9 grade Summary
Cell Theory
Over many years, many scientists observed and studied cells under the microscope.
As early scientists improved both microscopes and lenses, they could learn more and
more about cells. Some of these findings are listed in the table below.
Scientist (Year)
Finding
Hooke (1665)
identified and named cells
observed living cells; could see greater detail due to better
Leeuwenhoek (1674)
lenses
Schleiden (1838)
noted that plants are made of cells
Schwann (1839)
concluded that all living things are made of cells
Virchow (1855)
proposed that all cells come from other cells
The discoveries of these early scientists came together into the cell theory. Today’s
scientists agree with this cell theory. It says three things:
• All living things are made of cells.
• All cells come from other living cells.
• The cell is the most basic unit of life. There is nothing living that is smaller than a
cell.
Cells are very small. They are surrounded by a membrane that controls what enters
and leaves the cell. They have cytoplasm, a jellylike material that contains the
building blocks needed for life. And they are made of similar molecules. One of
these molecules is DNA, the genetic information.
There are two main types of cells, prokaryotic cells and eukaryotic cells. Prokaryotic
cells are extremely small. Their DNA floats in the cytoplasm, and they have no
distinct* internal parts. Prokaryotes, such as bacteria, are made of only one cell.
Eukaryotic cells have a nucleus, which is a membrane that separates DNA from the
cytoplasm. The nucleus is a type of organelle, a small part that carries out a specific
job in a cell. Eukaryotic cells have many types of organelles. Like the nucleus, most
organelles are covered by a membrane. Eukaryotes, such as plants and animals, are
made of one cell or many cells.
Cell Organelles
Proteins are very important, and many organelles work together to make them. These
organelles include the nucleus, endoplasmic reticulum, ribosomes, the Golgi
apparatus, and vesicles. Ribosomes are found in both eukaryotic and prokaryotic
cells. However, the other organelles—those surrounded by a membrane—are found
only in eukaryotic cells.
Nucleus: The nucleus stores and protects the DNA of the cell. DNA contains the
genes that are the instructions for making proteins.
The endoplasmic reticulum (ER): is a network of thin, folded membranes that
helps in the production of proteins and other molecules. The membranes are like a
maze; they fold back on themselves and have little spaces inside the folds. There are
two types of ER, smooth and rough. The rough ER looks bumpy because it has
ribosomes attached to it.
Ribosomes: are tiny organelles that link amino acids together to form proteins. They
are found on the surface of the ER and floating freely in the cytoplasm.
Golgi apparatus: is a stack of layers of membranes. In the Golgi apparatus, proteins
are changed, put into packages, and carried to other places in the cell.
Vesicles: are small sacs. They carry different molecules to where they are needed.
Vesicles are generally short-lived and are formed and recycled as needed.
Some organelles do jobs other than making proteins.
Mitochondria: are bean-shaped organelles that produce chemical energy that is
usable by a cell. They have two membranes. The inner membrane has lots of folds
that form compartments. Mitochondria also have their own ribosomes and DNA. At
one time, mitochondria may have been independent prokaryotes that were taken in
by larger cells.
Vacuoles: are sacs of fluids that store materials in a cell. These materials include
water, food molecules, ions, and enzymes. Many plant cells have a large central
vacuole. When filled with fluid, the central vacuole exerts pressure that can help
support the plant.
Lysosomes: are organelles that contain enzymes. They protect a cell by attacking
incoming bacteria or viruses. They also break down old cell parts.
Centrioles: are shaped like cylinders. They are made of tiny tubes in a circle. They
move when animal cells divide in two. Centrioles help form cilia and flagella,
structures that help cells to move or to move liquids past a cell. Centrioles are
surrounded by the centrosome. The centrosome is a small region of cytoplasm that
organizes proteins into fibers that help cells divide.
Plant cells have two parts that animal cells do not have. Plant cells have cell walls
and chloroplasts.
The cell wall: is a strong, rigid layer that protects, supports, and shapes the plant
cell. It surrounds the cell membrane. Some cell walls are very thick, such as those in
a tree. When joined with others, they can support a very large plant. Some cell walls
are thinner, like those in celery.
Chloroplasts: carry out photosynthesis that stores energy from sunlight as chemical
energy for the plant. The chloro- part of the name chloroplast means “green.”
Chloroplasts contain chlorophyll, a green pigment, that helps capture energy from
sunlight. Like mitochondria, chloroplasts have two membranes and their own
ribosomes and DNA.
Cell Membrane
The cell membrane is the package that a cell comes in. It is a thin layer that separates
the inside of the cell from the outside of the cell. It controls what comes into and
goes out of the cell. The cell membrane is made up of a double layer of
phospholipids. A phospholipid is made of three parts: a phosphate group, a glycerol,
and two fatty acid chains.
The phosphate group and glycerol make up the “head.” The fatty acids make up the
“tail.” In the cell membrane, phospholipids are arranged tail-to-tail so that the heads
face outward.
Fluid Mosaic Model
Scientists have developed the fluid mosaic model to describe the cell membrane.
The membrane is fluid because the phospholipids in each layer can move and slide.
It acts like a film of oil on the surface of water. The membrane is like a mosaic
because of all the different molecules embedded* among the phospholipids.
Together, they look like a mosaic.
Selective permeability
The cell membrane has selective permeability. This means that it allows some
materials, but not all, to cross it, or permeate it. In other words, the cell membrane
is semipermeable. You might own a semipermeable jacket. The jacket is waterproof;
it does not allow water in from the outside. But water vapor from your sweat can
exit through the fabric. Just like the jacket, a semipermeable cell membrane lets only
some materials through. In a cell, small nonpolar molecules can usually cross the
membrane by themselves. Small polar molecules can be carried by proteins. Large
molecules can be moved in vesicles.
Diffusion and Osmosis
Cells are constantly taking in and sending out substances. But cells do not have to
use energy to move all those molecules. Passive transport is the movement
(transport) of molecules without a cell using energy (passive).
A concentration gradient is the difference in the concentration of a substance from
one location to another. Molecules move from one place to another because of this
difference in concentration.
Diffusion is the movement of molecules from a place of higher concentration to a
place of lower concentration. When molecules diffuse, they are described as moving
down their concentration gradient*. The diffusion of molecules across the cell
membrane is a type of passive transport. It happens because of the natural motion of
particles. Diffusion does not need energy from a cell.
The diffusion of water molecules is called osmosis. The process of osmosis is
exactly the same as diffusion but refers only to water molecules. Water molecules
diffuse across a membrane from a place of higher water concentration to a place of
lower water concentration.
Active Transport, Endocytosis, Exocytosis
Cells can use energy to move molecules from an area of lower concentration to an
area of higher concentration. This process is called active transport.
Sometimes a material is too large to cross the membrane. Cells can use energy to
transport these materials in vesicles. A cell uses endocytosis to take in large
materials or liquids. The prefix endo- means “in.” In endocytosis, the cell membrane
starts to fold in, forming a pocket around a substance. The pocket breaks off inside
the cell, making a vesicle. The contents of the vesicle are then broken down or
released into the cell.
Phagocytosis is a type of endocytosis in which the cell membrane grows out to
surround large particles. Th word literally means “cell eating.” Phagocytosis plays
an important role in the immune system when white blood cells “eat” bacteria and
other invaders.
Exocytosis is the opposite of endocytosis. The prefix exo- means “out.” It is the
process that moves substances out of the cell. In exocytosis, a vesicle surrounds
materials that need to be removed. The vesicle then goes to the cell membrane, fuses
with it, and let’s go of the contents. Exocytosis is the cell’s way of getting rid of
wastes or secreting molecules.