Photosynthesis Flashcards
For the carbon fixation in the calvin cycle, how many turns of the cycle is needed to make 1 glucose molecule?
6
What electron carriers/complexes are involved in the electron transport chain for light-dependent reactions?
Primary electron acceptor
↓
plastoquinone (PQ)
↓
b6f complex
↓
plastocyanin (PC)
How much NADPH and ATP is made for every 2 electrons that pass through the electron transport chain for light-dependent reactions
1 NADPH and 1ATP is made by ATP synthase complex
When is a proton gradient created in the light-dependent reactions?
by the b6f complex in the ETC and the splitting of water
part of a leaf: chloroplasts are abundant, location of most of photosynthesis
palisade mesophyll cells
Where do light-dependent reactions take place
in the thylakoid
protein-rich semiliquid material in the interior of chloroplast
stroma
what enzyme is involved in photorespiration
rubisco
For reduction reactions of the calvin cycle, how many NADPH are needed for every 6 CO2
12
What is photoexcitation
electrons in chlorophyll molecules are initially at ground state however when a molecule absorbs a photon, one of the electrons is elevated to an orbital where it has more potential energy
What’s the purpose of the Calvin cycle?
to make glucose
part of a leaf: regulates exchange of CO2 and O2, allow water to escape by transpiration AND allows CO2 to diffuse into air spaces within the leaf’s mesophyll layers
stomata (stoma for singular)
What is photophosphorylation?
synthesizing ATP in the presence of light
ADP is phosphorylated in the stroma using light energy from photons
What is transpiration?
water loss; the evaporation of water from leaves
How might transpiration be harmful to a plant?
can cause dehydration; conditions that promote transpiration can cause guard cells to reduce the size of stomata
What are the 4 main stages of the light-dependent reactions?
- Photoexcitation (p680)
- Electron Transport Chain (chemiosmosis)
- Photoexcitation (p700)
- Splitting of Water
part of a leaf: creates openings called stomata
guard cells
unstacked thylakoids between grana
lamellae
Are the stomata open or closed during the day? Why is this so?
open; to ensure CO2 is available to the chloroplasts for photosynthesis
part of a leaf: transports water, minerals, carbohydrates
vein/vascular bundle
What is the final electron acceptor in light-dependent reactions?
NADP+
c4 plants: what is the role of bundle sheath cells? how do they help to limit the degree of photorespiration?
bundle-sheath cells are impermeable to CO2; as a result, CO2 is concentrated in the bundle-sheath cells where the calvin cycle takes place
~ this high CO2 concentration makes the Calvin cycle much more efficient than in C3 plants
Describe the steps of the Calvin cycle.
PHASE 1: Carbon Fixation
~ CO2 joins to Ribulose 1,5 - bisphosphate (RuBP) to form a 6C intermediate however the intermediate is too unstable and splits into two 3C molecules called 3 - phosphoglycerate (PGA) (THIS IS CATALYZED BY RUBISCO)
PHASE 2: Reduction Reactions
~ 6 3- phosphoglycerate molecules are phosphorylated by ATP to produce 6 molecules of 1,3 - bisphosphoglycerate (ATP turns into ADP)
~ 6 1,3 bisphosphoglycerate molecules are reduced by NADPH to produce 6 molecules of glyceraldehyde-3-phosphate (NADPH turns into NADP+)
~ I molecule of G3P (aka 2 G3P as 2 G3P make 1 glucose) exits the cycle
PHASE 3: Regeneration
~ the remaining 5 G3P molecules are rearranged to form 3 molecules of Ribulose 1,5 -bisphosphate (RuBP)
~ ATP turns into ADP as 3 ATP are used in this process
~ the RuBP is now available to join with the next CO2 in the next cycle
Size and shape of the guard cell changes with?
water concentration
What enzyme catalyzes the calvin cycle?
ribulose bisphosphate carboxylase; RUBISCO
part of a leaf allows light to pass to mesophyll
upper epidermis
Describe the consequence of photorespiration in terms of photosynthetic efficiency for plants
- as a result of photorespiration, all of the energy used to regenerate ribulose 1,5 bisphosphate is wasted which reduces the efficiency of photosynthesis
- atmospheric conditions influence the reduction of efficiency due to photorespiration
- for ex: in hot conditions, leaves begin to lose water through the stomata; to prevent further loss of water, stomata close and while it is closed, the oxygen formed in the light dependent reactions accumulates inside the leaves and carbon dioxide cannot enter; with this high ratio of oxygen to carbon dioxide, photorespiration increases significantly.
What is photorespiration
the reaction of oxygen with ribulose 1,5 bisphosphate in a process that reverses carbon fixation and reduces the efficiency of photosynthesis
what is C4 photosynthesis
C4 plants separate the initial uptake of carbon dioxide from the Calvin cycle into different types of cells
ex. corn
What enzyme causes the split of water into 1/2 Oxygen and 2 H+) for light-dependent reactions?
z complex
What is stomatal closing regulated by in the evening/night?
decrease in sucrose in guard cells
What is stomatal opening regulated by in the morning?
water and K+ ions
NADPH and ATP are used in the light independent reactions to fix CO2 into?
Glucose
part of a leaf: waxy and water resistant, protection
cuticle
Where do light-independent reactions occur (ex. calvin cycle)?
stroma of chloroplast
one of the two protein-based complexes composed of clusters of pigments that absorb light energy
photosystems
Describe the “temporal” adaptation in CAM plants and how the stomata are involved in this adaptation?
- CAM plants like water-storing plants (eg. cactus) use a biochemical pathway like the C4 plants, but the rxns take place in the same
~ carbon fixation is separated from the calvin cycle by time of day, rather than by different cell types - for plants that thrive in hot, desert conditions, their stomata remain CLOSED during the day and OPEN and night to prevent water loss
~ CO2 is then fixed while stomata are open
~ the rxns proceed until malate is formed which is stored in a large vacuole
~ malate exits the vacuole & is decarboxylated, freeing the CO2 which is then fixed again by rubisco and enters the Calvin cycle (when enough ADP and NADPH is made by light dependent rxns)
What are the 3 main stages of Light independent reactions (calvin cycle)?
- Carbon Fixation –> 2 turns of the cycle are needed to make 1 glucose molecule
- Reduction Reactions –> requires 12 NADPH for every 6 CO2
- RuBP Regeneration –> enzymes convert G3P into RIbulose 1,5 Bisphosphate
Describe the light-dependent reactions in detail.
- Photoexcitation (p680)
~ two photons strike photosystem II and excite 2 electrons from chlorophyll p680
~ the excited electrons are there captured by a primary electron acceptor and are then transferred to plastoquinone (PQ) and the Electron Transport Chain - Electron Transport Chain (chemiosmosis)
~ the 2 electrons pass through a proton pump which transports 4 protons from the stroma into the thylakoid lumen to create a proton gradient
~ this electrochemical gradient drives the PHOTOPHOSPHORYLATION of ADP to ATP
~ 1 ATP forms for every 4 protons that pass through ATPase
~ the electrons then flow to P700 to make ATP - Photoexcitation (p700)
~ two photons strike photosystem I and excite 3 electrons from chlorophyll p700 (replaced by electrons from p700)
~ these electrons then pass through another ETC (primary electron acceptor → carrier → 2H + NADP+ → NADPH)
~ the enzyme NADP reductase uses the 2 electrons and 1 proton from the stroma to reduce NADP+ into NADPH
~ NAP+ is the final electron acceptor - Splitting of Water
~ a z protein/complex splits water into 2 protons, 2 electrons, and 1 oxygen atom
~ the protons remain in the thylakoid space to add to the proton gradient
~ oxygen leaves as a byproduct
~ electrons are used for replacement to p680+
C4 photosynthesis: what happens in the mesophyll cells?
POMCPP
- In the outer layer of mesophyll cells, carbon dioxide is fixed by addition to phosphoenolpyruvate (PEP).
- This forms the product oxaloacetate which is then converted to malate and transported into the bundle sheath cells
- In the bundle sheath cells, malate is decarboxylated.
- The resulting pyruvate is transported back into the mesophyll cells and converted into phosphoenolpyruvate (PEP)
C4 photosynthesis: what is the product of carbon fixation
oxaloacetate
Describe the idea of stomata and electrochemical gradient.
- blue light activates receptors that stimulate proton pumps to actively drive protons out of guard cells
- this creates a H+ ion gradient
- the electrochemical gradient causes K+ ions to diffuse into the guard cells and water then follows by osmosis
- as the guard cells fill up with water and swell, they open due to a thicker cell wall on the inner side
What are carotenoids?
a large class of pigments which absorb blue and green light, making them yellow, orange, and red in colour.
Where does photosynthesis occur
in the chloroplast
How might transpiration be helpful to a plant?
- creates a transpiration pull that helps to move water, minerals, and other substances upwards
- produces an evaporative cooling effect that PREVENTS OVERHEATING
membrane-bound, flattened sacs that stack to form granum (columns)
thylakoid
What do chlorophyll molecules contain (in terms of structure)? Describe.
a porphyrin Ring and a phytol tail
Porphyrin Ring: magnesium ion in the centre, surrounded by hydrocarbon ring. This ring contains the electrons that absorbs light energy
phytol tail: hydrocarbon tail that anchors the molecule to a membrane
What is a pigment?
a compound that absorbs certain wavelengths of visible light while reflecting others.
What is cyclic phosphorylation?
** only happens in photosystem I
~ allows chloroplasts to make more ATP to drive the light-independent reactions
~ however, this does not release electrons to generate NADPH and without NADPH, carbon fixation cannot occur
Briefly outline the relationship between beta-carotene, vitamin A and retinal
Beta carotene is a pigment/carotenoid responsible for the orange colour of carrots. It can be converted into vitamin A which can then be converted into retinal which is the visual pigment in our eyes
Why are plants green?
Chlorophyll absorbs blue and red light while reflecting green light, making the plants green in colour
Describe the role of the reaction centre and antenna complex
- the antenna complex includes all the surrounding pigment molecules that gather light energy
~ transfers light energy to the reaction centre - the reaction centre is a complex of proteins and chlorophyll a molecules
~ when a reaction centre has recieved energy from the antenna complex, an electron becomes excited and is raised to a higher energy level
What is the value of accessory pigments?
pigments expand light absorption, protect chlorophyll from damage, and transfer absorbed energy to chlorophyll for photosynthesis.
