Light Reaction

Photosynthesis (basic inputs)

Light, water, and carbon dioxide

Photosynthesis (basic outputs)

Glucose and oxygen

Light reactions

The stage of photosynthesis that requires light directly; occurs on the thylakoid membrane

Dark reactions (light-independent reactions)

The stage of photosynthesis (Calvin cycle) that does not require light directly, though it depends on products from light reactions

PAR (Photosynthetically Active Radiation)

The visible light spectrum that plants can use for photosynthesis

Why plants appear green

Chlorophyll reflects green wavelengths of light rather than absorbing them

Chlorophyll a

A pigment especially effective at absorbing red and higher-energy wavelengths of light

Chlorophyll b

A pigment especially effective at absorbing blue wavelengths of light

Carotenoids

Orange-colored accessory pigments; transfer light energy to chlorophyll and act as antioxidants by trapping free radical oxygen

Where photosynthesis occurs (organelle)

Chloroplasts

Where photosynthesis occurs (specific membrane)

The thylakoid membrane, inside the chloroplast

Phospholipid bilayer

The structural basis of cell membranes; phosphate heads face water, lipid tails face each other, separating the inside and outside of a cell/compartment

Photosystem II

The first photosystem in the light reactions; this is where water splitting occurs

Water splitting (photolysis)

Occurs in Photosystem II; produces electrons, hydrogen ions, and oxygen as a byproduct

Source of the oxygen we breathe

Comes from water splitting in Photosystem II, not from carbon dioxide

Photosystem I

The second photosystem in sequence; uses excited electrons (passed from Photosystem II) along with light energy to help produce NADPH

ATP (from light reactions)

A chemical energy "currency" produced during the light reactions, generated via the hydrogen ion gradient across the thylakoid membrane

NADPH

A chemical energy carrier produced during the light reactions (from Photosystem I); used to power the Calvin cycle

Hydrogen ion gradient

Created when hydrogen ions become hyperconcentrated inside the thylakoid space during water splitting; drives ATP production

ATP synthase

The membrane protein that uses the hydrogen ion gradient (diffusion imbalance) to generate ATP

Diffusion

The movement of particles/molecules from an area of high concentration to an area of low concentration

Stomata

Specialized dermal cells (guard cells) in leaves that open and close to regulate gas exchange, primarily in response to light and water

Why stomata must open

To allow carbon dioxide to diffuse into the leaf, since CO2 is higher outside the leaf than inside

Tradeoff of open stomata

While CO2 diffuses in, water vapor simultaneously diffuses out, risking wilting if stomata stay open too long

Boundary layer

A zone next to any surface where air movement (and gas movement) slows down, including at the leaf surface

Intercellular airspace

The internal space within a leaf where gas movement is very slow and CO2 must dissolve before use

RuBisCO

The enzyme responsible for fixing atmospheric carbon dioxide into usable sugar; considered essential to nearly all life on Earth

RuBisCO's inefficiency

About 20% of the time, RuBisCO fixes oxygen instead of CO2, leading to photorespiration instead of sugar production

Photorespiration

The unproductive process that occurs when RuBisCO fixes oxygen instead of carbon dioxide; wastes energy and produces no usable product

Calvin cycle

The set of light-independent reactions that uses ATP and NADPH to fix CO2 into glucose via RuBisCO

Three phases of the Calvin cycle

Carbon fixation, reduction, and regeneration

Stroma

The fluid-filled space inside the chloroplast (outside the thylakoid) where the Calvin cycle occurs

First law of thermodynamics (as applied here)

Energy cannot be created or destroyed, only transferred/transformed (e.g., solar energy transformed into chemical energy in plants)