Biology 1B - Photosynthesis Flashcards
what is photosynthesis
process that converts atmospheric CO2 and H2O to carbohydrates
give a brief description of how solar energy is used in photosynthesis
solar energy is captured in chemical form as ATP and NADPH
ATP and NADPH are used to convert CO2 to hexose phosphates
what are phototrophs
photosynthetic organisms
what are the two reactions photosynthesis is divided into
light reaction
dark reaction
briefly describe light reactions
- require light
- take place in thylakoids and are membrane bound
- release O2 by splitting 2H2O molecules, with H+ from H2O used in the chemiosmotic synthesis of ATP
- and hydride ion H-, from H2O reduces NADP+ to NADPH
briefly describe dark reactions
- carbon fixation reaction
- can occur in light but doesn’t require it
- takes place in stroma in solution
- reduces gaseous CO2 to carbohydrates
- require energy of HADPH and ATP
describe the structure of chloroplasts
- have two surrounding membranes with intermembrane space between them
- have intermembrane staking (thylakoids) with stroma lamella connecting them
- orientation of these membrane stacks is important for light capture
describe chlorophyll
- most abundant pigment that captures light and most important in light harvesting
- chlorophyll a (Chl a) and b (Chl b) are in plants
- contain contain tetrapyrrole ring (chlorin) similar to heme but contains Mg2+
how does chlorophyl capture light
chlorophyl has alternating double and single bond structure due to Pi bonds
Electrons are shared across the whole electron cloud allowing more absorption across whole structure
higher chance of capturing light
what is the function of carotenoids (carotene pigments)
- another pigment that captures light ( type of antenna pigment)
- absorb light in blue-green spectrum where chlorophyll absorbs poorly (enhancing photosynthetic efficiency)
- exchange light energy (excited electrons) with chlorophyll until light is captured
in light reaction, how is light absorption organised
- photosystems I (PSI) and II (PSII)
- they operate in series (PSII first) connected by cytochrome bf complex (cascade of redox reactions that release energy)
describe the reaction centres in PSI and PSII
each contain a reaction centre (site of chemical reaction)
chlorophylls in each centre are paired to capture light
PSI - absorb light maximally at 700nm
PSII - absorb light maximally at 680nm
electrons are conducted from H20 to NADP+
describe the z scheme
- absorption of light energy converts P680 and P700 from poor to good reducing agents (excited molecules)
- light energy drives electron flow uphill
- NADP+ is ultimately reduced to NADPH
- light is captured by antenna complexes
- for each 2H2O oxidised to O2, 2NADP+ are reduced to 2NADPH
what is the z scheme
describes path of electron flow and reduction potentials of components in photosynthesis (PSI AND PSII)
what is Mitchell’s chemiosmosis theory
conversion of energy from electron transport via formation of transmembrane electrochemical gradient
how is ATP produced in light reactions
- through chemiosmosis
-protons driven by uH through chloroplast ATP synthase causing rotor to rotate, this conformational change drives synthesis of ATP by phosphorylating ADP - protons transfer across coupling membrane (photophosphorylation)
what is photophosphorylation
synthesis of ATP which is dependant on light using chloroplast ATP synthase
describe the structure of chloroplast ATP synthase
consists of two major particles
CF0 - spans membrane, contains pore for H+
CF1- protrudes into stroma and catalyses ATP synthesis from ADP and Pi
how is energy production balanced
cyclic electron transport
describe cyclic electron transport
- for every 4 electrons transferred to 2NADPH, only 2 ATP are produced
- for each CO2 reduced in carbohydrate synthesis, 2NADPH and 3ATP are needed
- imbalance between amount of ATP produced and needed
- cyclic electron transport yields ATP and not NADPH so creates better balance
how was the Calvin cycle discovered
by Melvin in 1948
used radioactive carbon-14 to trace the path of carbon during photosynthesis
briefly describe dark reaction photosynthesis (C3 plants)
- RuBisCO captures CO2 and attaches it to ribulose-1,5-bisphosphate, producing 2 molecules of 3-PGA
- ATP and NADPH from light reaction convert 3-PGA to G3P
- some G3P contributes to glucose formation
- allowing plants to store energy as carbohydrates
- remaining converted back to RuBP using ATP allowing cycle to continue
what is RuBisCO
- most abundant enzyme in nature
- consists of 8 large and 8 small subunits
- large are nuclear encoded and small are chloroplast encoded
what is RuBisCO secondary reaction it carries out
also reacts with O2 as well as CO2 but much greater affinity for CO2
causing oxygenation reactions producing toxic products
how do plants reduce build up of toxic compound produced by oxygenation reaction in C3 plants
photorespiration
what is on problem with RuBisCO
also binds with O2 as well as CO2
what are some alternative photosynthetic strategies to work around RuBisCO ability to bind to O2
using cyanobacteria from carboxysomes or pyrenoids to concentrate CO2 around RuBisCO
what is C4 photosynthesis
in 3% of land plants
fixes HCO3- with pyruvate in malate, then transport malate to a second cell type for decarboxylation and refixation by RuBisCO
more efficient as it fixes HCO3- first (avoids O2)
only major C4 crop is maize
what is CAM photosynthesis
- depends on temporal separation of the C4 and C3 fixation reactions
store C4 acids (malate) in vacuole for decarboxylation and refixation by RuBisCO during the day - (C3 fixation in day and C4 fixation in night)
what is the role of stomata in CAM photosynthesis
uses inverted cycle of stomatal opening/closing to save water and concentrate CO2 in the leaf
stomata open at night and close during day, concentrating CO2 inside leaf during day
what plants have CAM photosynthesis
only major CAM crop is pineapple
occurs in plants that need to conserve water
C3 plants can switch to CAM when under severe salt stress
what is a downside to C4 plants
metabolic cost
but are well suited to grow under high light and reduced water
