Midterm 2: Photosynthesis Slides Flashcards
Overall equation of photosynthesis
6 CO2 + 6 H2O + light = C6H12O6 + 6 O2
Where do CO2, water, and O2 come from in photosynthesis?
- CO2 enters leaves through stomata
- Water comes from soil
- O2 is freed from H2O
Stomata: what are they and what’s their role in photosynthesis?
They’re pores on the undersides of leaves that can open and close; they allow gases to leave and enter plant (gas exchange)
Describe the experiment used to confirm that water is the source of O2 produced by photosynthesis
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
Overview of light reactions
Light energy converted into chemical energy in the form of ATP and NADPH + H+
Overview of Calvin-Benson cycle (dark reactions)
CO2 plus ATP plus NADPH + H+ produced in light reactions used in Calvin-Benson cycle to produce sugars
Where is chlorophyll contained?
Thylakoid membranes
Properties of light (2)
- Form of electromagnetic radiation
- Exists as photons which exhibit wave-like properties
____ content of a photon is _____ _____ to the _____ of the light
Energy, inversely proportional, wavelength
Equation for speed of light (c)
Frequency x wavelength
What can happen to light when it strikes a molecule? (3)
Reflected, transmitted, or absorbed
Electromagnetic spectrum from lowest to highest energy
Microwaves/radio waves, IR, visible light (red to violet), UV, X rays, Cosmic/gamma rays
What does absorption spectrum mean?
Particular atom can only absorb photons corresponding to the atom’s available electron energy levels
What are the predominant pigments in green plants and which wavelengths do they absorb?
Chlorophyll a and b; absorb blue and red wavelengths
General structure of chlorophyll and b, and what do the parts do?
Chlorin ring containing Mg2+, and hydrocarbon tail; chlorin ring is light-harvesting part, hydrocarbon tail anchors molecule in thylakoid membrane
Accessory pigments
Absorb photons in the region between blue and red carotenoids
Action spectrum
Plots rate of photosynthesis carried out by organism against the wavelengths of light to which it is exposed
Resonance energy transfer
Mechanism by which electron is moved to reaction center from other chlorophyll molecules
Reaction center
Specific chlorophyll molecule where electron “boost” occurs
Excited chlorophyll is a ___ reducing agent; also give reaction with oxidizing agent A
Good; Chl* + A = Chl+ + A-
What happens to excited electron given up by excited chlorophyll (Chl)?
Shuttled along series of electron-carrier molecules in photosynthetic membrane; at proton-pumping channel, proton translocation results in ATP synthesis by chemiosmosis
Photosystem I
Reaction center contains P700 chlorophylls (absorbs light energy best at 700 nm and passes excited electrons to NADP+, reducing it to NADPH)
Photosystem II
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)
Which photosystem requires more energetic photons?
Photosystem II
Most important result of the experiment suggesting the need for two photosystems in noncyclic electron transport
With both 700 and 680 nm lights on, there was high rate of photosynthesis due to synergistic additive effect
What happens in noncyclic electron transport? (3)
- 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
Where does noncyclic electron transport take place?
Thylakoid membrane
Why is water splitting needed in noncyclic electron transport?
Photosystem II ends up with electron deficit, which water-splitting remedies
Overview of what happens in cyclic electron transport (3)
- 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
What process allows chloroplasts to form ATP and how does it work?
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
Structure of oxygen evolving (water splitting) complex of photosystem II
Four manganese, five oxygen, one calcium atom are linked in cluster where water is split - complex can pull electrons out of water
What happens in the fixation step of the Calvin-Benson cycle?
CO2 reacts with ribulose 1,5 bisphosphate (RuBP) to form two molecules of 3-phosphoglyceric acid (3PG) - rxn is catalyzed by RuBP carboxylase (rubisco)
What molecules does the Calvin-Benson cycle need and where does it take place?
Needs ATP and NADPH + H+ (Products of noncyclic electron transport) and takes place in stroma
How many turns of the Calvin-Benson cycle are needed to produce 1 molecule of glucose?
6 turns on successive CO2 molecules
In the fixation step of the Calvin-Benson cycle, what happens before the 2 3PGs are formed?
6-carbon intermediate formed but quickly breaks down into 2 3PGs (CO2 plus RuBP)
What is 3-phosphoglycerate reduced to and in how many steps?
Glyceraldehyde 3-phosphate (G3P); takes 2 steps
What kind of molecule is G3P (glyceraldehyde 3-phosphate)?
3-carbon sugar phosphate
What happens to G3P?
- 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)
What generally happens in photorespiration and what is its effect on photosynthesis?
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
What happens to glycolate formed during photorespiration?
Enters peroxisomes, is oxidized (forms glycine), and product enters mitochondria to be broken down, releasing CO2
When is oxygenase function of rubisco favored?
High temperatures and low CO2 levels
Why does rubisco sometimes add O2 to ribulose bisphosphate instead of CO2?
Probably has difficulty differentiating between them, as both are small molecules; also, O2 is about 500x more abundant in the atmosphere (has “advantage”)
How can rubisco’s selectivity for CO2 be improved?
Slower catalytic rate (e.g., 3 reactions/sec)
In plants, how do cellular respiration and photosynthesis interrelate? (2 ways)
- 3PG from Calvin cycle converted to pyruvate
- G3P from Calvin cycle converted to hexose phosphates 9like glucose-1-phosphate, which can enter glycolysis)
