VCE Biology Photosynthesis / Cellular Respiration

Photosynthesis

Photosynthesis is the process of converting sunlight energy into glucose (carbohydrate). 

• Overall Inputs: carbon dioxide, water.

• Overall Outputs: glucose (either used immediately by cellular respiration, or stored as starch), oxygen, water.

It is carried out by green plants, some bacteria (with chlorophyll) and some protists (with chlorophyll).

Autotrophs (Photoautotrophs and Chemoautotrophs)

Use photosynthesis to make their food (eg plants, algae, bacteria).

• Photoautotrophs: use sunlight energy for photosynthesis (eg, plants).

• Chemoautotrophs: use chemical energy for photosynthesis (eg, bacteria that use sulfide or methane).

Coenzymes

Coenzymes are organic, non-protein molecules that help enzymes during reactions. They bind to the enzyme's active site, donate energy or molecules, and are then reused. Coenzymes aren’t used up in the reaction—they’re recycled and can assist in many enzymatic reactions.

Photosynthesis Coenzymes

1. ATP

2. NADP+

ATP

ATP (adenosine triphosphate) is the main energy-carrying coenzyme in cells.

• It provides energy for reactions by losing a phosphate group, becoming ADP (adenosine diphosphate). 

• ADP can regain a phosphate to become ATP again. This repeated cycle of ATP to ADP and back is called coenzyme cycling, where ATP is "loaded" with energy and ADP is "unloaded."

•  Each ATP molecule can go through this cycle over 1,000 times a day.

NADP+

• It is unloaded until it picks up a hydrogen, becoming NADPH (loaded).

• NADPH can then donate the hydrogen, releasing a small amount of energy and turning back into NADP⁺.

• This cycle helps drive cellular reactions.

Mesophyll Cells

Mesophyll cells, which contain chloroplasts (the site of

photosynthesis), are divided into palisade and spongy layers.

Stomata

Stomata on the leaf surface allow carbon dioxide to enter and can close to reduce water loss. Water is absorbed by root hair cells and transported via the xylem to mesophyll cells for photosynthesis.

Chloroplast Structure

1. Inner and outer membrane

2. Stroma

3. Thylakoid

4. Thylakoid membrane

5. Grana

6. Lumen

7. Nucleoid

8. Ribosomes

9. Starch granules

Stroma

 Enzyme-rich fluid; site of light-independent reaction

Thylakoid

Membrane-bound disc with inner space called lumen

Thylakoid Membrane

Phospholipid bilayer with embedded chlorophyll, proteins, and pigments.

Grana

Stacks of Thylakoid (singular: granum)

Lumen

Space inside the thylakoid

Nucleoid

Circula DNA ring

Ribosomes

Site of protein synthesis

Starch granules

Storage form of glucose in plants.

Light Dependent Stage

Location: Occurs in the thylakoid membranes within the grana of chloroplasts

Requires: Light energy (sunlight) – cannot occur in darkness
Main purpose: To produce ATP and NADPH, which carry energy and hydrogen for the light-independent stage.

Light Dependent Stage Inputs

• Water (H₂O) – absorbed by roots and transported to leaves via xylem

• NADP⁺ – an unloaded coenzyme that will carry hydrogen

• ADP + Pi – to be recharged into ATP

• Light energy – absorbed by chlorophyll in thylakoid membranes

Light Dependent Stage Outputs

• Oxygen (O₂) – waste product from water splitting; diffuses out through stomata or used in cellular respiration

• NADPH – loaded coenzyme that carries hydrogen to the next stage

• ATP – energy-carrying molecule used in the next stage

Light Dependent Stage Steps

1. Light energy is absorbed by chlorophyll and other pigments in the thylakoid membrane

2. This energy is used to split water molecules (H₂O) into hydrogen (H⁺) and oxygen (O₂) – a process called photolysis

3. Oxygen is released as a by-product and exits the leaf or is used in aerobic respiration

4. Hydrogen ions (H⁺) are picked up by NADP⁺ to form NADPH

5. ADP + Pi are converted into ATP using energy from the light reactions (via ATP synthase)

6. Both NADPH and ATP move into the stroma for use in the light-independent stage

Light Independent Stage (Calvin Cycle)

Location: Occurs in the stroma of the chloroplast

Catalysed by multiple enzymes, including rubisco for carbon fixation
Main purpose: To use CO₂, NADPH, and ATP to produce glucose

Light Independent Stage (Calvin Cycle) Inputs

• Carbon dioxide (CO₂) – enters via stomata

• NADPH – brings hydrogen atoms and electrons

• ATP – provides energy for reactions

Light Independent Stage (Calvin Cycle) Outputs

• Glucose (C₆H₁₂O₆) – energy storage molecule

• Water (H₂O) – by-product

• NADP⁺ – returns to light-dependent stage

• ADP + Pi – also returns to light-dependent stage

Calvin Cycle Carbon Fixation

• CO₂ (6 molecules) enters the cycle

• Enzyme rubisco fixes carbon by attaching CO₂ to a 5-carbon molecule (RuBP)

• This unstable 6-carbon molecule splits into two 3-carbon molecules (3-PGA)

Calvin Cycle Carbon Reduction

• NADPH donates H⁺ ions and electrons → becomes NADP⁺

• ATP breaks into ADP + Pi, releasing energy

• This converts 3-PGA into PGAL (G3P) – a 3-carbon sugar

• One PGAL molecule exits the cycle to help form glucose

• Six CO₂ molecules are needed to make one glucose

Calvin Cycle Carbon Regeneration

• The remaining PGALs are used to regenerate RuBP

• ATP is required for this step

• This allows the cycle to continue

• Some oxygen atoms, left over from CO₂, combine with H⁺ from NADPH to form H₂O

Rubisco

• Quaternary structure enzyme composed of 8 large and 8 small polypeptide chains.

• Large chains contain the active site where carbon dioxide and RuBP bind and react.

• Small chains contribute to the activity and stability of the enzyme.

• Responsible for the initial changes to carbon dioxide in the Calvin cycle (the carbon fixation stage).

• Carbon fixation: Bonding of CO2 and a five-carbon molecule (RuBP) into 2 X 3-PGA.

• Converts CO2 into an organic compound.

Photorespiration

Rubisco is an inefficient enzyme that sometimes binds to oxygen instead of carbon dioxide, triggering a wasteful process called photorespiration. When Rubisco binds to O₂ instead of CO₂, photosynthesis is disrupted, reducing carbon fixation and glucose production. This wastes energy and negatively impacts the plant's growth, survival, and reproduction.

The likelihood of Rubisco binding to CO₂ or O₂ is influenced by:

1. Temperature

2. Substrate concentration

C3 Plants

Examples: Wheat, rice, barley, potato, most trees, common flowering plants.

Photosynthesis: Both light-dependent and independent stages occur in the mesophyll cells of the chloroplasts.

Habitat: Cool, temperate, and moist conditions.

Advantages: Efficient photosynthesis in cool, moist conditions with high CO₂.

Disadvantages: Inefficient in warm, dry conditions due to photorespiration (Rubisco binds to O₂ instead of CO₂).

Photorespiration: No adaptations to minimize it.

C4 Plants

Examples: Maize (corn), sugarcane, millet, native grasses.

Photosynthesis: Light-dependent stage occurs in mesophyll cells; CO₂ is fixed into a C4 molecule. The C4 molecule is transported to bundle sheath cells where the Calvin cycle occurs.

Adaptation: Physical separation of carbon fixation and the Calvin cycle. This ensures Rubisco is surrounded by CO₂, minimizing photorespiration.

Habitat: Warm, tropical, and humid conditions. Stomata close during the hottest part of the day.

Advantages: Efficient in hot, dry conditions with low CO₂ and reduced photorespiration.

Disadvantages: Requires more ATP to produce glucose.

C4 Carbon Fixation: Two fixation events—one in mesophyll cells (PEP carboxylase) and one in bundle sheath cells (Rubisco).

Evolutionary Advantage: C4 plants are more water-efficient and better adapted to high temperatures and CO₂-limited environments.

CAM Plants

Examples: Cacti, pineapple, agave, orchids.

Photosynthesis:

• Night: Stomata open, CO₂ enters and is fixed into C4 molecules (malate) by the enzyme PEP carboxylase. The malate is stored in vacuoles.

• Day: Stomata close to conserve water. Malate is converted back to CO₂, increasing CO₂ concentration around Rubisco, minimizing photorespiration, and enabling the Calvin cycle.

Adaptation: Temporal separation of carbon fixation (night) and the Calvin cycle (day) to reduce photorespiration and conserve water.

Habitat: Arid, desert environments.

• Advantages:

• Survives in very dry conditions.

• Minimizes water loss.

Disadvantages:

• Slower growth.

• Requires more energy to store CO₂ and convert malate back to CO₂.

Energy Use: Requires ATP to convert pyruvate to PEP, so more energy is used compared to C3 plants.

Water Conservation: Stomata only open at night when it's cooler, reducing water loss.

Photosynthesis Process: The light-dependent stage is similar to C3 and C4 plants, but CAM plants separate carbon fixation and the Calvin cycle over time rather than spatially across different cells.

C3, C4, CAM Table

Differences between C3, C4, CAM

Increased Light Availability Affecting the Rate of Photosynthesis

• Increased light availability raises the rate of photosynthesis.

• Light energizes chlorophyll and splits water in the light-dependent stage.

• The rate increases until a plateau is reached.

• At the plateau, another factor (e.g. CO₂ or temperature) becomes limiting.

• Enzymes become saturated at high light intensity, preventing further rate increase.

Decreased Light Availability Affecting the Rate of Photosynthesis

• Decreases rate of photosynthesis. 

• Less light absorbed, less water is split, less light dependent stage

• A limiting factor could be the amount of chlorophyll molecules that are absorbing the light.

Wavelength Affecting the Rate of Photosynthesis

• Chlorophyll absorbs red and blue wavelengths most effectively.

• Green light is mostly reflected, not absorbed, resulting in low photosynthetic rates under green light.

• Plants appear green because chlorophyll reflects green light.

• Chlorophyll is the main photosynthetic pigment.

• Other pigments (e.g. carotenoids) absorb additional wavelengths, broadening the usable light spectrum for photosynthesis.

High Temperature Affecting the Rate of Photosynthesis

If temp is above the optimal temperature of many photosynthetic enzymes, the enzymes may be denatured, affecting their active site and photosynthesis rate decreases. The rate of photorespiration also increases, because RuBisCo’s affinity for O2 increases. 

Low Temperature Affecting the Rate of Photosynthesis

At lower temperatures, the rate of photosynthesis slows. It is because molecules such as CO₂ and H₂O move slower. They have low kinetic energy.

Less enzyme-substrate collisions so photosynthesis reaction decreases.

High pH Affecting the Rate of Photosynthesis

Alters enzyme 3D structure and active site shape, reducing photosynthetic rate. Decreases CO₂ solubility in water; more CO₂ becomes bicarbonate (HCO₃⁻), less usable for some aquatic plants.

Low pH Affecting the Rate of Photosynthesis

Can denature or slow down enzyme activity. Disrupts chloroplast function and limits CO₂ fixation. More CO₂ is available in aquatic plants due to increased solubility at low pH.

C02 Levels Affecting the Rate of Photosynthesis

• Plants take in CO₂ via open stomata for photosynthesis.

• Closed stomata or low atmospheric CO₂ reduce the rate of photosynthesis.

• When CO₂ is abundant, more binds to RuBisCo → more Calvin cycle activity → more glucose produced.

• When CO₂ is low, less binds to RuBisCo → less Calvin cycle activity → reduced glucose production.

• Photosynthesis rate plateaus when enzymes are saturated or another factor (light, chlorophyll, temperature) becomes limiting.

CO2 Levels in C3 vs C4 vs CAM plants:

• C3 plants: No mechanism to reduce photorespiration; more affected by low CO₂.

• C4 and CAM plants: Evolved strategies to concentrate CO₂ around RuBisCo, minimizing photorespiration.

• Therefore, C4 and CAM plants are less sensitive to low CO₂ levels than C3 plants.

Water Availability Affecting the Rate of Photosynthesis

• Water enters through roots and water vapour exits through stomata, allowing CO₂ in and O₂ out.

• Low water availability causes stomata to close, reducing CO₂ uptake and slowing photosynthesis.

• When stomata are closed, CO₂ cannot enter for the Calvin cycle, and O₂ accumulates.

• Higher O₂ levels cause RuBisCo to bind to O₂ (photorespiration), reducing photosynthesis.

• Increasing water boosts photosynthesis until another factor becomes limiting.

• C4 and CAM plants are less affected by water shortage (unless extreme) due to water-conserving adaptations, unlike C3 plants.

Enzyme Inhibition Affecting the Rate of Photosynthesis

• Enzyme inhibitors slow photosynthesis by reducing enzyme activity.

• Competitive inhibitors block the active site; can be overcome by more substrate.

• Non-competitive inhibitors change enzyme shape; can't be reversed by more substrate.

• Many herbicides inhibit enzymes in light-dependent reactions, stopping photosynthesis.

• Binding disrupts electron transport or other key steps, halting photosynthesis and leading to plant death.

Purpose of CRISPR In Agriculture

• By 2050, global food demand is expected to double, yet arable land is nearly exhausted.

• Climate change worsens crop productivity, particularly affecting C3 plants, which undergo more photorespiration in warm climates.

• Increasing yields on the same land area without deforestation (which raises greenhouse gas emissions) is needed.

CRISPR-CAS9 As a Solution for the Future of Agriculture

CRISPR-Cas9 gene editing technology can help:

1. Engineer C3 plants to mimic more efficient C4 and CAM photosynthesis pathways.

2. Edit Rubisco genes, a key enzyme in carbon fixation, to reduce photorespiration and improve photosynthetic efficiency.

3. Modify chloroplasts for better energy capture and stomata for enhanced water use efficiency.

Applications and Benefits of CRISPR in Agriculture

• Stronger Crops: Tolerance to heat, cold, herbicides, and diseases.

• Better Yields: Larger, tastier, more nutritious, and longer-lasting produce.

• Lower Inputs: Needs less fertilizer, with improved nutrient uptake.

• Nutrient Boost: Enhanced vitamin/mineral content (biofortification).

• Climate Resilience: Drought and extreme weather tolerance.

• Less Waste: Longer shelf life reduces spoilage.

• New Products: Crops with novel traits (e.g., high-GABA tomatoes).

• No Foreign DNA: Edits without introducing external genes.

• Efficient & Affordable: Faster, more precise, and cost-effective than traditional breeding.