Photosynthesis

Question 1

What is the site of photosynthesis?

Answer 1

The chloroplasts of the leaf

Question 2

Name 12 adaptions of plants leaves that maximise the rate of photosynthesis?

Answer 2

1. Leaves are flat - provides a LSA to absorb as much light as possible
2. Leaves do not overlap - no shading so as much light is absorbed as possible
3. Thin - most of the light is absorbed in the first few centimetres
4. Thin - provide a short diffusion distance
5. Transparent cuticle and upper epidermis - light reaches the mesophyll cells
6. Mesophyll cells - long and narrow + have lots of chloroplasts - light ore likely to hit a chloroplast and be absorbed by chlorophyll
7. Many stomata - increases SA for gas exchange
8. Many stomata - provides a short diffusion distance for gasses
9. Stomata open and close in response to light intensity
10. Many air spaces in spongy mesophyll - rapid diffusion of CO2 in and O2 out
11. Network of xylem vessels - H2O to leaf cells
12. Network of phloem vessels - carry away sugars

Question 3

What is the equation for photosynthesis?

Answer 3

6CO2 + 6H20 = C6H12O6 + 602

Question 4

What is the first main step in photosynthesis?

Answer 4

1. Absorption of light energy by the chlorophyll (photoionisation)

Question 5

What is the second main step in photosynthesis?

Answer 5

The light dependent reactions, where the light energy is converted into chemical energy (energy within the bonds of a molecule). During the light dependent reaction, electrons gain energy and lose it through electron flow. This flow of electrons does a number of jobs: it causes water to split into protons, electrons and oxygen, and helps produce NADP, ATP ad oxygen

Question 6

What is the third main step in photosynthesis?

Answer 6

The light independent reaction uses the hydrogen from reduce NADP and energy from ATP to produce sugars and other organic molecules

Question 7

What are chloroplasts?

Answer 7

Organelles found in plant cells, they are usually disc shaped, and typically measure 2-10 micrometres in length and are about 1 micrometer thick. They also have a double membrane

Question 8

What is the endosymbiont theory and what evidence is there for this?

Answer 8

It is believed that chloroplasts were once prokaryotic organisms and that in the distant past, larger cells engulfed chloroplasts
Chloroplasts contain 70s ribosomes, has it’s own DNA in the form of plasmids and the outer membrane is thought to be similar to the cell wall of a prokaryote

Question 9

How does the grana allow chloroplast to carry out their role?

Answer 9

The light dependent reactions occur within the thylakoid membrane of the thylakoid

Question 10

Name 3 jobs of the thylakoid membrane

Answer 10

It holds the chlorophyll pigments
It is embedded with ATPsynthase enzymes
It acts as a barrier to ions, allowing a gradient to be established

Question 11

What is the stroma?

Answer 11

Liquid in the chloroplasts where reactions take place, this requires water so enzymes and substrates can dissolve and be used in chemical reactions

Question 12

What is the role of starch grains/amyloplasts?

Answer 12

Used for storing anh glucose that is made during photosynthesis

Question 13

What is a light dependent reaction?

Answer 13

It involves the absorption of light energy so it can be used to phosohorylate ADP and Pi into ATP and to slit water into H+, e- and O2 is a process called photolysis

Question 14

Why is chlorophyll green?

Answer 14

It reflects the green wavelength in visable light and absorbs all others

Question 15

OILRIG

Answer 15

Oxidation is loss, Reduction is gain

Question 16

Oxidation

Answer 16

When a molecules loses a hydrogen or an electron

Results in energy being given out

Question 17

Reduction

Answer 17

When a molecule gains a hydrogen or loses an electron

Results in energy being taken in

Question 18

ATP

Answer 18

Adenosine Triphosphate

Question 19

How is ATP synthesised?

Answer 19

At the centre of a chlorophyll molecule theres is a Mg atom. When light is absorbed by chlorophyll this excites 2 electrons and increases their energy level. These electrons are then emitted from the chlorophyll, as a result the chlorophyll has undergone photoionisation and has been oxidised. The electrons enter an electron carrier, these are proteins in the thylakoid membrane. As the carrier has gained electrons it has been reduced.

The electrons pass through the electron carriers through a series of reduction and oxidation reactions. Each electron carrier is at a lower energy level than the one before then electrons lose energy as they move through the electron carriers. This energy is then used to synthesis ATP through chemiosmosis

Question 20

What is the chemiosmotic theory?

Answer 20

Each thylakoid acts as a chamber into which protons are pumped from the stroma. The energy to carry out chemiosmosis is provided by the photolysis of water. Photolysis occurs in the thylakoid spaces increasing the proton concentration in the thylakoid space. This establishes and maintains a proton gradient between the thylakoid space and the stroma
The only route the proteins can take is through ATPsynthase channel proteins. As the protons move through the ATPsynthase enzymes, it changes the structure of the enzymes and catalyses the phosphorylation of ADP and Pi to create ATP

The protons diffuse through ATPsynthase into the stroma. The protons and electrons are taken up by Hydrogen carrying NADP, making it reduced

The reduced NADP carrie’s the electons to the light independent reaction. The ATP produces in the light dependent reaction provides energy for the light dependent reaction. Any reaction is produced can be used in aerobic respiration or diffuses out of the leaf

Question 21

Summarise what happens in PSII (Photosystem 2) in light dependent reactions

Answer 21

Light energy is absorbed by the chlorophyll
Electrons in the chlorophyll are raised to a higher energy level. The electrons are emitted from the chlorophyll (photoinosiation)
Electrons then pass through a number of electron carriers in the thylakoid membrane in a series of redox reactions
The electrons release energy at each stage of the electron transport chain
Some energy provided is used to move H+ from the stroma into the thylakoid space and form a H+ gradient (chemiosmosis)
The rest of the energy is lost as heat or given out as fluorescent light
H+ then diffuses through the thylakoid membrane via ATPsynthase
The movement of H+ through ATPsynthase provides the energy for the phosphorylation of ADP into ATP

Question 22

Summarise what happens in PSI (Phostosystem 1) in light dependent reactions

Answer 22

Light energy is absorbed by the chlorophyll molecule
Electrons in the chlorophyll are raised to a higher energy level and electrons are emitted (photoionisation)
The electrons then pass through a number of electron carriers in the thylakoid membrane in a series of redox reactions
NADP acts as the final electrom accepted and combines 2e- with H+ to form reduced NADP (NADPH + H+)
ATP and reduced NADP are then used in the light independent reaction
Photolysis of water produces 2H, 2e- and O2. The 2e- replaces the ones lost from the chlorophyll in PSII

Question 23

What is photolysis?

Answer 23

The separation of molecules due to light