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Passive Transport Starter • Name a cellular organelle that possesses a membrane and describe the purpose of the membrane. Golgi, mitochondria, chloroplast, nucleus, lysosomes Compartmentalise enzyme reactions/control substances in and out • Describe the purpose of cholesterol in the plasma membrane Prevents phospholipid tails from packing close together and preserves fluidity of membrane • Suggest why organisms living in polar regions have a high proportion of cholesterol in the membrane To keep membrane fluid and functioning correctly • List three substances that need to be transported into animal cells in order to survive. Oxygen, food, minerals, water • List two substances that need to be transported out of animal cells in order to survive Carbon dioxide, nitrogenous wastes Passive Transport Learning Objectives • Explain what is meant by passive transport • Explain what is meant by facilitated diffusion • Identify the role of membrane proteins in transport Success Criteria • Recall the definitions of diffusion, facilitated diffusion and osmosis (Grade D-E) • Compare diffusion and facilitated diffusion (Grade C) • Construct models/diagrams to show how the processes differ (Grade A-B) Transport across the cell membrane All cells are surrounded by a partially-permeable membrane that controls what substances can enter and exit the cell. A cell needs to be able to import the substances it needs to survive, and to export waste materials and substances that are needed outside the cell. There are several methods by which substances (molecules and ions) can cross the cell membrane: diffusion osmosis active transport. On your whiteboards write down what you understand from KS4 about each methods What is diffusion? Diffusion is the net movement of particles down a concentration gradient: from a region of high concentration to a region of low concentration. net movement of particles No metabolic energy is expended during diffusion so it is an example of passive transport. One example of diffusion is gas exchange across respiratory surfaces, such as the lungs of mammals and birds, and the gills of fish. Mind map – what factors affect diffusion!! Give examples where possible Diffusion Recap The molecules will diffuse both ways, but the net (overall) movement will be to the area of low concentration. There is a concentration gradient ..... particles move down it Diffusion is PASSIVE! There is NO energy involved. Diffusion occurs if the molecules involved can pass freely through a membrane. So they have to be small. If they’re not, facilitated diffusion is required! What factors affect diffusion? Rates of Diffusion The rate at which diffusion occurs is determined by several factors: 1. The size of the concentration gradient. The larger the difference in concentration, the faster diffusion will occur. 2. The thickness of the exchange surface. The thinner the exchange surface, the faster diffusion will occur. 3. The distance between the two areas. A shorter distance = faster diffusion. 4. The size of the molecules. Smaller molecules such as oxygen will diffuse quicker than large molecules like proteins. The rate of diffusion The rate of diffusion in a given direction across an exchange surface can be summarized by Fick’s law, which states that: surface area × difference in conc. rate of diffusion is proportional to: length of diffusion path (membrane thickness) Increasing the surface area across which the particles diffuse, or increasing the size of the concentration gradient will increase the rate of diffusion. Increasing the distance (or thickness of the membrane) over which diffusion takes place will decrease the rate. Facilitated Diffusion Facilitated diffusion uses the same principle as ordinary diffusion, except that protein carriers are involved. Small molecules like O2 and CO2 can simply diffuse across a membrane without any help. Larger molecules like amino acids and glucose can’t diffuse directly through the phospholipid bilayer. They still move down a concentration gradient, but because they’re so big, they move through carrier proteins or channel proteins. Facilitated diffusion is also passive (no energy). CARRIER or transmembrane Proteins OUTSIDE INSIDE Carrier proteins move large molecules in or out of the cell down a concentration gradient. 1. Molecules attach to the carrier protein. 2. The carrier changes shape. 3. It releases the molecule on the other side. CHANNEL Proteins OUTSIDE INSIDE Channel proteins form pores in the membrane for CHARGED PARTICLES to move down a concentration gradient. Only open in response to presence of molecule Facilitated Diffusion • Facilitated diffusion is specific. • i.e. A certain type of molecule will have a corresponding carrier or channel Glucose = glucose channel Amino acids = amino acid channel Facilitated diffusion Facilitated diffusion • Write down a definition of facilitated diffusion • What is the difference between carrier proteins and channel proteins? • Construct models/diagrams to show how the all the passive processes of transport occur - Diffusion - Facilitated diffusion using channel proteins - Facilitated diffusion using carrier proteins Plenary - whiteboards