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Structure and function of Cell Membrane
FLUID MOSAIC MODEL
Contains lipids and proteins. Lipid component referred to as the phospholipid bilayer. Phospholipids form a bilayer when placed in a water environment due to the phospholipid polarity. Phospholipids move freely within their plane of the bilayer resulting in fluidity within the membrane. The layers of the bilayer may have different phospholipid and cholesterol compositions. Proteins can be wholly or partially embedded in the phospholipid bilayer. Some proteins can move slowly through the membrane, others are fixed by attachment to the cytoskeleton. The cell membrane is asymmetrical since molecules such as glycoproteins and glycolipids project towards the outside of the cell. Fluid Mosaic - refers to the fluidity due to the phospholipid movement and the mosaic pattern due to the irregular placing of proteins in the membrane. The membrane is selectively permeable - it controls the entry and exit of molecule. The membrane layer is approx. 8-nm thick. The phospholipids are approx. nm and there is a gap between the two phospholipid layers which is bridged by integral proteins
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The bilayer is held together by hydrophobic interactions. The phospholipids cannot move to the other side of the bilayer (transverse movement) as that would mean the hydrophilic head would have to pass through the hydrophilic tail area - this in unlikely.
As temperature decreases the membrane becomes less fluid - less energy therefore less movement of the phospholipids and the phospholipids pack tightly together. The temperature at which the membrane changes from fluid to solid is known as the transitional temperature.
Another lipid is Cholesterol which can account for 50% of lipids in the membrane. Cholesterol enhances membrane fluidity by preventing close packing of the phospholipids. It has hydrophobic and hydrophilic areas which align themselves to the zones within the phospholipid bilayer. It also makes the membrane impermeable to most biological molecules by filling the gaps between the phospholipids.
Proteins
Proteins in the membrane have hydrophilic and hydrophobic zones. Membrane proteins align themselves with the phospholipids based on their polarity. Thus membrane proteins may be wholly or partially embedded in the membrane. Transmembranal proteins bridge the phospholipid bilayer.
Peripheral proteins are not embedded in the membrane but are attached to the surface. Proteins may move slowly within the membrane, however some are attached to the cytoskeleton and cannot move. Protein composition varies with the type of cell. Membrane proteins proteins can act as channels for molecules to pass through. They can act as carriers which pick up, and take molecules through the membrane. They may have an enzymic function. They can also act as junctions linking adjacent cells together. Membrane proteins help stabilise cell structure by attaching the membrane to the cytoskeleton. Another function of Membrane proteins can act as receptors for specific molecules (ie. hormones) and can have a role in cell - cell recognition.
Membranes have several different uses in the cell such as cell interface/boundary separating the living interior of a cell to the non living exterior. Prokaryotic and eukaryotic cells have a cell or plasma membrane. Membrane layers also surround eukaryotic organelles.
Keeps areas of different chemical composition apart. It helps to maintain the composition of different areas and Regulates the movement of molecules into and out of the cell and organelles, it also plays are role in cell recognition and communication processes. Membranes act as a barrier controlling the passage of molecules into and out of the cell and organelles.
They are selectively permeable, since not all substances can move through them. Small non-charged molecules, particularly if they are lipid-soluble can move freely through the membrane. Macromolecules and charged ions and molecules cannot freely move through the membrane. The membrane is generally +vely charged outside and -vely charged inside thus -ve ions tend to move out of the cell and +ve ions tend to move into the cell. Movement of molecules across the membrane is used for nutrient and ion uptake and excretion, removal of waste materials, transport of materials between cells, exchange of material between cell and environment.
The types of transport are -
Intracellular transport - into and out of organelles - Eukaryotic cells only - mitochondria, chloroplasts, nucleus, etc. Transcellular Transport - between adjacent cells or layers of cells - ie. intestinal tract epithelial cells.
Cellular Transport - uptake and excretion of material from the cell - eukaryotic and prokaryotic. PASSIVE Transport - molecules move from and area of high concentration to low concentration - no energy input required - unaided or with the help of a specialised protein carrier.
ACTIVE Transport - molecules move from an area of low concentration to high concentration - against the concentration gradient so energy is required - mediated by membrane proteins (pumps).
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