M5, C17 Energy for Biological Processes

Question 1

what is the word and balanced symbol equation for photosynthesis

Answer 1

carbon dioxide + water –light energy–> oxygen + glucose

6CO2 + 6H2O -> 6O2 + C6H12O6

Question 2

what does it mean when you say plants are autotrophic

Answer 2

they make their own organic molecules from inorganic molecules/CO2

e.g. plants make organic molecules from photosynthesis

Question 3

explain the importance of photosynthesis to life on earth

Answer 3

Produces oxygen which other organisms use for respiration.
Absorbs carbon dioxide which limits carbon dioxide levels in the atmosphere which could lead to global warming and climate change.
Plants are autotrophs which means they make their own food which heterotrophs can then eat.

Question 4

define heterotrophs

Answer 4

obtains organic molecules from other organisms

feeds on and digests another organism

Question 5

explain why the equation for photosynthesis is an oversimplification

Answer 5

It implies carbon dioxide and water combine and react which lead to the formation of oxygen and glucose. This is not true.
There are separate reactions- one which is light-dependent and one which is light-independent.
There are also lots of steps which happen in each of those 2 reactions.
The equation also doesn’t mention the pigment, chlorophyll, required to absorb the light energy for the light-dependent reaction.

Question 6

name all the parts of a leaf

Answer 6

from top to bottom:
waxy cuticle
upper epidermis
palisade mesophyll
spongy mesophyll with air spaces in between
vascular bundle
lower epidermis
guard cells and stomata
waxy cuticle

Question 7

how is the palisade mesophyll adapted for its function in the leaf

Answer 7

packed full of chloroplasts

absorbs the light energy required for photosynthesis

Question 8

why are their air spaces between the spongy mesophyll in the leaf

Answer 8

makes it easier for diffusion

Question 9

why is the waxy cuticle usually thicker on top of the leaf rather than on the bottom

Answer 9

because there is direct sunlight on the top of the leaf which increases the rate of evaporation
therefore the waxy cuticle has to be thicker to reduce water loss

Question 10

why is the upper epidermis thin

Answer 10

allows maximum sunlight to get through the leaf to the palisade cells so the chloroplasts can absorb maximum light energy for photosynthesis

Question 11

what are photosynthetic pigments

Answer 11

substances that absorb light at certain wavelengths

other wavelengths are reflected

Question 12

what are the 4 photosynthetic pigments found in chloroplasts

Answer 12

chlorophyll a
chlorophyll b
carotene
xanthophyll

(carotene and xanthophyll are also known as carotenoids)

Question 13

Why don’t plant rely on just one photosynthetic pigment

Answer 13

because different pigments absorb different wavelengths of light
so multiple are needed to absorb as much light energy as possible
also in different environments they may be different light sources so ensures at least one of the pigments can absorb the light energy

Question 14

name the features of the chloroplast

Answer 14

stroma
thylakoid - multiple are called granum
ribosome
chloroplast DNA
starch granule
drop of lipids
lamellae
inner and outer membrane

Question 15

where are the pigments found in the chloroplasts

Answer 15

in the thylakoid membranes

arranged in photosystems

Question 16

describe the similarities and differences between the 2 photosystems in the thylakoid membranes

Answer 16

Similarities:
Both photosystems are funnel shaped.
At the thinner end, there is a chlorophyll a molecule, called the primary pigment.
In the rest of the funnel are the accessory pigments which involves carotene, chlorophyll b and xanthophyll.
Light energy is transferred through the accessory pigments to the primary pigment.

Differences:
Light of wavelength 680nm is absorbed by the photosystem II (PSII).
Light of wavelength 700nm is absorbed by the photosystem I (PSI).

Question 17

give a step-by-step process of non-cyclic photophosphorylation for light-dependent photosynthesis reaction

Answer 17

1) Light strikes photosystem II, exciting a pair of electrons that leave the chlorophyll molecule from the primary pigment.
2) The electrons pass along a chain of electron carriers and the energy released is used to synthesise ATP.
3) Light has also struck PSI and a pair of electrons has been lost. The electrons from PSII replace them.
4) Also in PSII, the light initiates the enzyme attached to catalyse the photolysis reaction. This produces 2 hydrogen ions and 2 electrons to replace those lost in PSII.
5) The hydrogen ions diffuse down a concentration gradient along a channel protein. This initiates ATP synthase which catalyses the reaction to produce ATP.
6) The ATP and hydrogen ions are captured by the NADP in the stroma. The products made are ATP and NADPH (reduced NADP) which will be used in the light-independent stage.

Question 18

give a step-by-step process of cyclic photophosphorylation for light-dependent photosynthesis reaction

Answer 18

1) Light strikes PSI and a pair of electrons is excited from the chlorophyll molecule.
2) They pass along the chain of electron carriers back to the chlorophyll.
3) As they pass down the carriers, they produce energy.
4) The energy is used to actively pump hydrogen ions into the thylakoid.
5) The hydrogen ions diffuse down a concentration gradient back into the stroma through a channel protein.
6) The ATP synthase is activated to catalyse the reaction to produce ATP.

Question 19

Compare cyclic and non-cyclic photophosphorylation for the light dependent stage of photosynthesis.

a) photosystems involved
b) is photolysis involved
c) fate of electrons released from chlorophyll
d) products made

Answer 19

a) Only PSI is involved in cyclic but both are involved in non-cyclic
b) Photolysis is only involved non-cyclic
c) The electrons in cyclic return back to PSI. For non-cyclic, the electrons made by PSII go to PSI and the electrons from PSI are used by NADP to produce reduced NADP (NADPH)
d) Products of cyclic is just ATP. Products of non-cyclic are ATP and reduced NADP (NADPH)

Question 20

The light-independent stage of photosynthesis is known as the Calvin cycle. Give a step-by-step process of the Calvin cycle.

Answer 20

1) Carbon dioxide from the air diffuses into the leaf though open stomata. It diffuses through the air spaces in the spongy mesophyll and reaches the palisade mesophyll layer. It diffuses into the stroma.
2) Here, the carbon dioxide combines with a 5-carbon compound, ribulose bisphosphate (RuBP). The reaction is catalysed by rubisco.
3) The product is 2 molecules of a 3-carbon compound, glycerate 3-phosphate (GP).
4) GP is reduced and phosphorylated to another 3-carbon compound, triose phosphate (TP). ATP and reduced NADP from the light dependent reaction are used in this process.
5) The majority of TP is recycled back to RuBP by phosphorylation using ATP.

6) Food products are made from GP and TP.
Eg. TP makes glucose which can make fructose and together these make sucrose. These all make cellulose and starch. GP makes amino acids and fatty acids and TP makes glycerol which binds to the fatty acids to make lipids.

Question 21

define limiting factor

Answer 21

a factor present in the lowest amount that reduces the rate of photosynthesis

Question 22

what are the 3 limiting factors of plants

Answer 22

light intensity
carbon dioxide concentration
temperature

Question 23

what are the consequences of having a low light intensity to the photosynthesis reactions

Answer 23

less light would strike the photosystems meaning less electrons are excited from chlorophyll a
leads to less NADPH and ATP being produced
means less food will be made in the Calvin cycle

Question 24

what are the consequences of having low carbon dioxide concentration to the photosynthesis reactions

Answer 24

the light-dependent reaction will be unaffected
less carbon dioxide to react with the RuBP so less GP and TP made
meaning less food made

Question 25

what are the consequences of having a low temperature to the photosynthesis reactions

Answer 25

the enzymes of ATP synthase and rubisco will work slower

products made less quickly meaning less food made

Question 26

explain why the levels of GP and TP change when a plant changes from light to dark

Answer 26

when it’s dark, GP can still be made because carbon dioxide is unaffected
TP can only be made with the presence of ATP and NADPH but if there’s no light then the light-dependent reaction can’t happen so these things can’t be produced so TP levels drops
GP is usually converted to TP but because this can’t be made, GP levels increase

Question 27

one experiment to investigate the rate of photosynthesis is to count to the number of bubbles a pondweed produces when placed in a test tube of water
how can this experiment be made more accurate

Answer 27

Counting bubbles produces human errors and the bubbles are all different sizes. So a gas syringe should be attached to the top of the tube in order to fin dthe volume of gas the plant has produced. (note: this doesn’t stop the fact that some bubbles could be carbon dioxide produced during respiration)

Question 28

One experiment used to investigate the rate of photosynthesis is placing leaf discs in a dilute solution of sodium hydrogen carbonate and extracting the air from them using a syringe and seeing how long the leaves floated back to the top.
Why is sodium hydrogen carbonate used?
What does it mean if the leaf discs rise quicker?

Answer 28

Sodium hydrogen carbonate is used in order the provide the carbon dioxide for photosynthesis.

The quicker the leaves rise, the more photosynthesis they are doing because they are producing oxygen during photosynthesis and this refills the air sacs meaning they are more buoyant so will rise quicker.