Download 1.2 Cells: The Basic Units of Life

1.2 Cells: The Basic Units of Life
Cells are the basic structural and functional units of life.
There are many different kinds of cells, which are specialized
to carry out particular functions. In animals, muscle cells move
limbs by contracting, bone cells produce the hard material that
makes bones rigid, brain cells conduct electrical signals, and
skin cells form waterproof sheets that cover your body. In
plants, root cells absorb water from the environment, and green
cells in leaves produce food. Although they perform different
functions, cells have many common features.
General Structure of a Cell
A cell can be viewed as a factory with different departments,
each carrying out specialized tasks that help keep the cell
alive. Whether they are nerve cells in the brain of an animal,
or epidermal cells in the leaf of a plant, cells are bathed in
an aqueous (water-based) solution called extracellular fluid. On
the inside, a cell contains an aqueous solution called cytosol
in which a number of structures called cell organelles (little
organs) are suspended (Figure 1). A special organelle called the
nucleus acts as the control centre of the cell, issuing orders
that determine how the other organelles do their work. Cytoplasm
is the name given to the cytosol and cell organelles outside of
the nucleus. The contents of an animal cell (cytosol and cell
organelles) are separated from the extracellular fluid by a
thin, flexible film called the cell membrane or plasma membrane.
In addition to separating the interior of the cell from the
external environment, the cell membrane holds the contents of
the cell together and controls the movement of materials into
and out of the cell. Plant cells have an additional outer
covering called the cell wall (Figure 2). Cell walls are firm
yet porous structures that give plants their rigidity while
allowing water and dissolved materials to pass through easily.
Wood is composed of the cell walls of dead plant cells.
Figure 3 shows a typical plant cell and a typical animal cell.
Many of the smaller organelles identified in these diagrams
cannot be seen with a compound light microscope, but instead,
require the magnification and resolving power of a transmission
electron microscope.
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Membranes
In addition to the membrane that surrounds the cell (the cell
membrane), plant and animal cells have a number of organelles
that are surrounded by membranes such as the nucleus,
mitochondria, and chloroplasts. A membrane is composed of a
bilayer (double layer) of fat (lipid) molecules, called
phospholipids, and other compounds, called proteins and
carbohydrates (Figure 4).
The parts of a membrane are always moving, and the membrane is
said to be "fluid." That is why if a membrane is punctured with
an ultra-fine needle it will not leak. Instead, the
phospholipids and proteins will move in to fill the gap as the
needle is removed. The membrane is more like a strong soap
bubble than a rubber balloon.
Nucleus
As mentioned, the nucleus is the control centre of the cell and
directs all of the cell's activities. The nucleus is separated
from the cytoplasm by a porous double membrane called the
nuclear envelope, and is filled with nucleoplasm. Nucleoplasm is
a mixture of chemicals that stores information used by other
cell organelles in carrying out their functions. Nucleoplasm is
rich in compounds called nucleic acids. These include
ribonucleic acid (RNA) and deoxyribonucleic acid (DNA).
Ribosomes
Ribosomes are organelles used by the cell to produce proteins
(protein synthesis). Ribosomes are either floating in the
cytoplasm or attached to membranes. In general, free-floating
ribosomes produce proteins that are used inside the cell, and
membrane-attached ribosomes manufacture proteins for use outside
the cell. Ribosomes are so small (approximately 30 nm in
diameter) that even when viewed with a transmission electron
microscope they appear as small fuzzy dots (Figure 5).
Endoplasmic Reticulum
The endoplasmic reticulum (ER) is a complicated system of
membranous tubes and canals that connect with the nuclear
envelope (Figure 6, on the next page). The endoplasmic reticulum
is like a complex subway system with crisscrossing tunnels and
stations. As you can see in Figure 6, there are two types of
endoplasmic reticulum: rough endoplasmic reticulum (RER),
containing attached ribosomes, and smooth endoplasmic reticulum
(SER), with no ribosomes.
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Because the rough endoplasmic reticulum contains ribosomes, many
proteins are manufactured in it. The smooth endoplasmic
reticulum is thought to be a place where molecules of fat are
produced. The products of the endoplasmic reticulum become
enclosed in membrane-bound structures called vesicles. Vesicles
form when the membrane of the endoplasmic reticulum pinches off
at the ends. These vesicles then travel toward another
membranous organelle called a Golgi apparatus.
Golgi Apparatus
The Golgi apparatus is composed of several membranous tubes
that, under the microscope, usually look like a stack of
flattened balloons (Figure 7). It is named after Camillo Golgi,
the medical doctor who first identified it in 1898. The Golgi
apparatus chemically changes the fats and proteins produced in
the endoplasmic reticulum and then packages them in vesicles. In
many cases, the vesicles move through the cytoplasm, attach to
the cell membrane, and release their contents into the
extracellular fluid.
Lysosomes (found only in animal cells)
Some of the vesicles formed by the Golgi apparatus are called
lysosomes. Lysosomes are cell organelles containing proteins
that can break down the molecules that cells are made of into
their individual chemical components. Lysosomes are found only
in animal cells, and may be used to digest food particles
brought into the cell from the extracellular fluid. They are
also used to destroy potentially dangerous microorganisms, such
as bacteria and viruses, that may force themselves into the cell
(Figure 8). When an animal cell gets old, lysosomes break open
and decompose the entire cell. This process, in which the cell
could be said to "commit suicide," is called apoptosis. The
organism then uses the resulting compounds to build new cells.
Hence, lysosomes are sometimes referred to as "suicide sacs."
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Mitochondria
Mitochondria (singular: mitochondrion) are circular or rodshaped membranous organelles that float freely in the cytosol
(Figure 9). Many important chemical reactions occur in
mitochondria. These reactions contribute to cellular
respiration, a series of chemical changes that produces
compounds that cells use as a source of energy. Like the
nucleus, the mitochondrion contains two membranes: a smooth
outer membrane and a highly folded inner membrane. The folds of
the inner membrane are called cristae (singular: crista).
Cristae contain compounds that help carry out the reactions of
cellular respiration, and provide a large surface area on which
these reactions occur. Cells that require large amounts of
energy, such as muscle cells in animals and root tip cells in
plants, usually contain large numbers of mitochondria, each with
many cristae. Cells that do not require large amounts of energy,
such as most fat cells in animals and leaf cells in plants, have
smaller numbers of mitochondria with fewer cristae. The interior
space of a mitochondrion is filled with a protein-rich fluid
called matrix. Many chemical reactions of cellular respiration
also occur in the matrix.
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Plastids (found only in plant cells)
Plastids are free-floating membranous organelles found only in
the cytosol of plant cells. They are usually large enough to be
seen with a compound light microscope. Plastids store materials
that are useful to the plant cell and perform other important
functions that keep the cell alive. One of the most important
plastids is the chloroplast. Chloroplasts contain all of the
chemicals necessary to perform photosynthesis, a set of chemical
reactions that converts carbon dioxide and water (from the
extracellular fluid) into molecules that plant cells use as
food. Like mitochondria, chloroplasts possess two membranes, an
outer and an inner membrane. However, in chloroplasts, other
membranes are arranged into a system of interconnected
compartments called thylakoids, and thylakoids are usually
stacked on top of one another forming structures called grana
(singular: granum) (Figure 10).
The membranes of thylakoids contain molecules of green pigment
called chlorophyll. Chlorophyll is responsible for the green
colour of plants and is also responsible for the start of
photosynthesis. The space between thylakoid membranes and the
inner chloroplast membrane is filled with a protein-rich fluid
called stroma. Stroma contains compounds needed for
photosynthesis and DNA that allows chloroplasts to reproduce.
Other plastids in plant cells include amyloplasts and
chromoplasts. Amyloplasts are white or colourless plastids that
store the energy-rich products of photosynthesis in the form of
starch. Chromoplasts are colourful plastids containing the red,
orange, and yellow pigments commonly found in flowers, fruits,
and vegetables.
Vacuoles
Vacuoles are usually large, membrane-bound sacs filled with a
watery solution containing dissolved sugars, minerals, and
proteins. They are common in plant cells, where the pressure
inside one or two large vacuoles helps keep the cell membrane
pressed firmly against the cell wall (Figure 11). This pressure,
called turgor pressure, is responsible for the firm texture of
fresh vegetables such as celery stalks and carrot roots, and of
the stems and leaves of all plants. When plant cells lose water,
the vacuoles shrink, and the decrease in turgor pressure causes
structures such as stems and roots to become limp and wilted.