Cellular Respiration

Once students have investigated the similarities among cells and the structures that make up eukaryotic cells, they then conduct an experiment to explore how cells extract energy from food through the phenomena of cellular respiration, comparing how much carbon dioxide is produced by yeast when they consume different amounts of sucrose.

• Cells have certain requirements for life, including energy and nutrients. All cells extract energy from food molecules.
• All organisms, from bacteria to humans, need food because the molecules that make up food hold stored chemical energy, and can be used as building materials for growth and development.
• All cells perform cellular respiration, which is how they access energy from food molecules.
• In cellular respiration within eukaryotic cells, mitochondria use oxygen to convert some of the energy in glucose into energy that is stored in molecules called adenosine triphosphate, or ATP.

• How do we know that food contains chemical energy?
• Why is food so important for organisms?
• How do plants survive, given that they don’t need to eat food the way that animals do?
• What does food provide for organisms that help them to survive?
• How does gut bacteria help humans digest complex sugars?
• How does cellular respiration extract energy from glucose molecules?
• Why does cellular respiration involve chemical reactions
• How do we know that no atoms will be destroyed in cellular respiration, and that they will all be conserved?
• Why is cellular respiration essential for cells?

Misconception: Cells are completely separate from atoms and molecules.

Fact: Cells are matter, which means they are made up of atoms and molecules. For example, proteins are part of every cell. 

Misconception: Animal cells do not carry out essential life functions, such as obtaining energy and eliminating waste, themselves.

Fact: Cells are living things, so every cell carries out essential life functions. For example, every cell needs energy and the proper nutrients to survive, and it needs to eliminate waste. Every cell also builds all the proteins it needs to function.

Cell : the smallest unit of life; makes up single-celled and multi-celled organisms; surrounded by a cell membrane and filled with cytoplasm

Cell membrane : a protective membrane that surrounds a cell and selects which molecules can enter and exit the cell

Cellular respiration : the process in which cells extract energy from food

Mitochondria : organelles that are the power centers of the cell; combine sugar from food with oxygen from respiration to produce molecules the cell uses for energy (ATP)

Nucleus : a membrane-bound organelle that holds an organism’s genetic material (DNA)

Nutrients : chemicals that organisms need for the growth and maintenance of cells

Organelle : a small part that carries out specific functions in a cell

Protein : a large, complex molecule that plays many important roles in all organisms

Getting Energy from Food

Scientists have learned in recent years that bacteria in the gut play an essential role in this process. Andrej Joachimiak is one such scientist, and he is particularly interested in how bacteria help humans digest food through the use of enzymes. Enzymes are special proteins that speed up complex chemical reactions. Different kinds of bacteria produce different kinds of enzymes.

Joachimiak and his team focused on specific enzymes produced by bacteria in the human gut that help to break down more complex sugars into simple glucose. Glucose is a molecule made up of six carbon atoms, six oxygen atoms, and twelve hydrogen atoms. Sucrose is a sugar molecule made up of glucose and another simple sugar.

Glucose is an important ingredient in cellular respiration, which is the process in which cells extract energy from food. In cellular respiration within eukaryotic cells, mitochondria use oxygen to convert some of the energy in glucose into energy that is stored in molecules called adenosine triphosphate, or ATP. In addition to ATP molecules, carbon dioxide and water are also produced. The carbon dioxide is released as a waste product.

The energy found in glucose cannot be used by cells until it is stored in molecules of ATP. ATP is the molecule that carries energy where it is needed. Because of this, cellular respiration is always happening in cells. Without ATP, the cell would not be able to carry out its essential functions, including growing, dividing, and repairing or replacing worn-out organelles.

In humans, cells that need a lot of energy, such as muscle cells, have more mitochondria than cells that don’t need a lot of energy, such as skin cells. Both plant and animal cells store energy they don’t need immediately as fat. Humans generally store several weeks’ worth of energy in their cells.

Human cells are capable of breaking down complex sugar molecules into simple glucose. But the team of scientists studying bacterial enzymes realized that human body cells are more efficient at breaking down some food molecules more than others.

To understand this, it’s important to note that there are three primary components of food: carbohydrate, protein, and fat. Carbohydrates are a main source of glucose that your body uses for energy. There are different kinds of carbohydrates. Sugars give your body quick energy.

Starches give your body energy that lasts longer. Good sources of carbohydrates include whole grains, fruits, and vegetables. Regardless of the kind of carbohydrate, our bodies turn all carbohydrates into glucose that can be turned into energy through cellular respiration.

Because proteins are part of every cell in your body, they are constantly being used and replaced. Protein from food are broken down into molecules called amino acids, which are used to build new proteins. Fish, meats, eggs, and nuts are good sources of protein.

Fats help your body build tissue to protect important organs. Only 10 percent of fats are turned into glucose. Fats also serve as a backup fuel source. There are different kinds of fat. Good sources of healthier fats are found in nuts, vegetable oil, olive oil, certain fish, and avocados.

When people talk about food, they often mention how many calories a particular food has. Calories are the measure of energy that fuels your body, just as gasoline fuels your car. They are a form of potential energy that your body can use to produce heat and work. Without enough calories, your heart could not beat, your muscles could not move, and your brain could not regulate your body.

Energy comes from carbohydrates, proteins, and fats. One gram of carbohydrate has 4 calories, as does one gram of protein. One gram of fat has more than double—9 calories per gram. Experts recommend you get about half of your calories from carbohydrates, 1/5 from proteins, and 1/3 from fats.

Because every person is different, the number of calories your body needs depends on a number of factors, including your age, sex, weight, and muscle mass. It is important to get enough calories to support growth and development. However, if you eat more calories than your body uses, you will store the excess calories. Your body reaches equilibrium when you consume just enough calories to keep your body running properly.

In this lesson students carry out an experiment to discover how the concentration of sucrose affects the rate that yeast cells break down sucrose and use it for energy. Students conduct this experiment to figure out how the amount of sucrose affects the rate of cellular respiration in yeast.