5 Photosynthesis

Question 1

What does life depend on?

Answer 1

continuous transfers of energy

Question 2

what is the site of photosynthesis?

Answer 2

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

Question 3

what are the three raw materials of PS?

Answer 3

water
co2
light

Question 4

what adaptations does the leaf have for PS?

Answer 4

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

Question 5

overall equation for PS

Answer 5

6C02 + 6H20— light—> C6H12O6 + 602

carbon dioxide + water—> glucose + oxygen

Question 6

what are the three main stages of PS?

Answer 6

capturing light energy
light dependent reaction
light independent reaction

Question 7

chloroplast structure

Answer 7

They are surrounded by a double membrane. Inside
the chloroplast membranes are two distinct regions:
• The grana are stacks of up to 100 disc-like structures called thylakoids where the light-dependent stage of photosynthesis takes place. Within the thylakoids is the photosynthetic pigment called chlorophyll. Some thylakoids have tubular extensions that join up with thylakoids in adjacent grana. These are called intergranal lamellae.
• 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 8

what two purposes does light have in the LDR?

Answer 8

• to add an inorganic phosphate (Pi) molecule to ADP, thereby making ATP
• to split water into H+ ions (protons) and OH- ions. As the splitting is caused by light, it is known as photolysis

Question 9

oxidation

Answer 9

When a substance gains oxygen or loses hydrogen the process is called oxidation. The substance to which oxygen has been added or hydrogen has been lost is said to be oxidised.
Oxidation results in energy being given out

Question 10

reduction

Answer 10

When a substance loses oxygen, or gains hydrogen, the process is called reduction. In practice, when a substance is oxidised it loses electrons and when it is reduced it gains electrons
reduction results in energy being taken in.

Question 11

photoionisation stage of LDR

Answer 11

When a chlorophyll molecule absorbs light energy, it boosts the energy of a pair of electrons within this chlorophyll molecule, raising them to a higher energy level. These electrons are said to be in an excited
state. The electrons become so energetic that they leave the chlorophyll molecule altogether. As result the chlorophyll molecule becomes ionised and so the process is called photoionisation. The electrons that leave the chlorophyll are taken up by a molecule called an electron carrier. Having lost a pair of electrons, the chlorophyll molecule has been oxidised. The electron carrier, which has gained electrons, has been reduced.

Question 12

transfer of electrons down the electron transfer chain stage of LDR

Answer 12

The electrons are now 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.
Each new carrier is at a slightly lower energy level than the previous one in the chain, and so the electrons lose energy at each stage. Some of this energy is used to combine an inorganic phosphate molecule with an ADP molecule in order to make ATP.

Question 13

chemiosmosis stage of LDR

Answer 13

• Each thylakoid is an enclosed chamber into which protons (H+) 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 of water
• The photolysis of water also produces 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 he stroma.
• The protons can only cross the thylakoid membrane through ATP synthase channel proteins - the rest of the membrane is impermeable to protons. These channels form small granules on the membrane surface and so are also known as stalked granules.
• 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 with inorganic phosphate to form ATP.

Question 14

Photolysis of water stage of LDR

Answer 14

The loss of electrons when light strikes a chlorophyll molecule leaves it short of electrons. If the chlorophyll molecule is to continue absorbing light energy, these electrons must be replaced. The replacement electrons are provided from water molecules that are split using light energy. This photolysis of water also yields protons. The equation for chis process is:
2H2O —> 4H+ 4e- ­ + O2
These protons pass out of the thylakoid space through the ATP synthase channels and a re taken up by an electron carrier called NADP. On taking up the protons the NADP becomes reduced. The reduced NADP is the main product of the light-dependent stage and it enters the light independent reaction taking with it the electrons from the chlorophyll molecules. The reduced NADP 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 o f the leaf as a waste product of photosynthesis.

Question 15

site of LDR

Answer 15

the LDR of photosynthesis takes place in the thylakoids of chloroplasts. The thylakoids are disc-like
structures that are stacked together in groups called grana .

Question 16

how are Chloroplasts structurally adapted to their function of capturing sunlight and carrying out the LDR of
photosynthesis

Answer 16

• The thylakoid membranes provide a large surface area for the attachment of chlorophyll. electron carriers and enzymes that carry
out the light-dependent 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-dependent reaction.

Question 17

what determines the rate of PS?

Answer 17

carbon dioxide concentration, light intensity and well as temperature.

Question 18

LDR summary

Answer 18

1. Photons of light hit chlorophyll molecules in PSII causing the electrons to become excited.
   This is called photoionisation. The charge separation from this drives the process of photolysis.
2. Photolysis is the splitting of water with light. One molecule of water requires 4 photons of light to split. When water is split it produces 1 molecule of oxygen, 4 protons and 4 electrons. The oxygen either naturally diffuses out through the stomata or is used in
   respiration. The 4 electrons replace those lost from the chlorophyll, whilst the protons move into the stroma, later creating a proton gradient.
3. The excited electron then moves down a series of protein complexes. At one of the complexes the energy from the electron is used to pump 4 protons from the stroma to the thylakoid space.
4. The electron then moves down the chain further to PSI. Here more photons of light are absorbed causing the electron to move back up to a high energy level.
5. The electron then moves along the chain to another complex where the electron combines with a proton to form a hydrogen atom. This is then used to reduce NADP, forming reduced NADP.
6. The pumping of protons across the membrane means that there is now a greater concentration of protons in the thylakoid space than the stroma. As a result a proton gradient forms with a high concentration in the thylakoid space and a low concentration in the stroma. The protons move across the membrane by diffusion through a protein known as a stalked particle. The movement of these protons drives the process of photophosphorylation. The enzyme ATP synthase phosphorylates ATP from ADP and Pi.

Question 19

light independent reaction

Answer 19

1. Carbon dioxide from the atmosphere diffuses into the leaf through stomata and dissolves in water around the 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 bisphosphate (RuBP) a reaction catalysed by an enzyme called ribulose bisphosphate carboxylase, otherwise known as rubisco.
3. The reaction between carbon dioxide and RuBP produces two molecules of the 3-carbon glycerate 3-phosphate (GP).
4. Reduced NADP from the light-dependent 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-dependent 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-dependent reaction.

Question 20

co2 fixation stage of LIR

Answer 20

carbon dioxide that has diffused in through the stomata is fixed with ribulose bisphosphate (RuBP) in a process known as carboxylation. The enzyme Rubisco is needed in order to do this. A 6 carbon sugar is formed first, however this is very unstable and therefore forms 2 molecules of glycerate-3-phosphate.

Question 21

reduction phase of LIR

Answer 21

The 2 molecules of glycerate-3-phosphate contain a
COOH group and is therefore an acid. The reducing power of reduced NADP therefore reduces the glycerate-3-phosphate, with energy being provided by ATP. This therefore forms 2 molecule of triose phosphate. All of the NADP from the light dependent reaction has now been used with only some of the ATP being used.

Question 22

regeneration of RuBP stage of LIR

Answer 22

5 molecules of triose phosphate are used in order to regenerate 3 molecules of ribulose bisphosphate. The remaining amount of ATP from the light dependent stage is now used.

Question 23

organic molecule production stage of LIR

Answer 23

2 molecules of triose phosphate can combine to form

the intermediate hexose sugar fructose 1,6 bisphosphate where after it forms molecules of glucose.

Question 24

how many turns of the Calvin Cycle are required in order to produce 1 molecule of glucose per
molecule of CO2?

Answer 24

6

Question 25

site of LIR

Answer 25

takes place in the stroma of the chloroplasts.

Question 26

how is the chloroplast adapted to carry out the light-independent reaction of photosynthesis?

Answer 26

* The fluid of the stroma contains all the enzymes needed to carry out the light-independent reaction. Stromal fluid is membranebound 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-dependent 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 27

measuring photosynthesis practical

Answer 27

The rate of photosynthesis in an aquatic plant such as Canadian pondweed can be found by measuring the volume of oxygen produced by using a photosynthometer • The water bath is used to maintain a constant temperature throughout the experiment and can be adjusted as necessary. • Potassium hydrogen carbonate solution is used around the plant to provide a source of carbon dioxide. • A source of light, whose intensity can be adjusted, is arranged close to the apparatus, which is kept in an otherwise dark room. • The apparatus is kept in the dark for two hours before the experiment begins. • The light source is switched on and the plant left for 30 minutes to allow the air spaces in the leaves to fill with oxygen. • Oxygen released by the plant during photosynthesis collects in the funnel end of the capillary tube above the plant. • After 30 minutes this oxygen is drawn up the capillary tube by gently withdrawing the syringe until its volume can be measured on the scale, which is calibrated in mm3. • The gas is drawn up into the syringe, which is then depressed again before the process is repeated at the same light intensity five times, and the mean volume of oxygen produced per hour is calculated. • The apparatus is left in the dark for 2 hours before the procedure is repeated with the light source set at a different light intensity .

Question 28

no of carbon atoms in RuBP, GP and TP

Answer 28

5 3 3

Question 29

define limiting factor

Answer 29

factor that determines the max rate of a reaction, even if other factors change to become more favourable

Question 30

The photoionisation of chlorophyll also results in a release of energy. This energy is used in photosynthesis to drive three reactions:

Answer 30

Photophosphorylation - production of ATP from ADP and inorganic phosphate. Reduction - production of reduced NADP from NADP. Photolysis - splitting of water into protons, electrons and oxygen.

Question 31

electron transport chain

Answer 31

Light energy excites electrons in chlorophyll and the electrons are moved to a higher energy level (they are high-energy electrons). High-energy electrons are released from the chlorophyll and transferred to an electron carrier. Electron carriers are proteins located in the thylakoid membranes. When high-energy electrons are released they are transferred along a chain of electron carriers. The series of electron carriers is called the electron transport chain (ETC).

Question 32

the proton gradient (LDR)

Answer 32

As the electrons move down the ETC, they lose energy. This energy pumps protons from the stroma into the thylakoids. The protons are being transported against their concentration gradient and this requires energy. As protons build up inside the thylakoids, a proton gradient forms across the thylakoid membrane because the concentration of protons inside the thylakoids is greater than in the stroma.

Question 33

reduced NADP (LDR)

Answer 33

When light energy is absorbed, high-energy electrons are released. Some electrons are transferred directly to NADP. They are not passed along the ETC. The electrons react with a proton in the stroma to produce reduced NADP.

Question 34

Some triose phosphate molecules are converted to organic substances that the plant requires such as...

Answer 34

starch, cellulose, lipids, glucose, amino acids, and nucleotides

Question 35

TP converted into carbohydrates

Answer 35

Hexose sugars (monomers) are produced from two molecules of triose phosphate. E.g. Glucose. Hexose sugars can be joined together to form larger carbohydrates (polymers). E.g. Starch, cellulose.

Question 36

TP converted into lipids

Answer 36

Lipids are made from glycerol and fatty acid chains. Both components of lipids are synthesised from the products of the Calvin cycle: Triose phosphates are used in the synthesis of glycerol. Fatty acids are formed from glycerate 3-phosphate.

Question 37

temperature as LF of PS

Answer 37

Initially, the rate of photosynthesis increases as the temperature increase. Above about 45°C, the rate starts to fall. Enzymes such as rubisco play an important role in photosynthesis. At low temperatures, enzymes have less kinetic energy and are less likely to successfully collide with the substrate. This means fewer enzyme-substrate complexes are formed. Above the optimum temperature, enzymes will denature and the rate of photosynthesis will fall.

Question 38

light intensity as LF of PS

Answer 38

Increasing light intensity increases the rate of photosynthesis because more energy is provided. In low light intensities the light-dependent reaction cannot take place. This causes levels of reduced NADP and ATP to fall. Reduced NADP and ATP are necessary to convert GP to TP and to regenerate RuBP. Without reduced NADP and ATP the Calvin cycle cannot take place and rate of photosynthesis declines.

Question 39

CO2 conc as LF of PS

Answer 39

Increasing the CO2 concentration increases the rate of photosynthesis. At high CO2 concentrations, there is more carbon available for fixation in the Calvin cycle. If the rate of the Calvin cycle increases, more GP and TP is produced and more RuBP is regenerated. Very high CO2 concentrations can cause stomata to open wider causing more water to be lost. Too much water loss will then cause the stomata to close and the rate of photosynthesis to slow again.

Question 40

chlorophyll conc as LF of PS

Answer 40

High chlorophyll concentration gives a high rate of photosynthesis. If there is more chlorophyll, light can be absorbed at a faster rate. This will cause the light-dependent reaction rate to increase.

Question 41

Using Chromatography to Investigate Photosynthesis

Answer 41

Chromatography is a technique that can be used to identify which pigments are in the leaves of different plants. This allows us to identify what wavelengths of light a plant can absorb.

Question 42

The total no of ATP used for the production of a molecule of glucose is

Answer 42

18

Question 43

light compensation point

Answer 43

CO2 released during respiration equals that taken up during photosynthesis

Question 44

number of calvin cycles to produce one molecule of glucose and why

Answer 44

6 cycles to form 12 TP molecules, 10 of which will regenerate 5 RuBP molecules, 2 of which will form molecules of glucose this would require 18 ATP and 12 NADPH