Notes : 5.5 - 5.9 Photosynthesis

Question 1

Write the full equation for photosynthesis ?

Answer 1

Question 2

What does the specification say that photosynthesis is ?

Answer 2

requiring energy from
light to split apart the strong bonds in water molecules, storing the hydrogen in
a fuel (glucose) by combining it with carbon dioxide and releasing oxygen into
the atmosphere.

Question 3

What type of organisms are plants ?

Answer 3

• Autotrophic organisms - meaning they produce their own food through photosynthesis

Question 4

What type of organisms are animals ?

Answer 4

• Heterotrophic - Meaning they cannot produce their own food and must obtain nutrients and energy by consuming other organisms or organic matter. The term “heterotrophic” comes from the Greek words “hetero,” meaning “other,”

Question 5

Where do the light dependent reactions take place ?

Answer 5

In the thylakoid membranes.

Question 6

Main purpose of light dependent reactions.

Answer 6

o capture and convert light energy into chemical energy in the form of ATP and NADPH. These molecules are then used in the light-independent reactions (Calvin cycle) to produce glucose.

Question 7

Explain what happens during Light dependent reactions.

Answer 7

1. Light is absorbed by photosystem II (PSII).
2. This excites electrons the electron in chlorophyll a molecule in the reaction centre
3. The electron is passed along the electron transport chain.
4. The electrons which have been lost from PSII need to be replaced. This happens through the Photolysis of water occurs in PSII, where light energy splits water into oxygen (O₂), protons (H⁺ ions), and electrons:
   2H₂O → O₂ + 4H⁺ + 4e⁻
5. The electrons lose energy as they move along the chain, and this energy is used to pump protons (H⁺ ions) across the thylakoid membrane from the stroma into the thylakoid lumen, creating a proton gradient.
6. The proton gradient formed across the thylakoid membrane drives protons (H⁺ ions) to diffuse back into the stroma through the enzyme ATP synthase.
7. The flow of protons through ATP synthase causes ADP to be phosphorylated to form ATP (photophosphorylation) in a process called chemiosmosis. .
8. Meanwhile, Light is absorbed by PSI, exciting electrons again, which are then passed along a short electron transport chain
9. These electrons are finally accepted by NADP⁺, which combines with protons (H⁺) from the stroma to form reduced NADP (NADPH).
   NADP⁺ + 2e⁻ + H⁺ → NADPH.
10. The ATP and reduced NADP move into the stroma for the next stage of photosynthesis, the light independent reaction.

Question 8

Visualisation of PSII

Answer 8

Question 9

Products of the light independent reaction

Answer 9

NADPH
ATP
O₂

Question 10

Draw and label a chloroplast

Answer 10

Question 11

Where do the light independent reactions take place ?

Answer 11

In the stroma

Question 12

What are the three main stages of the light independent reaction (calvin cycle).

Answer 12

1. Carbon fixation
2. Reduction
3. Regeneration

Question 13

Explain the process of carbon fixation

Answer 13

• Carbon dioxide Fixation : Carbon dioxide is ‘fixed’ by adding it to, a 5-carbon molecule called, ribulose bisphosphate (RuBP), forming a 6-carbon molecule. This reaction is catalysed by an enzyme called Rubisco.
• Formation of Glycerate-3-phosphate : The resulting 6C molecule is unstable and immediately splits into two 3-carbon compounds called glycerate-3-phosphate (GP).

Question 14

Explain the process of reduction

Answer 14

• Where GP is converted into Triose phosphate (TP/GALP).
• ATP (from the LDR) phosphorylates GP into TP and provides the energy for this to happen.
• Reduced NADP (NADP) reduces GP into TP.
• Some GALP is converted glucose, but some is used to regenerate RuBP.
• For every 6 molecules of GALP, 1 is used to produce organic molecules whereas 5 will be used for RuBP regeneration.
• The calvin cycle goes around 6 times to make one glucose molecule.

Question 15

Explain the process of regeneration

Answer 15

• 5 out of every six molecules of GALP is used to regenerate RuBP.
• Regenerating RuBP requires energy which is generated by ATP hydrolysis.

Question 16

What other organic products does the calvin cycle produce ?

Answer 16

• LIpids : Made using glycerol which is synthesised from GALP and fatty acids which is synthesised from GP
• Amino acids: Some amino acids are made from GP
• Nucleic acids -The sugar in RNA (ribose) is made using GALP.

Question 17

Blurt everything you know about non-cyclic photophosphorylation.

Answer 17

• Involves both **PSII* and PSI.
• You produce O₂, NADPH and ATP

Question 18

Blurt everything you know about cyclic-phosphorylation

Answer 18

• Involves only **PSI*
• You produce ATP
• It’s called ‘cyclic’ because
  the electrons from the chlorophyll molecule aren’t passed onto NADP, but
  are passed back to PSI via electron carriers. This means the electrons are
  recycled and can repeatedly flow through PSI.
• Cyclic photophosphorylation generates two ATP molecules.

Question 19

Why do chloroplast in guard cells only contain PSI

Answer 19

• As the require ATP to actively load potassium lower than water potential. Water can move in by osmosis and the guard cells swell and open the stomata,

Question 20

Why is light used in cyclic phosphorylation ?

Answer 20

To excite the electron.

Question 21

In the dark. Explain why no glucose is produce.