C1.3

Question 1

The Reactants of Photosynthesis

Chlorophyll?

Answer 1

• Plant pigment that absorbs primary red and blue wavelengths of light, while reflecting green ones (appearance)
• chlorophyl b is most common
• Chlorophyll is found found in chloroplasts, specifically in the thylakoid membranes
• Absorb photons of light to initiate proccess of photosynthesis

Question 2

The Reactants of Photosynthesis

What is photolysis?

Answer 2

• The splitting of water to remove the hydrogen atom (uses the energy from photons of light to split the water).
• The oxygen is released as waste, and the hydrogen is passed through to the LDR.
• Hydrogens are necessary for the conversion of CO2 into sugar and to create ATP needed for carbon fixation.

Question 3

The Reactants of Photosynthesis

What is the chemical formula for photosynthesis?

And what TYPE of reaction is it?

Answer 3

6CO2 + 6H2O –light,chlorophyll—> C6H12O6 + 6O2
* Photosynteshsis is a REDOX reaction whereby water is oxidized when H-atoms are removed during hydrolysis and oxygen is released.
* Later carbon dioixde is reduced when the hydrogens from water are added to carbon dioxide, producing glucose.

Question 4

The Reactants of Photosynthesis

What is the purpose of water in photosynthesis?

Answer 4

• Water provides the source of hydrogen needed to produce ATP needed for carbon fixation & as hydrogen for glucose.
• Water is split by photosystem II, hydrogen divides into electrons and protons.
• Thus, water is the initial electron donor, the electrons faciliate the ETC that produces the ATP needed.

Question 5

The Reactants of Photosynthesis

How does the concentration of CO2 impact the rate of photosynthesis?

Answer 5

• Six CO2 is required to make every glucose molecule, thus a sufficient amount is needed for photosyntehsis.
• At low CO2 levels, acts as a limiting factor.
• Once sufficient, rate plateus as other factors become limtiing
• Once Rubisco fixes CO2 is already worked at its optimum rate

Question 6

The Reactants of Photosynthesis

How does the concentration of light intensity impact the rate of photosynthesis?

Answer 6

• Light intensity provides provides intial energy required to split water and create ATP, is thus also the hydrogen source needed for carbon fixation to make glucose.
• Low light intensity is a limiting factor such that increasing it increases the rate of photosyntehsis.
• Higher intensity, other favtors become limiting/ high oxygen production leads to photorespiration by binding to rubisco

Question 7

The Reactants of Photosynthesis

How does the concentration of temperature impact the rate of photosynthesis?

Think enzymes

Answer 7

• Photosynthesis relies on the enzymes, ATP synthase, rubisco and NADP reductase
• Low temperatures reduce kinetic energy thus reducing successful collisions.
• Higher temperatures break bonds in enzyme’s active sites leading to loss of function through denaturation

Question 8

Light Spectra and Photosynthesis

What is a pigment?

Answer 8

• Substance that provides colour, in plant context, light absorbing molecule
• Colour it emits depends on which light wavelenght it absorbed and which is reflects back.
• Pigements make up the photosytems of the chlorplasts which absorb light.

Question 9

Light Spectra and Photosynthesis

What is an Absorption Spectrum?

Answer 9

• Shows the absorption of light for each wavelength along the electromagnetic spectrum of visable light for a single pigment.
• Each individual pigment has its own absorption spectrum curve

Question 10

Light Spectra and Photosynthesis

What is an Action Spectrum

Answer 10

• Zooms out and plots the overall rate of photosynthesis across the different wavelengths of visable light.
• Action spectrum seen as one curve it is the result of the absorption curves all chlorophyll and accessory pigments

Question 11

Compare Absorption Spectra and Action Spectra.

See booklet for shape of spectrum, peaks of chlorophyll types.

Answer 11

• Absorption: single pigment, Y-axis is the absorption rate.
• Action: the result of the actions of ALL pigments that play a role in photosynthesis, Y-axis is rate of photosynthesis, shape is seen as a COMBINATION of all individual absorption spectra -> most closely relflects chlorophyll a
• BOTH: have light wavelength on X-axis

Question 12

The Light-Dependent Reactions

What are photosystems?

Answer 12

• Pigment-protein complexes located along thylakoid membranes of chloroplasts.
• Contain ~130 pigment molecules arranged in precise array to optimise wavelengths of light absorbed
• In centre, there’s a core complex containing the reaction centre that contains electrons that absorb the light energy from the pigments to become excited.

Question 13

The Light-Dependent Reactions

What is a reaction centre?

…special pair….excited electrons…

Answer 13

• Centre of a photosystem
• Pigments surrounding capture light and tranfer the energy through the system till it reaches this centre
• Special pair - two chlorophyll molecules that capture the energy from pigement array and use it to excite 2 electrons sitting in reaction centre
• These 2 excited electrons transferred into ETC (then replaced with two NEW electrons from water following photolysis)

Question 14

The Light-Dependent Reactions

What is PHOTOphosphorlyation?

Answer 14

• Solar energy as the source for the energy required for phosphorlyation.
• There’s two pathways (non-cyclic and cyclic - see later).

Question 15

The Light-Dependent Reactions

What are the excited electrons?

Answer 15

• Photons of light harvested by pigments –> electrons in pigment becomes excited (move to higher energy orbital)
• When returning to original orbital, they release energy which is absorbed by nearby pigments
• Pigments close arrangement allows energy to transfer through photosytem until reaching chlorophyll of reactoin centre.

Question 16

The Light-Dependent Reactions

What is the Ligh Receiving complex?

Question 17

The Light-Dependent Reactions

What is the difference in optimum wavelengths between photosystems?

Photosystm I and Photosystem II

Answer 17

• Photosystem II: higher wavelength absoprtion, drives inital electron excitation, ETC and produciton of ATP as a result.
• Photosystem I: captures photons of light and exicted electrons to continue along the membrane.

Question 17

The Light-Dependent Reactions

Special chlorophyll molecules in photosystems?

Answer 17

• Transfer of energy from pigements reaches special pair in reaction center
• Exicted electrons move up the reaction centre and to the First electron acceptor of the electron transport chain.
• Lost electrons are replaced by two electrons removed from a water that’s split from a special oxygen-evolving complex of Photosystem II using the energy of the excited chlorophyll.

Question 18

The Light-Dependent Reactions

How does photolysis occur at Photosytem II?

Oxygen Revolving Complex

Answer 18

• Within photosystem II, there’s an oxygen-evolving complex that can split water into hydrogen and oxygen.
• Complex requires the energy from the special chlorophyll pair
• When excited, they release an energised electron, becoming reducing agents able to pull electrons from water.
• THIS results in the splitting of water (photolysis) and replaces the electrons lost by the chlorophyll atoms.

Question 19

The Light-Dependent Reactions

Explain the electron transport chain in Light Reactions.

Similar to aerobic respiration, but here along the thylakoid

Answer 19

• First protein is called plastoquinone (pq) that ACCEPTS the electrons and transfers them to a cyochrome bf complex (transmembrane pump) that moves two protons into the thylakoid spaces as the electrons move through it.
• Then passes to plastocyanin (pc) that moves the electrons to the reaction centre of photosystem II

Question 20

The Light-Dependent Reactions

What is the role of the enzyme ATP synthase in Light reactions?

Answer 20

• Has a drum shape that’s activated when rotating protons move through them with their conc. gradient
• Causes the phosphorylation of ADP to ATP in the lower enzymatic section.
• Requires a positive proton gradient inside thylakoid space to produce ATP usingg chemiosmosis

Question 21

The Light-Dependent Reactions

What is the role of the enzyme NADP reductase in Light reactions?

Answer 21

• Follows Photosystem I and transfers electrons and protons onto NADP, reducing it for transfer.
• The protons and electrons used to generate ATP are also required for Calvin Cycle (to make glucose) and are thus transferred to stroma for carbon fixation by NADP.

Question 22

The Light-Dependent Reactions

What are the benefits of a structured array of pigments?

Answer 22

• Due to scattering of photos of light, more pigments absorb individual photons
• Different range of wavelength absorption supports presence of pigments wihin
• Pigements transfer energy between each other by fluroescence to reach spcial pair: CLOSE proximity allows this.

Question 23

Light-Dependent Reactions

Compare cyclic vs non-cyclic phosphorylation.

ATP yield, pathway

Answer 23

• NON-CYCLIC: NADP available, electrons CONTINUE from photosystem I to NADP reductase. ATP yield is small.
• CYCLIC: NADP not available (if all reduced/limited CO2 maybe)/ NADPH sufficient and more ATP needed, electrons from photosytem I can be transferred back to plasoquinone (pq), allowing ETC to proceed, HIGHER ATP yield.

Question 24

# Light-Independent Reactions What is carbon fixaiton?

Answer 24

* The proccess whereby an **inorganic gas** (CO2) is converted into an **organic sugar/compound** * The **first step of Calvin Cycle**, and involves attaching a CO2 to a **recyable sugar RuBP** which exists in **STROMA** * Proccess dependent on enzyme **rubisco**

Question 25

# Light-Independent Reactions What is and what does Rubisco do?

Answer 25

* **Attaches one Carbon Dioxide** molecule TO **one RuBP** sugar, fixing carbon, at the onset of CC. * Fixes only **3 carbon dioixde molecules** per second

Question 26

# Light-Independent Reactions What is the relationship between Calcin Cycle and Light?

Answer 26

* **Never uses light energy directly**, BUT is reliant upon the **ATP and NADPH** generated by light dependent reactions. * Thus in the ABSENCE of light, **Calvin cycle stops** and doesn't have energy it needed to progress * Likewise, for ligh dependent reactions to continue the **ADP and NADP** must be used in CC and recycled back. * **Insufficient CO2** for Calvin Cycle, light dependent reactions would also stop.

Question 27

# Light-Independent Reactions What is GP's roles in the Calvin Cycle?

Answer 27

* The **6 C-sugar** made is unstable and spontanesouly **lysis** into **two 3-carbon compounds** called glycertae-3-phophase (GP) * It's then **reduced by NADPH**

Question 28

# Light-Independent Reactions What is RuBP's roles in the Calvin Cycle?

Answer 28

* A **5 carbon sugar** that plays a role in carbon fixation; **carbon dioixde is attached to it** to form a **6-C sugar** * Essential for next step that converts that sugar into compounds used to make food sources. * It's limited, hus **must be regenerated**, so each round of Calcvin cycle **returns all RuBP used**

Question 29

# Light-Independent Reactions What is TP's roles in the Calvin Cycle?

Answer 29

* Once reduced by NADP (adds H & electrons), the remaining **three C-sugar is triose phosphate** * Equivalent to one half of a glucose molecules * For every **3 CO2** molcules, **6 TP** are made but **5 need to be regenerated** to RuBP and one is used towards **creaitng 'food'** for the plant (usually glucose)

Question 30

# Light-Independent Reactions Explain the regeneration of Triose phosphate | Needed becuase RuBP is limited

Answer 30

* Last step of CC converts **most of TP back into RuBP** * FIVE TP contain 15 carbons. * **ATP** used to break bonds and faciliate reformation back into **3 five-carbon RuBP** * Allows RuBP to be completely recycled & one TP to be a product.

Question 31

# Light-Independent Reactions How are carbohydrates and lipids formed from GP/G3P

Answer 31

* Most frequent product made is glucose (plants need large amounts of it for cell resp.) and formation of structural carbs. * **Six CO2** is required to form **2 TP** that can be combined for glucose. * Glucose can then be converted to sucros (travel via phloem/starch for storage) * Plants can also convert **TP into fatty and amino acids**

Question 32

# Light-Independent Reactions What are other limting factors in photosynthesis | Link between the two stages (light dependent vs independent)

Answer 32

* **Light intensity** limits the rate of **production of ATP and NADPH** by the light dependent reactions (therefore halting Calvin Cycle) * **Limited Carbon Dioxide** directly stops calvin cycle, leaving **NADPH AND ATP unused**, halting light dependent reactions.