Photosynthesis: Mechanisms and Pathways in Plants

Photosynthesis

the conversion of light energy to chemical energy that is stored in sugars or other organic compounds; occurs in plants, algae, and certain prokaryotes

Autotroph

an organism that obtains organic food molecules without eating other organisms or substances derived from other organisms. Autotrophs use energy from the sun or from oxidation of inorganic substances to make organic molecules from inorganic ones

Photoautotroph

an organism that harnesses light energy to drive the synthesis of organic compounds from carbon dioxide

Heterotroph

an organism that obtains organic food molecules by eating other organisms or substances derived from them

Photophosphorylation

the process of generating ATP from ADP and phosphate by means of chemiosmosis, using a proton-motive force generated across the thylakoid membrane of the chloroplast or the membrane of certain prokaryotes during the light reactions of photosynthesis

Chemiosmosis

An energy coupling mechanism that uses energy stored in the form of a hydrogen ion gradient across a membrane to drive cellular work, such as the synthesis of ATP. Under aerobic conditions, most ATP synthesis in cells occurs by chemiosmosis

Substrate-level phosphorylation

a method of ATP synthesis that occurs when a phosphate group is transferred from a substrate molecule to ADP to form ATP.

Oxidative phosphorylation

a process that uses energy released by the electron transport chain to power ATP synthesis.

Calvin cycle

a series of biochemical reactions that take place in the chloroplasts of photosynthetic organisms, where carbon dioxide is fixed into organic molecules.

PSI

Photosystem I, a protein and pigment complex that plays a crucial role in the light-dependent reactions of photosynthesis.

PSII

Photosystem II, a protein and pigment complex that captures light energy and uses it to extract electrons from water.

NADPH

a reduced form of nicotinamide adenine dinucleotide phosphate, used in the Calvin cycle as a reducing agent.

Glucose

a simple sugar that is an important energy source in living organisms and is a product of photosynthesis.

H₂O

water, a reactant in photosynthesis that is split to provide electrons.

O₂

oxygen, a byproduct of photosynthesis released into the atmosphere.

CO₂

carbon dioxide, a reactant in photosynthesis that is fixed into organic molecules.

H⁺ gradients

a difference in hydrogen ion concentration across a membrane, used in chemiosmosis to generate ATP.

rubisco

an enzyme that catalyzes the first step of the Calvin cycle, fixing carbon dioxide into organic molecules.

Anabolism

the set of metabolic pathways that construct molecules from smaller units, typically requiring energy.

Cellular respiration

the process by which cells convert glucose and oxygen into energy, carbon dioxide, and water.

Redox process

a chemical reaction involving the transfer of electrons between two species.

Photosynthesis

A 2-step process that converts light energy into chemical energy.

Light dependent reactions

Change electromagnetic energy from light into chemical bond energy in the form of ATP and NADPH.

Light independent reactions (Calvin cycle)

Use energy in the form of ATP and NADPH to reduce CO2 to form glucose.

Overall equation for photosynthesis

6 H2O + 6 CO2 -> 6 O2 + C6H12O6.

Chloroplast

An organelle found in plants and photosynthetic protists that absorbs sunlight and uses it to drive the synthesis of organic compounds from carbon dioxide and water.

Mesophyll

Leaf cells specialized for photosynthesis.

Stoma (stomata pl.)

A microscopic pore surrounded by guard cells in the epidermis of leaves and stems that allows gas exchange.

Stroma

The dense fluid within the chloroplast surrounding the thylakoid membrane and containing ribosomes and DNA.

Thylakoid

A flattened, membranous sac inside a chloroplast, often stacked in grana.

Chlorophyll

A green pigment located in membranes within the chloroplasts of plants and algae, participating in light reactions.

Light reactions

The first of two major stages in photosynthesis, occurring in the thylakoid membranes.

Calvin cycle

Occurs in stroma, using ATP and NADPH to incorporate CO2 into carbohydrate.

Guard cells

Cells that surround stomata and regulate gas exchange.

Thylakoid lumen

The interior space of the thylakoid where protons accumulate during light reactions.

Grana (granum)

Stacks of thylakoids in the chloroplast.

Photophosphorylation

The process of generating ATP from ADP and inorganic phosphate using light energy.

Carbon fixation

The incorporation of CO2 into organic molecules in the Calvin cycle.

NADP+

An electron acceptor that forms NADPH during light reactions.

ATP

A molecule that stores and transfers energy within cells.

NADPH

A reduced electron carrier that provides reducing power for the Calvin cycle.

O2

Oxygen produced as a byproduct of light reactions in photosynthesis.

Electrons (e-)

Negatively charged particles transferred during the light reactions.

H+

Protons that are transferred from water to CO2 during photosynthesis.

Potential energy

Energy stored in electrons as they move from water to CH2O.

Energy from light

The source of energy required for the light dependent reactions of photosynthesis.

Light Reactions

These reactions, which occur on the thylakoid membranes of the chloroplast or on membranes of certain prokaryotes, convert solar energy to the chemical energy of ATP and NADPH, releasing oxygen in the process.

Calvin Cycle

The second of two major stages in photosynthesis (following the light reactions), involving fixation of atmospheric CO2 and reduction of the fixed carbon into carbohydrate.

NADP+

The oxidized form of nicotinamide adenine dinucleotide phosphate, an electron carrier that can accept electrons, becoming NADPH.

NADPH

The reduced form of nicotinamide adenine dinucleotide phosphate; temporarily stores energized electrons produced during the light reactions.

Carbon Fixation

The initial incorporation of carbon from CO2 into an organic compound by an autotrophic organism (a plant, another photosynthetic organism, or a chemoautotrophic prokaryote).

Chloroplasts

Organelles that resemble mitochondria but have an extra compartment: the thylakoid.

Chlorophyll

Molecules that absorb the energy of sunlight.

Photosystems

A pair of structures that cooperate to generate both ATP and NADPH.

Light

A type of electromagnetic radiation (particle + wave).

Wavelength

The distance between crests of waves, such as those of the electromagnetic spectrum.

Electromagnetic Spectrum

The entire spectrum of electromagnetic radiation, ranging in wavelength from less than a nanometer to more than a kilometer.

Photon

A quantum, or discrete quantity, of light energy that behaves as if it were a particle.

Absorption Spectrum

The range of a pigments' ability to absorb various wavelengths of light; also a graph of such a range.

Chlorophyll a

A photosynthetic pigment that participates directly in the light reactions, which convert solar energy to chemical energy.

Chlorophyll b

An accessory photosynthetic pigment that transfers energy to chlorophyll a.

Action Spectrum

A graph that profiles the relative effectiveness of different wavelengths of radiation in driving a particular process.

Carotenoid

An accessory pigment, either yellow or orange, in the chloroplasts of plants and in some prokaryotes.

Absorption Spectra

Wavelengths that are absorbed by different pigments.

Action Spectrum

Rate of photosynthesis by whole plant at specific wavelengths.

Visible Light

Only a small part of the electromagnetic spectrum, which drives photosynthesis.

Pigments

Substances that absorb visible light and absorb specific wavelengths of light.

Carotenoids

Pigments that absorb at higher wavelengths and extend the range of photosynthesis.

Carotenoids

By absorbing wavelengths of light that chlorophyll cannot, carotenoids broaden the spectrum of colors that can drive photosynthesis.

Excited State

After an electron absorbs energy, it enters an excited state and is usually unstable.

Energy Release

Excited electrons in pigments can release energy as heat or light.

Electron Transfer

Excited electrons in pigments can be transferred to another molecule or 'captured'.

Water Oxidation

Electrons must be replaced by coming from the oxidation of water.

Chloroplasts

Chloroplasts resemble mitochondria but have an extra compartment: the thylakoid.

Photosynthesis Products

Photosynthesis generates, then consumes ATP and NADPH.

Chlorophyll

Chlorophyll molecules absorb the energy of sunlight.

Reaction Center

Excited chlorophyll molecules funnel energy into a reaction center.

Photosystems

A pair of photosystems cooperate to generate both ATP and NADPH.

Photon

A photon is a discrete amount of light energy.

Ground State vs Excited State

An electron can be in its ground state or an excited state.

Resonance Energy Transfer

Energy from light is transferred, exciting electrons in chlorophyll.

Light-Harvesting Complex

The light-harvesting complex consists of various pigment molecules bound to proteins.

Primary Electron Acceptor

A primary electron acceptor accepts excited electrons and is reduced as a result.

Photosystem II (PSII)

Photosystem II is a light-harvesting complex and a reaction center.

P680

P680 is the reaction center of Photosystem II that becomes unstable when excited.

Electron Transport Chain (ETC)

Excited electrons travel from PSII to PSI through the electron transport chain.

NADP+ Reduction

High energy electrons from PSI are used to reduce NADP+ to NADPH.

Z Scheme

The Z Scheme describes the combined actions of PS I and II that boost electrons to produce ATP and NADPH.

Thylakoid Space

The thylakoid space has a high concentration of H+ ions.

Stroma

The stroma has a low concentration of H+ ions.

Photosystem II

Absorbs light at 680 nm.

Photosystem I

Absorbs light at 700 nm.

Noncyclic electron flow

The predominant route of electron flow in photosynthesis.

Photophosphorylation

The process of synthesizing ATP as electrons fall down the electron transport chain.

Light-harvesting complex

A complex of proteins associated with pigment molecules that captures light energy and transfers it to reaction-center pigments in a photosystem.

Primary electron acceptor

A specialized molecule that shares the reaction-center complex with a pair of chlorophyll a molecules and accepts an electron from them.

Photosystem

A light-capturing unit located in the thylakoid membrane of the chloroplast, consisting of a reaction-center complex surrounded by numerous light harvesting complexes.

Cyclic Photophosphorylation

Electron cycling that releases energy to transport H+ and only produces ATP, not NADPH.

Calvin cycle

The process that uses ATP and NADPH to synthesize CH2O.

Thylakoid membrane

The membrane in chloroplasts where the photosystems are located.