CK-12 Life Science
Page 6
Three domains of life: Bacteria, Archaea, and Eukarya Archaea Bacteria Eukarya
Multicelluar No No Yes
Cell Wall Yes, without peptidoglycan Yes, with peptidoglycan Varies. Plants and fungi have a cell wall; animals do not.
Nucleus (DNA inside a membrane) No No Yes
Organelles inside a membrane No No Yes
Viruses
We have all heard of viruses. The flu and many other diseases are caused by viruses. But what is a virus? Based on the material presented in this chapter, are viruses living? No.
A virus is essentially nucleic acid surrounded by protein (Figure below). It is not made of a cell; it does not metabolize, it does not maintain homeostasis. Viruses need to infect a host cell to reproduce; they cannot reproduce on their own. However, viruses do evolve. So a virus is very different than any of the organisms that fall into the three domains of life.
Figure 2.31
These moon lander shaped complex virus infects bacteria.
Lesson Summary
Scientists have defined several major categories for classifying organisms: domain, kingdom, phylum, class, order, family, genus, and species.
The scientific name of an organism consists of its genus and species.
Scientists classify organisms according to their evolutionary histories and how related they are to one another - by looking at their physical features, the fossil record, and DNA sequences.
All life can be classified into three domains: Bacteria, Archaea, and Eukarya.
Review Questions
Who designed modern classification and invented the two-part species name?
In what domain are humans?
Quercus rubra is the scientific name for the red oak tree. What is the red oak’s genus?
In what domain are mushrooms?
Is it possible for organisms in two different classes to be in the same genus?
How are organisms given a scientific name?
Define a species.
What kingdoms make up the domain Eukarya?
What is the name for the scientific study of naming and classifying organisms?
What information do scientists use to classify organisms?
If molecular data suggests that two organisms have very similar DNA, what does that say about their evolutionary relatedness?
Can two different species ever share the same scientific name?
If two organisms are in the same genus, would you expect them to look much alike?
Further Reading / Supplemental Links
http://www.ucmp.berkeley.edu/history/linnaeus.html
http://www.physicalgeography.net/fundamentals/9b.html
http://www.pbs.org/wgbh/nova/orchid/classifying.html
Vocabulary
archaea
Microscopic one-celled organisms with no nucleus that tend to live in extreme environments.
bacteria
Microscopic one-celled organisms with no nucleus that live everywhere.
binomial nomenclature
The system for naming species in which the first word is the genus and the second word is the species.
cyanobacteria
Photosynthetic bacteria.
DNA
Deoxyribonucleic acid; Nucleic acid molecule that stores the genetic information.
Eukarya
Domain in which cells have a nucleus that includes plants, animals, fungi, and protists.
fossils
Ancient remains of living things; includes bone, teeth, and impressions.
nucleus
Tiny structure inside of some cells that walls off the DNA from the rest of the cell; DNA wrapped inside a membrane.
species
Group of organisms that can mate with one another to produce fertile offspring but do not mate with other such groups.
taxonomy
The science of naming and classifying organisms.
Points to Consider
This lesson introduced the diversity of life on Earth. Do you think it is possible for cells from different organisms to be similar even though the organisms look different?
Do you think human cells are different from bacterial cells?
Do you think it is possible for a single cell to be a living organism?
* * *
Chapter 3: Cells and Their Structures
Lesson 3.1: Introduction to Cells
Lesson Objectives
Explain how cells are observed.
Recall the cell theory.
Explain the levels of organization in an organism.
Check Your Understanding
What are the main characteristics of living things?
Name the four main classes of organic molecules that are building blocks of life.
Introduction
How do lipids, carbohydrates, proteins, and nucleic acids come together to form a living organism? By forming a cell. These organic compounds are the raw materials needed for life, and a cell is the smallest unit of an organism that is still considered living. Cells are the basic units that make up every type of organism. Some organisms, like bacteria, consist of only one cell. Other organisms, like humans, consist of trillions of specialized cells working together. Even if organisms look very different from each other, if you look close enough you’ll see that their cells have much in common. (Use of a microscope in Figure below helps to illustrate this.)
Figure 3.1
The outline of onion cells are visible under a light microscope.
Observing Cells
Most cells are so tiny that you can’t see them without the help of a microscope. The microscopes that students typically use at school are light microscopes. Robert Hooke created a primitive light microscope in 1665 and observed cells for the very first time. Although the light microscope opened our eyes to the existence of cells, they are not useful for looking at the tiniest components of cells. Many structures in the cell are too small to see with a light microscope.
When scientists developed more powerful microscopes in the 1950s, the field of cell biology grew rapidly. A light microscope passes a light beam through a specimen, but the more powerful electron microscope passes a beam of electrons through the specimen, allowing a much closer look at the cell (Figure below).
Transmission electron microscopes (TEM), which pass an electron beam through something, are used to look at a very thin section of an organism and allow us to study the internal structure of cells. Scanning electron microscopes (SEM), which pass a beam of electrons across the surface of something, show the details of the shapes of surfaces, giving a 3D image.
Electron microscopes showed many small structures in the cell that had been previously invisible with light microscopes. One drawback to using an electron microscope is that it only images dead cells. A light microscope can be used to study living cells.
Figure 3.2
An electron microscope allows scientists to see much more detail than a light microscope, as with this sample of pollen. But a light microscope allows scientists to study living cells.
Cell Theory
In 1858, after microscopes had become much more sophisticated than Hooke’s first microscope, Rudolf Virchow proposed that cells only came from other cells. For example, bacteria are composed of only one cell (Figure below ) and divide in half to replicate themselves. In the same way, your body makes new cells by the division of cells you already have. In all cases, cells only come from pre-existing cells.
This concept is central to the cell theory. The cell theory states that:
All organisms are composed of cells.
Cells are alive and the basic living units of organization in all organisms.
All cells come from other cells.
As with other scientific theories, the cell theory has been supported by thousands of experiments. And, since Virchow introduced the cell theory, no evidence has ever contradicted it.
Figure 3.3
Bacteria (pink) are an example of an organism consi
sting of only one cell.
Levels of Organization
Although cells share many of the same features and structures, as we will discuss in the next section, they also can be quite different. Each cell in your body is specialized for a specific task. For example:
Red blood cells (Figure below) are shaped with a pocket to increase their surface area for absorbing and releasing oxygen.
Nerve cells, which can quickly transmit the sensation of touching a hot stove to your brain, are elongated and stringy to allow them to form a complex network with other nerve cells (Figure below).
Skin cells (Figure below) are flat and fit tightly together.
As you can see, cells are shaped in ways that help them do their jobs. Multicellular (many-celled) organisms have many types of specialized cells in their bodies.
Figure 3.4
Red Blood cells are specialized to carry oxygen in the blood.
Figure 3.5
Neurons are shaped to conduct electrical impulses to many other nerve cells.
Figure 3.6
These epidermal cells make up the skin of plants. Note how the cells fit tightly together.
While cells are the basic units of an organism, groups of specialized cells can be organized into tissues. For example, your liver cells are organized into liver tissue, which is organized into an organ, your liver. Organs are formed from two or more specialized tissues working together for a common function. All organs, from your heart to your liver, are made up of an organized group of tissues.
These organs are part of a larger organization pattern, the organ systems. For example, your brain works together with your spinal cord and other nerves to form the nervous system. This organ system must be organized with other organ systems, such as the circulatory system and the digestive system, for your body to work. Organ systems are coordinated together to form the complete organism. As you can see (Figure below), there are many levels of organization in living things.
Figure 3.7
Levels of Organization, from the atom to the organism.
Lesson Summary
Cells were first observed under the light microscope, but today electron microscopes allow scientists to take a closer look at the internal structures of cells
The Cell Theory says that all organisms are composed of cells;
cells are alive and the basic living units of organization in all organisms; and
All cells come from other cells.
Cells are organized into tissues, which are organized into organs, which are organized into organ systems, which are organized to create the whole organism.
Review Questions
What type of microscope would you use to study living algae cells?
What type of microscope would you use to study the details on the surface of a cell?
What type of microscope would be best for studying internal structures of cells?
According to the cell theory, can we synthesize a cell in the laboratory from organic molecules?
Do all cells work exactly the same?
Put the following in the correct order from simplest to most complex: organ, cell, tissue, organ system.
Further Reading / Supplemental Links
Baeuerle, Patrick A. and Landa, Norbert. The Cell Works: Microexplorers. Barron’s; 1997, Hauppauge, New York.
Sneddon, Robert. The World of the Cell: Life on a Small Scale. Heinemann Library; 2003, Chicago.
Wallace, Holly. Cells and Systems. Heinemann Library; 2001, Chicago.
Vocabulary
cell
The smallest unit of an organism that is still considered living; the basic unit that make up every type of organism.
organ
A group of tissues that work together to perform a common function.
organ system
A group of organs that work together to perform a common function.
scanning electron microscope (SEM)
Microscope that scans the surface of a tissue or cell, showing a 3D image.
tissue
A group of specialized cells that function together.
transmission electron microscope (TEM)
Microscope used to look at a very thin section of an organism and allow us to study the internal structure of cells.
Points to Consider
Do you think there would be a significant difference between bacteria cells and your brain cells? What might they be?
Do you think a bacteria cell and brain cell have some things in common? What might they be?
Do you think cells are organized? What would be the benefit of organization?
Lesson 3.2: Cell Structures
Lesson Objectives
Compare prokaryotic and eukaryotic cells.
List the organelles of the cell and their functions.
Discuss the structure and function of the cell membrane and cytosol.
Describe the structure and function of the nucleus.
Distinguish between plant and animal cells.
Check Your Understanding
What is a cell?
How do we visualize cells?
Introduction
Understanding the structure and function of cells is essential to understanding how living organisms work. Cell biology is central to all other fields of biology, including medicine. Many human diseases and disorders are caused by the malfunction of people’s cells. Furthermore, toxins in the environment often act on specific cellular processes. The healthy functioning of the body and its organs is dependent on its smallest unit - the cell.
To better understand the biology of the cell, you will first learn to distinguish the two basic categories of all cells: prokaryotic and eukaryotic cells. You will also learn what makes a cell specialized; there are major differences between a “simple” cell, like a bacteria, and a “complex” cell, like a cell in your brain. To understand these differences, you need to first understand the basic components of the cell, which include the:
Cell membrane
Nucleus and chromosomes
Other organelles
Prokaryotic and Eukaryotic Cells
There are two basic types of cells, prokaryotic cells (Figure below), which include bacteria and archaea, and eukaryotic cells (Figure below), which include all other cells. Prokaryotic cells are much smaller and simpler than eukaryotic cells; eukaryotic cells can be considered to be “specialized.” Prokaryotic cells are surrounded by a cell wall that supports and protects the cell. In prokaryotic cells the DNA, the genetic material, forms a single large circle that coils up on itself. Prokaryotic cells also can contain extra small circles of DNA, known as plasmids. The two types of organisms consisting of prokaryotic cells belong to the domain Bacteria and the domain Archaea. These two domains were discussed in the Introduction to Living Things chapter.
Figure 3.8
Prokaryotes do not have a nucleus. Instead, their genetic material is a simple loop of DNA.
Figure 3.9
Eukaryotic cells contain a nucleus (where the DNA "lives," and surrounded by a membrane) and various other special compartments surrounded by membranes, called "organelles." For example, notice in this image the mitochondria, lysosomes, and peroxisomes.
Comparison of Prokaryotic and Eukaryotic Cells Feature Prokaryotic cells Eukaryotic cells
DNA Single “naked” circle;
plasmids
In membrane-enclosed
nucleus
Membrane-enclosed
organelles
No Yes
Examples Bacteria Plants, animals, fungi
The main difference between eukaryotic and prokaryotic cells is that eukaryotic cells store their DNA in a membrane-enclosed nucleus. The presence of a nucleus is the primary distinguishing feature of a eukaryotic cell. In addition to the nucleus, eukaryotic cells have other subcompartments, small membrane-enclosed structures called organelles. Membrane-enclosed organelles and a nucleus are absent in prokaryotic cells. Eukaryotic cells include the cells o
f fungi, animals, protists, and plants.
The Plasma Membrane and Cytosol
Both eukaryotic and prokaryotic cells have a plasma membrane. The plasma membrane is a double layer of specialized lipids, known as phospholipids, along with many special proteins. The function of the plasma membrane, also known as the "cell membrane," is to control what goes in and out of the cell.
Some molecules can go through the cell membrane in and out of the cell and some can't, so biologists say the membrane is semipermeable. It is almost as if the membrane chooses what enters and leaves the cell.
The cell membrane gives the cell an inside that is separate from the outside world. Without a cell membrane, the parts of a cell would just float away. A cell needs a boundary even more than we need our skin. Without a cell membrane, a cell would be unable to maintain a stable internal environment separate from the external environment, what we call homeostasis. You can learn more about cell membranes in the Cell Functions chapter.
Eukaryotic and prokaryotic cells also share an internal fluid-like substance called the cytosol. The cytosol is composed of water and other molecules, including enzymes that speed up the cell’s chemical reactions. Everything in the cell - the nucleus and the organelles - sit in the cytosol. The term cytoplasm refers to the cytosol and all the organelles, but not the nucleus.