Food

Have you ever wondered how things grow? Matter cannot be created out of nothing. So, for plants and animals (including people) to grow and to replace cells that have died, the plants/animals need to take in matter. Where does this matter come from?

The number one MAJOR source of materials for plants to grow is carbon dioxide, which plants get from the air! Most of the atoms that plants are made of come from carbon dioxide molecules in the air that become different types of molecules during chemical reactions in photosynthesis!

What else (besides CO₂ and water) do plants need to grow? Another source of matter for plants is minerals (made of atoms/molecules) that plants get from the soil. These minerals that plants take in will help build the new cells in plants. These minerals also provide some of the energy needed to make different kinds of cells in plants.

The Main nutrients plants need from soil (and often in fertilizers) are the elements: Nitrogen(N); Phosphorus (P); Potassium (K). These minerals are commonly found in fertilizers you can buy in stores.
- Nitrogen (N) is a gas at temperatures on Earth. Because of this, the Nitrogen in fertilizers must be bonded with other atoms in the form of a molecule (otherwise, it would just escape from the fertilizer into the air due to its lower density). Some common molecules in fertilizer that contain Nitrogen are Nitrate, NO₃ and Nitrate Nitrogen, which is just Nitrate with another Nitrogen atom: NO₃-N.
- Similarly, Phosphorus (P) in fertilizer is in the form of P₂O₅.
- And Potassium (K) is in the form K₂O (called potash).

Nutrients in soil, including the elements Nitrogen (N), Phosphorus (P), and Potassium (K), dissolve in water when it rains and are able to enter the roots of the plant. These nutrients travel to different parts of the plant, including the stem, leaves, and branches, where they are used by the plants as materials and energy to build and repair cells.

Carbon dioxide from the air is a very important part of plants. Many people do not realize that most of the atoms that make up plants come from carbon dioxide in the air!

What do these minerals (N, P, and K) do?

Nitrogen (N) is used by plants for growth. Nitrogen also helps give plants their green color.
- Nitrogen is very important because it is a part of chlorophyll molecules. A chlorophyll molecule is: C₅₅H₇₂O₅N₄Mg; it has 55 Carbon atoms, 72 Hydrogen atoms, 5 Oxygen atoms, 4 Nitrogen atoms, and one Manganese atom. Chlorophyll is the molecule that sunlight hits and gives energy to produce sugars from water and carbon dioxide (i.e., photosynthesis) in chemical reactions. Chlorophyll is what gives plants their green color.
  - But, most of a chlorophyll molecule is made of Carbon and Hydrogen atoms! (You need 55 Carbon atoms and 72 Hydrogen atoms for just one chlorophyll molecule!) Where does the Carbon and Hydrogen atoms come from? As you may have guessed, Carbon comes from CO₂ in the air. And the Hydrogen atoms come from water (H₂O)!
- Nitrogen is also a major component of amino acids, the building blocks of proteins. (Proteins are very important complex molecules that do most of the work in cells and are required for the structure, function, and regulation of the organism’s tissues and organs.)
Phosphorus (P) is used by plants to help form new roots, make seeds, fruit and flowers. It's also used by plants to help fight disease. In addition...
- Phosphorus (along with Oxygen) form phosphate molecules in ATP. ATP (Adenosine Tri-Phosphate) is a key source of energy for plants and animals. ATP breaks apart glucose in a very complicated series of chemical reactions. The end result of these reactions is to produce more ATP, which provides the energy to make various parts of plants, including leaves, flowers, seeds, etc. So, ATP and glucose react to form more ATP! This is how important ATP is for life! Because ATP is so important, we talk about it more below.
Potassium (K) helps plants make strong stems and grow fast. It's also used to help fight disease. In addition:
- Potassium helps control the opening and closing of little holes called stomata (see Vascular plants unit for more information on stomata), and therefore controls how much CO₂ enters and leaves the plants. CO₂ is necessary for photosynthesis to occur.
- Potassium also triggers the activation of enzymes (molecules that help speed up chemical reactions). Potassium is also essential for the production of the molecule Adenosine Triphosphate (ATP).

Sources of Energy in Plant/Animal Cells

Where exactly do plants and animals get the energy to repair cells and build new cells? Remember that energy is not a different kind of substance from the different types of atoms in the periodic table. Energy is (a) the actual motion of particles (atoms, molecules) or (b) the potential for greater motion of particles.

ATP molecules. A key source of energy is the molecule ATP (C₁₀H₁₆N₅O₁₃P₃), Adenosine Tri-Phosphate. ATP is critical for plants and animals in performing cellular work such as repairing cells and building new ones, including leave, flower, and seed cells.

But, how does ATP "make energy"? (Remember: "energy" is all about motion!) To answer, we must first see what ATP molecules are made of. The pictures below show two different ways to represent the ATP molecule. As you can see, ATP is made of three parts: adenine, ribose, and a group of three phosphates. (The adenine and ribose parts of ATP are together called "Adenosine", and the three phosphates make up the "Tri-Phosphate" part of "ATP.")

The key part of the ATP are the three phosphates, PO₄ (shown on the right side of both figures above). The last phosphate on ATP breaks away from the rest of the molecule (which is now ADP, or Adenosine Di-Phosphate, because it only has two phosphate molecules).
- Some of this lone phosphate's electrons move to a lower energy level as a result of the phosphate breaking its bond in ATP. Because energy must be conserved , this phosphate gains kinetic energy, or moves around faster. This kinetic energy due to motion can cause chemical reactions in plants and animals that require energy for the reaction to take place.
The kinetic energy from the last phosphate that has broken from the ATP (now ADP) molecule is also used by plants and animals to move other materials in and out of cells (and in animals, to make muscles contract!).

Q1: Which of the following is the most accurate explanation for the role of glucose in plant growth?

a) Glucose makes parts of the plant (stems, leaves, roots, etc.)

b) ATP makes glucose molecules. Glucose molecules provide the energy (due to their motion) to make parts of the plant.

c) Glucose and ATP react to form lots of ATP. ATP molecules break into a phosphate molecule and ADP molecule. The phosphate molecules provide energy (due to their motion) to make parts of the plant.

d) I really don't know. This is confusing!

Right! Glucose is not the DIRECT source of energy for making parts of the plant. Glucose molecules are important because they react with ATP molecules to make even more ATP molecules. ATP molecules are the DIRECT source of energy for making parts of the plant, like the seeds, petals, etc. (c) is the correct response. This energy from motion comes from the phosphate molecule (PO₄) breaking from the ATP molecule.

Actually, glucose is not the DIRECT source of energy for making parts of the plant. Glucose molecules are important because they react with ATP molecules to make even more ATP molecules. ATP molecules are the DIRECT source of energy for making parts of the plant, like the seeds, petals, etc. This energy from motion comes from the phosphate molecule (PO₄) breaking from the ATP molecule. So, (c) is the correct response.

What Plants Need to Grow