Chapter 11: Photosynthesis

Question 1

There are 3 main sequences in photosynthesis. What are they?

Answer 1

The absorption of light energy by the chlorophyll molecules in a plant cell

The light-dependent stage where light energy is directly used to form oxygen, ATP and reduced NADP (NADPH)

The light-independent stage where the products of the light-dependent stage are used to form triose-phosphate (TP) which goes on to form the products of photosynthesis (lipids, glucose)

Question 2

What do plants need energy for?

Answer 2

• Photosynthesis
• Active transport (uptake of minerals from soil via roots)
• DNA replication
• Cell division
• Protein synthesis

Question 3

What do animals need energy for?

Answer 3

Muscle contraction

Maintain a constant body temperature

Active transport (e.g. absorption of glucose)

DNA replication

Cell division

Protein synthesis

Question 4

Where is the site of photosynthesis?

Answer 4

The site of photosynthesis is the leaves of a plant. The leaf is the main photosynthetic structure in eukaryotic plants, while the chloroplasts are the cellular organelles where photosynthesis actually occurs.

Question 5

How is a leaf adaptated for effective photosynthesis?

Answer 5

1. A large surface area for absorbing sunlight
2. A thin structure so that diffusion pathways are short and because light is absorbed in the first few micrometres of the leaf
3. A transparent epidermis and waxy cuticle that lessens water loss through evaporation and lets sunlight through to the mesophyll cells
4. Long narrow upper mesophyll cells so that more of them can fit at the top of the leaf and packed with chloroplasts to absorb sunlight
5. Stomata are able to open and close depending on light intensity, which helps reduce water loss when the plant is not photosynthesising
6. Lots of air spaces in the lower mesophyll layer that allow quick diffusion of gases

Extras

1. Many stomata for gaseous exchange so that no mesophyll cell is too far away from one (short diffusion pathway)
2. A special arrangement of leaves makes leaves non-overlapping and stops leaves been overshadowed by each other
3. A network of xylem vessels that transport water to leaf cells and a network of phloem vessels that carry away the sugars produced during photosynthesis

Question 6

The raw materials for photosynthesis are…

Answer 6

Carbon dioxide

• colourless gas
• 0.04% of atmosphere
• enters microscopic pores/stomata in leaves

Water

• root hairs absorb water passively - no energy needed
• keeps plant tissue turgid
• maximising SA for light
• source of electrons
• solvent - all chemical reactions must occur in solution

Light

• catalyses the reaction
• absorbed by chloryphyll

Question 7

The products of photosynthesis are:

Answer 7

Glucose

• used to make other substances such as fats and proteins

Oxygen

• may be used in respiration
• may diffuse out of plant via stomata

NADPH

• gives hydrogen to independent stage

ATP

• gives energy to independent stage

Question 8

What is the chemical formula for photosynthesis?

Answer 8

6CO₂ + 6H₂O (+ energy) —- C₆H₁₂O₆ + 6O₂

Question 9

What is reduction?

Answer 9

Reduction is when:

• a molecule gains electrons (e-) or hydrogen (which is essentially an electron and a proton)
• a molecule loses oxygen

Question 10

What is oxidisation?

Answer 10

Oxidisation is when

• a molecule loses electrons (e-) or hydrogen
• a molecule gains oxygen

Question 11

What is the mnemonic for oxdiation and reduction?

Answer 11

Oxygen

Is

Loss… of electrons

Reduction

Is

Gain… of electrons

Question 12

Chlorophyll absorbs light energy from which part of the electromagnetic spectrum.

Answer 12

Chlorophyll absorbs light energy from the visible part of the electromagnetic spectrum

Question 13

Why do plants have different pigments?

Answer 13

Each pigment absorbs a different wavelength of light so that the total amount of light absorbed is greater than if just one pigment was used

Having more than one type of pigment increases the range of wavelengths of light that a plant can absorb

Question 14

Why do leaves appear green?

Answer 14

Only certain wavelengths of light are used for photosynthesis: 60nm and 700nm

The photosynthetic pigments chlorophyl a, chlorophyll b and carotene can only absorb red and blue light

Other wavelengths/green light is mostly reflected back, which is why plants look green.

Question 15

Facts: Light dependent stage

Answer 15

First stage of photosynthesis

Needs light energy

Site: thylakoid membrane

Products:

• ATP
• NADPH
• oxygen

Question 16

Facts: Light independent stage/Calvin cycle

Answer 16

Does not need light directly

Needs the products of light dependent stage

Products of light-dependent used to reduce/fix CO₂ into carbohydrate

Site: stroma of chloroplast

Simple sugars formed from CO2

ATP supplies energy

NADPH supplies hydrogen/protons

6 cycles needed to form 1 hexose 6C sugar

Question 17

Describe what happens in the light-dependent stage of photosynthesis

Answer 17

Light energy is absorbed by chlorophyll in photosystem II

• Chlorophyll becomes photoionised
• An electron is excited to a higher energy level and released from the chlorophyll molecule
• Electron passed down electron transport chain
• Energy released as it moves down
• Energy used to:

make ATP – photophosphorylation

ATP provides energy for light independent stage

reduce NADP – form reduced NADP or NADPH

NADPH carries hydrogen to light independent stage/has reducing power

Question 18

Reduced NADP (NADPH)

Answer 18

Coenzyme/electron carrier used in photosynthesis

Main product of light dependent reaction

Protons produced from photolysis of water

Pass through ATP synthase channel

Into thylakoid space

Protons taken up by NADP

NADP is reduced when it picks up the protons

Has gained OILRIG

Question 19

What is a coenzyme?

Answer 19

A coenzyme is a molecule that aids the functioning of an enzyme by transferring a chemical group from one molecule to another, i.e. it is a carrier

Question 20

What is photolysis?

Answer 20

Photolysis is the splitting of a molecule using light energy.

Water H₂O is split into

• oxygen O₂
• protons H⁺
• electron e^-

Replaces the electrons lost from chlorophyll

Question 21

Describe the chemiosmotic theory…

Answer 21

The process of electrons flowing down the electron transport chain, creating a proton gradient across the thylakoid membrane to drive ATP synthesis

Question 22

How is ATP made in the light dependent stage of photosynthesis?

Answer 22

Energy released from electrons passing down the electron transport chain is used to create an electrochemical/proton gradient across the thylakoid membrane

Protons transported/pumped into thylakoid

Protons build up in the thylakoid and move down their concentration gradient back into the stroma

Thylakoid membrane impermeable to protons

Have to pass through the ATP synthase enzyme embedded in the thylakoid membrane

The energy from this movement combines ADP and Pi to form ATP

This process of making ATP is photophoshorylation

Question 23

What is cyclic photophosphorylation?

Answer 23

Cyclic photophosphorylation is a type of photosynthesis that occurs in bacteria.

It only involves PSI

The electrons released from chrolophyll return to PSI via an electron transport chain/electron carriers

The energy released when the electron passes down the transport chain is used to make small amounts of ATP

No oxygen or reducing power in the form of reduced NADP is formed

Question 24

What happens to the rate of photosynthesis at different light intensities:

Answer 24

At high light intensity:

more light energy for the light-dependent stage meaning there is more light energy for ATP production and the production of reduced NADP

This means that the light-independent reaction can happen much more quickly as the products needed for it are being quickly supplied

As a result more sugars are produced that can be used for respiration

And as respiration increases, there is more energy for growth and there is a faster synthesis of new organic substance

Question 25

What is the importance of reduced NADP in photosynthesis?

Answer 25

Reduced NADP is needed for **biosynthesis**

Question 26

What is non-cyclic photophosphorylation?

Answer 26

Non-cyclic photophosphorylation is a type of photosynthesis that occurs in **plants** and **cyanobacteria**. **Two** photosystems are involved: **PSI** and **PSII** An electron from PSII is boosted to a higher energy level, **released**/emitted/ejected and passed **down an electron transport chain to PSI** and this electron transport chain generates a proton force used to make ATP The electrons lost are **replaced** by the **photolysis of water** (releasing oxygen) and does not return to PSII Hence this way of making ATP is called **non-cyclic** Light energy is absorbed by PSI and an electron is excited and emitted, going towards making **reduce NADP to NADPH** The ejected electron is **replaced by an electron from PSII**

Question 27

How is reduced NADP formed in the light dependent stage of photosysnthesis?

Answer 27

Reduced NADP/NADPH is formed when an **electron released from PSI** is **transferred** to NADP **along with a proton**/H+ ion from the stroma

Question 28

Describe what happens in the light independent stage of photosynthesis?

Answer 28

Carbon dioxide diffuses into the stroma from atmoshpere * CO₂ combines with _5-carbon_ molecule **RuBP ribulose biphosphate** * reaction catalysed by **enzyme rubisco** * results in a _6-carbon_ molecule * very unstable so splits/breaks down quickly * to give 2x 3-carbon molecules of **GP glycerate 3 phosphate** ATP and NADPH are used to convert GP to _3-carbon_ molecule **TP triose phosphate** * ATP is **hydrolysed** to provide energy to convert GP to TP * NADPH donates protons needed to reduce GP/for the reaction to happen For **every 1** molecule of CO₂ **2 TP** molecules are produced

Question 29

How many cycles of the Calvin cycle are needed to produce 1 hexose sugar and why.

Answer 29

**6** turns/cycles of the Calvin cycle are needed to produce 1 hexose sugar. Because... For **every 1 cycle** you get **2 molecules of TP** For **3 cycles** you get **6 molecules of TP** (2 x 3) But for **every 6 molecules of TP** **5** are used to regenerate RUBP Only 1 goes towards making a hexose sugar ratio of 5:1

Question 30

Carbohydrates or hexose sugars are made by...

Answer 30

joining **2 molecules of TP** triose phosphate molecules together

Question 31

Lipids are made using glycerol and fatty acids. * Glycerol is sysnthesised from... * Fatty acids are synthesised from...

Answer 31

Glycerol is sysnthesised from TP while fatty acids are synthesised from GP

Question 32

Some amino acids are synthesised from...

Answer 32

GP

Question 33

Name 3 photosynthetic pigments in the chloroplasts of plants.

Answer 33

chlorophyll a chlorophyll b carotene

Question 34

At what wavelength does photosystem I absorb light best?

Answer 34

Photosystem I absorb light best at a wavelength of 700 nm

Question 35

At what wavelength does photosystem II absorb light best?

Answer 35

Photosystem II absorbs light best at a wavelength of 680 nm

Question 36

What are photosynthetic pigments?

Answer 36

Photosynthetic pigments are coloured substances within chloroplasts that absorb light energy at different wavelengths

Question 37

Briefly explain how ribulose biphosphate RuBP is regenerated in the Calvin cycle. (2 marks)

Answer 37

For every 6 molecules of TP produced, 5 of them are used to regenerate RuBP The rest of the ATP from the light-dependent stage is needed to convert TP to regenerate RuBP

Question 38

Why is the Clavin cycle important?

Answer 38

It produces TP and GP TP and GP are converted into useful substances Such as: glucose, sucrose, starch, lipids, proteins That are needed for growth of the plant

Question 39

What are the optimum conditions for photosynthesis?

Answer 39

**High light intensity** * light provides **energy for light dependent stage** * higher light intensity = **more energy** **Temperature of 25 °C** * enzymes **denature at above 45 °C** and become **inactive at below 10 °C** * stomata close to avoid losing water = **less CO₂** **Carbon dioxide at 0.4%** * higher % of carbon = **higher rate of photsynthesis** * hgher than 0.4% = **stomata start to close** **Constant supply of water** * not enough = photosynthesis stops * too much = waterlogged cells * reducing uptake of minerals needed to make minerals fro making chlorophyll a

Question 40

What is the saturation point?

Answer 40

The saturation point is the point at which a factor is no longer the limiting the reaction Something else has become the limiting factor Increasing a variable makes no change to the dependent variable ?

Question 41

Describe what is going on in this graph. Light intensity is plotted against rate of photosynthesis.

Answer 41

At the start: 1. **Light is the limiting factor**/the rate of photosynthesis is limited by the light intensity 2. **as light intensity increases** the **rate of photosynthesis increases** 3. the graph reaches **saturation point** 4. increasing light intensity after this point **makes no difference** 5. something else has become the limiting factor 6. The **graph levels off**

Question 42

Describe what is going on in this graph. Temperature is plotted against rate of photosynthesis.

Question 43

Name 2 factors that can limit plant growth.

Answer 43

* Temperature * Carbon dioxide * Light intensity * Water

Question 44

Why is it important that argicultural growers know about factors that limit photosynthesis?

Answer 44

They are able to use the information to create optimum environments/conditions with the right levels of everything that plants need This **increases growth** So **increases yield** - more to eat - more income

Question 45

How do managment techniques in glasshouses help growers to create optimum conditions?

Answer 45

**CO₂** is added to the air by burning small amounts of **propane** **Light** gets through the glass while **lamps** provide light at night **Glasshouses** trap heat energy which **keeps the air warm** **Heaters and coolers** used to maintain a **constant** optimum temp **Air cirulation** used so that temperature is **even** throughout

Question 46

Why would plants in a greenhouse with added CO2 grow faster on average and be larger than plants grown in a greenhouse with no added CO2?

Answer 46

CO₂ is used by the plant for making glucose glucose provides energy the more CO₂ the more glucose more respiration more energy/ATP for cell division, DNA replication, protein synthesis i.e. growth

Question 47

Desribe 2 ways in which conditions can controlled in glasshouses to increase yields. (2 marks)

Answer 47

1. Burning 2.