Foundations in Biology Block 1A - 2.5 Biological membranes Passive transport

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Do now Name a cellular organelle that possesses a membrane and describe the purpose of the membrane. Describe the purpose of cholesterol in the plasma membrane Suggest why organisms living in polar regions have a high proportion of cholesterol in the membrane List three substances that need to be transported into animal cells in order to survive. List two substances that need to be transported out of animal cells in order to survive Golgi, mitochondria, chloroplast, nucleus, lysosomes Compartmentalise enzyme reactions/control substances in and out Prevents phospholipid tails from packing close together and preserves fluidity of membrane To keep membrane fluid and functioning correctly Oxygen, food, minerals, water Carbon dioxide, nitrogenous wastes

Passive Transport Learning Objectives Success Criteria Explain what is meant by passive transport Explain what is meant by facilitated diffusion Identify the role of membrane proteins in transport Recall the definitions of diffusion, facilitated diffusion and osmosis Compare diffusion and facilitated diffusion Construct models/diagrams to show how the processes differ

Transport across the cell membrane Boardworks AS Biology Transport Across Membranes 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 Teacher notes See the ‘Cell Membranes’ presentation for more information about the structure and function of cell membranes. osmosis active transport.

What is diffusion? Use the words: passive, concentration, gradient Give two examples of diffusion occurring in the body. State five factors that affect diffusion. Explain how they affect it. What is the equation for Fick’s law? What does Fick’s law state? State what facilitated diffusion is. Explain why some molecules use facilitated diffusion for transport across a membrane. Draw a diagram to help explain how carrier proteins and channel proteins carry out facilitated diffusion. Compare and contrast diffusion and facilitated diffusion.

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

There is a concentration gradient 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? Boardworks AS Biology Transport Across Membranes What factors affect diffusion? Teacher notes This simulation enables students to see how the rate of diffusion of particles across an exchange surface (membrane) is affected by surface area, thickness and concentration gradient. The orange strip across the centre of the box containing the particles represents the exchange surface. For each factor there are two simulations, each differing in one just one way: Surface area large: the exchange surface spans the full height of the box small: the exchange surface spans half the height of the box Thickness thin: the exchange surface is normal thickness thick: the exchange surface is much thicker (the particles move more slowly through this) Concentration gradient large: particles rapidly disappear once they have moved across the membrane, maintaining a large concentration gradient small: particles disappear much more slowly once they have moved across the membrane, leading to a smaller concentration gradient

Rates of Diffusion The rate at which diffusion occurs is determined by several factors: The size of the concentration gradient. The larger the difference in concentration, the faster diffusion will occur. The thickness of the exchange surface. The thinner the exchange surface, the faster diffusion will occur. The distance between the two areas. A shorter distance = faster diffusion. The size of the molecules. Smaller molecules such as oxygen will diffuse quicker than large molecules like proteins. 5. Temperature As temperature increases molecules have more kinetic energy

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. This is the same for Polar molecules such as ions these are insoluble in lipid as they can’t interact with the hydrophobic tails of the lipid bilayer, so they move through channel proteins. Facilitated diffusion is also passive (no energy). Polar molecules (opposite charges at either end of the molecule) = electrical charge

CARRIER or transmembrane Proteins OUTSIDE INSIDE Carrier proteins move large molecules in or out of the cell down a concentration gradient. Molecules attach to the carrier protein. The carrier changes shape. It releases the molecule on the other side. For example glucose

Only open in response to presence of molecule 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 Neurone – sodium and potassium – diffusion in and out of the axon – nerve impulses and also calcium channels a synapses. Chlorine channels in epithelial cells that line airways – help regulate mucus

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 Boardworks AS Biology Transport Across Membranes Facilitated diffusion Teacher notes Ions travel down an electrochemical gradient, rather than a concentration gradient.

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

Boardworks AS Biology Transport Across Membranes Plenary - whiteboards