During photosynthesis, a plant is able to convert solar energy into a chemical form. It does this by capturing light coming from the sun and, through a series of reactions, using its energy to help build asugar molecule called glucose.

Glucose is made of six carbon atoms, six oxygen atoms, and twelve hydrogen atoms. When the plant makes the glucose molecule, it gets the carbon and oxygen atoms it needs from carbon dioxide, which it takes from the air. Carbon dioxide doesn't have any hydrogen in it, though, so the plant must use another source for hydrogen. The source that it uses is water. There is a lot of water on the earth, and every water molecule is composed of two hydrogen atoms and one oxygen atom.

In order to take the hydrogen it needs to build glucose molecules, the plant uses the energy from the sun to break the water molecule apart, taking electrons and hydrogen from it and releasing the oxygen into the air. The electrons it takes are put into an electron transport system, where they are used to produce energy molecules called ATP that are used to build the glucose molecule-- all made possible by the sun's energy.

Thus, during photosynthesis a plant consumes water, carbon dioxide, and light energy, and produces glucose and oxygen. The sugar glucose is important because it is necessary for cellular respiration. During cellular respiration, the chemical energy in the glucose molecule is converted into a form that the plant can use for growth and reproduction. In the first step of respiration, called glycolysis, the glucose molecule is broken down into two smaller molecules called pyruvate, and a little energy is released in the form of ATP. This step in respiration does not require any oxygen and is therefore called anaerobic respiration. In the second step of respiration, the pyruvate molecules are rearranged and combined and rearranged again in a cycle. While the molecules are being rearranged in this cycle, carbon dioxide is produced, and electrons are pulled off and passed into an electron transport system which, just as in photosynthesis, generates a lot of ATP for the plant to use for growth and reproduction. This last step requires oxygen, and therefore is called aerobic respiration. Thus, the final result of cellular respiration is that the plant consumes glucose and oxygen and produces carbon dioxide, water, and ATP energy molecules.

At first, this doesn't seem to make any sense! If the plant can use the energy from the sun to make ATP, why does it go through all the trouble of then using up the ATP to make glucose, just so it can get ATP again? There are two reasons why the plant does this:
First, in addition to ATP, the plant needs materials to grow. Glucose is an important building block that is necessary to produce all of the proteins, DNA, cells, tissues, etc. that are important to life, growth, and reproduction.
Second, one problem with the sun is that it goes away every night, and during winter it isn't very bright. The plant needs energy all of the time. So, by producing glucose, the plant can store this molecule and then use it to produce energy during the night and over winter when there isn't enough sun to provide good photosynthesis.

It is very interesting how photosynthesis and cellular respiration help each other. During photosynthesis, the plant needs carbon dioxide and water-- both of which are released into the air during respiration. And during respiration, the plant needs oxygen and glucose, which are both produced through photosynthesis! So in a way, the products of photosynthesis support respiration, and the products of respiration support photosynthesis, forming a cycle.

While plants can complete this cycle by themselves, animals cannot, since animals aren't capable of photosynthesis! This means that animals have to survive solely through respiration. Also, since we animals can't produce glucose by ourselves, we have to get it from somewhere else-- from eating plants. We produce carbon dioxide that the plants need, and they produce the oxygen that we need, and then we eat them to get the glucose that we need. It seems that we need the plants a lot more than they need us!

Photosynthesis and respiration are complementary processes.

Let me briefly summarize what both processes do:

Photosynthesis:
Is the chemical process where plants can capture and organically fix the energy of the sun. This chemical reaction can be described by the following simple equation:
CO₂ + H₂O + light energy = Sugars + O₂
The main product of photosynthesis is a carbohydrate, such as the sugar glucose, and oxygen which is released to the atmosphere (the earth's atmosphere presently contains 20% of O₂).

Respiration:
Is the typical process where mitochondria of cells of organisms release chemical energy from sugar and other organic molecules through chemical oxidation. This process occurs in both plants and animals. In most organisms, respiration releases the energy required for all metabolic processes. This chemical reaction can be described by the following simple equation:
Sugars + O₂ = CO₂ + H₂O + released energy

Through the process of photosynthesis, green plants absorb solar energy and remove carbon dioxide from the atmosphere to produce carbohydrates (sugars). Plants burn these carbohydrates (and other products derived from them) through the process of respiration, the reverse of photosynthesis. Respiration releases the energy contained in sugars for use in metabolism. This process is not balanced because the oxygen used for respiration is much less than what is given off during the photosynthetic reaction and the carbon dioxide given off is much less than what is taken up during photosynthesis. This is because plants need the sugar molecules not only for energy but also for structural elements (plants are largely cellulose - which is a long sugar molecule).

I hope this helps you understand this complex process.

A lot of my college students still have trouble with this one. The reactions that happen in respiration and photosynthesis are different, but if we just look at what goes in and what comes out, they're opposites.

Here's photosynthesis:
Carbon dioxide (CO₂) and Water (H₂0) in, Oxygen and Sugar out.
Requires energy from the sun.

Here's cellular respiration:
Oxygen and Sugar in, CO₂ and H₂0 out. Releases energy from the sugar.

Plants can do both. When they have light, they use it as an energy source to put the pieces of CO₂ and H₂O to make sugar. They can put a bunch of sugars together to make starch (what foods are starchy?), cellulose (the stringy stuff you can't chew up), and wood.

When it's dark, they can do cellular respiration to break down the starch and sugar to release the energy they need.

Poor animals, we can only do cellular respiration. We need foods like starch, and oxygen, and we breathe out the CO₂ that's made. We don't get enough water from the process to take care of all of our needs so we have to drink more. Kangaroo rats don't have to drink water. They conserve water a lot better than we do.

Can plants live without animals? Can animals live without plants? Thanks for asking.