Energy and Cells


Living things are made of cells, which use energy to perform all the functions needed for life. All organisms need energy to grow, to make repairs, and to move. Where does that energy come from? Almost all the energy used by living things comes from the Sun. Energy takes many forms, such as light and chemical energy. Chemical energy is stored in organic substances such as foods and fuels. Each form of energy can be changed into another form.

The process by which organisms capture light energy from the Sun and change it into chemical energy is called photosynthesis. Organisms that carry out photosynthesis are called producers because they make, or produce, food. All autotrophs, or organisms that capture matter and energy from their surroundings and use it to make their own food, are producers. Because plants capture matter and energy from their surroundings and use it to make their own food, they are autotrophs and producers.


Most of the photosynthesis carried out in a plant takes place in the leaves but can also take place in green stems. The cells in leaves and green stems contain small structures, or organelles, called chloroplasts. Chloroplasts are the sites of photosynthesis. As shown in the diagram, chloroplasts have an inner and outer membrane that surround granum, or stacks of membranes called thylakoids. Thylakoids contain chlorophyll, the pigment that gives plants their green color and allows plants to capture the energy of sunlight. Chlorophyll is green because it reflects green light and absorbs other colors of light, particularly the red and blue light present in sunlight.

Materials of Photosynthesis

Plants need carbon dioxide and water to carry out photosynthesis. Carbon dioxide, a gas found in the air, enters the plant through stomata, small openings on the underside of leaves. Plants absorb water in their roots and then transport the water through xylem, up the stem to the leaves.

Photosynthesis Plant

During photosynthesis, a plant uses the energy of sunlight to convert carbon dioxide and water into glucose and oxygen, which are the products of photosynthesis. The plant uses the glucose as energy to carry out life functions. Some of the oxygen produced from photosynthesis is used by the plant during cellular respiration, and the rest exits the plant through the stomata.

The entire process of photosynthesis can be summarized by the following equation. It shows that six molecules of carbon dioxide and six molecules of water are needed to form one sugar molecule and six molecules of oxygen.

light energy + 6C02 + 6H20 → C6H12O6 + 602

Light-Dependent Reactions

Many chemical reactions take place during photosynthesis. During the light dependent reactions, chlorophyll molecules capture light energy from the Sun. A photon, or particle of light, hits an antenna-like structure that contains chlorophyll. This causes an electron to go to an excited state. The energy from the excited electron is used to split water molecules into hydrogen ions (H+), oxygen molecules (02), and electrons. An ion is an atom or molecule with an electrical charge. The plus sign next to the symbol for hydrogen (H) indicates that the hydrogen ion has a positive charge.

Two other types of molecules, NADPH and ATP (adenosine triphosphate), are produced during the light-dependent reactions. Enzymes, complex proteins that regulate chemical reactions in organisms, aid in production of ATP.

Calvin Cycle

The next step in photosynthesis, the Calvin cycle, combines the hydrogen ions and electrons with carbon dioxide from the air to form sugars and other carbohydrates. Energy for these reactions is from the NADPH and ATP produced previously. The Calvin cycle reactions take place in a space inside chloroplasts called the stroma. Several enzymes are used in the various chemical reactions that take place in the Calvin cycle.


Cellular Respiration

Plants get the energy they need from the food they make during photosynthesis. Animals, on the other hand, cannot make their own food, so they eat plants and other animals to get the energy they need.

Plants, animals, and other organisms must release the energy stored in the organic molecules of food. One way they do this is through cellular respiration, a process in which glucose and other organic molecules are broken down to release energy in the presence of oxygen.

Most of the process of cellular respiration occurs in mitochondria, where the enzymes necessary for the chemical reactions are stored. As shown in the diagram, a mitochondrion has an outer membrane and an inner membrane.


The process of cellular respiration is summarized in the following equation. Despite the simple equation, cellular respiration does not take place in one simple step. Instead the process occurs in a series of steps. In each step, cells can trap bits of energy and change that energy into a form the cell can use.

C6H12O6 + 602 = 6C02 + 6H20 + Energy

Stages of Cellular Respiration

In the first part of cellular respiration, known as glycolysis, two molecules of ATP provide the energy to break apart a glucose molecule. This produces two molecules of a different sugar, pyruvate, and two molecules of ATP, a molecule for storing energy. Glycolysis can also break apart other organic molecules such as proteins, carbohydrates, and fats. Glycolysis occurs outside the mitochondria.

Kreb’s Cycle

The next step of cellular respiration requires the presence of oxygen for the pyruvate molecules to enter the mitochondria. Inside the mitochondria, the pyruvate is changed into acetyl coenzyme A. The acetyl coenzyme A then enters the tri-carboxylic acid cycle, also known as the Krebs cycle. During this stage, the acetyl coenzyme A is then changed in a long string of chemical reactions aided by enzymes. The Krebs cycle produces carbon dioxide and energy and occurs in the inner folded membrane of the mitochondrion.

Oxidative Phosphorylation

The last stage of cellular respiration is called oxidative phosphorylation and occurs in the folded inner membrane of the mitochondrion. Hydrogen atoms from the Krebs cycle have lots of energy. During this last stage, the hydrogen atoms are split into protons and high-energy electrons. The electrons are passed down an electron-transport chain. This transfer of electrons provides energy for the production of ATP. Eventually the protons and electrons join with oxygen to form water.

During the entire process of cellular respiration, one molecule of glucose will produce 38 molecules of ATP. Some energy in this process will be released as heat. Carbon dioxide and water are the waste products of cellular respiration.

A Continuous Cycle

Did you notice anything familiar about cellular respiration? The materials needed for cellular respiration are the materials produced during photosynthesis. And the materials produced during cellular respiration are the materials needed for photosynthesis. The chemical equation for cellular respiration is the reverse of the chemical equation for photosynthesis, although the processes are not the reverse of each other. This means that oxygen and carbon dioxide are both cycled through respiration and photosynthesis.


Cellular respiration is said to be an aerobic process because it requires oxygen. When cells cannot get the oxygen they need, they use a process called fermentation to release energy stored in organic molecules. This process is said to be anaerobic because it does not require oxygen. Fermentation does not provide cells with as much energy as cellular respiration does. Two common types of fermentation are lactic-acid fermentation and alcoholic fermentation.

Lactic-Acid Fermentation

Lactic-acid fermentation is used to make certain foods such as yogurt and cheese. It also occurs in your muscles during strenuous exercise. As in cellular respiration, two molecules of ATP provide the energy to break down a glucose molecule into two molecules of pyruvate, which produces two molecules of ATP. However, during lactic-acid fermentation, the pyruvate is changed into lactic acid instead of more ATP molecules.

Alcoholic Fermentation

Alcoholic fermentation, which occurs in some single-celled organisms, is used to produce bread and beverages such as wine and beer. In alcoholic fermentation, glucose is changed into ethyl alcohol, or ethanol, two molecules of ATP, and carbon dioxide.