Photosynthesis Flashcards
Photosynthesis
The use of photons (light energy) to make sugars (glucose)
Carbon fixation
Inorganic carbon (CO2) is incorporated into an organic molecule (like a sugar molecule)
Photolysis
The term used to describe the splitting of water molecules by photosynthetic organisms, which releases electrons that can be excited/powered-up by solar energy
The energy from the excited electrons are then used to fix carbon dioxide into an organic glucose molecule
What type of reaction is photosynthesis?
Non-spontaneous, endergonic reaction (positive Gibbs free energy)
solar energy is absorbed
Stomata
Pores found in the bottom of the leaves and are the site of atmospheric gas exchange, which includes carbon dioxide and oxygen
Outer membrane of chloroplast
Outer plasma membrane
Composed of a phospholipid bilayer
Intermembranous space of chloroplast
The space between the outer and inner membranes
inner membrane of chloroplast
The inner plasma membrane
Composed of a phospholipid bilayer
Stroma
Fluid that fills the area inside the inner membrane
Where the calvin cycle occurs
Thylakoids
- membrane is a phospholipid bilayer structured organelle
- Suspended within the stroma
- Where light dependent reactions (non-cyclic) occur
- Individual membrane layers = thylakoids, an entire stack = granum
- A junction between two grana is called a lamella
- contains photosystems I and II, cytochromes and electron carriers
Thylakoid lumen
The interior of the thylakoid
Where H+ ions accumulate due to ETC
What is the goal of light dependent reactions?
Use light to make chemical energy/storage molecule (ATP) and a reduced electron carrier (NADPH)
What is the ATP for light dependent reactions used for?
It is not used to power the cell
It is consumed to power the Calvin cycle which makes glucose
The glucose then undergoes cellular respiration to produce ATP that does power the cell
Photosystems
large proteins embedded in the thylakoid membrane that contain special pigments (like chlorophyll and carotenoids) that are directly responsible for absorbing photons
What is another name for photosystmes I and II?
Photosystem I: P700
Photosystem II: P680
Named due to the wavelength in which they are able to absorb
Why does photorespiration occur?
Occurs when RuBisCo (which normally helps plants used carbon dioxide for energy) binds oxygen instead of carbon dioxide, producing a two-carbon molecule called phosphoglycolate which cannot be directly used in plants —> in order to fix this problem, plants undergo photorespiration
What is the result of photorespiration
Causes the cell to undergo oxygen fixation which derails the Calvin cycle, reduces the cell’s supply of fixed carbon, and wastes energy
What source of energy do C4 plants use to shuttle pyruvate back to the mesophyll cells?
Energy from the hydrolysis of ATP to AMP (equal to 2 ATP)
How many times does the Calvin cycle repeat to produce one molecule of glucose?
6 times
Where is malate stored in CAM plants?
Stored in the vacuole of the mesophyll cell for later use
it is not shuttles to a different cell (bundle-sheath cells) like C4 photosynthesis
When malate is decarboxylated in the bundle-sheath cells, what molecules are released?
Inorganic carbon dioxide —> which can now undergo the Calvin cycle with RuBisCo in an environment where oxygen is not prevalent (RuBisCo will have a lower risk of photorespiration)
Three-carbon pyruvate
What is required to convert 12 PGA to 12 G3P?
ATP and NADPH
Spatial isolation
Ensures carbon dioxide is sent to a location where oxygen will not compete for RuBisCo. Utilized in C4 photosynthesis
Temporal isolation
Ensures carbon dioxide is fixed at a different time so that oxygen will not compete for RuBisCo. Utilized by CAM photosynthesis
cyclic photophosphorylation
• only involves photosystem I and the first electron transport chain
• no electrons will reduce NADP+ to NADPH and therefore no NADPH is generated (extra ATP is made when cyclic photophosphorylation occurs)
The cell recycles electron from Photosystem I back to the first ETC (instead of the second ETC) which cause more pumping of protons into the thylakoid lumen to synthesize more ATP
What are the products of the light-dependent reactions in photosynthesis?
O2: comes from photolysis of H2O; emitted into the atmosphere
ATP and NADPH: needed to power the calvin cycle
General process of photorespiration:
Two-carbon molecule phosphoglycolate is shuttled to peroxisomes and mitochondria for conversion into a useful molecule called PGA (phosphoglycerate) which can enter the Calvin cycle
Fixed carbon is lost as carbon dioxide leading to an overall net loss of fixed carbon atoms and no new glucose is made
(Also called C2 photosynthesis since 2 carbon molecule produced)
What are the 4 steps of the Calvin cycle:
1) carbon fixation
2) reduction of 3-carbon intermediate
3) regeneration of RuBP
4) Carbohydrate synthesis
Step 1 of the Calvin cycle:
1) Carbon fixation: 6 CO2 + 6 RuBP —> 12 PGA (catalyzed by RuBisCo)
Carbon dioxide (1-C) combines with RuBP (5-C) to form a 6 carbon molecule which quickly breaks down into 2 PGA molecules (3-C/molecule)
Reaction is catalyzed by RuBisCo
What is the difference between cyclic photophosphorylation and non-cyclic photophosphorylation?
Non-cyclic photophosphorylation produces ATP and NADPH while cyclic photophosphorylation produces more ATP and no NADPH
what is the equation for photosynthesis?
6 CO2 + 6 H2O –> C6H12O6 + 6O2
where does cyclic photophosphorylation occur?
stroma lamellae
where does photolysis occur?
thylakoid lumen and then passes electrons to the thylakoid membrane for non-cyclic photophosphorylation
what are examples of crops that use C3 photosynthesis?
wheat, potato, rice and tomato
what are examples of crops that use C4 photosynthesis?
corn and sugarcane - avoid photorespiration in warm climates
where does chemiosmosis occur?
- across the thylakoid membrane
step 2 of the calvin cycle:
- reduction: 12 ATP + 12 NADPH convert 12 PGA –> 12 G3P
- PGA is phosphorylated by ATP and NADPH is reduced to form G3P
- energy is incoperated
- by products: NADP+ and ADP go into non-cyclic photophosphorylation
step 3 of calvin cycle:
- ** regeneration**: 6 ATP convert 10 G3P –> 6 RuBP
- most G3P is converted back to RuBP which allows the calvin cycle to repeat
step 4 of calvin cycle:
- carbohydrate synthesis
- the two remaning G3P are used to make glucose
overall reaction of the calvin cycle:
6CO2 + 18 ATP + 12 NADPH –> 18 ADP + 18 Pi + 12 NADP+ + 1 glucose (or 2 G3P)
