbohydrate sequence. Our immune systems and some pathogens take advantage of these membrane carbohydrates and membrane proteins to target particular cells. What do carbohydrates do?
Carbohydrate Can form a glycoprotein coat Function in cell recognition
CELL-CELL JUNCTIONS Cells within tissues can form a cohesive layer via intercellular junctions. These junctions provide direct pathways of communication between neighboring cells or between cells and the extracellular matrix. Cell-cell junctions are generally comprised of cell adhesion molecules (CAMs), which are proteins that allow cells to recognize each other and contribute to proper cell differentiation and development. Gap Junctions Gap junctions allow for direct cell-cell communication and are often found in small bunches together. Gap junctions (connexons) are formed by the alignment and interaction of pores composed of six molecules of connexin, as shown in Figure 8.6. They permit movement of water and some solutes directly between cells. Proteins are generally not transferred through gap junctions. Tight junctions prevent solutes from leaking into the space between cells via a paracellular route. Tight junctions are found in epithelial cells, and function as a physical link between the cells as they form a single layer of tissue. Tight junctions can limit permeability enough to create a transepithelial voltage difference based on differing concentrations of ions on either side of the epithelium. To be effective, tight junctions must form a continuous band around the cell; otherwise, fluid could leak through spaces between tight junctions. Desmosomes bind adjacent cells by anchoring to their cytoskeletons. Desmosomes are formed by interactions between transmembrane proteins associated with intermediate filaments inside adjacent cells. Desmosomes are primarily found at the interface between two layers of epithelial tissue. Hemidesmosomes have a similar function, but their main function is to attach epithelial cells to underlying (basement) membranes. From what yo have read, what is a cell to cell junction, GAP, Tight and Desmosoe ab hemidesmosomes?
Cell-Cell Junctions: regulate transport intracellularly and intracellularly. Gap Allow for rapid exchange of ions and other small molecules between adjacent cells. Tight Prevents paracellular transport Don't provide intercellular transport. Desmosomes and hemidesmosome Anchors layers of epithelial tissue together.
Endocytosis Endocytosis occurs when the cell membrane invaginates and engulfs material to bring it into the cell. The material is encased in a vesicle, which is important because cells will sometimes ingest toxic substances. Pinocytosis is the endo-cytosis of fluids and dissolved particles, whereas phagocytosis is the ingestion of large solids such as bacteria. Substrate binding to specific receptors embedded within the plasma membrane will initiate the process of endocytosis. Exocytosis Exocytosis occurs when secretory vesicles fuse with the membrane, releasing material from inside the cell to the extracellular environment. Exocytosis is important in the nervous system and intercellular signaling. For instance, exocytosis of neurotransmitters from synaptic vesicles is a crucial aspect of neuron physiology. Tell me about endocytosis, exocytosis, pinocytosis and phagocytosis?
Endocytosis and exocytosis Methods of engulfing material into cells or releasing material to the exterior of cells. Both through the cell membrane. Pinocytosis Ingestion of liquids into the cell in vesicles formed from the cell membrane. Phagocytosis Ingestion of larger solid molecules.
What do extracellular ligands do?
Extracellular Ligands Can bind to membrane receptors Function as channels or enzymes in second pathway messengers.
Phospholipids By substituting one of the fatty acid chains of triacylglycerol with a phosphate group, a polar head group joins the nonpolar tails, forming a glycerophospholipid, commonly called a phospholipid. Phospholipids spontaneously assemble into micelles (small monolayer vesicles) or liposomes (bilayered vesicles) due to hydrophobic interactions. Glycerophospholipids are used for membrane synthesis and can produce a hydrophilic surface layer on lipoproteins such as very-low-density lipoprotein (VLDL), a lipid transporter. In addition, phospholipids are the primary component of cell membranes. Phospholipids serve not only structural roles, but can also serve as second messengers in signal transduction. The phosphate group also provides an attachment point for water-soluble groups, such as choline (phosphatidylcholine, also known as lecithin) or inositol (phosphatidylinositol). Glycerophospholpids?
GLycerophospholipids Replace one fatty acid with a phosphate group. Lined to other hydrophilic groups.
LIPIDS The cell membrane is composed predominantly of lipids with some associated proteins and carbohydrates. At times, the cell membrane as a whole will be referred to as a phospholipid bilayer, as it is the primary component of this barrier around the cell. Within the cell membrane, there are a large number of phospholipids with very few free fatty acids. In addition, steroid molecules and cholesterol, which lend fluidity to the membrane, and waxes, which provide membrane stability, help to maintain the structural integrity of the cell. What are lipids?
Lipids They are primary membrane components. Both by mass and mole fraction There are triglycerides, glycerphospholipids, cholesterol and waxes.
MITOCHONDRIAL MEMBRANES Mitochondria are referred to as the "powerhouse" of the cell because of their ability to produce ATP by oxidative respiration. Mitochondria contain two membranes: the inner and outer mitochondrial membranes. Outer Mitochondrial Membrane The outer mitochondrial membrane is highly permeable due to many large pores that allow for the passage of ions and small proteins. The outer membrane completely surrounds the inner mitochondrial membrane, with the presence of a small intermembrane space in between the two layers. Inner Mitochondrial Membrane The inner mitochondrial membrane has a much more restricted permeability compared to the outer mitochondrial membrane. Structurally, the inner mitochondrial membrane contains numerous infoldings, known as cristae, which increase the available surface area for the integral proteins associated with the membrane. These proteins, discussed in Chapter 10 of MCAT Biochemistry Review, are involved in the electron transport chain and ATP synthesis. The inner membrane also encloses the mitochondrial matrix, where the citric acid cycle produces high-energy electron carriers used in the electron transport chain. The inner mitochondrial membrane contains a very high level of cardiolipin and does not contain cholesterol. Fro you have read, tell me about the outer and inner mitochondrial membrane?
Mitochondria membrane differs from the cell membrane Outer mitochondrial membrane Highly permeable to metabolic molecules and small proteins. Inner mitochondrial membrane Surround mitochondrial matrix. This is where the citric acid cycle produces electrons used in the electron transport chain Where many other enzymes important in cellular respiration are located. Does not contain cholesterol.
What is osmotic pressure?
Osmotic pressure Is a colligative property. Is the pressure applied to a pure solvent to prevent osmosis. Used to express the concentration of the solution. It is often better conceptualized as SUCKING pressure in which a solution is drawing water in, proportional to its concentration.
PROTEINS The fluid mosaic model also accounts for the presence of three types of membrane proteins. Transmembrane proteins pass completely through the lipid bilayer. Embedded proteins, on the other hand, are associated with only the interior (cytoplasmic) or exterior (extracellular) surface of the cell membrane. Together, transmembrane and embedded proteins are considered integral proteins because of their association with the interior of the plasma membrane, which is usually assisted by one or more membrane-associated domains that are partially hydrophobic. Membrane-associated (peripheral) proteins may be bound through electrostatic interactions with the lipid bilayer, especially at lipid rafts, or to other transmembrane or embedded proteins, like the G proteins found in G protein-coupled receptors. Transporters, channels, and receptors are generally transmembrane proteins. Where are protein located at? They act as what? What are tranmembrane proteins, Embeded proteins and Membrane associated proteins?
Proteins are Located in the cell membrane. Act as transporters, cell adhesion molecules and enzymes. Transmembrane proteins Can have one or more hydrophobic domains Functions as receptors or channels. EMbedded proteins Most likely part of a catalytic complex Involved in cellular communication Membrane associated proteins Act as recognition molecules or enzymes
Sodium-Potassium Pump There is a steady-state resting relationship between ion diffusion and the Na /K ATPase. One of the main functions of the Na+/K+ ATPase is to maintain a low concentration of sodium ions and high concentration of potassium ions intracellularly by pumping three sodium ions out for every two potassium ions pumped in. This movement of ions removes one positive charge from the intracellular space of the cell, which maintains the negative resting potential of the cell. As mentioned before, the cell membrane also contains leak channels that allow ions, such as Na+ and K+, to passively diffuse into or out of the cell down their concentration gradients. Cell membranes are more permeable to K+ ions than Na+ ions at rest because there are more K+ leak channels than Na+ leak channels. The combination of Na+/K+ ATPase activity and leak channels together maintain a stable resting membrane potential. THERES NO ANSWER TO THIS JUST READ AND CONTINUE.......
SPECIALIZED MEMBRANES Composition of cell membranes is fairly consistent. There are some cells that contain specialized membranes. MEMBRANE POtentiaL Maintained by the sodium-potassium pump and leak channels. Electrical potential Created by one ion can be calculated using the nernst equation. Resting potential at physiological temp. Can be calculated using the GOLDMAN-HODGKIN-KATZ-VOLTAGE-EQUATION Derived from the Nernst equation.
ACTIVE Requires energy in the form of ATP. Existing favorable ion gradient. These transports can be primary or secondary depending on the energy source. Secondary active transports Can be classified as symport or antiport Tell me about these 2?
SYM: both particles flow in the Same direction across the membrane. ANTI: particles flow the opposite directions.
PASSIVE Does Not require energy. Because the molecule is moving down its concentration gradient or from an area with higher concentrations to an area with lower concentrations. 3 types of passive transport. Simple, osmosis and facilitated. Tell me about these 3?
Simple diffusion Does Not require a transporter. Small, nonpolar molecules passively move from an area of high concentration to an area of low concentration until EQUILIBRIUM is achieved. Osmosis Describes the diffusion of water across a selectively permeable membrane. Facilitated diffusion Uses transport proteins. To move impermeable solutes across the cell membrane.
What is the plasma membrane?
The cell membrane is a phospholipid bilayer that regulates movement of solutes into and out of the cell. The plasma membrane Contains proteins embedded within the phospholipid bilayer.
GENERAL MEMBRANE STRUCTURE AND FUNCTION The phospholipid bilayer also includes proteins and distinct signaling areas within lipid rafts. Carbohydrates associated with membrane-bound proteins create a glycoprotein coat. The cell wall of plants, bacteria, and fungi contain higher levels of carbohydrates. The main function of the cell membrane is to protect the interior of the cell from the external environment. Cellular membranes selectively regulate traffic into and out of the cell and are involved in both intracellular and intercellular communication and transport. Cell membranes also contain proteins embedded within the lipid bilayer that act as cellular receptors during signal transduction. These proteins play an important role in regulating and maintaining overall cellular activity. MEMBRANE DYNAMICS The cell membrane functions as a stable semisolid barrier between the cytoplasm and the environment, but it is in a constant state of flux on the molecular level. Phospholipids move rapidly in the plane of the membrane through simple diffusion. This can be seen when fusing two membranes that have been tagged with different labels; the tags will migrate with their associated lipids until both types are equally intermixed. Lipid rafts are collections of similar lipids with or without associated proteins that serve as attachment points for other biomolecules; these rafts often serve roles in signaling. Both lipid rafts and proteins also travel within the plane of the membrane, but more slowly. Lipids can also move between the membrane layers, but this is energetically unfavorable because the polar head group of the phospholipid must be forced through the nonpolar tail region in the interior of the membrane. Specialized enzymes called flippases assist in the transition or "flip" between layers. Dynamic changes in the concentrations of various membrane proteins are mediated by gene regulation, endocytotic activity, and protein insertion. Many cells, particularly those involved in biosignaling processes, can up- or downregulate the number of specific cellular receptors on their surface in order to meet cellular requirements. WHat is the dynamic movement of the membrane? What are flippases? What are the dynamic movements of proteins and carbs within the membrane?
The membrane is not static. Lipids move freely in the plane of the membrane and can assemble into lipid rafts. Flippases are specific membrane proteins that maintain the bidirectional transport of lipids between the layers of the phospholipid bilayer in cells. Proteins and carbohydrates may also move within the membrane, but are slowed by their relatively large size.
Fatty Acids and Triacylglycerols Fatty acids are carboxylic acids that contain a hydrocarbon chain and terminal carboxyl group. Triacylglycerols, also referred to as triglycerides, are storage lipids involved in human metabolic processes. They contain three fatty acid chains esterified to a glycerol molecule. Fatty acid chains can be saturated or unsaturated. Unsaturated fatty acids are regarded as "healthier" fats because they tend to have one or more double bonds and exist in liquid form at room temperature; in the plasma membrane, these characteristics impart fluidity to the membrane. Humans can only synthesize a few of the unsaturated fatty acids; the rest come from essential fatty acids in the diet that are transported as triglycerides from the intestine inside chylomicrons. Two important essential fatty acids for humans are α-linolenic acid and linoleic acid. Saturated fatty acids are the main components of animal fats and tend to exist as solids at room temperature. Saturated fats are found in processed foods and are considered less healthy. When incorporated into phospholipid membranes, saturated fatty acids decrease the overall membrane fluidity. What are the function of triglycerides?
Triglycerides and free fatty acids Act as phospholipid precursors Found in low levels in the membrane
Waxes Waxes are a class of lipids that are extremely hydrophobic and are rarely found in the cell membranes of animals, but are sometimes found in the cell membranes of plants. A wax is composed of a long-chain fatty acid and a long-chain alcohol, which contribute to the high melting point of these substances. When present within the cell membrane, waxes can provide both stability and rigidity within the nonpolar tail region only. Most waxes serve an extracellular function in protection or waterproofing. Waxes?
Waxes Present in very small amounts if at all Most prevalent in plants Function n waterproofing and defense.
Sphingolipids Sphingolipids are also important constituents of cell membranes. Although sphingolipids do not contain glycerol, they are similar in structure to glycerophospholipids, in that they contain a hydrophilic region and two fatty acid-derived hydrophobic tails. The various classes of sphingolipids differ primarily in the identity of their hydrophilic regions. Classes of sphingolipids and their hydrophilic groups include ceramide, sphingomyelins, cerebrosides, and gangliosides. What are sphingolipids?
phingolipids are commonly believed to protect the cell surface against harmful environmental factors by forming a mechanically stable and chemically resistant outer leaflet of the plasma membrane lipid bilayer.
