Science (Grade 7) Cells

Basic Cell Characteristics | Nucleus, Mitochondria, Chloroplasts | Mitosis and Organism Development

Lesson Preview

There are two very special qualities of living beings.   

1.    The element carbon plays a vital role. Carbon is the only element that can combine with nitrogen and hydrogen to from stable and very large molecules such as DNA and proteins. Carbon can also that combine with oxygen to form a gas. This is important because carbon dioxide is one of the waste products of living beings and because it is a gas, it is easy to expel from the body. As far as we know, carbon is the only element capable of forming the “backbone of life”. Other than the elements named above, life also depends on sulfur, potassium, sodium, iron, calcium and magnesium, chlorine and trace amounts of copper and selenium and other elements.
  
2.    Living beings are composed of cells. Some beings have just one cell; some beings have many millions or billions of cells. Cells are highly complex with thousands of chemical reactions occurring at any one time. How did living cells arise from chemical elements in the air, sea and land? Since we cannot go back in time to find out, scientists have simulated what they believe to be the initial weather, atmospheric and geological conditions of earth soon after it formed and believe that very slowly complex molecules were formed which over thousands of years began to self-replicate and later form cells. The oldest fossils of cells are 3.5 millions years old.

To learn more about the origins of life, go to http://www.pbs.org/wgbh/nova/origins. Of special interest is the interview with astrophysicist Neil deGrasse Tyson.

This lesson is an introduction to biological cells.

 

Objectives

This lesson will explain:  

  1. cell functions
  2. the differences between plant and animal cells
  3. the role of the nucleus
  4. how energy is captured by the cells
  5. mitosis
  6. how cells differentiate.

 

Basic Cell Characteristics
CCSTD Science Grade 7 1.a., b

You are made up of cells. So are dogs and cats and trees and fish. Cells are the basic building blocks of life. The word cell was first coined by the biologist Robert Hooke when he was looking at a piece of cork under a microscope. The cork was divided into sections that looked like monastic cells, so Hooke called the sections he saw “cells”.

All cells have many functions, but each has a function in common, which is to help keep an organism alive. There are many different kinds of cells.

Each organism has its own kind of cells. Human cells are different from cow cells or tree cells or frog cells or turnip cells. And even within a specific organism, there are many different kinds of cells. Every human, for example, is made up of about 100 trillion cells -- brain cells and bone cells and stomach cells and many other kinds of cells.

For just one example of the specialization of cells, we can study skin cells -- they are flat and waterproof and are constantly being created in the lower epidermis (the lower layer of the outer layer of skin). Then they migrate to the upper epidermis (the outer layer of skin) where they flake off and die. Here are some other examples: Skeletal muscle cells function to provide movement for animals. Liver cells have many functions – one important one is to detoxify harmful chemicals in the body. Red blood cells function to transport oxygen from the lungs to the rest of the body. White blood cells help to protect us against infection.

There are several ways to classify cells, but the main division is between eukaryotic and prokaryotic cells.

Eukaryotic vs. Prokaryotic Cells

There are big differences between eukaryotic and prokaryotic cells, but they do have some things in common: 

  1. Both have DNA as their genetic material.
  2. Both are encased in membranes. The membranes of Prokaryotic cells and of some eukaryotic cells are surrounded by a strong cell wall.
  3. Both have a similar basic metabolism (the word metabolism refers to the formation and break down of chemicals in the body).

The major difference is that eukaryotes have a nucleus and prokaryotes do not.

Also, eukaryotic cells contain a wide variety of organelles (tiny organs or structures inside the cell ) that perform various functions. Some of the most important organelles are mitochondria, chloroplasts, lysosomes and ribosomes -- which we'll tell you about in our next section of this lesson.

But first, here is the basic structure of an eukaryotic cell:

  1. The membrane (the outside or coating) 
  2. The nucleus (the place where the DNA is stored and from where instructions are sent to the rest of the cell for the production of proteins. The nucleus has its own membrane)
  3. The cytoplasm or cytosol, which contains everything between the membrane and the nucleus (including organelles)

Eukaryotic cells are, on average, ten times as large as prokaryotic cells. Their DNA is more complex and is formed into chromosomes. And their organelles enable them to do more complex jobs.

3.5 Billion Years Ago

Many scientists think that prokaryotic cells arose about 3.5 billion years ago -- and that they were the first life forms on Earth; they believe this because they have found fossils of prokaryotic cells that are 3.5 billion years old. After about 2 billion years, aggregates of prokaryotic cells evolved into eukaryotic cells.

Today, the only prokaryotes we know of are bacteria (like the bacteria that give you strep throat and the bacteria in our digestive systems that help us to digest our food) and cyanobacteria (blue-green algae). To learn more about prokaryotic cells, go to http://www.biology4kids.com/files/micro_prokaryote.html

All other organisms are formed of cells that are eukaryotes. Different eukaryotes have different characteristics. Let’s begin by looking at the structure of plant cells.

 

 

As we said, most plants are made up of eukaryotic cells. But eukaryotic plant cells are different from eukaryotic animal cells. And the biggest difference is the cell membrane.

In animals (like us) the membrane is the actual outside of the cell. But plants have something extra -- a cell wall that goes all the way around the outside of the membrane, shown with the brown shading in the diagram above.

Remember, the main difference between plant cells and animals cells is this -- plant cells have a cell wall and animal cells don't. This cell wall is made of cellulose.

Cellulose is important in human nutrition since it provides fiber (roughage), which keeps your digestive system going. That's why it's important to eat a diet rich in plant material like fruits, whole grains and vegetables.

 

Experiments for Home and Classroom

Plants provide needed fiber in our diet. In this "Plant Parts Salad" activity, students are invited to delve a little more deeply into the role of various plant parts in our diet. http://www.hhmi.org/coolscience/vegquiz/index.html

 

Do you want to see more of this lesson?  Sign up for our free trial membership!
During your free 10-day trial, you will have access to all of eTap's features, including professional lessons and personal support from real people. If you're not delighted, you can cancel at any time during the free trial and you won’t be billed a dime. This is eTap: No hidden fees. No penalties. Just a great educational service to help children succeed.