lifes building blocks

discovery of cells and the cell theory

Cells were first discovered in the mid-1600s. The cell theory came about some 200 years later. You can see a re- enactment of some of the discoveries that led to the cell theory in this video: MEDIA Click image to the left or use the URL below. URL:

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structures found in all cells

All cells have certain parts in common. These parts include the cell membrane, cytoplasm, DNA, and ribosomes. The cell membrane is a thin coat of phospholipids that surrounds the cell. Its like the skin of the cell. It forms a physical boundary between the contents of the cell and the environment outside the cell. It also controls what enters and leaves the cell. The cell membrane is sometimes called the plasma membrane. Cytoplasm is the material inside the cell membrane. It includes a watery substance called cytosol. Besides water, cytosol contains enzymes and other substances. Cytoplasm also includes other cell structures suspended in the cytosol. DNA is a nucleic acid found in cells. It contains genetic instructions that cells need to make proteins. Ribosomes are structures in the cytoplasm where proteins are made. They consist of RNA and proteins. These four components are found in all cells. They are found in the cells of organisms as different as bacteria and people. How did all known organisms come to have such similar cells? The answer is evolution. The similarities show that all life on Earth evolved from a common ancestor.

prokaryotic and eukaryotic cells

Besides the four parts listed above, many cells also have a nucleus. The nucleus of a cell is a structure enclosed by a membrane that contains most of the cells DNA. Cells are classified in two major groups based on whether or not they have a nucleus. The two groups are prokaryotic cells and eukaryotic cells.

levels of organization

Cells and organelles are made of biochemical molecules. These include nucleic acids and proteins. Molecules, in turn, are made of atoms. Figure 3.6 shows these different levels of organization in living things. As you can see in Figure 3.6, living things also have levels of organization higher than the cell. These higher levels are found only in multicellular organisms with specialized cells. Specialized cells may be organized into tissues. A tissue is a group of cells of the same kind that performs the same function. For example, muscle cells are organized into muscle tissue. The function of muscle tissue is to contract in order to move the body or its parts. Tissues may be organized into organs. An organ is a structure composed of two or more types of tissue that work together to do a specific task. For example, the heart is an organ. It consists of muscle, nerve, and other types of tissues. Its task is to pump blood. Organs may be organized into organ systems. An organ system is a group of organs that work together to do the same job. For example, the heart is part of the cardiovascular system. This system also includes blood vessels and blood. The job of the cardiovascular system is to transport substances in blood to and from cells throughout the body. Organ systems are organized into the organism. The different organ systems work together to carry out all the life functions of the individual. For example, cardiovascular and respiratory systems work together to provide the individual with oxygen and rid it of carbon dioxide.

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why cells are so small

Cells with different functions often vary in shape. They may also vary in size. However, all cells are very small. Even the largest organisms have microscopic cells. Cells are so small that their diameter is measured in micrometers. A micrometer is just one-millionth of a meter. Use the sliding scale at the following link to see how small cells and cell parts are compared with other objects. Why are cells so small? The answer has to do with the surface area and volume of cells. To carry out life processes, a cell must be able to pass substances into and out of the cell. For example, it must be able to pass nutrients into the cell and waste products out of the cell. Anything that enters or leaves a cell has to go through the cell membrane on the surface of the cell. A bigger cell needs more nutrients and creates more wastes. As the size of a cell increases, its volume increases more quickly that its surface area. If the volume of a cell becomes too great, it wont have enough surface area to transfer all of its nutrients and wastes.

specialized cells

All living cells have certain things in common. Besides having the basic parts described above, all cells can perform the same basic functions. For example, all cells can use energy, respond to their environment, and reproduce. However, cells may also have special functions. Multicellular organisms such as you have many different types of specialized cells. Each specialized cell has a particular job. Cells with special functions generally have a shape that suits them for that job. Figure 3.5 shows four examples of specialized cells. Each type of cell in the figure has a different function. It also has a shape that helps it perform that function. The function of a nerve cell is to carry messages to other cells. It has many long arms that extend outward from the cell. The "arms" let the cell pass messages to many other cells at once. The function of a red blood cell is to carry oxygen to other cells. A red blood cell is small and smooth. This helps it slip through small blood vessels. A red blood cell is also shaped like a fattened disc. This gives it a lot of surface area for transferring oxygen. The function of a sperm cell is to swim through fluid to an egg cell. A sperm cell has a long tail that helps it swim. The function of a pollen cell is to pollinate flowers. The pollen cells in the figure have tiny spikes that help them stick to insects such as bees. The bees then carry the pollen cells to other flowers for pollination.

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eukaryotic cells

Eukaryotic cells are cells that contain a nucleus. They are larger than prokaryotic cells. They are also more complex. Living things with eukaryotic cells are called eukaryotes. All of them belong to the Eukarya Domain. This domain includes protists, fungi, plants, and animals. Many protists consist of a single cell. However, most eukaryotes have more than one cell. You can see a model of a eukaryotic cell in Figure 3.4. The cell in the figure is an animal cell. The nucleus is an example of an organelle. An organelle is any structure inside a cell that is enclosed by a membrane. Eukaryotic cells may contain many different organelles. Each does a special job. For example, the mitochondrion is an organelle that provides energy to the cell. You can see a mitochondrion and several other organelles in the animal cell in Figure 3.4. Organelles allow eukaryotic cells to carry out more functions than prokaryotic cells can.

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prokaryotic cells

Prokaryotic cells are cells that lack a nucleus. The DNA in prokaryotic cells is in the cytoplasm, rather than enclosed within a nuclear membrane. All the organisms in the Bacteria and Archaea Domains have prokaryotic cells. No other organisms have this type of cell. Organisms with prokaryotic cells are called prokaryotes. They are all single-celled organisms. They were the first type of organisms to evolve. They are still the most numerous organisms today. You can see a model of a prokaryotic cell in Figure 3.3. The cell in the figure is a bacterium. Notice how it contains a cell membrane, cytoplasm, ribosomes, and several other structures. However, the cell lacks a nucleus. The cells DNA is circular. It coils up in a mass called a nucleoid that floats in the cytoplasm.

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early observations of cells

British scientist Robert Hooke first discovered cells in 1665. He was one of the earliest scientists to study living things under a microscope. He saw that cork was divided into many tiny compartments, like little rooms. (Do the cells in Figure 3.1 look like little rooms to you too?) Hooke called these little rooms cells. Cork comes from trees, so what Hooke observed was dead plant cells. In the late 1600s, Dutch scientist Anton van Leeuwenhoek made more powerful microscopes. He used them to observe cells of other organisms. For example, he saw human blood cells and bacterial cells. Over the next century, microscopes were improved and more cells were observed.

development of the cell theory

By the early 1800s, scientists had seen cells in many different types of organisms. Every organism that was examined was found to consist of cells. From all these observations, German scientists Theodor Schwann and Matthias Schleiden drew two major conclusions about cells. They concluded that: cells are alive. all living things are made of cells. Around 1850, a German doctor named Rudolf Virchow was observing living cells under a microscope. As he was watching, one of the cells happened to divide. Figure 3.2 shows a cell dividing, like the cell observed by Virchow. This was an aha moment for Virchow. He realized that living cells produce new cells by dividing. This was evidence that cells arise from other cells. The work of Schwann, Schleiden, and Virchow led to the cell theory. This is one of the most important theories in life science. The cell theory can be summed up as follows: All organisms consist of one or more cells. Cells are alive and the site of all life processes. All cells come from pre-existing cells.

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instructional diagrams

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questions

The scientist who discovered that all living cells arise from other cells was

a. Hooke.

b. Schwann.

c. Schleiden.

-->  d. Virchow.

The cells of all living things have

a. a cell membrane.

b. cytoplasm.

c. ribosomes.

-->  d. all of the above

Which statement about prokaryotes is false?

a. They were the first type of organisms to evolve.

-->  b. They are the least numerous organisms today.

c. They have cells that lack a nucleus.

d. They are all single-celled organisms.

A group of cells of the same kind that perform the same function make up a(n)

a. organ.

-->  b. tissue.

c. organelle.

d. organ system.

Which level of organization is found in eukaryotes but not in prokaryotes?

a. atoms

b. molecules

c. cells

-->  d. none of the above

__Only eukaryotic cells contain DNA.

a. true

-->  b. false

__Cell theory was introduced as soon as cells were discovered.

a. true

-->  b. false

__All cells contain cytoplasm.

-->  a. true

b. false

__Prokaryotic cells lack ribosomes.

a. true

-->  b. false

All living cells can use energy.

-->  a. true

b. false

Some cells lack DNA.

a. true

-->  b. false

__Cells are classified in two major groups based on whether or not they have a cell membrane.

a. true

-->  b. false

The nucleus of a cell is an example of an organelle.

-->  a. true

b. false

__All single-celled organisms have prokaryotic cells.

a. true

-->  b. false

__All living cells can reproduce.

-->  a. true

b. false

As a cell gets bigger, its surface area increases more quickly than its volume.

a. true

-->  b. false

Some single-celled organisms have eukaryotic cells.

-->  a. true

b. false

__cell structure where proteins are made

a. cell membrane

b. organ

c. prokaryotic cell

d. organelle

e. eukaryotic cell

-->  f. ribosome

g. nucleus

__cell that contains a nucleus

a. cell membrane

b. organ

c. prokaryotic cell

d. organelle

-->  e. eukaryotic cell

f. ribosome

g. nucleus

__any structure inside a cell that is enclosed by a membrane

a. cell membrane

b. organ

c. prokaryotic cell

-->  d. organelle

e. eukaryotic cell

f. ribosome

g. nucleus

__thin coating of phospholipids that surrounds a cell

-->  a. cell membrane

b. organ

c. prokaryotic cell

d. organelle

e. eukaryotic cell

f. ribosome

g. nucleus

__membrane enclosed structure in a cell that contains most of the cells DNA

a. cell membrane

b. organ

c. prokaryotic cell

d. organelle

e. eukaryotic cell

f. ribosome

-->  g. nucleus

__structure composed of two or more types of tissues that work together to do a specific task

a. cell membrane

-->  b. organ

c. prokaryotic cell

d. organelle

e. eukaryotic cell

f. ribosome

g. nucleus

__cell that lacks a nucleus

a. cell membrane

b. organ

-->  c. prokaryotic cell

d. organelle

e. eukaryotic cell

f. ribosome

g. nucleus

The smallest unit of living things that can carry out the chemical reactions of life is the

a. organ system

b. organ

c. tissue

-->  d. cell

Cells were first discovered in the

a. 1500s

-->  b. 1600s

c. 1700s

d. 1800s

Scientists who contributed to the development of the cell theory included

a. Virchow

b. Schleiden

c. Schwann

-->  d. all of the above

The cell theory includes all of the following ideas except

-->  a. all cells contain a nucleus

b. all organisms consist of one or more cells

c. all cells come from pre-existing cells

d. cells are alive

Which parts do all cells have in common?

a. cell walls

-->  b. ribosomes

c. mitochondria

d. all of the above

What can all cells do?

a. carry messages

b. transport oxygen

-->  c. use energy

d. make food

Which level of organization is found in prokaryotes?

a. tissue

-->  b. molecule

c. organelle

d. organ

diagram questions

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