Photosynthesis
Definition and Word Equation
Photosynthesis is the synthesis of carbohydrates from raw materials using light energy.
Word Equation: carbon dioxide + water → glucose + oxygen
Chemical Equation: $$6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$$
Energy Source
- Chlorophyll is a green pigment in chloroplasts that transfers light energy to drive the reaction
Where It Occurs
- Chloroplasts in plant cells (mainly in leaves)
- Requires carbon dioxide, water and light
Use and Storage of Carbohydrates
Plants produce glucose during photosynthesis but use it for various purposes:
1. Starch (Storage)
- Main carbohydrate reserve in plants
- Stored in roots, stems and seeds
- Broken down to glucose when needed for energy
2. Cellulose (Structure)
- Major component of plant cell walls
- Provides rigidity and structural support
- Insoluble - does not affect water potential
3. Glucose (Respiration)
- Used immediately for cellular respiration
- Releases energy for cellular activities
- Provides ATP for metabolic processes
4. Sucrose (Transport)
- Transported in phloem from leaves to other parts
- Dissolved in water in phloem sap
- Moves to roots, fruits, growing shoots
5. Nectar (Pollination)
- Attracts pollinators like bees and butterflies
- Contains sugars as energy source for insects
Leaf Structure and Adaptations for Photosynthesis
Leaves are thin with large surface area for efficient photosynthesis. Specific structural features maximize light capture and gas exchange.
Key Leaf Structures
1. Epidermis
- Outer protective layer
- Transparent to allow light penetration
- Contains stomata for gas exchange
2. Cuticle
- Waxy waterproof layer above epidermis
- Prevents water loss
- Reduces transpiration
3. Stomata
- Pores on leaf surface (mostly on lower epidermis)
- Open for CO₂ to enter and O₂ to exit
- Regulated by guard cells
4. Guard Cells
- Kidney-shaped cells surrounding each stoma
- Contain chloroplasts
- Swell to open stomata, shrink to close them
5. Mesophyll
- Inner tissue layer where most photosynthesis occurs
- Palisade mesophyll (upper): tightly packed, many chloroplasts
- Spongy mesophyll (lower): loosely arranged, air spaces for gas diffusion
6. Air Spaces
- Between spongy mesophyll cells
- Allow CO₂ diffusion through leaf interior
- Connect to stomata for gas exchange
7. Vascular Bundles
- Xylem: transports water from roots to leaf
- Phloem: transports sucrose away from leaf
How Leaf Structures Adapt for Photosynthesis
| Structure | Adaptation | Benefit |
|---|---|---|
| Large surface area | Broad, flat leaf shape | Maximum light absorption |
| Thin leaves | Short diffusion distance | Efficient gas exchange |
| Palisade cells | Many chloroplasts, close to surface | High light capture |
| Stomata | Open to air | CO₂ entry for photosynthesis |
| Air spaces | Connected to stomata | CO₂ diffusion path |
| Veins | Xylem delivers water | Water supply for reaction |
| Cuticle | Waterproof layer | Prevents excessive water loss |
Factors Affecting Photosynthesis
Photosynthesis rate is influenced by environmental and internal factors. Understanding these helps explain plant growth patterns and agricultural practices.
1. Light Intensity
Effect
- Rate increases with light intensity up to a maximum
- Beyond maximum, rate plateaus (other factors become limiting)
Why
- Light provides energy for photosynthesis
- More photons = more chlorophyll excitation
- More ATP and NADPH produced
Graph Characteristics
- Linear increase initially
- Levels off when other factors limit rate
2. Carbon Dioxide Concentration
Effect
- Rate increases with CO₂ concentration up to saturation point
- Beyond saturation, rate stays constant
Why
- CO₂ is a raw material for photosynthesis
- More CO₂ = more substrate for reaction
- Enzymes (RuBisCO) become saturated
Graph Characteristics
- Increases to plateau
- Plateau occurs when enzymes saturated
3. Temperature
Effect
- Rate increases with temperature up to optimum (~25-30°C for most plants)
- Beyond optimum, rate decreases sharply
Why
- Temperature affects enzyme activity
- Higher T = more kinetic energy = more collisions
- Above optimum, enzymes denature
Graph Characteristics
- Bell-shaped curve
- Optimum at moderate temperature
- Sharp decline at high temperature
4. Chlorophyll Presence
Effect
- Chlorophyll required for light absorption
- Without chlorophyll, photosynthesis cannot occur
Examples
- Normal leaves: Green (chlorophyll present)
- Variegated leaves: Yellow/white patches lack chlorophyll
- Etiolated seedlings: Pale due to no light, low chlorophyll
Experiment
- Varying light/dark cycles affects chlorophyll production
- Hydrogencarbonate indicator changes colour with CO₂ levels
Investigating Photosynthesis
Hydrogencarbonate Indicator Test
Purpose
- Detects presence of CO₂ in air/water
- Changes colour based on CO₂ concentration
Indicator Colours
| CO₂ Level | Colour |
|---|---|
| High CO₂ | Yellow |
| Medium CO₂ | Orange |
| Low CO₂ | Purple |
How It Works
- Indicator absorbs CO₂ and changes colour
- Photosynthesizing plants remove CO₂ → colour shifts toward purple
- More photosynthesis = lower CO₂ = more purple
Experiment Setup
- Place plant in water with hydrogencarbonate indicator
- Seal in tube with light source
- Observe colour change over time
- Dark control shows no photosynthesis
Results Interpretation
- Purple: Active photosynthesis (low CO₂)
- Yellow: No photosynthesis (high CO₂)
- Orange: Intermediate level
Investigating Limiting Factors
Concept
A limiting factor is the environmental factor that is in shortest supply and thus restricts the rate of photosynthesis.
Blackman’s Principle of Limiting Factors
- Rate of process limited by slowest step
- When one factor is optimal, increasing another can increase rate
- When one factor is limiting, improving others won’t help
Example Scenario
- Low light intensity: Adding more CO₂ won’t increase rate (light is limiting)
- Optimal light, low CO₂: Adding CO₂ increases rate (CO₂ is now limiting)
- Optimal light and CO₂, low temperature: Raising temperature increases rate (temperature is limiting)
Limiting Factors Summary
| Factor | When It Limits | How to Identify |
|---|---|---|
| Light intensity | Dark or shaded conditions | Increasing light increases rate |
| CO₂ concentration | Low atmospheric levels | Increasing CO₂ increases rate |
| Temperature | Very low or very high | Rate drops if temperature changes |
| Water availability | Drought conditions | Stomata close to conserve water |
Plant Mineral Ions and Their Importance
Plants absorb mineral ions from the soil through root hair cells. These ions are essential for building organic molecules and maintaining physiological processes.
Nitrate Ions (NO₃⁻)
Importance
- Amino Acid Synthesis: Nitrate ions are essential for making amino acids, the building blocks of proteins
- Protein Production: Amino acids are linked together to form proteins needed for growth and cell structure
- Enzyme Production: Proteins include enzymes which catalyse metabolic reactions
Absorption Process
- Absorbed by root hair cells through active transport
- Energy from respiration is required
- Moves against concentration gradient (from soil where concentration is low to root cells where concentration is higher)
Deficiency Symptoms
- Slow growth: Limited protein synthesis affects growth
- Yellow leaves: Chlorosis due to lack of enzymes and chlorophyll production
- Stunted development: Overall reduced biomass production
Magnesium Ions (Mg²⁺)
Importance
- Chlorophyll Component: Magnesium is the central atom in chlorophyll molecule
- Photosynthesis Essential: Without magnesium, chlorophyll cannot be synthesized
- Light Absorption: Chlorophyll absorbs light energy for photosynthesis
Absorption Process
- Absorbed by root hair cells through active transport
- Energy from respiration is required
- Moves against concentration gradient
Deficiency Symptoms
- Yellow leaves: Chlorosis, especially in older leaves first
- Reduced photosynthesis: Less chlorophyll means less energy capture
- Poor growth: Overall decline in plant health
Other Important Mineral Ions
| Ion | Function | Deficiency Symptoms |
|---|---|---|
| Potassium (K⁺) | Enzyme activation, osmoregulation | Poor growth, weak stems |
| Phosphate (PO₄³⁻) | ATP production, DNA/RNA synthesis | Dark green leaves, stunted growth |
| Calcium (Ca²⁺) | Cell wall structure, cell division | Distorted new growth |
| Sulphate (SO₄²⁻) | Amino acid synthesis (cysteine, methionine) | Pale yellow leaves |
Practical Applications
Agriculture
- Greenhouses control light, temperature and CO₂ to maximize crop yield
- CO₂ enrichment increases photosynthesis rate
- Nitrate fertilizers support amino acid and protein production
Plant Health
- Yellow leaves indicate nitrogen deficiency (chlorophyll cannot be made)
- Pale leaves may indicate magnesium deficiency or lack of light
- Proper watering maintains turgor and stomata function
Glossary
| Term | Definition |
|---|---|
| Photosynthesis | Process converting light energy to chemical energy |
| Chlorophyll | Green pigment absorbing light energy |
| Stomata | Pores for gas exchange on leaf surface |
| Guard cells | Cells controlling stoma opening |
| Palisade mesophyll | Layer with many chloroplasts for photosynthesis |
| Limiting factor | Factor restricting process rate |
| Xylem | Tissue transporting water |
| Phloem | Tissue transporting sugars |
| Nitrate ions | Required for amino acid synthesis |
| Magnesium ions | Central atom in chlorophyll molecule |