Paper 2: Long processes

Question 1

light dependent reaction

Answer 1

occurs on the thylakoid membranes

light strikes molecule of chlorophyll, excites a pair of electrons to a higher energy level, chlorophyll is photoionised
electrons are accepted by an electron carrier in the transport chain in the thylakoid membrane
move along the ETC
each electron carrier has a higher affinity than the last for electrons so electrons move down the transport chain
energy released is used to synthesise ATP, photophosphorylation
Photon of light strikes a molecule of water: H2O-> 2H+ + 1/2 O2 + 2e-
Hydrogen reduces NADP to NADPH
Electrons returned to chlorophyll that originally lost them
Oxygen is lost as waste

Question 2

light independent

Answer 2

Occurs in the stroma
Carbon dioxide diffuses through the stomata, dissolves in the water surrounding the mesophyll cells, then diffuses through the cell surface membrane, cytoplasm and chloroplast membrane into the stroma
Carbon dioxide reacts with 5C RuBP catalysed by rubisco
Produces 2 molecules of glycerate 3 phosphate
NADPH reduces GP into triose phosphate using energy from ATP
NADP returned to the light dependent
Some TP are used to reform organic substances (1C is lost)
Most of TP is used using ATP to regenerate RuBP

Question 3

glycolysis

Answer 3

cytoplasm
hydrolyse 2 ATP, phosphorylate glucose to increase reactivity and lower activation energy
glucose splits into 2 x 3C triose phosphates
dehydrogenate TP to form NADH
enzymes convert 2TP to 2 pyruvate, generates 4 ATP

2 ATP, 2 NADH, 2 pyruvate

Question 4

link reaction

Answer 4

matrix
pyruvate is decarboxylated and dehydrogenated to form acetate
combines with coenzyme A to form acetyl coenzyme A

pyruvate + NAD + CoA -> acetyl coA + NADH + CO2

Question 5

krebs cycle

Answer 5

matrix
once per pyruvate, twice per glucose

acetyl coA joins oxaloacetate 4C to form 6C citrate
CoA back to link
citrate is decarboxylated and dehydrogenated, reduces NAD
forms 5C compound
decarboxylated and dehydrogenated into 4C molecule, forms 1 FADH and 3 NADH
produces ATP
4C oxaloacetate joins acetyl CoA

Question 6

oxidative phosphorylation/ETC

Answer 6

H atoms from glycolysis and krebs join NAD and FAD
NADH and FADH donate H+ electrons to first electron transport molecule
releases protons which are actively transported across inner mitochondrial membrane
same time electrons pass along ETC in redox reactions, losing energy which phosphorylated ADP to ATP and lost as heat
protons gather inbetween mitochondrial membranes, diffuse back into matrix through channel proteins
electrons combine with protons and oxygen at end of chain, forming water
oxygen is the final acceptor

Question 7

anaerobic respiration general

Answer 7

neither krebs or etc can continue as all coenzymes are reduced 
no NAD + or FAD to take up H atoms 
only atp produced is in glycolysis 
so NAD+ replenished by pyruvate
pyruvate accepts hydrogen from NADH
NAD+ oxidised reused in glycolysis

Question 8

plants and microorganisms anaerobic respiration

Answer 8

pyruvate decarboxylated losing CO2
forming ethanal
ethanal reduced by H supplied by NADH
forms ethanol

Question 9

animals anaerobic respiration

Answer 9

NADH from glycolysis can accumulate and be removed
pyruvate takes up 2 H from NADH
forming lactate
so NAD+ is regenerated
when oxygen is available lactate is oxidised back to pyruvate

Question 10

Respiration of lipids

Answer 10

hydrolysed into glycerol and fatty acis
glycerol is phosphorylated
converted into TP
TP converted to pyruvate and enters the link and krebs
fatty acids hydrolysed into 2C fragments, converted into acetyl CoA
also joins krebs

release twice as much energy as carbohydrates as they produce lots of hydrogen atoms

Question 11

Respiration of proteins

Answer 11

Hydrolysed into its constituent amino acids
Then deaminated and join the cycle at different points depending on how many carbons are in their carbon chain
3C go to Pyruvate
4C and 5C go to intermediates in Krebs

Question 12

Nitrogen cycle

Answer 12

Nitrogen fixation:

• nitrogen gas into nitrogen containing compounds, can occur naturally when lightning passes through atmosphere
• free-living nitrogen-fixing bacteria, reduce gaseous nitrogen to ammonia, manufactures amino acids, releases nitrogen rich compounds when they die and decay
• mutualistic free-living bacteria, live in the nodules on the roots of plants such as peas and beans, obtain carbohydrates from plant and plant acquires amino acids from bacteria

Ammonification:
-Production of ammonia form nitrogen containing compounds, Sa probiotic microorganisms feed on faeces and dead organisms releasing ammonium ions into the soil

Nitrification:

• plants use light energy to produce organic compounds
• nitrifying bacteria oxidize ammonium ions to nitrite ions NO2- and then nitrite to nitrate NO3-
• requires oxygen, soil must have several air spaces, farmers must ploughing land

Denitrification:

• when soils become waterlogged, low oxygen concentration
• increase in denitrifying bacteria
• anaerobic
• convert nitrates to gaseous nitrogen

Question 13

Phosphorus cycle

Answer 13

Doesn’t have a gaseous phase

• phosphorus exists as phosphate ions PO43- in the form of sedimentary rock deposits
• originate in the seas but brought to the land by uplifting of rocks geologically
• weathering and erosion of these rocks dissolves the ions
• plants absorb the ions
• animals feed on the plants
• excess phosphates are excreted by animals
• plants and animals die and decomposers break them own releasing phosphate ions into the water or the soil
• some ions remain in bones or shells of animals which take long periods of time to break down
• phosphate ions released by decomposition and dissolved out of rocks, transported by streams and rivers into lakes and streams where they form sedimentary rocks

Question 14

Eutrophication

Answer 14

Most lakes and rivers naturally low concentration of nitrate so nitrate ions are the limiting factor for plant and algal growth
Nitrate ion concentration increases as a result of leaching, no longer the limiting factor for plant and algal growth
Algae grow mainly at the surface so alga bloom forms
Dense surface of algae absorbs the light and prevents light penetrating to lower depths
Light then becomes the limiting factor for the growth of plants and algae at lower depths so they eventually die
Lack of dead plants no longer a limiting factor for the growth of saprobiotic bacteria so population grows
Saprobiotic bacteria require oxygen for their respiration creating an increased demand for oxygen
Concentration of oxygen is reduced and nitrates are released from decaying organisms
Oxygen is limiting factor for aerobic organisms
No aerobic organisms then less competition for anaerobic organisms so their population rises
Anaerobic organisms further decompose dead material releasing more nitrates and toxic waste such as hydrogen sulphide, makes water putrid

Question 15

Pacinian corpuscle

Answer 15

Responds to mechanical pressure
When pressure is applied, it becomes deformed and the membrane around its neurone becomes stretched
Stretching widens the stretch mediated sodium channels in the membrane and causes sodium ions to diffuse into the neurone
Influx of sodium ions causes depolarisation producing a generator potential
Generator potential creates an action potential

Question 16

Control of the heart rate

Answer 16

Impulse is sent from the sinoatrial node across both atria causing them to contract
Impulse is sent dow The non-conductive septum
Impulse reaches atrioventricular node
Pause to allow ventricles to fill
Impulse sent up bundle of his through purkinje fibres
Bundle of his conducts impulse through septum to base of ventricles and the purkinje fibres cause the ventricles to contract

Question 17

Control of blood by chemoreceptors

Answer 17

Found in the wall of the carotid arteries
Sensitive to changes in the pH due to changes in the CO2 concentration

Blood has higher than normal concentration of CO2 then the pH is lowered
Chemoreceptors in walls of carotid arteries and aorta detect this and increase the frequency of nervous impulses to the centre in the medulla oblongata that increases the heart rate
Centre increases the frequency of impulses via the sympathetic nervous system in the sinoatrial node, increases the production of electrical impulses by SAN increasing the heart rate
Increased blood flow causes leads to more carbon dioxide being removed by the lungs to the CO2 concentration returns to normal
Ph of the blood rises to normal and chemoreceptors reduce frequency of nerve impulses to the medulla oblongata
Medulla oblongata reduces frequency of impulses to SAN, reduces the heart rate

Question 18

Control of high blood pressure

Answer 18

Pressure receptors transmit more nervous impulses to the centre in the medulla oblongata that decreases the heart rate
Centre sends impulses via the parasympathetic nervous system to the SAN of the heart
Decrease in heart rate

Question 19

Control of low blood pressure

Answer 19

Pressure receptors transmit more nervous impulses to the centre in the medulla oblongata that increases the heart rate
Centre sends impulses via the sympathetic nervous system to the SAN
Increases thr heart rate

Question 20

Resting potential

Answer 20

-70 mV
Sodium ions actively transported out of the axon by sodium potassium pumps
Potassium ions are actively transported into the axon by sodium potassium pumps
3 sodium move out for every 2 potassium that move in
Outwards movement of sodium is greater, more sodium ions in the tissue fluid surrounding the axon than in the cytoplasm, creates an electrochemical gradient
Sodium ions begin to naturally diffuse back into the axon whilst the potassium begin to diffuse back out of the axon
Most of the gates in the channel that allow the potassium ions to move through are open, most of the gates that allow sodium to move through are closed

Question 21

Action potential

Answer 21

At resting potential some potassium voltage gated channels are open but the sodium gated channels are closed
Energy of the stimulus causes some sodium voltage gated channels in the axon membrane to open, sodium ions diffuse into the axon along their electrochemical gradient, trigger a reversal in the potential difference across the membrane, depolarisation
As sodium ions diffuse in more sodium channels begin to open, causes an influx of sodium ions by diffusion
Once action potential of +40mV has been established voltage gated sodium channels close and the potassium voltage gated channels open
Some potassium gated channels open the electrochemical gradient preventing further outwards movement of potassium is reversed and more potassium ion channels are opened, causes more potassium to diffuse out
Outward diffusion of potassium causes an overshoot of the electrical gradient, inside more negative than usual which is hyperpolarisation
Potassium voltage gated channels now close and sodium potassium pump pumps sodium out and potassium in, -65mV is re-established and axon is repolarised

Question 22

Passage of an action potential along a myelinated axon

Answer 22

Myelin sheath acts as an electrical insulator
Preventing action potentials from forming
Nodes of rancher all along the axon, action potentials “node hop” by Saltatory conduction to transmit the electrical impulse

Question 23

Refractory period

Answer 23

Once an action potential has been generated sodium ion movement is prevented for a period of time as the sodium-voltage gated channels are closed

Ensures that:

• action potentials are unidirectional, only move in a forwards direction so prevents them spreading in both directions, only pass from An active region to a resting region
• produces discrete impulses, no new action potential can be formed so that impulses are separated
• limits number of action potentials, separated from each other so limits the number of action potentials along an axon in a given time to limit the strength of the stimulus that is being detected

Question 24

Spatial summation

Answer 24

Number of different pre-synaptic neurone together release enough neurotransmitter to exceed the threshold value, trigger a new action potential

Question 25

Temporal summation

Answer 25

A single presynaptic neurone releases neurotransmitters many times over a short period of time, if concentration reaches the threshold then an action potential is triggered

Question 26

Inhibitory synapses

Answer 26

Pre-synaptic neurone releases neurotransmitter that binds to chloride ion protein channels on the post-synaptic neurone
Neurotransmitter causes the chloride ion protein channels to open
Chloride ions move into the postsynaptic neurone by facilitated diffusion
Binding of neurotransmitter causes th opening of nearby potassium k+ protein channels
Potassium ions move out of the postsynaptic neurone into the synapse
Chloride moving in and potassium moving out makes the inside of the postsynaptic membrane more negative and the outside more positive
Membrane potential reaches -80mV at resting potential
Reached hyperpolarisation
Much less likely that a new action potential will be created

Question 27

Cholinergic synapses

Answer 27

Arrival of action potential at the end of the pre-synaptic neurone causes calcium voltage gated channels to open and calcium ions move into the synaptic knob by facilitated diffusion
Influx of calcium ions into the presynaptic neurone causes the synaptic vesicles to fuse with the presynaptic membrane releasing acetylcholine into the synaptic cleft
Acetylcholine molecules diffuse across the narrow synaptic cleft very quickly because the diffusion pathway is short, acetylcholine then binds to receptor sites on sodium ion channels in the membrane of the post synaptic neurone
Causes sodium channels to open allowing sodium to rapidly diffuse in along a concentration gradient
Influx of sodium ions generates a new action potential

Acetylcholine esterase hydrolyses acetylcholine into choline and ethanoic acid (acetyl), diffuse back across the synaptic cleft and are recycled, also prevents new action potentials from forming
ATP released by mitochondria is used to recombine the acetyl and choline, stored in synaptic vessels

Question 28

Neuromuscular junctions

Answer 28

When nerve impulse arrives the synaptic vesicles fuse with the presynaptic membrane and release acetylcholine
Acetylcholine diffuses into the post synaptic membrane, altering its permeability to sodium ions, enter rapidly causing depolarisation
Acetyl choline is broken own by acetylcholine esterase to prevent overstimulation
Acetyl and choline are recycled

Question 29

Sliding filament mechanism

Answer 29

1. Muscle stimulation
   - action potential reaches neuromuscular junction causing calcium ion channels to open and calcium ions to diffuse into the synaptic knob
   - Calcium ions cause synaptic vesicles to fuse with the presynaptic membrane and release acetylcholine into the synaptic cleft
   - acetylcholine diffuses across the synaptic cleft and binds with receptors on muscle cell-surface membrane causing depolarisation
2. Muscle contraction
   - action potential travels deep into the fibre through a system of T-tubules, extensions of cell surface membrane and branch throughout sarcoplasm
   - tubules in contact with sarcoplasmic reticulum which has actively transported calcium ions from sarcoplasm leading to very low contractions of calcium ions in the cytoplasm
   - action potential opens calcium gated channels on sarcoplasmic reticulum and calcium ions diffuse into muscle sarcoplasm down a concentration gradient
   - calcium ions cause tropomyosin molecules that were blocking binding site on actin filament to pull away
   - ADP attached to myosin heads mean they can bind to actin filament and form a cross bridge
   - once attached to actin filament myosin heads change angle and pull actin filament along releasing the ADP
   - ATP attaches to myosin head, detaches it from the actin filament
   - calcium ions activate ATPase, hydrolyses ATP to ADP, provides energy for myosin head to return to original position
   - myosin head with ADP attached reattached further along actin filament, process is repeated
   - as myosin molecules are joined tail to tail in oppositively facing sets the movement of one set of myosin heads is the opposite direction to the other, actin to which they are attached also move in opposite directions
   - movement of actin in opposite directions pulls them towards each other, shortening distance between 2 adjacent Z lines
3. Muscle relaxation
   - when nervous stimulation ceases calcium ions are actively transported back into th endoplasmic reticulum using energy from ATP hydrolysis
   - reabsorption of calcium ions allows tropomyosin to block actin filament again
   - myosin heads are unable to bind to actin filaments and contraction ceases so muscle relaxes