Biology (1) 1501143 Past Paper 20241 PDF

Biology (1) 1501143 Dr Maysoun Qutob Applied Science Private University, 20241 1 Applied Science Private University 1  Membrane Structure and Function 2 Applied Science Private University 2  Plasma membrane...

Biology (1) 1501143 Dr Maysoun Qutob Applied Science Private University, 20241 1 Applied Science Private University 1  Membrane Structure and Function 2 Applied Science Private University 2  Plasma membrane Consists of a phospholipid bilayer. Exhibits selective permeability : it allows some substances to cross it more easily than others.  Cellular membranes are fluid mosaics of lipids and proteins Membranes consists of: the staple ingredients of Lipids (phospholipids) membranes, proteins carbohydrates. Phospholipids are the most abundant lipids in most membranes. A phospholipid is an amphipathic molecule, meaning it has both a hydrophilic (“water-loving”) region and a hydrophobic (“water- fearing”) region A phospholipid bilayer can exist as a stable boundary between two aqueous compartments because the molecular arrangement shelters the hydrophobic tails of the phospholipids from water while exposing the hydrophilic heads to water (Figure 7.2).  Most membrane proteins are amphipathic. Such proteins can reside in the phospholipid bilayer with their hydrophilic regions protruding. Figure 7.3 shows the fluid mosaic model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids. The Fluidity of Membranes Membranes are not static sheets of molecules locked rigidly in place. A membrane is held together mainly by hydrophobic interactions. Most of the lipids and some proteins can shift about sideways. Sideway movement phospholipids: rapid proteins: slow (figure 7.4) (proteins are much larger than lipids) Very rarely a lipid may flip-flop across the membrane, switching from one phospholipid layer to the other.  Membrane proteins move in a highly immobile directed manner driven along cytoskeletal held by their attachment fibers in the cell by motor to the cytoskeleton or to proteins the extracellular matrix  The temperature at which a membrane solidifies depends on the types of lipids it is made of. Membrane rich in phospholipids with : Unsaturated hydrocarbon tails: saturated hydrocarbon tails: remain fluid at low less fluid at low temperature temperature more viscous membrane Because of kinks in the tails where double bonds are located (cannot pack together)  Figure 7.5 Factors that affect membrane fluidity.  Cholesterol is wedged between phospholipid molecules in the plasma membranes of animal cells, has different effects on membrane fluidity at different temperatures (Figure 7.5b). Cholesterol can be thought of as a “fluidity buffer” for the membrane, resisting changes in membrane fluidity that can be caused by changes in temperature. 1. At relatively high temperatures; at 37°C (body temperature) cholesterol makes the membrane less fluid by restraining phospholipid movement. 2. it lowers the temperature required for the membrane to solidify by hindering the close packing of phospholipids.  The fluidity of a membrane affects both: 1. the permeability of membrane 2. the ability of membrane proteins to move to where their function is needed. Membranes must be fluid to work properly: When a membrane solidifies, its permeability changes, and enzymatic proteins in the membrane may become inactive if their activity requires movement within the membrane. Membranes that are too fluid cannot support protein function either. Extreme environments pose a challenge for life  Extreme cold Extreme hot Fish: have membranes with a Some bacteria and archaea: high proportion of unsaturated their membranes include hydrocarbon tails, enabling their unusual lipids that may prevent membranes to remain fluid. excessive fluidity at such high Plants: such as winter wheat, the temperatures (>90°C). percentage of unsaturated phospholipids increases in autumn, an adjustment that keeps the membranes from solidifying during winter. Certain bacteria and archaea can also change the proportion of unsaturated phospholipids in their cell membranes, depending on the temperature at which they are growing. Membrane Proteins and Their Functions A membrane is a collage of different proteins, often clustered together in groups, embedded in the fluid matrix of the lipid bilayer (i.e., RBCs >50 kinds of proteins in their plasma membrane). Different types of cells different sets of membrane proteins. There are two major populations of membrane proteins 1. Integral proteins 2. Peripheral proteins  Integral proteins (i.e., integrins) Peripheral proteins Penetrate the hydrophobic interior of the Are not embedded in the lipid bilayer at all. lipid bilayer. They are loosely bound to the surface of The majority are transmembrane the membrane, often to exposed parts of proteins, which span the membrane; integral proteins. other integral proteins extend only partway into the hydrophobic interior. The hydrophilic parts of the molecule are exposed to the aqueous solutions on either side of the membrane. Some proteins also have one or more hydrophilic channels that allow passage through the membrane of hydrophilic substances.  On the cytoplasmic side of the plasma membrane, some membrane proteins are held in place by attachment to the cytoskeleton. On the extracellular side, certain membrane proteins may attach to materials outside the cell (as with integrins) Membrane proteins carry out several different functions (Figure 7.7). The Role of Membrane Carbohydrates in Cell-Cell Recognition Cell-cell recognition, a cell’s ability to distinguish one type of neighboring cell from another, is crucial to the functioning of an organism. It is important: 1. In the sorting of cells into tissues and organs in an animal embryo. 2. It is the basis for the rejection of foreign cells by the immune system. 3. Function as markers that distinguish one cell from another (blood types designated A, B, AB, and O reflect variation in the carbohydrate part of glycoproteins on the surface of red blood cells. Cells recognize other cells by binding to molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane (forming glycolipids and glycoproteins). Membrane structure results in selective permeability There is a steady traffic of small molecules and ions moves across the plasma membrane in both directions. Sugars, amino acids, and other nutrients enter the cell, and metabolic waste products leave it. The cell takes in O2 for use in cellular respiration and expels CO2. The cell regulates its concentrations of inorganic ions such as Na+, K+, Ca2+, and Cl-, by shuttling them one way or the other across the plasma membrane. The Permeability of the Lipid Bilayer Nonpolar Ions and polar Hydrophobic (i.e., Hydrophilic hydrocarbons, CO2, O2) Cannot cross the membrane (i.e., Dissolve in bilayer membrane glucose and other sugars, H2O, and cross it easily charged atoms or molecules) No aid of membrane proteins Need for transport proteins (proteins built into membrane which play key roles in regulating transport)  Transport Proteins Channel Proteins Carrier Proteins Function by having a hydrophilic Hold onto their passengers and channel that certain molecules change shape in a way that shuttles or ions use as a tunnel through them across the membrane. the membrane. For example, glucose transporter a For example, Aquaporins specific carrier protein in the facilitates the passage of water plasma membrane of red blood molecules through the cells transports glucose across the membrane and increasing their membrane. is so selective that it passage rate (kidney cells) even rejects fructose, a structural Ion channels (gated channels: isomer of glucose. open or close in response to a stimulus (in nervous system)). The selective permeability of a membrane depends on both: 1. The discriminating barrier of the lipid bilayer 2. The specific transport proteins built into the membrane. Passive transport is diffusion of a substance across a membrane with no energy investment Molecules have a type of energy called thermal energy, due to their constant motion. One result of this motion is diffusion, the movement of particles of any substance so that they spread out into the available space. A substance will diffuse from where it is more concentrated to where it is less concentrated any substance will diffuse down its concentration gradient. ❖Diffusion is a spontaneous process, needing no input of energy. Figure 7.10.  When a substance is more concentrated on one side of a membrane than on the other, there is a tendency for the substance to diffuse across the membrane down its concentration gradient. One important example is the uptake of oxygen by a cell performing cellular respiration. The diffusion of a substance across a biological membrane is called passive transport because the cell does not have to expend energy to make it happen. The concentration gradient itself represents potential energy and drives diffusion. Effects of Osmosis on Water Balance The solution with a higher solute concentration has a lower free water concentration. Water diffuses across the membrane from the region of higher free water concentration (lower solute concentration) to that of lower free water concentration (higher solute concentration) until the solute concentrations on both sides of the membrane are more nearly equal. The diffusion of free water across a selectively permeable membrane is called osmosis. Water Balance of Cells Without Cell Walls Tonicity: The ability of a surrounding solution to cause a cell to gain or lose water. Depends in part on its concentration of solutes that cannot cross the membrane (nonpenetrating solutes) relative to that inside the cell. If there is a higher concentration of nonpenetrating solutes in the surrounding solution, water will tend to leave the cell, and vice versa.  If a cell without a cell wall (an animal cell), is immersed in an environment that is isotonic to the cell (iso means “same”), there will be no net movement of water across the plasma membrane (net movement = 0). Water diffuses across the membrane, but at the same rate in both directions. In an isotonic environment, the volume of an animal cell is stable (Figure 7.12a). In a solution that is hypertonic to the cell (hyper means “more,” referring to nonpenetrating solutes). The cell will lose water, shrivel, and probably die. This is why an increase in the salinity (saltiness) of a lake can kill the animals there. in a solution that is hypotonic to the cell (hypo means “less”), water will enter the cell faster than it leaves, and the cell will swell and lyse (burst).  Figure 7.13 The water balance of living cells. How living cells react to changes in the solute concentration of their environment depends on whether or not they have cell walls. (a) Animal cells, such as this red blood cell, do not have cell walls. (b) Plant cells do have cell walls. (Arrows indicate net water movement after the cells were first placed in these solutions.) Water Balance of Cells with Cell Walls The relatively inelastic cell wall will expand only so much before it exerts a back pressure on the cell, called turgor pressure, that opposes further water uptake. At this point, the cell is turgid (very firm), the healthy state for most plant cells. If a plant’s cells and surroundings are isotonic, there is no net tendency for water to enter and the cells become flaccid (limp); the plant wilts. A cell wall is of no advantage if the cell is immersed in a hypertonic environment a plant cell will lose water to its surroundings and shrink. As the plant cell shrivels, its plasma membrane pulls away from the cell wall at multiple places. This phenomenon, called plasmolysis, causes the plant to wilt and can lead to plant death.  Facilitated Diffusion: Passive Transport Aided by Proteins Many polar molecules and ions impeded by the lipid bilayer of the membrane diffuse passively with the help of transport proteins that span the membrane. This phenomenon is called facilitated diffusion. Most transport proteins are very specific: They transport some substances but not others. Channel proteins Aquaporins (kidney cells) Carrier proteins Ion channels (gated channels: open or close in Glucose transporter response to a stimulus (in nervous system)).