Chemistry of Life Lecture 20 (The Plasma membrane is a complex mixture of…
Chemistry of Life Lecture 20
The Plasma membrane is a complex mixture of lipid, protein and carbohydrate
The plasma membrane is the boundary that separates the living cell from its surroundings. The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others. Transport proteins are often responsible for controlling passage across cellular membranes.
Lipid molecules include fats, phospholipids and cholesterol
Lipids are the only true large biomolecule that does not include true polymers. A unified feature of lipids is that they mix poorly with water. the most biologically important lipids are fats, phospholipids and steroids. Lipids consist mostly of hydrocarbon regions. Fats are constructed from two types of smaller molecule.s : glycerol and fatty acids. Glycerol is a three carbon alcohol with a hydroxyl group attached to each carbon. A fatty acid consists a carboxyl group attached to a long carbon skeleton.
Fats separate from water because water molecules hydrogen bond to each other and exclude the fats. In a fat, three fatty acids are joined to a glycerol by an ester linkage, creating a triglyceride. The fatty acids in a fat can all be the same or two different or three different kinds. fatty acids vary in length, (number of carbons) and in the location and number of double bonds. Saturated fatty acids have the maximum number of hydrogen atoms possible and no double bonds. Unsaturated fatty acids have one or more double bonds.
The presence of double bonds affect the confirmation properties of a lipid.
Fats made from saturated fatty acids are called saturated fats and are usually solid at room temp. Most animal fats are saturated. Fats made from unsaturated fatty acids are called unsaturated fats or oils and are liquid at room temperature. Plant fats and fish fats are usually unsaturated.
Phospholipids generate the internal and external membranes of cells
In a phospholipid, two fatty acids and a phosphate are attached to a glycerol. The two fatty acids are hydrophobic but the phosphate and its attachments are hydrophilic. When phospholipids are added to water they self assemble into double layered sheets called bilayers. At the surface of a cell, phospholipids are arranged in a bilayer, with the hydrophobic tails pointing towards the interior. The phospholipid bilayer forms a boundary between the cell and its external environment.
Phospholipids form a bilayer in an aqueous solution
Phospholipids are the most abundant lipid in the plasma membrane. Phospholipids are amphipathic molecules containing hydrophobic and hydrophilc regions. The hydrophobic tails are sheltered inside the membrane whilst the hydrophilc heads are exposed to water on either side.
Cholesterol is an important component of cell membranes.
Steroids are lipids characterized by a carbon skeleton consisting of four fused rings. Cholesterol; a type of steroid, is a small component in animal cell membranes and a precursor from which steroids are synthesized. A high level of cholesterol in the blood may contribute to cardiovascular disease. The steroid cholesterol has different effects on the membrane fluidity of animal cells at different temperatures. At warm temperatures such as 37 degrees cholesterol restrains the movement of phospholipids. At cool temperatures, it maintains fluidity by preventing tight packing. Though cholesterol is present in plants they use related steroid lipids to buffer membrane fluidity
The fluidity of membranes
As temperature cools, membranes switch from a fluid state to a solid state. The temperature at which the membrane solidifies depends on the types of lipids present in that membrane. Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids. Membranes must be fluid to work properly. Membranes are usually about as fluid as salad oil. Cholesterol functions as a buffer for membrane fluidity. Variations in lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditions. Ability to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary.
Integral membranes proteins expose hydrophobic regions to the phospholipid bilayer.
Peripheral proteins are bound to the surface of the membrane. Integral proteins penetrate the hydrophobic core. Integral proteins that span the membrane are called transmembrane proteins. The hydrophobic regions of an integral protein consist of one or more stretches of non polar amino acids often coiled into an alpha helix.
Dynamic structure of the cell membrane containing an integral protein
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Aquaporin is a membrane spanning protein that shuttles water
Channel proteins called aquaporins greatly facilitate the passage of water molecules. Other transport proteins bind to molecules and change shape to shuttle them across the membrane. A transport protein is specific for the protein it transports.
Passive transport: diffusion across a membrane with no energy investment.