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C2 - Elements in Cells

I can list the main elements in cells and diagram how they cycle through the environment.


If you look at your hand, what do you see? Of course, you see skin, which consists of cells. But what are skin cells made of? Like all living cells, they are made of matter. In fact, all things are made of matter. Matter is anything that takes up space and has mass. Matter, in turn, is made up of chemical substances. In this lesson you will learn about the chemical substances that make up living things.

Chemical Substances

A chemical substance is matter that has a definite composition. It also has the same composition throughout. A chemical substance may be either an element or a compound.


An element is a pure substance. It cannot be broken down into other types of substances. Each element is made up of just one type of atom. An atom is the smallest particle of an element that still has the properties of that element.

There are almost 120 known elements. As you can see from Figure below, the majority of elements are metals. Examples of metals are iron (Fe) and copper (Cu). Metals are shiny and good conductors of electricity and heat. Nonmetal elements are far fewer in number. They include hydrogen (H) and oxygen (O). They lack the properties of metals.

Periodic Table of the Elements. The Periodic Table of the Elements arranges elements in groups based on their properties. The element most important to life is carbon (C). Find carbon in the table. What type of element is it, metal or nonmetal?


A compound is a substance that consists of two or more elements. A compound has a unique composition that is always the same. The smallest particle of a compound is called a molecule. Consider water as an example. A molecule of water always contains one atom of oxygen and two atoms of hydrogen. The composition of water is expressed by the chemical formula H2O. A model of a water molecule is shown in Figure below.

Water Molecule. A water molecule always has this composition, one atom of oxygen and two atoms of hydrogen.

What causes the atoms of a water molecule to “stick” together? The answer is chemical bonds. A chemical bond is a force that holds molecules together. Chemical bonds form when substances react with one another. A chemical reaction is a process that changes some chemical substances into others. A chemical reaction is needed to form a compound. Another chemical reaction is needed to separate the substances in a compound.

Where does the water come from that is needed by your cells? Or the carbon and nitrogen that is needed to make your organic molecules? Unlike energy, matter is not lost as it passes through an ecosystem. Instead, matter is recycled. This recycling involves specific interactions between the biotic and abiotic factors in an ecosystem.

Biogeochemical Cycles

The chemical elements and water that are needed by organisms continuously recycle in ecosystems. They pass through biotic and abiotic components of the biosphere. That’s why their cycles are called biogeochemical cycles. For example, a chemical might move from organisms (bio) to the atmosphere or ocean (geo) and back to organisms again. Elements or water may be held for various periods of time in different parts of a cycle.

  • Part of a cycle that holds an element or water for a short period of time is called an exchange pool. For example, the atmosphere is an exchange pool for water. It usually holds water (in the form of water vapor) for just a few days.
  • Part of a cycle that holds an element or water for a long period of time is called a reservoir. The ocean is a reservoir for water. The deep ocean may hold water for thousands of years.

The rest of this lesson describes three biogeochemical cycles: the water cycle, carbon cycle, and nitrogen cycle.

The Water Cycle

Water on Earth is billions of years old. However, individual water molecules keep moving through the water cycle. The water cycle is a global cycle. It takes place on, above, and below Earth’s surface, as shown in Figure below.

Like other biogeochemical cycles, there is no beginning or end to the water cycle. It just keeps repeating.

During the water cycle, water occurs in three different states: gas (water vapor), liquid (water), and solid (ice). Many processes are involved as water changes state in the water cycle.

Evaporation, Sublimation, and Transpiration

Water changes to a gas by three different processes:

  1. Evaporation occurs when water on the surface changes to water vapor. The sun heats the water and gives water molecules enough energy to escape into the atmosphere.
  2. Sublimation occurs when ice and snow change directly to water vapor. This also happens because of heat from the sun.
  3. Transpiration occurs when plants release water vapor through leaf pores called stomata (see Figure below). The water is a product of photosynthesis.

Plant leaves have many tiny stomata. They release water vapor into the air.

Condensation and Precipitation

Rising air currents carry water vapor into the atmosphere. As the water vapor rises in the atmosphere, it cools and condenses. Condensation is the process in which water vapor changes to tiny droplets of liquid water. The water droplets may form clouds. If the droplets get big enough, they fall as precipitation—rain, snow, sleet, hail, or freezing rain. Most precipitation falls into the ocean. Eventually, this water evaporates again and repeats the water cycle. Some frozen precipitation becomes part of ice caps and glaciers. These masses of ice can store frozen water for hundreds of years or longer.

Groundwater and Runoff

Precipitation that falls on land may flow over the surface of the ground. This water is called runoff. It may eventually flow into a body of water. Some precipitation that falls on land may soak into the ground, becoming groundwater. Groundwater may seep out of the ground at a spring or into a body of water such as the ocean. Some groundwater may be taken up by plant roots. Some may flow deeper underground to an aquifer. This is an underground layer of rock that stores water, sometimes for thousands of years.

The water cycle is demonstrated at http://www.youtube.com/watch?v=iohKd5FWZOE&feature=related (4:00).

The Water Cycle Jump can be viewed at http://www.youtube.com/watch?v=BayExatv8lE. (1:31).

KQED: Tracking Raindrops

We all rely on the water cycle, but how does it actually work? Scientists at University of California Berkeley are embarking on a new project to understand how global warming is affecting our fresh water supply. And they're doing it by tracking individual raindrops in Mendocino and north of Lake Tahoe. See http://www.kqed.org/quest/television/tracking-raindrops for more information.

The Carbon Cycle

Flowing water can slowly dissolve carbon in sedimentary rock. Most of this carbon ends up in the ocean. The deep ocean can store carbon for thousands of years or more. Sedimentary rock and the ocean are major reservoirs of stored carbon. Carbon is also stored for varying lengths of time in the atmosphere, in living organisms, and as fossil fuel deposits. These are all parts of the carbon cycle, which is shown in Figure below.

The Carbon Cycle. Carbon moves from one reservoir to another in the carbon cycle. What role do organisms play in this cycle?

The carbon cycle is discussed in the following video: http://www.youtube.com/watch?v=0Vwa6qtEih8 (1:56).

Carbon cycles quickly between organisms and the atmosphere. Cellular respiration releases carbon into the atmosphere as carbon dioxide. Photosynthesis removes carbon dioxide from the atmosphere and uses it to make organic compounds. Carbon cycles far more slowly through geological processes such as sedimentation. Carbon may be stored in sedimentary rock for millions of years.

The Nitrogen Cycle

Nitrogen makes up 78 percent of Earth’s atmosphere. It’s also an important part of living things. Nitrogen is found in proteins, nucleic acids, and chlorophyll. The nitrogen cycle moves nitrogen through the abiotic and biotic parts of ecosystems. Figure below shows how nitrogen cycles through a terrestrial ecosystem. Nitrogen passes through a similar cycle in aquatic ecosystems.

Nitrogen Cycle in a Terrestrial Ecosystem. Nitrogen cycles between the atmosphere and living things.

Plants cannot use nitrogen gas from the air to make organic compounds for themselves and other organisms. The nitrogen gas must be changed to a form called nitrates, which plants can absorb through their roots. The process of changing nitrogen gas to nitrates is called nitrogen fixation. It is carried out by nitrogen-fixing bacteria. The bacteria live in soil and roots of legumes, such as peas.

When plants and other organisms die, decomposers break down their remains. In the process, they release nitrogen in the form of ammonium ions. Nitrifying bacteria change the ammonium ions into nitrates. Some of the nitrates are used by plants. Some are changed back to nitrogen gas by denitrifying bacteria.

The nitrogen cycle is discussed at http://www.youtube.com/watch?v=pdY4I-EaqJA&feature=fvw (5:08).

Lesson Summary

  • Chemical elements and water are recycled through biogeochemical cycles. The cycles include both biotic and abiotic parts of ecosystems.
  • The water cycle takes place on, above, and below Earth’s surface. In the cycle, water occurs as water vapor, liquid water, and ice. Many processes are involved as water changes state in the cycle. The atmosphere is an exchange pool for water. Ice masses, aquifers, and the deep ocean are water reservoirs.
  • In the carbon cycle, carbon passes among sedimentary rocks, fossil fuel deposits, the ocean, the atmosphere, and living things. Carbon cycles quickly between organisms and the atmosphere. It cycles far more slowly through geological processes.
  • The nitrogen cycle moves nitrogen back and forth between the atmosphere and organisms. Bacteria change nitrogen gas from the atmosphere to nitrogen compounds that plants can absorb. Other bacteria change nitrogen compounds back to nitrogen gas, which re-enters the atmosphere.

Lesson Review Questions


1. What is a biogeochemical cycle? Name an example.

2. Identify and define two processes by which water naturally changes from a solid or liquid to a gas.

3. Define exchange pool and reservoir, and identify an example of each in the water cycle.

4. State three ways that carbon dioxide enters Earth’s atmosphere.

5. List all the ways that a single tree may be involved in the carbon cycle.

Apply Concepts

6. Assume you are a molecule of water. Describe one way you could go through the water cycle, starting as water vapor in the atmosphere.

7. Read the following passage, then apply information from the lesson to explain why the farmer plants peas:

A farmer has three fields in which she grows corn for market. Every year, she plants one of the fields with peas, even though she cannot make as much money selling peas as she can selling corn. She rotates the fields she plants with peas so that each field is planted with peas every 3 years.

Think Critically

8. Compare and contrast biological and geological pathways of the carbon cycle.

9. Explain why bacteria are essential parts of the nitrogen cycle.