Biomolecules

Question 1

describe the structure of amino acids

Answer 1

consists of a central carbon atom, amino group, carboxyl group, H atom and a variable R group. the R group can vary in size and charge and this is what gives each amino acid its unique chemical properties

Question 2

describe the properties of non-polar amino acids

Answer 2

• R group is hydrocarbon in nature and has no net charge
• amino acids are hydrophobic and unreactive, tend to be localised in the interior of the molecule (away from aqueous medium

Question 3

describe the properties of polar amino acids

Answer 3

• R groups have no net charge
• amino acids are hydrophilic in nature

Question 4

describe the properties of charged amino acids

Answer 4

• R groups of a.a are charged
• amino acids are either negatively-charged or positively-charged, making them hydrophilic
• acidic amino acids are negatively-charged while basic amino acids are positively-charged

Question 5

describe the buffering capacity of amino acids

Answer 5

• in neutral solutions, most amino acids exist in a form that contains both positive and negative charges (zwitterions)
• zwitterions carry both negative and positive charges on different atoms but has a net charge of zero
• amino acids exist as zwitterions in neutral aqueous solutions by the association of protons with the amino group, making it positively-charged, and the loss of protons from carboxyl group, making it negatively-charged
• since they have both acidic and basic properties in aqueous solutions, they are amphoteric
• this allows them to act as buffers in solution, resisting small changes in pH when an acid/alkali is added, by taking up or losing protons
• in acidic environment, COO- takes up protons from surroundings, causing pH to increase back to normal
• in alkaline environment, NH3+ donates protons to the surroundings, causing pH to decrease back to normal
• this buffering property is retained even when amino acids are bonded to each other to form proteins, because of the acidic and basic R groups of individual amino acids

Question 6

describe the formation of peptide bonds

Answer 6

a covalent bond formed between the amino group of one amino acid and the carboxyl group of another due to a condensation reaction, with the loss of a water molecule

Question 7

describe the formation and breakage of ionic bonds

Answer 7

• a bond formed between acidic and basic R groups of amino acids
• acidic and basic R groups are ionised at certain pHs, acidic R groups are negatively-charged while basic R groups are positively-charged
• electrostatic attraction occurs between two oppositely-charged R groups, forming an ionic bond
• ionic bonds may be formed between R groups of different polypeptides or between R groups at different parts of the same polypeptide
• this bond is much weaker than a covalent bond in an aqueous medium and can be broken by changing the pH of the medium
• pH changes can therefore disrupt the 3D conformation of the protein structure

Question 8

describe the formation of disulfide bonds

Answer 8

• covalent bond formed between sulfhydryl R groups, only the amino acid cysteine contains a -SH in its R group
• disulfide bonds may be formed between R groups of different polypeptides or between R groups of different parts of the same polypeptide
• disulfide bonds are strong and not easily broken
• proteins which have disulfide bonds in their structure will be able to withstand higher temperatures before being denatured

Question 9

describe the formation of hydrogen bonds

Answer 9

• relatively weak non-covalent bond between an electronegative atom and a hydrogen atom attached to another electronegative atom
• hydrogen bonds can be formed between different parts of the same polypeptide or between polypeptides (formed between -CO and -NH of polypeptide backbone, between -CO/-NH and R group or between R groups)
• hydrogen bond is weak but when it occurs frequently within a molecule, total effect increases molecular stability

Question 10

describe the formation of hydrogen bonds

Answer 10

• relatively weak non-covalent bond between an electronegative atom and a hydrogen atom attached to another electronegative atom
• hydrogen bonds can be formed between different parts of the same polypeptide or between polypeptides (formed between -CO and -NH of polypeptide backbone, between -CO/-NH and R group or between R groups)
• hydrogen bond is weak but when it occurs frequently within a molecule, total effect increases molecular stability

Question 11

describe the formation of hydrophobic interactions

Answer 11

• occur between non-polar/hydrophobic R groups
• if a polypeptide chain contains a number of these groups and is in an aqueous environment, the chain will tend to fold such that a maximum number of hydrophobic groups will come into close contact with each other and exclude water
• if the protein is globular, hydrophobic groups tend to point inwards towards the centre of the molecule while the hydrophilic groups face outwards into the aqueous environment, making the protein soluble
• membrane proteins which are embedded within the membrane are likely to have their hydrophobic regions found within the membrane, alongside the hydrophobic hydrocarbon tails of phospholipids. their hydrophilic regions face away from the membrane, alongside the hydrophilic phosphate heads

Question 12

explain primary structure and describe the types of bonds that hold the molecule in shape

Answer 12

• unique number and sequence of amino acids in a polypeptide chain
  amino acids are linked by peptide bonds and its sequence is specified by the base sequence of DNA in the nucleus
• primary structure determines the type and location of bonds present at higher levels of organisation in the protein
• it determines the overall 3D conformation and the function of a protein
• a change in one amino acid can change the amino acid sequence, leading to a change in type and location of bonds formed at higher levels of organisation in the protein, leading to a change in the specific 3D conformation and overall function of protein. e.g mutation in gene coding for haemoglobin causes sickle cell anaemia

Question 13

explain secondary structure

Answer 13

• local folding of a polypeptide into regular structures such as the alpha-helix and beta-pleated sheets
• these regular structures are the result of hydrogen bonds formed at regular intervals between the -CO and -NH groups of the polypeptide backbone (not amino acid R groups)

Question 14

explain alpha-helix structure

Answer 14

• an alpha-helix is formed when a hydrogen bond is formed between the O atom of the -CO group of an a.a residue and the H atom of the -NH group of the a.a that is situated four amino acid residues ahead in the linear sequence in the same chain
• one complete turn occurs for every 3.6 a.a
• it is very stable bc all -CO and -NH groups of the polypeptide backbone can participate in hydrogen bonding. regions with alpha-helices are rigid and rod-like
• R groups of some a.a in a polypeptide can interfere with the formation of an alpha-helix. a.a with bulky R groups can interfere with the formation of alpha-helix

Question 15

describe and explain the beta-pleated sheet

Answer 15

• different sections of the polypeptide chain can be folded to form adjacent strands, which can run either in the same directtion or in opposite directions, forming parallel or anti-parallel beta-pleated sheets
• these adjacent strands are held together by hydrogen bonds, formed between -CO and -NH groups of the polypeptide backbone to form a sheet
• hydrogen bonds can occur between two or more sections of the same polypeptiode or between two or more sections of different polypeptides
• within a sheet, all -CO and -NH groups are involved in hydrogen bonding, hence the structure is very stable and rigid
• bulky amino acids interfere with the formation of beta-pleated sheets, thus most a.a that are found in these sheets have compact R groups

Question 16

describe and explain tertiary structure

Answer 16

• formed with the bending, twisting and folding of the secondary structures of a polypeptide to form a specific 3D conformation
• specific 3D conformation is held together by hydrogen bonds, ionic bonds, hydrophobic interactions and disulfide bonds between the R grps of various a.a that lie close to each other in the 3D structure

Question 17

describe and explain quaternary structure

Answer 17

• association of two or more polypeptides
• separate polypeptides can be held together by hydrogen bonds, ionic bonds, disulfide bonds and hydrophobic interactions btwn R grps of a.a of different subunits

Question 18

describe denaturation

Answer 18

• change in specific 3D conformation of a protein molecule, molecule unfolds and no longer performs its normal biological function
• the a.a sequence (primary structure) is unaffected

Question 19

explain the effect of temperature on protein structure

Answer 19

disrupts weak hydrogen bonds, hydrophobic interaction and ionic bonds

Question 20

explain the effect of strong acids and alkalis and high concentrations of salt on protein structure

Answer 20

• disrupts hydrogen and ionic bonds
• breakage of peptide bonds may occur if protein is allowed to remain mixed with reagent for a long period of time

Question 21

explain the effect of heavy metals on protein structure (metallic ions)

Answer 21

interact w charged R grps of a.a and disrupt ionic bonds

Question 22

explain the effect of organic solvents and detergents on protein structure

Answer 22

disrupt hydrophobic interactions and hydrogen bonds

Question 23

describe the structure of collagen

Answer 23

• basic structural unit is tropocollagen, each tropocollagen consists of three polypeptides/alpha-chains
• each alpha-chain has a primary structure of -1000 a.a residues, and is made up of a repeating tripeptide sequence of glycine-X-Y, where X is proline and Y is either hydroxyproline or hydroxylysine
• glycine is found in every third position. it has a small R group that is small enough to fit into the restricted space where the three chains interact. this allows the three chains to wind together tightly and stabilise the triple helix structure
• regular distribution of proline and hydroxyproline residues along the polypeptide chain inserts kinks and disrupts the regular formation of intramolecular hydrogen bonding within the chain. thus, the polypeptide chain cannot fold into a regular alpha-helix structure
• three helical alpha-chains wind around each other to form a triple helix structure
• three alpha-chains are held in a helical conformation by an extensive network of hydrogen bonds between: -NH groups of glycine found in the polypeptide backbone of each strand and -CO groups found in the polypeptide backbones of the other two strands; and -OH groups found in R group of hydroxylysine or hydroxyproline
• different tropocollagen molecules are staggered and cross-linked covalently to form a collagen fibril. 20-100 collagen fibrils combine to form collagen fibres

Question 24

relate the structure of collagen to its function

Answer 24

• each alpha chain has -1000 a.a residues. collagen is made up of bundles of collagen fibres and fibrils and each alpha-chain of tropocollagen has a repeating tripeptide sequence of glycine-X-Y, where X is often proline and Y is hydroxyproline or hydroxylysine. collagen is a large and long molecule. glycine and proline are hydrophobic and polar groups (in the polypeptide bacbond and R groups) are already involved in hydrogen bonding between alpha-chains, so they are unavailable to form hydrogen bonds with water. both properties make collagen insoluble in water
• glycine is found in every third position in the alpha-chain and has a small R group which is small enough to fit into the restricted space where the three chains interact. the three alpha-chains can wind tightly together to stabilise the triple helix structure. hydrogen bonds form between polar groups (in the polypeptide backbone and R groups) between alpha-chains. this further stabilises the triple helix structure. different tropocollagen molecules are arranged in a staggered manner and cross-linked covalently to form a collagen fibril. many collagen fibrils bundle to form fibres. staggered arrangement of tropocollagen molecules, reinforced by covalent cross-links, reduces occurrence of weak spots within collagen fibrils. collagen has an inelastic but flexible structure with high tensile strength to withstand large pulling forces for support

Question 25

describe the structure of haemoglobin

Answer 25

• tetrameric protein, consisting of four subunits, two alpha-chains and two beta-chains. the secondary structure consists of only alpha-helices
• each haemoglobin subunit consists of a globin (protein) and haem (non-protein)
• globin has a globular structure, with hydrophobic a.a residues buried in the interior of the globular structure while hydrophilic a.a residues are found at the surface of the globin
• each globin has a deep hydrophobic cleft (haem binding pocket), which is lined with hydrophobic a.a residues to provide a hydrophobic environment for the haem group (hydrophobic)
• haem group consists of an iron ion, which is at the centre of the planar porphyrin ring. the haem is oriented such that one face of the iron ion is complexed to an a.a residue while the other face is accessible for oxygen binding
• tetramer is composed of two identical dimers. the two polypeptide chains in each dimer are held together by ionic bonds, hydrogen bonds and hydrophobic interactions. the two dimers are held together by weak hydrogen bonds, resulting in the ability of the two dimers to move with respect to each other
• this allows for cooperativity. when an oxygen molecule binds to (or is released) from one haemoglobin subunit, the binding (or release) induces a conformational change in the remaining subunits, which increases (or lowers) the affinity for oxygen of the remaining three oxygen binding sites respectively
• this facilitates the effective loading or unloading of oxygen

Question 26

relate the structure of haemoglobin to its function

Answer 26

• haemoglobin is a tetrameric protein. each subunit consists of globin and a haem group. the haem group contains Fe2+ for binding to oxygen. the four subunits in haemoglobin are each capable of oxygen binding. this maximises oxygen carrying capacity of each haemoglobin. oxygen molecule binds reversibly to Fe2+, without the oxidation of Fe2+. this allows the oxygen molecule to be released when needed
• each globin polypeptide is folded such that the bulk of the a.a residues with hydrophobic R groups are buried in the interior of the globular structure while the a.a residues with hydrophilic R grps are on the outside. this gives haemoglobin a globular structure. being globular allows many haemoglobin molecules to be packed into a single red blood cell, maximising oxygen carrying capacity of each red blood cell. being globular allows hydrophilic a.a residues at the surface of the molecule to form hydrogen bonds with water. haemoglobin is soluble in aqueous medium and hence an effective transporter of oxygen in blood
• each globin is folded into a specific 3D conformation with a deep hydrophobic cleft. haem resides in the hydrophobic cleft within each globin. the hydrophobic cleft allows haem, which is largely hydrophobic, to bind to globin stably. the hydrophobic cleft provides a hydrophobic and favourable environment for binding of oxygen molecule. haemoglobin is an effector transporter of oxygen molecules
• the two polypeptide chains in each dimer are held together by ionic bonds, hydrogen bonds and hydrophobic interactions. the two dimers are held together by weak hydrogen bonds, resulting in the ability of the two dimers to move with respect to each other. this allows for cooperativity. when an oxygen molecule binds to (or is released from) one haemoglobin subunit, the binding (or release) induces a conformational change in the remaining subunits, which increases (or lowers) the affinity for oxygen of the remaining three oxygen binding sites respectively. this facilitates the effective loading and unloading of oxygen

Question 27

describe the structure of GPLR

Answer 27

• consist of a single polypeptide chain that forms seven alpha-helices spanning the plasma membrane, with the N-terminus facing the exterior of cell while the C-terminus faces the cytosol
• consists of a binding site on the exterior of cell which binds to a signal molecule
• consists of a binding site at the cytoplasmic side of the cell which binds to G protein

Question 28

describe the activation of a GPLR

Answer 28

• in the absence of the extracellular signal molecule specific for the receptor, all three proteins are in inactive form. the inactive G protein has a GDP molecule bound to it
• when the specific signal molecule binds to the receptor, the receptor changes its 3D conformation and is activated and binds to G protein. activation of G protein occurs when GTP replaces GDP on G protein. activated G protein binds to and activates the enzyme, which triggers the next step in the pathway leading to the cell’s responses
• G protein then catalyses the hyrolysis of its bound GTP to GDP. G protein returns to its inactive state, dissociates from the enzyme, becoming available for reuse

Question 29

relate the structure of GPLR to its function

Answer 29

• consists of a single polypeptide chain that forms seven alpha-helices spanning the plasma membrane. this allows the hydrophobic R grps of a.a facing exterior of alpha-helices to interact with hydrophobic hydrocarbon tails of the phospholipid bilayer. this allows the receptor to be embedded on the plasma membrane to receive extracellular signals
• binding site for signal molecule facing exterior of the plasma membrane. this allows extracellular signal molecule to bind to the receptor at the plasma membrane and trigger a conformational change, thus activating it
• binding site for G protein facing cytoplasmic side of the plasma membrane. activated GPLR binds to G protein at the cytoplasmic side and activates G protein when GTP displaces GDP. this allows transmission of signals into the cell to generate appropriate cellular response

Question 30

differentiate between structural proteins and metabolic proteins

Answer 30

• structural proteins usually exist as long, straight fibres/sheets (fibrous proteins) while metabolic proteins usually exists as a globular structure (globular proteins)
• amino acid sequence of structural proteins is usually regular whereas metabolic proteins rarely exhibit regularities in their a.a sequences as specific sequence of a.a are required in order for proteins to fold into specific 3D conformation
• in a structural protein, a.a sequence may vary slightly btwn two different samples of the same protein whereas in a metabolic protein, the a.a sequence is highly specific and never varies btwn two samples of the same protein. substitution of just a single a.a residue can cause a major alteration in the function of a protein
• in a structural protein, the length of polypeptide chain may vary in two different samples of the same protein while in a metabolic protein, the length of polypeptide chain is always identical in two samples of the same protein
• structural proteins do not have a tertiary structure. the secondary structure is the most important, and some have quaternary structure. metabolic proteins have a tertiary structure. however, the quaternary structure may or may not be present
• in structural proteins, polypeptide chains are cross-linked at intervals to form long fibres/sheets while in metabolic proteins, polypeptide chain is tightly folded to form a globular structure
• structural proteins are insoluble in water, due to the large number of hydrophobic R groups of a.a residues on the exterior of their molecules. metabolic proteins or certain regions of the proteins are soluble in aqueous medium, as the hydrophilic R groups of a.a residues are exposed on the exterior of their molecules

Question 31

describe the structure of monosaccharides

Answer 31

• contain the carbonyl (-CO) group either as an aldehyde group (-CO as a terminal carbon with H attached) or as a ketone group (-CO as a non-terminal carbon of a carbon backbone)
• all other carbon atoms of the monosaccharide have a hydroxyl group (-OH) attached
• the alpha-glucose anomer has the hydroxyl group attached to the anomeric carbon below the plane of the ring and is opposite to 6’ carbon atom
• the beta-glucose anomer has the hydroxyl group attached to the anomeric carbon above the plane of the ring and on the same side as 6’ carbon atom
• the existence of alpha-glucose and beta-glucose leads to greater chemical variety and is of importance in e.g the formation of polysaccharides starch and cellulose

Question 32

describe the properties of monosaccharides

Answer 32

as aldehyde and ketone groups are easily oxidised, they are powerful reducing agents, all monosaccharides are reducing sugars

Question 33

state the function of monosaccharides

Answer 33

glucose is oxidised to release energy (ATP) in the process of respiration, while others are used as raw material for the synthesis of other types of small organic molecules

Question 34

describe the structure of disaccharides

Answer 34

carbohydrates made of two monosaccharides linked together by a covalent bond (glycosidic bond) through a condensation reaction with the loss of a water molecule

Question 35

describe the breakage of glycosidic bonds

Answer 35

hydrolysis: addition of water to a disaccharide, with its subsequent splitting into two monosaccharide units

Question 36

describe the functions of disaccharides

Answer 36

serve as energy-yielding and transport molecules
* lactose found exclusively in milk and is an important energy source for young mammals. it is digested slowly to give a slow steady release of energy
* sucrose is the form in which carbohydrate is transported in plants

Question 37

describe the structure of starch

Answer 37

• polymer of alpha-glucose and consists of amylose and amylopectin
• amylose is formed by condensation reactions between many alpha-glucose molecules, glucose residues are linked by alpha-1,4-glycosidic bonds. it is unbranched and each amylose chain is coiled into a helix. there is no cross-links between amylose chains as most OH groups of glucose residues project into interior of the helix. iodine is able to enter into the helices of amylose to form the blue-black complex
• amylopectin is formed by condensation reactions between many alpha-glucose molecules linked by alpha-1,4-glycosidic bonds. it is also coiled into a helix with no cross-linking between chains. however, amylopectin chains are shorter than in amylose and it has a branched structure, with branches formed by alpha-1,6-glycosidic bonds

Question 38

describe the functions of starch

Answer 38

• energy store in plants, from excess of any glucose produced during photosynthesis
• when needed, it is released from starch granules and degraded by enzymes to release glucose, which is the main respiratory substrate

Question 39

describe the structure of glycogen

Answer 39

• made of chains of alpha-glucose, which are linked by alpha-1,4-glycosidic bonds and alpha-1,6-glycosidic bonds at the branches
• it is helical and branched, with no cross-linking between glycogen molecules
• similar in structure to amylopectin but has shorter chains and is more highly branched

Question 40

describe the functions of glycogen

Answer 40

serves as energy store in animal cells

Question 41

relate the structure of starch and glycogen to their functions

Answer 41

• both are composed of several hundreds/thouands of glucose molecules linked by glycosidic bonds. upon hydrolysis, a large number of glucose molecules, which are the main respiratory substrates, are released to produce energy
• both are composed of several hundreds/thouands of glucose molecules linked by glycosidic bonds. for each molecule, most of the hydrophilic OH groups of glucose residues project into interior of the helices and form hydrogen bonds with one another. each molecule is verylarge. hydroxyl groups are not available to form hydrogen bonds with water so both are insoluble in water. they can be stored in large quatities without affecting the osmotic potential of cells, and do not easily diffuse out of cells
• molecules are helical in structure. amylopectin and glycogen are highly branched. helical and branched nature of chains allow for extensive coiling and entangling, thus allowing them to be compacted. this enables greater amount of carbohydrates can be stored per unit volume. branched structure of amylopectin and glycogen also allows many enzymes to act on it at any one time. they can be hydrolysed to release glucose quickly so that rate of respiration is increased

Question 42

describe the structure of cellulose

Answer 42

• polymer of beta-glucose molecules linked by beta-1,4-glycosidic bonds where adjacent residues are rotated 180 degrees, forming a straight chain of beta-glucose
• OH groups project outwards from each chain in all directions and form cross-links via hydrogen bonds with neighbouring chains. there are no available hydroxyl groups to form hydrogen bonds with water, hence cellulose is insoluble in water
• straight chains lie parallel to each other, such that cross-linking can occur along the entire length of the chains and bind the chains rigidly together, resulting in high tensile strength
• chains are bundled to form microfibrils, which are arranged in larger bundles to form macrofibrils

Question 43

relate the structure of cellulose to its functions

Answer 43

• each cellulose molecule contains about 10,000 residues, thus it is a large macromolecule. the hydrophilic OH groups of the glucose residues are involved in forming cross-links with adjacent chains. hydroxyl groups are not available to form hydrogen bonds with water, cellulose is insoluble in water
• adjacent beta-glucose molecules are rotated 180 degrees to form a straight chain. this allows the chains to cross-link with each other by forming hydrogen bonds between their OH groups, which are projected outwards on both sides of each chain. cellulose chains are bundled to form microfibrils, which are further arranged in larger bundles to form macrofibrils. macrofibrils are arranged in several layers in cell walls in a glue-like matrix made up of other polysaccarides. cellulose has tremendous tensile strength. cellulose in cell walls prevents cells from bursting when water enters by osmosis and also helps determine the shape of cells. the cellulose cell wall also allows for the development of turgidity, which help to support plants that lack wood
• the layers of cellulose are fully permeable to water and dissolved solutes and the gaps between layers of cellulose form channels. these channels can be filled with lignin. this allows for the passage of water and dissolved solutes which are important for the functioning of plant cells. lignified cellulose cell wall provides extra tensile strength

Question 44

describe the structure of triglycerides

Answer 44

• each triglyceride is made by condensation reactions between three fatty acids and a glycerol molecule, with the removal of water molecules
• condensation reaction results in the formation of ester bonds
• triglycerides with unsaturated fatty acids tend to be liquid at room temperature because the double bond introduces kinds into the fatty acid chains which prevent the triglycerides from close packing. these triglycerides are called oils
• triglycerides with longer fatty acid chains/saturated fatty acids, re more likely to be solid at room temperature because their fatty acid chains can pack closely together. these triglycerides are called fats

Question 45

describe the functions of triglycerides

Answer 45

• long term energy store. triglycerides have a much higher hydrogen to oxygen ratio than the same mass of carbohydrates. hence there are more energy rich C-H bonds which can be oxidised during respiration to produce ATP. hence, triglycerides contain much more energy per gram than either carbohydrates or proteins (triglycerides have a higher calorific value). triglycerides can be stored as droplets inside cell where their insolubility is very useful as they do not disperse into the cytoplasm. triglycerides act as long-term energy stores as they are oxidised only after carbohydrates are depleted
• excellent heat insulator (prevent excessive loss of heat)
• provide buoyancy as they are less dense than water
• mechanical protection
• source of metabolic water
• solvent for fat-soluble vitamins

Question 46

describe the structure and properties of phospholipids

Answer 46

• a phosphate group is attached to two fatty acids (hydrophobic)
• the phosphate group ionises and has a negative charge (hydrophilic)
• additional small molecules (charged/polar) can be linked to the phosphate group to form a variety of phospholipids
• amphipathic

Question 47

describe the functions of phospholipids

Answer 47

• formation of membranes: amphipathic nature of phospholipids enables them to form a phospholipid bilayer (main component of cell membranes). hydrophobic hydrocarbon tails are orientated such that a hydrophobic core is formed. thus, polar molecules/ions will not be able to pass through them
• regulate membrane fluidity (degree of saturation in fatty acid chains): phospholipids with saturated hydrocarbon tails can pack together closely and prevent membrane from becoming too fluid even at high temperatures. phospholipids with unsaturated hydrocarbon tails have kinks in the tails where the double bond is located. thus, the unsaturated hydrocarbon tails cannot pack together as closely as saturated ones and remains fluid even at low temperatures
• formation of glycolipids: oligosaccharides can associate with membrane lipids to form glycolipids, which function as markers that distinguish one cell from another (cell-cell recognition and cell-cell adhesion)

Question 48

describe the structure of cholesterol

Answer 48

consists of a carbon skeleton with four fused rings and a hydroxyl group at one end

Question 49

describe the functions of cholesterol

Answer 49

• maintaining membrane fluidity: OH group on cholesterol interacts with the charged phosphate group of membrane phospholipids. carbon skeleton of four fused rings and the hydrocarbon chain are embedded in the membrane, alongside the non-polar fatty acid chains. at relatively higher temperatures, cholesterol makes the membrane less fluid by restraining the movement of phospholipids. at low temperatures, cholesterol hinders solification by disrupting the regular packing of phospholipids
• decreases permeability of membrane to polar molecules and ions
• synthesis of other steroids
• synthesis of bile salts (act as an emulsifying agent and aids in digestion of fats)