Photosynthesis is the amazing process where plants and some other organisms use sunlight to turn water and carbon dioxide into food and oxygen.

Key idea: Not all forms of photosynthesis produce oxygen; some ancient and bacterial forms use different ingredients and release other byproducts like sulfur.

While most photosynthesis creates oxygen, some older or different types do not. For example, some bacteria use other chemicals instead of water and release sulfur instead of oxygen. This type of photosynthesis was common a very long time ago on Earth.

Some very simple organisms called archaea have an even simpler way to use light. They use a special pigment, similar to what helps us see, to absorb green light and create energy directly. They do not make sugars or release oxygen. Scientists think this might have been the very first way life learned to use sunlight for energy.

Key idea: The first step of photosynthesis involves pigments like chlorophyll capturing light energy, which then powers a series of reactions to create energy carrying molecules and release oxygen from water.

Photosynthesis starts when special colored molecules, called pigments, capture light energy. In plants, the most well known pigment is chlorophyll, which is why most plants look green. Chlorophyll is found inside tiny compartments in plant cells called chloroplasts, especially in the leaves.

When chlorophyll absorbs light, it gets excited and loses an electron. This electron then moves through a series of steps, like a tiny electrical current. This movement helps create two important energy carrying molecules: NADPH and ATP. During this step, water molecules are split apart, releasing oxygen as a byproduct.

After the light dependent reactions create NADPH and ATP, the plant uses these energy molecules in the next stage, called the light independent reactions. These reactions do not need light directly, but they rely on the energy from the first stage.

The main goal of this stage is to take carbon dioxide from the air and turn it into sugar. This happens through a series of steps known as the Calvin cycle. A special enzyme called RuBisCO helps capture the carbon dioxide. Using the energy from ATP and NADPH, the carbon dioxide is converted into simple three carbon sugars. These small sugars can then be combined to make bigger sugars like glucose, starch, or cellulose, which the plant uses for food or building its structure.

“Plants take carbon dioxide from the air and, using the energy captured from sunlight, turn it into sugar.”

Key idea: The overall equation for photosynthesis shows that carbon dioxide and water, powered by light, create sugar and oxygen.

The overall process of photosynthesis can be summarized by a simple equation. It shows that carbon dioxide and water, with light energy, produce sugar and oxygen.

Imagine you are baking a cake. Your ingredients are flour, sugar, and eggs. Your heat source is the oven. The cake is your final product. For photosynthesis, the ingredients are carbon dioxide and water. The heat source is light energy. The final product is sugar (food) and oxygen.

It is important to understand that photosynthesis is the opposite of cellular respiration. While photosynthesis builds up sugars and stores energy, cellular respiration breaks down sugars to release energy. Animals and plants both do cellular respiration.

“Photosynthesis is essentially the opposite of cellular respiration: one builds up sugars, the other breaks them down.”

Key idea: The understanding of photosynthesis has developed over centuries through the work of many scientists, revealing its key components and chemical steps.

Scientists have learned a lot about photosynthesis over hundreds of years. In the 1600s, Jan van Helmont showed that plants gain most of their mass from water, not just soil. Later, in the 1700s, Joseph Priestley and Jan Ingenhousz discovered that plants could 'restore' air that had been 'injured' by burning candles or breathing animals, especially when exposed to sunlight.

These early discoveries paved the way for understanding the basic ingredients and products of photosynthesis. Later, scientists like Cornelis Van Niel and Melvin Calvin (who won a Nobel Prize) figured out the detailed chemical steps and cycles involved in converting light energy and carbon dioxide into sugars.

Key idea: Factors like light, water, carbon dioxide, and temperature all influence the efficiency of photosynthesis, and plants have evolved different strategies to optimize this process in various environments.

Several things can affect how well photosynthesis works. These include the amount and type of light, how much water is available, the concentration of carbon dioxide in the air, and the temperature.

If there is not enough light, water, or carbon dioxide, the plant cannot make as much food. For example, if a plant does not get enough carbon dioxide, a process called photorespiration can happen. In photorespiration, the enzyme RuBisCO, which usually captures carbon dioxide, accidentally grabs oxygen instead. This wastes energy and does not produce sugar, which is bad for the plant.

Some plants, like corn and sugarcane, have special ways to deal with low carbon dioxide or hot, dry conditions. They have evolved different methods, like C4 photosynthesis or CAM (Crassulacean acid metabolism), to concentrate carbon dioxide around the RuBisCO enzyme, making photosynthesis more efficient and avoiding photorespiration.

⚠️Watch out: Many people think plants only need light and water. However, carbon dioxide is just as crucial for photosynthesis, and its availability can significantly limit how much food a plant can make.