- IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
- IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
C1.3 Photosynthesis
From light to chemical energy
It takes energy to make CO2 combine with H2O
This energy comes from sunlight
The energy is absorbed and used by chlorophyll
Light energy is transferred into chemical energy in carbon compounds
From inorganic to organic carbon compounds
Reminder 
Animals get their food by eating plants, or other animals
– Carnivores eat animals
– Herbivores eat plants
Plants make their own food
They combine carbon dioxide from the air with water and mineral ions from the soil
Plants do NOT get their food from the soil
The first stage by which plants make carbohydrates is called
PHOTOSYNTHESIS
Photosynthesis = Photo + synthesis = Light + Putting together
Photosynthesis = To make something (Sugar = glucose) using light
Equation of photosynthesis
Plants take in carbon dioxide from the air
They take up water (H2O) from the soil
The plants combine the \(CO_2\) with the \(H_2O\) to make the sugar, glucose \((C_6H_{12}O_6)\)
Word Equation of photosynthesis
Carbon dioxide + water → glucose + oxygen
Algae 
Cyanobacteria
Plants
+ algae
+ cyanobacteria
Can perform photosynthesis
Oxygen is released as a by-product (not the main point, although essential for life)
Atoms of oxygen come from the splitting of water
Energy = light energy
Gathered by pigment = chlorophyll
Photosynthetic pigments and light absorption
Leaf structure
Location of chlorophyll
Absorption and action spectra
Absorption spectrum = how light energy is absorbed by chlorophyll
Action spectrum = how light quality affects the rate of photosynthesis
- Absorption spectrum
- Action spectrum
Word Equation of photosynthesis
Carbon dioxide +Water → Glucose + Oxygen
How can we measure the rate of photosynthesis?
Measure how fast
– Carbon dioxide is consumed
– Oxygen is produced
unit = volume change / unit of time
mL / hour
Compare and contrast both spectra
Absorption spectrum of chlorophyll and action spectrum of photosynthesis overlap
– Chlorophyll is the main photosynthetic pigment
Instead of sunlight, artificial light sources can be used
As long as they contain blue and red light
Chlorophyll mostly + accessory pigments
Carotenoids and xanthophylls
All catch energy from photons
– Cooperation to absorb the most energy from sunlight
Multiple photosynthetic pigments
To cover the visible spectrum as much as possible
– To absorb as much
energy as possible
Limiting factors of photosynthesis
Simple pizza making
Three ingredients needed
how many pizzas can you make with…
6 dough + 6 tomato sauce + 6 grated cheese = ???
= 6 pizzas
What are the three main limiting factors of photosynthesis?
… and why?
How to test them through experiments?
Temperature
Carbon dioxide concentration
Temperature and Carbon dioxide concentration
Respiration vs. photosynthesis
Light intensity
Temperature, Carbon dioxide concentration and Light intensity
Temperature, Carbon dioxide concentration and Light intensity
Glasshouse production
Carbon dioxide concentration enrichment in enclosed greenhouses
Carbon dioxide concentration enrichment outdoors
FACE = FREE AIR CARBON DIOXIDE (CO2) ENRICHMENT
Carbon dioxide concentration enrichment
Research focus and outcomes
Photosystems (HL only)
The organelle chloroplast contains “organelles’ of its own
Thylakoids
Membrane and hollow lumen
Stacked into granum (pl. grana)
Double membrane = envelope
“cytoplasm” of chloroplast = chloroplasm = stroma
All reactions of photosynthesis happen in the chloroplast
Chloroplasts in plants
Chloroplasts in plants and photosynthetic pigments
Photosystems in thylakoid membrane in plants and cyanobacteria
In thylakoid membrane
Complex of several hundreds of pigments
Energy is funneled to a final chlorophyll molecule = Reaction center
Two types of photosystems
1. Photosystem I (PSI)
Reaction center absorbs wavelength 700 nm “P700”
2. Photosystem II (PSII)
Reaction center absorbs wavelength 680 nm “P680”
1. Energy funneled to reaction center
2. In reaction center, “Ground-state” electrons are “excited”
3. High energy electrons are released from reaction center
4. Used in the following reactions…
5. “Ground-state” electrons replaced by electrons from water molecules
A single chlorophyll molecule = not enough
– Photosystem = hundreds of them
Light-dependent reactions (HL only)
Photosynthetic Reactions
1. Light-dependent reactions
2. Light-independent reactions (Calvin Cycle)
Photolysis of water and production of oxygen
Light energy used to split water: photolysis
\(H_2O\) split into \(2H^{+} + \frac{1}{2}O_2\) and \(2e^{-}\)
Splitting of water happens inside lumen of thylakoids
Protons and electrons used in later stages
Oxygen released (waste product of photosynthesis)
Importance of oxygen released by photolysis of water
Oxygen fuels cell respiration
↓
Oxygen makes life of
many organisms possible
Electron transport chain, Chemiosmosis and ATP synthesis
1. Photolysis of water
2. Electron transport chain
3. Chemiosmosis
4. Formation of ATP
Photophosphorylation
5. Reduction of NADP+ into NADPH
NADPH and ATP given to
Light-independent reactions
1. Photolysis of water
Electrons of chlorophyll in PSII dislodged by light energy
Replaced by water’s electrons after photolysis
Photolysis: \(H_2O→ 2 e^{-} + 2 H^{+} + \frac{1}{2} O_2\)
2. Electron transport chain
Electrons transported by proteins within thylakoid membrane
Loss of energy by electrons
Light energy “re-boost” in PSI
Electrons transported by proteins within thylakoid membrane
End their transport in the stroma of the chloroplast
3. Chemiosmosis
During electron transport chain,
Protons pumped from stroma into thylakoid lumen
+ protons from photolysis of water
– Accumulation of protons in thylakoid lumen
– Gradient of protons lumen > stroma
Protons diffuse from lumen to stroma
By facilitated diffusion using proton channels: “Chemiosmosis”
4. Formation of ATP
Photophosphorylation
Protons diffuse from lumen to stroma
– Energy released
Energy used to phosphorylate ADP into ATP by enzyme ATP Synthase
Phosphorylation depends on light
5. Reduction of NADP+ into NADPH
Protons diffused from lumen to stroma + Electrons from PSI transported into stroma
Used to reduce NADP+ into NADPH by enzyme NADP+ reductase
Non-cyclic and cyclic photophosphorylation
Non cyclic (Electrons are not recycled)
Cyclic (Electrons are recycled)
Photophosphorylation (Phosphorylation that depends on light)
Electrons are recycled
Compare and contrast non-cyclic and cyclic photophosphorylation
| Non- cyclic Photosynthesis | Cyclic Photosynthesis |
| PS I and PS II involved | PS I only involved |
| Reaction centre is P680 | Reaction centre is P700 |
| Electron released are not cycled back | Electron released are cycled back |
| Photolysis of water takes place | Photolysis of water does not takes place |
| ATP and NADPH + \(H^{+}\) are synthesized | Only ATP synthesized |
| Phosphorylation takes place at only one place | Phosphorylation takes place at only two place |
| It is sensitive to DCMI and inhibits electron flow | It is not sensitive to di chloro di methyl urea (DCMI) |
Light-independent reactions (HL only)
\(CO_2\) from air fixed into carbohydrates
Also need ATP and NADPH from LDR
Do not need light
Can happen in dark, and under light
– Light-independent reactions
Happen in stroma of chloroplast
Produce sugar
Also produce ADP + Pi and \(NADP^{+}\)
Given to Light-dependent reactions
1. Carboxylation of RuBP
2. Reduction to G3P
3. Regeneration of RuBP
Production of sugar
NADPH and ATP given by
Light-independent reactions
1. Carboxylation of RuBP
\(CO_2\) from air fixed: inorganic to organic
Combined with Ribulose-bisphosphate (RuBP) to give a 6-carbon compound
Unstable: split into 2X 3-carbon compound
“Carboxylation” = adding carbon and oxygen
RuBP is carboxylated
Enzyme = Rubisco
RuBP carboxylase-oxygenase
Most abundant enzyme on Earth
In plants and cyanobacteria
Not effective in low \(CO_2\) concentrations.
2. Reduction to G3P
2X 3-carbon compound reduced to glycerate-3-phosphate (G3P)
Using NADPH and ATP from LDR
3. Regeneration of RuBP
To allow the Calvin cycle to continue
Need to regenerate RuBP
Available for next \(CO_2\) fixation
Using ATP rom LDR
Production of sugar
2X 3-carbon compound reduced to glycerate-3-phosphate (G3P)
Using NADPH from LDR
“Sugar” = triose phosphate (TP)
Do the numbers add up to produce glucose?
Need 6C to make one glucose
– Need 2 times 3 cycles to produce one glucose
Interdependence of the light-dependent and Light-independent reactions
The “only function” of light-dependent reactions
Is to (re)generate ATP and \(NADPH_2\) to fuel the Calvin cycle
What happens if there is no light?
Light-dependent reactions stop
– No production of ATP and NADPH
– Light-independent reactions stop
What happens if there is no \(CO_2\)?
- Light-independent reactions stop
– No production of ADP and \(NADP^{+}\)
– Light-dependent reactions stop
- Photosystem II stops functioning
– Light-dependent reactions stop
– No production of ATP and NADPH
– Light-independent reactions stop
Glucose, then what? (HL only)
