Rubisco in C3, C4, and CAM Photosynthesis (VCE SSCE Biology): Revision Notes

Rubisco in C3, C4, and CAM Photosynthesis

Introduction to Rubisco

Rubisco (ribulose bisphosphate carboxylase-oxygenase) is "a pivotal enzyme involved in initial carbon fixation during the light-independent stage of photosynthesis." This enzyme plays a crucial role in converting atmospheric carbon dioxide into organic compounds that plants can use. However, Rubisco has a significant flaw that affects photosynthetic efficiency, particularly under certain environmental conditions.

Photosynthesis stages review

Before exploring Rubisco's role, it's important to understand the two stages of photosynthesis:

Light-dependent stage: "the first stage of photosynthesis, where light energy splits water molecules into oxygen and hydrogen inside the thylakoid membranes." This stage produces ATP, NADPH, and oxygen.

Light-independent stage: "the second stage of photosynthesis where carbon dioxide is used to form glucose in the stroma of a chloroplast. Also known as the Calvin cycle, the dark stage, or the light-independent reactions." This stage uses the ATP and NADPH from the light-dependent stage to convert $\text{CO}_2$ into glucose.

Rubisco's role in the Calvin cycle

Rubisco controls the first reaction in the light-independent stage of photosynthesis. It catalyses the initial step of carbon fixation, where inorganic carbon dioxide is incorporated into organic molecules.

The Calvin cycle can be understood through three main stages:

1. Carbon fixation

Carbon fixation is "the process in living organisms where inorganic carbon, typically within carbon dioxide, is converted into organic compounds such as glucose."

Rubisco catalyses the reaction between $\text{CO}_2$ and a five-carbon molecule called RuBP (ribulose bisphosphate) to produce six molecules of a three-carbon compound called 3-PGA (3-phosphoglycerate). The term "inorganic" refers to "a compound that does not contain a carbon-hydrogen bond, e.g. carbon dioxide," whilst "organic" describes "a compound containing a carbon-hydrogen bond, e.g. glucose."

2. Reduction

The 3-PGA molecules are converted into different three-carbon molecules called G3P (glyceraldehyde-3-phosphate) using ATP and NADPH from the light-dependent reactions. One G3P molecule exits the cycle to contribute to glucose production, whilst the rest continue in the cycle.

3. Regeneration

The remaining G3P molecules are recycled using ATP to regenerate the RuBP needed to restart the cycle.

The problem with Rubisco: photorespiration

Whilst Rubisco typically performs its role effectively, it has a critical flaw. Sometimes, instead of binding to $\text{CO}_2$, Rubisco binds to oxygen ($\text{O}_2$). When this occurs, a wasteful process called photorespiration takes place.

Photorespiration is "a wasteful process in plants initiated by Rubisco that limits photosynthesis."

When Rubisco binds to $\text{O}_2$ instead of $\text{CO}_2$:

• The Calvin cycle cannot proceed
• No glucose is produced
• Energy is wasted in the photorespiration pathway
• Plant growth, survival, and reproduction are negatively affected

Factors affecting Rubisco binding

Two key factors determine whether Rubisco binds $\text{CO}_2$ or $\text{O}_2$:

Substrate concentration

A substrate is "the reactant of a reaction catalysed by an enzyme." The higher the concentration of a substrate, the more likely it will bind to an enzyme. Plants need:

• High $\text{CO}_2$ concentration around Rubisco (to promote photosynthesis)
• Low $\text{O}_2$ concentration around Rubisco (to prevent photorespiration)

Stomata (singular: stoma) are "small pores on the leaf's surface that open and close to regulate gas exchange." When stomata open, $\text{CO}_2$ enters whilst $\text{O}_2$ and water vapour exit. However, when plants close their stomata to conserve water, $\text{O}_2$ from the light-dependent stage accumulates inside cells, increasing photorespiration.

Temperature

Affinity is "the tendency of a molecule/atom to bind or react with another molecule/atom."

At normal or low temperatures, Rubisco's affinity for $\text{CO}_2$ is much greater than for $\text{O}_2$. However, as temperature increases, the bonds holding Rubisco together weaken, changing the enzyme's three-dimensional shape. This structural change causes Rubisco to have greater affinity for $\text{O}_2$, leading to more photorespiration.

When photorespiration increases

Photorespiration becomes more problematic in hot and dry conditions because:

1. Increased water loss causes stomata to close
2. Closed stomata trap $\text{O}_2$ from the light-dependent reactions inside leaves
3. $\text{O}_2$ concentration increases around Rubisco
4. Higher temperatures increase Rubisco's affinity for $\text{O}_2$
5. More $\text{O}_2$ is bound instead of $\text{CO}_2$, leading to increased photorespiration

To counter this problem, certain plants have evolved adaptations to increase the likelihood of Rubisco binding $\text{CO}_2$ rather than $\text{O}_2$. These adaptations classify plants into three groups: C3, C4, and CAM plants.

C3 plants

C3 plants are "plants with no evolved adaptation to minimise photorespiration."

C3 plants represent approximately 85% of plant species on Earth and undertake "standard" photosynthesis without any special adaptations to reduce photorespiration. The name "C3" comes from the three-carbon compound (3-PGA) produced during the initial carbon fixation step.

Characteristics of C3 plants

• All photosynthesis occurs within mesophyll cells (plant cells "found in leaves that contain large amounts of chloroplasts")
• No separation of carbon fixation steps
• Susceptible to photorespiration, especially in hot and dry conditions
• Most efficient in moderate, cool, or wet environments

C4 plants

C4 plants are "plants that minimise photorespiration by separating initial carbon fixation and the remainder of the Calvin cycle over space."

C4 plants have evolved a spatial adaptation where different steps of photosynthesis occur in different cell types. This separation helps concentrate $\text{CO}_2$ around Rubisco, reducing photorespiration.

How C4 photosynthesis works

C4 plants separate the photosynthetic process between two cell types:

1. In mesophyll cells (initial carbon fixation):

• Atmospheric $\text{CO}_2$ enters mesophyll cells
• An enzyme called PEP carboxylase (which cannot bind $\text{O}_2$) fixes $\text{CO}_2$ to a three-carbon molecule (PEP)
• This produces a four-carbon molecule called oxaloacetate (hence "C4")
• Oxaloacetate converts to malate, another four-carbon molecule
• Malate is transported to bundle-sheath cells

2. In bundle-sheath cells (remainder of Calvin cycle):

• Bundle-sheath cells are "plant cell types that are the site of most of the Calvin cycle in C4 plants"
• Malate breaks down, releasing $\text{CO}_2$
• The released $\text{CO}_2$ enters the Calvin cycle with Rubisco
• Glucose is produced
• Pyruvate (formed from malate breakdown) returns to mesophyll cells
• ATP converts pyruvate back to PEP, ready to fix more $\text{CO}_2$

Advantages and costs of C4 photosynthesis

Advantages:

• Mesophyll cells continuously pump $\text{CO}_2$ (as malate) to bundle-sheath cells
• High $\text{CO}_2$ concentration around Rubisco minimises photorespiration
• Maximises photosynthesis in hot, sunny conditions

Cost:

• Requires extra ATP to convert pyruvate to PEP
• Uses more energy than C3 photosynthesis
• However, in hot environments, the benefits outweigh the energy cost

CAM plants

CAM plants are "plants that minimise photorespiration by separating initial carbon fixation and the remainder of the Calvin cycle over time."

CAM stands for Crassulacean Acid Metabolism, named after the plant family where this pathway was first discovered. Instead of separating processes spatially like C4 plants, CAM plants separate them temporally (between night and day).

How CAM photosynthesis works

CAM plants separate photosynthetic steps between night and day within the same cells:

Night-time processes:

• Stomata open to allow $\text{CO}_2$ to enter (when it's cooler and more humid)
• PEP carboxylase fixes $\text{CO}_2$ to PEP, forming oxaloacetate (four-carbon molecule)
• Oxaloacetate converts to malate
• Malate is stored in vacuoles within mesophyll cells until daytime

Daytime processes:

• Stomata close to prevent water loss (critical in hot, dry environments)
• Malate exits vacuoles and breaks down, releasing $\text{CO}_2$
• Released $\text{CO}_2$ enters the Calvin cycle with Rubisco
• Glucose is produced
• Pyruvate forms and is converted back to PEP using ATP

Advantages of CAM photosynthesis

Water conservation:

• Stomata only open at night when conditions are cooler and more humid
• Dramatically reduces water loss
• Critical advantage in desert environments

Reduced photorespiration:

• Controlled release of $\text{CO}_2$ from vacuoles maintains high $\text{CO}_2$ concentration around Rubisco
• Minimises photorespiration despite closed stomata during the day

Cost:

• Like C4 plants, requires extra ATP to regenerate PEP
• Uses more energy than C3 photosynthesis

Comparison of C3, C4, and CAM plants

Feature	C3 plants	C4 plants	CAM plants
Limits photorespiration	No	Yes	Yes
Separation of $\text{CO}_2$ fixation and Calvin cycle	No separation	Between cells (spatial)	Between night and day (temporal)
Stomata open	During day	During day	At night
Advantages	Doesn't consume extra energy	Minimises photorespiration	Minimises photorespiration and reduces water loss
Disadvantages	Susceptible to photorespiration	Consumes extra energy	Consumes extra energy
Best adapted to	Moderate, cool, or wet environments	Hot, sunny habitats	Very hot, dry habitats
Examples	Most plants, wheat, rice, all trees	Corn, sugarcane, switchgrass	Cacti, pineapples, orchids

Key differences explained

C3 plants perform all photosynthesis steps in one location with no special adaptations, making them vulnerable to photorespiration but energy-efficient when conditions are favourable.

C4 plants spatially separate initial carbon fixation (mesophyll cells) from the Calvin cycle (bundle-sheath cells), pumping concentrated $\text{CO}_2$ to Rubisco and reducing photorespiration in hot, sunny conditions.

CAM plants temporally separate processes, fixing $\text{CO}_2$ at night when stomata are open and completing the Calvin cycle during the day when stomata are closed, making them extremely water-efficient.