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DAT > Biology- Photosynthesis > Flashcards

Biology- Photosynthesis Flashcards

1
Q

Photosynthesis

A

process of converting energy in sunlight to energy in chemical bonds, especially glucose

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2
Q

Photosynthesis Chemical Equation

A

6 CO2 + 6 H2O + light => C6H12O6 + 6 O2

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3
Q

How does photosynthesis begin?

A

various pigment molecules absorb energy from light; they include chorophyll a and cholorphyll b and the carotenoids. When the light is absorbed into 1 of these pigments, the energy from the light is incorporated into electrons within the atoms that make up the molecule

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4
Q

excited or energized electrons

A

unstable and almost immediately re-emit the absorbed energy.

The energy is then reabsorbed by e- of a nearby pigment molecule. Energy absorption and energy re-emission continues from one pig. molecule to another. The process ends when the energy is absorbed by one of two special chlorophyll (a) molecules, P680 and P700

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5
Q

P680 and P700

A

two chlorophyll molecules, named with numbers that represent the wavelengths at which they absorb their max. amounts of light (680 and 700 nanometers), are different from other chlorophyll molecules because of their association w/ nearby pigments.

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6
Q

P700 chlorophyll molecule

A

forms a pigment cluster called photosystem I (PS I)

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7
Q

P680 chlorophyll molecule

A

forms photosystem II (PS II)

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8
Q

Photophosphorylation

A

the process of making ATP from ADP and Pi (phosphorylation) using energy derived from light (photo)

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9
Q

Noncyclic Photophosphorylation

A
  1. Photosystem II.
  2. Primary electron acceptor.
  3. Electron Transport Chain.
  4. Phosphorylation.
  5. Photosystem I.
  6. NADPH.
  7. Splitting of Water

Photophosphorylation takes the energy in light and electrons in H2O to make the energy-rich molecules ATP and NADPH. Often called light-dependent reactions or light reactions

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10
Q

Noncyclic Photophosphorylation Equation

A

H2O + ADP + Pi + NADP+ +light => ATP + NADPH + O2 + H+

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11
Q

Noncyclic photophosphorylation (Step 1)

A

Photosystem II. Electrons trapped by P680 in photosystem II are energized by light.

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12
Q

Noncyclic photophosphorylation (Step 2)

A

Primary electron acceptor. Two energized electrons are passed to a molecule called the primary electron acceptor.

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13
Q

Noncyclic photophosphorylation (Step 3)

A

Electron transport chain. Electrons pass through an ETC, which consists of proteins that pass e- s from one protein carrier to another.

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14
Q

Ferredoxin and Cytochrome

A

some carrier proteins include nonprotein parts containing iron

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15
Q

Noncyclic photophosphorylation (Step 4)

A

Phosphorylation. As the 2 electrons move “down” the ETC, they lose energy. The energy lost by the e- as they pass along the ETC is used to phosphorylate, on avg., about 1.5 ATP molecules.

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16
Q

Noncyclic photophosphorylation (Step 5)

A

Photosystem I. The ETC terminates w/ PS I (w/ P700). Here the electrons are again energized by sunlight and passed to a primary electron acceptor (different from the one associated w/ PS II)

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17
Q

Noncyclic photophosphorylation (Step 6)

A

NADPH. the 2 e- s pass through a short ETC. At the end of the chain, the 2 e- s combine w/ NADP+ and H+ to form NADPH. NADPH is a coenzyme. Like NADH in respiration, NADPH is an energy-rich molecule.

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18
Q

Where do they occur: NADH and NADPH?

A

NADH in cellular respiration

NADPH in photosynthesis

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19
Q

Noncyclic photophosphorylation (Step 7)

A

Splitting of Water. The 2 e- s that came from PSII are now incorporated into NADPH. The loss of these 2 e- s from PSII is replaced when H2O is split into 2 e- s, 2 H+, and 1/2 O2. A manganese-containing protein complex catalyzes the reaction. The 2 e- s from H2O replace the lost ones from PSII, one of the H+ provides the H in NADPH, and the 1/2 O2 contributes to the oxygen gas that is released

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20
Q

Cyclic Photophosphorylation

A

occurs when e- s in PS I are recycled. Energized e- s from PS I join w/ protein carriers and generate ATP as they pass along the ETC. E- s return to PS I. Here they can be energized again to participate in cyclic/noncyclic photophosphorylation. Cyclic photophosphorylation occurs simulataneously w/ noncyclic photophosphorylation to generate additional ATP. 2 e-s passing through cyclic photophosphorylation generate about 1 ATP

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21
Q

Calvin Cycle

A

fixes CO2 by taking inorganic CO2 and incorporating it into an organic molecule that can be used. The function of the pathway is to produce 1 molecule of glucose (C6H12O6). The Calvin cycle must repeat 6 times, and use 6 CO2 molecules

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22
Q

Calvin Cycle Steps

A
  1. Carboxylation
  2. Reduction
  3. Regeneration
  4. Carbohydrate Synthesis
23
Q

Calvin Cycle (Step 1)

A

Carboxylation: 6 CO2 combine w/ RuBP to produce 12 PGA. The enzyme rubisco catalyzes the merging of CO2 and RuBP.

The Calvin cycle is referred to the C3 photosynthesis because the first product made, PGA (phosphoglyercate), has 3 carbon atoms. Other names are the Calvin-Benson Cycle and the carbon reduction cycle

24
Q

Calvin Cycle (Step 2)

A

Reduction: 12 ATP and 12 NADPH are used to convert 12 PGA to 12 G3P. The energy in ATP and NADPH molecules is incorporated into G3P, thus making G3P a very energy-rich molecule. ADP, Pi, and NADP+ are released and then re-energized in non-cyclic photophosphorylation.

25
Q

Calvin Cycle (Step 3)

A

Regeneration: 6 ATP are used to convert 10 G3P to 6 RuBP. Regenerating the 6 RuBP originally used to combine w/ 6 CO2 allows the cycle to repeat.

26
Q

Calvin Cycle (Step 4)

A

Carbohydrate synthesis. Note that 12 G3P were created in step 2, but only 10 were used in step 3. The 2 remaining G3P are used to build glucose, a common energy-storing molecule.

-Other monosaccharides like fructose and maltose can also be formed. In addition, glucose molecules can be combined to form disaccharides like sucrose and polysaccharides like starch and cellulose.

27
Q

What is often called the light-independent reactions or dark reactions?

A

the Calvin Cycle; the process does not use light directly, but it cannot occur in the absence of light

28
Q

Give a summary of the Calvin Cycle

A

the Calvin cycle takes CO2 from the atm. and the energy in ATP and NADPH to create a glucose molecule

29
Q

Calvin Cycle Equation

A

6CO2 + 18 ATP + 12 NADPH + H+ => 18 ADP + 18 Pi + 12 NADP+ + 1 glucose

30
Q

What do chloroplasts consist of?

A
  1. Outer Membrane
  2. Intermembrane Space
  3. Inner membrane
  4. Stroma
  5. Thylakoids
  6. Thylakoid lumen
31
Q

What happens in chloroplasts?

A

chloroplasts are the sites where the light-dependent and light-independent reactions of photosynthesis occur

32
Q

Outer membrane

A

consists of a double layer of phospholipids

33
Q

Intermembrane space

A

this is the narrow area between the inner and outer membranes

34
Q

inner membrane

A

second membrane that also has a double phospholipid bilayer

35
Q

stroma

A

the stroma is the fluid material that fills the area inside the inner membrane. The Calvin-cycle occurs here, fixing carbon from CO2 to generate carbohydrate precursors (G3P)

36
Q

Where does the Calvin Cycle occur?

A

In the chloroplast, more specifically in the stroma

37
Q

Thylakoids

A

suspended within the stroma are stacks of membranes. Individual membrane layer are thylakoids. The membranes of thylakoids contain the protein complexes (including the photosystems PS I and PS II), cytochromes, and other e- carriers of the light-dependent reactions

38
Q

granum

A

an entire stack of thylakoids

39
Q

thylakoid lumen

A

this is the inside, or lumen, of the thylakoid. H+ ions (protons) accumulate here.

40
Q

Chemiosmosis

A

the mechanism of ATP generation that occurs when energy is stored in the form of a proton concentration gradient across a membrane

41
Q

Chemiosmosis steps

A
  1. H+ ions (protons) accumulate inside thylakoids
  2. A pH and electrical gradient across the thylakoid membrane is created
  3. ATP synthases generate ATP
  4. The Calvin cycle produces G3P using NADPH and CO2 and ATP
42
Q

Chemiosmosis (Step 1)

A
  1. H+ ions (protons) accumulate inside thylakoids. H+ are released into the lumen of the thylakoid when H2O is split by PS II. Also, H+ are carried from the stroma into the lumen by a cytochrome in the ETC
43
Q

Chemiosmosis (Step 2)

A

As H+ accumulate inside the thylakoid, the pH decreases. Since some of these H+ come from outside the thylakoids (from the stroma), the H+ concentration decreases in the stroma and its pH increases. This creates a pH gradient consisting of differences in the concentration of H+ across the thylakoid membrane from a stroma pH 8 to a thylakoid pH 5 (a factor of 1000). Since H+ are + charged, their accumulation on the inside of thylakoid creates an electric gradient (or voltage) as well

44
Q

Chemiosmosis (Step 3)

A

ATP synthases generate ATP. The pH and electrical gradient represent potential energy like water behind a dam.

Similar to a dam, channel proteins, called ATP synthases, allow the H+ to flow through the thylakoid membrane and out to the stroma.

The energy generated by the passage of the H+ (like the water through turbines in a dam) provides the energy for the ATP synthases to phophorylate ADP to ATP. The passage of about three H+ is required to generate 1 ATP

45
Q

Chemiosmosis (Step 4)

A

The Calvin Cycle produces G3P using NADPH and CO2 and ATP. At the end of the ETC following PS I, e- combine w/ NADP+ and H+ to produce NADPH. With NADPH, ATP, and CO2, two G3P are generated and subsequently used to make glucose or other carbohydrates

46
Q

Photorespiration

A

fixation of oxygen, usually through rubisco which has active sites for CO2 and O2.

47
Q

What two problems does photorespiration lead to?

A
  1. CO2- fixing efficiency is reduced, because instead of fixing only CO2, rubisco fixes some O2 as well
  2. The products formed when O2 is combined w/ RuBP do not lead to the production of useful, energy-rich like glucose. Instead, specialized cellular organelles, the peroxisomes, are found near chloroplasts, where they function to break down photorespiration products. Thus, considerable effort is made by plants to rid the cell of the products of photorespiration
48
Q

C4 Photosynthesis

A
  1. When CO2 enters the leaf, it is absorbed by the usual photosynthesizing cells, the mesophyll cells. Instead of being fixed by rubisco into PGA, the CO2 combines w/ PEP to form OAA (oxaloacetate or oxaloacetic acid).
  2. The fixing enzyme is PEP carboxylase. OAA, the first product of this pathway, has 4 carbon atoms.
  3. OAA is converted to malate, and the malate is shuttled through the plasmodesmata to specialized cells within the leaf, the bundle sheath cells.
  4. In the bundle sheath cells, tbe malate is converted to pyruvate and CO2
  5. The pyruvate is then shuttled back to the mesophyll cells where 1 ATP (broken down to AMP, instead of ADP) is required to convert the pyruvate back to PEP.
  6. The process repeats. The overall effect is to move CO2 from mesophyll cells to the bundle sheath cells. Thus, CO2 is spatially segregated.
49
Q

What is the purpose for moving CO2 to bundle sheath cells?

A

The purpose is to increase efficiency of photosynthesis. The bundle sheath cells surround the leaf veins and are themselves surrounded by densely packed mesophyll cells.

Since bundle sheath cells rarely make contact with an intercellular space, very little O2 reaches them. When malate delivers CO2 to a bundle sheath cell, rubisco begins the Calvin cycle (C3 photosynthesis). Because little O2 is present, rubisco can fix CO2 w/o competition from O2. So little photorespiration takes place.

50
Q

In C4 photosynthesis, what must happen so that photosynthesis can occur?

A

Stomata must open to allow CO2 to enter. However, when they are open, H2O can escape. The higher rate of photosynthesis among C4 plants allows them to reduce the time that the stomata are open, thereby, reducing H2O loss.

Thus, C4 plants are found in hot, dry climates, where they possess an adaptive advantage over C3, which compensates for the additional energy requirement 1 ATP to AMP for C4.

51
Q

What are two advantages of C4 photosynthesis?

A

It minimizes photorespiration and reduces water loss. C4 photosynthesis occurs in about a dozen plant families. Sugarcane, corn, and crab grass are examples.

52
Q

CAM Photosynthesis (crassulacean acid metabolism)

A

Pathway is almost the same as C4, except for the following:

  1. PEP carboxylase still fixes CO2 to OAA, as in C4. Instead of malate, however, OAA is converted to malic acid
  2. Malic acid is shuttled to the cell’s vacuole (not moved out of the cell to bundle sheath cells as in regular C4).
  3. At night, stomata are open, PEP carboxylase is active, and malic acid accumulates in the cell’s vacuole.
  4. During the day, stomata are closed (the reverse of other plants). At this time, malic acid is shuttled out of the vacuole and converted back to OAA (requiring 1 ATP to ADP), releasing CO2. The CO2 is now fixed by rubisco, and the Calvin Cycle proceeds. Thus in CAM, CO2 is temporally segregated
53
Q

What is an advantage of CAM photosynthesis?

A

It can proceed during the day while the stomata are closed, greatly reducing H2O loss. As a result, CAM provides an adaptation for plants that grow in hot, dry environments with cool nights (such as deserts)

DAT (18 decks)

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