SCI203A: Biology | Unit 3: Cell Biology | Lesson 20: Respiration and Photosynthesis

Mastery Map:

photosynthesis are chemically opposite, they occur by different processes, and they have oxygen and carbon dioxide cycling between cellular respiration.

The equations for photosynthesis and for cellular respiration are similar, but the processes are quite different.

Although the chemical equations are very similar, it is important not to start thinking that the processes are the same.

The chemical equation for cellular respiration is opposite of that for photosynthesis. Now that you are an expert on reading chemical equations...well, at least the one for photosynthesis.....

C6 H12 6O2--> 6CO2 + 6H2O + ENERGY looks much simpler than the process actually is because it only shows the beginning and end products, not what happens in between. Remember, cellular respiration includes several stages: glycolysis, the Krebs cycle, and the electron transport chain. The chemical equation for cellular respiration is the opposite of the equation for photosynthesis.

Understanding the numbers in the chemical equation for photosynthesis is important. The subscript numbers tell you how many atoms of that element are in a molecule.

For example, in water-H2O, there are 2 atoms of hydrogen and 1 atom of oxygen. If there is no number, assume that there is 1 atom of that element. The number on the left of a molecule tells you how many of that total molecule is used. For photosynthesis, you can say that sunlight plus 6 molecules of carbon dioxide (CO2) and 6 molecules of water (H2O) are used to make 1 molecule of glucose (C6 H12 O6) and 6 molecules of oxygen (O2).

Early organisms did not undergo photosynthesis or cellular respiration. If you could go back in time, don't pick the earliest stages of earth's formation. Why is that so??

It is because you wouldn't be able to breathe. The entire planet formed from molten rock. As it cooled, oceans formed and landforms solidified. The environment of early earth was much different than it is today-namely, there was no free oxygen in the atmosphere. When life first appeared about 3.8 billion years ago, oxygen was not present in a free form. Organisms used anaerobic processes to produce energy.

Photosynthesis evolved, and oxygen began to be pumped into the atmosphere. Approximately 2.8 billion years ago, photosynthesis evolved.

Like mini-machines, microscopic organisms started pumping oxygen into the atmosphere in larger and larger quantities. Without any life-forms using the oxygen for cellular respiration yet, the oxygen accumulated.Why do you think this is so? Estimated growth of free oxygen in the earth's atmosphere.

SCI203A: Biology | Unit 3: Cell Biology | Lesson 20: Respiration and Photosynthesis

Photosynthesis and cellular respiration have chemically opposite equations and are accomplished through different processes. Breathing is something you usually don't think about doing. Think about it now. Take a deep breath through your nose. Try to picture tens of millions of oxygen atoms cascading into your lungs and then dispersing to trillions of cells. Now think about countless people and animals all doing the same. Why doesn't the oxygen supply ever run out?

Photosynthesis and cellular respiration are inverted processes that cycle materials back and forth between them.

Photosynthesis uses materials that cellular respiration produces, and vice versa. The chemical equations for both processes are mirror images, but the physical details of their processes are not. Oxygen and carbon dioxide play critical roles in both processes, and cycle back and forth between them.

Oxygen and carbon dioxide cycle back and forth between photosynthesis and cellular respiration.

The untouched bamboo takes in the exhaled carbon dioxide to pump into the Calvin cycle for more glucose production. Oxygen cycles back and forth between the panda and the stalks-between cellular respiration and photosynthesis. Don't forget that plants use much of the glucose they make. Plants undergo both photosynthesis and cellular respiration. Animals only undergo cellular respiration.

With oxygen in the atmosphere, the ozone layer formed, shielding the earth from harmful ultraviolet rays and once oxygen was abundant, cellular respiration evolved and life-forms became more complex.

When cellular respiration evolved, it changed the possibilities for life on earth. Instead of making 2 molecules of ATP during anaerobic respiration, or fermentation, organisms could now earn a whopping 38 molecules of ATP for each molecule of glucose they used. It is because of this new wealth of chemical energy that life-forms had extra energy to spend on new and complicated structures and processes.

With the abundance of oxygen, the ozone layer formed to protect the earth from UV rays. Early earth also featured the intense ultraviolet rays of the sun that were beating down on it unfiltered.

With the abundance of oxygen, an ozone layer developed around earth's atmosphere. Ozone is a molecule made out of 3 oxygen atoms. The oxygen you breathe is made out of 2 oxygen atoms. Like an umbrella shielding the planet, the ozone layer prevents much of the UV rays from the sun from reaching the earth's surface. The combination of this protection and the availability of oxygen gave rise to a rapid increase in new and more complex life-forms.

The overall process of photosynthesis can be shown in a chemical equation. The overall process of photosynthesis, including light-dependent reactions and light-independent reactions-the Calvin cycle, can be represented in a single chemical equation.

You probably recognize that the letters represent atoms of different elements: C for carbon, O for oxygen, and H for hydrogen. Notice that there are two types of numbers-the subscript numbers on the right of an atom and the regular numbers on the left of some molecules. The process of photosynthesis can be expressed in this chemical equation.