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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.
(Click the forward arrow to continue...)
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:
Carbon dioxide passes from the air into small openings in the bottom of
leaves called stomata.
The roots absorb water, which travels through vessels in the
stem and to the leaves. Water is necessary for photosynthesis.
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.
The energy from light causes chlorophyll molecules to lose electrons.
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.
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+).
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!
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!).
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.