3.1 Photosynthesis

Question 1

Formula for photosynthesis

Answer 1

6 CO2 + 6 H2O + light energy > C6 H12 O6 + 6 O2

Ie carbon dioxide + water + light energy > glucose and oxygen

Question 2

Stroma

Answer 2

Aqueous matrix inside chloroplast.
Components and solutes dispersed into stroma
Rich in proteins
Contains enzymes necessary for cellular processes
Contains chloroplast DNA
Contains ribosomes and molecules needed for protein synthesis as granule.
Calvin Cycle takes place in Stroma

Question 3

Thylakoids

Answer 3

Internal membrane system called thylakoid membrane.
Thylakoids are internal, membrane bound compartments by thylakoid membrane.
Contains thylakoid lumen, this contains plastocyanins and other molecules for electrons.
Location of chlorophyll - light reaction of photosynthesis takes place here.
Mostly formed into stacks called Grana - can be free floating

Question 4

Granum - singular

Grana - plural

Answer 4

Thylakoids arranged in the form of discs stacks one above the other.
Connected to each other by inter-Grana or stroma thylakoids.
In higher plants can be organised into a granum-stroma membrane assembly.
Granum - stacked thylakoid discs.
Connected by stroma thykaloids.

Question 5

Two stages of photosynthesis.

Answer 5

Light dependent reaction
Calvin cycle (light independent reaction)

Question 6

Light dependent reaction.

Answer 6

Takes place in Thylakoid membranes of chloroplast.
Light hits pigment molecules.
Electrons are energised within.
Energised electrons drive two important reactions.
i. Conversion of ADP molecule to ATP molecule.
ii splitting of water into Hydrogen and Oxygen
Two free hydrogen atoms combine with NADP to create NADPH2.
Oxygen gas is released as a waste product.

Question 7

Photosynthesis -light dependent reaction.

Brief description

Answer 7

Light dependent reactions take place in the thalkaloid in the chloroplast.

Reactions harness energy from the sun to produce ATP and NADPH

NADPH and ATP are energy carrying molecules that travel into stroma where Ca.vin cycle reaction take place.

Question 8

Definition of photosynthesis

Answer 8

Process by which:

Green plants trap light energy from sun.
Convert this energy into chemical energy stored in molecules.
Eg. Glucose which is manufactured from carbon dioxide and water.

Question 9

Respiration - definition

Answer 9

Process by which:

Molecules such as glucose are broken down to yield energy.
End products are water and carbon dioxide.

Question 10

ATP (adenosine triposphate) and ADP (adenosine diphosphate)

Answer 10

ATP stores energy in one of its bonds.
When ATP is converted into ADP a phosphate ion is lost and energy is released in a controlled manner.
To trap energy, the reverse occurs.
ATP provides a readily available source of energy.
Energy can be moved to part of a cell where it is required to drive reactions.

Question 11

Calvin Cycle

Stage one - Carbon fixation

Answer 11

i. Takes place in stroma.Does not need light but needs energy produced in light dependent stage of photosynthesis.
ii. 3 molecules of carbon dioxide, RuBisCO (an enzyme) and RuBP are present to initiate Calvin cycle.
iii. RuBisCO catalyses reaction between carbon dioxide and RuBP.
iv. Forms a 6 carbon compound which is converted into two, 3 carbon compounds.
v. This is called carbon fixation; carbon dioxide is fixed from inorganic form to organic molecules.

Question 12

C4 Pathway

Part one

Answer 12

Begins with PEP molecule.
PEP is a 3-C chemical that is converted into OAA when combined with CO2.
OAA is converted into malic acid and transported from mesophyll cells in leaf into the bundle sheath cells.
In bundle sheath cells OAA is broken down into PEP and CO2 .
CO2 enters Calvin Cycle.
PEP returns to mesophyll cell.
Resulting sugars are now next to leaf veins and can readily be used by plant.

Question 13

Common C4 plant

Answer 13

Zea mays

Question 14

C4 Pathway

Part two

Answer 14

Capture of CO2 by PEP is mediated by enzyme called PEP carboxylase.
PEP carboxylase has a stronge affinity for CO2 than does RuBP carboxylase.
When CO2 levels decline below RuBP threshold RuBP is catalysed with
O2 instead of CO2.
This produces glycolic acid.
Glycol acid cannot be broken down by photorespiration.
Does not produce NADPH nor ATP.
Dismantle Calvin Cycle.

Question 15

C3 plants

Answer 15

C3 plants are successful in areas if moderated light intensity, not excessive temperatures and CO 2 levels of approx 200 00m or higher plus optimal soil water availability.

Eg Pisum sativum, Solanum tuberosum & Daucus carota.

Question 16

CAM Plants

Answer 16

CAM photosynthesis fixes carbon in certain plants.
Acts as adaption enabling them to tolerate dry, arid conditions.
Stomata in leaves are shut during day to reduce evapotranspiration.
Stomata are open at night to collect CO 2.
CO 2 is stored in chemical form called malate.
It is utilised during photosynthesis taking place during the day.

Question 17

Benefits of CAM photosynthesis.

Answer 17

Stomata are closed during the day to reduce evapotranspiration.
Ideal for growth in arid and semi arid environments.

Eg Sedum, Cacti, and Orchids

Question 18

Calvin cycle.
Stage 2.
Reduction

Answer 18

i. ATP and NADP use stored energy to convert 3-PGA into G3P.
ii. This is called a reduction reaction as an electron is gained.
iii. ADP and NAD+ resulting from reduction return to light dependent reaction to be re-energised.

Question 18

Calvin cycle
Stage 3
Regeneration

Answer 18

i. One G3P molecule leaves Calvin cycle.
ii. It contributes to formation of carbohydrates molecule, usually glucose C6 H12 06
iii. As carbon dioxide has 6 carbon atoms it takes six turns of the Calvin cycle to make one carbohydrate molecule.
iv. Remaining G3P molecules regenerate RuBP. System is able to to prepare for carbon fixation step. ATP is used in regeneration of RuBP