5.1 Photosynthesis

Question 1

What is the word and symbol equation for photosynthesis?

Answer 1

Carbon dioxide + Water -> Glucose + Oxygen

6CO2 + 6H2O -> C6H12O6 + 6O2

(With light over the arrow)

Question 2

How is CO2 and H2O acquired for photosynthesis?

Answer 2

CO2 - diffuses into the plant from the atmosphere through the stomata

H2O - moves via osmosis from the soil into the root hair cells, eventually moving through the xylem in transpiration

Question 3

How is a leaf adapted for carrying out photosynthesis?

Answer 3

Large surface area
Minimal leaf overlapping
Palisade cell is long and narrow absorbing more light
Spongy mesophyll with air gaps allow for a short diffusion distance
Guard cells on stomata all to open and close
Transparent cuticle to let light through
Network of xylem

Question 4

What is involved in the structure of a chloroplast?

Answer 4

Inner/outer membrane 
Thylakoid membrane
Granum (stack of thylakoid membrane)
Intergranal lamella
Stroma 
Starch grains
DNA

Question 5

What are the three stages of photosynthesis?

Answer 5

Capturing light energy
Light dependent reaction
Light independent reaction

Question 6

What takes place in the grana (thylakoid membrane)?

Answer 6

Site of light dependent reaction

Question 7

What takes place in the stroma?

Answer 7

Site of light independent reaction

Question 8

How do we remember redox?

Answer 8

OIL RIG

Oxidation is loss
Reduction is gain

(Of electrons)

Question 9

What is the aim of the light dependent reaction?

Answer 9

To add an inorganic phosphate molecule to ADP to form ATP

To split water into H+ ions (protons) in photolysis

Question 10

Describe the first stage of the light dependent reaction?

Answer 10

Light energy (photon) is absorbed by chlorophyll 2 (II)
Chlorophyll II is oxidised as the light energy excites the electrons (raising to a higher energy state) therefore losing 2 electrons - photoionisation
The primary acceptor gains the 2 electrons from the chlorophyll and is therefore reduced
To keep the reaction going photolysis of water takes place where the light splits water into: 2 electrons (to reduce the chlorophyll), 2 H+ and oxygen - which diffuses back out

Question 11

Describe the second stage of the light dependent reaction?

Answer 11

From the primary acceptor 2 electrons are transferred along many electron carriers, which releases energy used to create ATP leading to each carrier being at a lower energy level therefore the electrons lose energy
(The electron carriers undergo redox)
Light energy is absorbed by chlorophyll 1 (I) and it also gains the 2 electrons
Chlorophyll I loses 2 electrons to another primary acceptor which is reduced
The 2 electrons combine with the 2H+ (from photolysis of water) and NADP to form NADPH2

Question 12

What are all the products of the light dependent reaction?

Answer 12

NADPH2
Oxygen
ATP

Question 13

How are chloroplasts adapted for the light dependent reaction?

Answer 13

Thylakoid membrane provides a large surface area for attachment of chlorophyll, electron carriers and enzymes
Network of proteins hold the chlorophyll precisely
Granal membranes have ATP synthase channels to catalyse ATP
Contains DNA and ribosomes to quickly manufacture proteins involved in the reaction

Question 14

What is the light independent reaction also called?

Answer 14

The Calvin cycle

Question 15

Describe the first 2 stages of the Calvin cycle/light independent reaction?

Answer 15

CO2 is added to RuBP called carboxylation catalysed by rubisco
A 6 carbon unstable molecule is produced and quickly broken down into 2 x 3 carbon molecules called GP
GP is reduced by reduced NADP and energy supplied from ATP (both from light dependent reaction)
NADP - back to the first photosynthesis stage
GP is reduced to 2 x 3 carbon molecules called TP

Question 16

Describe the last stage of the Calvin cycle/light independent reaction?

Answer 16

1/6 of TP goes to form glucose (used to make starch, amino acids etc…)
5/6 of TP is involved in regeneration
ATP (from the light dependent reaction) is hydrolysed and the Phosphate group (Pi) is used to make RuBP

Question 17

How is the second stage of photosynthesis linked to the first?

Answer 17

The second doesn’t need light but it needs the products from the light dependent reaction

Question 18

Where does the CO2 come from in the light independent reaction?

Answer 18

Diffuses through the stomata of the leaf from the atmosphere
It dissolves in water around the walls of the mesophyll cells
It diffuses through the cell surface membrane, cytoplasm, chloroplast membrane and into the stroma

Question 19

How are chloroplasts adapted for the light independent reaction?

Answer 19

The fluid stroma contains a high concentration of enzymes

Stroma surrounds the grana therefore the products of the first stage can readily diffuse into the stroma

Contains DNA and ribosomes to quickly manufacture proteins involved in the reaction

Question 20

In a question what does adding CO2 into the reaction do? (Potentially via hydrogen carbonate)

Answer 20

Acts as an immediate source of CO2

Can often be tracked (radioactively labelled) by the carbon from the CO2 into it’s products i.e glucose

Question 21

What is significant about RuBP?

Answer 21

It’s not very efficient as it also has an affinity for oxygen

Question 22

What are some factors affecting photosynthesis?

Answer 22

Light intensity
Carbon dioxide concentration
Temperature
Presence of chlorophyll

Question 23

What is a limiting factor?

Answer 23

The factor in shorter supply which limits the maximum rate of photosynthesis and growth

Question 24

On a rate of photosynthesis graph what tends to be the initial limiting factor?

Answer 24

The one along the bottom

Question 25

How does increasing light affect the rate of photosynthesis?

Answer 25

More water is split in photolysis and excites the electrons quicker

Question 26

How does increasing CO2 concentration affect the rate of photosynthesis?

Answer 26

More RuBP is turned into GP using the CO2 with rubisco in carboxylation

Question 27

How does increasing temperature affect the rate of photosynthesis?

Answer 27

The heat energy excites the electrons

Keeps enzymes working at optimum conditions

Question 28

What is the light compensation point?

Answer 28

Where the volume of oxygen produced and carbon dioxide absorbed in photosynthesis
Is exactly balanced by
The volume of oxygen absorbed and the carbon dioxide produced in respiration

No net exchange of gases