Question 1

Q1. By looking at a plant do you tell whether it is a C3 or C4 plant? How?

Accepted Answer

No, it is generally not possible to definitively tell whether a plant is a C3 or C4 plant by merely looking at its external morphology (gross features). While some C4 plants might have certain adaptations like thicker leaves or different growth habits, these are not reliable indicators for all species.

The distinction between C3 and C4 plants primarily lies in their internal leaf anatomy and biochemical pathways of carbon fixation.

**How to differentiate:**
To distinguish between C3 and C4 plants, one needs to examine their internal leaf anatomy, specifically looking for:

1.  **Kranz Anatomy:** C4 plants exhibit a specialized leaf anatomy called 'Kranz anatomy' (Kranz means 'wreath' in German). This involves:
    *   **Bundle Sheath Cells:** A prominent ring of large, thick-walled bundle sheath cells surrounds the vascular bundles. These cells are rich in chloroplasts (often agranal) and store starch.
    *   **Mesophyll Cells:** The mesophyll cells are arranged in a concentric layer around the bundle sheath cells, forming a 'wreath-like' structure. They have smaller chloroplasts (granal).
    *   **Reduced Intercellular Spaces:** C4 leaves generally have fewer intercellular spaces compared to C3 leaves.

2.  **Absence of Kranz Anatomy:** C3 plants, on the other hand, do not possess Kranz anatomy. Their mesophyll cells are not differentiated into distinct layers around vascular bundles, and bundle sheath cells, if present, are not prominent or specialized for photosynthesis in the same way as in C4 plants.

Therefore, a microscopic examination of a leaf cross-section is required to identify the characteristic Kranz anatomy of C4 plants.

Question 2

Q2. A plant in your garden is a C3 plant. Which pathways would you consider to be crucial to improve its yield?

Accepted Answer

For a C3 plant, the primary limitation to its yield, especially in hot and dry conditions, is the process of photorespiration. Therefore, to improve the yield of a C3 plant, the crucial pathways and processes to focus on would be those that minimize photorespiration and maximize the efficiency of carbon dioxide fixation.

Here are the key pathways/aspects to consider:

1.  **Minimizing Photorespiration:**
    *   **The Problem:** In C3 plants, the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) has an affinity for both CO2 and O2. When O2 concentration is high relative to CO2, RuBisCO acts as an oxygenase, leading to photorespiration. This process consumes ATP and NADPH, releases CO2, and does not produce sugar, thus reducing photosynthetic efficiency.
    *   **Improvement Strategy:** Genetic engineering or breeding efforts could focus on modifying RuBisCO to increase its affinity for CO2 and decrease its affinity for O2, especially at higher temperatures. Alternatively, mechanisms to increase CO2 concentration around RuBisCO within the chloroplasts could be explored.

2.  **Enhancing Calvin Cycle (C3 Cycle) Efficiency:**
    *   **The Problem:** The Calvin cycle is the main pathway for sugar synthesis. Its efficiency is directly linked to the amount of CO2 available and the activity of its enzymes.
    *   **Improvement Strategy:** Increasing the activity or quantity of key enzymes of the Calvin cycle, such as RuBisCO, ATP synthase, and NADP reductase, could enhance the rate of CO2 fixation and subsequent sugar production. Ensuring optimal conditions for these enzymes (e.g., temperature, pH) is also important.

3.  **Optimizing Light-Dependent Reactions:**
    *   **The Problem:** The Calvin cycle requires ATP and NADPH produced during the light-dependent reactions. Inefficient light harvesting or electron transport can limit the supply of these energy carriers.
    *   **Improvement Strategy:** Improving the efficiency of light absorption by chlorophylls and carotenoids, enhancing the electron transport chain, and optimizing the production of ATP and NADPH would indirectly boost the Calvin cycle's performance.

4.  **Improving Water Use Efficiency:**
    *   **The Problem:** C3 plants tend to open stomata more widely for CO2 uptake, leading to significant water loss through transpiration. Under water stress, stomata close, limiting CO2 availability and increasing photorespiration.
    *   **Improvement Strategy:** Developing varieties with improved water use efficiency (e.g., better root systems, stomatal control that balances CO2 uptake with water loss) would allow the plant to maintain higher photosynthetic rates even under moderate drought conditions.

In summary, the most crucial pathway to target for yield improvement in a C3 plant is the **reduction of photorespiration** by enhancing the carboxylase activity of RuBisCO relative to its oxygenase activity, thereby maximizing the efficiency of the **Calvin cycle**.

Question 3

Q3. Why is the colour of a leaf kept in the dark frequently yellow, or pale green? Which pigment do you think is more stable?

Accepted Answer

When a leaf is kept in the dark, its color frequently turns yellow or pale green due to the following reasons:

1.  **Chlorophyll Degradation:** Chlorophyll, the primary green pigment responsible for photosynthesis, is a relatively unstable pigment. It requires light for its synthesis and is continuously broken down and resynthesized in the presence of light. When a leaf is kept in the dark, the synthesis of new chlorophyll molecules stops, but the existing chlorophyll continues to degrade. As the green chlorophyll pigments break down, their concentration decreases.

2.  **Unmasking of Accessory Pigments:** Leaves contain other accessory pigments besides chlorophyll, primarily carotenoids (which include carotenes and xanthophylls). These pigments are yellow, orange, or red in color. In a healthy, light-exposed leaf, the abundant green chlorophyll masks the colors of these accessory pigments. However, as chlorophyll degrades in the dark, the yellow and orange colors of the more stable carotenoids become visible, giving the leaf a yellow or pale green appearance.

3.  **Absence of New Chlorophyll Synthesis:** Light is essential for several steps in chlorophyll synthesis. Without light, the metabolic pathways for chlorophyll production are inhibited, preventing the replenishment of the degraded chlorophyll.

**Which pigment is more stable?**

**Carotenoids** are more stable than chlorophyll. This is evident because:

*   They persist in the leaf even after chlorophyll has degraded (as seen in leaves kept in the dark or during autumn senescence).
*   They are less susceptible to photodegradation and play a crucial role in protecting chlorophyll from photo-oxidation by dissipating excess light energy.

Question 4

Q4. Look at leaves of any ‘sun-plant’ and ‘shade-plant’. Can you tell which of these has a higher rate of photosynthesis? Why?

Accepted Answer

By observing the leaves, it is generally possible to infer that a **'sun-plant'** would have a higher potential rate of photosynthesis compared to a 'shade-plant'.

**Why?**

Sun plants and shade plants have evolved distinct anatomical and physiological adaptations to optimize photosynthesis in their respective light environments. These adaptations allow sun plants to maximize light capture and carbon fixation under high light intensities, while shade plants are adapted to efficiently utilize low light.

Here's why sun plants typically have a higher rate of photosynthesis:

1.  **Thicker Leaves and More Layers of Palisade Mesophyll:** Sun plants often have thicker leaves with more layers of palisade mesophyll cells. This allows for greater light absorption and a higher density of chloroplasts per unit leaf area.

2.  **Higher Chlorophyll Content (per unit area):** While shade plants may have more chlorophyll per chloroplast to capture scarce light, sun plants generally have a higher overall chlorophyll content per unit leaf area, enabling them to absorb more light energy.

3.  **More Stomata:** Sun plants typically have a higher density of stomata on their leaf surfaces. This facilitates greater CO2 uptake, which is crucial for maintaining a high rate of photosynthesis under high light conditions.

4.  **Higher Activity of Photosynthetic Enzymes:** Sun plants often have higher concentrations and activities of photosynthetic enzymes, such as RuBisCO and those involved in the electron transport chain. This allows for a faster rate of biochemical reactions in the Calvin cycle.

5.  **Higher Light Saturation Point:** Sun plants are adapted to utilize high light intensities and reach their light saturation point at much higher light levels compared to shade plants. This means they can continue to increase their photosynthetic rate as light intensity increases, up to very high levels.

6.  **Better Developed Vascular System:** A more extensive vascular system in sun plants can efficiently transport water to the photosynthetic cells and sugars away from them, supporting higher metabolic rates.

In contrast, shade plants are adapted to low light conditions. They typically have thinner leaves, fewer stomata, and reach light saturation at much lower light intensities. Their photosynthetic machinery is optimized for efficiency at low light, but they can be damaged by high light (photoinhibition) and cannot achieve the high photosynthetic rates of sun plants.

Question 5

Q5. Figure 13.10 shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions:
(a) At which point/s (A, B or C) in the curve is light a

Accepted Answer

The graph (Figure 13.10) illustrates the relationship between light intensity and the rate of photosynthesis, demonstrating the concept of limiting factors as proposed by Blackman.

**(a) At which point/s (A, B or C) in the curve is light a limiting factor?**

Light is a limiting factor at points **A and B**.

*   **Region A:** In this initial phase, the rate of photosynthesis is directly proportional to the light intensity. As light intensity increases, the rate of photosynthesis increases linearly. This indicates that light is the primary factor restricting the overall rate.
*   **Region B:** In this transitional phase, the rate of photosynthesis continues to increase with increasing light intensity, but the rate of increase begins to slow down. This suggests that while light is still a limiting factor, other factors are starting to become limiting as well, or the plant is approaching its maximum capacity for light utilization.

**(b) What could be the limiting factor/s in region A?**

In region A, light is the primary limiting factor. However, other factors that could potentially be limiting the rate of photosynthesis even in this low light region (or would become limiting if light were not the primary constraint) include:

*   **Carbon dioxide (CO2) concentration:** If CO2 levels are very low, even abundant light cannot drive photosynthesis faster.
*   **Temperature:** Photosynthetic enzymes have optimal temperature ranges. If the temperature is too low or too high, enzyme activity will be reduced.
*   **Water availability:** Severe water stress can lead to stomatal closure, reducing CO2 uptake.
*   **Internal factors:** These include the amount of chlorophyll, the number and activity of photosynthetic enzymes (e.g., RuBisCO), and the overall health and genetic potential of the plant.

**(c) What do C and D represent on the curve?**

*   **C represents the Light Saturation Point:** This is the point where the rate of photosynthesis reaches its maximum and no longer increases with further increases in light intensity. At point C, the photosynthetic machinery is fully saturated with light, meaning all available reaction centers are operating at their maximum capacity.

*   **D represents the region where other factors become limiting:** Beyond point C (including point D), the curve plateaus. This indicates that light is no longer the limiting factor. Instead, other factors such as CO2 concentration, temperature, water availability, or the capacity of the enzymatic reactions of the Calvin cycle are now limiting the rate of photosynthesis. Even if light intensity increases further, the rate of photosynthesis will not increase because one or more of these other factors are holding it back.

Question 6

Q6. Why is the colour of a leaf kept in the dark frequently yellow, or pale green?

Accepted Answer

When a leaf is kept in the dark, it does not receive light, which is essential for photosynthesis. Chlorophyll, the green pigment responsible for absorbing light energy, is highly unstable and requires light for its synthesis. In the absence of light, the existing chlorophyll pigments start to degrade. Since the synthesis of new chlorophyll stops, the green colour fades away. The yellow and pale green colours are due to the unmasking of other accessory pigments like carotenoids (yellow, orange, red pigments) and xanthophylls (yellow pigments), which are more stable and present in the leaf but are normally masked by the abundant green chlorophyll. As chlorophyll degrades, these underlying pigments become visible, giving the leaf a yellow or pale green appearance.

Question 7

Q7. Define the law of limiting factors.

Accepted Answer

The Law of Limiting Factors was proposed by F.F. Blackman in 1905. It states that: "When a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest factor." In simpler terms, if a biochemical process (like photosynthesis) is affected by multiple factors, the rate of the process will be determined by the factor that is in shortest supply or at its minimum optimal level. Increasing the quantity of other factors will not increase the rate of the process until the limiting factor is also increased to a point where it is no longer limiting.

Question 8

Q8. What are the factors that affect photosynthesis?

Accepted Answer

The rate of photosynthesis is influenced by a combination of internal (plant-dependent) and external (environmental) factors. According to Blackman's Law of Limiting Factors, the rate of photosynthesis will be limited by the factor that is in shortest supply.

Question 9

Q9. Why is it that normally C-3 plants respond to CO2 enrichment by showing increased rates of photosynthesis while C-4 plants do not?

Accepted Answer

The differential response of C3 and C4 plants to CO2 enrichment is primarily due to their distinct CO2 fixation pathways, the enzymes involved, and their susceptibility to photorespiration.

Question 10

Q10. What are the adaptations that have evolved in C-4 plants to increase the efficiency of photosynthesis?

Accepted Answer

C4 plants have evolved several significant anatomical and biochemical adaptations that allow them to photosynthesize more efficiently, especially in hot, dry environments with high light intensity. These adaptations help them minimize photorespiration and maximize CO2 fixation.

Question 11

Q11. Why is the colour of a leaf kept in the dark frequently yellow, or pale green?

Accepted Answer

The yellow or pale green colour of a leaf kept in the dark is primarily due to the degradation of chlorophyll and the unmasking of other pigments.

1.  **Chlorophyll Degradation:** Chlorophyll, the primary green pigment responsible for photosynthesis, requires light for its synthesis and stability. When a leaf is kept in the dark, the synthesis of new chlorophyll stops, and the existing chlorophyll molecules begin to break down. This process is called senescence.
2.  **Unmasking of Carotenoids:** Leaves contain other pigments like carotenoids (yellow, orange, red) and xanthophylls (yellow) in addition to chlorophyll. Under normal light conditions, the abundant green chlorophyll masks the colours of these accessory pigments. However, as chlorophyll degrades in the dark, the yellow and orange colours of the carotenoids become visible, giving the leaf a yellow or pale green appearance. If the degradation is not complete, it might appear pale green.

Question 12

Q12. Why is the rate of photosynthesis higher in red light than in green light?

Accepted Answer

The rate of photosynthesis is significantly higher in red light compared to green light due to the differential absorption of these wavelengths by photosynthetic pigments, primarily chlorophylls.

1.  **Absorption Spectrum of Chlorophyll:** Chlorophyll a and chlorophyll b, the main photosynthetic pigments, absorb light most efficiently in the blue-violet and red regions of the visible spectrum. They show very high absorption peaks around 430-470 nm (blue) and 640-670 nm (red).
2.  **Green Light Reflection/Transmission:** Conversely, chlorophylls absorb very little green light (around 500-600 nm). Instead, green light is largely reflected or transmitted by the leaves, which is why leaves appear green to our eyes.
3.  **Energy for Photosynthesis:** Since red light is strongly absorbed by chlorophylls, the energy from red photons is effectively captured and utilized to drive the light-dependent reactions of photosynthesis, leading to a higher rate of ATP and NADPH production. Green light, being poorly absorbed, contributes very little energy to the photosynthetic process, resulting in a much lower rate.

Question 13

Q13. What is the law of limiting factors? Explain it with graphical representation.

Accepted Answer

### Law of Limiting Factors

The Law of Limiting Factors was proposed by F.F. Blackman in 1905. It states that: **"When a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest factor."**

In simpler terms, if multiple factors influence a biochemical process (like photosynthesis), the rate of that process will be determined by the factor that is present in the least optimal amount, even if other factors are present in abundance. Increasing the quantity of the limiting factor will increase the rate of the process, but only until another factor becomes limiting.

**Explanation with Examples:**

Consider photosynthesis, which is influenced by several factors such as light intensity, carbon dioxide concentration, temperature, and water.

1.  **Light Intensity:** If a plant is exposed to low light intensity, the rate of photosynthesis will be low, even if CO2, water, and temperature are optimal. Light is the limiting factor. Increasing light intensity will increase the rate of photosynthesis. However, beyond a certain point, even if light intensity continues to increase, the rate of photosynthesis will plateau because another factor (e.g., CO2 concentration) becomes limiting.
2.  **Carbon Dioxide Concentration:** At low CO2 concentrations, the rate of photosynthesis is limited by the availability of CO2, even under high light and optimal temperature. Increasing CO2 concentration will boost the photosynthetic rate until another factor (like light or temperature) becomes limiting.
3.  **Temperature:** Enzymes involved in photosynthesis (e.g., RuBisCO) have optimal temperature ranges. Below or above this range, their activity decreases, limiting the rate of photosynthesis, even if light and CO2 are abundant.
4.  **Water:** Although water is a reactant in photosynthesis, its direct effect as a limiting factor is usually not on the reaction itself but indirectly through its effect on stomatal opening. Water stress causes stomata to close, reducing CO2 uptake and thus limiting photosynthesis.

### Graphical Representation

To illustrate the law of limiting factors, consider a graph showing the rate of photosynthesis as a function of one factor (e.g., light intensity) while keeping other factors constant.

**Description of the Graph:**

*   **X-axis:** Limiting Factor (e.g., Light Intensity)
*   **Y-axis:** Rate of Photosynthesis

1.  **Phase 1 (Steep Rise):** At low levels of the factor (e.g., low light intensity), the rate of photosynthesis increases almost linearly as the factor increases. In this region, the factor on the X-axis is the primary limiting factor.
2.  **Phase 2 (Plateau):** As the factor continues to increase, the curve eventually flattens out and reaches a plateau. At this point, increasing the factor further does not increase the rate of photosynthesis. This indicates that another factor (e.g., CO2 concentration or temperature) has now become limiting. The rate is now limited by this *new* limiting factor, not the one on the X-axis.
3.  **Effect of Changing a Second Factor:** If we were to repeat the experiment but with a higher concentration of the *second* limiting factor (e.g., higher CO2 concentration), the entire curve would shift upwards. The initial linear rise would be steeper or extend further, and the plateau would be reached at a higher rate of photosynthesis, demonstrating that the overall maximum rate is now higher because the previous limiting factor (CO2) has been alleviated.

This graph clearly shows that the rate of photosynthesis is not solely dependent on one factor but is a complex interplay, with the slowest or least available factor dictating the overall speed.

Question 14

Q14. Compare C3 and C4 pathways.

Accepted Answer

### Comparison of C3 and C4 Pathways

The C3 and C4 pathways are two different mechanisms plants use to fix carbon dioxide during photosynthesis. They represent adaptations to different environmental conditions.

**Summary of Differences:**

*   **Efficiency in Hot Climates:** C4 plants are more efficient in hot, dry, and high-light environments due to their ability to concentrate CO2 in bundle sheath cells, minimizing photorespiration. C3 plants suffer from significant photorespiration under these conditions.
*   **Enzyme Affinity:** PEPcase in C4 plants has a much higher affinity for CO2 than RuBisCO, allowing them to fix CO2 even at very low atmospheric concentrations.
*   **Anatomical Adaptation:** Kranz anatomy in C4 plants provides a spatial separation of initial CO2 fixation (mesophyll) and the Calvin cycle (bundle sheath), which is crucial for their efficiency.

Question 15

Q15. Stomata of desert plants remain closed during day time. How do they take up carbon dioxide for photosynthesis?

Accepted Answer

Desert plants, also known as succulents or CAM (Crassulacean Acid Metabolism) plants, have evolved a unique adaptation to conserve water by keeping their stomata closed during the hot, dry daytime. They take up carbon dioxide for photosynthesis through a specialized mechanism:

1.  **Night-time CO2 Uptake:** During the cooler night hours, when transpiration rates are significantly lower, the stomata of CAM plants open. They absorb atmospheric CO2.
2.  **Initial CO2 Fixation:** The absorbed CO2 is then fixed by the enzyme PEP carboxylase (PEPcase) into a 4-carbon organic acid, typically malic acid, in the cytoplasm of mesophyll cells. This is similar to the initial fixation step in C4 plants.
3.  **Storage of Organic Acids:** The malic acid (or other organic acids) is then stored in large vacuoles within the mesophyll cells.
4.  **Daytime Decarboxylation and Calvin Cycle:** During the day, when light is available for the light-dependent reactions, the stomata remain closed. The stored malic acid is transported out of the vacuole and decarboxylated (broken down) to release CO2. This internally released CO2 is then utilized by RuBisCO in the Calvin cycle (C3 pathway) within the same mesophyll cells to produce sugars.

This temporal separation of CO2 uptake (night) and CO2 fixation in the Calvin cycle (day) allows CAM plants to photosynthesize while minimizing water loss, making them highly adapted to arid environments.

Question 16

Q16. Why is the RuBisCO enzyme the most abundant enzyme in the world?

Accepted Answer

RuBisCO (Ribulose-1,5-bisphosphate Carboxylase/Oxygenase) is indeed considered the most abundant enzyme on Earth. This remarkable abundance can be attributed to several critical factors:

1.  **Central Role in Carbon Fixation:** RuBisCO is the primary enzyme responsible for carbon fixation in the Calvin cycle (C3 cycle), which is the metabolic pathway by which atmospheric carbon dioxide is converted into organic compounds. It catalyzes the initial and most crucial step where CO2 combines with a five-carbon sugar, ribulose-1,5-bisphosphate (RuBP), to form two molecules of 3-phosphoglycerate (3-PGA). This process is fundamental to nearly all life on Earth, as it forms the basis of the food chain.

2.  **Ubiquitous Presence in Photosynthetic Organisms:** RuBisCO is found in the chloroplasts of virtually all photosynthetic organisms, including plants, algae, and cyanobacteria. Since photosynthesis is the process that sustains most ecosystems and life forms, the enzyme at its core must be widely distributed.

3.  **High Concentration within Chloroplasts:** Within the chloroplasts of C3 plants, RuBisCO constitutes a significant proportion of the total soluble protein, often making up 30-50%. This high concentration is partly due to its relatively slow catalytic rate (low turnover number) compared to many other enzymes. To achieve sufficient rates of carbon fixation, plants must produce large quantities of RuBisCO.

4.  **Dual Nature and Inefficiency:** RuBisCO has a dual nature; it can act as both a carboxylase (fixing CO2) and an oxygenase (reacting with O2). The oxygenase activity leads to photorespiration, a wasteful process that reduces photosynthetic efficiency. To compensate for this inefficiency and ensure adequate CO2 fixation, plants produce even more of the enzyme.

5.  **Vast Biomass of Photosynthetic Life:** The sheer volume of photosynthetic biomass globally, from microscopic phytoplankton to vast forests, means that the total amount of RuBisCO required to support this immense biological activity is enormous, making it the most abundant protein on the planet.

Question 17

Q17. What are the characteristics of a C4 plant?

Accepted Answer

C4 plants are a group of plants that have evolved a specialized photosynthetic pathway to efficiently fix carbon dioxide, particularly in hot, dry environments with high light intensity. They exhibit several distinct characteristics that differentiate them from C3 plants:

### 1. Kranz Anatomy
This is the most distinctive anatomical feature of C4 plants. 'Kranz' is a German word meaning 'wreath', referring to the arrangement of cells. It involves:
*   **Bundle Sheath Cells:** These are large, thick-walled cells that form a wreath-like layer around the vascular bundles. They are characterized by having a large number of chloroplasts (often agranal or with reduced grana), thick, impermeable walls (to prevent gaseous exchange), and lack intercellular spaces.
*   **Mesophyll Cells:** These cells are loosely arranged around the bundle sheath cells. They contain chloroplasts with well-developed grana and have intercellular spaces.

### 2. Dual Carboxylation System (Two-Step Carbon Fixation)
C4 plants employ two distinct carboxylation steps, occurring in different cell types:
*   **Primary CO2 Fixation (in Mesophyll Cells):** Atmospheric CO2 is first accepted by Phosphoenolpyruvate (PEP), a 3-carbon compound, in the mesophyll cells. This reaction is catalyzed by the enzyme PEP carboxylase (PEPCase), which has a high affinity for CO2 and does not bind O2. The first stable product formed is a 4-carbon compound, Oxaloacetic Acid (OAA).
*   **Secondary CO2 Fixation (in Bundle Sheath Cells):** OAA is converted into other 4-carbon compounds (like malate or aspartate) and transported to the bundle sheath cells. Here, these 4-carbon compounds are decarboxylated, releasing CO2. This CO2 is then fixed by RuBisCO into the Calvin cycle (C3 cycle), which operates within the bundle sheath cells.

### 3. Absence or Minimization of Photorespiration
Due to the efficient CO2 pump mechanism (PEP carboxylase) in mesophyll cells, a high concentration of CO2 is delivered to the bundle sheath cells, where RuBisCO is located. This high CO2:O2 ratio around RuBisCO effectively suppresses its oxygenase activity, thereby significantly reducing or eliminating the wasteful process of photorespiration.

### 4. High Photosynthetic Efficiency
C4 plants exhibit higher photosynthetic rates compared to C3 plants, especially under conditions of high temperature, high light intensity, and low atmospheric CO2 concentration. This is primarily because they avoid the energy-wasting process of photorespiration.

### 5. Better Water Use Efficiency
C4 plants can achieve higher rates of photosynthesis with less water loss through transpiration compared to C3 plants. Their efficient CO2 fixation allows them to keep their stomata closed for longer periods, conserving water, while still maintaining a high internal CO2 concentration for photosynthesis.

### 6. High Optimum Temperature
The enzymes involved in the C4 pathway, particularly PEP carboxylase, are adapted to function optimally at higher temperatures, making C4 plants well-suited for tropical and subtropical regions.

### Examples
Common examples of C4 plants include maize (corn), sugarcane, sorghum, millet, and many tropical grasses.

Chapter 13: Photosynthesis in Higher Plants