Chapter 6: Photosynthesis

What is photosynthesis?

Photosynthesis is the process where energy in sunlight is captured and drives the synthesis of carbohydrates.

In plants, what are the primary photosynthetic organs?

In plants, the leaves are the photosynthetic organs.

What are the internal membranes within chloroplasts called, and how are they organized?

Internal membranes within chloroplasts are called thylakoids, which are stacked together in columns called grana.

What is the stroma in a chloroplast?

Stroma is the liquid that surrounds the thylakoids within a chloroplast.

What are the three main stages of photosynthesis?

The three main stages are capturing energy from sunlight, converting light energy to chemical energy (ATP & NADPH), and using ATP and NADPH to make carbohydrates from CO2.

What are the light-dependent reactions of photosynthesis responsible for?

Light-dependent reactions convert light energy into chemical energy in the form of ATP and NADPH.

What is another name for the light-independent reactions and what do they produce?

Light-independent reactions are also known as the Calvin cycle and they use ATP and NADPH to make carbohydrates from CO2.

What are the inputs and outputs of the light-dependent reactions?

Inputs are light energy and H2O (water); outputs are ATP, NADPH, and O2 (oxygen).

What are the inputs and outputs of the light-independent reactions (Calvin cycle)?

Inputs are ATP, NADPH, and CO2 (carbon dioxide); output is glucose.

What are the packets of energy that comprise light called?

Light is comprised of packets of energy called photons.

What is the main photosynthetic pigment in plants?

The main pigment in plants is chlorophyll.

Where do the light-dependent reactions occur within the chloroplast?

The light-dependent reactions occur on the thylakoid membranes.

How is light energy processed in a photosystem?

Light energy is first captured by any one of the pigments in a photosystem and is passed along until it reaches the reaction center.

What happens in Photosystem II during the light-dependent reactions?

Photosystem II captures a photon of light, releases an excited electron to the electron transport system (ETS), and uses H2O as the source of electrons, producing ATP.

What happens in Photosystem I during the light-dependent reactions?

Photosystem I absorbs another photon of light, releases an excited electron to a second ETS, which then produces NADPH.

What is the source of electrons for Photosystem II?

Water (H2O) is the source of electrons for Photosystem II.

What is the primary purpose of the light-independent reactions (Calvin Cycle)?

The purpose of the light-independent reactions (Calvin Cycle) is to use chemical energy from light-dependent reactions (ATP and NADPH) to make carbohydrates from CO2.

What is the initial output molecule from the Calvin Cycle?

The initial output from the Calvin Cycle is a 3-carbon molecule.

What enzyme catalyzes carbon fixation in the Calvin cycle?

Carbon fixation is catalyzed by an enzyme called Rubisco.

Why is Rubisco considered a popular target for bioengineering?

Engineering Rubisco to be more efficient could pull more CO2 out of the atmosphere (combating climate change) and increase sugar production in plants (producing more food).

Describe the first step of the Calvin cycle, involving CO2 and RuBP.

The Calvin cycle begins when a carbon atom from a CO2 molecule is added to a 5-carbon molecule (RuBP), forming an unstable 6-carbon molecule that immediately splits into 3-carbon molecules.

How is energy from ATP and NADPH used in the Calvin cycle?

Energy from ATP and hydrogens from NADPH are added to the 3-carbon molecules, which are then reduced and either combine to make glucose or are used to make other molecules.

Why do plants in hot climates sometimes have trouble with C3 photosynthesis?

In hot climates, plants close stomata to prevent water loss, leading to O2 buildup and decreased CO2 inside the leaves, causing photorespiration.

What is photorespiration?

Photorespiration is a process where Rubisco binds O2 instead of CO2, causing the Calvin cycle to 'waste' a cycle, especially when CO2 levels are low and O2 levels are high.

How do C4 plants adapt to hot climates to prevent photorespiration?

C4 plants do carbon fixation in a separate cell layer, which prevents Rubisco from encountering O2, thus avoiding photorespiration.

How do CAM plants adapt to hot climates to prevent photorespiration?

CAM plants use the C4 pathway at night (when it's cooler) to fix CO2 and then use the C3 pathway during the day, which helps them prevent water loss and photorespiration.