a level biology: lipids and proteins (biological molecules part 2)

are lipids polar or non-polar?

• non-polar

• the electrons in the outer orbitals (the electrons that form bonds) are more evenly distributed than in polar molecules, so there are no negative or positive areas in the molecules

• this also means that lipids are insoluble if solid (don’t dissolve in water) and immiscible if liquid (don’t mix with water)

what are the uses of lipids in biology?

• energy storage (adipose cells) (because lipids are insoluble in water and are hydrophobic (do not form hydrogen bonds with water), so they make good storage molecules in aqueous body environments)

• energy source for respiration

• water source for respiration

• protection of vital organs

• thermal or electrical insulation

• plasma membranes of cells

• waterproofing feathers or leaves

• buoyancy for aquatic animals (as they are less dense than water)

what is the structure of a triglyceride?

• one glycerol (an alcohol) and three fatty acid (a carboxylic acid) tails (which can be saturated or unsaturated)

• the glycerol and fatty acids are joined by an ester bond (which occurs via the condensation reaction, esterification)

• (they are non-polar)

what is meant by monounsaturated?

there is one double C=C bond in the hydrocarbon chain

what is meant by polyunsaturated?

there are two or more double C=C bonds in the hydrocarbon chain

are unsaturated fats liquid or solid at room temperature, and why?

• liquid (oil)

• because: the double bond(s) mean the molecule can kink or bend, so the molecules cannot pack closely together to form a solid

what are cis fats?

fats where hydrogens appear on opposite sides of the molecule

what are trans fats?

fats where the hydrogens appear on the same sides of the molecule

what is the structure of a phospholipid?

• one glycerol molecule with two fatty acid tails and a phosphate ion

what are the properties of phospholipids?

• amphipathic/amphiphilic (have both hydrophobic and hydrophilic regions):

• the phosphate group is polar and so is hydrophilic

• the fatty acid tails are non-polar and so are hydrophobic

• since there are both hydrophobic and hydrophilic parts, phospholipid molecules form monolayers or bilayers (where a hydrophobic core is formed) when in water

what is the purpose of phospholipids?

• they make up the plasma membrane

• because: their amphipathic properties allow them to form a hydrophobic core as a barrier against water-soluble molecules

• + the hydrophilic phosphate heads form hydrogen bonds with water so the cell membrane can contain water-soluble molecules while keeping them separate from the outside molecules)

why are some of the fatty acid tails in the plasma membrane unsaturated?

• for flexibility:

• the tails can change shape and so the lipids are fluid at lower temperatures, allowing the membrane proteins to move + it allows small molecules to pass through the membrane

how many water molecules do both phospholipids and triglycerides release when formed via a condensation reaction?

3

what are sterols?

• steroid alcohols (molecules with 4 C rings and an OH group)

• they are amphiphilic (the hydroxyl group is polar and hydrophilic; the rest of the molecule is non-polar and hydrophobic)

• E.g. cholesterol

where is cholesterol produced and found? what is its purpose?

• produced in the liver and intestines

• found in plasma membranes, in between phospholipids

• regulates stability and fluidity of the membrane

what is the test for lipids (when the sample is solid)?

• crush the solid samples and put them in separate test tubes

• use a pipette to add ethanol to each sample so there is a layer of liquid above the sample (the layer should be about the same volume in each test tube)

• shake the tubes with bungs in

• allow the solid to settle to let the lipid dissolve in the ethanol

• record observations

• add a set volume of distilled with with a pipette to each sample and shake gently to mix, then record observations

• positive result: a milky white emulsion forms

• negative result: solution remains clear and colourless

what is the test for lipids (when the sample is liquid)?

• put the same volume of each sample into a different test tube

• use a pipette to add a set volume of ethanol to each sample

• shake the tubes with bungs in

• record observations

• add a set volume of distilled with with a pipette to each sample and shake gently to mix, then record observations

• positive result: a milky white emulsion forms

• negative result: solution remains clear and colourless

what are the purposes of proteins in biology?

• cell growth and repair

• cell structure (carrier proteins in plasma membranes)

• hormones

• enzymes

• immunoproteins (e.g. immunoglobulins)

• transport proteins (e.g. haemoglobin)

• structural proteins (e.g. collagen, keratin)

• contractile proteins (e.g. myosin)

how many different amino acids are there?

20

what is the general structure of an amino acid?

• amino group (basic) (NH₂ and sometimes a H⁺ too)

• carboxyl group (acidic) (COOH, when in solution it loses the H⁺ to leave O^-)

what are peptide bonds?

covalent bonds between amino acids

what is the primary structure of a protein?

the sequence in which the amino acids are joined, determined by the info carried in DNA

what does the sequence of amino acids affect?

the way the polypeptide folds to give the protein’s shape

what is the secondary structure of a protein?

the 3d folding of the polypeptide, determined by the hydrogen bonds formed between negatively-charged nitrogen and oxygen atoms with positively-charged hydrogen atoms

what are the two types of secondary structure?

• alpha helix

• beta-pleated sheet

where are the hydrogen bonds formed in an alpha helix?

between every 4th peptide bond

where are the hydrogen bonds formed in a beta-pleated sheet?

between parallel peptide bonds (because the protein is folded so that two parts of the polypeptide chain are parallel to each other)

what is the tertiary structure of a protein?

the further conformational changes to shape of the secondary structure, which results in additional bonds forming between R groups (side chains)

what are the four types of bonds formed in the tertiary structure of a protein (excluding peptide bonds, since those are also in primary structure)?

• disulphide bridge: (most rare) very strong covalent bonds formed between sulphur-containing side chains

• ionic bond: (less common) weak-ish bonds between oppositely-charged side chains

• hydrophobic interactions: (common) weak interactions between non-polar side chains

• hydrogen bond: (most common) weak bonds between very polar side chains

what is the quaternary structure of a protein?

the 3d arrangement of two or more polypeptides (in proteins that have more than one polypeptide chain)

what are the four types of bonds in the quaternary structure?

same as in tertiary structure:

• disulphide bridge: (most rare) very strong covalent bonds formed between sulphur-containing side chains

• ionic bond: (less common) weak-ish bonds between oppositely-charged side chains

• hydrophobic interactions: (common) weak interactions between non-polar side chains

• hydrogen bond: (most common) weak bonds between very polar side chains

what is an example of a protein with a quaternary structure?

haemoglobin

what are the two types of proteins?

• globular (e.g. insulin, catalase, haemoglobin)

• fibrous (e.g. keratin, elastin, collagen)

what are the properties of globular proteins?

• compact

• roughly spherical (because their non-polar/hydrophobic side chains are orientated towards the centre of the protein, away from the aqueous surroundings, while their polar/hydrophilic side chains face the outside of the protein)

• soluble in water

what are conjugated proteins?

• globular proteins containing a non-protein component called a prosthetic group

• (as opposed to simple proteins, which don’t have a prosthetic group)

• E.g. haemoglobin

what are the properties of haemoglobin?

• quaternary structure (two alpha helixes and two beta-pleated sheets)

• globular protein

• conjugated protein

• each polypeptide chain has one haem prosthetic group

• haem contains Fe²⁺ and so binds to oxygen (each haemoglobin molecule can bind to 4 O₂ molecules)

• soluble in water

what is the function of catalase?

breaks down hydrogen peroxide (a common by-product of metabolism)

how many haem prosthetic groups are in catalase?

4

what are the properties of fibrous proteins?

• long

• very organised structure (due to repeating sequence of amino acids)

• insoluble in water (due to having a lot of hydrophobic side chains in their primary structures)

• strong

what is the purpose of keratin?

• making up hair, nails, horns, feathers

• because: there is a lot of cysteine amino acids forming many disulphide bridges, making it very strong

what is the purpose of elastin?

• making up tendons, bones, skin

• because: it is made up of many stretchy tropoelastin molecules, allowing it to stretch and rebound to the original size

what is the purpose of collagen?

• tendons, bones, cartilage, walls of arteries

• because: the triple helix quaternary structure gives it a lot of strength and flexibility

how do you test for proteins?

• put a set volume of each sample into separate test tubes

• hold one of the tube’s at a 45 degree angle and slowly add a small volume of Biuret’s reagent using a syringe, making sure the reagent forms a layer on top of the sample

• record any colour changes seen at the interface between the reagent and the sample

• shake gently and record observations again

• repeat for the other sample

• positive result: solution turns from blue to lilac, so there are peptide bonds present

• negative result: solution remains blue