Photosynthesis in C3, C4 and CAM plants

Different groups of plants differ in the way they introduce CO2 into photosynthesis .  We can group these into C3, C4 and CAM plants.

All of these plants, however, require the enzymeRuBisCO for photosynthesis to occur.  RuBisCO is a crucial enzyme in photosynthesis, found in the Calvin cycle.  RuBisCO is involved in carbon fixation and catalyses the reaction between RuBP and carbon dioxide (CO₂).

C3, C4 and CAM plants all undertake photosynthesis, but have adapted characteristics to undergo photosynthesis that suit different environments.

Use this page to revise the following concepts of photosynthesis in C3, C4 and CAM plants:

RuBisCO

RuBisCO is a crucial enzyme in the process of photosynthesis .

RuBisCO is one of the most abundant proteins on Earth, as it is found in all plants, algae, and some bacteria. This reflects its vital role in the global carbon cycle.

It plays a central role in the Calvin cycle, which is responsible for fixing carbon dioxide (CO₂) from the atmosphere into organic molecules, and transforming it into organic compounds, like glucose.

An image of the Calvin cycle. In the top left-hand corner there is an image of chloroplast indicating carbon dioxide enters the chloroplast, and then within the chloroplast the Calvin cycle occurs, and then sugar leaves the chloroplast. In the rest of the image the Calvin cycle is represented in more detail starting with the first stage, carbon fixation, moving to the second stage, reduction, and finishing with the third stage, regeneration of RuBP

Photosynthesis in C3 plants

C3 plants make up around 95% of all plant species on the planet. They photosynthesise by capturing sunlight during the day, splitting water into energy and oxygen. Using the released energy in the Calvin cycle they build carbon dioxide up into glucose.

Click on the information hotspots below for further explanations.

Photosynthesis in C4 plants and their adaptations for photosynthesis

C4 plants are a group of plants that have adapted to efficiently carry out photosynthesis in hot and dry environments.

They utilise a specialised pathway known as C4 photosynthesis, which enhances their ability to fix carbon dioxide (CO₂) and minimise photorespiration.

Characteristics of C4 photosynthesis are as follows:

1. Leaf Structure: C4 plants typically have a distinct leaf anatomy, with two types of cells involved in photosynthesis: mesophyll cells and bundle sheath cells. This allows for efficient CO₂ fixation and transport. an image showing a cross section of the mesophyll in both a C3 and C4 plant. Each cross section has a zoomed in section illustrating the chemical process within the chloroplast.

Source: Young, S. N. R., & Lundgren, M. R. (2023). C4 photosynthesis in Paulownia? A case of inaccurate citations. Plants, People, Planet, 5(2), 292–303. https://doi.org/10.1002/ppp3.10343

2. Multi Stage Photosynthesis Process: Click on parts of the diagram depicting photosynthesis in C4 plants to find out more about that process.

Photosynthesis in CAM plants and their adaptations for photosynthesis

CAM (Crassulacean Acid Metabolism) plants are a specialised group of plants adapted to extremely arid conditions. CAM plants are highly efficient in their use of water, allowing them to thrive in dry environments where other plants might struggle.

They have developed a unique photosynthetic pathway that allows them to minimise water loss while still capturing carbon dioxide (CO₂).

Click on parts of the diagram below depicting photosynthesis in CAM plants to find out more about the process.

Compare and Contrast C3, C4 and CAM plants

Use the following summary table to complete the activity below.

Graphic providing an overview of C3, C4 and CAM photosynthesis, showing the relationship between carbon dioxide, the stomata, the mesophyll if relevant, and the Calvin cycle for each.

C3 PlantsC4 PlantsCAM Plants
Plant Types Wheat, Rice, Barley. Rye, Oats, Soybean, Sugar Beet, Potato. Maize, Sugarcane, Millet, Sorghum, Native Grasses.Cacti, Succulents, Orchids, Pineapples.
HabitatCool, temperate and moist conditions.Warm, temperate and tropical regions.Hot, arid and drought regions.
Carbon Fixation

Carbon dioxide enters the leaf through the stomata.

The enzyme RuBisCO catalyses the reaction between CO₂ and RuBP in the first step of the Calvin cycle to create 2 3-carbon molecules known as PGA.

Carbon dioxide enters the leaf through the stomata.

The enzyme PEP carboxylase converts CO₂ and PEP, through a series of conversions, to a 4-carbon compound known as malate.

This occurs in the mesophyll cells.

At night carbon dioxide enters the leaf through the stomata.

The enzyme PEP carboxylase converts CO₂ and PEP, through a series of conversions, to a 4-carbon compound known as malate.

This occurs in the mesophyll cells.

Stomata BehaviourClosed during hot, dry conditions.Closed during the hottest part of the day.Open only at night.
AdvantagesEfficient photosynthesis if cool, moist and high CO₂ concentrations.

Efficient photosynthesis if hot, dry and low CO₂ concentrations.

Low photorespiration.

Survive in very dry conditions.

Minimise water loss.

Disadvantages

High photorespiration.

Decreased photosynthetic efficiency.

Need more ATP energy to produce glucose.

Slower growth.

Operate less energy efficiently by the conversion of stored acids back to CO₂.