There are two types of metabolic reactions:
- Catabolism: breaking down larger molecules into smaller molecules. e.g. digestion.
- Anabolism: building up smaller molecules into larger ones. e.g. photosynthesis.
Nutrients
- Carbohydrates, fats, proteins are macronutrients - they are are needed in large quantities.
- Vitamins and minerals are micronutrients - they are needed in small quantities.
- Water Makes up 70% of a cell
- Fibre*
Use of chemical groups in the body
- Carbohydrates
Energy storage and supply, structure (e.g. cellulose) - Proteins
Structure, transport, enzymes, antibodies, hormones - Lipids
Membranes, energy supply, thermal insulation, protective layers, electrical insulation in neurones, some hormones - Vitamin and minerals
Form part of some larger molecules, takes part in some metabolic reactions, acts as co-enzymes - Nucleic Acid
Information molecules, carries instruction for life - Water
Takes part in many reactions, support in plants, solvent for most metabolic reactions, transport
*Fibre is a carbohydrate, it does not provide nutrients nor energy, it adds bulk to the diet making it easier for gut muscles to push food along - lower risks of constipation and intestinal cancer.
Proteins
Amino acids
- Proteins are made up of individual molecules (monomers) called amino acids.
- They are joined to forma longer chain called polypeptide - which can be combined to form a protein.
- Polypeptides and proteins are therefore polymers.
- There are 20 different amino acids.
- All amino acids contains: Carbon, Hydrogen, Oxygen and Nitrogen.
- In addition, some contains sulphur.
Structure
- They have an amino group (NH2) and a carboxyl group (COOH)
- These roups along with a H atom are attached to a central carbon atom called the alpha-carbon.
- The alpha-carbon atom has an R group attached to it.
- This R group is a variable group - it's different in each of the twenty amino acids.
Where do animals get their amino acids?
- Animals need proteins in their diet.
- These are digested to amino acids and used to produce proteins.
- Excess amino acids can not be stored as the amino group makes them toxic and thus it is removed my deamination in the liver.
Where do plants get their amino acids?
- Plants make the amino acids they need
- They use nitrate from the soil to produce amino groups
- These are added to the organic groups made from photosynthesis
Formation of a dipeptide
- Amino cids can link together by forming a peptide bond
- A peptide bond is formed when the carboxyl group of one amino acid combines with the amino group of another with the elimination of H2O
- this is called a condensation reaction
- When two amino acids are joined by a peptide bond they form dipeptide
Making polypeptides and proteins
- Polypeptides and proteins are synthesised on the ribosomes - protein synthesis.
- This process uses mRNA.
- This puts amino acids together in the right order.
- Different mRNA molecules make different proteins.
Levels of Structures
- Primary structure
This is the sequence of amino acids in a polypeptide molecule.
The sequence of amino acids is vital because it determines the ultimate shape of the protein and therefore its function.
- Secondary structure
This refers to the regular arrangement of the polypeptide chain.
The alpha-helix is where the polypeptide chain is loosely coiled in a regular spiral.
The beta-pleated sheet is were the polypeptide chains are more extended than in the alpha helix.
The C=O and N-H groups from the peptide bond regions are held near to each other so that they for many hydrogen bonds.
These hold the coils of the alpha-helix and the beta-pleated sheets together and make it a stable structure.
- Tertiary structure
This is the further folding of the secondary structure which gives it a 3D compact shape.
It depends on the properties of the variable R-groups in the polypeptide chain.
- R-group bonding
Disulphide bond - the amino acid, cysteine, contains sulphur. where two cysteines are found close together a covalent bond called disulphide bond forms between chains.
Ionic bonds - occurs between oppositely charged R groups
Hydrogen bonds - occur between some R groups - these can be easily broken.
Hydrophobic/philic interactions - hydrophobic amino acids will be most stable if they are held together with water excluded. Hydrophilic amino acids end to be found on the outside of globular proteins.
- Quaternary structure
Consists of two or more different polypeptide chains which are held together by bonds between the R groups.
- Globular proteins
- 3D feature: rolls up to form balls (compact)
- Soluble in water - hydrophilic
- Metabolic
- Examples: Enzymes, plasma proteins, hormones, antibodies
- Fibrous proteins
- Forms fibres (long)
- Insoluble - hydrophobic
- Structural
- Examples: Collagen, keratin
The effect of heat
- Heat increases kinetic energy in a molecule. - causing molecules to vibrate and breaking some bonds maintaining tertiary structure.
- If enough heat is applied the structure can unravel and the protein can no longer function - denatured.
- Even when cooled it will not take on its original shape/arrangement,
Protein hydrolysis
- Protein breakdown is catalysed by enzymes.
- These enzymes are known as protease enzymes.
- Hormone regulation: hormones need to be broke down so that their effect is not permanent.
- Ageing: skin loses elasticity and becomes wrinkled due to inability to rebuild the protein collagen.
- Collagen
- Fibrous protein
- 3 polypeptide chains
- Twisted triple helix
- polypeptides held together by hydrogen bonds between chains
- this forms a collagen fibril
- many fibrils forms a fibre
- Haemoglobin
- 4 polypeptide chains, 2 alpha and 2 beta
- Each one also has an iron prosthetic group attached
- Globular therefore soluble
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