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Topics included in this booklet are; Plasma membranes Diffusion Osmosis Active Transport Plasma Membranes All cells are surrounded by a membrane (called the plasma membrane), a barrier which is responsible for controlling the entry and exit of substances to and from the cell. The plasma membrane has two parts; Bimolecular leaflet of phospholipids which make up the bulk of the membrane. Proteins which are found at intervals along the membrane. These two parts are found in the ……………………………………………. Model of membrane structure. 1 There are two types of proteins in the plasma membrane; EXTRINSIC PROTEINS – these proteins are found on the outer and inner surfaces of the membrane but do not penetrate the whole membrane. INTRINSIC PROTEINS – these proteins penetrate the whole plasma membrane. The functions of the membrane proteins are; 1. To provide channels by which water soluble molecules may pass across the membrane. The bulk of the membrane is made up of phospholipids. Lipid soluble molecules can pass across the membrane freely as they can dissolve in the membrane. Molecules that are not lipid soluble, particularly charged molecules must pass across the membrane via pores crated by intrinsic protein molecules in order to avoid the hydrophobic tails of the phospholipid molecules. These molecules use carrier proteins to enter the cell. Each molecule has its own specific carrier determined by the shape of the protein molecules. Because the movement of the molecule is aided by the protein this is called FACILITATED DIFFUSION. NB Small non-lipid soluble molecules can pass between the phospholipid molecules. 2. To enable cells to recognise molecules on the outside of the cell. Some molecules are very large and cannot enter cells directly e.g. hormones, which are made of proteins. The metabolism of the cell may need to be altered due to the presence of one of these molecules on the cell surface. Proteins are able to recognise certain molecules outside the cell and signal their presence to the inside of the cell. This process is aided by the presence of the glycocalyx. 3. To act as pumps Some of the proteins pump molecules into the cell against the concentration gradient. This is ACTIVE TRANSPORT. The plasma membrane is called a PARTIALLY PERMEABLE MEMBRANE because it is permeable to some molecules and impermeable to others. 2 There are three ways in which molecules may pass across the membrane; DIFFUSION OSMOSIS ACTIVE TRANSPORT Why is the plasma membrane structure described as the fluid mosaic model? What property of proteins allows them to recognise molecules? What is the glycocalyx? Which of the processes of entry into the cell are Active? Passive? Examples of molecules which may enter the cell by Diffusion Osmosis Active transport What are the functions of the plasma membrane? Diffusion All particles in liquids and gases are in constant random motion. This motion results in a net movement of particles from a region of high concentration to a region of lower concentration. Diffusion is the passive movement of substances along a concentration gradient. Diffusion continues until EQULIBRIUM is reached. At this point the particles are spread evenly in the available space. Random movement of particles still occurs but there is no net movement in any particular direction. 3 Particles can also diffuse across a membrane as long as the membrane has pores that are larger than the particles. The rate of diffusion can be affected by a number of factors; FACTOR EFFECT ON DIFFUSION Temperature Physical movement Concentration Gradient Distance Pressure Size of Molecules These factors affect diffusion across membranes too. There are also additional factors that can affect the rate of diffusion across a membrane; 4 FACTOR EFFECT ON DIFFUSION ACROSS A MEMBRANE Thickness of membrane Diameter and number of pores (permeability) Surface area of membrane FICKS LAW is a formula by which we can work out the rate of diffusion; Rate of diffusion = surface area x concentration difference Thickness of membrane Facilitated Diffusion Large lipid insoluble molecules such as glucose and amino acid cannot enter the cell by diffusion across the plasma membrane. They must be aided across the membrane by intrinsic proteins called carrier proteins. These proteins provide channels through which these molecules can pass across the membrane in a process called FACILITATED DIFFUSION. Specific carrier protein molecules transport each substance across the membrane. The proteins recognise the molecules they interact with by their shape. Facilitated diffusion only occurs down a concentration gradient and is a passive process. 5 Gated channel proteins – a special case of facilitated diffusion Mineral ions also move across membranes by facilitated diffusion but they travel along an electrochemical gradient. This is a gradient which occurs between two regions with different charges. If there is a high concentration of Na + ions in one area that area becomes positively charged. A high concentration of Cl- ions will make an area negatively charged. If these regions are either side of a membrane then the Na+ will move from the positive to the negative region and Cl- form the negative to the positive region, down electrochemical gradients. Proteins in the membrane called CHANNEL PROTEINS allow ions to pass through the membrane. Each channel protein has a particular shape and electrical charge and will only allow one type of ion through. Channel proteins can be open or closed and so are called GATED CHANNELS. Osmosis Not all substances can pass through the cell surface membrane. Water molecules do but larger solute molecules do not. This means that the membrane is partially permeable. The movement of water molecules through a partially permeable membrane from a region of high concentration of water molecules to a region of lower concentration of water molecules is called osmosis. 6 Key terms in describing osmosis are Water potential Solute potential Pressure potential Water Potential Water molecules, like all other molecules are mobile (they move around constantly). In pure water or in solutions containing very few solute molecules the water can move freely the water molecules have ………………………………………………………. In a solution with many solute molecules, the movement of water molecules is restricted because of the interactions (bonding) between the solute and water molecules the water molecules have ………………………………………………… Therefore any solution in which the water molecules have high average kinetic energy will have a greater tendency to lose water than those with low free kinetic energy because the water molecules are able to move around more. When describing free kinetic energy biologists use the term …………………………..(Symbol ) Define osmosis using the term water potential. Measuring water potential The water potential of pure water (symbol w) (at atmospheric pressure) is 0 KPa (……………….………………) Any addition of solute molecules decreases the water potential of the solution therefore makes the water potential negative. Match each of the solutions with the correct measurements of water potential Description Water potential Pure water = 50KPa 0.5M glucose solution = 0KPa 1M glucose solution = 200KPa 2M glucose solution = 100KPa 7 Look at the following diagram Label the water and solute molecules Label one of the solutions pure water and the other glucose solution Which solution has the highest water potential? In which direction will osmosis occur? Look at the following diagram X Y -405 KPa Z -395KPa -380KPa In which direction(s) will osmosis occur between these three cells? Between which pair of cells will the net rate of water movement be greatest? Explain your answer. Solute potential The effect that solute molecules have on water potential is referred to a solute potential. Any addition of solute molecules decreases the water potential of a solution so solute potential (symbol s) is always a negative value. The water potential of a solution can be calculated as follows; Water potential of solution = water potential of pure water + solute potential 8 = w + s Remembering that w = 0 what is the s of cells x,y & z above? Cell X = Cell Y = Cell Z = Pressure potential A w = 0 B w = 0 Both of these cells contain pure water. will osmosis occur between these two cells? will there be any movement of water between these two cells? What happens if one of the cells is squeezed? A =0 B =0 What happens to the pressure inside cell b? What happens to the free kinetic energy of the water molecules inside cell b? What happens to the water potential inside cell b? Will osmosis now occur? In which direction? The contribution of external pressure on water potential is called pressure potential (symbol p). At atmospheric pressure p = 0. p is 9 always a positive value because any increase in pressure increases water potential. As p also contributes to water potential the equation for the water potential of a solution is now as follows; Water potential = water potential + solute potential + pressure potential of pure water of solution Write this equation using the appropriate symbols. Since the water potential of pure water is always 0 the equation can be simplified as Note that this equation only applies to the calculation of in plant cells as this the only instance in which pressure potential can develop due to the presence of the cell wall. Calcuation of - worked example A s = -600 KPa p = 400 KPa Cell A Cell B = s = p = = -600 + 400 = = -200 KPa = Osmosis will occur from cell A to cell B. 10 B s = -800KPa p = 400 KPa s + p -800 + 400 -400 KPa Calculate and the direction of osmosis for the following cells. A A A s = -600 KPa p = 300 KPa B s = -700KPa p = 200 KPa s = -400 KPa p = 100 KPa B s = -600KPa p = 600 KPa s = -700 KPa p = 500 KPa B s = -700KPa p = 500 KPa Water potential and animal cells Isotonic solutions have the same concentration of water molecules as animal cells, so the rate at which water molecules diffuse into and out of the cell is the same. There is no NET movement of water molecules Hypertonic solutions have a lower concentration of water molecules compared to the inside of a cell and so there is a net movement of water molecules out of the cell. The cell shrinks this is called crenation. Hypotonic solutions have a higher concentration of water molecules compared to the inside of the cell so there is net movement of water molecules into the cell. This causes the cell to swell. A cell which is full of water but which has not burst is said to be turgid. Lysis is the term given to the bursting of the cell. 11 Plant cells and osmosis The vacuole of a plant cell usually has a high concentration of ions and so has a very negative water potential. This low water potential causes water to enter the cell by osmosis. The cytoplasm and vacuole both swell and the cell develops a high pressure potential. The pressure exerted by the vacuole on the cell wall is called TURGOR PRESSURE. This pressure keeps the cell TURGID (rigid) and is the main method of support in young plants. If the cells lose water the leaves and stem wilt. If a fully turgid plant cell is placed into a hypertonic solution (one that has a lower concentration of water molecules compared to the inside of the cell) then the following changes occur; The cell starts to lose water by osmosis The pressure potential of the cell drops to zero The cytoplasm shrinks and the plasma membrane starts to peel away from the cell wall this is the point of INCIPIENT PLASMOLYSIS. Further loss of water results in the plasma membrane pulling away from the cell wall completely, this is FULL PLASMOLYSIS. Active Transport Active transport is the movement of molecules across the plasma membrane against a concentration gradient. This is an active process and uses intrinsic protein molecules as molecular pumps. It requires energy in the form of ATP produced by respiration therefore cells which carry out lots of active transport have many mitochondria. 12 Active transport is dependent on the energy produced by respiration. Any factor that increases the rate of respiration will increase the rate of active transport. Poisons such as cyanide which act as respiratory inhibitors will stop active transport. 13