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FUNCTIONS OF A CELL
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The image at the right shows a model of a spaceship from
a popular movie. You may not be able to tell by looking at
it, but it was constructed completely out of plastic building
blocks.
Imagine taking apart this model. You might begin by
removing large sections. But, suppose you keep breaking
down the model until the pieces can no longer be broken
down further. What do you have? Can you picture the
smallest building blocks that make up this model?
Plastic building blocks
were used to make this
model of a spaceship.
Now imagine taking apart a leaf from a plant. Suppose
that you do this with the help of a microscope. What is the
smallest unit you can reduce the leaf to and still identify it as part of a plant? Do you need
the microscope to be able to see this unit?
The Cell Theory
When broken down to its tiniest building block, a leaf is reduced to individual cells. In fact,
any living thing can be reduced to cells. Once scientists discovered that cells are the basic
unit of living things, they developed the cell theory. This theory covers three main ideas.
• All organisms are made up of one or more cells.
The cell is the smallest unit that makes up an organism.
Some organisms are composed of only one cell. These
are called unicellular, or single-celled organisms. The
prefix “uni-” means one, like a unicycle with one wheel.
Other organisms are composed of many cells. These
are known as multicellular organisms. The prefix “multi-”
means more than one. For example, multiple items
means more than one item.
• Cells are the basic units of structure and function
in organisms. The cell is the smallest living unit of an
organism. Cells are made of smaller structures called
atoms, but atoms are nonliving units. The cell represents
the basic building block of all organisms.
A bean plant is
multicellular, as shown
by a close-up of the bean
plant’s root cells.
• Cells come only from other living cells. A single cell can divide to produce two cells.
This process of cell division provides a way for cells to increase in number. Cell division
is the process by which living cells produce new cells. New cells can only come from
existing cells.
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FUNCTIONS OF A CELL
what do you think?
A long time ago, people were confused when
they observed flies emerging from rotting
meat. They assumed that the flies arose
spontaneously from the meat. This idea was
called spontaneous generation.
In 1668, an Italian scientist named Francesco
Redi did an experiment to test the idea of
spontaneous generation. He prepared three
identical jars with raw meat. He left one jar
open. He covered the second jar with cloth
and the third jar with heavy paper. Flies were
able to enter the first jar but not the other two
jars. After a few days, Redi observed that
flies emerged from meat in the first jar but not in the others. Which of the three main ideas
of the cell theory does this observation support? Explain your reasoning.
Discover Science: Cell Discovery and Technology
How were scientists able to develop the cell theory? The invention of the microscope
occurred before cell theory was developed. Once microscopes were made with sufficient
magnification, scientists could observe cells. These observations led to the cell theory.
Robert Hooke was the first scientist to observe
cells. Hooke built a simple microscope and used
it to observe a variety of organisms. He described
his findings in a book titled, Micrographia that was
published in 1665. In his book, Hooke provides an
illustration of the structure of cork, which is made
from plant stems. He describes the repeating
small structures making up the cork as “cells.” The
structures reminded him of the cells or rooms in a
monastery.
A scanning electron microscope
allows scientists to study the
structure of bacteria.
Since Hooke’s time, more powerful microscopes
have been developed. Today, scientists use electron
microscopes to observe bacterial cells, which are
tiny, simple cells without a nucleus. Bacterial cells are 10 to 100 times smaller than more
complex cells that have a nucleus. Electron microscopes can also be used to view the
structures of tiny organelles within eukaryotic cells.
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FUNCTIONS OF A CELL
Cellular Energy
Earlier, you learned that one of the main
ideas of the cell theory was that the cell
is the smallest living unit in organisms.
In order to live, organisms must take in
energy from their environment. Their cells
convert this energy into forms that they
can use to grow and reproduce. Animals
also use energy to move about their
environment. Some animals use energy
to sense their environment and to process
information. For example, an animal may
use energy to sense seasonal changes in
temperature and react by seeking cooler
or warmer places to live. These are all
forms of biological energy.
Cells use metabolic processes to convert
the energy in food into useful forms
of energy. Waste products from these
processes are expelled.
So, how do cells convert the energy
obtained from food into useful energy?
Metabolism is the overall process that cells use to make these conversions. Metabolism
involves many chemical reactions. These reactions are linked so that the products (what is
made in a reaction) of one reaction are the reactants (what goes into a reaction) of another.
Some products of metabolism are not useful and are expelled as waste.
Metabolic Reactions: Cellular Respiration
The term metabolism refers to all of the chemical reactions
that take place inside a cell. One important subset of these
reactions is cellular respiration. In eukaryotes, cellular
respiration occurs in the mitochondria. In prokaryotes,
cellular respiration occurs in the cytoplasm near the cell
membrane.
In cellular respiration, carbohydrates, fats, and proteins are
broken down. Energy is released and used by cells. The
gas carbon dioxide, which is a compound with the formula
CO2, is also released during cellular respiration.
The mitochondria carry
out cellular respiration
in eukaryotes. They are
often called the “power
factories” of cells.
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FUNCTIONS OF A CELL
look out!
Photosynthetic organisms carry out cellular respiration, too. It is important to remember that
both plant and animal cells need the energy released during cellular respiration. Animals
obtain the substances broken down during respiration by eating plants or other animals.
In contrast, plants obtain these substances through photosynthesis. They use energy from
sunlight to convert carbon dioxide and water into food substances. These food substances
are broken down during cellular respiration.
Important Cell Structures
A single cell is made up of many different parts that work
together. Each individual part of a cell performs a specific
function or set of functions. This is similar to the way in which
an organism is made up of many structures that perform
specific functions. For example, some structures of a frog
include legs that help it jump, eyes that allow it to see, and
skin that protects its body and helps it breathe.
You learned that mitochondria are involved in the
transformation of energy from food into a form cells can use.
Let’s learn about some other important cell structures found in
eukaryotic cells.
• Cell membrane: A cell is surrounded by a cell membrane. This structure regulates
which substances can enter and exit the cell. All things needed by a cell, such as
nutrients and water, enter through the cell membrane. Waste products leave a cell
through the cell membrane. Skin acts in a similar way to a cell membrane. Skin allows
some substances to enter the body and prevents some substances from entering the
body. Skin also lets some substances, like sweat, out of the body.
• Cell wall: Plant cells have a stiff cell wall that surrounds the cell membrane. The cell
wall provides protection and support for the cell. Its function is similar to the function of
a skeleton in an organism. Animal cells do not have a cell wall.
• Nucleus: The nucleus is the control center of a eukaryotic cell. It controls most of the
cell’s activities, including cell division and growth. The nucleus is like the “brain” of a
cell.
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FUNCTIONS OF A CELL
• Cytoplasm: The cytoplasm is a gel-like substance that includes all the living material
in a cell except the nucleus. Many of the necessary cell reactions take place in the
cytoplasm.
• Lysosomes: The function of the lysosome is to break down large food molecules into
smaller food molecules. Lysosomes also break down waste products and old cell parts.
They can be thought of as “garbage collectors” for the cell.
• Vacuoles: Plant and animal cells contain vacuoles. However, most plant cells have a
larger central vacuole. Animal cells have smaller vacuoles. These structures are the
“storage houses” of a cell. They store water and food materials.
• Endoplasmic reticulum: This structure works as a kind of factory for making important
substances for the cell like proteins and lipids. The endoplasmic reticulum also acts as
a “highway” for the cell; it transports these substances throughout the cell.
• Golgi apparatus: The Golgi apparatus is structure that modifies and packages proteins.
It can be thought of as the “shipping center” of a cell.
• Chloroplasts: Plant cells have green chloroplasts. These structures help trap energy
from sunlight. This energy is used during photosynthesis. Animal cells do not have
chloroplasts.
What do you know?
Now that you have explored the functions of cells, use your knowledge to compare a whole
organism to a cell. Complete the chart by writing the correct terms from the word box
below in the chart on the next page. Note that there are more words than you will need to
complete the chart and some words may be used twice.
Word Box
• Endoplasmic reticulum
• Golgi apparatus
• Lungs
• Lysosome
• Small intestine
• Mouth
• Mitochondria
• Urinary system
• Cell membrane
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FUNCTIONS OF A CELL
Process
Human organ or
structure
Cell organelle or
structure
Take in food
Digest food
Respiration
Process waste for
disposal
Now take some time to compare animal and plant cells. Read each description in the top
row of the chart. Decide whether the description applies to each type of cell. Circle Y for
yes and N for no. Then explain your answer in the space provided in each block of the
table.
Animal and Plant Cells
Animal cell
Cannot be broken Obtains energy
Carries out
down further into by eating other
cellular
a living unit
organisms
respiration
Y
N
Y
N
Y
N
Distributes
materials in
the cell
Y
N
Plant cell
Y
Y
N
Y
N
Y
N
N
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FUNCTIONS OF A CELL
connecting with your child
Exploring Cellular Respiration and Mitochondria in Different Cell Types
You can observe cellular respiration with
Here are some questions to discuss with
your child by following this procedure.
students before they begin their research:
1. Place one-quarter cup of warm tap
water in a sealable plastic sandwich
bag.
• Why might different cells in the human
body have different numbers of
mitochondria?
2. Add one teaspoon of table sugar and
one tablespoon of active dry yeast to
the water in the bag. Carefully press
to remove most of the air in the bag
and then seal the bag.
• Which cell types do you think have large
numbers of mitochondria compared to
others?
3. Gently mix the contents. Then, place
the bag in a warm spot for about
30 minutes.
4. Check the bag and record your
observations. What change suggests
the yeasts have been carrying out
cellular respiration?
Here are some questions to discuss with
students after they conduct their research:
• Were your predictions correct?
• What surprised you about the information
you found?
To expand students’ understanding of the
role of mitochondria in human cells, have
them conduct research to locate information
about the numbers of mitochondria in
various human cells. Students can research
the average numbers of mitochondria found
in muscle cells, blood cells, skin cells, heart
cells, and liver cells. Encourage students to
think about the reasons for any differences
and to research what scientists understand
to be reasons for the differences.
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