I. Cell Structure and Function
A. Life Is Cellular
1. The Discovery of the Cell
a)
Early Microscopes
It was not until the mid-1600s that scientists began to use microscopes to
observe living things.
(1)
In 1665, Englishman Robert Hooke used an early compound microscope
to look at a thin slice of cork, a plant material.
(2)
Under the microscope, cork seemed to be made of thousands of tiny,
empty chambers.
(3)
Hooke called these chambers “cells” because they reminded him of a
monastery’s tiny rooms, which were called cells.
(4)
b)
The Cell Theory
(1)
All living things are composed of cells.
(2)
Cells are the basic units of structure and function in living things.
(3)
New cells are produced from existing cells.
2. Prokaryotes and Eukaryotes
a)
Typical cells range from 5 to 50 micrometers in diameter.
The tiniest mycoplasma bacteria are only 0.2 micrometers across, so
small that they are difficult to see under even the best light microscopes.
(1)
The giant amoeba may be 1000 micrometers in diameter, large enough to
be seen with the unaided eye as a tiny speck in pond water.
(2)
Cells fall into two broad categories, depending on whether they contain
a nucleus.
b)
The nucleus (plural: nuclei) is a large membrane-enclosed structure
that contains the cell’s genetic material in the form of DNA.
c)
A membrane is a thin layer of material that serves as a covering or
lining.)
(1)
d)
The nucleus controls many of the cell’s activities.
e)
Eukaryotes are cells that contain nuclei.
f)
Prokaryotes are cells that do not contain nuclei.
g)
Prokaryotes
Prokaryotic cells are generally smaller and simpler than eukaryotic cells,
although there are many exceptions to this rule.
(1)
(2)
Prokaryotic cells have genetic material that is not contained in a nucleus.
Despite their simplicity, prokaryotes carry out every activity associated
with living things.
(3)
(4)
h)
The organisms we call bacteria are prokaryotes.
Eukaryotes
Eukaryotic cells are generally larger and more complex than prokaryotic
cells.
(1)
Eukaryotic cells generally contain dozens of structures and internal
membranes, and many are highly specialized.
(2)
Eukaryotic cells contain a nucleus in which their genetic material is
separated from the rest of the cell.
(3)
(4)
Plants, animals, fungi, and protists are eukaryotes.
B. Eukaryotic Cell Structure
1. Comparing the Cell to a Factory
a)
In some respects, the eukaryotic cell is life a factory.
Because many of these structures act as if they are specialized organs,
these structures are known as organelles, literally “little organs.”
b)
Cell biologists divide the eukaryotic cell into two major parts: the
nucleus and the cytoplasm.
c)
(1)
The cytoplasm is the portion of the cell outside the nucleus.
(2)
The nucleus and cytoplasm work together in the business of life.
2. Nucleus
In the same way that they main office controls a large factory, the
nucleus is the control center of the cell.
a)
The nucleus contains nearly all the cell’s DNA and with it the coded
instructions for making proteins and other important molecules.
b)
The nucleus is surrounded by a nuclear envelope composed of two
membranes.
c)
The nuclear envelop is dotted with thousands of nuclear pores, which
allow material to move into and out of the nucleus.
d)
Like messages and instructions moving in and out of a main office, a
steady stream of proteins, RNA, and other molecules move through the
nuclear pores to and from the rest of the cell.
(1)
e)
The granular material you see in the nucleus is called chromatin.
(1)
Chromatin consists of DNA bound to protein.
(2)
Most of the time, chromatin is spread throughout the nucleus.
When a cell divides, however, chromatin condenses to form
chromosomes, which contain the genetic information that is passed from one
generation of cells to the next.
(3)
f)
Most nuclei also contain a small, dense region known as the nucleolus.
(1)
The nucleolus is where the assembly of ribosomes begins.
3. Ribosomes
One of the most important jobs carried out in the cellular “factory” is
making proteins.
a)
b)
Proteins are assembled on ribosomes.
Ribosomes are small particles of RNA and protein found throughout the
cytoplasm.
c)
They produce proteins by following coded instructions that come from
the nucleus.
d)
e)
Each ribosome, in its own way, is like a small machine in a factory.
4. Endoplasmic Reticulum
Eukaryotic cells also contain an internal membrane system known as
the endoplasmic reticulum, or ER.
a)
The ER is the site where lipid components of the cell membrane are
assembled, along with proteins and other materials that are exported from
the cell.
b)
The portion of the ER involved in the synthesis of proteins is called
rough ER.
c)
(1)
It is given this name because of the ribosomes found on its surface.
The other portion of the ER is known as smooth ER because ribosomes
are not found on its surface.
d)
The smooth ER contains collections of enzymes that perform specialized
tasks, including the synthesis of membrane lipids and the detoxification of
drugs.
(1)
5. Golgi Apparatus
Proteins produced in the rough ER move next into an organelle called
the Golgi apparatus, discovered by Italian scientist Camillo Golgi.
a)
The function of the Golgi apparatus is to modify, sort, and package
proteins and other materials from the endoplasmic reticulum for storage in
the cell or secretion outside the cell.
b)
The Golgi apparatus is somewhat like a customization shop, where the
finishing touches are put on proteins before they are ready to leave the
“factory.”
(1)
From the Golgi apparatus, proteins are then “shipped” to their final
destination throughout the cell or outside of the cell.
(2)
6. Lysosomes
Even the neatest, cleanest factory needs a cleanup crew, and that’s what
lysosomes are.
a)
b)
Lysosomes are small organelles filled with enzymes.
On function of lysosomes is the digestion, or breakdown, of lipids,
carbohydrates, and proteins into small molecules that can by used by the
rest of the cell.
c)
7. Vaculoes
Every factory needs a place to store things, and cells contain places for
storage as well.
a)
Some kinds of cells contain saclike structures called vacuoles that store
materials such as water, salts, proteins, and carbohydrates.
b)
In many plant cells there is a single, large central vacuole filled with
liquid.
c)
Vacuoles are also found in some single-celled organisms and in some
animals.
d)
8. Mitochondria and Chloroplasts
a)
All living things require a source of energy.
b)
Mitochondria
(1)
Nearly all eukaryotic cells, including plants, contain mitochondria.
Mitochondria are organelles that convert the chemical energy stored in
food into compounds that are more convenient for the cell to use.
(2)
Mitochondria are enclosed by two membranes—an outer membrane and
an inner membrane.
(3)
(4)
c)
Mitochondria are like the powerhouse of the cell.
Chloroplasts
(1)
Plants and some other organisms contain chloroplasts.
Chloroplasts are organelles that capture the energy from sunlight and
convert it into chemical energy in a process called photosynthesis.
(2)
(3)
Chloroplasts are the biological equivalents of solar power plants.
9. Cytoskeleton
A supporting structure and a transportation system complete our
picture of the cell as a factory.
a)
Eukaryotic cells have a structure—the cytoskeleton—that helps support
the cell.
b)
The cytoskeleton is a network of protein filaments that helps the cell to
maintain its shape.
c)
d)
The cytoskeleton is also involved in movement.
10. Centrioles
Centrioles are located near the coleus and help to organize the cell
division.
a)
b)
Centrioles are not found in plant cells.
C. Cell Boundaries
1. All cells are surrounded by a thin, flexible barrier known as the cell
membrane.
2. Many cells also produce a strong supporting layer around the membrane
known as a cell wall.
3. Cell Membrane
The cell membrane regulates what enters and leaves the cell and also
provides protection and support.
a)
The composition of nearly all cell membranes is a double-layered sheet
called a lipid bilayer.
b)
c)
There are two layers of lipids, hence the name bilayer.
The lipid bilayer gives cell membranes a flexible structure that forms a
strong barrier between the cell and its surroundings.
d)
In addition to lipids, most cell membranes contain protein molecules
that are embedded in the lipid bilayer.
e)
Some of the proteins form channels and pumps that help to move
material across the cell membrane.
f)
Many of the carbohydrates, attached to the proteins, act like chemical
identification cards, allowing individual cells to identify one another.
g)
4. Cell Walls
Cell walls are present in many organisms, including plants, algae, fungi,
and many prokaryotes.
a)
b)
Cell walls lie outside the cell membrane.
The main function of the cell wall is to provide support and protection
for the cell.
c)
d)
Most cell walls are made from fibers of carbohydrate and protein.
5. Diffusion Through Cell Boundaries
a)
Measuring Concentration
The cytoplasm of a cell contains a solution of many different substances
in water.
(1)
(2)
Recall that a solution is a mixture of two or more substances.
(3)
The substances dissolved in the solution are called solutes.
The concentration of a solution is the mass of solute in a given volume of
solution, or mass/volume.
(4)
b)
Diffusion
In a solution, particles move constantly. They collide with one another
and tend to spread out randomly.
(1)
As a result, the particles tend to move from an area where they are more
concentrated to an area where they are less concentrated, a process known
as diffusion.
(2)
When the concentration of the solute is the same throughout a system,
the system has reached equilibrium.
(3)
Because diffusion depends upon random particle movement, substances
diffuse across membranes without requiring the cell to use energy.
(4)
6. Osmosis
Osmosis is the diffusion of water through a selectively permeable
membrane.
a)
b)
How Osmosis Works
Water will tend to move across the membrane until equilibrium is
reached. At that point, the concentrations of water and sugar will be the
same on both sides of the membrane.
(1)
When this happens, the two solutions will be isotonic, which means
“same strength.”
(2)
When the solution is more concentrated with sugar it is hypertonic, which
means “above strength.”
(3)
(4)
When the dilute sugar solution is hypotonic, it is “below strength.”
7. Facilitated Diffusion
Cell membranes have protein channels that make it easy for certain
molecules to cross the membrane.
a)
Red blood cells, for example, have a cell membrane protein with an
internal channel that allows glucose to pass through it. Only glucose can
pass through this channel, and it can move through in either direction.
b)
c)
This process is known as facilitated diffusion.
Hundreds of different protein channels have been found that allow
particular substances to cross different membranes.
d)
Although facilitated diffusion is fast and specific, it is still diffusion, and
therefore does not require energy.
e)
8. Active Transport
As powerful as diffusion is, cells sometimes must move materials in the
opposite direction—against a concentration gradient.
a)
b)
This is accomplished by a process known as active transport.
c)
As its name implies, active transport requires energy.
d)
Molecular Transport
Small molecules and ions are carried across membranes by proteins in
the membrane that act like energy-requiring pumps.
(1)
e)
Endocytosis and Exocytosis
Endocytosis is the process of taking material into the cell by means of
infolding, or pockets, of the cell membtane.
(1)
Phagocytosis means “cell eating.” In phagocytosis, extensions of
cytoplasm surround a particle and package it within a food vacuole.
(2)
Tiny pockets form along the cell membrane, fill with liquid, and pinch off
to form vacuoles within the cell. This is known as pinocytosis.
(3)
During exocytosis, the membrane of the vacuole surrounding the
material fuses with the cell membrane, forcing the contents out of the cell.
(4)
D. Diversity of Cellular Life
1. Unicellular Organisms
Cells are the basic living units of all organisms, but sometimes a single
cell is a little more than that.
a)
b)
Single-celled organisms are also called unicellular organisms.
Unicellular organisms do everything that you would expect a living
thing to do. They grow, respond to the environment, transform energy,
and reproduce.
c)
2. Multicellular Organisms
a)
Organisms that are made up of many cells are called multicellular.
All multicellular organisms depend on communication and cooperation
among specialized cells.
b)
Cells throughout an organism can develop in different ways to perform
different tasks.
c)
d)
This process is called cell specialization.
3. Levels of Organization
The levels of organization in a multicellular organism are individual
cells, tissues, organs, and organ systems.
a)