Chapter 5.5: Life Is Supported by the Sun: Light Energy Captured during Photosynthesis Converts Carbon Dioxide to Carbohydrates

Flashcards from Chapter 5.5 of Principles of Life, 3rd Edition.

Photosynthesis

An anabolic process utilizing solar energy to convert carbon dioxide and water into carbohydrates, similar in equation to the reverse of aerobic catabolism

Light reactions

The reactions in photosynthesis that convert light into chemical energy via ATP and NADPH, producing O₂ as a byproduct

NADPH

An electron carrier produced in the light reactions of photosynthesis, similar in structure to NADH but with an additional phosphate group

Carbon-fixation reactions (dark reactions)

Reactions that utilize the formed ATP and NADPH with CO₂ to produce carbohydrates, proceeding after the light reactions

Photons

Individual packets of light energy, the strength of which depends on the source’s electromagnetic wavelength

Visible spectrum

The range of light used in photosynthesis, avoiding bond breakage but preserving the necessary amount of energy for reactions

Pigments

Molecules that may absorb photons in specific sections of the visible spectrum, allowing the associated energy to be absorbed via excited electrons

• Divided into chlorophyll a and b for photosynthesis

Absorption spectra

A graph showing which wavelengths of light of a certain pigment or substance can be absorbed, roughly corresponding to its action spectrum

Chlorophyll

A pigment used in photosynthesis featuring a magnesium ion surrounded by a ring as well as a hydrocarbon tail for thylakoid membrane anchoring

• Absorbs red and blue light, with accessory pigments absorbing intermediate wavelengths

Antenna complex

The combination of several light-harvesting complexes to capture light energy through pigments

Reaction center

The area where energy is transferred from the antenna complex to electron acceptors after exciting a chlorophyll electron

Photosystem

The total structure comprising the antenna complex and reaction center together

Photosystem II

The first photosystem in the electron transport chain, passing an excited electron to photosystem I

• Replenishes electrons through the breaking of O—H bonds in water, with O then released and H used for the proton gradient

Photosystem I

The second photosystem in the electron transport chain, using an excited electron to reduce NADP to form NADPH in one pathway and create a proton gradient for ATP-forming reactions in another

Photophosphorylation

The use of ATP synthase powered by an electron-generated H⁺ gradient to produce ATP from ADP, in between photosystem II and I, supplying energy for later carbon fixation

Calvin cycle

The process of CO₂ reduction to produce carbohydrates following the light reactions, consisting of:

• CO₂ fixation by rubisco with RuBP to form the three-carbon 3PG

• Reduction of 3PG to the three-carbon sugar G3P, using ATP and NADPH

• Regeneration of RuBP through the phosphorylation of produced RuMP via most of the G3P (one-sixth is exported for use in photosynthesis)

Rubisco

The enzyme that catalyzes the fixation of CO₂ with RuBP to form the three-carbon 3PG molecule

Photorespiration

The fixation of O₂ instead of CO₂ by rubisco into 3PG, oxidizing a carbon molecule entirely into CO₂ for release later on

• Releases carbon into the atmosphere in a less efficient process, affecting plants with closed stomata more (typically in hot, dry environments)

• Mitigated by some plants by using an enzyme that does not react with O₂ for carbon fixation

Starch

A polysaccharide formed of linked glucose units used for energy storage in plants

Autotrophs

Organisms that take part in photosynthesis, providing energy for heterotrophs