Photosynthesis Lecture Notes
Aquatic Ecosystems
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Photoautotrophs create sugars using energy, photosynthesis:
Heterotrophs consume and utilize energy within autotrophs in process called cellular respiration
is usable, stores energy chemical energy
is not useful to a cell; collects & makes things acidic
Autotrophs perform both cellular respiration and photosynthesis
Relationship (ratio) between Ps and CR
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If Ps > CR then plants store energy, have energy to grow and reproduce
If Ps = CR then plant survives, no excess energy to grow, reproduce
If Ps < CR then plant uses energy faster than its made; it starves and dies
Which of the above is most adaptive for autotrophs? Why? For Heterotrophs? Why?
o Ps > CR b/c store energy & grow & reproduce
o Ps > CR b/c provides food (stored energy) for the Heterotrophs
 How is ratio regulated? What reaction changes daily? Why
o Regulated by amount of sunlight; if a lot of light, then Ps > CR (generally)
o Regulated by amount of
;
; temperature, if warmer then chemical reactions occur
better
 Max. rate of Ps = Gross Productivity (GP)
 GP – CR = Net (primary) productivity
o Net (primary) productivity is energy obtained by heterotrophs from consumption of
autotrophs!
 How can Photosynthesis & Cellular respiration rates be measured?
o Measure amount of oxygen in ecosystem
 Product of photosynthesis, reactant of CR
 Measured using ‘ampule’ method
o Measure amount of CO2 in ecosystem
 CO2 + H2O ←→ H2CO3 ←→ H++HCO–3
 Photosynthesis increased pH; more basic ←
 Cellular respiration decreases pH: more acidic →
Ratio
Amount O2
Amt CO2
pH
↑
↓
↑
Ps > CR
Ps = CR
=
=
=
↓
↑
↓
Ps < CR
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What information will the flask wrapped in aluminum foil provide?
o Negative control; only cellular respiration
In which flask should pH vary the most on a daily basis? Why?
o No screen, largest amount of light energy and then same CR as the others
In a real aquatic ecosystem, what abiotic factor what do the screens simulate?
o Depth, shade (algae bloom, other plants, other competition); murky water
Which flask is the positive control? Which is the negative control? Why?
o Pos: no screen; neg: foil
Purpose of water bath?
o Water has a high specific heat & prevent temperature fluctuations
o Temp affects the rate of photosynthesis
Other controls
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o Amount of plant matter; duration of the light; water; size of flask; same probes; same type of
screen (same mesh size); equal amounts of both plants
Plants
o Hydrophila difformis & Bacopa monnieri
Lab Data
Flask
Light
1 screen
3 screens
DO (mg/L) start
6.1
6.1
6.1
DO end (mg/L)
9
6
8.5 should have been 3
Foil
6.1
.5
Net productivity = (light) flask DO – Initial DO reading
 Gross productivity = net flask DO + dark (CR) flask DO
 Respiration = Initial DO reading – Dark Flask DO
 Determine NET productivity for all flasks
Photosynthesis Big Picture
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Light reactions need light; dark reactions don’t
Divided into 2 main stages
o Light reactions (fig 10.11, 10.12, 10.14, 10.16)
 Use water, light
 Create ATP, NADPH, oxygen (waste)
 Redox process
 Occurs at thylakoid membrane & inner thylakoids space
o Light-Independent Reactions: Calvin Cycle
 Occurs in stroma of chloroplast
 Occurs independent of light
 Temperature dependent
 Use ATP, NADPH from light reactions
 Makes PGAL (G3P) → CHO
Net productivity
9 – 6.1 = 2.9
6 – 6.1 = –0.1
8.5 – 6.1 = 2.4
3 – 6.1 = –3.1
.5 – 6.1 = -5.6
Chloroplast & Photosynthetic Pigments
Pigment Excitation: Photosystem
Organization
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Photon hits pigments molecule and ‘excites’ a
pair of electrons to a higher orbital
Photon increases potential energy of eElectrons are passed among pigments until
they are passed to a specific chlorophyll a of
reaction center
The reaction center chlorophyll is oxidized and
passes e- to primary electron acceptor which is
reduced
Cyclic Photophosphorylation
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Photosystem I: reaction center chlorophyll = P700: chlorophyll a
P700 absorbs light at λ=700nm
P700 found in algae; most primitive Photosystem
PS I makes ATP only, no NADPH (directly)
Fd (ferredoxin) passes e- from P700 through cytochome complex and then returns e- to oxidized
P700
Cytochrome comples performs chemiosmosis
o H+ ions move from stroma into inner thylakoid space to create a gradient of H+ inside
thylaoid relative to stroma
o H+ allowed to diffuse down concentration gradient (back into stroma) through ATP synthase
Non-Cyclic Photophosphorylation
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Photosystem II, then PS I
Photosystem utilizes P680 as reaction center chlorophyll
PS II makes both ATP & NADPH
Occurs in ALL plans (mosses → pine → angiosperms)
P680 is a strong oxidizing agent and stimulates enzyme to split water to replace e- of oxidized
pigment (P680) & make O2
Meanwhile, e- passed down electron transport chain of cytochrome complex; move H+ ions form
stoma to inner thylakoid space (chemiosmosis)
H+ diffuse back into stroma through ATP synthase
Electrons then used to reduce P700 of Photosystem I
Photosystem I passes e- to reduce NADP+ with help of reductase enzyme to make NADPH
Light-independent Reactions – Calvin Cycle
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Also known as Calvin Cycle or C3 Cycle
Required 9 ATP + 6 NADPH → 1 PGAL (G3P) a 3 carbon sugar used to make glucose
One turn of Calvin Cycle
1. Carbon fixation stage
a. CO2 enters and with help of enzyme Rubisco (ribulose bisphosphate carboxylase oxygenase)
combines with RuBP (ribulose bisphosphate) to form 3-phosphoglycerate (PGA) a 3-C
molecule
2. Reduction Stage
a. PGA is reduced by ATP and NADPH to form PGAL
3. Regeneration of CO2 Acceptor Stage
a. Most of the PGAL stays in Calvin Cycle & is converted from 3C PGAL to 5C RuBP with help
of more ATP
 For every one CO2 you get nothing; you need 3 cranks and 3 CO2 to achieve a PGAL
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o The source of H+ is from NADPH
Questions
1. Why is the C3 cycle temperature dependent?
a. b/c enzyme are temp. dependent; enzymes denature
b. RUBISCO has an optimal temperature to work at
2. Name the molecules that start the Calvin cycle.
a. ATP, NADPH, CO2
3. How many CO2 are fixed in 1 turn of cycle
a. 1 CO2 per turn
4. How many CO2 used to make 1 PGAL
a. 3
5. How many turns of the Calvin cycle are required to make 1 molecule of GLUCOSE
a. Six turns
6. How many PGAL (G3P) to make one glucose?
a. 2
7. Where in plant does Calvin cycle occur?
a. Stroma
8. How many ATP, NADPH are required to make glucose?
a. 18 ATP, 12 NADPH
9. Where does the energy come from to fix carbon in Calvin cycle?
a. ATP & NADPH from light reactions
10. Can carbon-fixation occur in dark or if cloudy? Why?
a. Yes, b/c as long as ATP & NADPH & CO2 are available, then the chemical reactions can occur
Photorespiration
1. Modern land plants are constantly fighting dehydration
2. Water is lost from plant’s stomata which are usually open during the day to allow CO2 into
mesophyll cells of leaf for photosynthesis (dark reactions, carbon fixation)
a. Also, water evaporates to “suck” water up into the plant
3. Water loss from plants through stomata = transpiration
4. Plants reduce water loss by closing stomata… but now the plants ‘starve’ (can’t fix carbon) b/c they
use up their supply of CO2 quickly and can’t get more CO2 b/c stomata are closed
5. Meanwhile, O2 accumulates in the leaf mesophyll from light reactions of Ps (photolysis) that
continue because it’s sunny
6. RUBISCO will bind RuBP to CO2 or O2 depend upon concentrations
7. If RUBISCO binds RuBP to oxygen → photorespiration
a. Photorespiraton produces a 2-carbon molecule which exists the chloroplast (& Calvin cycle)
and must be metabolized by mitochondria or peroxisome to convert back to CO2…
b. The problem is now RuBP is lost from the Calvin cycle b/c the 2-C molecule exits the
chloroplast
8. Plants that close stomata and do the wasteful process of photorespiration are called C3 plants
9. C3 plants include: rice, wheat, soybeans … important agricultural plants that produce less food for US
when it’s hot, bright, and dry outside
Adaptations to avoid Photorespiration
Spatial Separation
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C4 plants: Spatial separation of Carbon-fixation
o aka Hatch – Slack pathway
Value: CO2 fixed as a 4
carbon molecule
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Fix carbon dioxide as a 4 carbon organic molecule BEFORE fixation occurs in Calvin cycle
Include sugar cane, corn, and most grasses
Unique anatomy/arrangement of leaf cells:
Bundle sheath cells: located deep in leaf around vascular tissue
o Do Calvin cycle exclusively
Mesophyll cells: fix carbon into 4-C molecule, then pass to bundle sheath cells via plasmodesmata
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This reaction occurs even when stomata are closed during hot, dry, bright days
Able to store more carbon (4 carbon) for when the stomata
CAM Plants: Crussulacean Acid Metabolism
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Temporal separation of Carbon-fixation
For plants that live in arid environments: succulents, jade, pineapple
Fix carbon at night only into 4C organic acid: OAA
Only open stomata at night to obtain CO2 when temperature is cooler, so less transpiration (H2O
loss) occurs
Generalizations:
C4 plant, Calvin cycle, Carbon fixation, PEP carboxylase, OAA< Malate, Pyruvate, CO2, Mesophyll Cell,
Bundle Sheath Cell, Photorespiration, RUBISCO
CAM Plant, Arid environment, Time of day, Carbon fization, OAA, Malate, Pyruvatem, CO2,
Transporation, Stomata, Calvin cycle
Review Diagram