biol 1406 chapter 8

photosynthesis

converts light energy from the sun into chemical energy (glucose) for plants

photosynthesis equation

6CO₂ (carbon) + 2H₂O (water) + light → C₆H₁₂O₆ (glucose) + 6H₂O (water) + 6O₂ (oxygen)

where does photosynthesis occur

in chloroplasts of leaf cells

what organisms carry out photosynthesis

cyanobacteria, algae, all land plants

main stages of photosynthesis

light-dependent reactions and light-independent reactions (calvin cycle)

light-dependent reactions

require light, occur in thylakoid membrane, capture energy from sunlight, makes ATP, O₂, and reduces NADP⁺ to NADPH

light-independent reactions (calvin cycle)

do not require light, occur in stroma, use ATP and NADPH to synthesize organic molecules from CO₂

what did Van Helmont’s experiment show

plant growth comes mainly from water, not soil

what did Joseph Priestley discover

plants restore the air by releasing oxygen, the rat experiment

what did Jan Ingenhousz discover

plants release oxygen in the presence of light, plants release carbon dioxide in the dark, only green parts of plants produce oxygen

what did F.F. Blackman discover

photosynthesis is a two-step process, light is a limiting factor at low light intensities, but temperature and CO2 concentration are the limiting factors at higher light intensities

pigments

molecules that absorb specific wavelengths of light

relationship of energy content and wavelength

inversely proportional, decrease in wavelength = increase in energy

absorption spectrum

range and efficiency of photons a molecule is capable of absorbing

pigments used in green plant photosynthesis

chlorophylls (a and b) and carotenoids

chlorophyll a

main pigment in plants and cyanobacteria, the only pigment that can act directly to convert light energy to chemical energy, absorbs violet/blue and red light

chlorophyll b

secondary pigment that absorbs light wavelengths that chlorophyll a does not absorb

carotenoids

absorb light primarily in the blue/green to transfer this energy to chlorophyll, protects against light damage, is responsible for the yellow/orange/red color in fruits and vegetables

two parts of photosystems

antenna complex (light-harvesting complex) and reaction center

antenna complex (light-harvesting complex)

consists of hundreds of accessory pigment molecules, gather photons and feed the captured light energy to the reaction center

reaction center

contains one or more chlorophyll a molecules, passes excited electrons out of the photosystem to the electron acceptor

two photosystems in chloroplasts

photosystem I (P700) and photosystem II (P680), these carry out a noncyclic transfer of electrons that is used to generate both ATP and NADPH

photosystem I (P700)

functions like sulfur bacteria

photosystem II (P680)

can generate an oxidation potential high enough to oxidize water

cyclic photophosphorylation

purpose is to generates ATP only and helps balance ATP/NADPH ratio, only photosystem I (PSI) involved, cyclic electron flow (electrons return to PSI), only produces ATP, no external electron donor required, PSI is electron acceptor, proton gradient formation via cytochrome b6f complex

noncyclic photophosphorylation

purpose is to produce ATP, NADHPH, and O₂ for the calvin cycle, both photosystem I (PSI) and photosystem II (PSII), electrons do not return and instead end up in NADPH, produces ATP, NADPH, and O₂, water is the electron donor, NADP⁺ is electron acceptor, proton gradient formation via cytochrome b6f complex

chemiosmosis

process where a proton gradient drives ATP synthase to make ATP

three phases of calvin cycle

carbon fixation, reduction, and regeneration of RuBP (1,5-bisphosphate)

carbon fixation

RuBP (1,5-bisphosphate) + CO₂ → PGA (3-phosphoglycerate), rubisco (enzyme) fixes carbon

reduction

PGA (3-phosphoglycerate) is reduced to glyceraldehyde 3-phosphate (G3P)

regeneration of RuBP

G3P (glyceraldehyde 3-phosphate) is used to regenerate RuBP (1,5-bisphosphate)

calvin cycle output

CO₂ is fixed, ATP provides energy while NADPH donates electrons, one G3P (glyceraldehyde 3-phosphate) is made per three turns, two G3P (glyceraldehyde 3-phosphate) are needed (six turns), RuBP (1,5-bisphosphate) is regenerated, one glucose per six turns

two enzymatic activities of rubisco

carboxylation and photorespiration

carboxylation

addition of CO₂ to RuBP (1,5-bisphosphate), produces two 3-PGA molecules (3-phosphoglycerate), leads to glucose synthesis, efficient and desired pathway

photorespiration

rubisco binds O₂ to RuBP (1,5-bisphosphate), favored when stoma are closed in hot conditions, creates low CO₂ and high O₂, phosphoglycolate must be recycled which requires ATP and releasing CO₂ and reduces efficiency, consumes ATP and NADPH without generating glucose

types of photosynthesis

C₃, C₄, CAM

C₃

primary enzyme is rubisco, first product is 3-phosphoglycerate (3-PGA, 3-carbon), CO₂ fixation in mesophyll cells, photorespiration occurs when O₂ binds to rubisco to reduce efficiency, best for cool/moist environments, examples are wheat, rice, soybeans, trees

C₄

primary enzyme is PEP carboxylase, first product is oxaloacetate (4-carbon), CO₂ fixation is mesophyll cells and bundle-sheath cells, photorespiration is reduced due to spatial separation of CO₂ fixation and calvin cycle, best for hot/sunny environments, examples are corn, sugarcane, sorghum

CAM

primary enzyme is PEP carboxylase, first produce is malate (f-carbon), CO₂ fixation location in mesophyll cells, photorespiration is reduced by fixing CO₂ at night, best for dry/arid environments, examples are cacti, succulents, pineapples