Photosynthesis Overview and Mechanisms

The capturing of light energy by chlorophyll and other pigments, producing ATP and NADPH. This occurs within the thylakoids of chloroplasts.

The use of ATP and NADPH produced in the light reaction to convert carbon dioxide into sugars. This occurs in the stroma of chloroplasts.

A pigment found in plants that absorbs light energy, particularly in the blue and red wavelengths. It is crucial for photosynthesis.

A group of pigments in plants that absorb light energy in various wavelengths, working alongside chlorophyll.

A type of chlorophyll that absorbs light primarily at 700 nm. It plays a key role in the light reaction by transferring electrons.

The transfer of energy from light to chemical energy in the form of ATP, which is used to power the Calvin cycle.

A process where plants consume oxygen and release CO2, reducing the efficiency of photosynthesis. It occurs when oxygen is present instead of carbon dioxide.

A photosynthetic pathway found in some plants, where CO2 is initially fixed into a 4-carbon compound. It's adapted for hot, dry environments.

The arrangement of mesophyll cells in C4 plants, where the chloroplasts are located in a ring around the vascular bundle.

A photosynthetic pathway that enhances carbon fixation efficiency in hot, dry conditions by using an initial four-carbon compound.

An enzyme that decarboxylates malate to pyruvate, releasing CO2 for use in the Calvin cycle.

A photosynthetic pathway where carbon dioxide is captured and stored at night, and used during the day to minimize water loss through transpiration.

The process where CAM plants store organic acids in their vacuoles at night and release CO2 during the day for photosynthesis.

Photosystem II (PSII) is a protein complex found in the thylakoid membranes of chloroplasts. It is responsible for capturing light energy and using it to split water molecules, releasing oxygen as a byproduct.

The reaction center of Photosystem II is a specialized chlorophyll molecule called P680. It absorbs light energy most efficiently at a wavelength of 680 nanometers.

Light-harvesting complexes (LHCs) are protein-bound pigment molecules that surround the reaction center in Photosystem II. Their role is to absorb light energy and transfer it to the reaction center efficiently.

When a chlorophyll molecule absorbs a photon of light, an electron within the molecule is energized and moves to a higher energy state, becoming excited.

The excited state of an electron in chlorophyll is transient and unstable. It quickly returns to its ground state, releasing the excess energy as heat.

The oxygen-evolving complex (OEC) is a key component of Photosystem II. It catalyzes the splitting of water molecules, releasing oxygen and hydrogen ions.

Water splitting by the OEC releases hydrogen ions (H+) into the thylakoid lumen, contributing to the proton gradient that drives ATP synthesis during photophosphorylation.

Photophosphorylation is the process of producing ATP using light energy. It involves a series of electron transfers and proton gradients within the thylakoid membrane of chloroplasts.

Photophosphorylation can be categorized into two types: cyclic and non-cyclic. Both involve electron transport and proton gradients, but they differ in their electron flow pathways.

The proton motive force, generated by the proton gradient across the thylakoid membrane during photophosphorylation, drives the synthesis of ATP by ATP synthase, an enzyme embedded in the membrane.

Cyclic photophosphorylation uses only Photosystem I, cycling electrons back through the electron transport chain, generating ATP without producing NADPH or oxygen.

Non-cyclic photophosphorylation produces both ATP and NADPH, using both Photosystem I and II, and releases oxygen as a byproduct.

The Calvin cycle requires more ATP than NADPH. When the Calvin cycle slows down due to low ATP, NADPH accumulates, triggering a shift to cyclic electron flow to increase ATP production.

The Calvin cycle is the light-independent stage of photosynthesis, where carbon dioxide is converted into glucose, using ATP and NADPH produced during the light-dependent reactions.

The Calvin cycle occurs in the stroma of chloroplasts, the fluid-filled space surrounding the thylakoid membranes.

In the carbon fixation phase, carbon dioxide molecules are attached to RuBP (ribulose bisphosphate), a 5-carbon sugar, by the enzyme RuBisCO.

The unstable 6-carbon compound formed in carbon fixation splits into two molecules of 3-phosphoglycerate (PGA), a 3-carbon molecule.

In the reduction phase, 3-phosphoglycerate is converted to glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar, using ATP and NADPH.

Only one out of the six G3P molecules produced is used to form glucose. The other five G3P molecules are rearranged to regenerate RuBP, the starting molecule for the cycle.

RuBisCO is a crucial enzyme that catalyzes the attachment of carbon dioxide to RuBP, initiating the Calvin cycle.

The Calvin cycle requires 9 ATP and 6 NADPH molecules to synthesize 1 molecule of G3P (glyceraldehyde-3-phosphate), a crucial intermediate in sugar production.

Photorespiration is a process that occurs in C3 plants when they close their stomata and oxygen levels rise, causing Rubisco to bind to oxygen instead of carbon dioxide. This leads to decreased photosynthetic output.

Photorespiration lowers photosynthetic output by wasting valuable energy (ATP) and releasing carbon dioxide, leading to lower sugar production.

C4 plants have a specialized carbon fixation mechanism where they initially form a four-carbon compound, minimizing photorespiration and enhancing photosynthesis, especially in hot, arid climates.

Bundle sheath cells in C4 plants tightly surround the veins of the leaf, forming a protective sheath and playing a crucial role in carbon fixation.

Mesophyll cells are the primary photosynthetic cells in C4 plants, located between the epidermis and bundle sheath cells, where the initial carbon fixation step occurs.

C4 plants have evolved an innovative carbon fixation mechanism, known as the Hatch-Slack pathway, to avoid photorespiration and enhance photosynthesis, particularly in hot, dry environments.

CAM plants are named for their unusual mode of carbon fixation called crassulacean acid metabolism, a unique adaptation to arid environments that separates the processes of carbon fixation and sugar production over time.

CAM plants have evolved an innovative carbon fixation mechanism, known as crassulacean acid metabolism (CAM), that allows them to minimize water loss by opening their stomata only at night, absorbing CO2, and storing it as malate.

CAM plants store carbon dioxide absorbed at night in the form of malate, a four-carbon organic acid, allowing them to continue photosynthesis during the day when stomata are closed to conserve water.