Chapter 6 - Photosynthesis Flashcards
1
Q
Photosynthesis
A
- Where primary producers (Autotrophs E.g., plants) get their energy
- Carried out by plants and some protistans (Algae)
- Uses the kinetic energy from sunlight (photons) to make glucose from CO2 and H2O
2
Q
Endosymbiosis
A
- One organism living inside another and both benefiting from that relationship
- “Endo” = Inside
- “Symbiosis” = Living
- Made photosynthesis possible (photosynthetic bacterium entered eukaryotic cell a long time ago and formed chloroplasts. Why able do photosynthesis now)
3
Q
Chloroplasts
A
- Located in the cells in a plants leaves
- Where photosynthesis occurs
- Has two membranes
4
Q
Stroma
A
- Fluid-filled interior of chloroplast (‘cytoplasm’)
5
Q
Thylakoids
A
- In the chloroplasts
- Internal membrane folded into a series of flat, interconnected discs
6
Q
Granum
A
- Stack of thylakoid discs in the chloroplast
7
Q
Chlorophyll
A
- Pigment in the chloroplasts
- Pigment is a molecule that absorbs light
- Thylakoids contain chlorophyll which gives green plants their colour
8
Q
Sunlight
A
- A form of kinetic energy
- Light is made up of electromagnetic energy which travels in waves
- Visible light: 400-700 nm wavelength
- This wavelength is:
o Short enough to have sufficient energy to excite e- in pigments –> transfer energy
o Long enough that there is not too much energy that might damage the cell
9
Q
Chlorophyll Light Absorption
A
- Absorbs most red, orange, blue, indigo, and violet light
- Green and some yellow are reflected – which is why plants look green
10
Q
Photosystems
A
- Specialised protein complexes
- Clusters of chlorophyll pigment
- Responsible for absorbing light energy and transferring electrons
- Primarily PSII and PSI
- Located in thylakoid membrane
11
Q
Light Dependent Reaction
A
- Converts solar energy to chemical energy
- Takes place in thylakoids
- Inputs: Light energy, H2O
- Outputs: O2, ATP and NADPH
- Include Photosystems
12
Q
Step #1 – Light absorption
A
- Light energy is absorbed by chlorophyll pigments within the photosystem II (PSII)
- This excited the e- in the reaction centre
- Light energy ejects 2 e- from PSII
13
Q
Step #2 – Water Splitting (Photolysis)
A
- To replace the lost e-, PSII split H2O molecules to replace lost 2e-
- This releases oxygen as a by product
- Providing e- to continue the chain
14
Q
Step #3 – Electron Transfer
A
- The excited e- (ejected) are passed along an electron transport chain (ETC) within the thylakoid membrane
15
Q
Electron Transport Chain
A
- Collection of proteins e- pass through in a series of redox reactions (reactions involving the transfer of elections)
- This ultimately generates a proton gradient that drives energy release
16
Q
Step #4 – Proton Gradient Formation
A
- As e- move along the ETC, H+ builds up inside the thylakoid
- Captured energy is used to pump H+ from the stroma into the thylakoid compartment through transporter proteins, creating a concentration gradient
- Important to generate ATP through ATP synthase
17
Q
Step #5 – Photosystem I (PSI)
A
- e- from the ETC reach another protein containing chlorophyll called Photosystem I (PSI)
- There, they are further energised by light absorption
18
Q
Step #6 NADPH Formation
A
- e- again are “excited” and move along to another ETC
- The high-energy e- from PSI are used to reduce NADP+ (NADP+ + H+) to NADPH
- This is another energy carrier molecule
19
Q
Step #7 ATP Synthesis
A
- H+ that build up inside the thylakoid, due to the proton gradient, move across the gradient through an enzyme ‘ATP Synthase’
- The movement of H+ through ATP synthase provides energy needed to convert ADP to ATP
- ADP + Pi (inorganic phosphate) –>ATP
20
Q
Light Independent Reaction (Calvins Cycle or Dark Reaction)
A
- “Synthesis” part of photo synthesis
- Use chemical energy from light dependent reactions (“photo”)
- Occurs in the stroma in the light or dark
- Inputs: CO2, ATP, NADPH
- Output’s: Glucose, NADP+, ADP
21
Q
Rubisco
A
- Enzyme that catalyses the first step of carbon fixation
- Key enzyme for converting inorganic carbon into organic
22
Q
RuBP (Ribulose bisphosphate)
A
- A five-carbon sugar that acts as a CO2 acceptor molecule
- Facilitating carbon fixation
23
Q
PGA (phosphoglycerate)
A
- Three carbon molecule
- Plays a crucial role as an intermediate in the Calvin Cycle
24
Q
PGAL (Phosphopglyceraldehyde)
A
- Three carbon compound
- Plays essential role in glucose formation and regeneration of RuBP
25
Q
Stage #1 – Carbon Fixation
A
- Carbon fixation means to combine CO2 molecules into organic molecules
1. Enzyme protein Rubisco attaches CO2 to RuBP
2. Produces PGA
26
Q
Stage #2 Reduction
A
- Energy stored in ATP and NADPH is released
- PGA is reduced to PGAL
- Two PGAL combine to make glucose
27
Q
Stage #3 Regeneration (regenerates RuBP)
A
- Stages 1 & 2 use CO2 to make glucose, but it is the Calvin “Cycle”
- RuBP must be regenerated so the cycle can start again
1. Energy stored in ATP is released
2. PGAL is used to regenerate RuBP for the next cycle
28
Q
Photosynthesis without light
A
- In the dark and winter (when plants drop their leaves) they use the glucose they made to get ATP through aerobic respiration
- Plants have 3 pathways for this: C3, C4, and CAM (based on the number of carbon atoms in the first stable product of carbon fixation during photosynthesis)
- All three produce PGAL which can be turned into other organic molecules like glucose, cellulose, starch etc.
29
Q
C3 Pathway
A
- The first CO2 acceptor is a 3-carbon molecule
- Needs a continuous supply of CO2
- CO2 can’t be “fixed” in these plants for long
- Stomata always open –> lose water continuously
- Most common pathway, only possible in humid climates with moderate temps and CO2 levels
- E.g., Rice, potato, wheat, most trees
30
Q
C4 Pathway
A
- The first CO2 acceptor is a 4-carbon molecule
- Store CO2 for a short time
- Stomata can close for some part of the day to reduce water loss
- Dominate in warm, sunny, moderately humid climates, low CO2
- E.g., Corn, sugarcane, crab grass, many grasses
31
Q
CAM Pathway
A
- Very different pathway to make PGAL
- CO2 is stored all night long (H2O levels are higher and Temperature lower)
- Results in much less water loss
- Dominant plants in desert conditions (hot, dry environments with water conservation)
- Cactus, pineapple, orchids
32
Q
Why We Need Plants
A
- Plants pull CO2 from the atmosphere
- Produced by respiration, fossil fuel combustion, etc.
- Plants release O2 into the atmosphere
- Used for respiration by animals and plants
33
Q
Earth’s Atmosphere and CO2
A
- Early Earth’s atmosphere had no O2.
- Biomolecules (proteins, phospholipids, etc.) formed spontaneously, leading to living organisms.
- Photosynthesis released O2 –> stopped spontaneous biomolecule formation due to oxidation.
- O2 is highly reactive, forming free radicals that damage tissues and break apart organic molecules.
- Organisms evolved mechanisms to neutralize O2 by adding H+, converting it into water.
- Eventually, bacteria evolved a metabolic pathway (i.e. glycolysis and citric acid) to strip H+ from organic molecules (carbs, proteins, fatty acids), releasing energy that can be turned into ATP
- O2 accepts H+ at the end, forming water and detoxifying O2.
- These bacteria later became endosymbionts in eukaryotic cells (mitochondria).
- Mitochondria use O2 to produce 36 ATP per glucose molecule, compared to 2 ATP without O2.
