4. Photosynthesis + Flashcards
1
Q
Photosynthesis
A
- process of converting energy in sunlight to energy in chemical bonds, especially glucose
- general equation:
6CO2 + 6H2O + light -> C6H12O6 + 6O2 - equation is reverse of cellular respiration except energy is from light
2
Q
General Process of Photosynthesis
A
- light-absorbing pigments in plant cells absorb energy from light within a narrow arrange of wavelength (diff pigments = diff wavelength, different pigments act together to optimize energy absorption.)
- when light is absorbed, energy is incorporated into electrons within atoms that make up the molecule. these “excited electrons” are unstable and re-emit the absorbed energy. energy is reabsorbed by nearby pigments. energy bounces from one pigment to next.
- finally, energy is absorbed by one of two special chlorophyll a molecules, P680 and P700. # = wavelength. these are different from other chlorophyll because of their association w/ other pigments. Chlorophyll P700 forms pigment cluster called Photosystem I (PS I), chlorophyll P680 forms Photosystem II (PSII)
3
Q
Photophosphorylation
A
- making ATP from ADP and Pi using light energy (photo)
4
Q
Noncyclic Photophosphorylation
A
- electrons trapped by P680 in PSII are energized by light (2 electrons)
- two energized electrons are passed to primary electron acceptor (primary because it’s first)
- two electrons pass through e- transport chain. some carrier proteins in the chain, like ferredoxin and cytochrome, include nonprotein parts containing iron.
- as the two electrons move “down” e- transport chain, they lose energy. energy is used to phosphorylate on average about 1.5 ATP molecules.
- electron transport chain terminates w/ PSI (P700). here, electrons are again energized by light and passed to primary electron acceptor (a different one from previous steps)
- the two electrons pass through a short chain (different chain) and at the end combine w/ NADP+ and H+ to form NADPH. NADPH is a coenzyme. Like NADH, it’s energy rich. P in NADPH represents phosphorus
- the two electrons from PS II are now in NADPH. They are replaced by electrons from splitting of H2O. (splitting of H2O = two electrons go to PSII, one H+ goes to make NADPH, 1/2 O2 contributes to oxygen that is release.
In summary, photophosphorylation takes the energy in light and the electrons in H2O to make the energy-rich molecules ATP and NADPH. Because the reactions require light, they are often called the light-dependent reactions, or light reactions.
H2O + ADP + Pi + NADP+ + light ——>
ATP + NADPH + O2 + H+
5
Q
Cyclic Photophosphorylation
A
- occurs when electrons energized in PSI are “recycled.” In this sequence, energized electrons from PSI join w/ protein carriers and generate ATP as they pass along the e- transport chain
- this is in contrast to noncyclic photophosphorylation where the electrons incorporate into NADPH
- electrons return again to PSI and can be energized again to do cyclic or noncyclic path.
- cyclic occurs simultaneously w/ noncyclic to generate additional ATP
- two electrons passing through cyclic generate on average about 1 ATP
6
Q
Calvin Cycle
A
- fixes CO2
- function is to produce a single molecule of glucose. in order to accomplish this, the Calvin cycle must repeat six times, and use 6 CO2 molecules
- no light is directly used in the cycle, that’s why it’s also known as light-independent reactions or dark reactions….but cannot occur in absence of light because it is dependent upon products (ATP and NADPH) of photophosphorylation.
- In summary, takes CO2 from atmosphere and energy in ATP and NADPH to create a glucose molecule
7
Q
Calvin Cycle Steps
(Calvin-Benson Cycle)
(Carbon Reduction Cycle)
A
***note: all molecules have been multiplied by 6, the number of turns the cycle occurs to produce one molecule of glucose
- Carboxylation: 6 CO2 combine with 6 RuBP to produce 12 PGA. enzyme rubisco catalyzes merging of CO2 and RuBP. Calvin cycle is called C3 photosynthesis because the first product formed, PGA is 3 carbons.
- Reduction: 12 ATP and 12 NADPH are used to convert 12 PGA to 12 G3P. energy in ATP and NADPH is incorporated into G3P. ADP, Pi, NADP+ released.
- Regeneration: 6 ATP are used to convert 10 G3P to 6 RuBP. regenerating the 6 RuBP originally used allows cycle to repeat
- Carbohydrate Synthesis. 12 G3P were created but only 10 G3P were recycled. the other two G3P are used to build glucose. other monosaccharides can be formed as well.
Summary:
6CO2 + 18 ATP + 12 NADPH + H+ ——->
18 ADP + 18 Pi + 12 NADP+ + 1 glucose
8
Q
Chloroplasts
A
- site of both light-dependent and light-independent reactions
- outer membrane. double layer of phospholipids
- intermembrane space
- inner membrane. double layer of phospholipids
- stroma. fluid material inside inner membrane. Calvin cycle occur here fixing carbon from CO2 to generate carbohydrate precursors (G3P).
- thylakoids. suspended w/in stroma are pankcake like membranes. individual membrane layers are called thylakoid; an entire stack is a granum. the membranes of the thylakoids contain the protein complexes (including PSI and PSII), cytochromes, and other e- carriers of the chain
- thylakoid lumen. inside of the thylakoid. H+ accumulate here.
9
Q
Chemiosmosis in Chloroplasts
A
- ATP generation when energy is stored in the form of a proton gradient across a membrane.
- analogous to ATP generation in mitochondria
- H+ (proton) released into the lumen of thylakoid when water is split. Also, H+ are carried from stroma into lumen by a cytochrome in the electron transport chain.
- A pH and electrical gradient across the thylakoid membrane is created. As H+ accumulate inside the thylakoid, the pH decreases. Since some H+ is coming from stroma, the pH there increases. Since H+ is positively charged, an electrical (or voltage) gradient is created.
- ATP synthase generates ATP. Channel proteins, called ATP synthase, allow H+ to flow through thylakoid membrane and out to the stroma. The energy generate by passage of H+ provides the energy for ATP synthase to phosphorylate ADP to ATP.
- At the end of the e- transport chain following PSI, electrons combine w/ NADP+ and H+ to produce NADPH. With NADPH, ATP, and CO2, two G3P are generated and subsequently use to make glucose or other carbohydrates.
10
Q
Photorespiration
A
- because of its critical function in catalyzing the fixation of of CO2 in all photosynthesizing plants, rubisco is the most common protein on earth. In addition of fixing CO2, it is also able to fix oxygen…this is called photorespiration, which leads to problems. fixing O2 is a waste and diverts from fixing CO2.
- specialized organelles, the peroxisomes are found near chloroplasts and function to break down photorespiration products
11
Q
C4 Photosynthesis Advantages
A
- minimizes photorespiration
- reduces water loss
- C4 plants common in hot, dry climates, where they possess an adaptive advantage over C3 plants.
- “add on” to C3 photosynthesis
12
Q
Photosynthesizing Cells
A
Mesophyll cells
13
Q
C4 Photosynthesis Steps
A
- instead of being fixed by rubisco into PGA, CO2 combines w/ PEP (phosphoenolpyruvate) to form OAA (oxaloacetate or acetic acid). the enzyme here is PEP carboxylase
- OAA, the first product of this pathway has 4 carbons, thus the name C4 photosynthesis
- OAA is then converted to malate
- malate shuttled through plasmodesmata to specialised cells w/in the leaf, the bundle sheath cells. here, malate is converted to pyruvate and CO2. rubisco can now begin the calvin cycle (C3 photosynthesis) more efficiently because there is less competition from O2.
- pyruvate is then shuttled back to mesophyll cells where one ATP is required to convert pyruvate -> PEP. then the process repeats
- overall effect: move CO2 from mesophyll cells to bundle sheath cells. Very little oxygen reaches the bundle sheath cells making photosynthesis more efficient.
- in order for photosynthesis to occur, leaf pores, called stomata must open to allow CO2 to enter. H2O can escape however. The higher rate of photosynthesis in C4 plants allow them to reduce time stomata are open and reduce H2O loss. –> C4 plants found in hot dry climates.
14
Q
CAM Photosynthesis
A
- CAM = crassulacean acid metabolism
- “add on” to C3 photosynthesis
- almost identical to C4 photosynthesis, w/ some changes:
- PEP carboxylase still fixes CO2 to OAA, as in C4. instead of malate however, OAA is converted to malic acid. minor difference (malate is ionized form of malic acid)
- malic acid is shuttled to vacuole of cell (not moved to bundle sheath cells as in regular C4)
- at night, stomata are open, PEP carboxylase is active and malic acid accumulates in the cells’ vacuole.
- during the day, stomata are closed (reverse of other plants). at this time, malic acid is shuttled out of the vacuole and converted back to OAA (requiring 1 ATP), releasing CO2. CO2 can now be fixed by rubisco, and the calvin cycle proceeds.
15
Q
Advantage of CAM
A
- photosynthesis occurs during the day while stomata are closed, greatly reducing H2O loss.
- as a result, CAM provides an adaptation for plants that grow in hot, dry environments w/ cool nights (such as deserts).
- the name crassulacean acid metabolism comes from the discovery of CAM in the succulent plants of the family Crassulaceae and the discovery of the accumulation of malic acid in vacuols during the night.
- CAM is also found in other plant families, including cacti.
