Bio consolidation

Question 1

Describe the differences in the role of the vesicles that fuse with the forming face and the vesicles that are formed at the maturing face. [4]

Answer 1

Cis face

• Vesicles contain proteins and lipids from transported ER and sER that will undergo chemical modification within the golgi body Examples of modification: glycosylation, phosphorylation

Trans face

• Packaging, sorting and transport function: vesicles containing modified products will be transported to the cell membrane where they fuse and release the products to the outside of the cell via exocytosis

Question 2

Outline how enzymes are packaged into vesicles and release to the outside of the cell [4]

Answer 2

(1) Enzymes synthesised in rER are packaged into transport vesicles pinched off from rER and fuse with cis/forming face of Golgi apparatus

(2) At Golgi apparatus, modification, sorting and packaging of enzymes occur;

(3) Secretory vesicles (A: Golgi vesicles) containing enzymes buds off trans/maturing face of Golgi apparatus;

(4) Secretory vesicles move to and fuse with cell surface membrane, releasing the enzymes via exocytosis; [Points 5 and 6 – 1m max]

**(5) Vesicles move towards cell surface membrane with the help of microtubules;

(6) ATP is required for exocytosis;

Question 3

Phospholipid

Answer 3

1. A phospholipid consists of 1 phosphoric acid / phosphate group and 2 fatty acids combined with 1 glycerol phosphoester bonds; (R: if quantity of components not stated)
2. Each fatty acid forms an ester bond with glycerol (total of 2 ester bonds) and phosphoric acid forms a phosphoester bond with glycerol;

Question 4

Explain how the structure and properties of triglyceride is related to its role in living organisms [3]

Answer 4

1. 3 long hydrocarbon chains/ large number of carbon-hydrogen bonds for energy store/ which can be oxidised to provide energy/ form ATP;
2. The hydrocarbon chains are also hydrophobic / non-polar which enables the molecule to be insoluble in water such that it will not affect the water potential of the cells;
3. Being non-polar / dehydrated means triglyceride can be packed compactly within cells as energy store;
4. AVP – maximum two

● Large number of hydrogen atoms which yields metabolic water upon oxidation

● Less dense than water which provides buoyancy to aquatic mammals

Question 5

Describe how the structure of glycogen/ starch relates to its function as an energy storage molecule. [3]

must mention storage molecule in answer

Answer 5

Folding of glycogen molecule such that OH grps are in the the interior of the the molecule
* Insoluble in water so does not exert osmotic effect on the cell.
Glucose residues joined by −1,4-glycosidic bonds
* The bonds can be hydrolysed by enzymes.
* The bonds results in helical coil so that structure is more compact for storage.
Anomeric carbon of each glucose monomer is involved in glycosidic bond formation
* Stable compound/unreactive
Amylopectin/glycogen highly branched
* Branching provides many sites for enzymes to act on, allows rapid release of glucose

Question 6

Describe how the structure of phospholipid contributes to its function in membranes [3]

focus is on membrane as a whole

Answer 6

(1) hydrophilic phosphate head interacts with aqueous medium inside and outside the cell

(2) hydrophobic hydrocarbon tails interact with each other via hyrdeophobic interactions forming the hydrophobic core which repels charged/ hydrophilic molecules from passing through

(3) unsaturated fatty acids within the tails possess kinks which prevent close packing of phospholipid thus increasing permeability of membrane to non-polar molecules

Question 7

Explain the model

fluid mosaic

Answer 7

ppl molecules arranged in bilayer, fatty acid tails interact with eo via weak hydrophobic interactions –> lateral movement in bilayer –> membrane is fluid

cholesterol scattered unevenly in the bilayer

transmembrane protein embedded in the bilayer making the membrane asymmetrical –> mosiac structure

Question 8

Function of Cholesterol,
Transmembrane proteins & Glycoproteins

Answer 8

Cholesterol - regulation of membrane fluidity
Transmembrane proteins

Channel protein/ carrier protein for transport of hydrophilic / polar / charge substances across the membrane via specific channels

Glycoproteins
Serving as specific cell surface markers for cell-cell recognition processes;;

Cell-cell adhesion for purposes such as forming tissues

Question 9

Explain how the structure of a phospholipid molecule makes it suitable for its function in cell membranes.

focus is on phospholipid/ ability to interact with surrounding proteins

Answer 9

(1) Non-polar hydrocarbon chains of phospholipid bilayer can form hydrophobic interactions with non-polar R groups of amino acid residues found on exterior surface of proteins

(2) Charged phosphate head of phospholipid bilayer/aqueous solutions on either side of membrane can interact (e.g hydrogen bond, ionic bond) with charged /polar R groups of **amino acid residues **found on the exterior surface of proteins

Question 10

functions of membranes in general

Answer 10

Phospholipid bilayer of organelles serve to compartmentalise the substances transported and provide optimum internal environment in vesicle ——> substances do not get affected by reactions in the cell

Question 11

Explain how the structure of the vesicle allows it to serve its function shown in Fig. 1. 1. [3]

Answer 11

1. Structure of vesicle: made up of a phospholipid bilayer;
2. Phospholipids (PL) provides fluidity, which allows for fusion of vesicle membrane with cell surface membrane for release of enzymes to the outside of the cell;
3. The hydrophilic phosphate heads of the PL interact with the aqueous environment/cytoplasm and within the vesicle for stability;

Question 12

Describe how large insoluble Identify the process particles may be taken into a cell.

Answer 12

1. phagocytosis/ endocytosis;;
2. cell surface membrane invaginates;
3. enclosing the large particles;
4. membrane then pinches off;
5. to form a phagocytic vacuole;

Question 13

how are transmembrane proteins held in the membrane?

Answer 13

1. A (is protein) is held in the membrane by weak hydrophobic interactions;;
2. They have regions of hydrophobic amino acid residues of protein which interact with fatty acid tails of phospholipid;;

Question 14

Explain why A is necessary for potassium ions and glucose to pass through cell surface membrane.

Identify the properties of substances: size, charged, polarity

Answer 14

1. Potassium ions are charged;;
2. glucose is polar;;
3. cannot pass through the hydrophobic core of the cell membrane;;
4. transmembrane proteins are specific to the type of substance they transport;;
5. transport the substances against concentration gradient;;

Question 15

Describe how amino acids at different positions in amino acid sequence are brought close together in the structure of protein

IMPTTT

Answer 15

1. Folding of polypeptide chain (primary structure) to give rise to secondary structure,
2. α helix and β-pleated sheet
3. via H bonding between C=O and N-H of polypeptide backbone
4. Further folding into 3 o structure
5. brings amino acid residues that are far apart on the polypeptide chain close together
6. maintained by H bonds, ionic bonds, disulfide bonds and hydrophobic interactions between R groups of amino acid residues

Question 16

Relating structure of cellulose to its function as structural scaffold

Answer 16

Large molecule
* Insoluble in water so does not exert osmotic effect on the cell.
Alternate glucose monomers are inverted Glucose residues are linked by b(1→4) glycosidic bonds
* The bonds results in long, straight chains.
Hydroxyl group project outwards from cellulose chain in all directions
* Allows hydrogen bonds to be formed between chains.
- forms extensive cross linkages in microfibrils and allows microfibrils to arranged in larger bundles to form macrofibrils.
- results in high tensile strength.

Question 17

formation of alpha-1,4 glycosidic bond;

Answer 17

1. The OH group of carbon atom 1; on a -glucose molecule;
2. and the OH group of carbon atom 4; on another -glucose molecule
3. react to form a alpha-1,4 glycosidic bond;
4. with the elimination of 1 water molecule;
5. during a condensation reaction;
6. The reaction is catalysed by enzymes;

Question 18

Explain why different mRNA can result in the same amino acid sequences. [3]

Answer 18

1. Genetic code is degenerate*;
2. Where more than one codon codes for the same amino acid;
3. There are 64 codons and 20 amino acids

Question 19

effect of pH on enzyme

Answer 19

Ionic bonds and hydrogen bonds between R groups are disrupted

Distorts specific 3-dimensional conformation of protein and hence active site is distorted, resulting in denaturation

Question 20

effect of temp on enzyme

Answer 20

(1) High temperature will cause excessive intramolecular vibrations from the high temperature → disrupt intramolecular bonds like hydrogen bonds, ionic bonds, hydrophobic interactions [No disulfate!!] that maintain three-dimensional conformation of the protein

(2) lowered ES complexes → lower rate of enzymatic reaction

Question 21

explain the mode of action of enzyme (relate to induce fit hypothesis)

Answer 21

1. catalyses the hydrolysis (what) to (what)
2. Binding of substrate to the active site of enzyme causes enzyme to have a slight conformational change, resulting in the substrate fitting more precisely in the active site; R

ref. to the induced-fit hypothesis;

3. Formation of the enzyme-substrate complex lowers the activation

energy of triglyceride hydrolysis;

4. The ester bond between formed between (what & what)
   placed under physical stress;
5. And catalytic residues in the active site alter the distribution of electrons within the bond

Question 22

Explain the induced fit mechanism. [2]

Answer 22

1. when substrate enters/binds to the active site*
2. it causes active site to change conformation thus providing a more precise fit
3. this allows catalysis to occur/allows interaction of catalytic R groups of active site with substrate;

Question 23

Explain how the enzyme amino-acyl-tRNA synthetase is specific [2]

imptt

Answer 23

1. Enzyme aminoacyl-tRNA synthetase has a specific active site* that recognizes a tRNA with a specific anticodon* and a specific amino acid
2. Both of which have a shape and charge complementary to its active site
3. This double specificity ensures that there is a correct match between amino acid and tRNA

Question 24

Describe how a primer strand is synthesised [3]

Answer 24

1. Use DNA as a template
2. Ribonucleotides form complementary base pairs with the dna template
3. Formation of phosphodiester bonds between the ribonucleotides
4. Synthesis in the 5’ to 3’ direction

Question 25

Explain how the end replication problem arises.

Answer 25

1. DNA polymerase only extends in a 5’ → 3’ direction
2. unable to fill gap after the last RNA primer is removed at 5’ end of newly synthesised strand
3. as no upstream 3’ OH
4. resulting in shorter daughter strand compared to the template strand

Question 26

State the function of DNA and describe how its properties allow it to perform this function.

Answer 26

1. DNA stores genetic information;;
2. DNA must be chemically stable to encode information without being easily changed by age/nutrition/ environment;;
3. DNA must be able to replicate accurately so that the information can be pass down to the next generation;;

Question 27

function of compartmentalisation

Answer 27

• Provides localised environment to facilitate metabolic processes occurring simultaneously
• allows of development of proton gradient across membrane for the transport of of substances
• isolate harmful substances from the rest of the cell

Question 28

Biological Species Concept

Answer 28

1. A group of organisms capable of interbreeding;
2. To produce viable, fertile offspring;
3. Organisms in one species is reproductively isolated from organisms of another species;
4. Due to reproductive barriers such as physiological or behavioural isolation

Question 29

Ecological Species Concept

Answer 29

A group of organisms sharing the same ecological niche;

And are adapted to the same set of resources; in the same habitat;

No two species can share the same ecological niche;

Question 30

Morphological Species Concept

Answer 30

8. A group of organisms sharing a unique set of structural features;
9. Organisms of one species is morphologically distinct from organisms of another species

Question 31

describe antigen presentation

Answer 31

antigen-presenting cell engulfs bacterial cell via phagocytosis forming phagosome

phagosome fuses with lysosome;

lysosomal enzymes break down antigens on surface of bacterial cell into short peptides;

antigen peptides bind to Class II Major Histocompatability Proteins (MHC) proteins;

inside the vesicle from Golgi apparatus forming peptide-MHC complexes;

peptide-MHC complexes are transported to the** cell surface** of the antigen-presenting cells for antigen presentation;

Question 32

G1 checkpoint

Answer 32

To ensure cell enters cell cycle only in presence of/sufficient growth factors;

and no DNA damage before DNA replication in S phase

Question 33

G2 checkpoint

Answer 33

4. To ensure all DNA are replicated completely and accurately;
5. And appropriate cell size before mitosis occurs;

Question 34

M checkpoint

Answer 34

Ensures that all chromosomes are attached to microtubules at their kinetochores before anaphase;

For proper separation of chromosomes during anaphase;

To ensure genetic stability during cell division;

Question 35

Explain the need for the tight control of the mitotic cell cycle

Answer 35

1. To prevent uncontrolled cell division (resulting in cancer);

Tight control via checkpoints including

(at G1 checkpoint)

2. To ensure cell enters cell cycle only in presence of/sufficient growth factors;
3. and no DNA damage before DNA replication in S phase;

(at G2 checkpoint)

4. To ensure all DNA are replicated completely and accurately;
5. And appropriate cell size before mitosis occurs;

(at M checkpoint)

6. Ensures that all chromosomes are attached to microtubules at their kinetochores before anaphase;
7. For proper separation of chromosomes during anaphase;
8. To ensure genetic stability during cell division;