Cellular Respiration – Energy

Energy Flow in Living Things

To survive, all living things need energy. Energy is the ability to do work or cause change. Work involves moving something against a force, or going from one place or position to another while overcoming some force to get there.

Take a look at two ways your body does work by expending energy:


Cells do work on a very small scale for many reasons, including:

  1. Growth and Repair
  2. Active Transport
  3. Reproduction

All of these require energy.

The ultimate source of energy for all organisms is the sun. Photosynthetic organisms convert the energy from sunlight into chemical energy (sugars) that is stored in food.   Then that food is broken down (in eukaryotes) in the mitochondria to release ATP, an energy storage molecule that is useable by the cell. Prokaryotes are able to use energy from food, but they do so in a primitive way, which we will discuss later.

Take a look at the picture below and trace the movement of energy from the sun into ATP.


What did you learn by interacting with the diagram? Answer the questions below:

  1. What is the source of energy for life on Earth?
  2. Which product of Photosynthesis stores energy?
  3. What form of chemical energy is useable by the cell?
  4. Is energy recycled?

You have probably noticed in the diagram that Cell Respiration and Photosynthesis are reciprocal processes that depend upon each other for raw materials (matter) while they are also moving energy into useable forms for living things.

If we were to write out a simplified “recipe” or chemical equation for each process, they might look something like this:


Translation: Six molecules of water plus six molecules of carbon dioxide (in the presence of light) produces one molecule of sugar (glucose) and six molecules of oxygen.


Translation: One molecule of sugar (glucose) plus 6 molecules of oxygen produces 6 molecules of water, 6 molecules of carbon dioxide, and ATP.

Stop and Think: Did you notice that the products of one equation almost perfectly match the reactants of the other?

Metabolism is the sum total of all chemical reactions within organisms. Photosynthesis and cell respiration are some of those chemical reactions. There are two types of metabolic reactions:

  • Catabolic reactions break down large molecules into smaller ones. Usually these release energy.
  • Anabolic reactions build larger molecules from atoms or smaller molecules. These require an input of energy.

Is photosynthesis anabolic or catabolic? What about cellular respiration? Hover over each equation above to check your answer.

Classifying Organisms Based on Energy Sources


We can classify, or group, organisms based on which processes they perform:

  • Autotrophs (or Producers) use photosynthesis to collect and store energy originally from sunlight. They store energy in the chemical bonds of organic compounds, primarily the carbohydrate sugar known as glucose. They have chloroplasts and mitochondria; once they make the glucose in photosynthesis, they can use some of it in the mitochondria to generate ATP. Examples of autotrophs include algae and plants. Some bacteria can be autotrophs, though they do not have a chloroplast.
  • There are a few organisms deep in the ocean that obtain energy from inorganic compounds, or chemicals. These are called chemoautotrophs, or chemotrophs. These types of organisms tend to live in extreme environments, like the deep sea vents spewing superheated rock. Bacteria on and around these vents are able to use the harsh chemicals to produce energy.
  • Heterotrophs (or Consumers) need to consume food to get their energy. They use cell respiration to convert chemical energy from their food (glucose) into ATP so that the cells can do work that supports life. Heterotrophs have mitochondria, but not chloroplasts. Heterotrophs include animals, fungi, and many unicellular organisms.

Without autotrophs, all other living things would die. Without producers you cannot have consumers. Autotrophs store food and contribute to the basis of the food chain for other things to eat. Only part of the energy from the sun is used by autotrophs to make food, and only part of that energy can be passed on to other consumers. A great deal of the energy is LOST as heat. Enough energy is passed from autotroph to heterotroph to give the heterotroph the energy it needs.

Can you find the autotrophs, or producers in this food web? What about the consumers (or heterotrophs)?


ATP Is The Energy Currency of the Cell

The activities of the cell are powered by a chemical fuel called adenosine triphosphate (also known as ATP). An ATP molecule is made up of adenine, ribose (a sugar), and three phosphate groups:


A simpler way to represent this molecule would be:


How does the ATP molecule store chemical energy? ATP will release energy when it breaks the bond holding onto a phosphate group. Look at the picture below. When the last phosphate group is removed, energy is released from that chemical bond, resulting in ADP (adenosine diphosphate) and a free phosphate group. Note that ADP has only two phosphate groups:


The ADP molecule can be recycled to store energy again. Since the cell can add or subtract a phosphate group, it has a way of storing and releasing energy.

Cells normally have only a small amount of ATP available. ATP cannot be used to store energy for long periods of time. It is more efficient for a cell to keep a small supply of ATP. Cells can regenerate ATP from ADP as needed by using the energy in carbohydrates like glucose.


Other High Energy Carriers

Sometimes, cells are unable to directly collect energy and place it into ATP. They can, however, collect high energy electrons from other molecules during catabolism. The electrons are stored in electron carriers such as NADH, FADH2 and NADPH. Later, these molecules can donate the high energy electrons they collect to a process that generates ATP! They serve an important purpose as a “middle man” for energy storage.



from GVL Biology http://cms.gavirtualschool.org/Shared/Science/Biology17/CellEnergetics/Biology_CellEnergetics_Shared2.html