Biology Revision
Lipids
Triglycerides
They are good for storage because the long hydrocarbon tails of the fatty acids contain lots of chemical energy so when they are broken down a load of energy is released.
In animals and plants triglycerides are used as energy storage molecules.
They are good for storage because they're insoluble. They bundle together as insoluble droplets in cells because the fatty acid tails are hydrophobic, so they face inwards, and the glycerol heads are hydrophilic, shielding themselves from the water.
Lipids
Phospholipids
Phospholipids are found in cells membranes of all organisms. Cell membranes control what enters and leaves a cell.
Phospholipid heads are hydrophilic and their tails are hydrophobic, so they form a double layers with their heads facing out towards the water on either side.
The centre of the bilayer is hydrophobic, so water-soluble substances can't easily pass through it - the membrane acts as a barrier to those substances
Lipids
Cholesterol
In eukaryotic cells, cholesterol molecules help strengthen the cell membrane by interacting with the phospholipid bilayer.
Cholesterol has a small size and a flattened shape - this allows cholesterol to fit in between the phospholipid molecules in the membrane.
They bind to the hydrophobic tails of the phospholipids, causing them to pack more closely together. This helps to make the membrane less fluid and more rigid.
Esterification
Triglycerides are synthesised by the formation of an ester bond between each fatty acid and they glycerol molecule.
Each ester bond is formed by a condensation reaction.
The process in which triglycerides are synthesised is called esterification.
Triglycerides break down when the eater bonds are broken. each ester bond is broken in a hydrolysis reaction.
Fatty acids
There are two kinds of fatty acids - saturated an unsaturated
Saturated fatty acids don't have any double bonds between their carbon atoms. The fatty acid is 'saturated' with hydrogen.
Unsaturated fatty acids have at least one double bond between carbon atoms, which cause the chain to kink.
Phospholipid
Phospholipids are macromolecules made up of two fatty acid molecules and one phosphate group
The phosphate group is hydrophilic and the fatty acid tails are hydrophobic
Proteins
Primary structure - the sequence of amino acids in the polypeptide chain.
Secondary structure - hydrogen bonds form between nearby amino acids in the chain. this makes it coil into an alpha helix or fold into a beta pleated sheet.
Tertiary structure - the coiled or folded chain of amino acids is often coiled and folded further. More bonds form between different parts of the polypeptide chain. For proteins made from a single polypeptide chain, the tertiary structure forms their final 3D structure.
Quaternary structure - some proteins are made of several different polypeptide chains held together by bonds. The quaternary structure is the way these polypeptide chains are assembled together. For proteins made from more than one polypeptide chain, the quaternary structure is the protein's final 3D structure.
Amino acids
All amino acids have the same general structure - a carboxyl group(-COOH) and an amino acid group(-NH2) attached to a carbon atom. The difference between different amino acids is the variable group (R) they contain.
All amino acids contain the chemical elements carbon, oxygen, hydrogen and nitrogen. Some also contain sulfur.
1) Amino acids are linked together by peptide bonds to form dipeptides and polypeptides.
2) A molecule of water is released during the reaction - it's a condensation reaction.
3) the reverse of this reaction adds a molecule of water to break the peptide bond - it's a hydrolysis reaction.
Bonding and structural levels
Primary structure - held together by peptide bonds between amino acids.
Secondary structure - held together by hydrogen bonds between amino acids.
Tertiary structure - this is affected by different kinds of bonds:
Ionic bonds - attractions between negatively-charged R-groups and positively charged R-groups on different parts of the molecule.
Disulfide bonds - when two molecules of cysteine come close together the sulfur atom in one cysteine bonds to the sulfur in the other cysteine, forming a disulfide bond.
Hydrophobic and hydrophilic interactions - when hydrophobic R-groups are close together in the protein they clump together meaning that the hydrophilic R-groups are more likely to be pushed to the outside, which affects how the protein folds up into its final structure.
Hydrogen bonds - these weak bonds form between slightly positively-charged hydrogen atoms in some R-groups and slightly negatively-charged atoms in other R-groups on the polypeptide chain.
Quaternary structure - this is determined by the tertiary structure of the individual polypeptide chains being bonded together. Because of this, it can be influenced by all the bonds mentioned above.
Globular proteins
The hydrophilic R-groups on the amino acids tend to be pushed to the outside of the molecule. This is caused by the hydrophobic and hydrophilic interactions in the protein's tertiary structure.
This makes globular proteins soluble, so they're easily transported in fluids.
Examples of globular proteins are Haemoglobin, Insulin and Amylase.
Examples
Haemoglobin is a globular protein that carries oxygen around the body in red blood cells. it's known as a conjugated protein because it's a protein with a non-protein group attached. The non-protein group part is called a prosthetic group. each of the four polypeptide chains in the haemoglobin has a prosthetic group called haem. A haem group contains iron, which binds to.
Insulin is a hormone secreted by the pancreas. it helps regulate the blood glucose level. Its solubility is important it means it can be transported in the blood to the tissues where it acts. An insulin molecule consists of two polypeptide chains, which are held together by disulfide bonds.
Amylase is an enzyme that catalyses the breakdown of starch in the digestive system. it is made of a single chain of amino acids. Its secondary structure contains both alpha-helix and beta-pleated sheet sections. Most enzymes are globular proteins.
Fibrous proteins
Fibrous proteins are insoluble and strong. They're structural proteins and are fairly unreactive.
Examples
Collagen - found in animal connective tissues. It is a very strong molecule. Minerals can bind to the protein to increase its rigidity.
Keratin - found in many of the external structures of animals. It can either be flexible or hard and tough.
Elastin - found in elastic connective tissue. It is elastic, so it allows tissues to return to their original shape after they have been stretched.