CELL MEMBRANES and TRANSPORT www.assignmentpoint.com

The Cell www.assignmentpoint.com

Learning Objectives Describe the fluid mosaic model of membrane structure and explain the underlying reasons for this structure. Outline the roles of phospholipids, cholesterol, glycolipids, proteins and glycoproteins in membranes. Outline the roles of the plasma membrane, and the roles of membranes within cells. Describe and explain how molecules can get in and out of cells (cross cell membranes) by the processes of diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis. Describe the effects on animal and plant cells of immersion in solutions of different water potential. Describe the features of the gaseous exchange surface of mammalian lung. Describe the features of root hairs that enable the uptake of ions by active transport. www.assignmentpoint.com

Cell membrane All living things are surrounded by a membrane. A cell membrane is also known as plasma membrane. Controls exchange of materials such as nutrients and waste between cells and their environment. Has other important functions for example to enable cells to receive hormones. To understand the function of anything in biology, you must study the structure first! www.assignmentpoint.com

Cell Membranes from Opposing Neurons (TEM x436,740). Nerve cell Gap between cells Cell membrane { } cell membrane 7nm wide Nerve cell www.assignmentpoint.com

Cell membranes are made of PHOSPHOLIPIDs HYDROPHILIC heads (water liking) -Attracted to the water called POLAR HYDROPHOBIC tails (water fearing) -Not attracted to the water called NON-POLAR A Phospholipid www.assignmentpoint.com

Phospholipids are important structural components of cell membranes Phospholipids are important structural components of cell membranes. Phospholipids are modified so that a phosphate group (PO4-) replaces one of the three fatty acids normally found on a lipid. The addition of this group makes a polar "head" and two nonpolar "tails". www.assignmentpoint.com

A phospholipid HYDROPHILIC HEAD At the other end of the phospholipid is a phosphate group and several double bonded oxygens. The atoms at this end of the molecule are not shared equally. This end of the molecule has a charge and is attracted to water. It is POLAR HYDROPHOBIC TAILS The two long chains coming off of the bottom of this molecule are made up of carbon and hydrogen. Because both of these elements share their electrons evenly these chains have no charge. They are NON POLAR. Molecules with no charge are not attracted to water; as a result water molecules tend to push them out of the way as they are attracted to each other. This causes molecules with no charge not to dissolve in water. 3D model of a Phospholipid www.assignmentpoint.com

A Phospholipid Bilayer Phospholipids can form: BILAYERS -2 layers of phospholipids with hydrophobic tails protected inside by the hydrophilic heads. The PHOSPHOLIPID BILAYER is the basic structure of membranes. www.assignmentpoint.com

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Structure of the cell membrane Phospholipids Cell membranes are made mainly of phospholipids. They have: HYDROPHILIC heads (water liking) -Attracted to the water POLAR HYDROPHOBIC tails (water fearing) -Not attracted to the water NON-POLAR Phospholipids can form BILAYERS -2 layers of phospholipids with hydrophobic tails protected inside by the hydrophilic heads. The PHOSPHOLIPID BILAYER is the basic structure of membranes. www.assignmentpoint.com

Diagram representing the cell membrane Remember the membrane is 7nm wide www.assignmentpoint.com

Fluid mosaic model Cell membranes also contain proteins within the phospholipid bilayer. This ‘model’ for the structure of the membrane is called the: FLUID MOSAIC MODEL FLUID- because individual phospholipids and proteins can move around freely within the layer, like it’s a liquid. MOSAIC- because of the pattern produced by the scattered protein molecules when the membrane is viewed from above. www.assignmentpoint.com

Diagram of a cell membrane www.assignmentpoint.com

TEM of freeze-fractured cell membrane. The fracture occurs between the two phospholipid layers. You can clearly see the exposed proteins sticking out of the two layers. Individual phospholipids are too small to see. www.assignmentpoint.com

Cell Membranes from Opposing Neurons (TEM x436,740). } Phospholipid Bilayer 7nm wide www.assignmentpoint.com

Features of the fluid mosaic model Double layer – BILAYER of phospholipids which can move about by ………………………… in their own ………………………. Phospholipid tails point inwards forming a ……. ………. ………………………… interior. The phospholipid heads point outwards facing the aqueous (water containing) medium surrounding the membrane. Some phospholipids fatty acid tails are ……………………….. – straight so fit together tightly. Some are ………………………… – bent so fit together ……………………. The more unsaturated tails there are the more ……………… the membrane becomes. The lower the temp, the ……………….fluid. Most protein molecules …………………. like icebergs in the layers, some are fixed to ………………………. inside the cell and don’t float. Some proteins are embedded in the outer layer, some in the inner layer and some ………………… the two layers. Hydrophobic and Hyrdophilic parts of the protein molecules sit next to the …………………………… and ………………………….. portions of the ……………………………… of the membrane. This ensures the proteins stay in the membrane. The membrane is ……nm thick on average. Some phospholipids have carbohydrates attached to them called- ……………………….. Some of the proteins have carbohydrates attached to them called– …………………… The membrane also contains molecules of ……………………………….. www.assignmentpoint.com

Features of the fluid mosaic model Double layer – BILAYER of phospholipids which can move about by DIFFUSION in their own MONOLAYER Phospholipid tails point inwards forming a NON-POLAR HYDROPHOBIC interior. The phospholipid heads point outwards facing the aqueous (water containing) medium surrounding the membrane. Some phospholipids fatty acid tails are SATURATED – straight so fit together tightly. Some are UNSATURATED – bent so fit together loosely. The more unsaturated tails there are the more ‘fluid’ the membrane becomes. The lower the temp, the less fluid. Most protein molecules float like icebergs in the layers, some are fixed to structures inside the cell and don’t float. Some proteins are embedded in the outer layer, some in the inner layer and some span the two layers. Hydrophobic and Hyrdophilic parts of the protein molecules sit next to the Hydrophobic and Hydrophilic portions of the phospholids of the membrane. This ensures the proteins stay in the membrane. The membrane is 7nm thick on average. Some phospholipids have carbohydrates attached to them – GLYCOLIPIDS Some of the proteins have carbohydrates attached to them – GLYCOPROTEINS The membrane also contains molecules of CHOLESTEROL www.assignmentpoint.com

Roles of components of cell membranes Using the following headings produce a table on A4 to summarise roles of t he different types of molecules found in the cell membrane. Try not to copy, pick out the relevant information and write it in note form You may want to use bullet points, different coloured pens etc basically whatever helps you to remember them. There will be a short test on this next lesson! Use pages 53-54. Component Function Phospholipids Cholesterol Proteins Glycolipids and Glycoproteins www.assignmentpoint.com

Summary Cell membranes have a basic structure composed of a PHOSPHOLIPID BILAYER. Phospholipds have HYDROPHOBIC (non-polar) tails and HYDROPHILIC (polar) heads. The best model of the cell membrane is called the FLUID MOSAIC MODEL The average thickness of the membrane is 7nm. The fatty acid tails of phospholipids can be SATURATED (straight) or UNSATURATED (bent) Proteins can float or be fixed and also have hydrophobic and hydrophilic portions. Some proteins and phospholipids have carbohydrates attached to them to form GLYCOPROTEINS AND GLYCOLIPIDS. Phospholipids form the bilayer, act as barrier to most water soluble substances Cholesterol regulates the fluidity of the membrane, gives mechanical stability and help to prevent ions from passing through the membrane. Proteins act as transport proteins to act as channels for substances to move into or out of the cell. Some act as membrane enzymes and some have important roles in membranes of organelles. Glycolipids and Glycoproteins help to stabilise membrane structure, some act as receptor molecules eg for hormones and neurotransmitters or as antigens for other cells to recognise them. www.assignmentpoint.com

Movement of selected molecules across the cell membrane www.assignmentpoint.com

Moving amoeba www.assignmentpoint.com