Biology Past Paper - Cell Movement

Unit 3 - Movement into & out of the cells 3.1 Diffusion in biology Diffusion What is Diffusion in Biology? Diffusion is the movement of molecules from a region of its higher concentration to a region of its lower concentration Molecules move down a concentrat...

Unit 3 - Movement into & out of the cells 3.1 Diffusion in biology Diffusion What is Diffusion in Biology? Diffusion is the movement of molecules from a region of its higher concentration to a region of its lower concentration Molecules move down a concentration gradient, as a result of their random movement For living cells, the principle of the movement down a concentration gradient is the same, but the cell is surrounded by a cell membrane which can restrict the free movement of the molecules The cell membrane is a partially permeable membrane - this means it allows some molecules to cross easily, but others with difficulty or not at all The simplest sort of selection is based on the size of the molecules Diffusion helps living organisms to: obtain many of their requirements get rid of many of their waste products carry out gas exchange for respiration Examples of diffusion in living organisms You will need to learn examples of substances that organisms obtain by diffusion Don’t forget that plants require oxygen for respiration at all times, as well as carbon dioxide for photosynthesis when conditions for photosynthesis are right (e.g. enough light and a suitable temperature) Site Molecules moving From To Small intestine Digested food Lumen of small Blood/ lymph in products - glucose, intestine villi found covering amino acids, fatty small intestine acids and glycerol walls etc. Leaf Oxygen Air spaces between Mitochondria in all mesophyll cells cells  Leaf Carbon dioxide Air spaces between Chloroplasts in mesophyll cells mesophyll cells Leaf Water vapour Stomatal pores Air outside stomata Lungs Oxygen Alveolar air space Blood in capillaries around alveoli Lungs Carbon dioxide Blood in capillaries Alveolar air space around alveoli Where does the energy for diffusion come from? All particles move randomly at all times This is known as Brownian motion The energy for diffusion comes from the kinetic energy of this random movement of molecules and ions 3.2 Factors that influence diffusion Surface area to volume ratio The bigger a cell or structure is, the smaller its surface area to volume ratio is, slowing down the rate at which substances can move across its surface Many cells which are adapted for diffusion have increased surface area in some way - eg root hair cells in plants (which absorb water and mineral ions) and cells lining the ileum in animals (which absorb the products of digestion) Distance The smaller the distance molecules have to travel the faster transport will occur This is why blood capillaries and alveoli have walls which are only one cell thick, ensure the rate of diffusion across them is as fast as possible Temperature The higher the temperature, the faster molecules move as they have more energy This results in more collisions against the cell membrane and therefore a faster rate of movement across them Concentration Gradient The greater the difference in concentration on either side of the membrane, the faster movement across it will occur This is because on the side with the higher concentration, more random collisions against the membrane will occur 3.3 Water Water as a Solvent Water is important for all living organisms as many substances are able to dissolve in it (it is a solvent) This makes it incredibly useful and essential for all life on Earth Water is important as a solvent in the following situations within organisms: Dissolved substances can be easily transported around organisms - eg xylem and phloem of plants and dissolved food molecules in the blood Digested food molecules are in the alimentary canal but need to be moved to cells all over the body - without water as a solvent this would not be able to happen Toxic substances such as urea and substances in excess of requirements such as salts can dissolve in water which makes them easy to remove from the body in urine Water is also an important part of the cytoplasm and plays a role in ensuring metabolic reactions can happen as necessary in cells 3.4 Osmosis Osmosis All cells are surrounded by a cell membrane which is partially permeable Water can move in and out of cells by osmosis Osmosis is the diffusion of water molecules from a dilute solution (high concentration of  water) to a more concentrated solution (low concentration of water) across a partially permeable membrane In doing this, water is moving down its concentration gradient The cell membrane is partially permeable which means it allows small molecules (like water) through but not larger molecules (like solute molecules) Osmosis (extended) Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane It can get a little confusing to talk about the 'concentration of water' when we also talk about solutions being ‘concentrated’ (having a lot of solute in them), so instead we can say that a dilute solution has a high water potential (the right-hand side of the diagram below) and a concentrated solution has a low water potential (the left-hand side of the diagram below) 3.5 Osmosis experiments Osmosis Experiments Immersing plant cells in solutions of different concentrations The most common osmosis practical involves cutting cylinders of root vegetables such as potato or radish and placing them into distilled water and sucrose solutions of increasing concentration The cylinders are weighed before placing into the solutions They are left in the solutions for 20 - 30 minutes and then removed, dried to remove excess liquid and reweighed If the plant tissue gains mass: Water must have moved into the plant tissue from the solution surrounding it by osmosis  The solution surrounding the tissue is more dilute than the plant tissue (which is more concentrated) If plant tissue loses mass: Water must have moved out of the plant tissue into the solution surrounding it by osmosis The solution surrounding the tissue is more concentrated than the plant tissue (which is more dilute) If there is no overall change in mass: There has been no net movement of water as the concentration in both the plant tissue and the solution surrounding it must be equal Remember that water will still be moving into and out of the plant tissue, but there wouldn’t be any net movement in this case Investigating osmosis using dialysis tubing Dialysis tubing (sometimes referred to as visking tubing) is a non-living partially permeable membrane made from cellulose Pores in this membrane are small enough to prevent the passage of large molecules (such as sucrose) but allow smaller molecules (such as glucose and water) to pass through by diffusion and osmosis This can be demonstrated by: Filling a section of dialysis tubing with concentrated sucrose solution Suspending the tubing in a boiling tube of water for a set period of time Noting whether the water level outside the tubing decreases as water moves into the tubing via osmosis Water moves from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane 3.6 Osmosis in animal & plants Osmosis in Plant Tissues When water moves into a plant cell, the vacuole gets bigger, pushing the cell membrane against the cell wall Water entering the cell by osmosis makes the cell rigid and firm This is important for plants as the effect of all the cells in a plant being firm is to provide support and strength for the plant - making the plant stand upright with its leaves held out to catch sunlight The pressure created by the cell wall stops too much water entering and prevents the cell from bursting  If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant wilts Osmosis in Animals & Plants (extended) Plant cells in solutions of different concentrations When plant cells are placed in a solution that has a higher water potential (dilute solution) than inside the cells (e.g. distilled water) then water moves into the plant cells via osmosis These water molecules push the cell membrane against the cell wall, increasing the turgor pressure in the cells which makes them turgid When plant cells are placed in a concentrated solution (with a lower water potential than inside the cells) water molecules will move out of the plant cells by osmosis, making them flaccid If plant cells become flaccid it can negatively affect the plant's ability to support itself If looked at underneath the microscope, the plant cells might be plasmolysed, meaning the cell membrane has pulled away from the cell wall Animal cells in solutions of different concentrations Animal cells also lose and gain water as a result of osmosis As animal cells do not have a supporting cell wall, the results on the cell are more severe If an animal cell is placed into a strong sugar solution (with a lower water potential than the cell), it will lose water by osmosis and become crenated (shrivelled up) If an animal cell is placed into distilled water (with a higher water potential than the cell), it will gain water by osmosis and, as it has no cell wall to create turgor pressure, will continue to do so until the cell membrane is stretched too far and it bursts 3.7 Active transport Active Transport Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration Importance of Active Transport: Extended Energy is needed because particles are being moved against a concentration gradient, in the opposite direction from which they would naturally move (by diffusion) Active transport is vital process for the movement of molecules or ions across membranes Including: uptake of glucose by epithelial cells in the villi of the small intestine and by kidney tubules in the nephron uptake of ions from soil water by root hair cells in plants 3.8 Proteins Protein Carriers: Extended Active transport works by using carrier proteins embedded in the cell membrane to pick up specific molecules and take them through the cell membrane against their concentration gradient: 1. Substance combines with carrier protein molecule in the cell membrane 2. Carrier transports substances across membrane using energy from respiration to give them the kinetic energy needed to change shape and move the substance through the cell membrane 3. Substance released into cell