C1.3 Photosynthesis

What is photosynthesis?

Turning light energy into chemical energy

What organisms use photosynthesis?

 Plants, algae, and cyanobacteria

What does photosynthesis produce?

to produce sugars (glucose) and other carbon compounds

Where does photosynthesis take place?

all reactions of photosynthesis takes place in the chloroplast

What is the photosynthesis equation?

 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

What starts the photosynthesis process?

the process begins in the photosystems.

What are Photosystems?

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

Where are photosystems found

on the membranes within chloroplasts

What serves as the reaction center?

One specific chlorophyll a molecule within the photosystem

Why do photosystems use arrays of many pigment molecules?

Photosystems use arrays of many pigment molecules to absorb enough light energy to photoactivate the central chlorophyll molecule.

What are the advantages of many pigment molecules in a photosystem?

• 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

What happens to chlorophyll at the reaction center of Photosystem II?

It releases electrons, creating a very unstable molecule in its oxidized state.

Why does the reaction center in Photosystem II become a powerful oxidizing agent?

Because of its oxidized state after losing electrons.

What process allows Photosystem II to split water molecules?

Photolysis

What is generated as a byproduct of the photolysis of water in Photosystem II?

O₂ (oxygen).

How are the electrons lost by Photosystem II replaced?

By the electrons produced during the photolysis of water.

What holds the pigments in place in a photosystem?

the pigments are held by proteins

What is the overall role of photosystems?

• 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

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)

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

To produce ATP through chemiosmosis.

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

In the thylakoids of the chloroplasts in plants.

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

Photophosphorylation

What is photophosphorylation?

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.

What happens in the light-dependent stage?

• 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.

what can photophosphorylation be?

Cyclic or noncyclic

What happens in cyclic photophosphorylation?

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.

What happens in noncyclic photophosphorylation? 

• 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.

What Is NADP?

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

What does NADP do in photosystem 1?

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).

In non-cyclic photophosphorylation used to reduce NADP to NADPH?

the electrons excited and lost from photosystem I

Why is the process of reducing NADP to reduced NADP is essential in photosynthesis

it essential because without it, the light-independent stage of photosynthesis could not occur.

whats required to produce carbon compounds in the light-dependent stage of photosynthesis

ATP and NADPH

What is the overall role of chloroplasts in plant cells?

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

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.

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

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

Why is the intermembrane space within thylakoids important?

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

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.

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

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.

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.

What is Rubisco?

An enzyme that catalyzes RuBP.

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

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

Where does the light independent cycle take place?

Takes place in the stroma of the chloroplast

What are the products of light dependent reactions?

• O2

• ATP

• NADPH