Photosynthesis

Question 1

Describe the site of photosynthesis in eukaryotic plants

Answer 1

• The leaf is the main photosynthetic structure in eukaryotic plants.
• Chloroplasts are the cellular organelles within the leaf where photosynthesis takes place.

Question 2

Describe how leaves are adapted to bring together the three raw materials of photosynthesis (water, carbon dioxide,light) and remove its products (oxygen and glucose)

Answer 2

1) a large surface area that absorbs as much sunlight as possible.
2) an arrangement of leaves on the plant that minimises overlapping and so avoids the shadowing of one leaf by another.
3) Thin as most light is absorbed in the first few micrometers of the leaf and the diffusion distance for gases is kept short.
4) A transparent cuticle and epidermis that let light through to the photosynthetic mesophyll cells beneath.
5) Long, narrow upper mesophyll cells packed with chloroplasts that collect sunlight.
6) Numerous stomata for gaseous exchange so that all mesophyll cells are only a short diffusion pathway from one.
7) Stomata that open and close in response to changes in light intensity.
8) Many air spaces in the lower mesophyll layer to allow rapid diffusion in the gas phase of carbon dioxide and oxygen
9) A network of xylem that brings water to the leaf cells, and phloem that carries away the sugars produced during photosynthesis.

Question 3

Summarise the three key phases of photosynthesis (in chronological order)

Answer 3

1) Capturing of light energy by chloroplast pigments such as chlorophyll.
2) The light dependant reaction, in which some of the light energy absorbed is conserved in chemical bonds. During the process, an electron flow is created by the effect of light on chlorophyll, causing water to split (photolysis) into protons, electrons and oxygen. The products are reduced NADP, ATP and oxygen.
3) The light independent reaction, in which these protons are used to produce sugars and other organic molecules.

Question 4

What are the substances used in the light dependant stage of photosynthesis

Answer 4

• H2O
• ADP
• NADP
• Phosphate Pi

Question 5

What are the end products of the light dependant stage of photosynthesis

Answer 5

• NADPH
• ATP
• Oxygen

Question 6

What are the substances used in the light independent stage of photosynthesis

Answer 6

• CO2
  -NADPH
• ATP

Question 7

What are the end products of the light independent stage of photosynthesis

Answer 7

• C to make organic substances
• ADP +Pi which reform ATP

Question 8

What is the basic structure of a chloroplast

Answer 8

• Chloroplasts are small, disc-shaped organelles that are surrounded by a double membrane.
• Within the double membrane there are two distinct regions: The grana and the stroma

Question 9

Explain what the grana and thylakoids are and the role they play in photosynthesis

Answer 9

• The grana are stacks of up to 100 disc shaped structures called thylakoids
• Thylakoids are the site of the light dependant stage of photosynthesis.
• Thylakoids contain the photosynthetic pigment chlorophyll.
• Some thylakoids have tubular extensions that hook up with thylakoids in adjacent grana.
• These are called inter-granal lamellae.

Question 10

Explain what the stroma is and the role it plays in photosynthesis

Answer 10

• The stroma is a fluid filled matrix where the light independent stage of photosynthesis takes place.
• Within the stroma are a number of other structures such as starch grains.

Question 11

For what two purposes is light energy used for in the light dependant stage of photosynthesis

Answer 11

1) To add an inorganic phosphate molecule to ADP, thereby making ATP.
2) To split water into H+ ions (protons) and OH- ions. This is called photolysis.

Question 12

What three things can oxidation refer to

Answer 12

• loss of electrons
• gain of oxygen
• loss of hydrogen

Question 13

What three things can reduction refer to

Answer 13

• gain of electrons
• loss of oxygen
• gain of hydrogen

Question 14

Describe the first part of the light dependant stage of photosynthesis- how ATP is made

Answer 14

1) When light is absorbed by chlorophyll upon hitting photosystem II, it causes electrons to become excited and raise to a higher energy level.
2) This causes the electrons to leave the chlorophyll, meaning the chlorophyll becomes ionised via photoionisation.
3) The electrons that leave the chlorophyll are taken up by an electron carrier.
4) Having lost a pair of electrons, the chlorophyll has become oxidised whilst the electron carrier that has gained them has been reduced.
5) The electrons are passed along a number of electron carriers in a series of oxidation-reduction reactions.These electron carriers form a transfer chain that is located in the membranes of the thylakoids.
6) Each new carrier is at the slightly lower energy level than the previous one in the chain, and so electrons lose energy at each stage.
7) Some of this energy is used to combine an inorganic phosphate molecule with an ADP molecule to make ATP by a mechanism that is explained by chemiosmotic theory.

Question 15

Describe the chemiosmotic theory by which ATP is produced in the light dependant stage of photosynthesis.

Answer 15

• Each thylakoid is an enclosed chamber into which protons are pumped from the stroma using protein carriers in the thylakoid membrane called proton pumps.
• The energy to drive this process comes from electrons released when water molecules are split by light (photolysis)
• The photolysis of water also produced protons which further increases their concentration inside the thylakoid space.
• Overall this creates and maintains a concentration gradient of protons across the thylakoid membrane with a high concentration inside the thylakoid space and a low concentration in the stroma.
• The protons can only cross the thylakoid membrane through ATP synthase channel proteins (stalked granules)- the rest of the membrane is impermeable to protons.
• As the protons pass through these ATP synthase channels they cause changes to the structure of the enzyme which then catalyses the combination of ADP and Pi to form ATP.

Question 16

Describe the photolysis of water and how the products are used in the light dependant stage of photosynthesis

Answer 16

1) Photolysis replaces the electrons that are lost when light hits the chlorophyll.
2) Photolysis is the splitting of water using light: it produces protons, electrons and oxygen
3) The protons produced pass out of the thylakoid space through the ATP synthase channels and are taken up by an electron carrier called NADP.
4) On taking up the protons, the NADP becomes reduced.

Question 17

How are the products of the light dependant stage of photosynthesis (NADPH and oxygen) used after this stage

Answer 17

• The reduced NADP is the main product of the light-dependant stage and it enters the light independent reaction, taking with it the electrons from the chlorophyll molecules.
• The NADPH is important because it is a further potential source of chemical energy to the plant.
• The oxygen by-product from the photolysis of water is either used in respiration or diffuses out of the leaf as a waste product of photosynthesis.

Question 18

Where does the light dependant stage of photosynthesis take place

Answer 18

In the thylakoids/granum of the chloroplasts

Question 19

What are the three key processes that you must cover when describing the light dependant stage of photosynthesis

Answer 19

1) The making of ATP starting with the excitement of electrons.
2) The chemiosmotic theory of ATP formation
3) The photolysis of water

Question 20

Describe how the chloroplasts are adapted to carry out the light dependant reaction of photosynthesis

Answer 20

• The thylakoid membranes provide a large surface area for the attachment of chlorophyll, electron carriers and enzymes that carry out the light dependant reaction.
• A network of proteins in the grana hold the chlorophyll in a very precise manner that allows maximum absorption of light.
• The granal membranes have ATP synthase channels within them, which catalyse the production of ATP. They are also selectively permeable which allows establishment of a proton gradient.
• Chloroplasts contain both DNA and ribosomes so they can quickly and easily manufacture some of the proteins involved in the light-dependant reaction.

Question 21

Where does the light independent stage of photosynthesis take place

Answer 21

The stroma of the chloroplasts

Question 22

Describe the light independent stage of photosynthesis (Calvin cycle)

Answer 22

1) Carbon dioxide from the atmosphere diffuses into the leaf through stomata and dissolved in water around walls of the mesophyll cells. It then diffuses through the cell-surface membrane, cytoplasm and chloroplast membranes into the stroma of the chloroplast.
2) In the stroma, the carbon dioxide reacts with the 5-carbon compound ribulose bisphophate (RuBP) a reaction catalysed by an enzyme called ribulose bisphosphate carboxylate, otherwise known as rubisco.
3) The reaction between the carbon dioxide and RuBP produces two molecules of the 3-carbon glycerate-3-phosphate (GP).
4) Reduced NADP from the light dependant reaction is used to reduce glycerate-3-phosphate to triose phosphate (TP) using energy supplied by ATP
5) The NADP is re-formed and goes back to the light-dependant reaction to be reduced again by accepting more protons.
6) Some triose phosphate molecules are converted to organic substances that the plant requires such as starch, cellulose, lipids, glucose, amino acids, and nucleotides.
7) Most triose phosphate molecules are used to regenerate ribulose bisphosphate using ATP from the light-dependant reaction.

Question 23

Describe how the chloroplast is adapted to carry out the light-independent stage of photosynthesis

Answer 23

• The fluid of the stroma contains all the enzymes needed to carry out the light independent reaction. Stromal fluid is membrane bound in the chloroplast which means a chemical environment which has a high concentration of enzymes and substrates can be maintained within it- as distinct from the environment of the cytoplasm.
• The stroma fluid surrounds the grana and so the products of the light-dependant reaction in the grana can readily diffuse into the stroma.
• It contains both DNA and ribosomes so it can quickly and easily manufacture some of the proteins involved in the light-independent reaction.

Question 24

What are the four stages that aqa splits the light dependant reaction into

Answer 24

1. Photolysis of water
2. Photoionisation of chlorophyll
3. Chemiosmosis
4. Production of ATP and reduced NADP