13.1. Photosynthesis as an energy transfer process

Question 1

Photosynthesis

Answer 1

transfers light energy into chemical potential energy of organic molecules. This energy can then be released for work in respiration

Question 2

Equation for Phosynthesis

Answer 2

6CO2 + 6H2O – light energy in the presence of chlorophyll –> C6H12O6 + 6O2

carbon + water –> carbohydrate + oxygen

Question 3

Where does photosynthesis take place in a plant?

Answer 3

chloroplasts in mesophyll cells

Question 4

Pigment

Answer 4

a substance that absorbs light of some wavelengths but not others
- the wavelengths that it does not absorb are reflected from it

Question 5

Photosynthetic Pigments

Answer 5

1) Primary Pigments - Chlorophylls - A and B

2) Accessory Pigments - Carotenoids - B-carotene and Xantophylls

Question 6

Absorbance Spectrum

Answer 6

a graph of the absorbance of different wavelengths of light by a pigment

Question 7

Action Spectrum

Answer 7

a graph of the rate of photosynthesis at different wavelengths of light

Question 8

Why does chlorophyll look green?

Answer 8

• chlorophyll reflects green light

- other wavelengths of light are absorbed

Question 9

Chlorophyll A

Answer 9

• most abundant pigment in most plants
• absorption peaks at 340nm (blue) and 662nm (red)
• emits an electron when it absorbs light

Question 10

Chlorophyll B

Answer 10

• similar to chlorphyll A
• its absorption peaks are 453nm and 642nm
• similar role to chlorophyll A but not as abundant

Question 11

Carotene

Answer 11

• orange pigments that protect chlorophyll from damage by formation of single oxygen atoms
• absorb mainly in the blue-violet region of the spectrum
• pass on some of the energy from the light to chlorophyll

Question 12

Xanthophyll

Answer 12

• capture energy from wavelengths of light that are not absorbed by chlorophyll

Question 13

2 sets of reactions in photosynthesis

Answer 13

1) light dependent reactions - light energy is necessary

2) the light independent reactions - light energy is not needed.

Question 14

Conditions for light dependent reactions to take place

Answer 14

• light energy

- presence of suitable pigments that absorb certain wavelengths of light

Question 15

NADP

Answer 15

• a coenzyme used in photosynthesis

- accepts hydrogen to be reduced

Question 16

What occurs in the light dependent stage?

Answer 16

Energy from light is trapped by chlorophyll, and then the energy is used for:

• photolysis - split apart the strong bonds in water molecules to release hydrogen (protons) and oxygen
• phosphorylation (cylic or non-cyclic) - synthesis of ATP
• hydrogen ions reduce a carrier molecule called NADP
• ATP and reduced NADP are passed from the light dependent to the light independent reactions

Question 17

Types of Phosphorylation

Answer 17

Photophosphorylation of ADP to ATP can be cyclic or non-cyclic, depending on the pattern of electron flow in one or both types of photosystem.

Question 18

Cyclic Photophosphorylation

Answer 18

• involves only photosystem I
• light is absorbed by photosystem I and is passed to the primary pigment.
• an electron in the chlorophyll molecule is excited to a higher energy level and is emitted from the chlorophyll molecule. This is called photoactivation
• Instead of falling back into the photosystem and losing its energy as thermal energy, the excited electron is captured by an electron acceptor and passed back to a chlorophyll molecule via a chain of electron carriers.
• During this process, enough energy is released to synthesise ATP from ADP and an inorganic phosphate group (Pi) by the process of chemiosmosis
• The ATP then passes to the light independent reactions

Question 19

Non-cyclic Photophosphorylation

Answer 19

• Non-cyclic photophosphorylation involves both photosystems in the so-called ‘Z scheme’ of electron flow.
• Light is absorbed by both photosystems andexcited electrons are emitted from the primary pigments of both reaction centres.
• These electrons are absorbed by electron acceptors and pass along chains of electron carriers, leaving the photosystems positively charged.
• The primary pigment of photosystem I absorbs electrons from photosystem II.
• Its primary pigment receives replacement electrons from the splitting (photolysis) of water
• As in cyclic photophosphorylation, ATP is synthesised as the electrons lose energy while passing along the carrier chain

Question 20

Photolysis of Water

Answer 20

• Photosystem II includes a water-splitting enzyme that catalyses the breakdown of water:
• H2O → 2H+ + 2e− + 1/2 O2
• Oxygen is a waste product of this process.
• The hydrogenions combine with electrons from photosystem Iand the carrier molecule NADP to give reduced NADP.
• Reduced NADP passes to the light independent reactions and is used in the synthesis of carbohydrate.
• The photolysis of water can be demonstrated by the Hill reaction

Question 21

Light independent stage

Answer 21

• made up of a cycle of reactions known as the Calvin Cycle

- takes place in the stroma of a chloroplast, where Rubisco is found

Question 22

What occurs in the light independent stage?

Answer 22

• CO2 diffuses into the stroma from the air spaces within the leaf
• it enters the active site of Rubisco, where CO2 combines with a 5-carbon compound RuBP (ribulose biphosphate)
• the products of this reaction are two 3C molecules called GP (glucose phosphate)
• the combination of CO2 with RUBP is called carbon fixation
• energy from ATP and hydrogen from reduced NADP are then used to reduce GP to TP (triose phosphate)
• TP is the first carb to be produced in photosynthesis
• most of the triose phosphate is used to regenerate RuBP so that more CO2 can be fixed
• the rest of the TP is used to make glucose, other carbohydrates, lipids or amino acids - whatever the plant needs

Question 23

Separation of Photosynthetic Pigments by Chromatography

Answer 23

• to get the leaf extract, grind up leaves using a pestle and mortar
• place a many drops of the pigment solution producing a concentrated area at the centre of the pencil line drawn on the absorptive paper
• dip the edge of the absorptive paper into suitable organic solvent (water) and make sure the pigment spot is not immersed
• as the solvent ascends the porous paper, it carries the pigments with it at different rates
• In this way they become separated from one another and can be identified with their different colours and positions
• the relative distances traveled by the various pigments depend on which solvent and separation medium is used
• pigments can be identified by Rf values and colour of the pigments

Question 24

Rf value

Answer 24

distance travelled by pigment spot / distance travelled by solvent