photosynthesis

Question 1

why are plants , algae and cyanobacteria autotrophic?

Answer 1

Plants, algae and cyanobacteria photosynthesise, they convert sunlight into chemical energy to synthesise
large organic molecules from simple inorganic molecules (H2O and CO2) – (photo)autotrophic nutrition

Question 2

why are plants algae and cyanobacteria producers?

Answer 2

o producers – providing organic molecules and energy to other, non-photosynthetic heterotrophs.
Energy needed for active transport, DNA rep, cell division, protein synthesis, muscle contraction,
maintenance of body temp

Question 3

phtosynthesis formula

Answer 3

: 6CO2 + 6H2O + energy from light -> C6H12O6 + 6O2 (chlorophyll)

Question 4

why is CO2 an example of carbon fixation

Answer 4

carbon fixation – CO2 converted to sugars. The carbon for all organic
molecules is provided by carbon fixation. Carbon fixation is endothermic (requires energy) and is reduction
(requires electrons). Carbon fixation helps regulate CO2 conc in the atmosphere/oceans.

Question 5

why are fungi animals and many protoctists and bacteria are heterotrophs

Answer 5

heterotrophs – obtain energy

by digesting large organic molecules of food to be used as respiratory substrates.

Question 6

respiration formula

Answer 6

Respiration is oxidation, releasing energy – exothermic.

Respiration: C6H12O6 + 6O2 -> 6H2O + 6CO2 + energy

Question 7

whats a compensation point?and the condition to be met

Answer 7

When photosynthesis and respiration occur at the
same rate there is no net gain or loss of carbohydrate.
- Respiration rate is constant (through out day and night),
photosynthesis varies with light intensity (only in day).
- The light intensity needs to be sufficient to allow photosynthesis
at a rate that replenishes the carbohydrate stores used up by
respiration.

Question 8

why do shade plant reach compensation period sooner than sun plants

Answer 8

Shade plants utilise light of lower intensity than sun plants can. When exposed to light after being in
darkness shade plants reach their compensation point sooner than sun plants, which require higher
light intensity to reach optimum for photosynthesis.

Question 9

name 3 structure inside the chloroplast

Answer 9

Granum: inner part made of stacks of thylakoid
membranes, where light-dependent stage takes place.
Stroma: fluid filled matrix, where light-independent
stage takes place.
Thylakoid: flattened membrane-bound sacs found
inside chloroplasts; contains photosynthetic
pigments/photosystems – is the site of the light-dependent stage.

Question 10

why do photosynthetic bacteria have no chloroplast

Answer 10

Plants and algae have chloroplasts, photosynthetic bacteria do not. They are disc
shaped and 2-10 μm long. They have double membrane (chloroplast envelope)
and an intermembrane space; the outer membrane is highly permeable. Inner has
transport proteins.

Question 11

where does the light dependent stage take place

Answer 11

grana

Question 12

The grana are connected by intergranal lamellae.

With many grana and many chloroplasts there is a large SA for:

Answer 12

The distribution of photosystems that contain the photosynthetic pigments that absorb sunlight
energy.
- The electron carriers and ATP synthase enzymes needed to convert light energy into ATP.

Question 13

why are proteins embedded in thylakoid

Answer 13

Proteins embedded in thylakoid membranes hold photosystems in place.

Question 14

why is the grana surrounded by the stroma?

Answer 14

The grana are surrounded by the

stroma, products of the light-dependent stage can easily pass into the stroma for light-independent stage.

Question 15

where does the light independent stage takes place

Answer 15

stroma

Question 16

whats in the stroma

Answer 16

Fluid filled matrix that contains the enzymes needed to catalyse the reactions of the light-independent
stage, as well as starch grains, oil droplets, 70S ribosomes (due to endosymbiosis) and circular DNA loops
that contains genes coding for proteins needed for photosynthesis – these are synthesised in the chloroplast
ribosomes.

Question 17

whats a photosynthetic pigment

Answer 17

: pigment that absorbs specific wavelengths of light and traps the energy associated
with light, they include chlorophylls a and b, carotene and xanthophyll.

Question 18

whats a phtosystem? and why are pigments embeddedin thylakoid membrane

Answer 18

A funnel-shaped collection of accessory pigments (light-harvesting systems) that pass their
energy absorbed from light to a primary pigment (chlorophyll a) reaction centre at the base of the funnel.
Pigments are embedded in thylakoid membrane (held in specific place by proteins). Involved in the lightdependent stage of photosynthesis.

Question 19

how do we see pigments

Answer 19

Each pigment absorbs light of a particular wavelength and reflects other wavelengths, they appear the
colour they are reflecting. The energy associated with the light is captured and funnelled down to the
primary pigment reaction centre. Chlorophyll = mix of chlorophyll a and b.

Question 20

whats a primary pigment

Answer 20

: reaction centres, where electrons are excited during the light-dependent stage of
photosynthesis.

Question 20

whats a primary pigment

Answer 20

: reaction centres, where electrons are excited during the light-dependent stage of
photosynthesis.

Question 20

whats a primary pigment

Answer 20

: reaction centres, where electrons are excited during the light-dependent stage of
photosynthesis.

Question 21

what .Chlorophyll a ?

Answer 21

– chlorophyll has a long hydrocarbon chain, a porphyrin group containing Mg.
2 forms of chlorophyll a, both appear blue-green. They are both situated at the centre of photosystems and
absorb red light, but have different absorption peaks:
- P680 is found in photosystem II and its peak of absorption is light of wavelength 680nm.
- P700 is found in photosystem I and its peak of absorption is light of wavelength 700nm.
Chlorophyll a also absorbs some blue light, of wavelength approx. 440nm.

Question 22

whats an accessory pigment. and give 3 examples

Answer 22

make up light-harvesting systems, surround the reaction centres and transfer energy to
them to increase energy available for electron excitement.
- Chlorophyll b – absorbs light of wavelength 500nm and 640nm. It appears yellow-green.
- Carotenoids –
- Carotene absorb blue light of wavelengths 400-500nm. They reflect yellow/orange light.
- Xanthophylls absorb blue & green light of wavelengths 375-550nm. They reflect yellow light.

Question 23

describe how to separate photosynthetic pigments

Answer 23

Thin layer chromatography has a mobile phase (liquid solvent – e.g. ethanol), and a stationary phase
(chromatography plate, plate with thin layer of silica gel on top).
- Grind up leaves with anhydrous sodium sulfate and propanone.
- Transfer liquid to test tube, add petroleum ether and shake, 2 layers form, top layer is mixture of
pigments and petroleum ether.
- Pencil line at bottom of chromatography plate, build up a concentrated spot of liquid at that point by
adding several drops.
- Once point of origin (spot) is completely dry put plate in solvent (mix of propanone, cyclohexane and
petroleum ether). As solvent spreads up plates pigments separate.
- Mark solvent front at end with pencil and dry in well-ventilated place.
- Calculate Rf values of pigments and look up identity.
Solvent is flammable (no flames near), don’t use leaves people are allergic too.

Question 24

what are the stages of the light dependent stage

Answer 24

This stage consists of:
1. Light harvesting at the photosystems
2. Photolysis of water
3. Photophosphorylation (non-cyclic or cyclic) – the production of ATP in the presence of light
- The formation of reduced NADP (non-cyclic)
Oxygen, the by-product of photosynthesis is also produced in the light-dependent stage.

Question 25

describe the photolysis of water

Answer 25

In PSII there is an enzyme that, in the presence of light, splits water molecules into protons (H+
), electrons
and oxygen – photolysis: 2H2O -> 4H+
+ 4e-
+ O2 (H2O is oxidised to O2)
- Water is the source of protons (H+
) that will be used in photophosphorylation
- Water donates electrons to chlorophyll to replace those lost when light strikes chlorophyll
- The source of the by-product – oxygen.
- Some is used in aerobic respiration. But during periods of high light intensity the rate of
photosynthesis is greater than respiration, most of the oxygen by-product diffuses out of the
leaves, through stomata.
- Keeps plant cells turgid, enabling them to function.

Question 26

whats a Photophosphorylation

Answer 26

Photophosphorylation is the generation of ATP from ADP and inorganic phosphate, in the presence of light.
There are two types:
- Non-cyclic – involves PSI and PSII. It produces ATP, oxygen and reduced NADP (from NADP).
- Cyclic – involves only PSI. Produces ATP in smaller quantities than non-cyclic.

Question 27

what happens in both phtosystems

Answer 27

in both are linked by electron carriers embedded in thylakoid membranes that accept and
donate excited electrons forming an electron transport chain.
- They are proteins with iron ions at their centre.
- Iron ion combines with electrons getting reduced to Fe2+, then donates an electron to next carrier in
the chain to become reoxidised to Fe3+.

Question 27

what happens in both phtosystems

Answer 27

in both are linked by electron carriers embedded in thylakoid membranes that accept and
donate excited electrons forming an electron transport chain.
- They are proteins with iron ions at their centre.
- Iron ion combines with electrons getting reduced to Fe2+, then donates an electron to next carrier in
the chain to become reoxidised to Fe3+.

Question 28

describe what happens in the non cyclic photophosporylation

Answer 28

1. Light is absorbed by PSII (P680), its energy is channelled to the primary pigment reaction centre.
   - The light energy excites a pair of electrons (to higher energy level) in the chlorophyll.
   - The energised electrons escape from the chlorophyll and are captured by an electron carrier
   embedded in the thylakoid membrane and move along the electron transport chain to PSI.
2. Photolysis of water produces electrons, H+
   , and oxygen.
   - These electrons move to PSII to replace the
   electrons that leave PSII to move along the
   ETC.
3. As electrons are passed along the chain of electron
   carriers embedded in the thylakoid membrane,
   they release energy.
4. This energy is used to actively pump protons across
   the thylakoid membrane into the thylakoid space.
   - As protons accumulate in the thylakoid
   space, a proton gradient forms across the
   thylakoid membrane.
5. Protons diffuse down their conc gradient through
   special channels in the membrane associated with ATP synthase enzymes.
   - The flow of protons causes ADP and Pi to join forming ATP – chemiosmosis. Energy is
   transferred from the protons flowing to allow a phosphate bond to be created
6. Eventually the electrons in the ETC are absorbed by a molecule of chlorophyll in PSI (low energy lvl).
   - The electrons are re-excited to an even higher energy level.
   - A protein-iron-sulphur complex called ferredoxin accepts the electrons from PSI.
7. The electrons and a proton (H+
   ) are transferred to NADP in the stroma to form reduced NADP.
   - The reduction of NADP is catalysed by NADP reductase.
   The light energy has been converted into chemical energy in the form of
   ATP by photophosphorylation. Water is electron donor, red NADP is final
   electron acceptor. ATP and reduced NADP are now in the stroma ready
   for the light-independent stage of photosynthesis.

Question 29

what happens in cyclic photosphorylation

Answer 29

This only uses PSI (P700). As light strikes PSI, a pair of electrons in the
chlorophyll at the reaction centre are excited to a higher energy level.
Electrons pass along an electron carrier system and then back to PSI. The passage of electrons along the
electron carriers produces a small amount of ATP is generated.
No photolysis of water occurs, so no protons or oxygen are produced, no reduced NADP is generated.
Chloroplasts in guard cells contain only PSI. They produce ATP that actively brings K+ into cells, lowering the
water potential so water follows by osmosis, causing guard cell to swell, opening the stomata.

Question 30

whats the Calvin cycle

Answer 30

Metabolic pathway of the light-independent stage, occurring in the stroma of chloroplasts. CO2
is fixed, using the products of the light-dependent stage, to make organic compounds.

Question 31

whats the role of CO2

Answer 31

CO2 is the source of carbon for the production of all organic molecules found in the carbon-based life forms
on Earth. These organic molecules may be used as structure (e.g. cell membranes, antigens, enzymes,
muscle proteins, cellulose cell walls) or act as energy stores (starch/glycogen).

Question 32

how does the CO2 enter the leaf ?and how can that benefit the plant

Answer 32

Carbon dioxide enters the leaf through the stomata and diffuses through the spongy mesophyll layer
to the palisade layer, into the palisade cells, through their thin cellulose cell walls, and then through
the chloroplast envelope into the stroma.
- The fixation of CO2 in the stroma maintains a conc gradient that aids this diffusion. CO2 that is a byproduct of respiration of plant cells may also be used in this stage.

Question 33

describe the calvin cycle

Answer 33

CO2 combines with ribulose bisphosphate (RuBP) – a 5-
carbon compound. This reaction is catalysed by RuBisCO
(ribulose bisphosphate carboxylase-oxygenase).
2. RuBP, is carboxylated, forming an unstable intermediate
6C compound that immediately breaks down into 2
molecules of glycerate-3-phosphate (GP) – 3C
compound. The CO2 has now been fixed.
3. GP is then reduced, using ATP and H+ ions from the
reduced NADP made in the light-dependent stage, to
triose phosphate (TP) – different 3C compound.
- Energy from ATP is used at the rate of 2 ATP for
every molecules of CO2 fixed. NADP is reoxidised.
4. 10 of every 12 TP molecules are used to regenerate
RuBP, this process requires ATP. The remaining 2 TP
molecules are the product.
- Remaining TP can be converted into many useful
organic compounds (e.g. glucose).
6 turns of the cycle are needed to produce 2 TP to make 1 glucose because 10 out of 12 TP used to
regenerate RuBP. 6 turns of the cycle uses 17 ATP and 12 reduced NAD

Question 34

what useful complex organic molecule can TP and GP be converted to

Answer 34

Carbohydrates – hexose sugars (e.g. glucose) are made by joining 2 TP molecules.
- Larger carbohydrates (starch, sucrose and cellulose) are made by joining these hexose sugars
in different ways.
- Lipids – glycerol can be synthesised from TP, and fatty acids (from GP) to make lipids.
- Amino acids – made from GP & TP.
- The rest of the TP is recycled to regenerate RuBP.
- 5 molecules of the 3C compound TP interact to form 3 molecules of the 5C compound RuBP.

Question 35

why does the calvin cycle only run on daylight , when it doesn’t directly require sunlight

Answer 35

The products of light-dependent stage (ATP and red NADP) are needed for Calvin cycle. Although it does not
directly use light energy it soon stops if there is no light, as products from light-dependent stage are not
available to reduce the CO2 in the Calvin cycle.

Question 36

how does the light dependent stage affect the RuBisCO in the calvin cycle

Answer 36

During the light-dependent stage H+ are pumped from the stroma into the thylakoid spaces, so the
conc in the stroma falls, raising the pH to approx. 8, which is the optimum for RuBisCO.
- RuBisCO is also activated by the presence of extra ATP in the stroma.
- In daylight the conc of Mg ions increase in the stroma; these ions act as cofactors for RuBisCO.
- The ferredoxin that is reduced by electrons in PSI activates enzymes involved in the Calvin cycle.

Question 37

limiting factors(raw material edition)

Answer 37

Limiting factors include raw materials (CO2/H2O), energy source
(light intensity), and availability of chlorophyll, electron carriers
and enzymes. Turgidity of cells and temp are also important. At
any given moment the rate of a metabolic process is limited by
the factor that is present at its lowest level.

Question 38

whats a saturation point. give example

Answer 38

In graphs photosynthesis rate increases until reaches saturation
point – another factor is limiting rate. Graphs of lower CO2 conc
and temp are lower, so in these cases the low CO2 conc/temp is a limiting factor.

Question 39

how does light intensity affect the rate of photosynthesis as a limiting factor

Answer 39

Light is needed to provide energy for the first (light-dependent) stage producing ATP and red NADP. Only
certain wavelengths of light (blue and red) are absorbed by photosynthetic pigments and used in
photosynthesis.
Light also causes stomata to open for gaseous exchange and transpiration – leading to the uptake of water
and its delivery to leaves.
At a constant favourable temp and CO2 conc, light intensity is the
limiting factor

Question 40

effects on low light intensity in the calvin cycle

Answer 40

GP cannot be reduced to TP – as ATP and red NADP from
light-dependent stage is short supply.
- TP levels fall and GP accumulates (as it is still being made).
- If TP levels fall, RuBP cannot be regenerated so RuBP falls.

Question 41

when is co2 a limiting factor

Answer 41

On warm , sunny, windless day CO2 conc is usually limiting factor.
At night light intensity is.

Question 42

Effect of low CO2 concentration on the Calvin cycle

Answer 42

If CO2 conc is low conversion of RuBP to GP is slow (less CO2 to combine with RuBP), RuBP
accumulates.
- GP and TP cannot be made so they fall and RuBP rises as it is formed form TP and GP.
Note: Light-dependent stage also slows as less energy available from glucose made from TP.

Question 43

why is photosynthesis very sensitive to enzyme

Answer 43

Photosynthesis involves many enzymes (ATP synthase, RuBisCo) and so is very sensitive to temperature

Question 44

Effect of temperature on Calvin cycle

Answer 44

All Calvin cycle reactions are catalysed by enzymes, at low temp all reactions are slower, TP, GP and
RuBP levels all fall.
- At very high (45°C +) temperatures enzymes denature so TP, GP and RuBP levels all fall.

Question 44

Effect of temperature on Calvin cycle

Answer 44

All Calvin cycle reactions are catalysed by enzymes, at low temp all reactions are slower, TP, GP and
RuBP levels all fall.
- At very high (45°C +) temperatures enzymes denature so TP, GP and RuBP levels all fall.

Question 45

Effects of changing temp on respiration:

Answer 45

From low temp to 25-30 °C, if plants have enough water, CO2 and light, rate increases with temp.

Question 46

effects of high temperatures:

Answer 46

Photorespiration - O2 competes with CO2 for the enzyme RuBisCO’s active site. Reduces the amount
of CO2 being accepted by RuBP and so reduces quantity of GP and TP produced, initially causing an
accumulation of RuBP. However due to a lack of TP, RuBP can’t be regenerated. Reduces rate of
light-independent stage.
- Chloroplast membranes could be damaged – Calvin cycle enzymes could be released into cell
reducing rate of light-independent stage.
- Stomata close – to avoid losing too much water, photosynthesis slows as less CO2 available.
- Thylakoid membranes may be damaged – reduces rate of light-dependent stage by reducing number
of electron carriers available for electron transfer.
- Chlorophyll could be damaged – reduce amount of pigments that can absorb light energy, reduce
energy available, reduce rate of light-dependent stage.

Question 47

whats water stress

Answer 47

Water stress: the condition a plant will experience when water supply is limiting

Question 48

describe how water stress can affect the rate of photosynthesis

Answer 48

Roots are unable to take up enough water to replace that lost by transpiration.
- Cells loose water and become plasmolysed.
- Tissues become flaccid and leaves wilt (tissues become flaccid; cells become plasmolysed).
- Plant roots produce abscisic acid that translocates to leaves causes stomata to close reducing
gaseous exchange – less CO2 available.
- Less light energy absorbed (due to wilting), less CO2 and less water for photosynthesis – rate
decreases.

Question 49

whats the Measuring photosynthesis rate

Question 50

whats the Measuring photosynthesis rate. and why does O2 production rate has its limitits

Answer 50

Rate of material uptake (CO2) or rate of production of by-product (O2) – vol per unit time.
- Measuring O2 production rate has limits, some O2 is used for respiration and there may be dissolved
nitrogen in gas collected (not just O2 collected)

Question 51

what things we aft to make sure to do when we set a photosynthometer

Answer 51

Set up so it is air tight, no air bubbles in
capillary tubing. Gas given off by plant
collects in flared end of capillary tube. As
experimenter manipulates the syringe, the
gas bubble can be moved next to ruler in
tube, measure distance moved. If radius of
capillary tube know volume of O2 produced
can be calculated.

Question 52

describe how to Investigate the effects of light intensity on the rate of photosynthesis:

Answer 52

IV: Light intensity DV: Rate of photosynthesis CV: other factors.
- Darkened room so only light is from light source. Water bath for constant temp.
- Same length of plant and make sure no bubbles in capillary tube. Place the plant (Elodea) cut end
facing upwards in a boiling tube and add 2 drops of sodium hydrogencarbonate soln.
- Place light source as close as possible and measure distance d.
- Light intensity = 1/d2 or use light meter to measure intensity. Note units for light intensity are
lux (l).
- Allow the plant to acclimatise for 10 mins and then position capillary tube (flared end) over cut end
of plant stem and collect gas for 5 mins.
- Measure distance bubble moves in capillary tube.
- Repeat and calc mean for different distances.
This experiment can be used to measure other factors wavelength of light (coloured filters), temp and CO2
conc.