Photosynthesis Flashcards
what are the bonds in ATP, and what happens when broken?
phosphoanhydride bond, ATP to ADP, releases large amounts of energy
state the three methods of phosphorylating ADP to form ATP
photophosphorylation, substrate-level phosphorylation, oxidative phosphorylation (capture energy from exergonic reactions)
what are photoautotrophs?
organisms that use an inorganic form of carbon as raw materials, producing sugars through photosynthesis
what is the chemical reaction of photosynthesis?
carbon dioxide + water [sunlight + chlorophyll] = glucose + oxygen + water
what are the three reasons that photosynthesis is important?
- how solar energy is captured
- source of complex organic molecules for heterotrophic organisms
- releases oxygen as by-product for aerobic respiration
what is the major photosynthetic organ in plants?
leaf
if max surface area of each leaf is exposed to sunlight, what are the three things the plant can do?
obtain light energy from the sun, exchange gases, translocate liquids
what are the three main tissue types in plants, and their functions?
dermal: protective outer covering, facilitate gaseous exchange and uptake of ions + water
vascular: structural support, transport of water and solutes between organs
ground: making and storing food
state the three main cell types in ground tissue
parenchyma, collenchyma, sclernchyma
describe the function and structure of parenchyma
living, serves as packing tissue
most common type in plants
carries out metabolic functions like synthesis and storage of food
spherical cells with thin cellulose primary cell wall
mature cells have large central vacuole
some specialised to contain chloroplasts for photosynthesis (eg. palisade and spongy mesophyll in leaves)
in leaves, stems, roots, fleshy tissue of fruits
describe the function and structure of collenchyma
living, flexible structural support
elongated, unevenly thickened cellulose cell wall (thicker than parenchyma)
mature cells can still elongate, found just below epidermis (cortex) and/or surrounding vascular bundles
describe the function and structure of sclerenchyma
dead at maturity, structural support
elongated, evenly thickened cell walls (thicker than collenchyma)
cell walls contain waterproof lignin, specialised ones transport water
two types: sclereids and fibres
mature cells in parts of plant that stopped growing
describe the size and shape of the chloroplast and its envelope MEMORISE
- lens-shaped (in plants)
- 5-10 micrometers in length, 4-7 micrometers in width
- double membrane
what is a stroma in chloroplasts? MEMORISE
- gel-like matrix enclosed by chloroplast envelope
- contains circular DNA, 70S ribosomes, starch granules, oil droplets and enzymes used in Calvin cycle
what is a thylakoid in chloroplasts? MEMORISE
- flattened sacs or pouches
- photosynthetic pigments and electrons carriers are embedded within membrane
space enclosed within thylakoid known as the thylakoid lumen or thylakoid space
what is a granum in chloroplasts? MEMORISE
- a stack of thylakoids
- increases surface area and amount of pigments available for light-dependent reaction of photosynthesis
- connected by integranal lamellae (singular: lamella), making thylakoids one single continuous compartment
what is chlorophyll, its molecular structure, and what makes different types of chlorophyll?
main photosynthetic pigment, absorbs largely red and blue-violet light (reflects green)
molecular structure (two parts):
1. hydrophilic porphyrin ring with a flat, light-absorbing hydrophilic head with an MAGNESIUM in centre (Mg2+ deficiency reduces chlorophyll deficiency, causes yellowing)
2. hydrophobic hydrocarbon tail that projects into thylakoid membrane to keep chlorophyll embedded
what are the characteristics of chlorophyll a, and why is it the most important?
chlorophyll a is the major pigment in photoautotrophs, absorbs red and blue (only one direct in light-dependent rxn)
any molecule that absorbs light to transfer to chlorophyll a is an accessory pigment (most commonly chlorophyll b)
what are carotenoids, the light they absorb, and its two main types?
accessory pigments (pass light energy onto chlorophyll a of the reaction centre)
yellow, orange, red or brown pigments, absorb strongly in blue-violet range
two types: carotene and xanthophylls (460-550nm of visible light spectrum)
what are the two main functions of accessory pigments (eg. chlorophyll b, carotenoids)
broadening spectrum of light for photosynthesis: absorb intermediate wavelengths of light, broadening the spectrum of colours
photoprotection: absorbing excessive light and dissipating it, preventing auto-oxidation of chlorophyll, preventing photobleaching (damage of chlorophyll pigments due to excessive light),
what are the absorption and action spectrums of photosynthetic pigments, and what do they show? MEMORISE
- absorption: graph of amount of light absorbed at different wavelengths by a pigment
- action: effectiveness of diff wavelengths at driving photosynthesis
what is photosynthesis, and name its three main stages + where they occur + what happens during them
energy transduction, light to chemical energy
- light harvesting stage (thylakoid membranes)
light energy is captured with a mixture of pigments - light-dependent (thylakoid membranes)
light excites and displaces an electron from chlorophyll
light energy converted to chemical energy through flow of electrons, coupled to ATP synthesis, NADPH produced, photolysis of water - light-independent (stroma)
chemical energy of ATP and NADPH used in reduction of CO2, producing sugars
what occurs during the excitation of chlorophyll by light, and what are the three ways it returns to ground state?
ground (stable) to excited (unstable)
- transferring the energy (but not the electron) directly to neighbouring chlorophyll molecule by resonance energy transfer: during light harvesting
- boosting an electron to higher energy level, transferring to electron acceptor, then taking up another low-level electron from electron donor (eg splitting of water)
- when light energy is absorbed by an isolated chlorophyll molecule in solution, not in nature: energy is lost by converting it into heat (molecular motions) or a combi of heat and light on a longer wavelength (fluorescence)
what are photosystems, and their three closely-linked components?
in the thylakoid membrane, photosynthetic pigments are arranged into photosystems to trap light and convert it into useful forms
- light-harvesting complexes (LHCs): collected by 200-300 pigment molecules that capture, absorb and transfer light energy to the reaction centre
- reaction centre: a pair of special chlorophyll a molecules that irreversibly trap energy, by passing an excited electron to an adjacent chain of electron acceptors in the same complex
- primary electron acceptor: found in reaction centre, involved in electron transfer
what are the similarities and differences of the two photosystems?
functionally and spatially distinct, differ in wavelengths they absorb
PSII: special chlorophyll a is P680, absorbs maximally 680nm wavelength of light
PSI: special chlorophyll a is P700, absorbs maximally 700nm wavelength of light
both are identical in special chlorophyll a molecule
differ in light-absorbing properties bc of association with different accessory pigments and proteins in thylakoid membrane, affecting electron distribution
what generally occurs in the electron transport chain (ETC)?
ETCs found at thylakoid membranes, between photosystems
electron carriers transfer electrons from one carrier to another, can be coenzymes or protein molecules
a series of electron carriers = ETC, passed down by a series of redox reactions (each carrier is oxidised by gaining an electron, and oxidised by donating it to the next carrier)
ETC allows transfer to be done in several energy-releasing steps instead of one, some energy released to make ATP
what occurs during the light-harvesting stage of photosynthesis?
light of appropriate wavelengths strike any molecule within a photosystem
light energy is absorbed by that pigment molecule, exciting it
resonance energy transfer: excitation energy is transferred within the photosystem
what generally occurs in the light-dependent reaction (location, products, function, steps), and name the two ways it can proceed
occurs in thylakoid membrane of chloroplasts
synthesise NADPH and ATP (with trapped chemical energy) using captured light energy
NADPH and ATP used in light-independent reaction to fix carbon dioxide and trap energy in glucose
- non-cyclic photophosphorylation
- cyclic phosphorylation
then the ATP is synthesised through chemiosmosis
what does NADPH stand for?
reduced nicotinamide adenine dinucleotide phosphate
how does non-cyclic photophosphorylation occur in the light-dependent reaction of photosynthesis? MEMORISE
involves both PSII and PSI in “Z-scheme of electron flow”, producing ATP and NADPH
- photon of light strikes a pigment molecule in LHC, relaying energy via resonance energy transfer until it reaches one of the pair of special chlorophyll a molecules in PSII reaction centre, exciting one of the P680 electrons to a higher energy state
- photoexcited electron captured by primary electron acceptor, each P680 is missing an electron
- an enzyme splits water into two electrons, two H+, one O2 in photolysis of water, electrons replenish PSII’s deficit
- energised electron passes from PSII to PSI via first ETC
- while passing thru ETC, electrons release energy, pumping proteins against conc gradient from stroma into thylakoid space (generating proton gradient to drive ATP synthesis) through photophosphorylation
- more light strikes pigment molecule in PSI, exciting electron in special chlorophyll a in PSI reaction centre, passed onto primary electron acceptor and creating electron deficit in P700, replenished by electron from PSII
- excited electron passed down second ETC through ferredoxin
- NADP reductase (enzyme) transfers electrons from Fd to NADP (requires 2 electrons)
produces O2, NADPH, ATP
how does cyclic photophosphorylation occur in the light-dependent reaction of photosynthesis? MEMORISE
involves photosystem I only, produces ATP only (no NADPH)
- light absorbed by LHC and passed onto chlorophyll a (P700) in reaction centre of PSI
- PSI releases a photoexcited electron, passed onto primary electron acceptor (ferredoxin), cycled back to cytochrome (b-f) complex in electron transport chain from PSII to PSI
- synthesised ATP from ADP and Pi, needed in light-independent stage
done when there is no more NADP+ to reduce into NADPH in second ETC, so energy utilised to make more ATP instead
define chemiosmosis
chemiosmosis is the process in which energy stored in the form of a hydrogen ion gradient across the membrane is used to drive cellular work such as the synthesis of ATP
how does chemiosmosis occur? MEMORISE
- energy released from the ETC used to pump protons from stroma to thylakoid space, establishing proton gradient
- proton gradient drives synthesis of ATP by ATP synthase complex
- H+ diffuse down gradient from thylakoid space across membrane into stroma through ATP synthase complex
what is the location, purpose, and state the four phases of the light-independent reaction / Calvin cycle
location: stroma of chloroplasts
reduce CO2 using ATP (energy source) and NADPH (reducing agent) produced in the light-dependent reaction
- carbon dioxide uptake and fixation
- reduction of phosphoglyceric acid (PGA)
- regeneration of carbon dioxide acceptor (RuBP)
- product synthesis and sugar formation
what occurs during carbon dioxide fixation in the Calvin cycle (reactants, catalyst, process, products)
CO2 gas diffuses from the environment through the stomata into the cytoplasm of (spongy and palisade) mesophyll cells and into the chloroplasts
carboxylation of RuBP: CO2 is fixed when it combines with a 5-carbon carbon dioxide acceptor, ribulose bisphosphate (RuBP) to form an unstable 6-carbon intermediate, catalysed by ribulose bisphosphate carboxylase oxygenase (rubisco)
unstable intermediate breaks down spontaneously into two molecules of a three-carbon compound called phosphoglyceric acid (PGA) / 3-phosphoglycerate / glycerate-3-phosphate (GP
what occurs during the reduction of PGA during the Calvin cycle?
each PGA molecule is phosphorylated to form 1,3-biphosphoglycerate (ATP gives away its phosphate group to form ADP)
NADPH donates a pair of electrons to become NADP+, further reducing 1,3-bisphosphoglycerate to form glyceraldehyde-3-phosphate (GALP or G3P) or triose phosphate (TP): hydrogen from NADPH, energy from ATP
what occurs during the regeneration of carbon dioxide acceptor (RuBP) in the Calvin cycle?
for every 3 molecules of CO2 that enter the Calvin cycle, three RuBP are carboxylated, six TP formed
net gain of carbohydrate: only one TP, since other five are used to regenerate three molecules of RuBP used in CO2 fixation step, investing three more molecules of ATP
RuBP is regernated
what occurs during product synthesis and sugar formation in the Calvin cycle?
TP spun off from light-independent reaction becomes starting material for metabolic pathways
two TP molecules make one hexose sugar (requiring six turns of the Calvin cycle)
what is the overall equation of the Calvin cycle?
3 CO2 + 6 NADPH + 5 H2O + 9 ATP → glyceraldehyde-3-phosphate (G3P) + 2 H+ + 6 NADP+ + 9 ADP + 8 Pi (Pi = inorganic phosphate)
what is the fate of photosynthetic products (TP, PGA)?
PGA and TP are intermediates in glycolysis (cellular respiration)
carbohydrates synthesis: TP made into hexose sugars (respiratory substrates for ATP production or polymerised as starch for storage or cellulose for structure)
lipids synthesis: PGA added to coenzyme A to form acetyl CoA for cellular respiration, TP made into glycerol to form triglycerides and phospholipids
proteins synthesis: PGA and TP are important aa precursors
what is the principle of limiting factors?
rate of biochemical process is limited by slowest reaction in the series (factor nearest its minimum value)
list the eight factors that influence the rate of photosynthesis
light
CO2 concentration
temperature
chlorophyll concentration
specific inhibitors (eg. DCMU, cyanide)
water
oxygen
pollution
what is the compensation point of a plant, and what occurs then?
point at which the rate of photosynthesis is equal to the rate of respiration, no net gaseous exchange between the plant and its environment (reached at low light intensities like sunrise and sunset)
