photosynthesis

Question 1

Photosynthesis definition

Answer 1

process by which energy in the form of light from the sun is used to build complex organic molecules (e.g glucose)

carbon dioxide + water –> glucose + oxygen

Question 2

Energy transfer

Answer 2

light energy is transferred to chemical energy trapped in the bonds of the complex organic molecules produced

Question 3

structure + function of chloroplasts

Answer 3

the network of internal membranes provides a large surface area to maximise the absorption of light

membranes form flattened sacs (thylakoids) which are stacked to form grana.

Grana are joined by membranous channels (lamellae)

light is absorbed by complexes of pigments which are embedded in the thylakoid membrane

fluid in the chloroplast (stroma) - the site of many chemical reactions resulting in the formation of complex organic molecules - it contains enzymes, sugars and organic acids

carbohydrates produced by photosynthesis and not used straight away are stored as starch grains in the stroma

Question 4

Photosynthetic pigments including chlorophyll

Answer 4

pigment molecules absorb specific wavelengths of light and reflect others - different pigments absorb + reflect different wavelengths which is why they are different colours

Chlorophyll - usually absorbs red + blue light and reflects green

Pigments include chlorophyll a, chlorophyll b, xanthophylls and carotenoids

pigments are found in the thylakoid membranes (attached to proteins) - the protein + pigment is called a photosystem

proteins + pigments form a light harvesting system (antennae complex) which absorbs light of different wavelengths and transfers this energy to the reaction centre

chlorophyll a is located in the reaction centre - where the reactions involved in photosynthesis take place

photosystem - light harvesting system + reaction centre

Question 5

primary pigments

Answer 5

they are reaction centres where electrons are excited during the light dependent reaction

Question 6

accessory pigments

Answer 6

make up light harvesting systems - surround reactions centres + transfer light energy to them to boost the energy for electron excitement to take place

Question 7

Photosystem 1 optimum wavelength for absorbing light

Answer 7

700 nm

Question 8

Photosystem 2 optimum wavelength for absorbing light

Answer 8

680 nm

Question 9

what are the 2 stages of photosynthesis? (Overview)

Answer 9

Light dependent stage:
- energy from sunlight is absorbed and used to make ATP
- Hydrogen from water is used to reduce coenzyme NADP to reduced NADP

Light independent stage:
- H from reduced NADP and CO2 are used to build organic molecules
- ATP is needed to provide energy for the reaction

Question 10

use of coenzymes in photosynthesis

Answer 10

Coenzyme - a molecule that aids the function of an enzyme - usually by transferring a chemical group from 1 molecule to another

In photosynthesis:
- NADP transfers hydrogen from one molecule to another

Question 11

photolysis definition

Answer 11

the splitting of a molecule using light

Question 12

Photophosphorylation definition

Answer 12

adding phosphate to a molecule using light

Question 13

where does the light dependent reaction take place?

Answer 13

thylakoid membrane

Question 14

Photosystems are linked by…

Answer 14

electron carriers

Question 15

electron carriers definition

Answer 15

proteins that transfer electrons

Question 16

electron transport chain definition

Answer 16

a chain of proteins through which electrons flow

• formed from photosystems and electron carriers

Question 17

Non- Cyclic photophosphorylation produces …

Answer 17

ATP
Reduced NADP
O2

Question 18

Stages of the light dependent reaction

Answer 18

1) LIGHT ENERGY EXCITES ELECTRONS IN CHLOROPHYLL:

• light energy is absorbed by PS II
• Light energy excites electrons in chlorophyll
• the electrons move to a higher energy level
• these high-energy electrons move along the electron transport chain to PS I

2) PHOTOLYSIS OF WATER PRODUCED H+ ions, electrons and O2
- as the excited electrons from chlorophyll leave PS II they have to be replaced
- Light energy splits water into H+, electrons and O2

REACTION EQUATION:
H2O –> 2H+ + 1/2 O2

3) ENERGY FROM THE EXCITED ELECTRONS MAKES ATP:

• the excited electrons lose energy as they move along the electron transport chain
• this energy is used to transport protons into the thylakoid, via membrane proteins (proton pumps) so the thylakoid has a higher proton concentration than the stroma (forms a proton gradient across the membrane)
• protons move down their concentration gradient into the stroma via an enzyme (ATP synthase). The energy from this movement combines ADP and Pi to form ATP

4) Generates reduced NADP

• Light energy is absorbed by PS I which excites the electrons to a higher energy level
• The electrons are transferred to NADP along with a proton from the stroma forming reduced NADP

Question 19

oxygen evolving complex

Answer 19

forms part of PS II
- an enzyme that catalyses the breakdown of water

Question 20

cyclic photophosphorylation

Answer 20

• only uses PS I
• the electrons from chlorophyll aren’t passed onto NADP - instead they are passed back to PS I via electron carriers
• doesn’t produce O2 or reduced NADP - only small amounts of ATP

Question 21

where does the light independent reaction take place?

Answer 21

stroma

Question 22

Stages of the light independent reaction (Calvin cycle)

Answer 22

• doesn’t rely on light energy directly but does rely on the products of the light-dependent reaction

1) CO2 IS COMBINED WITH RIBULOSE BISPHOSPHATE TO FORM 2 MOLECULES OF GLYCERATE 3-PHOSPHATE

• CO2 enters the leaf via the stomata and diffuses into the stroma of the chloroplast
• CO2 is combined with RuBP (a 5-C compound), giving an unstable 6-C compound (carbon fixation). This breaks down into 2 molecules of a 3-C compound - Glycerate-3-phosphate (GP)
• RuBisCo catalyses this reaction between CO2 and RuBP

2) ATP AND REDUCED NADP ARE REQUIRED FOR THE REDUCTION OF GP TO TP

• ATP from the LDR provides the energy to turn GP into a different 3-C compound - triode phosphate (TP)
• This reaction required H+ ions which come from reduced NADP. Reduced NADP is recycled to NADP for use in the LDR
• TP can be converted into many useful compounds e.g glucose

3) RuBP IS REGENERATED
- 5 out of every 6 molecules of TP are used to regenerate RuBP
- this requires the rest of the ATP produced by the LDR

Question 23

Summary of the Calvin cycle

Answer 23

FIXATION:
CO2 is fixed in the first step

REDUCTION
GP is reduced to TP by the addition of H from reduced NADP

REGENERATION:
- RuBP is regenerated from the recycled TP

Question 23

Why are the issues with the enzyme RuBisCo

Answer 23

• it is inefficient as it is competitively inhibited by O2 and lots of it is needed to carry out photosynthesis successfully

Question 24

Regeneration of RuBP

Answer 24

For 1 glucose molecule to be made, 6 CO2 molecules need to enter the Calvin cycle - this results in the production of 12 TP molecules 2 TP molecules are removed to make glucose 10 TP molecules (10 x 3 carbons) are recycled to regenerate 6 RuBP molecules (6 x 5 carbons) The cycle has to turn 6 times to make 1 hexose sugar 6 turns of the Calvin cycle require 18 ATP molecules and 12 reduced NADP

Question 25

What are GP and TP used to make

Answer 25

Carbohydrates - hexose sugars are made by joining 2 TP molecules - larger carbohydrates are made by joining hexose sugars together in different ways Lipids: - glycerol is synthesised from TP - fatty acids are synthesised from GP Amino acids: - some are made from GP

Question 26

Optimum conditions for photosynthesis

Answer 26

1) High light intensity of a certain wavelength - the higher the light intensity, the more energy it provides - photosynthetic pigments only absorb red and blue light 2) Temp - around 25 degrees C - if temp is too low (below 10 degrees) RuBisCo becomes inactive but if it is too high (above 45) it may denature - chlorophyll could be damaged - this can reduce the amount of pigment that can absorb light energy - thylakoid membranes may be damaged - this could reduce the L-D stage reactions by reducing the number of sites for electron transfer - the membranes around the chloroplast could be damaged - this could cause enzymes used in the Calvin cycle to be released which would reduce the light-independent stage reactions 3) CO2 ar 0.4% - increasing CO2 increases rate of photosynthesis but any higher conc means the stomata will start to close

Question 27

How does light intensity affect the rate of reaction?

Answer 27

As light intensity increases, ATP and reduced NADP are produced at a higher rate In low light intensities: - products of the L-D reaction (ATP + reduced NADP) will be in short supply - the conversion of GP to TP and RuBP is slow - the level of GP increases and levels of TP and RuBP fall (as they're used to make GP) - there is less TP available to regenerate RuBP

Question 28

How does CO2 concentration affect the rate

Answer 28

Increasing the CO2 concentration: - increases the rate of carbon fixation in the Calvin cycle - increases the rate of TP production At low CO2 concentrations: - conversion of RuBP to GP is slow (as there is less CO2 to be fixed) - levels of RuBP rise - levels of GP and TP fall ( as they're used to make RuBP)

Question 29

how does temperature affect the rate

Answer 29

As temperature increases: - the rate of enzyme controlled reactions increases until the enzymes denature - increased rate of carbon fixation - when the enzymes denature, levels of RuBP, GP and TP fall At low temperatures: - reactions are slower as enzymes work more slowly - levels of RuBP, GP and TP fall

Question 30

how does water stress affect rate

Answer 30

- if plants don't have enough water, their stomata will close to preserve water - less CO2 will enter the leaves - slows down rate

Question 31

Using TLC to separate photosynthetic pigments

Answer 31

Stationary phase - thin layer of silica gel applied to gas Mobile phase - liquid solvent - Different solubilities result in of pigments in the mobile phase and differing interactions with the stationary phase lead to them moving at different rates Method: 1) Grind up several leaves with anhydrous sodium sulfate and some propane 2) Transfer the liquid to a test tube and add some petroleum ether. Gently shake the tube - 2 distinct layers will form in the liquid - the top layer is the pigments mixed with the petroleum ether 3) transfer some liquid from the top layer into a second test tube with some anhydrous sodium sulfate. 4) Draw a horizontal pencil line near the bottom of the chromatography plate. Build up a single concentrated spot of the solution from step 3 on the line (point of origin) 5) Once the point of origin is dry, put the plate into a glass beaker with some prepared solvent (a mixture of propanone, cyclohexane and petroleum ether). The point of origin should be above the solvent. 6) Put a lid of the beaker and leave the plate to develop - as the solvent spreads up the plate, the different pigments move with it but at different rates 7) When the solvent has nearly reached the top, take the plate out and mark the solvent front with a pencil and leave it to dry in. well-ventilated plate 8) Calculate the Rf value and look them up in a database to identify what the pigments are

Question 32

chemiosmosis definition

Answer 32

the process of electrons flowing down the electron transport chain and creating a proton gradient to drive ATP synthesis