Photosynthesis (CQC)

Question 1

What is the equation for photosynthesis

Answer 1

carbon dioxide + water –> glucose + oxygen

Question 2

how to perform chromatography on a leaf.

Answer 2

• tear up a leaf into small pieces
• grind the leaf pieces and propanone to extract the leaf pigments
• transfer a sample of extract to a watchglass
• evaporate to dryness with hot air from a hairdryer
• add a few drops of propanone to dissolve the pigments
• build up a concentrated spot 10mm from the end of a strip of chromatography paper by transferring tiny drops of solution.
• suspend the strip in a tube with the base dipping into the running solvent
• remove the strip from the tube when the running solvent has nearly reached the top. draw a pencil line to show how far the solvent moved.
• the pigment in each spot can be identified from its colour and its rf value. Rf is the distance moved by a spot, as a proportion of the distance moved by the solvent. Pigments move at different rates. the rate depends on whether a pigment is more attracted to the hydrophobic running solvent or to the hydrophilic chromatography strip. typical rf values are shown on the diagram.

Question 3

what happens when chlorophyll absorbs light?

Answer 3

Pigments such as chlorophyll have a structure that allows an electron within a molecule to jump from one energy level up to a higher energy level, using energy obtained by absorbing a photon of light. This is how solar energy is transformed into chemical energy in photosynthesis. The electron is said to be “excited”. photosynthetic pigments can pass excited electrons on to other molecules. Much of the energy carried by the excited electron ends up in glucose or other carbon compounds.
However, only specific wavelengths of light are able to raise the electron to the higher level, other wavelengths are reflected.

Question 4

what is an absorption and action spectrum?

Answer 4

Absorption?
A graph showing relative amounts of photosynthesis at different wavelengths of light is an absorption spectrum.

What is the action spectrum?
The action spectrum shows that some green light is used in photosynthesis, even though chlorophyll absorbs little. This is due to pigments such as xanthophyll and carotene harvesting some wavelengths that chlorophyll does not.

Question 5

How does light intensity limit the rate of photosynthesis?

Answer 5

• as light intensity increases, the rate of the light-dependent reactions increases
• more electrons in chlorophyll are excited
• increased rate of photolysis
• increased rate of ATP and reduced NADP production

the graph which displays this increases readily until a plateau.

Question 6

how does increasing temperature impact the rate of photosynthesis

Answer 6

• Enzymes and substrate molecules gain kinetic energy
• more successful collisions near the optimum temperature
• increase in rate of light-dependent reactions (NADP reductase is working more efficiently)
• if temperature is too high enzymes denature and water loss increases causing stomata to close (no CO2 for light-independent reaction)

Its graph increases to a maximum and then decreases

Question 7

how does increasing carbon dioxide concentration increase the rate of photosynthesis/

Answer 7

• increase in carbon dioxide concentration, increases the rate of carbon fixation in the light independent reaction. More CO2 combines with RuBP to form GP, this is catalysed by rubisco
• normally present at 0.04%

its graph increases to a maximum and then plateaus

Question 8

how can you measure the rate of photosynthesis?

Answer 8

Rate of photosynthesis= change in concentration/ time

There are two main ways of measuring the rate of photosynthesis

1. oxygen production: counting bubbles
2. carbon dioxide uptake: measuring water acidity

Question 9

how can light intensity be altered?

Answer 9

change the distance from the lamp

Question 10

how can carbon dioxide be altered

Answer 10

add sodium hydrogen carbonate

Question 11

how can temperature be altered?

Answer 11

submerge in a hot water bath

Question 12

what are photosystems?

Answer 12

Photosystems are molecular arrays of chlorophyll and other accessory pigments with special chlorophylls as the reaction centre, from which pairs of excited electrons are emitted. Photosystems are located in thylakoid membranes. In photosynthetic eukaryotes, thylakoids are flattened and arranged in stacks inside chloroplasts. They have two types of photosystems (PSI and PSII) with different structures and functions.

PSI is positioned where the thylakoid membrane is exposed to the surrounding stroma, whereas PSII does not need to be in contact with the stroma. Cyanobacteria also have thylakoids with PSI and II but they are not located in chloroplasts and the pigments are arranged differently.

Question 13

what happens in the light dependent reaction?

Answer 13

1. light is absorbed by chlorophyll in PSII. An electron in chlorophyll is excited, raised to a higher energy level and transferred to an electron carrier, causing chlorophyll to become positively charged as it loses an electron
2. photolysis of water. Light energy splits water into protons, electrons and oxygen. Oxygen diffuses out, and the electrons replace those lost by PSII
3. Electrons pass between elctron carriers down the electron transport chain. Energy lost by electrons drives the reaction ( ADP +Pi -> ATP) in phosphotylation. This reaction is catalysed by ATP synthase.
4. Light is absorbed by PSI exciting another electron. Protons from the photolysis of water and these electrons combine with NADP to form reduced NADP. This reaction is catalysed by NADP reductase

The ATP and NADPH is required for the light independent reaction.

Question 14

Which two ways can the electrons be supplied by for chemiosmosis?

Answer 14

1. Cyclic Phosphorylation: energy is absorbed by chlorophyll in PSI, electrons are excited to a higher energy level and transferred to an electron acceptor. The electron then moves down the electron transport chain, releasing energy. The energy released is used to move protons into the thylakoid space. Protons then diffuse through ATP synthase, driving the reaction ADP+ Pi –> ATP. The electrons are then passed back to PSI (not NADP, so no reduced NADP made)
2. Non-cyclic phosphorylation: pairs of exicted electrons are emitted by PSII and after passing along the ETC, they flow to PSI, whereafter they are used to reduce NADP

Question 15

what wavelength of light does PSII deal with?

Answer 15

680nm

Question 16

What wavelength of light does PSI deal with?

Answer 16

700nm

Question 17

what are the differences between cyclic and non cyclic phosphorylation?

Answer 17

1. Cyclic phosphorylation only occurs in PSI, whereas non cyclic occurs in PSII and PSI
2. they both take place in the thylakoid membrane
3. Cyclic phosphorylation doesnt require water, whereas non-cyclic does
4. cyclic phosphorylation only uses ATP synthase, whereas non-cyclic uses both ATP synthase and NADP reductase
5. They both involve chemiosmosis
6. They both produce ATP
7. Only non-cyclic phosphorylation produces NADPH

Question 18

what features make the thylakoid membrane good for its job?

Answer 18

• Impermeable to protons so a proton gradient can develop
• Encloses a small volume of fluid so a gradient develops rapidly
• Made of phospholipids so can hold photosystems composed of hydrophobic pigment molecules
• Holds other components in the correct relative positions - ATP synthase, electron carriers and NADP reductase

Question 19

Why are high concentration of rubisco required in the chloroplasts?

Answer 19

High concentrations of Rubisco are needed in the stroma of chloroplasts, because it works relatively slowly and is not effective in low carbon dioxide concentrations. Because of this and because of the abundance of photosynthesizing organisms. Rubisco is the most abundant enzyme on earth.

Question 20

What happens in the light independent reaction (calvin cycle)

Answer 20

1. Carbon dioxide diffuses into the stroma of the chloroplast and is then fixed by being converted into a more complex carbon compound. This happens in a carboxylation reaction catalysed by RUBISCO (enzyme).
2. Carbon dioxide is combined with ribulose bishopsphate (RuBP) (5C). The product of this reaction is an unstable six-carbon compound which splits immediately to form two molecules of glycerate 3-phosphate (G3P)
3. G3P is converted into triose phosphate in a reduction reaction in the stroma. The hydrogen needed for this is supplied by reduced NADP (NADPH). Energy is required and is supplied by ATP (these are from the light dependent reaction)
4. Five molecules of triose phosphate are converted by a series of reactions into three molecules of RuBP. This process requires the use of energy in the form of ATP. For every six molecules of Triose phosphate made by the light- independent reactions, five are used to regenerate RuBP, and one exits the cycle as glucose.