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Cell Structure and Function
Can you name something that you know exists even though you can’t see it with
your own eyes? A drop of pond water has tiny swimming organisms and small bits
of plant material, but we can’t always see them with our eyes. How do we know
there are tiny things in a drop of pond water? We can use a microscope to view the
pond water. There are instruments people use every day to help them see things
they wouldn’t usually be ab le to see. Have you ever used a pair of binoculars or a
magnifying glass? Have you ever had an x-ray taken of an injury? Do you need to
wear glasses or contact lenses to see clearly? Vision systems are even being
developed to restore vision to bind people. In this chapter, you will take a journey
into a small word that was discovered when the microscope was invented – the
world of cells. Imagine you could shrink yourself and walk into a tiny cell. What
is it like inside a cell? It’s a fascinating journey!
7.1 What are Cells?
Look closely at the skin on your arm. Can you see that it is made of cells? Of
course not! Your skin cells are much too small to see with your eyes. Now look at
one square centimeter of your arm. That square centimeter contains about 100,000
skin cells. Cells are so small that they weren’t even discovered until the invention
of the microscope. What are cells and how were they discovered?
You Are Made Of Cells
A cell is the basic unit of structure and function in a living thing. Your body is
composed of trillions of cells. You have skin cells, muscle cells, nerve cells, blood
cells, and many other types as well. Each type of cell has a unique structure and
function, but they all share similarities.
Each cell in your body shares the characteristics of all living things. Each cell can
respond, grow, reproduce, and use energy. Like larger organisms, cells respond to
changes in their surroundings in ways that keep them alive.
Finding Out
About Cells
How did we learn
about cells? It all
started with the
invention of the microscope in the late 1500s. English scientist Rober
Hooke was the first to record his observations of cells. In 1663, he took a
think slice of cork and placed it under a microsope that he built. Cork is
made from the bark of the cork oak tree, but its cells are no longer alive.
Hooke made detailed sketches of his observations. An artist’s version of
one of his sketches is shown in Figure 7.2. Hooke called each of the square
structures a cell because they reminded him of tiny rooms.
Anton van Leeuwenhoek was nota scientist. He was a Dutch craftsmn who
made lenses. Yet with skill and curiosity, van Leeuwenhoek made some fo the
most important discoveries in biology. He used his lenses to builde a simple
microscope. With his microscope, he looked at pond water, blood, and scrapings
from his teeth. He was the first to observe single-celled protists, blood cells, and bacteria.
As microscopes improved, scientists made more discoveries. In 1839, two German scientists, Mtthais
Schleiden and Theodore Schewann , viewed plant and animal tissues under a microscope. They concluded that
all plant and animals were made up of cells.
Cell Image
Cel
ls
usu
ally do not glow. Scientists use fluorescent proteins to make cells glow. The
cells absorb these proteins like stains. The fluorescence microscope uses filters
that only let in light that matches the fluorescing material being studied. All other
types of light are blocked out. The fluorescing areas shine out against a dark
background, making certain cell structures glow. The mouse egg cell in Figure
7.3 have been treate to show DNA as a glowing blue.
The Cell Theory
Schleiden and Scchwann’s theory was widely accepted by other scientists. But
where did cells come from? In the 1800s, it was believed that living things came from nonliving objects. Did
cells come from some tiny, noniving object/ In 1855, a German physician named Rudolf Virchow proposed
that cells can only come from other cells.
Rudolf Virchow
The
work of Hooke, van Leeuwenhoek, Schleiden, Schwann, Virchow, and others led to an improtant theory in life
science. The cell theory explains that all living things are made of one or more cells, that arise from existing
cells.
Similarities Among Cells
Organisms can be an individual animal, plant, or single-celled life form. Some organisms are made of only a
single cell, or unicellular. Unicellular is a single-celled organism that consists of only one cell. You are made
of billions of cells, or multicellular. Multicelluar is an oganism that is made up of many cells. In multicellular
organisms like you, there are many different types of specialized cells. Specialized Cell a recells that differ in
structure (size, shape...) and function (the role they perform in the organism). For example, the cells that line the
retina of your eye have a structure and function that is very different from your skin cells. About 200 different
types of specialized cells make up the tissues and organs in your body,
Even though there are many different types of cells, they all share similar characteristic. These include:
1. All cells are surrounded by a cell membrane. The cell membrane is a
barrier between the inside of the cell and its environment. It also
controls the movement of materials into and out of the cell.
2. All cells contain organelles. An organelle is a structure inside a cell
that helps the cell perform its functions. Although all cells contain
organelles, they don’t’ all contain the same kinds.
3. All cells contian cytoplasm. The cytoplasm is a fluid mixture that
contains the organelles. It also contains the compounds cells need to
survive such as water, salts, enzymes, and other carbon compounds.
4. All cells contain DNA. The cell theory states that all cells come from
other cells. When cells reprodue, they make copies of their DNA and
pass it on to the new cells. DNA contains the instructions for making
new cells and controls all cell functions.
Classifying Cells
Based on the organization of their structures, all living cells can be be classified into two groups: prokaryotic
and eukaryotic. Animals, plants, fungi, and protozoans all have eukaryotic cells. Only bacteria have
prokaryotic cells.
Prokaryotic Cells
Prokaryotes are any single cellular organism that does not have a nucleus. Prokaryotes are the smallest and
simplest cells. A prokaryote is a single-celled organism that lacks a nucleus and other internal compartments.
Because prokaryotes lack many specialized internal compartments, they cannot carry out many specialized
functions (hence why they are simpler), and because they lack these structures, they are much smaller than
eukaryotes (their size is usually about 0.5-2 µm).
Prokaryotes are the most primitive of cells (meaning they are the oldest and simplest), and they lived at least
3.5 billion years ago. For nearly 2 billion years, prokaryotes were the only organisms on Earth. The most
familiar example of prokaryotes is bacteria.
In prokaryotic cells, the cytoplasm is everything inside the cell membrane. Prokaryotes have a cell wall
surrounding their cell membrane. The cell wall is very important because it gives prokaryotic cells their shape.
The cell wall is typically made up of polysaccharides connected by
short chains of amino acids. Prokaryotes also have a cytoskeleton, but
it is very simple and aides in cell movement. Many (not all)
prokaryotes also have a flagella, which are long, threadlike structures
that protrude from the cell’s surface to enable the cell to move at faster
speeds.
The cell wall is often times covered by a capsule, which is also made
out of polysaccharides. The capsule is very sticky, and allows the
prokaryote to stick to teeth, skin, food, intestines, etc. Prokaryotes may
also have a pilus, which are sticky projections.
The DNA shape of prokaryotes is different than eukaryotes because it
consists of a single, circular molecule of DNA. The DNA is also free
and loose within the cell because it is not housed in a nucleus.
Eukaryotic Cells
Eukaryotes are any multi cellular organism whose cells contain
a nucleus and other organelles. Eukaryotic cells were the first
cells to appear on earth that had specialized internal
compartments. Eukaryotic cells evolved about 2.5 billion years
ago, and eukaryotic cells are defined by having a nucleus.
The specialized internal compartments that are found in eukaryotic
cells are known as “organelles” meaning “little organs”. There are
many different organelles in eukaryotic cells, and they are defined
as a structure that carries out specific activities in the cell.
Examples of organelles are mitochondria, endoplasmic reticulum,
and the Golgi body, to name a few.
In Eukaryotic cells, the cytoplasm is defined as everything inside the cell membrane and outside of the nucleus. The
cytosol is the fluid that is contained in the cytoplasm. The cytoskeleton of eukaryotic cells is very complex and
supports the cell’s shape and movement.
Eukaryotic cells are very large in comparison to other types of cells (about 10 µm). Eukaryotic cells are also very
complex compared to other cells because they contain many specialized organelles that each has a specific function.
Though all cells have DNA, eukaryotic cells are the only cell type that has an organelle known as the nucleus (as
mentioned above). The nucleus houses and protects the DNA.
Prokaryotic Cells
Eukaryotic Cells
Bacteria
All other cells
No nucleus
Nucleus
Organelles not membrane-covered
Membrane-covered organelles
DNA is bunched up in the center of
the cell
DNA is found in the nuncleus