Photosynthesis

Photosynthesis is important! As I'm sure you know, people and other animals need oxygen to live. Without oxygen, we're all in trouble! How do we get oxygen? A major source of oxygen comes from plants. Oxygen molecules (O₂) are made during the process of photosynthesis, which occurs in plants.

But what is Photosynthesis? Unlike animals, plants can make their own food (sugars). The process by which food (sugar) is made in plants is called photosynthesis. 

Let’s break the word down into its Latin roots:

• Photo means light (as in "photons" or "photography": making pictures with light).
• Synthesis means to put together or combine (like when you "synthesize" information).

So, photosynthesis means to "put together by light." In photosynthesis, sugar is made by light.

Photosynthesis is a  chemical process. During photosynthesis, sunlight is a major source of energy for the atoms in carbon dioxide (CO₂)  and water (H₂O) molecules to combine (synthesize) to make sugar molecules. Oxygen molecules (O₂) are also made during photosynthesis.

The sugars produced are used by plants as their food to help do things like grow and repair damage in cells.

Photosynthesis is a very complicated process. In this unit, we will simplify it and discuss the main steps involved. The chemical equation below shows a simplified summary of the process of photosynthesis: CO₂ and H₂O are eventually converted into glucose (and other sugars) and oxygen.

Simplified Summary of Photosynthesis:

As a reminder, in photosynthesis, carbon dioxide and water are converted into glucose and oxygen:

Steps of Photosynthesis. To make sugars, plants need sunlight, water and carbon dioxide (and a bunch of other things we won't worry about now). Let’s learn some important steps of photosynthesis:

1. Carbon dioxide passes from the air into small openings in the bottom of leaves called stomata.
2. The roots absorb water, which travels through vessels in the stem and to the leaves. Water is necessary for photosynthesis.
3. Sunlight is absorbed by molecules in the leaves called  chlorophyll. Chlorophyll is what gives the leaves their green color. Chlorophyll is found within the special organelles (little organs) of the plant cell called chloroplasts, which are in the leaves and other parts of the plant.
4. The energy from light causes chlorophyll molecules to lose electrons. 
5. Chlorophyll molecules then become positively charged. Chlorophyll molecules are really really good at pulling electrons from other molecules (due to both the electric force and atoms in chlorophyll molecules "wanting" complete energy circles). So, chlorophyll molecules end up pulling electrons from nearby H₂O (water) molecules.
6. This causes the H₂O molecules to split into oxygen and hydrogen atoms (another chemical reaction). Free oxygen atoms from this process combine to make an oxygen molecule, O₂. Each hydrogen atom has lost an electron to the chlorophyll molecule, making it positively charged (H⁺).
7. Oxygen (O₂) is a "waste product" of this reaction and is not used by plants. O₂ is released from the small holes at the bottom of leaves (the stomata) into the atmosphere. But this is good news for people and animals!
8. Hydrogen and carbon dioxide are used in the process of making glucose and other sugars that are food for the plant. Two different ways of representing a glucose molecule are shown below. The glucose that is produced also goes into making the rigid cell walls of plants. Glucose is stored for future use in the plant’s leaves, roots, or fruit (which we may like to eat because it is sweetened by glucose!).
9. Another molecule, called ATP, is also made during photosynthesis. ATP molecules  are one source of energy for the chemical reactions in photosynthesis. ATP molecules also provide energy for the growth and development of plants.

Two different ways of representing a glucose molecule are shown below.

What are plants made of? (Hint: Think about what plants use in photosynthesis.)

Most people would be surprised to learn that plants are mostly made from carbon dioxide (CO₂), which comes from the air!!  This is why plants (including trees) are so important to our environment: they remove CO₂ (a gas that absorbs energy from sunlight) from the air. This helps keep the Earth from getting too warm.