Midterm 2: Photosynthesis Slides

Question 1

Overall equation of photosynthesis

Answer 1

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

Question 2

Where do CO2, water, and O2 come from in photosynthesis?

Answer 2

• CO2 enters leaves through stomata
• Water comes from soil
• O2 is freed from H2O

Question 3

Stomata: what are they and what’s their role in photosynthesis?

Answer 3

They’re pores on the undersides of leaves that can open and close; they allow gases to leave and enter plant (gas exchange)

Question 4

Describe the experiment used to confirm that water is the source of O2 produced by photosynthesis

Answer 4

Two plants in separate jars: plant one given isotopically labeled “heavy” water and unlabeled CO2, plant two given isotopically labeled CO2 and unlabeled water - oxygen released by plant one was labeled

Question 5

Overview of light reactions

Answer 5

Light energy converted into chemical energy in the form of ATP and NADPH + H+

Question 6

Overview of Calvin-Benson cycle (dark reactions)

Answer 6

CO2 plus ATP plus NADPH + H+ produced in light reactions used in Calvin-Benson cycle to produce sugars

Question 7

Where is chlorophyll contained?

Answer 7

Thylakoid membranes

Question 8

Properties of light (2)

Answer 8

• Form of electromagnetic radiation

- Exists as photons which exhibit wave-like properties

Question 9

____ content of a photon is _____ _____ to the _____ of the light

Answer 9

Energy, inversely proportional, wavelength

Question 10

Equation for speed of light (c)

Answer 10

Frequency x wavelength

Question 11

What can happen to light when it strikes a molecule? (3)

Answer 11

Reflected, transmitted, or absorbed

Question 12

Electromagnetic spectrum from lowest to highest energy

Answer 12

Microwaves/radio waves, IR, visible light (red to violet), UV, X rays, Cosmic/gamma rays

Question 13

What does absorption spectrum mean?

Answer 13

Particular atom can only absorb photons corresponding to the atom’s available electron energy levels

Question 14

What are the predominant pigments in green plants and which wavelengths do they absorb?

Answer 14

Chlorophyll a and b; absorb blue and red wavelengths

Question 15

General structure of chlorophyll and b, and what do the parts do?

Answer 15

Chlorin ring containing Mg2+, and hydrocarbon tail; chlorin ring is light-harvesting part, hydrocarbon tail anchors molecule in thylakoid membrane

Question 16

Accessory pigments

Answer 16

Absorb photons in the region between blue and red carotenoids

Question 17

Action spectrum

Answer 17

Plots rate of photosynthesis carried out by organism against the wavelengths of light to which it is exposed

Question 18

Resonance energy transfer

Answer 18

Mechanism by which electron is moved to reaction center from other chlorophyll molecules

Question 19

Reaction center

Answer 19

Specific chlorophyll molecule where electron “boost” occurs

Question 20

Excited chlorophyll is a ___ reducing agent; also give reaction with oxidizing agent A

Answer 20

Good; Chl* + A = Chl+ + A-

Question 21

What happens to excited electron given up by excited chlorophyll (Chl)?

Answer 21

Shuttled along series of electron-carrier molecules in photosynthetic membrane; at proton-pumping channel, proton translocation results in ATP synthesis by chemiosmosis

Question 22

Photosystem I

Answer 22

Reaction center contains P700 chlorophylls (absorbs light energy best at 700 nm and passes excited electrons to NADP+, reducing it to NADPH)

Question 23

Photosystem II

Answer 23

Reaction center contains P680 chlorophylls (absorbs light energy best at 680 nm, oxidizes water molecules, and passes energized electrons through series of carriers to produce ATP)

Question 24

Which photosystem requires more energetic photons?

Answer 24

Photosystem II

Question 25

Most important result of the experiment suggesting the need for two photosystems in noncyclic electron transport

Answer 25

With both 700 and 680 nm lights on, there was high rate of photosynthesis due to synergistic additive effect

Question 26

What happens in noncyclic electron transport? (3)

Answer 26

- Use of photosystems I and II to produce NADPH + H+ and ATP - Electrons from H2O replenish chlorophyll molecules that gave up electrons - O2 is by-product of H2O breakdown

Question 27

Where does noncyclic electron transport take place?

Answer 27

Thylakoid membrane

Question 28

Why is water splitting needed in noncyclic electron transport?

Answer 28

Photosystem II ends up with electron deficit, which water-splitting remedies

Question 29

Overview of what happens in cyclic electron transport (3)

Answer 29

- Forms ATP, but not NADPH - At end of redox chain, electrons are returned to electron-deficient chlorophylls with no O2 produced - Produces additional (backup) ATP for reactions of Calvin-Benson cycle

Question 30

What process allows chloroplasts to form ATP and how does it work?

Answer 30

Chemiosmosis: electrons move through redox reactions and release energy; energy pumps protons across thylakoid membrane from stroma, establishing pH gradient (proton-motive force); ATP synthase couples diffusion of protons back to stroma with ATP synthesis

Question 31

Structure of oxygen evolving (water splitting) complex of photosystem II

Answer 31

Four manganese, five oxygen, one calcium atom are linked in cluster where water is split - complex can pull electrons out of water

Question 32

What happens in the fixation step of the Calvin-Benson cycle?

Answer 32

CO2 reacts with ribulose 1,5 bisphosphate (RuBP) to form two molecules of 3-phosphoglyceric acid (3PG) - rxn is catalyzed by RuBP carboxylase (rubisco)

Question 33

What molecules does the Calvin-Benson cycle need and where does it take place?

Answer 33

Needs ATP and NADPH + H+ (Products of noncyclic electron transport) and takes place in stroma

Question 34

How many turns of the Calvin-Benson cycle are needed to produce 1 molecule of glucose?

Answer 34

6 turns on successive CO2 molecules

Question 35

In the fixation step of the Calvin-Benson cycle, what happens before the 2 3PGs are formed?

Answer 35

6-carbon intermediate formed but quickly breaks down into 2 3PGs (CO2 plus RuBP)

Question 36

What is 3-phosphoglycerate reduced to and in how many steps?

Answer 36

Glyceraldehyde 3-phosphate (G3P); takes 2 steps

Question 37

What kind of molecule is G3P (glyceraldehyde 3-phosphate)?

Answer 37

3-carbon sugar phosphate

Question 38

What happens to G3P?

Answer 38

- Most recycled back to ribulose bisphosphate (takes 12 of 15 reactions in Calvin cycle) - Some leaves chloroplast for cytoplasm to be converted to hexoses (glucose and fructose)

Question 39

What generally happens in photorespiration and what is its effect on photosynthesis?

Answer 39

Rubisco catalyzes reaction of RuBP with O2 (in addition to with CO2) to form glycolate (2CO) - kind of like mistakenly using O2 instead of CO2; this reduces efficiency of photosynthesis

Question 40

What happens to glycolate formed during photorespiration?

Answer 40

Enters peroxisomes, is oxidized (forms glycine), and product enters mitochondria to be broken down, releasing CO2

Question 41

When is oxygenase function of rubisco favored?

Answer 41

High temperatures and low CO2 levels

Question 42

Why does rubisco sometimes add O2 to ribulose bisphosphate instead of CO2?

Answer 42

Probably has difficulty differentiating between them, as both are small molecules; also, O2 is about 500x more abundant in the atmosphere (has "advantage")

Question 43

How can rubisco's selectivity for CO2 be improved?

Answer 43

Slower catalytic rate (e.g., 3 reactions/sec)

Question 44

In plants, how do cellular respiration and photosynthesis interrelate? (2 ways)

Answer 44

- 3PG from Calvin cycle converted to pyruvate | - G3P from Calvin cycle converted to hexose phosphates 9like glucose-1-phosphate, which can enter glycolysis)