Photosynthesis

Question 1

Photosynthetic plants are described as_________

Answer 1

Photosynthesis carried out by plants and algae
These organisms are described as autotrophic

Question 2

Animals are referred to as ___________-

Answer 2

Organisms such as animals cannot photosynthesise
They get their nutrients by eating plants or other animals
Known as heterotrophic organisms

Question 3

word equation for photosynthesis

Answer 3

carbon dioxide + water -> glucose + oxygen

—-> (above - energy from light)

co2 and water are reacted together
This required energy, which comes from light
products of photosynthesis - carbohydrate glucose + oxygen

great deal of energy stored in glucose chemical bonds
this energy was transferred from the light energy trapped during photosynthesis

Question 4

where do the reactions of photosynthesis take place

Answer 4

The reactions of photosynthesis take place in chloroplasts

Question 5

Describe what is meant by the light-dependent reactions

Answer 5

Photosynthesis actually consists of a series of different chemical reactions (consists of two sets of reactions)

We can divide these reactions into two groups

The light-dependent reactions take place on the thylakoid membranes
The light-dependent reactions can only happen in the presence of light

In the light-dependent reactions, light energy is harvested and used to produce ATP and a reduced hydrogen carrier (reduced NADP)

Hydrogen carrier in photosynthesis is the coenzyme NADP

In the light dependent reactions, NADP is reduced forming reduced NADP
Also in the light-dependent reactions, water is split by photolysis, forming oxygen

Question 6

Describe what is meant by the light-independent reactions

Answer 6

The light-independent reactions take place in the stroma
The light-independent reactions do not require light to function

(however) The light-independent reactions use the ATP and reduced NADP produced in the light-dependent reactions

In the Light-independent reactions, glucose is produced from carbon dioxide

Question 7

What happens if light is not present

Answer 7

LIR’s do not require light to function
However the LIR’s do require the ATP and NADPH produced by the LDR’s
So this means that if light is not present, the LDR’s stop.
The levels of ATP and NADPH then fall. And this causes the LIR’s to stop

Question 8

What is meant by a light harvesting system

Answer 8

reaction centre is surrounded by accessory pigments and proteins
referred to as the light harvesting system

Together the accessory pigments and their proteins are referred to as a light-harvesting system. Light harvesting system is also called an antennae complex

_______________

LDR’s take place in the thylakoid membranes
These membranes contain light-harvesting pigments such as chlorophyll and a range of proteins needed for the LDR’s

Chlorophyll is a pigment which absorbs visible light
There are several different forms of chlorophyll.
We will be focussing mainly on chlorophyll A

Graph shows how chlorophyll A absorbs different wavelengths of light
Chlorophyll A absorbs mainly red and blue light
Green light is not absorbed (it is reflected), which is why plants appear green

Question 9

What is meant by a reaction centre

Answer 9

A chlorophyll A molecule and its associated proteins are called the reaction centre

A molecule of chlorophyll A in the thylakoid membrane
Chlorophyll A is the central molecule in the LDR’s
So chlorophyll A is referred to as the primary pigment
The chlorophyll A works with a range of different proteins and other molecules. And together, these are called the reaction centre

The reaction centre is surrounded by other pigment molecules. These molecules include other forms of chlorophyll such as chlorophyll B, carotenoids and xanthophylls.
These molecules are referred to as accessory pigments.

Working with a number of proteins, the accessory pigments absorb light energy. The energy is then passed to the chlorophyll A molecule in the reaction centre.

Different accessory pigments absorb different wavelengths of light
So by using a range of pigments, this ensures that as much light energy is harvested as possible

Together the accessory pigments and their proteins are referred to as a light-harvesting system. Light harvesting system is also called an antennae complex

Question 10

What is a photosystem

Answer 10

Together the reaction centre and the light-harvesting system are referred to as a photosystem

Question 11

How many photosystems do the thylakoid membranes have

Answer 11

The thylakoid membranes actually contain two slightly different photosystems
called photosystem I (PS I) and photosystem II (PS II)

Photosystem I - absorbs light with 700nm wavelength

Photosystem II - absorbs light with 680 nm wavelength

Question 12

Describe photoionisation

Answer 12

when light energy is absorbed, this energy is passed to the chlorophyll A molecule in the reaction centre
This energy causes a pair of electrons in the chlorophyll A molecule to move to a higher energy level. These electrons are said to be excited

If enough energy is absorbed, then these electrons leave the chlorophyll A molecule. This process is referred to as photoionisation

These electrons are now carrying a great deal of energy. This energy will be used to make ATP and reduced NADP in the LDR’s

Question 13

Describe the LDR’s in photosyntehsis

including
Non cyclic phosphorylation
Cyclic phosphorylation
Photolysis

Answer 13

Light is harvested by photosystems on the thylakoid membranes

LDR’s involve two different photosystems. PS II and PSI

First, light energy is absorbed by PS II and a pair of electrons is excited

This pair of electrons now passes through an electron transport chain via a series of oxidation and reduction reactions

At the end of the electron transport chain, the electrons are passed to PS I

As the electron pair moves through the electron transport chain, the energy of the electrons is used to produce ATP by chemiosmosis (ADP + Pi -> ATP)

Light energy is now absorbed by PS I, and the pair of electrons is excited again. The electrons now make their way through an electron transport chain, again generating ATP by chemiosmosis (ADP + Pi -> ATP)

At the end the electron pair is added to NADP in the stoma forming reduced NADP
(NADP + H+ + 2e- -> NADPH) - REDUCED NADP

at the end (of the electron transport chain)

PSII -> electron transport chain -> PSI -> electron transport chain

The energy from light has been used to produce ATP. This process is called photophosphorylation

Because the pair of electrons that left photosystem II have not returned to photosystem II, this is non-cyclic photophosphorylation

Remeber
Photosystem II has lost a pair of electrons
To replace the electron pair an enzyme in photosystem II splits a molecule of water.
The energy for this process ultimately came from the light that was absorbed at the start.
Splitting water like this is called photolysis
This produced two electrons which replace the electron pair lost from photosystem II.
Photolysis also produces oxygen
This is the source of oxygen in photosynthesis

H2o -> 2e- + 1/2O2 + 2H+
* photolysis of water produces protons, electrons and oxygen.

-> (under or above) - photolysis

The reactions above are called non-cyclic photophosphorylation

However under certain conditions, cyclic photophosphorylation can take place.
During cyclic photophosphorylation, light energy is absorbed by Photosystem I. This causes a pair of electrons on photosystem I to be excited
These electrons pass through the electron transport chain, generating ATP (ADP + Pi -> ATP)
At the end, the electron pair returns back to Photosystem I.

Because the electron pair from photosystem I returned back to photosystem I, this is cyclic photophosphorylation

Cyclic photophosphorylation takes place when the chloroplast requires an increased amount of ATP

Both non-cyclic and cyclic photophosphorylation produce ATP. A key part of this process involves electron transport chains

https://images.nagwa.com/figures/explainers/357165907648/10.svg

Question 14

Describe what occurs in the electron transport chians

Answer 14

As the electrons pass through the electron transport chains, the energy of the electrons is used to pump protons (H+ ions) from the stroma into the thylakoid interior.

stroma
H+ H+
| |
PSII ETC PSI ETC
| |
\/ \/
H+ H+

thylakoid interior

PSII ETC PSI etc. (on thylakoid membrane)

The thylakoid membrane is impermeable to protons so they cannot diffuse back. So the concentration of protons is greater in the thylakoid interior than in the stroma.
This is called an electrochemical proton gradient
This proton gradient is enhanced by two other processes

During non-cyclic photophosphorylation, a proton is removed from the stroma and added to NADP to make reduced NADPH

NADP + H+ + 2e- -> reduced NADP
(H+ - proton in stroma)
This lowers the concentration of protons in the stroma

Secondly during photolysis, protons produced from water in the thylakoid interior.
(H2O -> 2e- + 1/2 O2 + 2H+)
-> (above or below) - photolysis

Both of these processes enhance the proton gradient, produced by the electron transport chains

This proton gradient is now used to generate ATP

The enzyme ATP synthase is found on the thylakoid membrane. ATP synthase contains an ion channel through the centre. Protons can diffuse down the gradient through the ion channel into the stroma.
This movement of protons is used by ATP synthase to generate ATP from ADP + Pi)

ADP + Pi –> ATP
-> (above or below ) ATP synthase

This process is called chemiosmosis
Chemiosmosis is how ATP is generated in photosynthesis

https://images.nagwa.com/figures/explainers/357165907648/10.svg

https://s3.eu-west-2.amazonaws.com/elements.cognitoedu.org/3e8fc59a-db74-4e87-82c1-b1152a685ed8/non-cyclic-photophosphorylation-diagram.png

https://materialsfutures.org/fileCLZW/journal/article/clzw/2022/4/mfaca346f20_hr.jpg

Question 15

Describe the light-independent reactions in photosynthesis (the Calvin Cycle)

Answer 15

Light-independent reactions take place in the stroma
The LIR’s use the ATP and NADPH produced by the LDR’s

During the LIR’s, carbon dioxide is used to make glucose
The LIR’s do not require light

The Light-independent reactions are actually a cycle. It is called the Calvin Cycle

The first stage of the Calvin Cycle is called fixation
In this stage, the 5 carbon molecule ribulose bisphosphate (RuBP) reacts with carbon dioxide.

This reaction is catalysed by the enzyme Ribulose Bisphosphate Carboxylase (Rubisco)

From this reaction, two molecules of glycerate-3-phosphate (2 x GP) are produced. Each molecule of GP has 3 carbon atoms

The next stage is called Reduction
In this stage, each molecule of glycerate-3-pohosphate is reduced forming the three carbon sugar Triose Phosphate (2 x TP)

This reaction uses reduced NADP and ATP from the light-dependent reactions

The reduced NADP provides the hydrogen needed for reduction and the ATP provides energy
(Reduced NADP -> NADP)
(ATP -> ADP + Pi)

Triose phosphate is a really important molecule
Some of the TP is removed from the cycle and used by the plant cell to make glucose and other molecules such as lipids and amino acids (or nucleotides)

The final stage of the Calvin Cycle is called regeneration
In this stage, the RuBP is regenerated using ATP
(ATP -> ADP + Pi)

The regeneration stage is really important as this allows the cycle to continue

Each turn of the Calvin Cycle takes in or fixes one carbon atom from carbon dioxide
A major product of the Calvin cycle is glucose which contains six carbon atoms
So this means that the Calvin cycle has to turn six times to make one molecule of glucose

https://s3.eu-west-2.amazonaws.com/elements.cognitoedu.org/6fa15b93-5d8b-4860-be18-eb6fa911ab84/calvin-cycle-diagram.png

Most TP is used to regenerate RuBP using ATP.
The rest of the TP can be used to make other organic molecules.

For each turn of the Calvin cycle, five carbons are used to regenerate RuBP (5C) and only one carbon, from CO2 (1C), is available to make new organic compounds. This means six full turns of the Calvin cycle are needed to make one molecule of glucose (6C).

Question 16

Describe limiting factors in photosynthesis

Question 17

State limiting factors of photosynthesis

Answer 17

Light energy, CO2, water are all essential for photosynthesis (limiting factors of photosynthesis)

Question 18

What happens to rate of photosynthesis, if we increase the light intensity

Answer 18

When the light intensity is very low, the rate of photosynthesis is also very low

That is because the light intensity will be too low for the light-dependent reactions to function at their maximum rate

Because of this, the levels of ATP and reduced NADP will also be low so the LIR’s will also be running slowly

As we increase the light intensity, the rate of photosynthesis increases.
That is because the light-dependent reactions have increased in rate, producing more ATP and reduced NADP.
Because of this, the LIR’s are also running at a faster rate

So if we increase light intensity, then the rate of photosynthesis increases, we know that light intensity is the limiting factor for photosynthesis

As we continue to increase the light intensity, at a certain point, the rate of photosynthesis stops increasing. Now another factor is the limiting factor e.g. conc. of co2

Question 19

As we continue to increase the light intensity, at a certain point, the rate of photosynthesis stops increasing. Now another factor is the limiting factor e.g. conc. of co2

how can we see if conc. of co2 is really a limiting factor, or it is something else

Answer 19

As we continue to increase the light intensity, at a certain point, the rate of photosynthesis stops increasing. Now another factor is the limiting factor e.g. conc. of co2

We can see if this is the case by repeating the experiment with a higher co2 concentration

In this case, we can see that the rate of photosynthesis increased showing that CO2 conc. was the limiting factor

https://o.quizlet.com/T5EfCwFLJTb9YskLc037Tw.png

https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcTRu7wQLCRmxF9bWNia-tz4XNFFJUkTK19QhCg0vaFKcN_YwZOg

Question 20

Explain how CO2 conc. is a limiting factor, if light intensity is high

effect of CO2 on rate of photosynthesis

Answer 20

At very high light intensities, the LDR’s are running at a very fast rate
This means that the levels of ATP and reduced NADP are very high

So in this case, the light-independent reactions could run at a very fast rate

However, the light independent reactions also require CO2.
So in this case, the conc. of CO2 was acting as the limiting factor

Question 21

How can we tell if a factor is a limiting factor

Answer 21

When we increase the level of a factor and this increases the rate of a reaction, then we know that, that factor was limiting the rate of that reaction

So if we increase light intensity, then the rate of photosynthesis increases, we know that light intensity is the limiting factor for photosynthesis

Question 22

Describe the effect of temperature on the rate of photosynthesis

Answer 22

A key enzyme in the Calvin cycle is Rubisco, which catalyses the fixation of carbon dioxide.

If the temperature is too low, then Rubisco will not be functioning at a fast rate.

So this will limit the rate of the LIR’s and the overall rate of photosynthesis

Question 23

Explain how that if the light intensity if low then the overall rate of photosynthesis is less affected by temperature

Answer 23

At low light intensity, the overall rate of photosynthesis is limited by the rate of the LDR’s

These (The LDR’s) are less dependent on enzymes and so are less effected by temperature

So at low light intensity, temperature has less of an effect on the overall rate of photosynthesis

(so if both light intensity and temp. are low. It is better to increase light intensity first, before temperature)

Question 24

Explain how water is a limiting factor in photosynthesis

Answer 24

Water is not usually considered to be a limiting factor even though water is required for photosynthesis

If water becomes scarce, then plants close their stomata to reduce the rate of transpiration

However, this also prevents CO2 from diffusing into the leaves. So in this case the conc. of CO2 becomes the limiting factor

Question 25

Describe how limiting factors can affect the concentrations of key molecules in the Calvin cycle

Answer 25

At low light intensity, the LDR's is slow and the concentration of ATP and reduced NADP fall. Less GP can now be converted to TP So the concentration of GP increases and the concentration of TP decreases As RuBP is made from triose phosphate, the concentration of RuBP also decreases If the concentration of CO2 falls, then less GP can be formed This means that less TP will also be formed However the conc. of RuBP will increase, as less RuBP is reacting with CO2 A key enzyme in the Calvin cycle is Rubisco, which catalyses the fixation of carbon dioxide. If the temperature is too low, then Rubisco will not be functioning at a fast rate. So this will limit the rate of the LIR's and the overall rate of photosynthesis