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Lesson 10.2 The Cell
Microscopes helped scientists discover that cells can be grouped
into two types. There are prokaryotic (proh ka ree AH tihk) cells and
eukaryotic (yew ker ee AH tihk) cells.
The cells of plants, animals, fungi, and protists are eukaryotic
cells.The genetic material of eukaryotic cells is surrounded by a
membrane. Every eukaryotic cell also has organelles—other parts
that are surrounded by a membrane and have specialized functions.
Eukaryotic cells are usually larger than prokaryotic cells.
The genetic material in a prokaryotic cell is not surrounded by
amembrane. This is the most
important feature of a prokaryotic cell. Prokaryotic cells also do not
have many of the cell parts other cells have. Most prokaryotic cells
are unicellular organisms and are called prokaryotes.
Cell Shape and Movement
Cells come in many shapes and sizes. The size and shape of a cell is part of the function of the cell.
Some cells, such as human red-blood cells, can be seen only by using
a microscope. The cells can pass easily through small blood vessels
because of their small size. Their disk shapes are important for
carrying oxygen. Nerve cells have parts that jut out. These projections
on nerve cells can send signals over long distances. Some plant cells
are hollow. These hollow cells make up tubelike structures that can
carry water and dissolved substances to parts of the plant.
The size and shape of a cell make it possible for the cell to carry
out its functions. The parts that make up a cell have their own
functions as well. A cell’s parts are like the players on a football team
who perform different tasks on the playing field. A cell is made up of
different parts that perform different functions to keep the cell alive.
All cells have some parts, or structures, in common. One of these
structures is a cell membrane. A cell membrane is a flexible covering
that protects the inside of a cell from the environment outside the cell.Cell membranes are made of proteins and phospholipids.
Every cell has a cell membrane. But some cells also have a cell
wall. Plant cells, fungal cells, bacterial cells, and some protists have cell walls. A cell wall is a stiff
structure outside the cell membrane. A cell wall protects a cell from
viruses and other harmful organisms. In some plant and fungal cells,
a cell wall helps the cell keep its shape and gives it support.
Cytoplasm and the Cytoskeleton
The fluid inside a cell is made of water, salts, and other molecules
and is called the cytoplasm. The cytoplasm contains a cell’s
cytoskeleton. The cytoskeleton is made of threadlike proteins that are joined together. The cytoskeleton
is a framework that gives a cell its shape and helps it move.
If you look at a cell using a microscope, you might see structures
on the outside of the cell. These appendages might look like hairs
orlong tails. They often help a cell move. Flagella (fluh JEH luh)
(singular, flagellum) are long and taillike. They whip back and forth
to move the cell. Cilia (SIH lee uh) (singular, cilium) are short,
hairlike structures. They can move a cell or move molecules away from a cell. The cilia in your windpipe
move harmful particles away from your lungs.
The organelles of eukaryotic cells have different functions in the
cell. Organelles help a cell carry out different functions at the same
time. These functions include getting energy from food, storing
information, and getting rid of waste material.
The largest organelle inside most eukaryotic cells is the nucleus. The nucleus is the part of a eukaryotic
cell that directs cell activities and contains genetic information stored in DNA. DNA is in structures called chromosomes. The number of
chromosomes in a nucleus is different for different species of
organisms. The nucleus also contains proteins and an organelle called the
nucleolus (new KLEE uh lus). The nucleolus makes ribosomes,
organelles that help produce proteins. Two membranes form the
nuclear envelope that surrounds the nucleus. The nuclear envelope
has many pores. Certain molecules, such as ribosomes and RNA,
move into and out of the nucleus through these pores.
You learned that proteins are important molecules in cells.
Proteins are made of small organelles called ribosomes. A ribosome
is not surrounded by a membrane. Ribosomes are in the cytoplasm
of a cell. Ribosomes can be attached to an organelle called the
endoplasmic reticulum (en duh PLAZ mihk · rih TIHK yuh lum), or
ER. ER with ribosomes on its surface is called rough ER. Rough ER, is where proteins are produced. ER
without ribosomes on its surface is called smooth ER. It makes lipids
such as cholesterol. Smooth ER also helps remove harmful
substances from a cell.
All living things must have energy to survive. Cells process some
energy in specialized organelles called mitochondria (mi tuh KAHN
dree uh) (singular, mitochondrion).
Most eukaryotic cells contain hundreds of mitochondria. Some cells
in a human heart can contain 1,000 mitochondria.
The cells of some organisms, such as plants and algae,
contain organelles called chloroplasts (KLOR uh plasts). Chloroplasts
are membrane-bound organelles that use light energy and make food,
a sugar called glucose, from water and carbon dioxide in a process
called photosynthesis (foh toh SIHN thuh sus). The sugar has stored
energy that can be used when the cells need it.
A mitochondrion is surrounded by two membranes. Chemical
reactions within mitochondria release energy. This energy is stored
in high-energy molecules called ATP—adenosine triphosphate (uh
DEH nuh seen · tri FAHS fayt). The energy in ATP molecules is used
by the cell for growth, cell division, and transporting materials.
Lesson 10.2 Goes with Student Page
Processing, Transporting, and Storing Molecules
The Golgi (GAWL jee) apparatus is an organelle that looks like a
stack of pancakes. It gets proteins ready for their specific jobs. It then
packages the proteins into tiny membrane-bound, ball-like structures
called vesicles. Vesicles are organelles that transport substances to
other parts of the cell. Some vesicles in an animal cell are called
lysosomes. Lysosomes help break down and recycle different parts
ofthe cell. Some cells also have structures called vacuoles (VA kyuh wohlz).
Vacuoles are organelles that store food, water, and waste materials
for a cell. A plant cell usually has one large vacuole . Some animal
cells have many small vacuoles.