1. Biochemistry Unit
The Cell Membrane

2. 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.

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

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

5. 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.

6. 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!

7. 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.

8. Fluid Mosaic Model

9. 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.

10. 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.

11. 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 ………………………………..

12. 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

13. 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
Component Function
Phospholipids
Cholesterol
Proteins
Glycolipids and Glycoproteins