Download Plant and Animal Cells

2.1
Plant and Animal Cells
Biology as a science is built on three simple but very important ideas. These
three ideas form the cell theory. The cell theory states that
1. All living things are made up of one or more cells and their products.
2. The cell is the simplest unit that can carry out all life processes.
cell theory a theory that all living things
are made up of one or more cells, that
cells are the basic unit of life, and that all
cells come from pre-existing cells
3. All cells come from other cells; they do not come from
non-living matter.
All living things are made up of cells, but these cells may be very simple or
very complex. The simplest organisms are archaea and bacteria. These simple,
single-celled life forms are called prokaryotes (Figure 1(a)). The cells do not
have a nucleus. More complex cells can exist as single-celled organisms or
multicellular organisms. The cells of these organisms, known as eukaryotes,
have a more complex internal organization, including a nucleus. Eukaryotes
include all protists, fungi, animals, and plants, from the tiniest Amoeba to the
longest whale and the tallest tree (Figure 1(b) to (d)). The cells of eukaryotes
are much larger than the cells of prokaryotes: tens to thousands of times
larger. There are even some eukaryotes—beyond the scope of this unit—that
are made up of one huge cell with very many nuclei.
ORGANISM
without a
nucleus
with a
nucleus
prokaryote
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many cells
single-celled
organism
(a)
eukaryote a cell that contains a nucleus
and other organelles, each surrounded by
a thin membrane
eukaryote
one cell
(e.g., E.coli))
prokaryote a cell that does not contain
a nucleus or other membrane-bound
organelles
multicellular
organism
(e.g., Amoeba)
animal
(b)
plant
(e.g., whale)
(c)
(e.g., pine tree)
(d)
Figure 1 The relationship between
prokaryotes and eukaryotes. The
bacterium (a) is a prokaryote. The
Amoeba (b), the whale (c), and
the pine tree (d) are all eukaryotes.
Cell Structure
Your body is made up of many specialized organs that carry out all the
processes needed to live. In the same way, a eukaryotic cell also has specialized
parts, called organelles, that carry out specific functions necessary for life.
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organelle a cell structure that performs a
specific function for the cell
2.1 Plant and Animal Cells
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Structures Common to Plants and Animal Cells
All cells have to perform the same basic activities to stay alive: use energy,
store materials, take materials from the environment, get rid of wastes, move
substances to where they are needed, and reproduce. Each organelle has a
specific function within the cell. Just as workers in a factory or a hospital
coordinate their efforts to achieve a purpose, the various organelles of a cell
work together to meet the needs of the cell—and the whole organism.
Figure 2 shows the organelles in a typical plant cell and a typical animal cell.
ribosome
chloroplast
endoplasmic
reticulum
cell wall
Golgi body
cytoplasm
nucleus
mitochondrion
cell membrane
vacuole
nuclear
membrane
vacuole
cytoplasm
ribosome
mitochondrion
cell membrane
endoplasmic
reticulum
nucleus
nuclear
membrane
Golgi body
(a) Plant cell
(b) Animal cell
Figure 2 Plant and animal cells have many of the same organelles, but there are some differences.
CytoPlASm
All the organelles inside the cell are suspended in the cytoplasm. The
cytoplasm is mostly water, but it also contains many other substances that
the cell stores until they are needed. Many chemical reactions take place
within the cytoplasm, which can change from jelly-like to liquid, allowing
organelles to be moved around.
Figure 3 This TEM image of a cell
highlights the cell membrane in green.
Cell membrAne
The cell is surrounded by a flexible double-layered cell membrane (Figure 3).
The function of the cell membrane is both to support the cell and to allow
some substances to enter while keeping others out. For example, water
and oxygen molecules can easily pass through the cell membrane, but
larger molecules, such as proteins, cannot. Because of this ability, the cell
membrane is called a “semi-permeable membrane.”
A similar membrane also surrounds most organelles in a eukaryotic cell.
nuCleuS
Figure 4 The large nucleus is easily
visible inside this starfish cell.
DNA (deoxyribonucleic acid) the
material in the nucleus of a cell that
contains all of the cell’s genetic
information
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The nucleus is a roughly spherical structure within the cell (Figure 4). The
nucleus contains genetic information that controls all cell activities. This
genetic information is stored on chromosomes. Chromosomes contain DNA
(deoxyribonucleic acid), the substance that carries the coded instructions
for all cell activity. When a cell divides, the DNA is copied so that each new
cell has a complete set of DNA.
Chapter 2 • Cells, Cell Division, and Cell Specialization
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mitoChondriA
Cells contain many mitochondria (singular: mitochondrion) (Figure 5).
Mitochondria are sometimes called the “power plants” of the cell because they
make energy available to the cell. Active cells, such as muscle cells, have more
mitochondria than less active cells, such as fat-storage cells. Cells store energy
as a form of glucose (a sugar). The mitochondria contain enzymes that help
to convert the stored energy into an easily usable form. This process is called
cellular respiration and requires oxygen. The waste products of this reaction
are carbon dioxide and water.
glucose + oxygen → carbon dioxide + water + usable energy
Figure 5 The mitochondrion (17 000 3)
is the large, reddish, oval structure in
this TEM image.
Cells in which cellular respiration has to happen very fast, such as muscle
cells and cells in the liver, have many mitochondria. In contrast, cells that are
fairly inactive—that do not have to respire quickly—tend to have very few
mitochondria. Fat cells may have only one or two mitochondria.
endoPlASmiC retiCulum
The endoplasmic reticulum is a three-dimensional network of branching tubes
and pockets (Figure 6). It extends throughout the cytoplasm and is continuous
from the nuclear membrane to the cell membrane. These fluid-filled tubes
transport materials, such as proteins, through the cell.
Endoplasmic reticulum is important in many types of cells. In the brain
it assists with the production and release of hormones. In the muscles the
endoplasmic reticulum is involved with muscle contraction.
Figure 6 The endoplasmic reticulum
(5 500x), coloured brown in this TEM,
transports materials throughout the cell.
GolGi bodieS
Golgi bodies collect and process materials to be removed from the cell
(Figure 7). They also make and secrete mucus. Cells that secrete a lot of
mucus, such as cells lining the intestine, have many Golgi bodies.
VACuoleS
A vacuole is a single layer of membrane enclosing fluid in a sac. The functions
of vacuoles vary greatly, according to the type of cell. These functions include
containing some substances, removing unwanted substances from the cell,
and maintaining internal fluid pressure (turgor) within the cell. (The special
role of plant vacuoles is explained below.) Animal cells may have many small
vacuoles that are often not visible. Mature plant cells usually have one central
vacuole that is visible under a microscope.
Some animal cells can change their shape to wrap around and surround
smaller objects to bring them inside the cell. Amoeba do this to obtain food.
Some white blood cells engulf bacteria to kill them. During the engulfing
process, a portion of the cell membrane turns inside out and forms a vacuole
inside the cell until the engulfed object is digested. Then any waste material is
ejected from the cell as the vacuole again joins up with the cell membrane.
organelles in Plants Cells only
Plant cells and animal cells have many structures in common, but there are
also some differences. Plant cells have some organelles that animal cells do not
have (Figure 8).
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Figure 7 Golgi body (30 0003)
(a)
(b)
Figure 8 Plant cells (a) have a cell
wall, large vacuoles, and chloroplasts
(5 5003). Animal cells (b) do not
(2 5003).
2.1 Plant and Animal Cells
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CELL WALL
The cell wall is found just outside the cell membrane of a plant cell. It is a
rigid but porous structure made of cellulose. This cell wall provides support
for the cell and protection from physical injury. This tough material may hold
together long after the plant has died. The paper in this book is composed
mostly of cellulose from the cell walls of trees.
VACuole
Plant cells usually have one large vacuole, which takes up most of the space
inside the cell. When these are full of water, turgor pressure keeps the cells
plump, which keeps the plant’s stems and leaves firm. If the water level drops,
however, the vacuoles lose turgor pressure and the cells become soft. The plant
stems and leaves become limp and droopy until the water is replaced.
ChloroPlAStS
Many plant cells that are exposed to light, such as the cells of leaves, have
structures called chloroplasts (Figure 9). Chloroplasts contain chlorophyll
and give leaves their green colour. More importantly, chloroplasts absorb light
energy. This light energy is used in photosynthesis—the process of converting
carbon dioxide and water into glucose and oxygen.
Figure 9 Chloroplasts in plant cells
(2503)
IN
carbon dioxide + water + energy (sunlight) → glucose + oxygen
Photosynthesis allows plants to obtain their energy from the Sun so that
they can make their own food. Plant cells rely on mitochondria to metabolize
glucose, just as animal cells do.
SUMMARY
• The cell theory states that all living things are
made up of cells, the cell is the simplest unit that
can carry out all life processes, and all cells are
reproduced from other cells.
• Eukaryotic cells contain organelles that carry
out specific life functions.
• The cell membrane, cytoplasm, nucleus,
mitochondria, endoplasmic reticulum, Golgi
bodies, and vacuoles occur in both plant and
animal cells.
• The simplest single-celled organisms, including
bacteria, are prokaryotes. More complex
organisms, including multicellular organisms,
are eukaryotes.
CHECK
• Structures found only in plant cells are
chloroplasts, a large vacuole, and the cell wall.
YOUR LEARNING
2. Are your cells prokaryotic or eukaryotic? Explain. K/U
6. Not all plant cells contain chloroplasts. What is the most
likely reason for this? K/U
3. What is the most obvious difference between prokaryotic
and eukaryotic cells? K/U
7. Plant cells are surrounded by a cell wall. What is the
function of this structure? K/U
4. How does the nucleus coordinate cell activities?
8. Plant cells can make their own “food”—glucose. Why do
plant cells have mitochondria? K/U
1. Summarize the cell theory in your own words. K/U
K/U
5. When you exercise, you breathe harder and faster. Using
your knowledge of organelles, explain why this happens.
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A
Chapter 2 • Cells, Cell Division, and Cell Specialization
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