unit 3 biology mock topics

23/11/25

What is ATP and what is it made up of?

Adenosine Triphosphate
pentose sugar - ribose
nitrogenous base - adenine
3 phosphate groups

what can ATP be described as and why

the universal energy currency - it is used in all cells for all chemical reactions.

name 3 uses of ATP

Biosynthesis
Active transport
Muscle contraction

Name 3 ways in which ATP is suited to its role

• Inert: doesn't break down or react unless needed to.

• Releases energy in usable quantities: to minimise waste.

• It can be easily hydrolysed: quick

• small/soluble: can move easily to deliver energy where its needed.

how is ATP broken to ADP and Pi

by hydrolysis, an exergonic reaction - energy is released

this is done using the enzyme ATP synthetase which is located in internal membranes of mitochondria and chloroplasts

define chemiosmosis

the flow of protons down an electrochemical gradient through ATP synthetase

describe the electron transport chain (7 steps)

1) Electrons are released from reduced coenzymes.
2) The process of electron transfer along the electron transport chain provides energy.
3) This energy is used for the active transport of protons from the matrix/stroma to the intermembrane space or thylakoid space.
4) this generates an electrochemical gradient.
5) protons will diffuse back down the gradient through ATP synthetase.
6) this provides the energy needed to form ATP from ADP and Pi by oxidative phosphorylation
7) Oxygen is the final electron acceptor, forming water, allowing the flow of electrons to continue.

3 similarities and 3 differences between ETC in mitochondria and chlorplasts.

1) Use of ATP synthetase
2) protons provide energy for ATP synthesis
3) proton pumps across inner membrane of organelle

1) site of ETC: M= cristae, C= thylakoid membrane
2) protons pumped into: M- matrix, C- stroma
3) coenzymes: M= NAD,FAD, C= NADP

why is ATP more useful than glucose

glucose provides more than needed = waste.
breakdown of glucose takes longer = ATP hydrolysis is a single reaction.

name the reactants and products of photosynthesis

reactants - carbon dioxide and water
products - oxygen and glucose

4 adaptations of leaves for photosynthesis

1. large surface area - to maximise light captured

2. thin - light can penetrate through leaf

3. stomatal pores - allow carbon dioxide to diffuse in

4. air spaces in spongy mesophyll and palisade mesophyll - allow co2 to diffuse into photosynthesising cells.

what is the primary adaptation of palisade mesophyll

cylindrical and at a right angle to leaf surface

why? - leaves can accommodate more palisade mesophyll.

2 adaptations of chloroplasts for photosynthesis

1. large surface area due to grana - maximise absorption of light.

2. chloroplasts can move within palisade cells. -

• can move towards the top on dull days.

• bottom when high light intensity to protect pigments from bleaching.

whats a transducer and why can a chloroplasts be called one

converts one form of energy into another.

chloroplasts can convert energy in photons of light into chemical energy

what are the 2 classes of pigment - give an example for each

1. carotenoids

• beta carotene

• xanthophylls

1. chlorophyll

• chlorophyll a

• chlorophyll b

what is the function of a photosynthetic pigment

to absorb light energy at different wavelengths and begin its conversion to chemical energy.

whats the equation to calculate retention factor

Rf = distance travelled by pigment / distance travelled by solvent front.
must be between 0 and 1

define homeostasis

the mechanism by which a constant internal environment is maintained and the ability to return to a set point.

what factors should be maintained in the internal environment

solute potential
core body temperature
glucose concentration
pH

why does the internal environment need to be maintained

protects cells from any changes in the external environment.
Ensuring reactions continue at a constant rate and allows normal cell functioning.

what is the mechanism by which self-regulation occurs in homeostasis.

negative feedback

1) a receptor detects the factors level and its deviation from the set point
2) receptor sends a message to coordinator
3) coordinator communicates with effector. (e.g muscle) which responds
4) factor returns to set point

negative feedback with glucose concentration process

1) glucose is detected above its set point
2) insulin is secreted
3) this reduces glucose concentration by converting it to glycogen

1) glucose is detected below its set point
2) glucagon is secreted (glucaGONe = glucose is gone)
3) converts glycogen to glucose

negative feedback with decreased core body temperature

1) temperature falls below set point
2) increased muscle movement through shivering generates heat
3) blood vessels constrict (vasoconstriction) allows retention of heat.

positive feedback example in the body

pregnancy - oxytocin stimulates contraction of the uterus. increased contractions stimulate increased oxytocin production.

define excretion

the removal of metabolic waste made by the body

give examples of excreted products

urea, excess water and salts

define osmoregulation

the control of the water potential of the body's fluids by the regulation of the water content of the body.

whats the difference between egestion and excretion

Egestion is the removal of undigested food that has not been used by the body, whereas excretion is the removal of metabolic waste products produced inside the body.

how is urea produced

1) excess amino acids, which haven't been assimilated to proteins, are deaminated in the LIVER.
2) the remaining amine group is converted to urea.

the structure of the nephron

afferent arteriole, glomerulus, bowman's capsule, proximal convoluted tubule, descending loop of henle, ascending loop of henle, distal convoluted tubule, collecting duct.

in what part of the kidney would you find the loop of henle and collecting duct

medulla

what is an antenna complex

an array of protein and pigment molecules within the thylakoid membranes of the grana that transfer energy from a range of light wavelengths to chlorophyll a at the reaction centre.

what is absorption spectra

a graph showing how much light is absorbed by a particular pigment at different wavelengths.

what is action spectra

a graph showing the rate of photosynthesis at different wavelengths

whats the relationship between absorption and action spectra

there is a close correlation between the two on a graph.

this shows that it is the wavelengths of light that are absorbed that are used in photosynthesis.

briefly describe the process of light harvesting

1) the antenna complex, containing photosynthetic pigments, harvests light.
2) excitation is passed from one pigment to another, to the reaction centre
3) at the reaction centre, chlorophyll a becomes excited and emits an electron.

where does photosynthesis occur

light dependent - the thylakoid membranes on the grana within the chloroplast
light independent - stroma

what are the requirements of the light dependent stage

light and water

what is photophosphorylation

the addition of a phosphate ion to ADP using light

what are the two pathways which the light dependent stage can occur and which photosystems are involved

non-cyclic photophosphorylation - PSII AND PSI
cyclic photophosphorylation - PSI only

describe the process of non-cyclic photophosphorylation

1) photosystem II absorbs light
2) the photons of light excite the electrons and raise them to higher energy level (photoionisation)
3) electrons pass to electron acceptors, reducing them (gain electrons).
4) electrons are passed down the electron transport chain to generate the proton gradient needed for oxidative phosphorylation.
5) The electrons are then passed to photosystem I
6) PSI absorbs light which excited electrons to a higher energy level from ground state
7) excited electrons are passed to electron acceptors
8)these electrons are passed to NADP with H+ from photolysis
9) reduced NADP is formed

what are the products of non-cyclic photophosphorylation and why are they needed

reduced NADP: reducing power synthesise co2 to glucose
2ATP: chemical energy
oxygen: by-product which diffuses out the leaf through stomata.

whats the process of cyclic photophosphorylation

1) photosystem 1 absorbs light
2) electrons in chlorophyll a are excited
3) excited electrons are passed to the electron acceptor
4) electrons pass through electron transport chain producing ATP
5) electrons return to PS1

What happens to the charges of photosystems in non-cyclic photophosphorylation. how is it fixed?

• electrons are not returned to PSI as they're passed to NADP

• this leaves chlorophyll in PSI with a positive charge.

• this is neutralised by the electrons from PSII (ETC)

• this leaves PSII with a positive charge.

• this is neutralised by electrons from photolysis

what is photolysis and what happens to the products

the splitting of water molecules by light, producing hydrogen, oxygen and electrons.

hydrogen - used to reduce NADP
electrons - used to neutralise photosystem II
oxygen - waste product diffuses out of the leaf through stomata

what helps generate a proton gradient for the ETC

proton pumps - push protons into the thylakoid space
photolysis - H+ is produced in the thylakoid space
protons are removed by NADP in the stroma

1) therefore, theres an electrochemical gradient where there are more protons in the thylakoid membrane than the stroma
2) H+ diffuses through ATP synthetase back into the stroma and ATP is produced

where do the light independent reactions take place

in solution in the stroma

whats RuBP?

Ribulose bisphosphate
5C molecule

describe the light independent stage of photosynthesis

1) Carbon dioxide combines with Ribulose Bisphosphate catalysed by Ribulose bisphosphate carboxylase.
2) an unstable 6C compound is formed
3) this splits into 2 glycerate-3-phosphate
4) Glycerate-3-phosphate is reduced to triose phosphate by reduced NADP, requiring ATP
5) NAHP is reformed
6) SOME triose phosphate is converted to glucose.
7) most triose phosphate converted to ribulose phosphate
8) finally ribulose bisphosphate is regenerated from ribulose phosphate using ATP.

what is the name of the enzyme which catalyses co2 with RuBP

Ribulose Bisphosphate Carboxylase (rubisco).

importance is evident by its abundance in the atmosphere.

how can carbohydrates be manufactured from light independent stage

1) triose phosphate is converted to fructose bisphosphate
2) this can be converted to glucose
3) to be transported around the plant - this can be combined to fructose to make sucrose.

alpha glucose = starch for storage
beta glucose = cellulose for cell walls

how can lipids be formed from the calvin cycle

1) glycerate-3-phosphate can make Acetyl CoA.
2) this can form fatty acids

1) Triose phosphate can be converted to glycerol

glycerol and fatty acids (x3) can form triglycerides by condensation

why are triglycerides needed in plants (x3)

1) energy storage
2) osmotic balance: insoluble= doesn't affect water potential
3) waterproofing (e.g. waxy layers)

how can proteins be formed from calvin cycle

1) glycerate-3-phosphate can form amino acids

why are proteins needed in plants (x3)

1) formation of enzymes such as rubisco and ATP synthetase
2) structural proteins
3) seed storage

what conditions are needed for photosynthesis

carbon dioxide
water
light intensity
suitable temperature

whats a limiting factor

a factor which limits the rate of a physical process by being in short supply. Controls the rate

when is carbon dioxide needed in photosynthesis

in the calvin cycle to combine with ribulose bisphosphate using Rubisco (ribulose bisphosphate carboxylase )

what happens to photosynthesis as carbon dioxide concentration increases

the rate of the independent stage increases so photosynthesis increases.

why does co2 concentration eventually decrease in photosynthesis

1) Co2 is taken in through stomata
2) carbonic acid is produced
3) this dissociates to H+ and HCO3-
4) H+ increases which decreases the pH
5) To avoid this leading to proteins denaturing, the stomata will close at 1% co2 to prevent co2 uptake

what happens to rate of photosynthesis when light intensity changes

1) in darkness - light dependent stages cant occur so not ATP or reduced NADP. rate is limited (light indep. slowed )

2) Increasing light intensity - the light dependent stages can occur so rate increases.

3) 10,000 lux - maximum light absorption, any further increase has no effect - rate is constant

4) extreme light intensity - rate of photosynthesis decreases because photosynthetic pigments are damaged. less efficient light absorption so light dependent fails.

whats the light compensation point

light intensity where the rate of photosynthesis is equal to the rate of respiration
there is no net gas exchange.
c02 in = co2 out
o2 in = 02 out

why is it favourable for the light intensity to be above the light compensation point

because it means more glucose is being made (photosynthesis) than used (respiration ). so it can be stored

what happens to rate of photosynthesis with changes in temperature

1) increasing temperature - rate increases as kinetic energy increases
2) above optimum - enzymes (such as rubisco) denature and rate decreases

what is the limiting factor when there are several limiting factors

the one which is closest to its minimum value

how is nitrogen taken up by plants

As nitrates.

name 3 roles of nitrogen in plants

1) Synthesis of proteins
2) Component of chlorophyll
3) Component of nucleic acids.

what would happen to a plant if nitrogen was in short supply

chlorosis. yellowing of the leaves due to insufficient chlorophyll production.

slowed growth - less protein production

what is magnesium used for in plants (x2)

1) component of chlorophyll
2) facilitating transport of phosphorus (flower development)

what would happen to a plant with low magnesium

chlorosis - yellowing of leaves due to lack of chlorophyll
fail to produce fruit - lack of phosphorus distribution

advantage of a plant having multiple different photosynthetic pigments

They allow the plant to absorb a wider range of wavelengths of light, which increases the overall rate of photosynthesis.

how many calvin cycles are needed to produce 1 molecule of glucose. explain…

1) each calvin cycle produces 2 triose phosphate
2) for every 6 triose phosphates formed ( 3 cycles) 5 are used for the regeneration of RuBP.
3) so it would take SIX CYCLES to give 2 triose phosphates
4) 2 triose phosphate give 1 glucose

what is the first stage in filtration

Ultrafiltration in the bowman' capsule

why do the capillaries of the glomerulus have a high … pressure

• afferent arteriole has a wider diameter than efferent arteriole.
• the hearts contraction increases the HYDROSTATIC pressure of arterial blood.

what separates the blood entering the glomerulus and the space in the bowman's capsule?? x3

1) one cell thick endothelium with fenestrations

2) glomerular basement membrane - selective barrier acting like a sieve

3) Bowmans capsules' podocytes (squamous epithelial). gaps between these are called filtration slits.

in what direction does filtrate move in ultrafiltration. how is this maintained

from the glomerulus to the bowman's capsule under HIGH PRESSURE (hence, ultra…)

DOWN A PRESSURE GRADIENT - from an area of higher hydrostatic pressure to an area of lower hydrostatic pressure.

what is found in the glomerular filtrate

glucose, salts, water, urea, amino acids

which molecules are retained in the glomerulus

proteins, blood cells and platelets due to their size and action of capillary walls and basement membranes

what is the water potential of the efferent arteriole and why

low because there is a high protein concentration and water has been lost in ultrafiltration

where does selective reabsorption occur

proximal convoluted tubule

how is the proximal convoluted tubule adapted to its role (x4)

1) Large surface area - cuboidal epithelium with microvilli.
2) many mitochondria - provide ATP for active transport
3) Close association with capillaries - maintain concentration gradient.
4) tight junctions - prevent seepage of reabsorbed material back into filtrate.

how does the PCT maintain its pH

it exchanges HCO3- AND H+

HCO3- = increase pH

H+ = decrease pH

what substances are reabsorbed through the PCT back into the blood

All of the glucose and amino acids
Most of the water and salts
Some of the urea

how are glucose and amino acids reabsorbed

Secondary active transport
Co-transport mechanism with Na+

the coupled movement of Na+ down its concentration gradient with movement of glucose/amino acids

1) Na+ and glucose move into the cuboidal PCT cell by facilitated diffusion
2) Glucose accumulates inside the cell
3) there is a higher concentration of glucose in the cell than in the blood.
4) glucose moves into the blood capillaries by facilitated diffusion, down its concentration gradient. `

how is the sodium ion concentration gradient maintained between the PCT and cuboidal cell

sodium is pumped into the capillaries to reduce their concentration in the cuboidal cell.

how is water reabsorbed from the PCT to the blood

by osmosis.
reabsorbed ions lower the water potential in the blood.

water moves from a higher to a lower water potential

why is glucose needed in the body

Aerobic respiration for the production of ATP

what is the name for low blood glucose and what are the consequences

Hypoglycaemia

dizziness and confusion = brain relies on glucose so without it it cannot function properly

what is the name for high blood glucose and what are the consequences

hyperglycaemia

Thickened blood = higher glucose concentration in the glomerular filtrate, low water reabsorption

hence, more urine output.

define isotonic

Isotonic means that two solutions have the same water potential or the same concentration of solutes

where is the PCT filtrate and the blood isotonic

at the base of the PCT - maximum selective absorption has taken place

what is the loop of henle described as and why

hairpin counter-current multiplier

Counter-current because flow is in opposite directions in the two limbs: up in ascending limb and down in descending limb.

multiplier: multiplies the concentration of solutes in the medulla.

are the walls of the ascending loop permeable or impermeable to water

impermeable

what does the ascending loop of henle do

it actively transports sodium and chloride ions out of the filtrate, into the medullary tissue fluid.

this increases the concentration of salts in the tissue fluid and therefore lowers the water potential.

what is the water potential like at the top of the ascending loop of henle

lower water potential as it passes up, due to the pumping out of salts.

what is the descending loop of henle and whats its function

its permeable to water and SLIGHTY permeable to salts

1) as filtrate flows down the limb, water moves out by osmosis from a higher to a LOWER water potential (created by action of ascending limb).

2) water is carried away by the vasa recta

what are the vasa recta? why is important

capillaries that surround the loop of Henle

1. delivers nutrients to medulla cells
2. carries reabsorbed water

what is the water potential at the apex of the loop of henle

it is at its lowest water potential.
as solute concentration is high.

why is the counter current flow an advantage

enables the maximum concentration to build up at the apex.

what is the main difference between the structure of the DCT and PCT.

PCT has cuboidal epithelial with microvilli whereas DCT has few or no microvilli.