CONCEPTUAL Life Science
Cells and Tissues
[Introductory note: This is a full unit in the course as I teach it. The parts on animal tissues could be easily incorporated into the
introduction to section on the human animal. Not so easy to do with the plants because the only real emphasis on plants that there is
will be with the energy and photosynthesis unit. So, I am entering it here. We may want to think about the photosynthesis some more
and come up with something like the green plant makes food for us, or green plants and photosynthesis.
As an expedient, I have typed in all of the notes that I use with my students. In the lecture book I use, there are drawings for nearly
every tissue. These have been omitted here. I can always draw them later if we decide to go that route. FHO 3/18/04]
PYRAMID OF BIOLOGICAL STRUCTURE
Figure CT-1. The pyramid of biological structure.
Living things exhibit structure and organization. The basic unit of structure and
function is the cell. Cells are organized into units containing similar cells. These are
called tissues. Different types of tissues are found in an organ. For example, an organ
such as the stomach has epithelium, muscle and nerve tissue in it. Organs are organized
into structural units called organ systems. The stomach is part of the digestive system.
The digestive system also contains other organs. Therefore, a living creature, such as a
human, is an organism containing various organ systems with their component organs,
made of tissues of which the basic units are the cells.
PLANT TISSUES
I.
Meristematic tissues
Meristematic tissues have embryonic, undifferentiated cells. Undifferentiated is a
term used to mean that the cells have not changed into other cells yet.
CT-1
CT-2
When embryonic cells change into other cells they undergo a process called
differentiation. Differentiation is the process of becoming different. In meristematic
tissue, the cells have the ability to divide using mitosis but they have not changed into
other tissue types yet. They eventually will become cells of other types. Flowering
plants such as trees have meristems located in three places: the shoot tip, the root tip and
the cambium layer between the bark of the tree and the wood.
A.
Shoot tip
There is a meristem at the end of each branch of a tree. It is protected by a bud.
The cells in the shoot tip meristem (also known as the apical meristem) produce
embryonic cells that will eventually develop into the stems, branches and leaves of the
tree. The bud has one year’s worth of growth inside. In the spring when the trees deploy
their leaves, the bud begins to grow and the leaves develop and deploy quickly. During
the summer, the tree makes another set of leaf buds for next year so that next spring, it is
ready to deploy its leaves at the appropriate time.
B.
Root tip
There is a meristem at the tip of each root. The meristem enables the root to grow
deeper into the soil. It also produces the other root tissues. The root tip meristem is
protected by a root cap so that friction with the soil does not wear it down.
After the cells of the root tip divide, they begin to elongate. This produces a zone
of elongation in the root. This production of new cells and elongation process permits the
root to penetrate deeper into the soil.
C.
Cambium
The cambium is found between the xylem and phloem layers of woody stems. It
produces new xylem and phloem cells. This activity increases the diameter of the tree.
Since cambium is active only during the growing season, the xylem tissue forms in rings.
A tree generally produces one ring of xylem per year. Therefore you can
determine the age of the tree by counting the rings. The appearance of rings is produced
by placing this spring’s new, large-diameter cells immediately adjacent to last summer’s
old, small-diameter cells.
II.
Permanent tissues
A.
Surface tissues
1.
Epidermis
CT-3
The epidermis contains living cells and is found on the surfaces of young stems
and leaves. The lower epidermis of leaves contains specialized occlusive cells known as
guard cells that surround openings in the leaves called stomates. “Stomate” is derived
from Latin and means hole. The guard cells regulate the size of the holes in the leaves
and permit exchange of gases (CO2. O2) and water vapor with the air. The epidermis
generally has a waxy outer surface to prevent loss of water by the plant except when the
stomates are open.
2.
Periderm
The periderm contains non-living cells that have thick secondary walls. The walls are
waterproofed and made impermeable with a material called suberin. Periderm is a tissue
that replaces the epidermis on older stems of the plant. The suberin-containing secondary
walls of the periderm prevent loss of water by the plant.
B.
Fundamental tissues
1.
Parenchyma
Parenchyma contains living cells that are capable of cell division. It is the least
specialized of the plant tissues. These cells have thin primary walls. Parenchyma is used
as a filler tissue in such plant structures as corn stalks.
2.
Collenchyma
The cells of collenchyma are usually alive. They have secondary wall material
that is deposited in the corners giving them a characteristic appearance under the
microscope. This type of tissue is found in certain types of stems. It serves as a support
tissue.
3.
Sclerenchyma
a.
Fibers
The fibers of sclerenchyma are dead. These cells have very thick secondary walls
and very small lumens. A lumen is a cylindrical space surrounded by a cylindrical
structure. The space inside a garden hose is a lumen. “Lumen” means light—sort of like
the light at the end of the tunnel.
The very thick walls of fiber cells give them much strength. This type of tissue
serves as a structural support tissue in stems. It is often associated with vascular bundles.
CT-4
b.
Sclereids
The cells of sclereids are also dead. Like the fibers, they have very thick
secondary walls with very tiny lumens. Sclereids are spherical. They are part of such
plant structures as peach pits.
4.
Endodermis
The endodermis is a layer of living cells found in the root. It has a waterproofing
layer known as the Casparian strip containing lignin and suberin, two impermeable
materials. The Casparian strip prevents materials from leaking from the outer tissues of
the root into the center where the vascular tissue is located. The plant uses the
endodermis to regulate what materials enter the plant and are transported up to the stem
and leaves.
C.
Vascular tissues
1.
Xylem
Xylem cells are dead. Xylem is a transport tissue that conducts water and
minerals upward in the plant. It also serves as a support tissue. Xylem is the major
component of wood. Xylem tissue in a woody stem (such as a tree trunk) has rays of
parenchyma cells in it.
2.
Phloem
Phloem contains living cells that conduct nutrients both upward and downward.
Phloem cells always are found in pairs. There is a large conducting cell called a sieve
tube and a smaller cell alongside known as the companion cell. The cytoplasm
communicates between the sieve tube and the companion cell. The sieve tube is
responsible for conducting materials. The companion cell contains the nucleus that
controls the pair of cells.
In a tree, the phloem is found in the bark. When the vascular tissues, xylem and
phloem, turn to the outside to connect to stems and leaves, the phloem is always on the
bottom and the xylem is always on top. This is apparent in a microscopic view of a
cross-section of a leaf. In the leaf, the vascular bundles always have the xylem on the top
and the phloem underneath the xylem.
ANIMAL TISSUES
I.
Epithelium
CT-5
Epithelium is a tissue that lines or covers all body surfaces, both inside and
outside. This includes the skin, the inside and outside of the blood vessels, cuctgs, the
digestive tract and other body organs.
A.
Simple epithelium
Simple epithelium always contains a single layer of cells attached to a membrane.
As this membrane is underneath the cells, it is called the basement membrane. There are
three kinds of epithelial cells that differ in their shapes.
1.
Simple squamous epithelium
“Squamous” means flat. Simple squamous epithelium contains a single layer of
flat cells attached to a basement membrane. An example of this type of tissue is
peritoneum, the tissue that surrounds and suspends the digestive organs in the abdominal
cavity.
2.
Simple cuboidal epithelium
“Cuboidal” means having the shape of a cube. This tissue contains a single layer
of cube-shaped cells on a basement membrane. An example of this type of tissue is the
lining of the collecting duct in the kidney.
3.
Simple columnar epithelium
“Columnar” means that the cells are tall, like columns. This tissue contains a
single layer of tall cells on a basement membrane. An example of this type of tissue is
the intestinal lining.
B.
Stratified epithelium
“Stratified” means that the cells of the tissues are found in layers. All three types
of stratified epithelium start out as several layers of cuboidal cells on a basement
membrane. The other layers determine which type of tissue it is.
1.
Stratified squamous epithelium
Stratified squamous epithelium contains several layers of cuboidal cells on a
basement membrane. As you proceed away from the membrane the cells get
progressively flatter. An example is skin.
CT-6
2.
Stratified cuboidal epithelium
In stratified cuboidal epithelium, the cells in all layers of the tissue are cuboidal.
An example is the lining of the ducts of the sweat glands.
3.
Stratified columnar epithelium
In stratified columnar epithelium, all layers of the tissue are cuboidal except for
the top layer that contains tall cells. An example is the lining of the ducts of the
mammary glands.
II.
Connective tissue
Connective implies holding things together or joining things. Thus we expect to
find tendons, ligaments and cartilage classified as connective tissue. In addition, the
category of connective tissues contains body-wide tissues such as bone, blood and lymph.
In most cases, the cells that make up the tissue are surrounded by some kind of matrix or
material characteristic of the tissue.
A.
Tissues that connect body structures
1.
Loose connective tissue (areolar)
Loose connective tissue contains cells surrounded by a matrix of collagenous
fibers, elastic fibers and lymph. Collagenous fibers are made of collagen, a type of
structural body protein. Lymph is also called tissue fluid. It is a liquid that is found in
between the different parts of the body in the spaces known as the connective tissue
spaces.
2.
Dense connective tissue
Dense connective tissue is composed primarily of collagenous fibers. Dense
connective tissue is found in the dermis of the skin, tendons and ligaments. A tendon
connects a muscle to a bone while a ligament connects one bone to another bone.
3.
Adipose tissue
Adipose tissue is made of fat cells. Each cell has a thin ring of cytoplasm that
surrounds a large vacuole containing a fat droplet.
4.
Cartilage
There are three kinds of cartilage. Each consists of cells surrounded by a matrix
of small fibers.
CT-7



B.
Yellow elastic cartilage is found in the arteries and between the rungs of the
trachea.
Hyaline cartilage forms the ridge of the nose and the rings of the trachea.
Elastic cartilage is found in the external ear, the epiglottis and the Eustachian
tube.
Bone
1.
Description of bone tissue
Bone cells are surrounded by a matrix of calcium phosphate. There are tunnels in
the bone called Haversian canals that contain the blood vessels that supply blood to the
bone cells. Bone has the most mineral matter. There are two types of bone tissue known
as spongy bone and compact bone.
2.
Spongy bone
The ends of the long bones are made of spongy bone. “Spongy,” when it is used
in biology, means “having the appearance of a sponge.” It does not mean that the tissue
is soft and flexible. Spongy bone is hard but has the appearance of a sponge.
3.
Compact bone
Compact bone does not have the spaces in it that give it a spongy appearance.
The long parts of the long bones are composed of compact bone tissue.
C.
Vascular tissue
Vascular tissue consists of cells surrounded by a liquid matrix. There are two
types of vascular tissue that are called blood and lymph.
1.
Blood
Blood is found in blood vessels. It is carried by the circulatory system.
2.
Lymph
Lymph consists of fluid that is found outside of the blood vessels. It is also
known as tissue fluid. It is collected via the lymphatic system and eventually returns to
the circulatory system.
III.
Muscles
A.
Smooth muscle
CT-8
Smooth muscles are found in all involuntary organs except the heart. Examples
of organs include the diaphragm and the arteries. Smooth muscle does not contain
striations.
B.
Striated muscle
Striated muscles are attached to the bones of the skeleton. Each striated muscle
cell is multinucleate because it has many nuclei. Striated muscles contain striations,
which are lines that produce a cross-banding effect. These lines result from the
orientation of the muscle proteins within the cell. The striated skeletal muscles are also
known as voluntary muscles.
C.
Cardiac muscle
The heart is the only organ that contains cardiac muscle. This muscle cell type
has distinct cells that are separated by intercalated discs. The intercalated discs partition
the muscle into cells. Cardiac muscle cells also contain striations.
IV.
Nerve tissue
The neuron is the cell of the nervous system. Each axon is surrounded by a
sheath of membranes.