C1.3 Photosynthesis


Photosystems

  1. What is photosynthesis?

A: Turning light energy into chemical energy


  1. What organisms use photosynthesis?

A: Plants, algae, and cyanobacteria


  1. What does photosynthesis produce?

A: to produce sugars (glucose) and other carbon compounds


  1. Where does photosynthesis take place?

A: all reactions of photosynthesis takes place in the chloroplast


  1. What is the photosynthesis equation

A: 


  1. What starts the photosynthesis process?

A: the process begins in the photosystems. 


  1. What are Photosystems?

A: are molecular arrays of chlorophyll and accessory pigments within protein complexes, located in membranes


  1. Where are photosystems found

A: on the membranes within chloroplasts


  1. On specific chlorophyll a molecule within the photosystem serves as the reaction center



  1. Photosystems, only through its use of arrays of many pigment molecules can enough light energy be absorbed to photoactivate the central chlorophyll molecule


  1. What are the advantages of many pigment molecules in a photosystem

A:    - Absorbs a wider range of wavelengths, maximizes energy absorption for photosynthesis

  • Funnels energy efficiently to the reaction center, where chlorophyll molecules is located

  • Activates chlorophyll a to release electrons for the light-dependent phase of photosynthesis

  • Prevents inefficiency without multiple pigments; energy absorption would be insufficient for effective photosynthesis


  1. The release of electrons from chlorophyll at the reaction center of photosystem 2 creates a very unstable molecule in its oxidized state. 

  2. The reaction center because of its oxidized state now becomes a powerful oxidizing agent

  3. The reaction center in photosystem 2 becomes a powerful oxidizing agent, allowing it to split water molecules using a process called photolysis

  4. Photolysis of water in photosystem2 generates O2 has a byproduct and replaces the electrons lost by photosystem 2

  5. What holds the pigments in place in a photosystem?

A: the pigments are held by proteins

  1. What is the overall role of photosystems?

A: - its the key to light dependent phase

  • Enables organisms to harness light energy

  • Produces O2 as a byproduct

  • Drives the production of ATP and NADPH


  1. What are antenna complexes?

They surround the reaction center and the antenna pigments  collect light energy from the sun (not all are 680 nm) and transfer it to reaction centers (thats specified for 680nm) 


Thylakoids

  1. What is the main purpose of the light-dependent stage of photosynthesis?

Ans:  To produce ATP through chemiosmosis.


  1. Where does the light-dependent stage of photosynthesis take place?

Ans: In the thylakoids of the chloroplasts in plants.


  1. What process is used to produce ATP in the light-dependent stage of photosynthesis?

Ans: Photophosphorylation.


  1. What is phosphorylation?

Ans: The process of generating ATP using light energy. It occurs during the light-dependent reactions of photosynthesis, where light energy is used to phosphorylate ADP to ATP.


  1. What happens in the light-dependent stage?

Ans:

  • Light energy excites electrons in the reaction center of the photosystem.

  • The excited electrons are transferred through an electron transport chain (ETC) of carrier proteins.

  • As electrons move along the ETC, they decrease in energy; the released energy pumps protons (H⁺ ions) from the stroma into the thylakoid lumen.

  • The accumulation of protons in the thylakoid lumen creates an electrochemical gradient across the thylakoid membrane.

  • This proton gradient provides the potential energy to drive the ATP synthase complex, which phosphorylates ADP to produce ATP.

  • Since light energy initiates this process, it is called photophosphorylation.


  1. Phosphorylation can be either cyclic or non-cylic


  1. What happens in cyclic phosphorylation? 

Ans: Light energy excites electrons in photosystem I (PSI). These electrons move along the electron transport chain (ETC), where they pump H⁺ ions to establish a proton gradient that makes ATP through ATP synthase. After moving along ETC, the electrons return to PSI, replacing those initially lost. It is termed "cyclic" because the electrons are lost from and return to the same photosystem.


  1. What happens in noncyclic phosphorylation? 

Ans: 

  • light energy excites electrons in photosystem II (PSII).

  • The excited electrons are passed along the electron transport chain (ETC), where they pump H⁺ ions, establishing a proton gradient that drives ATP synthesis.

  • The electrons then enter photosystem I (PSI), where they are re-energized with light energy.

  • The re-energized electrons are transferred to ferredoxin and used to reduce NADP to NADPH.


  1. What Is NADP?

Ans: A coenzyme that plays a key role in cellular respiration by being an electron carrier


  1. What does NADP do in photosystem 1?

Ans: In photosystem 1 NADP accepts two electrons from photosystem 1 as well as two H+ from the stroma to become reduced NADP (and that is NADPH). 


  1. In non-cyclic photophosphorylation, the electrons excited and lost from photosystem I are used to reduce NADP to NADPH


  1. The process of reducing NADP to reduced NADP is essential in photosynthesis because, without it, the ___ stage of photosynthesis could not occur. A: light-independent


  1. Both ___ and ___ are required to produce carbon compounds in the light-independent stage of photosynthesis. A: ATP; reduced NADP


  1.  In the light-independent stage of photosynthesis, ___ and ___ are used to produce ___. A: ATP; reduced NADP; carbon compounds



  1.  What is the overall role of chloroplasts in plant cells?

A: Chloroplasts are the organelles responsible for photosynthesis, hosting both the light-dependent and light-independent stages.

  1. What role do thylakoids play in the light-dependent stage of photosynthesis?

Thylakoids are flattened sacs that contain photosystems, the electron transport chain (ETC), and ATP synthase, which enable photolysis, ATP production via chemiosmosis, and the reduction of NADP.

  1. What is the function of the thylakoid membrane in the light-dependent stage of photosynthesis?

A: The thylakoid membrane houses the photosystems, ETC, and ATP synthase, essential for producing ATP and NADPH in the light-dependent stage.

  1. Why is the intermembrane space within thylakoids important?

A: The intermembrane space allows for the buildup of an electrochemical (proton) gradient, which drives ATP production through chemiosmosis.


  1. What is the role of the thylakoid intermembrane space in the light dependent stage of photosynthesis? 

Photolysis occurs in the thylakoid intermembrane space, splitting water to release oxygen, protons (H⁺), and electrons.


  1. What is the role of grana in the light-dependent reactions of photosynthesis?

A: Grana, stacks of thylakoids, provide a large surface area to accommodate numerous photosystems, ETCs, and ATP synthase complexes, maximizing efficiency in the light-dependent stage.


Calvin Cycle: 


  1. What is RuBP? 

RuBP is a 5 carbon molecule that combines with carbon dioxide in a reaction catalysed by the enzyme rubisco to initiate the process of carbon fixation.

  1. What is Rubisco?

  • An enzyme that catalyzes RuBP. 


  1. Why is there a high concentration of Rubisco in the stroma?

  • Because its a slow enzyme so and calvin cycle is inefficient due to its high requirement of energy

  • Cannot discriminate between O2 and CO2 so its inefficient

  • To speed up the process many molecule of Rubisco are needed


  1. What are the 3 main parts in the light independent cycle? 

  • Carbon fixation: Converting CO2 to organic compounds. The catalysing enzyme Rubisco joins CO2 with RuBP to form G3P

  • Reduction: ATP driven reactions then form intermediates before NADPH driven reactions form triose phosphate

  • Regeneration: For every “6 turns of the wheel” 2 triose phosphates will leave the chloroplast and 10 triose phosphates continue through the last stages of the cycle to form RuBP

  1. does the light independent cycle take place? 

Takes place in the stroma of the chloroplast


  1. What are the products of light dependent reactions?

  • O2

  • ATP

  • NADPH