Biology 101 - Photosynthesis: Energy from Sunlight

Comprehensive vocabulary flashcards covering the mechanisms of photosynthesis, the Calvin cycle, light reactions, and C3/C4/CAM plant adaptations.

Photosynthesis

An anabolic process by which energy from sunlight is captured and used to convert carbon dioxide ($CO_{2}$) into more complex carbon-containing compounds.

Autotrophs

Organisms such as plants, algae, and cyanobacteria that use water and carbon dioxide to make their own food (carbohydrates) for use or conversion into other molecules.

Heterotrophs

Organisms that must consume other organisms, such as animals, fungi, and most bacteria, to obtain food.

Oxygenic Photosynthesis

A form of photosynthesis carried out by plants, algae, and cyanobacteria in aerobic environments, following the equation: $6 CO_{2} + 12 H_{2}O \rightarrow C_{6}H_{12}O_{6} + 6 O_{2} + 6 H_{2}O$.

Non-oxygenic Photosynthesis

A process where molecules other than water (such as $H_{2}S$, $S^{2-}$, $H_{2}$, $Fe^{2+}$, or arsenic-derived compounds) donate protons and electrons.

Light Reactions

Pathway occurring in the thylakoid that converts light energy into chemical energy as $ATP$ and the reduced electron carrier $NADPH$.

Carbon-fixation Reactions

Also known as the Calvin cycle or light-independent reactions; these take place in the stroma and use $ATP$, $NADPH$, and $CO_{2}$ to produce carbohydrates.

Photons

Particles of light that behave as discrete packets of energy; the amount of energy is inversely proportional to the wavelength.

Pigments

Molecules that absorb specific wavelengths in the visible range and scatter or transmit others, which determines the color we see.

Absorption Spectrum

A plot of the specific wavelengths of light absorbed by a particular pigment.

Action Spectrum

A plot showing the rate of photosynthesis against different wavelengths of light, typically measured by the amount of $O_{2}$ released.

Chlorophyll a

The major pigment in photosynthesis which has a hydrocarbon tail used to anchor it in the protein complex of the thylakoid membrane.

Photosystem

A protein complex in the thylakoid membrane consisting of light-harvesting complexes (antenna systems) and a reaction center.

Resonance

The form of chemical energy used to pass absorbed energy between pigment molecules until it reaches chlorophyll $a$ in the reaction center.

Photosystem II (PSII)

A system in noncyclic electron transport containing $P680$ chlorophyll that absorbs light best at $680\,nm$.

Photosystem I (PSI)

A system in electron transport containing $P700$ chlorophyll that absorbs light best at $700\,nm$.

Cyclic Electron Transport

A pathway that uses only photosystem I and electron transport to produce $ATP$ instead of $NADPH$, where the electron returns to the original chlorophyll.

Photophosphorylation

A chemiosmotic mechanism in which the transport of protons ($H^{+}$) across the thylakoid membrane into the lumen creates an electrochemical gradient used by $ATP$ synthase.

Rubisco

Ribulose bisphosphate carboxylase/oxygenase; the enzyme that catalyzes the binding of $CO_{2}$ to $RuBP$ at the start of the Calvin cycle.

3-phosphoglycerate (3PG)

The first stable three-carbon sugar phosphate formed in the Calvin cycle after the breakdown of a six-carbon intermediate.

Glyceraldehyde 3-phosphate (G3P)

The direct product of the Calvin cycle; it can be exported to the cytoplasm for glycolysis or converted into hexoses like glucose, fructose, and sucrose.

Ribulose bisphosphate (RuBP)

The five-carbon acceptor molecule that binds with $CO_{2}$ and is regenerated at the end of the Calvin cycle.

Thioredoxin

A small protein that, when reduced by electrons from ferredoxin, activates Calvin cycle enzymes by reducing their disulfide bridges.

Photorespiration

A process occurring in the light where Rubisco acts as an oxygenase, adding $O_{2}$ to $RuBP$ and resulting in a $25\%$ loss of fixed carbon.

Phosphoglycolate

A toxic inhibitor formed when Rubisco reacts with $O_{2}$; it must be converted to $3PG$ through reactions in the peroxisome and mitochondria.

C3 Plants

Plants like roses and rice where the first product of carbon fixation is $3PG$ and whose Rubisco acts as an oxygenase on hot, dry days.

C4 Plants

Plants like corn and sugarcane where the first product of carbon fixation is the four-carbon oxaloacetate, helping to avoid photorespiration on hot days.

PEP Carboxylase

An enzyme in C4 and CAM plants that catalyzes the reaction of $CO_{2}$ and $PEP$ to form oxaloacetate and lacks oxygenase activity.

CAM Plants

Crassulacean acid metabolism plants, such as cacti and succulents, that open stomata at night to fix $CO_{2}$ and close them during the day to conserve water.

Bundle Sheath Cells

Specialized cells in C4 plants where $CO_{2}$ concentration is increased to ensure Rubisco functions as a carboxylase.