Photosynthesis and Respiration Study Guide

Updated on Sep 21, 2011


The process of photosynthesis performed by plants is the fundamental way Earth's life interacts with the nonliving, physical environment. Photosynthesis takes lifeless energy along with inorganic, inanimate chemicals and converts them into organic matter that feeds all life forms. In a companion set of reactions, animals (and plants) use organic matter and oxygen to derive energy to live. This is called respiration.

A Plant Experiment

Around the mid-1600s, a Dutch doctor, Johann Baptista van Helmont (1579–1644), planted a willow tree. He did his planting as part of a controlled experiment, so he carefully weighed the plant, the soil, and the amount of water he used. After several years, the plant had gained over 150 pounds, but the weight of the soil had not changed. To van Helmont and people of his day, this result showed that the water had been turned into the matter of the tree. It wasn't until the end of the following century, more than a hundred years later, that we discovered the real weight gain came from the plant absorbing gasses from the air and using those with energy from sunlight to grow in size.


The biochemical process that plants use to grow and gain weight is called photosynthesis, which is a highly necessary instance of a biochemical reaction. In this process, plants absorb carbon dioxide from the air and form a union of this gas with water, using sunlight energy and specialized chemicals called enzymes. The process allows the plants to produce sugar (glucose) and oxygen.

Photosynthesis Equation

written using the chemist's shorthand, the photosynthesis equation looks like this:


Metabolism is the collection of all the physical and chemical processes in which a living being is engaged. Metabolism is one of the functions that distinguishes life from nonlife. Living beings maintain themselves at a complex level of organization, and metabolism helps them do this. Nonliving objects are more simply constructed and do not engage in metabolism.

Turning Energy into Matter

The really fascinating aspect of photosynthesis is that raw sunlight energy, a very nonliving thing, is absorbed by plants to form the chemical bonds between simple, inanimate compounds that yield an organic foodstuff (the sugar) upon which all life is dependent. This sugar, called glucose, is the chemical basis for the formation all living compounds. Plants use other biochemical reactions to turn glucose into useful molecules, and herbivore animals that eat plants use glucose and other plant-based compounds to create the molecules they need. Carnivorous animals that eat other animals also form compounds from those food molecules, but at the basis of it all is the capture and conversion of sunlight energy into organic matter during photosynthesis.

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