*Modified from TELO content.



Before we begin looking at specific biological reactions, let’s take a step back and examine a change involving objects we can see with our eyes and hold with our hands. Suppose you are interested in purchasing a pizza store and want to know if it’s a good business decision. You want to investigate without the present owner knowing because you fear the owner will raise the price. So, instead of going into the store, watching what happens or asking to examine the books that record expenses and profits, you decide to watch the store from outside.

You observe how often trucks arrive with pizza dough, pizza toppings (cheese, pepperoni, etc.), and other supplies. You also observe how often pizzas leave the store, either with customers or with delivery people to deliver pizzas to customers.

In this analogy, the pizza supplies are the reactants and the boxed pizzas that are delivered to customers are the end products. The workers within the store that shape the dough, add the toppings and place the pizzas in ovens and finally in boxes are the equivalent of the enzymes.

Although we don’t actually see the workers doing their job, we can infer that if the store is using large quantities of reactants (dough and toppings) and/or making large numbers of end products (boxed pizzas) that the workers (enzymes) must be very active. Also, we can describe the change by identifying how different reactants (dough and toppings) are changed.

We could represent this change in different ways. We could show this overall change similar to a biochemical equation.

a biochemical equation showing dough on the left connecting by a long horizontal arrow to a slice of pizza on the right. Above and below the arrow is pepperoni and cheese, respectively. Curved lines from the cheese and pepperoni join the horizontal arrow. 


 by MIT OpenCourseWare

We can also describe this change using a flowchart. In this case we are describing individual changes in the order that they occur.

A flowchart showing pepperoni, cheese and dough on the left. They are each at the end of an arrow that points right to a box describing the preparation of the pizza: cutting, grating and rolling. Another arrow points to the right to a chef holding a pizza. Then another arrow points to a box describing baking: melting cheese and crust browning. Finally, a last arrow points right to a slice of pizza. 


 by MIT OpenCourseWare

 At this point in the Activity you should know about the last big question in this course: How do small steps lead to changes?  Like for the other questions, sometimes you will discuss your answers with the class. Other times you will keep track of your answers for yourself. Thinking about and recording the answers to these questions will direct the way you work to complete the main tasks in this course.

How do small steps lead to change?

Choose a change that is part of your everyday life. Find a picture or sketch a diagram to show this change like the examples outlined above. Describe the change as either a biochemical equation or a flowchart. Share this with the class by posting below in the comment section.

How do small steps lead to changes?

Changes 1. Is the change that you shared with your class a physical change or a chemical change? How can you tell?


Energy in Reactions

In Biology there are several types of energy that are important to understand. As you learn more about Biology in future years you will learn about more types of energy. For now, these are the most common types of energy important to this course:

Light used by many autotrophs is a form of electromagnetic energy. You will remember that visible light is the middle portion of the electromagnetic spectrum.

The different types of electromagnetic waves organized from left to right by wavelength: Gamma rays (10-12), X-rays (10-10), Ultraviolet (10-8), Infrared (10-6 to 10-4), Microwave (10-2), Radio waves (1 to 106). Above gamma rays is a symbol for radioactivity. Above X-rays is a symbol for the X-ray image showing the bones in a hand. Above ultraviolet and infrared is the spectrum of visible light, from violet to red. Above microwave is a picture of a microwave oven. Above radio wave is a symbol for a radio tower. Below the row of electromagnetic waves is a long wave showing frequency from greatest frequency on the left below gamma rays to smallest frequency on the right below radiowaves.

Heat is also known as thermal energy.

Potential energy is the energy stored in the bonds of biological molecules. For example, when we say that energy is transferred from a molecule of glucose to ATP, what is happening is that the potential energy is transferred from the bonds in glucose to the bonds in ATP.

Kinetic energy is the energy of a moving object. A good example of this is how ATP is used by a motor protein to move a vesicle  along a microtubule in a cell.

Two cartoon pictures side-by-side. In the left picture is a woman pulling the string back in a loaded bow and arrow. “Potential energy” is written above the arrow. In the right picture is a woman holding a bow with the string released and the arrow flying forward. “Kinetic energy” is written above the arrow.

The reactions that occur in the body are collectively known as metabolism.  In everyday life we also use the term metabolism to describe how quickly a person can burn food. In this Activity we are looking at the types of reactions, not the speed of those reactions. That being said, all metabolic reactions involve changes of energy.

How does understanding change?

Understanding 20. Fast metabolism, or slow metabolism. Describe what you understand these terms to mean. Since there is no “correct” answer to this question, is there a specific example from everyday life that you’re basing your answer on?

Metabolism can be further divided into two groups:

  • anabolic – a reaction that makes a larger molecule from smaller ones reactions and
  • catabolic – a reaction that makes smaller molecules from a larger one.

Watch either one of these video to identify differences and similarities between anabolism and catabolism. You will use this information in the next section.





Interestingly, one common biochemical reaction is neither anabolism nor catabolism: isomerization.

The isomerization of citrate into isocitrate is an important reaction that occurs in the mitochondria as part of cellular respiration. The reactant is a six-carbon molecule called citrate. Citrate is made of a carboxyl group connected to a second carbon with two hydrogen, a third carbon connected to a hydroxyl and a carboxyl, a fourth carbon with two hydrogen, and a final carbon in a carboxyl group. The reaction arrow shows a reversible forward-backward change. The product is a six-carbon molecule called isocitrate. Isocitrate is made of a carboxyl group connected to a second carbon with two hydrogen, a third carbon connected to a hydrogen and a carboxyl, a fourth carbon with a hydrogen and hydroxyl, and a final carbon in a carboxyl group. 


 The isomerization of citrate into isocitrate is an important reaction that occurs in the mitochondria as part of cellular respiration.  by Simon Fraser University

Even though it looks like individual functional groups are being oxidized or reduced, the total number of carbon, hydrogen and oxygen atoms in the molecule does not change. Prove it to yourself and count the numbers of each atom!