Cells and the Cell Theory
Robert Hooke was the first
person to describe cells. In February 23,
1665, he built a microscope
to look at tiny objects. One
day, he looked at a thin slice
of cork. Cork is found in the
bark of cork trees. The cork
looked like it was made of
little boxes. Hooke named
these boxes cells, which
means “little rooms” in Latin.
Hooke’s cells were really the Objectives
outer layers of dead cork Explain the Cell Theory.
cells. Hooke’s microscope
and his drawing of the cork
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The Cell Theory
Almost 200 years passed before scientists concluded
that cells are present in all living things. Scientist Matthias
Schleiden (mah THEE uhs SHLIE duhn) studied plants. In
1838, he concluded that all plant parts were made of
cells. Theodor Schwann (TAY oh dohr SHVAHN) studied
animals. In 1839, Schwann concluded that all animal
tissues were made of cells. Soon after that, Schwann
wrote the first two parts of what is now known as the cell
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Allorganisms are made up of 1 or more
The Cell is the basic unit of all living things.
Later, in 1858, Rudolf Virchow (ROO dawlf FIR koh), a doctor, stated
that all cells could form only from other cells. Virchow then added
the third part of the cell theory.
Cells come from existing Cells
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Many Small Cells
Thereis a physical reason why most cells
are so small. Cells take in food and get rid
of wastes through their outer surface. As a
cell gets larger, it needs more food and
produces more waste. Therefore, more
materials pass through its outer surface.
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Cell Size Continues….
As the cell’s volume increases, its surface area grows
too. But the cell’s volume grows faster than its surface
area. If a cell gets too large, the cell’s surface area will
not be large enough to take in enough nutrients or
pump out enough wastes. So, the area of a cell’s
surface—compared with the cell’s volume—limits the
cell’s size. The ratio of the cell’s outer surface area to
the cell’s volume is called the surface area–to-volume
ratio, which can be calculated by using the following
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Two Kinds of Cells
Allcells have cell membranes, organelles,
cytoplasm, and DNA. But there are two
basic types of cells—cells without a
nucleus and cells with a nucleus. Cells
with no nucleus are prokaryotic (proh KAR
ee AHT ik) cells. Cells that have a nucleus
are eukaryotic (yoo KAR ee AHT ik) cells.
Prokaryotic cells are further classified into
two groups: bacteria (bak TIR ee uh) and
archaea (AHR kee uh).
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Identify the different parts of a eukaryotic cell.
Explain the function of each part of a eukaryotic cell.
Even though most cells are small, cells are still
complex. A eukaryotic cell has many parts
that help the cell stay alive.
Plant cells and animal cells are two types of
eukaryotic cells. These two types of cells have
many cell parts in common. But plant cells
and animal cells also have cell parts that are
different. Compare the plant cell in Figure 1
and the animal cell in Figure 2 to see the
differences between these two types of cells.
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Plant and Animal Cells
a cell is the smallest unit that can perform
all life processes.
Cellsare covered by a membrane that
separates the inside of the cell and the
cytoplasm from the outside.
cells have DNA, and many store the
DNA in a nucleus.
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1. List three advantages of being multicellular.
2. Describe the four levels of organization in living things.
3. Explain the relationship between the structure and function of a part of an organism
The Benefits of Being Multicellular
You are a multicellular organism. This means that
you are made of many cells. Multicellular
organisms grow by making more small cells, not
by making their cells larger. For example, an
elephant is bigger than you are, but its cells are
about the same size as yours.
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Benefits of Larger Cells
Read page 20 in the textbook and
provide an explanation to each one.
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A tissue is a group of cells that work together to
perform a specific job. The material around and
between the cells is also part of the tissue. The
cardiac muscle tissue, shown in Figure 2, is made
of many cardiac muscle cells. Cardiac muscle
tissue is just one type of tissue in a heart.
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Tissues Working Together
A structure that is made up of two or
more tissues working together to perform
a specific function is called an organ. For
example, your heart is an organ. It is
made mostly of cardiac muscle tissue. But
your heart also has nerve tissue and
tissues of the blood vessels that all work
together to make your heart the powerful
pump that it is.
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Organs Working Together
A group of organs working together to perform a
particular function is called an organ system. Each
organ system has a specific job to do in the body.
For example, the digestive system is made up of
several organs, including the stomach and
intestines. The digestive system’s job is to break
down food into small particles. Other parts of the
body then use these small particles as fuel. In turn,
the digestive system depends on the respiratory
and cardiovascular systems for oxygen.
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Organs working together
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Structure and Function
Structure and Function
In organisms, structure and function are related. Structure is the
arrangement of parts in an organism. It includes the shape of a part
and the material of which the part is made. Function is the job the part
does. For example, the structure of the lungs is a large, spongy sac. In
the lungs, there are millions of tiny air sacs called alveoli. Blood vessels
wrap around the alveoli, as shown in Figure 4. Oxygen from air in the
alveoli enters the blood. Blood then brings oxygen to body tissues.
Also, in the alveoli, carbon dioxide leaves the blood and is exhaled.
The structures of alveoli and blood vessels enable them to perform a
function. Together, they bring oxygen to the body and get rid of its
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• Explain the process of diffusion.
• Describe how osmosis occurs.
• Compare passive transport with active transport.
• Explain how large particles get into and out of cells.
What happens if you pour dye on top of a layer of gelatin? At first,
it is easy to see where the dye ends and the gelatin begins. But
over time, the line between the two layers will blur, as shown in
Figure 1. Why? Everything, including the gelatin and the dye, is
made up of tiny moving particles. Particles travel from where they
are crowded to where they are less crowded. This movement
from areas of high concentration (crowded) to areas of low
concentration (less crowded) is called diffusion (di FYOO zhuhn).
Dye particles diffuse from where they are crowded (near the top
of the glass) to where they are less crowded (in the gelatin).
Diffusion also happens within and between living cells. Cells do
not need to use energy for diffusion.
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Figure 1 The particles of the dye and the gelatin slowly mix by diffusion.
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Diffusion of Water
The cells of organisms are surrounded by and filled with fluids that
are made mostly of water. The diffusion of water through cell
membranes is so important to life processes that it has been
given a special name—osmosis (ahs MOH sis).
Water is made up of particles, called molecules. Pure water has
the highest concentration of water molecules. When you mix
something, such as food coloring, sugar, or salt, with water, you
lower the concentration of water molecules. Figure 2 shows how
water molecules move through a membrane that is
semipermeable (SEM i PUHR mee uh buhl). Semipermeable
means that only certain substances can pass through. The
picture on the left in Figure 2 shows liquids that have different
concentrations of water. Over time, the water molecules move
from the liquid with the high concentration of water molecules to
the liquid with the lower concentration of water molecules.
Figure Page 35 in text.
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The Cell and Osmosis
Osmosis is important to cell functions. For example, red blood cells
are surrounded by plasma. Plasma is made up of water, salts,
sugars, and other particles. The concentration of these particles is
kept in balance by osmosis. If red blood cells were in pure water,
water molecules would flood into the cells and cause them to
burst. When red blood cells are put into a salty solution, the
concentration of water molecules inside the cell is higher than the
concentration of water outside. This difference makes water move
out of the cells, and the cells shrivel up. Osmosis also occurs in
plant cells. When a wilted plant is watered, osmosis makes the
plant firm again.
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Moving Small Particles
Small particles, such as sugars, cross the cell membrane
through passageways called channels. These channels
are made up of proteins in the cell membrane. Particles
travel through these channels by either passive or
active transport. The movement of particles across a
cell membrane without the use of energy by the cell is
called passive transport, and is shown in Figure 3. During
passive transport, particles move from an area of high
concentration to an area of low concentration.
Diffusion and osmosis are examples of passive transport.
Figure on page 36 in textbook.
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A process of transporting particles that requires the cell to use
energy is called active transport. Active transport usually involves
the movement of particles from an area of low concentration to
an area of high concentration.
Moving Large Particles
Small particles cross the cell membrane by diffusion, passive
transport, and active transport. Large particles move into and out
of the cell by processes called endocytosis and exocytosis.
The active-transport process by which a cell surrounds a large
particle, such as a large protein, and encloses the particle in a
vesicle to bring the particle into the cell is called endocytosis (EN
doh sie TOH sis). Vesicles are sacs formed from pieces of cell
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When large particles, such as wastes,
leave the cell, the cell uses an active-
transport process called exocytosis (EK
soh sie TOH sis). During exocytosis, a
vesicle forms around a large particle
within the cell. The vesicle carries the
particle to the cell membrane. The vesicle
fuses with the cell membrane and
releases the particle to the outside of the
cell. Figure on page 36