Photosynthesis uses light energy to synthesise organic molecules

Question 1

What are the two stages of photosynthesis?

Answer 1

• Light dependent stage

- Light independent stage

Question 2

Explain the light-dependent stage of photosynthesis

Answer 2

• Converts light energy to chemical energy
• The photolysis of water releases electrons and protons
• Energy carried by electrons establishes a proton gradient across the thylakoid membrane
• The energy is used to phosphorylated ADP, which generates ATP, in photophosphorylation
• The protons and electrons reduce NADP

Question 3

Describe the light-independent stage of photosynthesis

Answer 3

The ATP and reduced NADP reduce carbon dioxide and produce energy containing glucose

Question 4

Explain the distribution of chloroplasts in relation to light trapping:
What does the inner membrane fold inwards to make?
Where are photosynthetic pigments located?
Where do the light-independent reactions take place?
Why do starch grains appear white?

Answer 4

• Photosynthesis takes place in chloroplasts
• Chloroplasts are surrounded by a double membrane
• The inner membrane folds inwards to make thylakoid lamellae
• These combine in stacks of up to 100 disc-shaped structures, forming grana, where the photosynthetic pigments are located and where the reactions of the light-dependent stage of photosynthesis take place
• The stroma is the fluid filled interior bathing the thylakoids and grana, where the light-independent reactions take place
• Starch grains in the chloroplast appear white because the stain used for electron micrographs, osmium tetroxide, binds to lipid, but not to carbohydrates

Question 5

Explain the distribution of chloroplasts in relation to light trapping:
Where are they mainly found?

Answer 5

Chloroplasts need light and so they are found only in those parts of the plant that are exposed to light- the leaves and the stem. The leaf is the main organ of photosynthesis and the chloroplasts are found largely in the palisade mesophyll. They also occur in the spongy mesophyll, but the only epidermal cells containing chloroplasts are the guard cells

Question 6

structural features of leaves and their significance for photosynthesis

Answer 6

• Large surface area_ Capture as much light as possible
• Thin_ Light penetrates though the leaf
• Stomatal pores_ Allow carbon dioxide to diffuse into the leaf
• Air spaces in the spongy mesophyll_ Allow carbon dioxide to diffuse to photosynthesising cells
• Spaces between palisade cells_Allow carbon dioxide to diffuse to photosynthesising cells

Question 7

Structural features of plant cells and their significance for photosynthesis

Answer 7

• Cuticle and epidermis are transparent; cellulose cell walls are thin_ Light penetrates to the mesophyll
• Palisade cells have a large vacuole_ Chloroplasts form a single layer at the peripheral of each cell do not shade each other
• Palisade cells are cylindrical, elongated at right angles to the surface of the leaf_ Leaves can accommodate a large number of palisade cells; light only passes thorough two epidermal walls and one palisade wall before reaching chloroplasts. If the cells were stacked horizontally, light would be absorbed by passing through many cell walls, preventing it reaching chloroplasts

Question 8

Structural features of chloroplasts and their significance for photosynthesis

Answer 8

• Chloroplasts have a large surface area_ maximum absorption of light
• Chloroplasts move within palisade cells_ chloroplast move towards the top of the cell on dull days, for maximum absorption of light. If the light intensity is very high, they move to the bottom of the cell, protecting pigments from bleaching
• Chloroplasts rotate within palisade cells_ Thylakoids maximise the absorption of light
• Pigments in the thylakoids are in single layer at the surface of the thylakoid membrane_ pigments maximise their absorption of light
• About five times as many chloroplasts in palisade cells than spongy mesophyll cells_ Palisade cells are at the top of the leaf and they are exposed to light than the spongy mesophyll cells, so chloroplasts capture as much light as possible

Question 9

Chloroplasts are transducers:

What are transducers? (Give an example)

Answer 9

A transducer changes energy from one form to another.
A light bulb, for example, transduces electrical energy into light and heat and an engine transduces the chemical energy in petrol into kinetic energy, heat and sound. Biological transducers are much more efficient than such artificial devices and waste less energy in the conversions that they make. Chloroplasts are transducers, turning energy in the protons of light into chemical energy, made available through ATP and incorporated into molecules such as glucose

Question 10

What is a pigment?
What pigments trap light energy in chloroplasts?
Why is it useful to have more than one pigment?

Answer 10

• A molecule that absorbs specific wavelengths of light
• In the chloroplasts light energy is trapped by photosynthetic pigments
• Different pigments traps different wavelength, this allows a large range of wavelengths to be absorbed and is consequently more useful than if there were just one pigment, absorbing a small range of wavelengths

Question 11

How many classes of pigments that act as transducers in flowering plants?

Answer 11

The are two main classes of pigments which are carotenoids and the chlorophylls

Question 12

What is an absorption spectrum?

Answer 12

The different pigments can be shown to absorb different wavelengths of light by making separate solutions of each and shining light through them.
- An absorption spectrum is a graph that indicates how much light a particular pigment absorbs at different wavelengths

Question 13

What wavelengths of light does chlorophylls and carotenoids absorb?

Answer 13

• Chlorophylls absorb light energy mainly in the red and blue-violent regions of the spectrum an reflect green, giving leaves their colour
• Carotenoids absorb light energy from blue-green region of the spectrum, they appear yellow-orange

Question 14

What is an action spectrum?

Answer 14

Absorption at a particular wavelength does not indicate, however, whether that wavelength is actually used in photosynthesis
- An action spectrum is a graph that shows the rate of photosynthesis at different wavelengths of light, as measured by the mass of carbohydrate synthesised by plants exposed to different wavelengths

Question 15

What happens when an action spectrum is superimposed on the absorption spectrum?

Answer 15

• A close correlation between the two can be seen. This suggests that the pigments responsible for absorbing the light are used in photosynthesis

Question 16

Photosystems lie in the plane of a thylakoid membrane. Each comprises: An Antenna complex

Answer 16

Containing the photosynthetic pigment, chlorophylls and carotenoids are anchored into the phospholipids of the thylakoid membrane, held together by protein molecules in clusters of up to 400 molecules. Each cluster is called an antenna complex. The combination of pigments allows light at a range of wavelengths to be absorbed

Question 17

Photosystems lie in the plane of a thylakoid membrane. Each comprises: A reaction centre

Answer 17

within the antenna complex. It contains two molecules of the primary pigment, chlorophyll a. When the chlorophyll a molecules absorb light, their excitation allows each one to emit an electron

Question 18

There are two types of reaction centre: Photosystem I (PSI)

Answer 18

Is arranged around a chlorophyll a molecule with an absorption peak of 700 nm. It is called P700

Question 19

Photosystem II (PSII)

Answer 19

Which was discovered after PSI, hence the numbering, is arranged around a chlorophyll a molecule with an absorption peat of 680nm. It is also called P680

Question 20

What absorbs the photons?

Answer 20

Some photons are absorbed by chlorophyll a directly but many are absorbed first by chlorophyll b and the carotenoids, which are the accessory (or antenna) pigments. The photons excite the accessory pigments and energy is passed through them to the reaction centre, where electrons of chlorophyll a are excited and raised to a higher energy level. Chlorophyll a is the most significant molecule of the reaction centre because it passes energy to the subsequent reactions of photosynthesis. It is referred to as the primary or core, pigment

Question 21

Photosynthesis includes a sequence of reactions that take place on the thylakoid membranes, using light as an energy source and using water. This sequence is the light dependent stage of photosynthesis and it produces:

Answer 21

• ATP, which provides the chemical energy transduced from light energy to synthesise energy-rich molecules such as glucose
• Reduced NADP, which provides the reducing power to synthesise molecules such as glucose from carbon dioxide
• Oxygen, a by-product, derived from water. Oxygen diffuses out of the chloroplast, out of the photosynthetic cells and out of the leaf through the stomata

Question 22

What reactions constitute the light-independent stage?

Answer 22

The reactions using ATP and reduced NADP, making molecules such as glucose, occur in solution in the stroma. They can happen in the light but do not require it. These reactions constitute the light-independent stage, and include a cycle of reactions called the Calvin cycle, named after one of their discoverers

Question 23

What is phosphorylation?
What is photophosphorylation?
- What are the pathways of photosynthesis?

Answer 23

• Phosphorylation is the addition of a phosphate ion to ADP.
• The term phosphorylation implies that the energy for this reaction comes fro light comes light. Photosynthesis has two pathways for phosphorylation. Non-cyclic phosphorylation involves both PSI and PSII and the pathway of electrons is linear. Cyclic phosphorylation uses PSI only and electrons go through a cycle

Question 24

The passage of electrons: Cyclic phosphorylation

Answer 24

• PSI absorbs photons, which excites electrons in the chlorophyll a molecules in its reaction centre
• These are emitted and picked up by an electron acceptor, which passes them down a chain of electron carriers back to PSI. The energy released as electrons pass through the electron transport chain phosphorylates ADP to ATP
• Electrons have flowed from PSI to the electron acceptor, back to PSI so this phosphorylation is described as cyclic phosphorylation

Question 25

The passage of electrons: Non cyclic phosphorylation

Answer 25

In an alternative pathway, electrons are transferred from the electron acceptor to oxidised NADP in the stroma, which, with protons from the photolysis of water, is reduced

• The electrons have not been returned to PSI so its chlorophyll is left with a positive charge
• The positive charge is neutralised by electrons from PSII. They have been excited to a high energy level by light absorption, picked up by an electron acceptor and passed down the electron transport chain to PSI
• Electron passage down the electron transport chain makes energy available for phosphorylation of ADP. As the electrons from PSII move is one direction only (PSII->electron acceptor->electron transport chain->PSI) this is non-cyclic phosphorylation
• The chlorophyll in PSII is left with a positive charge and that is neutralised by the electrons released in the photolysis of water

Question 26

Define ‘Z scheme’

Answer 26

The pathway taken by electron sin non-cyclic photophosphorylation

Question 27

Describe what happens in the photolysis of water

Answer 27

In the thylakoid spaces, water molecules absorb light, which indirectly causes them to dissociate into hydrogen, oxygen and electrons:
H20-> 2H+ + 2e- +1/2 O2
-Photolysis is enhanced by a protein complex in PSII, which is the only known enzyme that causes water to be oxidised

Question 28

Explain what happens during the photolysis of water

Answer 28

• The electrons replaced those form PSI
• The protons from water and electrons from PSI reduce NADP
• Oxygen diffuses out of the chloroplast and cell, out through the stomata as a waste product

Question 29

Explain the passage of protons and phosphorylation (1)

Answer 29

• As electrons pass through a proton pump in the thylakoid membrane, they provide energy to pump protons from the stroma into the thylakoid space
• The protons join H+ ions from the photolysis of water and accumulate
• They generate an electrochemical gradient, since there are more inside the thylakoid space than there are outside, in the stroma
• This gradient is a source of potential energy

Question 30

Explain the passage of protons and phosphorylation (2)

Answer 30

• Chemiosmosis occurs. The H+ ions diffuse down their electrochemical gradient through ATP synthetase in the thylakoid membrane, into the stroma.
• This makes available the energy derived from light and carried by the electrons
• As they pass through ATP synthetase, ADP is phosphorylated to ATP

Question 31

Explain the passage of protons and phosphorylation (3)

Answer 31

• Once in the stroma, H+ ions are passed to oxidised NADP, reducing:
  NADP + 2H+ + e—> reduced NADP
• This removal H+ ions, in conjunction with the protein pump, contributes to maintaining the proton gradient across the thylakoid membranes

Question 32

In summary, thee factors maintain the proton gradient between the thylakoid space and the stroma:

Answer 32

• The proton pump associated with the electron transport chain pushing protons into the thylakoid space
• The photolysis of water in the thylakoid space
• The removal o protons from the stroma reducing NADP

Question 33

What can the light dependent stage of photosynthesis be summarised as:

Answer 33

NADP+H2O + 2ADP + 2Pi–> reduced NADP + 1/2 02 +2H20 + 2ATP

Question 34

Describe the light independent stage of photosynthesis

Answer 34

The light-independent stages occurs in solution in the stroma of the chloroplast and it involves many reactions, each catalysed by a different enzyme. The reactions use the products of the light-dependent stage

Question 35

The reactions in the light-independent stage use the products of the light dependent stage. Explain this concept

Answer 35

• ATP is a source of energy
• Reduced NADP is the source of the reducing power, reducing carbon dioxide
• A five-carbon acceptor molecule, ribulose bisphosphate, combines with carbon dioxide, catalysed by the enzyme ribulose bisphosphate carboxylase, abbreviated to rubisco.
• It is the most abundant protein in the biosphere, and the high concentration reflects its importance
• An unstable six-carbon compound is formed
• The six-carbon compound immediately splits into two molecules of three carbon compound glycerate-3-phosphate (GP)
• GP is reduced to triose phosphate by reduced NADP. Reducing a molecule requires energy and in this case, the energy is provided by the ATP made in the light-dependent stage. Triose phosphate is the first carbohydrate made in photosynthesis
• NADP is reformed
• Some of the triose phosphate is converted to glucose phosphate, and then into starch by condensation
• Most of the triose phosphate goes through a series of reactions which regenerates RuBP so the cycle can continue. ATP made in the light-dependent stage provides the energy for this to happen

Question 36

Product synthesis: Carbohydrates

Answer 36

• The first hexose made is fructose phosphate
• This can be converted to glucose and combined with the glucose to make sucrose, for transport around the plant
• The alpha glucose molecules may be converted to starch, for storage, or to beta glucose, which is polymerised into cellulose for cell walls

Question 37

Product synthesis: Fats

Answer 37

• Acetyl coenzyme A (AcCoA) can be synthesised from glycerate-3-phosphate, made in the Calvin cycle, and converted to fatty acids
• Triose phosphate can be converted directly to glycerol, another 3C compound
• Fatty acids and glycerol undergo condensation reactions to form triglycerides

Question 38

Product synthesis: Proteins

Answer 38

• Glycerate-3-phosphate can also be converted into amino acids for protein synthesis
• The amino group is derived from NH4 + ions , made from nitrate ion s(NO3-) taken in at the roots and transported throughout the plant

Question 39

Limiting factors in photosynthesis

Answer 39

Plants need a suitable environment to be efficient at photosynthesis. They need:

• The reactants carbon dioxide and water
• Light at a high enough intensity and of suitable wavelengths
• A suitable temperature

Question 40

Explain the importance of limiting factors with reference to photosynthesis

Answer 40

If any of those factors is lacking, photosynthesis cannot take place. Each factor has an optimum value, at which the rate of photosynthesis is highest. If any is at a sub optimal value, at which the rate of photosynthesis is reduced. In that case, the value of that factor is controlling the rate of photosynthesis. If it is higher, photosynthesis will happen faster. The factor is a limiting factor, because it is limiting, or controlling, the rate of photosynthesis. If it is higher, photosynthesis and is therefore not a limiting, or controlling, the rate of photosynthesis

Question 41

Limiting factors of photosynthesis: Explain the effect of CO2 concentration

Answer 41

• As the CO2 concentration increases from zero, the rate of the light-dependent reactions increases, and so the rate of photosynthesis increases, showing that CO2 concentration is a limiting factor. If the concentration is increased above about 0.5%, the rate of photosynthesis remains constant, implying that carbon dioxide concentration is not affecting the rate of photosynthesis and is therefore not a limiting factor at these concentrations. The rate decreases above about 1% as the stomata close, preventing carbon dioxide uptake

Question 42

Explain the effect of carbon dioxide concentration on specific types of plants

Answer 42

• Carbon dioxide concentration is usually a limiting factor for photosynthesis in terrestrial plants, because its concentration in the air is about 0.04%.
• The photosynthesis s of crop plants is most efficient at 0.1% carbon dioxide
• Experiments with tomatoes show even faster photosynthesis at 0.5%, but only in the short term
• Aquatic plants and algae can increase the concentration of carbon dioxide in their own cells with carbonic anhydrase, so that it is not a limiting factor in their photosynthesis. The optimum concentration for algae is approximately 0.1 mol dm3 i.e. about 0.1%
• The rate of the slowest reaction is a sequence determines the overall rate of the process. It is called the rate-limiting step. In the light-dependent reactions of photosynthesis, the reaction catalysed by rubisco is the rate limiting step

Question 43

Explain the effect of light intensity concentration on specific types of plants

Answer 43

Light intensity is a significant factor in controlling the rate of photosynthesis

• If the plant is in darkness, the light-independent reactions of photosynthesis are possible but the light dependent reactions are not, and so no oxygen is evolved
• As the light intensity increases, the light-dependent reactions occur with increasing efficiency, and so overall, the rate of photosynthesis increases. Light intensity is controlling the rate of photosynthesis and so it a limiting factor
• At a certain intensity, around 10000 lux, the reactions of the light-dependent stage are at their maximum rat. Higher light intensity does not produce faster reactions and so the rate of photosynthesis remains constant. Light intensity is not a limiting factor
• If the light intensity is even higher, the rate of photosynthesis will decrease because photosynthetic pigments are damaged. They will not absorb light so efficiently and so the light-dependent stage fails

Question 44

Sun and shade plants

Answer 44

Different species have adapted so that their photosynthesis is most efficient at different light intensities. Sun plants, such as Salvia, are most efficient at photosynthesis in high light intensity and shade plants such as lily of the valley are most efficient at low light

• Photosynthesis can be assessed in terms of carbon dioxide uptake into photosynthetic cells
• As the light intensity decreases, the rate of light dependent reactions decreases
• The rate of the light-independent reactions also decreases and so the rate of carbon dioxide uptake decreases
• At a particular light intensity, so little carbon dioxide is needed that respiration provides all that is required and none is absorbed.
• Similarly, all the oxygen needed for respiration is provided by photosynthesis
• There is, therefore, no gas exchange. The light intensity at which this happens is called the light compensation point. It occurs at a lower light intensity for shade plants than for sun plants, so they can grow in more shaded habitats than sun plants

Question 45

Limiting factors of photosynthesis: Explain the effect of temperature on the rate of photosynthesis

Answer 45

• Increased temperature increases the rate of photosynthesis because the kinetic energy of the molecules involved increases
• Above a particular temperature, which is different in different species, the enzymes progressively denature and the rate of photosynthesis and it is therefore a limiting factor

Question 46

Limiting factors of photosynthesis: Explain the effect of water

Answer 46

• When water is scarce, a plant’s cells plasmolyse, stomata close, wilting occurs and many physical functions are affected. Experiments with water -deficient plants show that even slight water deprivation can reduce the carbohydrate made, so water availability is a limiting factor in photosynthesis. But as so many systems are affected, it is not straightforward to see its effect on photosynthesis alone

Question 47

Explain how limiting factors can combine

Answer 47

• Sometimes, a plant has optimal values for all these environmental factors and then its photosynthesis will occur efficiently and it will grow well
• But if a plant has several of these factors limiting, for example a wood anemone in an oak forest on a cool spring morning, when temperature and light intensity are low, its ability to perform photosynthesis is reduced and it will grow slowly
• The actual rate of photosynthesis is controlled by the factor that is nearest to its minimum value

Question 48

Describe mineral nutrition

Answer 48

Inorganic nutrients are needed by plants and be limiting factors for metabolism if in short supply

Question 49

Minerals have various roles: Give an example of a mineral with a structural role

Answer 49

Calcium in the middle lamella of cell walls

Question 50

Minerals have various roles: Give an example of how minerals are used In the synthesis of compounds needed for the growth of the plant

Answer 50

As enzyme activators, such as the magnesium requirements by ATPase and DNA polymerase

Question 51

Minerals have various roles: Give an example of how they may form an integral part of a molecule

Answer 51

Magnesium in chlorophyll, iron in the carriers of the electron transport chain and manganese in photostem II

Question 52

Give examples of macronutrients

Answer 52

• Nitrogen
• Potassium
• Sodium
• Magnesium
• Calcium
• Nitrate
• Phosphate
  They are required in greater quantities than micronutrients, for example manganese and copper

Question 53

Nitrogen

Answer 53

Most of the nitrogen in the soil is in the humus, in organic molecules of decaying organisms. Inorganic nitrogen occurs as ammonium ions NH4+ or nitrate ions, NO3-. Nitrogen is largely taken up by roots as nitrates, although Rhizobium in root nodules delivers ammonium ions in the plant. The ions are transported in the xylem and delivered to the cells. Nitrate is converted into ammonium ions, which become the amino the amino (NH2) groups of amino acids. Amino acids are transported in the phloem and used for the synthesis of proteins, chlorophylls and nucleotides

Question 54

What are some of the symptoms of nitrogen deficiency

Answer 54

• Because of its role in protein synthesis, symptoms of nitrogen deficiency include reduced growth in the whole part
• Nitrogen is a component of chlorophyll and so its deficiency also causes chlorosis, a yellowing of the leaves due to inadequate chlorophyll production
• Chlorosis first appears in the older leaves

Question 55

Explain the role of magnesium in plants

Answer 55

• Magnesium is absorbed as Mg2+ and it is transported in the xylem
• It is required by all tissues, but especially leaves
• Magnesium forms part of the chlorophyll molecule and so the main symptom of magnesium deficiency is chlorosis
• This begins between the veins of older leaves as existing magnesium in the plant is mobilised and transported to newly formed leaves
• Magnesium ions are also important enzyme activators such as for ATPase