8.1: Background

There are two great processes that make the world of life go around: photosynthesis and respiration. One is about converting solar energy into chemical energy (to make the sugar, glucose); the other is about converting this chemical energy into a useful form (to convert glucose into ATP) to drive everything us organisms do. One seems to be the opposite of the other, and when you consider where the “first” process begins and where the other ends, then these two processes indeed form a circle. If there is actually a “circle of life” this is it: photosynthesis makes energy and respiration uses it. All cells (and thus all organisms) perform respiration (often in an organelle called the mitochondria), whereas only certain kinds of organisms that have a specific organelle called a chloroplast carry out photosynthesis. Those organisms that are capable of photosynthesis are known as producers, and all the remaining organisms can be termed consumers.

Photosynthesis can be defined as the transfer and storage of solar energy to a chemical form called glucose (a type of sugar). Glucose (and other kinds of sugar) is an arrangement of atoms of carbon (C), hydrogen (H) and oxygen (O). Much like a battery stores energy used to power your portable CD player, glucose is designed to be a temporary holding pen for some of the energy that arrives from the sun. From the standpoint of chemistry, photosynthesis is written like this:

CO\(_2\) + H\(_2\)O \(\xrightarrow{\mbox{solar energy}}\) carbohydrates + O\(_2\)

Carbon dioxide comes from the air, the water comes from the soil or the surrounding environment, the glucose is either used by the plant or gets stored (we eat the stored stuff), and the oxygen gets released into the air. This reaction requires energy input from the sun.

Respiration can be defined as the release of the stored energy from glucose; this stored energy is transferred to a molecule called ATP that is used to drive any process in your cells that needs energy input.

carbohydrates + O\(_2\) \(\rightarrow\) CO\(_2\) + H\(_2\)O + ATP

This reaction releases energy in the form of ATP.

An example of this reaction occurs during intense physical exertion. While contracting muscles, you need lots of ATP. This is because your muscles need ATP to do what they are supposed to do (i.e., contract and release, contract and release). Where does this ATP come from? You obtain it by retrieving sugars that are stored in your liver (they got there by digesting more complex foods in your digestive tract) and carrying them via your bloodstream to your muscles where respiration occurs to move the energy from glucose (that began as energy in the sun) to ATP which make your muscle cells work.

Note that the balance sheet is even. The only discrepancy is the energy budget. A lot more solar energy is available than gets stored as glucose, and more energy is available in the glucose than gets transferred to ATP. All the energy that is not stored is “lost” as heat. Have you ever noticed that you warm up during intense physical exertion? Duh.

In an earlier lab, you learned about osmosis and diffusion. These processes are free; they require no energy because stuff is moving from a high concentration to a low concentration. It just happens. However, life often requires that we move stuff the opposite way: from a low concentration to a high concentration. It also requires that this movement happens in an organized fashion. This takes energy. In short, life needs energy. ATP is this energy.

In this lab you will perform a simple procedure that will illustrate the extent to each of these processes in an aquatic plant.