Carbon dioxide (\(text{CO}_2\)) is an odorless, colorless gas present in the atmosphere at a concentration of approximately 400 parts per million. This simple molecule is the primary ingredient plants use to manufacture the organic compounds necessary for their existence. Plants draw \(text{CO}_2\) from the air and convert it into complex sugars that serve as their sole source of food. Without the carbon atom supplied by atmospheric \(text{CO}_2\), plant life could not construct its body or sustain its metabolic processes.
The Fundamental Process: Photosynthesis
The central role of carbon dioxide is executed during the light-independent reactions of photosynthesis, often referred to as the Calvin Cycle. This cycle takes place within the stroma inside the chloroplasts of the plant cell. Here, energy stored from the light-dependent reactions (ATP and NADPH) is used to “fix” the carbon atom from \(text{CO}_2\) into an organic molecule.
Carbon fixation begins when the enzyme RuBisCO catalyzes the attachment of a \(text{CO}_2\) molecule to a five-carbon sugar (RuBP). This initial six-carbon compound is unstable and immediately splits into two molecules of 3-phosphoglyceric acid (3-PGA). This step converts inorganic carbon from the atmosphere into a usable organic form.
The 3-PGA molecules are then converted into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar, using the energy from ATP and NADPH. The plant diverts this G3P out of the cycle to synthesize glucose (\(text{C}_6text{H}_{12}text{O}_6\)) and other carbohydrates. The carbon atoms provided by the \(text{CO}_2\) form the backbone of the newly created sugar molecule.
How Carbon Dioxide Enters the Plant
The leaf surface is covered in a waxy cuticle that is impermeable to gas, necessitating specialized structures for \(text{CO}_2\) uptake. Carbon dioxide enters the plant primarily through microscopic pores on the leaves called stomata, which function as controlled gateways for gas exchange.
The movement of \(text{CO}_2\) from the outside air into the leaf interior is achieved through simple diffusion. The process is driven by a concentration gradient, as the continuous use of \(text{CO}_2\) in photosynthesis maintains a lower concentration inside the leaf than in the surrounding atmosphere. Gas molecules naturally flow inward through the open stomata and into the intercellular air spaces of the leaf.
Each stoma is bordered by a pair of guard cells, which regulate the size of the opening. Guard cells adjust their internal water pressure to open and close the pore in response to environmental cues, such as light and water availability. This regulation is a balance, as opening the stomata to let in \(text{CO}_2\) inevitably allows water vapor to escape, a process called transpiration. The \(text{CO}_2\) then dissolves in the moist cell walls before diffusing into the chloroplasts.
Building Blocks for Plant Structure
Once the carbon from \(text{CO}_2\) is fixed into glucose, that sugar molecule becomes the foundational material for virtually all plant compounds and functions. The plant uses some of the newly created glucose immediately to fuel its growth and maintenance activities through cellular respiration. This process releases chemical energy (ATP) that powers the plant’s metabolism.
Excess glucose is converted into more complex molecules for either storage or structure. For energy reserves, glucose molecules are linked together to form starch, a large, insoluble polysaccharide stored in roots, stems, and seeds.
For physical structure, the fixed carbon is converted into rigid polymers that make up the plant body. Glucose is polymerized into cellulose, the primary component of plant cell walls that provides mechanical strength and rigidity. It is also converted into lignin, a complex polymer that contributes to the hardness of wood and provides structural support to vascular tissues. Because carbon from atmospheric \(text{CO}_2\) is the source of these compounds, it accounts for approximately 45% of a plant’s dry weight.