Paper 1: Long processes

Question 1

Semi-conservative replication

Answer 1

DNA helicase breaks the hydrogen bonds between the complementary base pairs between the two strands within the double helix
Causes the double helix to unwind
Each strand of the parental DNA separate and can act as a template strand
Free floating DNA nucleotides in the nucleoplasm are attracted to their complementary base pairs on the template strands of the parental DNA
DNA polymerase catalyses the formation of the phosphodiester bond by a condensation reaction between the adjacent nucleotides
2 sets of daughter DNA contains one strand of the parental DNA and one newly synthesised strand

Question 2

Mitosis

Answer 2

G1- growth
S- DNA synthesis
G2- growth and preparation
Mitosis:
Prophase: chromosomes condense and become visible, nuclear membrane breaks down
Metaphase: chromosomes align along the equator of the cell, chromosomes attach to the spindle fibres via the centromere
Anaphase: spindle fibres contract and shorten, centromere divides into two and pulls the chromatids to the opposite poles of the cell
Telophase: chromosomes at each pole of the cell become longer and thinner and visible, spindle fibres disintegrate and nuclear membrane begins to reform
Cytokinesis: cytoplasm splits

Creates 2 genetically identical cells

Question 3

Binary fission

Answer 3

Circular dna and plasmids are tightly coiled
Circular dna uncoils and duplicates, plasmids also duplicate
Cytoplasm elongates, separating the DNA, each pole of the cell receives a full set of dna and plasmids
Cytoplasm begins to split, equatorial plate begins to form
Cell wall beings to form and the cell splits into two new daughter cells
Genetically identical cells

Question 4

Facilitated diffusion

Answer 4

Passive process
Proteins are used to transport molecules
Movement of ions and polar molecules which can’t use simple diffusion
Uses proteins channels and carrier proteins
Protein channels form tubes filled with water and allows water soluble ions to pass through the membrane, still selective as the channel proteins only open in the prescience of certain ions when they bind to the protein
Carrier proteins will bind with a molecule which causes a change in the shape of the protein

Question 5

Active transport

Answer 5

Uses a carrier proteins spanning the cell membrane
Molecule binds to a receptor complementary to the shape of the protein
ATP binds to the carrier protein from the inside of the cell and is hydrolysed into ADP and Pi
The attached Pi causes the carrier protein to change shape and release the molecule into the other side
Phosphate ion is then released and the protein returns to its original shape

Question 6

Co-transport of glucose and sodium in the ileum

Answer 6

Normally more glucose in the epithelial cells than in the lumen
Sodium ions are actively transported out of the epithelial cells into the blood
Reduces the concentration of sodium ions in the epithelial cell
Sodium ions can then diffuse from the lumen down their concentration gradient into the epithelial cell
Protein the sodium ions diffuse through is a co-transporter protein, so either glucose or amino acids also attach and are transported into the epithelial cell against their concentration gradient
Glucose then moves by facilitated diffusion from the epithelial cell to the blood

Question 7

Phagocytosis

Answer 7

Pathogens release chemoattractants which attract the phagocytes causing the phagocyte to move towards the pathogen
Receptors on the phagocyte will bind non specifically to the antigens on the pathogens surface membrane
Phagocyte changes shape and engulfs the pathogen
Forms a phagosome
Lysosome within the phagocyte will fuse with the phagosome, releasing lysozymes
This lytic enzyme hydrolyses the pathogen
Soluble products are absorbed and the antigens are processed and presented on the phagocytes cell surface membrane

Question 8

Cell mediated response

Answer 8

T CELLS
T cells respond to antigens on the surfaces of cells (infected body cells, macrophage which is presenting the processed antigens, transplanted cells, cancer cells)

Once a pathogen has been engulfed and destroyed by a phagocyte the antigens are positions on the cell surface, now an antigen presenting cell
Helper T cells have receptors on their surface which bind to the antigens on APC
Once attached this activates the helper T cell to divide by mitosis to replicate an make large numbers of clones
Cloned helper T cells differentiate into different cells:
- some remain as helper T cells and activate b lymphocytes
-some stimulate macrophages to perform more phagocytosis
- some become memory cells for that antigen
-some become cytotoxic T cells

Question 9

Cytotoxic T cells

Answer 9

Destroy abnormal or infected cells
Release performing which embeds in the cell surface membrane and makes a pore
So any substances can enter and leave the cells
Causes cell death

Question 10

Humoral response

Answer 10

B cells must be activated:

• antigens in the blood collide with their complementary antibody on a B cell
• b cell takes in the antigen by endcytosis and presents it on its own cell surface membrane
• b cell collides with a helper T cell and activates clonal selection
• undergo mitosis to make large amounts of cells which differentiate into plasma or memory cells

Plasma cells make antibodies
Memory cells rapidly divides into plasma cells when re-infected with the same pathogen to make large numbers of ntibodies rapidly

Question 11

HIV replication

Answer 11

Following infection HIV enter bloodstream and circulates around body
Protein on hiv binds to cd4 proteins on t helper cells
Protein capsid fuses with the cell surface membrane, rna ad enzymes of hiv enters the t helper cells
HIV reverse transcriptase converts viral rna to dna
DNA moves into t helper cells nucleus via nuclear pore and inserted into cells dna
HIV dna in nucleus creates messenger rna using cells enzymes, contains instructions for making new viral proteins and rna to get into new hiv
mRNA passes out of nucleus through nuclear pore and uses cell’s protein synthesis mechanisms to make HIV particles
HIV buds away from t helper cells with a piece of its cell surface membrane forming lipid envelope

Question 12

ELISA test

Answer 12

Apply sample to surface to all the antigens will attach
Wash surface several times to remove any unattached antigens
Add antibody that is specific to the antigen we are trying to detect and leave the two to bind together
Wash surface to remove excess antibody

Question 13

Inspiration

Answer 13

External intercostal muscles contract while internal intercostal muscles relax
Ribs are pulled upwards and outwards increasing the volume of the thorax
Diaphragm muscles contract causing it to flatten, also increases the volume of the thorax
Decreased thoracic pressure
Atmospheric pressure increases
Atmospheric pressure greater than pulmonary pressure so air is forced into the lungs

Question 14

Expiration

Answer 14

Internal intercostal muscles contract, external intercostal relax
Ribs move down and inwards
Decreases thoracic volume
Diaphragm muscles relax ad pushed up into dome shape
Volume of thorax is further decreased
Decreased volume of the thorax increased the pressure in the lungs
Pulmonary pressure is now greater than that of the atmosphere, air is forced out of the lungs

Question 15

Salivary digestion

Answer 15

Saliva enters mouth from salivary glands and is thoroughly mixed with the food during chewing
Saliva contains salivary amylase, starts to hydrolyse any starch to maltose, contains mineral salts to maintain ph
Food swallowed and enters stomach where conditions are acidic, acid denatures the amylase and prevents further hydrolysis of the starch
Food passed into small intestine, mixed with pancreatic juice
Pancreatic juice=pancreatic amylase, continues the hydrolysis of any remaining starch to maltose, alkaline salts are produced to maintain ph
Muscles in intestinal wall push food along the ileum, produces maltose, called membrane bound disaccharidase, maltose hydrolyses maltose

Question 16

Emulsification

Answer 16

Lipid digestion

Bile salts break down lipids into micelles

Question 17

Protein digestion

Answer 17

Endopeptidases hydroylses peptide bonds between amino acids in the central region of protein molecule forming series of peptide molecules
Exopeptidases hydrolyse peptide bonds on the ends of amino acids formed by endopeptidases, progressively release dipeptides and single amino acids
Dipeptidases hydrolyse the bond between the two amino acids of a dipeptide, membrane bound and part of the cell surface membrane of the epithelial cells lining the ileum

Question 18

Absorption of triglycerides

Answer 18

Once formed during digestion monoglycerides and fatty acids remain in association with bile salts, called micelles
Through movement of material within lumen of the ileum, micelles come into contact with epithelial cells lining villi
Micelles break down, releasing monoglycerides and fatty acids, as these are non-polar they easily diffuse across cell surface membrane into epithelial cells
Once inside the monoglycerides and fatty acids are recombined into triglycerides
Associate with cholesterol and lipoproteins to form chylomicrons
Move out of the epithelial cells by exocytosis, enter lymphatic capillaries called lacteals

Question 19

Loading and unloading of oxyge n

Answer 19

At gas exchange surface carbon dioxide is constantly being removed
Ph is slightly raised due to low CO2 conc
Higher ph changes shape of haemoglobin into one that enables it to load oxygen readily
Shape increases the affinity of haemoglobin for oxygen so isn’t released while being transported in the blood to the tissues
In tissues the carbon dioxide is produced by respiring cells
Carbon dioxide is acidic in solution so ph of blood within the tissues is lowered
Changes the shape of the haemoglobin into one with lower affinity for oxygen
Haemoglobin releases oxygen into respiring tissues

Question 20

cardiac cycle

Answer 20

diastole:
- blood returns to atria through pulmonary vein and vena cava
- atria fill and pressure increases
- when pressure exceeds pressure in the venctricles the atrioventricular valves open
- blood passes into the ventrciles aided by gravity
- muscular walls are relaxed at this stage, ventricle walls recoil and reduce pressure in the venctricles,
- causes lower pressure than that in the aorta and pulmonary artery
- semi-lunar valves close

atrial systole:

• contraction of the atrial walls, recoil of the ventricle walls
• forces remaining blood into ventricles from atria
• muscle of the ventricle walls remains relaxed

ventricular systole:

• delay to allow the ventricles to fill with blood
• walls contract simultaneously
• increases blood pressure within them forcing shut the atrioventricular valves and prevents the backflow of blood into the atria
• atrioventricular valves closed the pressure in the ventricles rises further
• once it exceeds that in the aorta and pulmonary artery, blood is forced from ventricles into these vessels
• ventricles have thick muscular walls, meaning they can contract forcefully, creating high pressure necessary to pump blood around the body

Question 21

aortic pressure

Answer 21

rises when ventricles contract as blood is forced into the aorta
gradually falls but never below 12kpa
due to the elasticity of its wall which creates a recoil action
essential if blood is to be constantly delivered to tissues
produces a temporary rise in pressure at start of the relaxation phase

Question 22

ventricular pressure

Answer 22

low at first but gradually increases as the ventricles fill with blood as the atria contract
left atrioventricular valve close and pressure rises dramatically as thick muscular walls of ventricles contract
as pressure rises above that of the aorta, blood is forced into the aorta past the semilunar valves
pressure falls as the ventricles empty and walls relax

Question 23

atrial pressure

Answer 23

always relatively low because thin walls of the atrium can’t create much force
highest when contracting but drops when left atrioventricular valve closes and its walls relax
atria fill with blood which leads to a gradual build up of pressure until a slight drop when left atrioventricular valve opens and some blood moves into the ventricle

Question 24

ventricular volume

Answer 24

rises as the atria contract and ventricles fill with blood
drops suddenly as blood is forced out into the aorta when semi-lunar valve opens
volume increases again as ventricles fill with blood

Question 25

formation of tissue fluid

Answer 25

blood pumped by the heart passes along arteries and arterioles and then capillaries
heart pumping creates hydrostatic pressure at arterial end
causes tissue fluid to move out of the blood plasma
outward pressure is opposed by hydrostatic pressure of tissue fluid outside capillaries which resists outward movement of liquid and the lower water potential of the blood due to the plasma proteins causes water to move back into the blood
combined effect produces an outwards pressure that pushes tissue fluid out at arteriole end
only forces small molecules out

Question 26

return of tissue fluid

Answer 26

loss of tissue fluid from capillaries reduces hydrostatic pressure within them
by time blood has reached venous end of capillary network its hydrostatic pressure is usually lower than that of tissue fluid outside of it
tissue fluid forced back into capillaries by higher hydrostatic pressure outside them
plasma has lost all water and still contains proteins, lower water potential than tissue fluid
water leaves tissue by osmosis down water potential gradient

Question 27

movement of water across cells of a leaf

Answer 27

mesophyll cells lose water to air spaces by evaporation due to heat supplied by the sun
cells have a lower water potential and so water enters by osmosis from neighbouring cells
loss of water form these neighbouring cells lowers their water potential
take in water from neighbouring cells by osmosis

Question 28

cohesion-tension

Answer 28

water evaporates from mesophyll cells due to heat from the sun leading to transpiration
water molecules form hydrogen bonds between one another and hence stick together, cohesion
water forms a continuous unbroken column across the mesophyll cells and down the xylem
as water evaporates from mesophyll cells in the lead into the air spaces beneath the stomata, more molecules of water are drawn up behind it as a result of this cohesion
column of water is therefore pulled up into the xylem as a result of transpiration, transpiration pull
transpiration pull puts xylem under tension so negative pressure within the xylem

Question 29

mass flow theory

Answer 29

1. transfer of sucrose into sieve elements from photosynthesising tissue
   - sucrose made from products of photosynthesis in cells with chloroplasts
   - sucrose diffuses down concentration gradient by facilitated diffusion from photosynthesising cells into companion cells
   - hydrogen ions are actively transported from companion cells into spaces within cell walls using atp
   - hydrogen ions diffuse down conc gradient through carrier proteins into the sieve tube elements
   - sucrose transported along with hydrogen ions by co-transport
2. mass flow of sucrose through sieve tube elements
   -sucrose produced by photosynthesising cells is actively transported into sieve tubes
   -causes sieve tubes to have a lower water potential
   -xylem has a much higher water potential so water moves from xylem to sieve tubes by osmosis, creates high hydrostatic pressure
   -respiring cells sucrose is used in respiration or converted to starch
   -cells have low sucrose content so sucrose actively transported from sieve tubes, lowering water potential
   -water also moves in by osmosis
   -lowers hydrostatic pressure
   -high hydrostatic pressure at source and low at sink
   mass flow of sucrose down hydrostatic gradient
3. transfer of sucrose from sieve tube elements into storage or other sink cells
   - sucrose is actively transported by companion cells out of sieve tubes into sink cells

Question 30

transcription

Answer 30

dna helicase unwinds double helix and breaks hydrogen bonds between the bases
each strand can act as template strands
free floating nucleotides in the nucleoplasm line up with their complementary base on the template strand
rna polymerase catalyses the formation of the sugar phosphate backbone
forms pre-mrna
pre-mrna then spliced and introns are removed
mrna leaves nucleus via nuclear pore

Question 31

translation

Answer 31

ribosome attaches to the starting codon on the end of an mrna molecule
trna with complementary anticodon moves to the ribosome and pairs with the codon on mrna, trna carries a specific amino acid
trna with the anticodon then binds to next codon on mrna, carries a specific amino acid
2 amino acids are joined by a peptide bond using enzyme and atp
synthesises a polypeptide
chain until stop codon is produced and is released

Question 32

chromosome mutations

Answer 32

changes in the whole set: 3 or more sets of chromosomes in an organism rather than normal 2, called polyploidy and normally occurs in plants
changes in individual chromosomes: homologous pairs fail to separate in meiosis, called non-disjunction and normally results in gamete having one more or less chromosomes

Question 33

independent segregation

Answer 33

in meiosis 1 each chromosome lines alongside its homologous partner
when they arrange themselves it is random
one of each pair will go to daughter cell
combination is random so combination of maternal and paternal is a matter of chance

Question 34

crossing over

Answer 34

chromatids of each pair twist round each other
tensions are created and portions of chromatids break off
broken portions may rejoin with chromatids of its homologous partner
usually is the equivalent portion of homologous chromosomes that are exchanged
new genetic combinations of maternal and paternal alleles are produced

Question 35

natural selection

Answer 35

random mutation in an allele of a certain gene
may posses an advantage
individuals are better adapted
more likely to survive and reproduce
and pass on the allele to their next generations
this generation more likely to survive and reproduce as they are more advantageous
at the expense of the individuals with the less favourable alleles
frequency of the favourable allele increases