Photosynthesis

Question 1

Write out the overall chemical reactions for photosynthesis and respiration.

Answer 1

Photosynthesis:
6CO2 + 6H20 ↔ C6H12O6 + 6O2
Respiration:
C6H12O6 + 6O2 ↔ 6CO2 + 6H20 (+ATP)

Question 2

Describe the relationship between photosynthesis and respiration, and use this to explain the importance of photosynthesis for consumers as well as producers.

Answer 2

Photosynthesis produces glucose. Respiration uses the glucose to release energy. This energy/ glucose is passed to the consumers when they eat the producers.

Question 3

Use the concept of “bond energy” to explain why photosynthesis requires energy from the sun and stores energy whereas respiration releases energy that can be used to make ATP.

Answer 3

In respiration, large organic molecules are broken down to small inorganic molecules. Small inorganic molecules have much stronger bonds than big organic molecules so they release a lot of energy when formed. As a result, the total energy required to break all the bonds in a complex organic molecule is less than the total energy released in the formation of all the bonds in the smaller inorganic products, so there is an excess of energy which is released. This is an exothermic reaction.

In photosynthesis, large organic molecules are made from small inorganic molecules. It requires more energy to break the bonds of the small inorganic molecules that is released in the formation of the large organic molecules. Therefore energy is required from the sun. This is an endothermic reaction.

Question 4

Describe the structure of chloroplasts.

Answer 4

• Double membrane
• The interior fluid is called the stroma
• Thylakoids, internal network of membranes
• a granum is a stack of thylakoid membranes
• grana are joined together by inter-granal lamellae
• surface of thylakoid membranes has lots of chlororphyll.

Question 5

Name the two main stages of photosynthesis and state where each occurs in a chloroplast.

Answer 5

1) LDR - happens in photosystems which are embedded in the thylakoid membranes.
2) LIR - Takes place in the stroma.

Question 6

Define the term “photosynthetic pigment”.

Answer 6

• pigment molecules which can absorb light
• different pigments can absorb different wavelengths of light
• found in the chloroplasts
• used in the light dependent stage of photosynthesis

Question 7

Define the term “light harvesting system/ antennae complex’

Answer 7

A group of protein and chlorophyll molecules found in the thylakoid membrane of the chloroplasts in a plant. The role of system is to absorb or harvest light energy of different wavelengths and transfer this energy quickly and efficiently to the reaction centre.

Question 8

Define the term“reaction centre”.

Answer 8

The site in the chloroplast that receives the energy trapped by chlorophyll and accessory pigments and initiates the electron transfer process. Chlorophyll a is located in the reaction centre.
(the light harvesting system and reaction centre are collectively known as a photosystem)

Question 9

Define the term “photosystem”.

Answer 9

Protein complexes involved in the absorption of light energy and electron transfers in photosynthesis.

Question 10

Name the photosynthetic pigment in the reaction centre of a photosystem.

Answer 10

Chlorophyll a

Question 11

Name 3 types of photosynthetic pigments found in the antennae complex/ light harvesting system.

Answer 11

chlorophyll b, carotenoides, xanthophylls,

Question 12

Explain why it is useful for photosynthetic organisms to have many different photosynthetic pigments.

Answer 12

Because different photosynthetic pigments absorb different wavelengths. This means that they can adjust to different intensities of light.

Question 13

Label and annotate an absorption spectrum graph to explain what it shows.

Answer 13

Graph: wavelength of light on x, absorbance at y.
Pattern: Peaks at red and dark blue, troughs at green and light blue.
This shows that the plant’s photosynthetic are most suited to absorbing red/ dark blue light and cannot absorb green light.

Question 14

Describe the purpose of chromatography.

Answer 14

To show the different photosynthetic pigments in a plant extract. The pigments are separated on the chromatography paper because the rate at which they diffuse up the paper is varied.
An RF values can be calculated for each pigment (RF values is distance moved by pigment/ over distance moved by solvent. This value will help you identify the different pigments: least soluble will have moved the least distance and so will have a smaller RF value.

Question 15

Describe a step by step method for conducting thin layer chromatography to separate and identify photosynthetic pigments.

Answer 15

1) Draw pencil on chromatography paper.
2) Obtain leaf extract by grinding leaves with propanone until a dark green solution forms.
3) Transfer leaf extract with a capillary tube to the strip of chromatography paper, creating a small spot on the centre of the pencil line.
4) Put the strip in a chromatography tube with a solvent in the bottom. The solvent in the tube shouldn’t reach the spot of plant extract.
5) Leave it and remove the strip once the solvent is about one centimeter from the top.
6) Mark the solvent line and each pigment line with pencil.
7) Calculate RF values to identify the different pigments.

Question 16

Explain what determines how far a particular molecule travels in chromatography.

Answer 16

Depends on solubility of molecule and interactions (hydrogen bonds).

Question 17

Draw a diagram to summarise the light-dependent stage of photosynthesis and state where this occurs.

Answer 17

Non-cyclic photophosphorolation:

1) Light is absorbed by pigments in PSII. This absorbed light excites electrons at the reaction centre of the photosystem.
2) The excited are released from PSII and are passed on to the electron transport chain. ATP is produced by the process of chemiosmosis.
3) Electrons lost from PSII are replaced by the photolysis of water.
4) At PSI, the electrons receive another boost of energy and so become excited. They are then released from the reaction centre of PSI.
5) The electrons lost from PSI are replaced by electrons coming along the electron transport chain from PSII.
6) The electrons leaving the electron transport chain following PSI are accepted, along with hydrogen ions, by the coenzyme NADP, forming reduced NADP (uses enzyme NADP reductase).

Question 18

Name the two useful products, the waste product, and the requirements, of the light-dependent stage of photosynthesis.

Answer 18

Two useful products:

• ATP
• Reduced NADP

Waste product:
- Oxygen from the photolysis of water

Question 19

Define the term “phosphorylation”.

Answer 19

The addition of a phosphate group to a molecule.

Question 20

Define the term “photophosphorylation”.

Answer 20

The addition of a phosphate group to a molecule using light energy.

Question 21

Define the term “cyclic photophosphorylation”.

Answer 21

Synthesis of ATP involving only PSI.

Question 22

Define the term “non-cyclic photophosphorylation”.

Answer 22

The synthesis of ATP and reduced NADP involving PSII and PSI.

Question 23

Define the term “photolysis”. Give the equation.

Answer 23

Water molecules are split into hydrogen ions, electrons and oxygen molecules using energy from the sun.

2H2O → 4e- + 2H+ + O2

Question 24

Draw, label and annotate a diagram to show the process of cyclic phosphorylation (describe the process).

Answer 24

Process is the same up to PSI but then the electrons that leave PSI are returned to PSI instead of being used to form reduced NADP.
This means PSI can still lead to the production of ATP without any electrons supplied from PSII.

Question 25

Describe the process of photolysis and what the products of photolysis are used for. (Cyclic photophosphorlyation)

Answer 25

1) There is an enzyme which is part of PSII which catalyses the breakdown (photolysis) of water. The products are electrons, hydrogen ions and oxygen.
2) The electrons are used to replace the lost electrons from the reaction centre of PSII.
3) The H+ ions build up in the lumen of the thylakoid. This causes the concentration of H+ ions to build up across the membrane = H+ ion concentration gradient.
4) This concentration gradient causes the H+ ions to move back through the thylakoid membrane down the concentration gradient, via the channel protein and enzyme, ATP synthase.
5) The H+ ions passing through the enzyme catalyses the production of ATP.
6) Once the H+ ions have returned to the stroma, they combine with NADP and an electron from PSI to form reduced NADP (catalysed by NADP reductase).

Question 26

Draw a diagram to summarise the light-independent stage of photosynthesis and state where this occurs.

Answer 26

Calvin cycle - takes place in the stroma of chloroplasts

1) CO2 diffuses into stroma through the stomata and spongy mesophyll.
2) The CO2 combines with a five-carbon molecule called Ribulose Biphosphate (RuBP). The carbon in the carbon dioxide it therefore ‘fixed’ - it is incorporated into an organic molecule.
3) The enzyme ribulose bisphosphate carboxlyase (RuBisCo) catalyses this carbon fixation and an unstable 6-carbon molecule is formed. (RuBisCo is a very inefficient enzyme as it is competively inhibited by oxygen, so a lot of it is need for photosynthesis).
4) The unstable 6-carbon molecule immediately breaks down, forming two 3-carbon glycerate 3-phosphate (GP) molecules.
5) Each GP molecule is converted into another 3-carbon molecule, triose phosphate (TP), using a hydrogen atom from reduced NADP and the energy supplied by ATP.
6) Most of the TP is recycled to regenerate RuBP so that the calvin cycle can continue. The rest is used in the synthesis of glucose.

Question 27

What is required for the Calvin Cycle to take place?

Name the useful product of, the three requirements of, and the molecules that are returned to the light-dependent stage from, the light-independent stage of photosynthesis.

Answer 27

• CO2, from the atmosphere
• RuBP, regenerated in Calvin Cycle
• ATP, from light-dependent stage
• reduced NADP, from light-dependent stage

Question 28

What are the products of the Calvin?

Answer 28

• TP (triose phosphate)
• Regenerated RuBP
• ADP
• Oxidised NADP

Question 29

What is returned to the light-dependent reaction from the calvin cycle?

Answer 29

• ADP

- Oxidised NADP

Question 30

Define RuBisCo

Answer 30

A key enzyme involved in the carbon fixation stage of the calvin cycle.

Question 31

Define GP

Answer 31

A carbohydrate, three-carbon sugar which is used in the synthesis of many bio molecules.

Question 32

Define carbon fixation.

Answer 32

When carbon is incorporated into an organic molecule.

Question 33

Describe the roles of ATP and reduced NADP in the Calvin cycle.

Answer 33

ATP - provides energy for the conversion of GP into TP, and the regeneration of RuBp.
reduced NADP - provide hydrogen atoms to convert GP into TP.

Question 34

Explain how RuBP is regenerated in the Calvin cycle.

Answer 34

For 1 glucose molecule to be produced, 6 carbon dioxide molecules have to enter the calvin cycle, resulting in 6 full turns of the cycle.
For every cycle, 2 TP molecules are produced, so 12 in total. 2 of these 12 TP molecules are removed to make glucose, the remaining 10 are recycled to regenerate 6 RuBp molecules (used in 6 turns of the calvin cycle).
The energy for this regeneration is provided by ATP.

Question 35

Describe the uses of triose phosphate.

Answer 35

• regeneration of RuBp
• it is the starting point for the synthesis of glucose and other biological molecules such as other carbohydrates, lipids, proteins and nucleic acids.

Question 36

Label and annotate an action spectrum graph to explain what it shows.

Answer 36

• action spectrum: rate of physiological activity plotted against wavelength of light.
• wavelength of light (x axis), relative effectiveness of photosynthesis (y axis).
• The peaks show which wavelenghts of light are best for photosynthesis, the troughs show which are the worst.

Question 37

Describe the difference between an absorption spectrum graph and an action spectrum graph.

Answer 37

They both show roughly the same thing. Absorption spectrum plots relative absorption against wavelength of light so shows which wavelengths absorb the most light.

Question 38

Define the term “limiting factor”.

Answer 38

A factor that limits the rate of a process.

Question 39

State 5 factors that affect the rate of photosynthesis.

Answer 39

• Light intensity
• Temperature
• PH
• Carbon dioxide concentration.
  (- Water)

Question 40

Explain why wavelength of light cannot be a limiting factor for photosynthesis whereas light intensity, temperature and carbon dioxide concentration can be.

Answer 40

• Chloroplasts contain many different photosynthetic pigments (i.e. chlorophyll a/b, xanthophyll, carotenoids) which absorb different wavelengths of life.

Question 41

Describe how light intensity can affect the rate of photosynthesis.

Answer 41

• Light is needed as an energy source for the LDR. As light intensity increases, ATP and reduced NADP are produced at a higher rate. If there is no light, ATP and red NADP will not be produced and therefore the LIR will not take place.

Question 42

Describe how temperature can affect the rate of photosynthesis.

Answer 42

• Affects rate of enzyme-controlled reactions. As temperature increases, the rate of enzyme activity increases until the point when the proteins denature. -
• An increase in temperature increases the rate of enzyme-controlled reactions such as carbon fixation.

Question 43

Explain how graphs of the rate of photosynthesis under different environmental conditions can show what is limiting the rate of photosynthesis under a particular set of conditions. (F)

Answer 43

see graph sheet in folder - try to explain it to yourself.

Question 44

Explain how water stress limits the rate of photosynthesis but water availability itself is not considered to be a limiting factor.

Answer 44

• When the plant undergoes water stress , the stomata on the leaves will close. This stops the diffusion of CO2 into the plant, reducing the rate of LIR and eventually stopping photosynthesis.
• Although water is required for photosynthesis, it is never considered a limiting factor because for the water potential to have become low enough to limit the rate of photosynthesis the plant will already have closed its stomata and ceased photosynthesis.

Question 45

Describe how the rate of photosynthesis can be measured.

Answer 45

• Use a data logger to get an accurate reading for the volume of oxygen produced, volume of CO2 lost, or increase in dry mass of the plant.

Question 46

Describe a step by step method to investigate how one environmental factor affects the rate of photosynthesis.

Answer 46

1) Factors affecting rate of photosynthesis can be investigated using live pond weed (elodea) .
2) Keep pondweed in the dark before use.
3) Place pondweed in a conical flask with sodium hydrogen carbonate, which acts as the CO2 supply.
4) Leave apparatus to equilibrate for 10 mins before readings are taken.
5) Use oxygen sensor (connected to data logger) to take readings of concentration of oxygen in the solution. Sensor may need to be calibrated first using the oxygen concentration of the air.
6) Software can be set up to take readings at desired intervals/ for a desired length of time.

Question 47

Define the term “compensation point” in relation to light intensity and describe how it can be determined for a particular photosynthetic organism.

Answer 47

Compensation point - rate of respiration = rate of photosynthesis. For example, O2 is used at the same rate it is produced.

At high light intensities, the LDR works at a much faster rate so chloroplasts produced O2 faster than it is used in by the mitochondria in respiration - O2 conc rises. In low light intensities, O2 production drops because the LDR slows down, yet respiration in the mitochondria continues - O2 conc falls. A compensation point has been reached when the O2 concentration in the atmosphere of the plant remains constant. This can be measured using an oxygen sensor.

Question 48

Explain how changing carbon dioxide concentration affects the concentration of RuBP, TP and GP in the Calvin cycle.

Answer 48

As carbon dioxide is an essential substrate of the calvin cycle, low concentrations will lead to reduced concentrations of GP (as there’s less carbon dioxide to be fixed) and TP. The concentration of RuBp will temporarily increase as it is still being formed from TP but not being used to fix carbon dioxide.

Question 49

Explain how changing temperature affects the concentration of RuBP, TP and GP in the Calvin cycle.

Answer 49

All reactions making up the calvin cycle are catalysed by enzymes, for example, RuBisCO in carbon fixation. At lower temps enzyme and substrate molecules have less kinetic energy resulting in fewer successful collisions and a reduced rate of reaction. This means decreasing temperature results in lower concentrations of GP, TP and RuBP.
The same effect will be seen at very high temperatures as enzymes will be denatured.

Question 50

Describe methods for increasing the rate of photosynthesis in a green house.

Answer 50

Light - artificial lighting may be used in winter, and blinds can be rolled up or down to control the amount of light entering.

Humidity - if too high it encourages the growth of fungi and other organisms. Humidity can be raised using a mist spray, and can be lowered by opening ventilators or increasing the temp.

Water in soil - amount of water in soil must be controlled. Too much water and plants are water logged, too little and they wilt,

Temp - adjusting heating, opening and closing ventilators.

CO2 in air - can provide extra CO2 by burning gas/ paraffin, or it can be released from cylinders.

Question 51

Describe the differences between non-cyclic and cyclic photophosphorylation.

Answer 51

Non-cyclic:

• Uses both PSII and PSI
• Produced red. NADP and ATP
• Electrons from PSII do not return to the cycle
• Involves photolysis of water

Cyclic:

• Uses only PSI
• Produces only ATP
• Electrons from PSII are returned to the cycle
• Does not involve the photolysis of water.