Chapter 10: Photosynthesis

Merged flashcards from Chapter 10 of Pearson's Campbell Biology, Twelfth Edition.

Photosynthesis

A process that uses light to change carbon dioxide and water into organic molecules and oxygen for cellular respiration

• Occurs within chloroplasts

• Nourishes the entire living world directly or indirectly

Chloroplasts

The organelles within cells where photosynthesis occurs

Autotrophs

Organisms that sustain themselves without eating anything derived from other organisms

• Producers of the biosphere that produce organic molecules from CO₂ and other inorganic molecules

• Includes most plants as well as some algae, protists, and prokaryotes

Heterotrophs

Organisms that obtain organic material from other organisms

• Consumers of the biosphere that eat other living things, dead organic material, or feces

• Almost all of these depend on photoautotrophs (directly or indirectly) for food and O₂

Decomposers

Subsection of heterotrophs that eat dead organic material or feces

Fossil fuels

Fuel stores under the earth formed from the remains of organisms that died hundreds of millions of years ago, representing ancient stores of the sun’s energy

Chloroplasts

Organelles within plants and other photosynthetic organisms that are structurally similar to and likely evolved from photosynthetic bacteria

• Structure allows for chemical reactions of photosynthesis

Leaves

The area of most photosynthesis in plants

Mesophyll

The interior tissue of the leaf where chloroplasts are mainly found

Stomata

Pores in a plant’s leaf where CO₂ enters and O₂ exits for gas exchange

Veins

Structures within a plant that transport water from the roots and export sugar to nonphotosynthetic parts of the plant

Stroma

Dense fluid within the chloroplast envleloped by two membranes

Thylakoids

Connected sacs in the chloroplast that compose a third membrane system, stacked in columns called grana

Chlorophyll

The pigment that gives leaves their green color that resides in thylakoid membranes

Photosynthesis

A complex series of reactions represented by the equation 6CO₂ + 12H₂O + Light energy → C₆H₁₂O₆ + 6O₂ + 6H₂O

• Represents the effective reverse of cellular respiration in electron flow to make sugar in two stages

• Oxidizes H₂O and reduces CO₂ in an endergonic process powered by light

Water

Substance used in photosynthesis as it is split into hydrogen and oxygen

• Electrons from hydrogen are used to create sugar molecules while releasing O₂ as a by-product

• Hydrogen may also be obtained via other sources, such as from H₂S that creates S₂ as a waste product

Light reactions

The first part of photosynthesis that occurs in the thylakoids

• Splits H₂O, recieving electrons and protons as H⁺

• Releases O₂ as a by-product

• Reduces the electron acceptor NADP⁺ to NADPH

• Generates ATP from ADP by photophosphorylation

NADP⁺

An electron acceptor reduced in the light reactions in the thylakoids to NADPH for transfer to oxygen

Photophosphorylation

The generation of ATP from ADP in the light reactions in the thylakoids

Calvin cycle

Second part of photosynthesis within the stroma that makes sugar from CO₂, using ATP and NADPH generated from light reactions

• Begins with carbon fixation, incorporating CO₂ into organic molecules

• Reduces fixed carbon to carbohydrate by transferring electrons from NADPH

Carbon fixation

The incorporation of carbon from CO₂ into organic molecules via the Calvin cycle in the stroma

Chloroplasts

Solar-powered chemical factories

• Thylakoids transform light energy into the chemical energy of ATP and NADPH, providing energy and reducing power needed by the Calvin cycle to make sugar

Electromagnetic energy (electromagnetic radiation)

Energy that travels in rhythmic waves via electromagnetic forces; light is one example

Wavelength

A measure of the distance between crests of electromagnetic waves

• Can range from less than a nanometer (gamma rays) to more than a kilometer (radio waves)

Electromagnetic spectrum

The entire range of electromagnetic energy, or radiation

Visible light (visible spectrum)

The electromagnetic spectrum between wavelengths 380 nm to 740 nm which drive photosynthesis and produce the colors seen by the human eye

Photons

Discrete particle-like units of light

• Each particle has a fixed quantity of energy inversely related to the wavelength of the light — shorter wavelengths have more energy per photon

Pigments

Substances that absorb visible light; differs for every type

• Wavelengths that are absorbed disappear, while wavelengths that are not absorbed are reflected or transmitted

• Most leaves appear green because the pigment chlorophyll absorbs violet-blue and red light while reflecting and transmitting green light

• Light causes electrons in a pigment to go to an unstable excited state, which can later fall back to a ground state releasing energy as heat or light

Spectrophotometer

A device that measures a pigment’s ability to absorb various wavelengths, sending light through pigments and measuring the fraction of light transmitted at each wavelength

Absorption spectrum

A graph plotting a pigment’s light absorption versus wavelength

Chlorophyll a

The key light-capturing pigment that participates directly in light reactions

• Absorbs violet-blue and red light for photosynthesis for most energy while reflecting green light

Chlorophyll b

An accessory pigment to chlorophyll a

Carotenoids

A separate group of accessory pigments to chlorophyll a, absorbing violet and blue-green light and thus broadening the spectrum for photosynthesis

• Some can also absorb excessive light that would otherwise damage chlorophyll or react with oxygen

Action spectrum

A profile of the relative effectiveness of different wavelengths

• Higher for violet-blue and red light in photosynthesis

• Broader for overall photosynthetic processes in combination with chlorophyll b, due to a slight structural difference in the pigment molecules

Fluorescence

The release of excess energy by electrons in the form of light

Photosystem

A reaction-center complex surrounded by light-harvesting molecules

Photosystem II (PS II)

Also known as P680, it is the first photosystem in the thylakoid membrane that functions best with absorbing light with chlorophyll a at 680 nm

• Named for its secondary discovery to Photosystem I

Photosystem I (PS I)

Also known as P700, it is the second system in the thylakoid membrane that functions best with absorbing light with chlorophyll a at 700 nm

• Named for it being discovered first before Photosystem II

Calvin cycle

An anabolic process within the stroma of the chloroplast that builds sugar from smaller molecules using ATP with the reducing power of electrons carried by NADPH

• Has the three phrases of carbon fixation, reduction, and regeneration of the CO₂ acceptor (RuBP)

• Consumes 9 ATP and 6 NADPH overall to make 1 G3P molecule for other molecular synthesis processes, which are regenerated with light reactions

Glyceraldehyde 3-phosphate (G3P)

The sugar that is produced from carbon that enters as CO₂

• One of these requires three complete cycles to fix three CO₂ molecules

Carbon fixation

The binding of CO₂ to a five-carbon sugar named ribulose biphosphate, catalyzed by RuBP carboxylase-oxygenase (rubisco)

• This is then split into two molecules of 3-phosphoglycerate for each CO₂ fixed

Rubisco

Enzyme that catalyzes the binding of CO₂ to ribulose biphosphate in carbon fixation

Reduction

The phosphorylation of each molecule of 3-phosphoglycerate by six ATP and six NADPH to produce a G3P sugar

• Results in a 3:6 CO₂ input to G3P output ratio

• Only one G3P molecule can be counted as a net gain of carbohydrate as it is passed on, the rest remain to regenerate the CO₂ acceptor RuBP

Regeneration of RuBP

The rearrangement of the five molecules of G3P in a complex series of reactions to yield three molecules of RuBP

• This uses three additional ATP molecules