Biochemistry Unit The Cell Membrane

Cell Membrane Functions Allows some materials to easily pass through the membrane while preventing larger or unwanted materials from entering. Selects which materials can enter or exit depending on its internal and/or external condition. Therefore the cell membrane is selectively permeable.

Cell Membranes from Opposing Neurons (TEM image) Nerve cell Gap between cells Cell membrane { } cell membrane 7nm wide Nerve cell

Structure: Phospholipid Bilayer Composed of two layers of phospholipids. External and internal environments are aqueous…

Structure: Phospholipid Bilayer The cell membrane has a fluid like consistency comparable to olive oil and is flexible. Phospholipids are able to move freely within their monolayer.

Structure: Cholesterol Cholesterol molecules prevent the cell membrane from becoming too mushy or too solid. Prevents small molecules from passing directly through Without cholesterol the animal cell would need a cell wall!

Structure: Proteins Protein molecules are also embedded within the cell membrane, they act as: Special carrier molecules Receptor sites that receive hormones Gatekeepers allow molecules to pass through the membrane. Carry special carbohydrates and are called glycoproteins. These specialized structures help cells identify themselves to other cells or hormones. Cells communicate with each other. The presence of specialized protein markers on the outside of the cell membrane helps hormones reach their target destination. Without the protein markers, the immune system would not know which cells to destroy.

Fluid Mosaic Model

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.

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. http://www.wisc-online.com/objects/ViewObject.aspx?ID=AP1101

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 ………………………………..

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

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