Transport in Plants

PHOTOSYNTHESIS IN HIGHER PLANTS

Overview and Importance

**Photosynthesis** is a physicochemical process by which green plants use light energy to synthesize organic compounds (food) from carbon dioxide and water. It is **autotrophic nutrition** – the ability of organisms to manufacture their own food using inorganic raw materials and an external energy source (light).

**Why photosynthesis is important:**

Primary source of all food on Earth; all heterotrophs depend on it

Releases oxygen into the atmosphere, essential for respiration of all living organisms

Basis of energy flow in all ecosystems

Uses solar energy as the ultimate energy source for all life forms

All animals, including humans, depend on plants for food. Plants are called **autotrophs** because they synthesize their own food through photosynthesis, while all other organisms are **heterotrophs** – they depend on autotrophs for nutrition.

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11.1 WHAT DO WE KNOW?

**Basic observations from simple experiments:**

**Chlorophyll requirement:** Green pigment in leaves is essential for photosynthesis. Variegated leaves or partially covered leaves show starch formation only in green regions when exposed to light.

**Light requirement:** Only illuminated parts of leaves show starch formation, proving photosynthesis is light-dependent.

**CO₂ requirement:** When a leaf part is enclosed in a tube with KOH-soaked cotton (which absorbs CO₂), that portion does not produce starch, while the exposed part does. This proves **CO₂ is essential** for photosynthesis.

**Key inference:** Three factors are absolutely required: chlorophyll, light, and carbon dioxide.

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11.2 EARLY EXPERIMENTS

Priestley's Experiment (1770)

**Joseph Priestley (1733-1804)** conducted groundbreaking experiments showing the essential role of air in plant growth.

**Observations:**

A candle burns in a closed bell jar until the air is "fouled" and the flame extinguishes

A mouse suffocates in a closed space because air becomes unsuitable for breathing

When a mint plant is placed in the same bell jar, the candle continues to burn and mouse survives

**Conclusion:** Plants restore to the air whatever breathing animals and burning candles remove. Plants must release something that makes air suitable for respiration.

**Historical significance:** Priestley discovered **oxygen in 1774**, later understanding that plants release this gas.

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Ingenhousz's Experiment (1730-1799)

**Improvement on Priestley's experiment:** Ingenhousz repeated the experiment in two conditions – darkness and sunlight.

**Observations:**

In **sunlight:** Small bubbles formed around green parts of aquatic plants

In **darkness:** No bubble formation

Bubbles were identified as **oxygen**

**Key findings:**

**Sunlight is essential** for the air-purifying process in plants

Only **green parts release oxygen**, not all plant tissues

Photosynthesis is **light-dependent process**

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Von Sachs' Contribution (1854)

**Julius von Sachs** provided evidence that glucose is produced during plant growth.

**Discoveries:**

**Chlorophyll** (green substance) is located in special structures called **chloroplasts** within plant cells

**Green parts synthesize glucose** from CO₂ and water

Glucose is typically stored as **starch** in plant tissues

Location of glucose synthesis = location of chloroplasts

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Engelmann's Experiment (1843-1909)

**T.W. Engelmann** created the first **action spectrum of photosynthesis** using spectral analysis.

**Method:**

Used a prism to split light into spectral components

Illuminated green alga *Cladophora* in suspension of aerobic bacteria

Bacteria accumulated where most O₂ was evolved (marker of photosynthesis)

**Results:**

**Maximum bacterial accumulation** in **blue and red regions** of spectrum

Minimal accumulation in green region

This creates the **action spectrum** – resembles absorption spectra of chlorophyll a and b

**Significance:** First demonstration that photosynthesis is most efficient in blue and red wavelengths.

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Cornelius van Niel's Contribution (1897-1985)

**Microbiologist van Niel** conducted studies on purple and green bacteria, revolutionizing photosynthesis understanding.

**Key hypothesis:** Photosynthesis is essentially a **light-dependent reaction** where hydrogen from oxidizable compound reduces CO₂ to carbohydrates.

**General equation:**

2H₂A + CO₂ → A + CH₂O + H₂O (where H₂A is hydrogen donor)

**Critical discovery:** In green plants, **H₂O is the hydrogen donor** (oxidized to O₂), but in some bacteria, **H₂S** is the donor (oxidized to sulphur or sulphate instead of O₂).

**Evidence:** Using **radioisotopic techniques** with ¹⁸O isotope, scientists proved that **oxygen released comes from water**, not from CO₂.

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11.3 WHERE DOES PHOTOSYNTHESIS TAKE PLACE?

Location

Photosynthesis occurs in:

**Green leaves** (primary site)

**Other green parts** of plants (green stems, sepals, bracts)

**Chloroplasts** – the specific organelle within cells

The Photosynthetic Apparatus

**Mesophyll cells** contain numerous chloroplasts that align themselves to receive optimum light:

**Parallel to walls:** When low light intensity available

**Perpendicular to incident light:** In bright light conditions for maximum light capture

Chloroplast Structure and Division of Labour

**The chloroplast contains** (as studied in Chapter 8):

**Membranous system:**

**Grana:** Stacked thylakoid discs

**Stroma lamellae:** Individual thylakoid membranes in stroma

**Thylakoid membranes:** Site of light reactions

**Functions:**

Trap light energy

Synthesize ATP and NADPH

**Stroma (matrix):**

Contains enzymes for dark reactions

Synthesizes sugar (glucose) from ATP and NADPH

Glucose stored as starch

Light Reactions vs Dark Reactions

**Light Reactions (Photochemical Reactions):**

Occur in **thylakoid membranes**

**Directly light-driven** – require photons

Products: **ATP, NADPH, O₂**

No CO₂ fixation occurs here

**Dark Reactions (Calvin Cycle/Carbon Reactions):**

Occur in **stroma**

**Indirectly light-dependent** – use products (ATP, NADPH) from light reactions

**NOT truly "dark reactions"** – misleading name; they require ATP and NADPH from light reactions

Don't occur in complete darkness

Products: **Glucose, starch**

**Critical point:** Both reactions are interdependent; light reactions provide energy currency for dark reactions.

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11.4 HOW MANY TYPES OF PIGMENTS ARE INVOLVED IN PHOTOSYNTHESIS?

Pigment Separation

**Paper chromatography** of leaf pigments reveals **four pigments:**

1. **Chlorophyll a** (bright/blue-green)

Most abundant

Chief pigment of photosynthesis

Reaction centre in both photosystems

2. **Chlorophyll b** (yellow-green)

Accessory pigment

Transfers energy to chlorophyll a

3. **Xanthophylls** (yellow)

Accessory pigments

Absorb light, transfer energy to chlorophyll a

4. **Carotenoids** (yellow to yellow-orange)

Accessory pigments

Provide photoprotection

Also called beta-carotene (orange)

**Question:** Why are leaves not uniformly green? Different pigments with different absorption maxima create various shades of green visible.

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Absorption and Action Spectra

**Pigments** are substances that absorb light at specific wavelengths.

**Absorption spectrum:**

Graph showing wavelengths of light absorbed by pigment

**Chlorophyll a:** Maximum absorption in **blue (430-450 nm) and red (640-680 nm) regions**

Minimal absorption in green region (500-600 nm) – why chlorophyll appears green to our eyes

**Action spectrum:**

Graph showing rate of photosynthesis at different wavelengths

**Maximum photosynthesis:** Blue and red wavelengths

Resembles chlorophyll a absorption spectrum

**Important finding:** Action spectrum of photosynthesis does NOT perfectly match absorption spectrum of chlorophyll a alone:

Photosynthesis occurs at wavelengths not maximally absorbed by chlorophyll a

Other pigments contribute to photosynthesis

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Role of Accessory Pigments

**Functions:**

1. **Absorb light** at wavelengths not efficiently absorbed by chlorophyll a (especially green wavelengths)

2. **Transfer absorbed energy** to chlorophyll a

3. **Extend the range** of wavelengths usable for photosynthesis

4. **Protect chlorophyll a** from photo-oxidation damage

5. Make photosynthesis **more efficient** by broadening light absorption range

**Result:** The combination of chlorophyll a and accessory pigments maximizes light capture across visible spectrum, explaining why photosynthesis occurs throughout visible spectrum though maximally in blue-red regions.

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11.5 WHAT IS LIGHT REACTION?

Definition and Key Events

**Light reactions** (also called photochemical phase) are reactions in photosynthesis that:

Directly require light energy (photons)

Occur in **thylakoid membranes**

Result in:

**Water splitting** (photolysis)

**Oxygen release**

**ATP synthesis** (energy currency)

**NADPH formation** (reducing power)

Photosystems and Light Harvesting Complex

**Photosystems:** Discrete photochemical units embedded in thylakoid membranes containing protein complexes and pigments.

**Two photosystems:**

**Photosystem II (PS II):**

Named based on discovery sequence, NOT functional sequence

Reaction centre: **P680** (chlorophyll a with absorption peak at 680 nm)

Located primarily in **grana**

Associated with water-splitting complex (oxygen-evolving complex)

**Photosystem I (PS I):**

Reaction centre: **P700** (chlorophyll a with absorption peak at 700 nm)

Located in **stroma lamellae** and grana margins

Connected to NADP⁺ reduction

Light Harvesting Complex (Antennae)

**Structure:**

**Hundreds of pigment molecules** bound to proteins

All pigments except **one chlorophyll a molecule** form antenna system

Single chlorophyll a molecule = **reaction centre**

**Function:**

Absorb photons of different wavelengths

**Transfer energy** to reaction centre via **resonance energy transfer**

Concentrate light energy at reaction centre

Enable **efficient light harvesting**

**Reaction centre significance:**

Only reaction centre chlorophyll can undergo **photoexcitation** and lose electrons

Accessory pigments absorb light but don't directly lose electrons

Reaction centres are different in PS I (P700) and PS II (P680)

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11.6 THE ELECTRON TRANSPORT

The Z-Scheme of Light Reactions

**The Z-scheme** describes electron flow through photosystems in non-cyclic photophosphorylation.

Step-by-step Electron Transport Process

**1. Photosystem II (PS II) Activation:**

Reaction centre chlorophyll a (**P680**) absorbs **red light (680 nm)**

Electrons become **excited** and jump to higher orbital (farther from nucleus)

P680 becomes P680⁺ (oxidized, electron-deficient)

**2. Electron Acceptor in PS II:**

Excited electrons are picked up by **primary electron acceptor**

Electrons move downhill through **electron transport chain** containing cytochromes

Movement is **downhill** in terms of redox potential (energy released)

**3. Connection to PS I:**

Electrons from PS II electron transport chain reach **Photosystem I**

Electrons replace those that were removed from PS I

**Electrons are NOT consumed** – circulated through system

**4. Photosystem I (PS I) Activation:**

Reaction centre chlorophyll a (**P700**) absorbs **red light (700 nm)**

Electrons become excited and transfer to **another acceptor molecule**

This acceptor has greater redox potential than previous carriers

**5. NADP⁺ Reduction:**

Excited electrons move **downhill again** to **NADP⁺ molecule**

**NADP⁺ + 2e⁻ + H⁺ → NADPH + H⁺** (or NADPH)

NADPH becomes the reducing power for dark reactions

This is the **endpoint of electron transport**

Z-Scheme Visualization

The characteristic **Z-shaped** graph emerges when all electron carriers are arranged on a redox potential scale:

Electrons move from low to high potential in PS II (uphill, using light energy)

Electrons move downhill through transport chain

Electrons move uphill again in PS I (using light energy)

Electrons move downhill to NADP⁺

**Significance:** Each photosystem uses light energy to push electrons uphill against the redox potential gradient.

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11.6.1 SPLITTING OF WATER (PHOTOLYSIS)

**Question:** How does PS II continuously supply electrons for removal?

**Answer:** Water molecules are **split** to provide replacement electrons.

**Water Splitting Reaction:**

**2H₂O → 4H⁺ + 4e⁻ + O₂**

Or represented as:

**2H₂O → 4H⁺ + [O] + 4e⁻** (where [O] is atomic oxygen, combines to O₂)

Mechanism

**Location:** Water-splitting complex (oxygen-evolving complex) associated with **Photosystem II**, embedded in **inner side of thylakoid membrane**

**Process:**

1. Water molecules bind to water-splitting complex in PS II

2. Light energy drives removal of electrons from water

3. **4 electrons** are released and replace those removed from P680

4. **4 protons (H⁺)** are released into **thylakoid lumen**

5. **Oxygen (O₂)** is released as byproduct

Products Released

**Water splitting produces three products:**

1. **Electrons:** Replace electrons removed from PS II reaction centre

2. **Protons (H⁺):** Accumulate in **thylakoid lumen** (inside thylakoid space)

3. **Oxygen (O₂):** Released into thylakoid lumen initially, then to atmosphere

**Why inside the lumen first?** Water-splitting complex is on inner (lumen-facing) side of thylakoid membrane; protons and O₂ are released into the enclosed lumen space before diffusing out.

**Stoichiometry clarification:** Overall equation shows 12 water molecules as substrate because:

Light reactions split 2 water molecules

This produces 1 O₂ molecule and 4 electrons/4 protons

For complete glucose synthesis (dark reaction), multiple turns of light reactions needed

The 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ equation aggregates all reactions

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11.6.2 CYCLIC AND NON-CYCLIC PHOTOPHOSPHORYLATION

**Photophosphorylation** is the synthesis of ATP from ADP and inorganic phosphate (Pi) using light energy.

**ATP + Energy ← ADP + Pi**

**Two types based on electron flow pattern:**

Non-Cyclic Photophosphorylation

**Definition:** Both **PS II and PS I function in series**, electrons flow in one direction (non-cyclic).

**Key features:**

Electrons flow: **PS II → electron transport chain → PS I → NADP⁺**

**Two photosystems** work sequentially

**One light photon** absorbed per electron in each photosystem (two total per electron progressing through both)

**Products:**

**ATP** synthesized

**NADPH + H⁺** synthesized

**O₂** released

**Ratio:** Approximately **3 ATP and 2 NADPH** produced per glucose synthesis

**Occurrence:** Primarily in grana membranes where both photosystems are present

**Location of PS II and PS I:** Both located in grana thylakoids, connected by electron transport chain

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Cyclic Photophosphorylation

**Definition:** Only **PS I is functional**; electrons cycle within photosystem instead of flowing to NADP⁺.

**Electron flow path:**

1. **PS I excited** by light (700 nm)

2. Electrons transfer to acceptor

3. Electrons pass through **electron transport chain** containing cytochromes

4. Electrons **return to P700** (not going to NADP⁺)

5. Cycle repeats

**Products:**

**ATP only** (synthesized)

**NO NADPH** produced

**NO O₂** released

**When it occurs:**

When **only wavelengths > 680 nm** available (PS II not excited)

When plant needs more ATP than NADPH

Likely location: **Stroma lamellae** membranes

**Difference in membrane composition:**

**Grana membranes:** Contains PS I, PS II, NADP reductase

**Stroma lamellae:** Contains PS I only; LACKS PS II and NADP reductase enzyme

This structural difference allows cyclic flow in stroma lamellae

**Physiological significance:**

Maintains **ATP:NADPH ratio** needed for dark reactions

Provides flexibility in energy production matching metabolic demand

PS I can function independently in cyclic mode

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11.6.3 CHEMIOSMOTIC HYPOTHESIS

**Key principle:** ATP synthesis in chloroplasts, like in mitochondria, is linked to **proton gradient** across membrane.

Mechanism of Proton Accumulation

**Location:** Thylakoid membrane and lumen

**Source of protons:**

1. **Water splitting:** 2H₂O produces 4H⁺ in thylakoid lumen

2. **Electron transport:** Electron transport chain complexes pump additional H⁺ from stroma into lumen

**Result:** **Proton accumulation inside thylakoid lumen** (opposite to mitochondria where protons accumulate in matrix)

Proton Gradient Creation

**Outside (stroma):** Low H⁺ concentration (high pH ≈ 8)

**Inside (thylakoid lumen):** High H⁺ concentration (low pH ≈ 4.5)

**Difference:** pH gradient of approximately 3-4 units creates **electrochemical gradient**

ATP Synthesis from Gradient

**Process:**

1. Protons accumulate in lumen due to water splitting and electron transport

2. **ATP synthase** enzyme complex spans thylakoid membrane

3. Protons flow **down concentration gradient** through ATP synthase

4. Proton flow drives **rotational mechanism** of ATP synthase

5. **ADP + Pi → ATP** using energy from proton gradient

**Chemiosmotic coupling:** The proton gradient (proton motive force) is the **intermediate energy carrier** that couples electron transport to ATP synthesis.

Comparison with Mitochondrial Oxidative Phosphorylation

| Feature | Chloroplast (Photosynthesis) | Mitochondrion (Respiration) |

|---------|------------------------------|----------------------------|

| **Membrane** | Thylakoid | Inner mitochondrial membrane |

| **Proton location** | Accumulate in lumen (inside) | Accumulate in matrix (inside) |

| **Driving force** | Light energy (photons) | Chemical energy (electron donors) |

| **Electron source** | Water splitting | NADH, FADH₂ oxidation |

| **Endpoint** | NADP⁺ reduction | O₂ reduction |

| **Process** | Photophosphorylation | Oxidative phosphorylation |

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11.7 WHERE ARE ATP AND NADPH USED?

Dark Reactions Location

ATP and NADPH produced in light reactions are used in the **stroma** where **dark reactions (Calvin cycle)** occur.

Dark Reaction Overview (Preview)

**The Calvin cycle** (carbon fixation cycle) uses:

**ATP:** Energy for carboxylation and reduction steps

**NADPH:** Reducing power for reduction of 3-PG to G3P

**CO₂:** Carbon source

**Products:** Glucose and other carbohydrates stored as starch

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11.8 THE C₄ PATHWAY

Problem with C₃ Pathway (Calvin Cycle)

Before understanding C₄ pathway, recall that C₃ plants (like wheat, rice, pea):

First stable product of CO₂ fixation is **3-phosphoglycerate (3-PG)** – a 3-carbon compound

Use **Ribulose-1,5-bisphosphate (RuBP)** as CO₂ acceptor

Enzyme: **RuBisCO**

**Limitation:** In high temperatures and bright light, **photorespiration** occurs (discussed in 11.9), reducing photosynthetic efficiency.

What is C₄ Pathway?

**Definition:** An alternative photosynthetic pathway in which **first stable product of CO₂ fixation is a 4-carbon compound** (oxaloacetate).

**First product:** **Oxaloacetate (4-carbon)** rather than 3-PG (3-carbon)

**Enzyme:** **PEP carboxylase** (not RuBisCO as primary carboxylase)

Has higher affinity for CO₂

Does not catalyze photorespiration

Plants Using C₄ Pathway

**Examples:**

**Sugarcane** (*Saccharum officinarum*)

**Maize/Corn** (*Zea mays*)

**Sorghum**

**Crabgrass**

Many tropical grasses

C₄ Pathway Steps

**1. CO₂ Fixation (Mesophyll cell):**

**PEP (3-carbon) + CO₂ → Oxaloacetate (4-carbon)**

Enzyme: PEP carboxylase

Location: Mesophyll cell cytoplasm

**2. Oxaloacetate reduction:**

Oxaloacetate → Malate (4-carbon)

Uses NADPH

**3. Transport:**

**Malate translocates** from mesophyll to bundle sheath cells via plasmodesmata

**4. Decarboxylation (Bundle sheath cell):**

**Malate → Pyruvate + CO₂**

CO₂ concentration increases in bundle sheath cells

**5. Calvin Cycle (Bundle sheath cell):**

**CO₂ + RuBP → 3-PG**

Enzyme: RuBisCO

Standard C₃ pathway continues in bundle sheath cells

**3-PG → G3P → Glucose**

**6. Pyruvate regeneration:**

Pyruvate returns to mesophyll cells

Regenerates PEP for next cycle

Advantages of C₄ Pathway

**1. CO₂ concentration:** PEP carboxylase activity concentrates CO₂ in bundle sheath cells where RuBisCO works, even if atmospheric CO₂ is low

**2. Photorespiration reduction:** High CO₂ concentration in bundle sheath suppresses photorespiration

**3. Efficiency:** More efficient in hot, bright conditions where C₃ plants struggle

**4. Water efficiency:** Requires less water to fix same amount of CO₂

**5. Nitrogen efficiency:** Lower RuBisCO requirement (less protein needed)

Anatomical Adaptations

**Kranz Anatomy:** Special leaf anatomy in C₄ plants

Two photosynthetic cell layers:

**Mesophyll cells:** Outer layer, CO₂ fixation here

**Bundle sheath cells:** Inner layer around vascular bundle, Calvin cycle here

**Large intercellular spaces:** Maximize CO₂ diffusion

**Numerous plasmodesmata:** Connect mesophyll and bundle sheath cells for metabolite transport

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11.9 PHOTORESPIRATION

Definition

**Photorespiration** is an oxygenase activity of RuBisCO enzyme that consumes O₂ and releases CO₂, occurring in presence of light but not producing ATP or NADPH (wasteful compared to true respiration).

Also called **oxidative photosynthetic carbon loss** or **C₂ photosynthesis**.

Why Photorespiration Occurs

**RuBisCO dual specificity:**

**RuBisCO** (Ribulose-1,5-bisphosphate carboxylase/oxygenase) enzyme has **two catalytic sites**

Can accept both **CO₂** (carboxylation) and **O₂** (oxygenation)

**When photorespiration increases:**

High **O₂:CO₂ ratio**

High **temperature** (enzyme favors oxygenase activity over carboxylase activity)

Low **light intensity** (indirectly increases O₂)

**Bright, hot, dry conditions** (typical of midday in tropical/subtropical regions)

The Photorespiratory Pathway

**Step 1: Oxygenation of RuBP (Chloroplast)**

**RuBP + O₂ → 3-PG (3-C) + 2-phosphoglycolate (2-C)**

Enzyme: RuBisCO (oxygenase activity)

Only one 3-PG formed instead of two (in carboxylation)

**Step 2: Phosphoglycolate metabolism (Chloroplast, Peroxisome, Mitochondrion)**

2-phosphoglycolate → Glycolate

Glycolate enters **peroxisomes**

Oxidation of glycolate to **glyoxylate**

**Transamination:** Glyoxylate → **Glycine** (amino acid)

**Step 3: Further metabolism (Mitochondrion)**

2 Glycine molecules → **Serine** (amino acid)

**Serine → Glycerate** (3-C)

Glycerate returns to chloroplast

**Step 4: Regeneration of 3-PG (Chloroplast)**

Glycerate → **3-PG**

Enters Calvin cycle

Only net result: **1 C is lost as CO₂**, energy (ATP, NADPH) is consumed without producing carbohydrates

Net Effect of Photorespiration

**Per 2 molecules of RuBP consumed:**

**1 CO₂ released** (completely oxidized, wasted)

**1 3-PG produced** (enters Calvin cycle)

**ATP consumed** (no ATP production – energy loss)

**NADPH consumed** (no NADPH production – reducing power loss)

**Overall impact:** Photorespiration **reduces photosynthetic efficiency by 25-50%** in C₃ plants under high O₂/low CO₂ conditions.

Photorespiration in C₃ vs C₄ Plants

**C₃ plants (Wheat, Rice, Pea, most crops):**

**Photorespiration is significant** under hot, bright conditions

RuBisCO exposed to atmospheric O₂:CO₂ ratio (≈21:0.04)

Efficiency reduced by photorespiration

**C₄ plants (Sugarcane, Maize, Sorghum):**

**Photorespiration minimal**

Bundle sheath cells have high CO₂ concentration

High CO₂:O₂ ratio favors RuBisCO carboxylase activity

Photorespiration virtually suppressed

**Evolutionary significance:** C₄ pathway likely evolved as adaptation to reduce photorespiration losses in hot, bright environments.

Location of Photorespiratory Enzymes

**Chloroplast:** RuBisCO (oxygenase), initial steps

**Peroxisome:** Glycolate oxidation

**Mitochondrion:** Glycine and serine metabolism

**Chloroplast again:** Final steps, regeneration of 3-PG

**Significance of multiple compartments:** Requires coordinated transport of metabolites between organelles.

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11.10 FACTORS AFFECTING PHOTOSYNTHESIS

Introduction

The rate of photosynthesis is not constant but varies with environmental and internal factors. Understanding these factors is crucial for optimizing crop production and understanding plant ecology.

1. Light Intensity

**Definition:** Amount of light energy per unit area per unit time (measured in lux or photons m⁻² s⁻¹).

**Effects on photosynthesis rate:**

**At low light intensity:**

Photosynthesis rate is **directly proportional** to light intensity

**Light is limiting factor**

Rate increases linearly with increasing light

**At saturation point (optimum light):**

Photosynthesis rate reaches **plateau**

Further increase in light does **NOT increase rate**

Light no longer limiting; another factor becomes limiting

**Compensation point:** Light intensity where photosynthesis rate equals respiration rate (net photosynthesis = 0)

**Saturation point:** Higher light intensity where further increase doesn't increase photosynthesis

**Graph characteristics:**

X-axis: Light intensity

Y-axis: Rate of photosynthesis

Shape: **Hyperbolic curve** – steep rise then plateau

**Light saturation varies with plant type:**

**Sun plants** (heliophytes): Require high light intensity for saturation (e.g., sugarcane)

**Shade plants** (sciophytes): Saturate at low light intensity (e.g., forest understorey plants)

2. Carbon Dioxide Concentration

**Definition:** Percentage or ppm of CO₂ in atmosphere (currently ≈415 ppm, 0.04%).

**Effects on photosynthesis rate:**

**At low CO₂ concentration:**

Rate **directly proportional** to CO₂ concentration

**CO₂ is limiting factor**

RuBisCO enzyme not saturated with substrate

**At optimum/saturation CO₂:**

**Rate plateaus** (around 800-1000 ppm for most plants)

Further increase in CO₂ does NOT increase rate significantly

Another factor becomes limiting

Some plants show **slight decrease** at very high CO₂

**Limiting factor principle:**

Initially rate increases with CO₂ (light and enzymes sufficient)

At saturation, light or temperature becomes limiting

**Rate determined by least available factor** (Liebig's law of minimum)

**Relationship to photosynthesis compensation point:**

CO₂ concentration below which photosynthesis = respiration

Different for different plants (C₃ vs C₄)

**Practical applications:**

**Greenhouses:** Artificially increase CO₂ to 600-1000 ppm to increase crop yield

**Historical significance:** Atmospheric CO₂ increase from 280 ppm (pre-industrial) to 415+ ppm (present) has increased photosynthesis but also caused climate change

3. Temperature

**Definition:** Degree of thermal agitation of molecules; affects enzyme activity.

**Effects on photosynthesis rate:**

**Enzyme activity relationship:**

Both light and dark reactions involve **enzymes**

Enzyme activity increases with temperature (up to optimal point)

**Q₁₀ value** (rate increase per 10°C rise) typically 2-3 for enzymatic reactions

**Temperature optimum:**

**Most plants:** 25-35°C for photosynthesis rate

**Varies with habitat:**

Temperate plants: Optimum ≈ 20-25°C

Tropical plants: Optimum ≈ 30-40°C

Alpine plants: Optimum ≈ 15°C

**Below optimum:**

Enzyme activity increases with temperature

Rate of photosynthesis increases

**At optimum:**

Maximum enzyme activity

Maximum photosynthesis rate

**Above optimum:**

**Enzyme denaturation** occurs

**Chlorophyll destruction** (photoinhibition)

**Stomata closure** to prevent water loss

Photosynthesis rate **drops sharply**

Enzyme irreversibly damaged above ~50°C

**Dark vs Light reactions response:**

**Light reactions:** Temperature increase has modest effect (limited by quantum requirements)

**Dark reactions (Calvin cycle):** **More temperature-sensitive** (enzyme-dependent)

Above optimum, dark reactions inhibited first

**Graph characteristics:**

Typically **bell-shaped curve**