The link between photosynthesis and cellular respiration is an inverse relationship, both are opposites of each other. Photosynthesis is the process by which carbon dioxide is converted into organic compounds from sunlight. The most frequent compound is sugar. Photosynthesis occurs in plants, algae, and some bacteria. Cellular respiration, in contrast, takes glucose (sugar) and other organic compounds oxidizing them to create carbon dioxide. Each cycle depends on one another in order for the "entire cycle" to take place. The completed cycle ensures life continues to exist on the planet. Both are necessary for living organisms.
Photosynthesis is a natural process that provides all aerobic creatures on earth with oxygen. Without photosynthesis, humans would cease to exist. Photosynthesis helps maintain a natural amount of oxygen in the atmosphere. The process of photosynthesis is also the source of carbon in organic compounds in human bodies, as well as animals. Photosynthesis provides the oxygen for life and for the process of cellular respiration.
Cellular respiration can be defined as metabolic reactions in organic cells which converts biochemical energy into adenosine triphosphate, better known as ATP. Cellular respiration also releases carbon dioxide into the atmosphere allowing plants, algae, and bacteria to process the carbon dioxide back into oxygen. Photosnythesis and cellular respiration is a life cycle that keeps the natural ecological balance of the earth in place.
Photosynthetic organisms synthesize their food directly from carbon dioxide (produced through cellular respiration) using light energy from the sun. Plants, algae, and bacteria release oxygen after sugar is synthesized. Carbon dioxide is converted into sugars through a process known as carbon fixation, which is defined as the process of converting carbon dioxide to a solid compound.
The simplest equation for photosynthesis can be summed up as such:
Carbon dioxide + light energy → carbohydrates (sugar) + oxygen
The Krebs cycle (citric acid) explains the process of cellular respiration. The cycle begins with a two-carbon compounds that changes into a six-carbon compound (citrate). The citrate undergoes chemical transformations within the cell, while losing two carbon dioxide molecules in the process. This is how carbon dioxide is released into the air for photosynthetic organisms to begin photosynthesis. Most of the energy created through cellular respiration is transferred as positive electrons forming NADH+, a co-enzyme called Nicotinamide adenine dinucleotide at the end of the cycle.
The cycle of photosynthesis and cellular respiration make one large cycle that sustains life on the planet. Both photosynthesis and cellular respiration are required to exist in symbiosis. Without photosynthesis, cellular respiration cannot occur, and without cellular respiration, photosynthesis cannot occur. Nature has created a delicate balance in both of these cycles.
About.com - Cellular Respiration