Longer Processes

Question 1

describe the induced fit model of enzyme action

Answer 1

1. substrate binds to active site
2. enzyme active site changes shape to substrate
3. puts pressure on bonds and bends them
4. lowers activation energy
5. products removed and enzyme returns to original shape

Question 2

describe the process of semi-conservative replication

Answer 2

1. DNA helicase breaks hydrogen bonds between complementary base pairs between the polynucleotide strands causing the DNA double helix to unravel
2. each separated parental DNA strand acts as a template
3. free floating DNA nucleotides within the nucleus are attracted to the complementary base pairs on the template strands
4. adjacent nucleotides joined together by condensation reaction to form phosphodiester bonds - catalysed by DNA polymerase
5. hydrogen bonds form between bases of original and new strands

Question 3

outline what happens in prophase

Answer 3

1. chromosomes condense and become visible
2. nucleolus disappears
3. centrioles separate and move towards opposite poles of cell (animals only)

Question 4

outline what happens in metaphase

Answer 4

1. chromosomes align along equator of cell
2. spindle fibres released from poles attach to centromere + chromatids

Question 5

outline what happens in anaphase

Answer 5

1. spindle fibres retract and pull the centromere + chromatids they are bound to towards opposite poles - requires ATP
2. causes centromere to divide in 2

Question 6

outline what happens in telophase and cytokinesis

Answer 6

1. chromosomes become longer and thinner again
2. spindle fibres disintegrate + nucleolus starts to reform
3. cytoplasm divides in 2

Question 7

outline the cotransport of glucose and sodium ions in the ileum

Answer 7

1. Na ions actively transported out of epithelial call and into blood, reducing conc of Na ions in epithelial cell
2. Na ions diffuse from lumen down conc gradient and into epithelial cell
3. Na ions diffuse through a cotransporter protein, so either glucose or amino acids also attach + are transported into epithelial cell against their conc gradient
4. glucose moves by facilitated diffusion from epithelial cell and into blood

Question 8

outline the process of phagocytosis

Answer 8

1. chemicals/debris released by pathogens attract phagocytes causing them to move toward cells
2. phagocytes attach to the chemicals via receptors
3. phagocytes change shape to move around + engulf pathogen
4. pathogen is contained within a phagosome vesicle
5. a lysosome within the phagocyte will fuse with the phagosome + release its contents
6. lysozyme enzyme is released into the phagosome
7. pathogen is destroyed
8. soluble products are absorbed and used by phagocyte

Question 9

outline the process of cell-mediated response

Answer 9

1. once pathogen engullfed + destroyed by phagocyte, the antigens are positioned on the cell surface - is now an APC
2. helper T cells attach to the antigens via receptors
3. once attached, this stimulates the helper T cells to divide by mitosis, producing large numbers of clones
4. cloned helper T cells differentiate into different cells

Question 10

outline the process of humoral response

Answer 10

1. antigens in blood collide with complementary antibody on B cell
2. B cell takes in antigen by endocytosis + presents it on cell surface
3. B cell collides with helper T cell receptor, which activates B cell to go through clonal selection
4. B cells undergo mitosis to make large numbers of cells which then differentiate into memory or plasma cells
5. plasma cells make antibodies
6. B memory cells can divide rapidly into plasma cells when reinfected with same pathogen to make large numbers of antibodies rapidly

Question 11

outline the replication of HIV in helper T cells

Answer 11

1. HIV transported around in blood until it attaches to a CD4 protein on the helper T cells
2. HIV protein capsule fuses with helper T cell membrane, enabling the RNA and the enzymes (reverse transcriptase) from HIV to enter
3. reverse transcriptase copies the viral RNA into a DNA copy + moves to the helper T cell nucleus
4. here, mRNA is transcribed + the helper T cell starts to create viral proteins to make new virus particles

Question 12

outline the process of the ELISA test

Answer 12

1. virus proteins (antigens) are added to well of a 96 well plate
2. the antigens bind to the plastic, coating the bottom of the wells
3. the primary antibody is added to the well - these are from the patient’s serum sample
4. excess antibodies washed away, leaving only antibodies bound to the antigens - prevents non-specific binding
5. a secondary antibody complementary to the first is added to the wells, also has an enzyme attached
6. excess secondary antibody washed away, removing any unbound antibodies
7. substrate added to wells
8. secondary antibody containing enzyme will cause a colour change when exposed to the substrate
9. presence of colour indicates presence of antibodies in test sample, meaning the patient will have been exposed to the antigen

Question 13

outline the process of inspiration

Answer 13

1. external intercostal muscles contract, pulling ribs upwards and outwards
2. internal intercostal muscles relax
3. diaphragm contracts downwards from dome position
4. lung volume increases
5. air pressure in lungs (pulmonary pressure) is lower in comparison to atmospheric pressure
6. air moves into lungs from atmospheric pressure to lower pressure

Question 14

outline the process of expiration

Answer 14

1. external intercostal muscles relax
2. internal intercostal muscles contract, pulling ribs downwards and inwards
3. diaphragm relaxes + returns from dome position
4. air pressure in lungs (pulmonary pressure) is higher in comparison to atmospheric pressure
5. lung volume decreases to increase the pressure
6. air moves out of lungs

Question 15

outline the movement of water up the xylem

Answer 15

1. water evaporates out of stomata - loss of water creates a lower pressure and lowers water potential
2. water is pulled up by the xylem to replace it due to the negative pressure
3. cohesion created by hydrogen bonds causes water to form a column within the xylem
4. water molecules adhere to the walls of the xylem which helps to pull the water column upwards
5. pulling up of water creates tension, pulling the xylem in to become narrower, which increases the root pressure so more water is drawn up

Question 16

Describe the cohesion-tension theory of water transport in the xylem

Answer 16

1. water lost from leaf due to transpiration
2. this lowers the water potential of leaf cells
3. water pulled up the xylem creating tension
4. water molecules cohere by hydrogen bonds
5. this forms a continuous water column
6. water molecules adhere to the walls of the xylem, helps to pull column upwards
7. pulling up of water creates tension, pulling the xylem in to become narrower - increases root pressure so more water drawn up

Question 17

outline the process of transcription

Answer 17

1. DNA helicase breaks hydrogen bonds between bases, helix unwinds and bases are exposed
2. only one strand of the DNA acts as a template
3. free mRNA nucleotides in nucleus align opposite exposed complementary DNA bases
4. RNA polymerase joins together adjacent RNA nucleotides by forming phosphodiester bonds
5. pre-mRNA is formed
6. introns spliced out of pre-mRNA, leaving only exons

Question 18

outline the process of translation

Answer 18

1. once modified mRNA has left nucleus, it associates with a ribosome in the cytoplasm
2. ribosome moves to start codon
3. tRNA brings specific amino acid
4. tRNA anticodons bind to complementary mRNA codons
5. ribsome moves along to next codon
6. amino acids join by peptide bonds with use of ATP
7. continues until ribosome reaches stop codon - will detach

Question 19

describe the mass flow hypothesis for the mechanism of translocation in plants

Answer 19

1. at source sucrose is actively transported into the sieve tube element
2. by companion cells
3. lowers water potential in sieve tube element + water enters by osmosis from xylem
4. produces high hydrostatic pressure
5. mass flow towards sink as sucrose moves down hydrostatic pressure gradient
6. at sink sugars are unloaded + either used in respiration or converted for storage

Question 20

outline the digestion of lipids

Answer 20

1. lipids are emulsified by the bile
2. lipases hydrolyse triglycerides into fatty acids and monoglycerides by hydrolysing the ester bonds in lipids
3. lipases are made in the pancreas and work in the small intestine
4. bile salts produced in liver emulsify lipids into small lipid droplets - this increases the surface area for faster hydrolysis by lipases
5. after being hydrolysed, the monoglycerides and fatty acids associate with the bile salts to form micelles

Question 21

outline the absorption of lipids

Answer 21

1. micelles contain bile salts, fatty acids and monoglycerides
2. micelles make fatty acids and monoglycerides more soluble in water.
3. fatty acids and monoglycerides are carried by micelles to cell lining of the ileum
4. this maintains a higher conc of fatty acids at the lining of the ileum so they are reabsorbed by diffusion into microvilli in epithelial cells
5. fatty acids and monoglycerides link to reform triglycerides
6. triglycerides associate with lipoproteins + cholesterol inside the golgi apparatus to form chylomicrons in the golgi apparatus
7. vesicles move to cell membrane
8. chylomicrons leave the epithelial cell move into a lacteal (lymph capillary) which transports them to the rest of the body

Question 22

outline the digestion of carbohydrates

Answer 22

1. amylase produced by pancreas + salivary glands hydrolyses polysaccharides into maltose through the hydrolysis of glycosidic bonds
2. digestion begins in mouth, continues in duodenum and is completed in ileum
3. sucrase + lactase (membrane bound enzymes) hydrolyse sucrose and lactose into monosaccharides