IB MYP 4-5 Biology-Photosynthesis- Study Notes - New Syllabus
IB MYP 4-5 Biology-Photosynthesis- Study Notes – New syllabus
IB MYP 4-5 Biology-Photosynthesis- Study Notes – IB MYP 4-5 Biology – per latest IB MYP Biology Syllabus.
Key Concepts:
- Light-dependent and light-independent reactions
- Chloroplast structure and function
- Factors affecting rate (light intensity, CO₂ concentration, temperature)
Photosynthesis
Light-Dependent and Light-Independent Reactions
What is Photosynthesis?
Photosynthesis is the biological process by which green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose.
Overall Equation: \[ 6\mathrm{CO}_2 + 6\mathrm{H}_2\mathrm{O} + \text{light energy} \rightarrow \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{O}_2 \]
- Occurs in chloroplasts
- Requires chlorophyll to absorb light
- Involves two stages:
- Light-dependent reactions
- Light-independent reactions (Calvin Cycle)
Stage 1: Light-Dependent Reactions 
- Location: Thylakoid membranes of chloroplasts
- Time: Daylight (requires light)
- Inputs: Water (H₂O), light energy, NADP⁺, ADP + Pi
- Outputs: Oxygen (O₂), ATP, NADPH
- Key Steps:
- Light energizes electrons in chlorophyll
- Photolysis of water releases oxygen
- Electrons pass through ETC, forming ATP
- NADP⁺ is reduced to NADPH
- Purpose: Produce ATP and NADPH for the Calvin Cycle
Experiment Design – Testing Photosynthesis Rate
- Independent variable examples: light intensity, CO₂ concentration, temperature
- Dependent variable: oxygen production rate or bubble count (in aquatic plants)
- Common control variables: species of plant, distance from light, same water temperature
- Hypothesis example:
If light intensity increases, then the rate of photosynthesis will also increase, because more photons energize chlorophyll molecules for ATP and NADPH formation.
Stage 2: Light-Independent Reactions (Calvin Cycle) 
- Location: Stroma of chloroplasts
- Time: Can occur anytime (uses light stage products)
- Inputs: Carbon dioxide (CO₂), ATP, NADPH
- Outputs: Glucose, ADP, NADP⁺
- Key Steps:
- Carbon fixation by RuBisCO with RuBP
- G3P production using ATP and NADPH
- Some G3P makes glucose, rest regenerates RuBP
- Purpose: Convert CO₂ into glucose using energy carriers
Comparison Table
| Stage | Light-Dependent Reactions | Light-Independent Reactions |
|---|---|---|
| Location | Thylakoid membrane | Stroma of chloroplast |
| Requires light? | Yes | No (indirectly) |
| Inputs | Light, H₂O, NADP⁺, ADP | CO₂, ATP, NADPH |
| Outputs | O₂, ATP, NADPH | Glucose, ADP, NADP⁺ |
| Key Molecules Formed | ATP and NADPH | Glucose |
Why Is Photosynthesis Important?
- Provides oxygen for aerobic organisms
- Creates energy-rich organic compounds for ecosystems
- Reduces atmospheric CO₂
- Forms the base of almost all food chains
Chloroplast: Structure and Function
What Are Chloroplasts?
Chloroplasts are double-membrane-bound organelles found in plant cells and algae. They are the sites of photosynthesis, where light energy is converted into chemical energy stored in glucose.
Basic Functions of Chloroplasts
- Capture light energy using chlorophyll
- Convert light energy into ATP and NADPH (in thylakoids)
- Fix carbon dioxide into glucose (in the stroma)
- Produce oxygen as a by-product of splitting water
Chloroplast Structure
Chloroplasts have a highly organized internal structure that supports photosynthesis efficiently.
- Outer Membrane: Smooth and semi-permeable, allows small molecules and ions to pass freely
- Inner Membrane: Selectively permeable, contains transport proteins for movement into the stroma
- Stroma: Fluid-filled matrix, site of the Calvin Cycle, contains enzymes, DNA, ribosomes, starch grains
- Thylakoids: Flattened disc-like sacs, contain chlorophyll, site of light-dependent reactions
- Grana (Granum): Stacks of thylakoids, increase surface area for capturing light, connected by lamellae
- Thylakoid Membrane: Houses photosystems, electron transport chains, and ATP synthase
- Chlorophyll: Green pigment in thylakoid membrane; absorbs blue and red light, reflects green
Chloroplast DNA:
Chloroplasts have their own circular DNA and ribosomes. They can replicate independently and support the endosymbiotic theory, which suggests chloroplasts were once free-living bacteria.
Summary Table: Structure and Function
| Structure | Function |
|---|---|
| Outer Membrane | Protection, regulates entry of molecules |
| Inner Membrane | Controls exchange with stroma |
| Stroma | Site of Calvin Cycle, contains enzymes and DNA |
| Thylakoids | Site of light-dependent reactions |
| Grana | Maximize surface area for light absorption |
| Chlorophyll | Absorbs light energy for photosynthesis |
| DNA & Ribosomes | Protein synthesis and replication |
Why Are Chloroplasts Important?
Chloroplasts power primary productivity in plants, produce oxygen for aerobic life, support food chains with glucose production, and regulate global carbon dioxide levels.
Factors Affecting the Rate of Photosynthesis
Definition: The rate of photosynthesis refers to how quickly plants convert carbon dioxide and water into glucose and oxygen using light energy. This rate is influenced by environmental conditions known as limiting factors.
1. Light Intensity
- Light provides energy for the light-dependent reactions in thylakoid membranes.
- Increasing light intensity raises the rate of photosynthesis until it levels off.
- Saturation Point: A stage where more light no longer boosts the rate because another factor becomes limiting.
2. Carbon Dioxide Concentration
- CO₂ is a raw material for the Calvin Cycle, used in carbon fixation to form glucose.
- Higher CO₂ levels increase the rate until enzymes like RuBisCO become saturated.
- Common limiting factor in greenhouses and enclosed environments.
3. Temperature
- Enzymes controlling photosynthesis function optimally between 25°C and 35°C.
- Low temperatures slow enzyme activity, high temperatures may denature enzymes.
- Extreme heat can also cause stomata to close, reducing CO₂ intake.
Interactive Effects
All three factors often interact. Even in bright sunlight, photosynthesis can be limited by low CO₂ or unsuitable temperatures. Improving one factor may increase yield temporarily, until another factor becomes the new limiting point.
Investigating Limiting Factors – Practical Setups and Analysis
Include:
- Method Outline for Cabomba experiment:
- Place aquatic plant under lamp
- Vary distance to change light intensity
- Count bubbles for 5 mins at each distance
- Repeat 3–4 trials per condition
- Expected Graph Shape:
- Light intensity vs rate = rapid rise → plateau
- CO₂ concentration vs rate = increasing → levels off
- Temperature vs rate = bell curve
- Reason for trends:
- Light and CO₂: saturation occurs
- Temperature: enzyme activity peak followed by denaturation
Summary Table
| Factor | Role in Photosynthesis | Effect on Rate |
|---|---|---|
| Light Intensity | Provides energy for light-dependent reactions | Increases rate until saturation point |
| Carbon Dioxide | Used in Calvin Cycle for glucose synthesis | Increases rate until enzyme saturation |
| Temperature | Affects enzyme activity | Rises to an optimum, then falls |
Key Concept: At any given time, one factor controls the rate of photosynthesis – this is the limiting factor. Enhancing that specific factor can increase the rate until another one takes over as the limiting factor.