Cells: The Living Units

Four concepts collectively known as the cell theory: Organelles

 A cell is the basic structural and functional - These are permanent structures with
unit of living organisms. When you define cell characteristic morphology that are highly
properties, you define the properties of life. specialized in specific cellular activity.
 The activity of an organism depends on both Inclusions –they are the secretions and storage
the individual and the combined activities of products of cells.
its cells.
 According to the principle of Plasma Membrane
complementarity of structure and function, - Plasma membrane is a thin outer membrane,
the biochemical activities of cells are dictated which maintains the integrity of the cell. It
by their shapes or forms, and by the relative keeps the cell and its contents separate and
number of the subcellular structures they distinct from the surrounding. It is a double
contain. layered measuring about 4.5 nm and made of
 Cells can only arise from other cells. (All cells phospholipids, cholesterol, glyco-lipid, &
come from the division of pre-existing cell) carbohydrate (oligosaccharides). The bi-layer
is self-sealing. If a needle is injected and
CELL DIVERSITY
pulled out, it automatically seals.

Functions:

1. Separate the cytoplasm inside a cell from

extra cellular fluid.
2. Separate cell from one another
3. Provide an abundant surface on which
chemical reaction can occur.
4. Regulate the passage of materials in to and
out of cells. It also let some things in and
keeps others out. The quality selective
permeability

Movement across-cell membrane place in two ways.

 Passive movements –uses energy

 Active movements –consumes energy in the
form of ATP.

Passive movement: includes

Regardless of these differences, all cells have the a) Simple diffusion, the random movements of
same basic parts and some common functions. For molecules from area of high concentration to
this reason, it is possible to speak of a generalized, or the area of low concentration.
composite cell. Example: air in alveoli of lung
A human cell has three main parts: b) Facilitated diffusion, larger molecules, which
are not soluble in lipid need protein channel
1. The plasma membrane: the outer boundary to pass through the plasma membrane. No
of the cell which acts as a selectively direct energy needed.
permeable barrier. Example: Amino acid passes through the cell
2. The cytoplasm (si′to-plazm): the intracellular membrane.
fluid packed with organelles, small structures c) Osmosis, a special type of diffusion referring
that perform specific cell functions. to the passage of water through a selectively
3. The nucleus (nu′kle-us): an organelle that permeable membrane from an area of high
controls cellular activities. Typically the water concentration to lower water
nucleus lies near the cell’s center. concentration.
 d) Filtration, small molecules pass through however are found as colloids. Colloids are
selectively permeable membrane in response particles that remain suspended in the
to force of pressure. surrounding medium.
Example: filtration in the kidney in the
Organelles
process of urine formation.
- Organelles are specialized portion of the cell
Active movements across membranes
with a characteristic shape that assume
- Substances move through a selectively specific role in growth, maintenance, repair
permeable membrane from areas of low and control.
concentration on side of a membrane to an
a. Nucleus
area of higher concentration on the other
- Oval in shape and is the largest structure in
side. This is against concentration gradient.
the cell. Contain the hereditary factor in the
Therefore, it requires energy.
cell. Hence it controls cell activity & structure.
a. Active Transport
Most cell contain single nucleus but some like
- Until equilibrium substances could move by
matured Red Blood cell do not contain.
passive movement. But if equilibrium reached
However Muscle cell contain several
and still more molecules are needed, they
nucleuses. The nucleus separated from other
must be pumped through the membrane
cell structure by double membrane called
against concentration gradient.
nuclear membrane. Pores over the nuclear
- This process requires the use of ATP. One
membrane allow the nucleus to communicate
example of such processes is Sodium–
with the cytoplasm.
Potassium pump and calcium pump. In this
- In the nucleus a jelly like fluid that fill the
process all follows similar process. These are
nucleus is karylymph (neucleoplasm), which
molecules bind to carrier protein, molecule-
contain the genetic material called
carrier complex pass through the membrane,
chromosome. Nucleus also contain dark,
assisted by an enzyme & ATP and carrier
somewhat spherical, non-membrane bound
protein returns to its original shape & repeat
mass called nucleolus. It contains DNA, RNA
the process.
and protein, which assist in the construction
b. Endocytosis
of ribosome.
- Pocketing in by plasma membrane. It includes:
b. Ribosome
 Pinocytosis –cell drinking - Tiny granules, composed of Ribosomal RNA
(rRNA). They are site of protein synthesis
 Receptor–mediated Endocytosis-
c. Endoplasmic reticulum
Endocytosis with the help of receptor.
- It is a double membrane channel. It is
 Phagocytosis –cell eating. continuous with the nuclear membrane. It
involved in intracellular exchange of material
c. Exocytosis with the cytoplasm. Various products are
- Opposite to Endocytosis. transported from one portion of the cell to
- To remove out undigested particles. another via the endoplasmic reticulum. So it is
Cytoplasm considered as intracellular transportation. It is
also storage for synthesized molecules.
- It is a matrix or ground substance in which Together with the Golgi complex it serves as
various cellular components are found. It is synthesis & packaging center.
thick semi-transparent, elastic fluid containing 1. Granular E.R.
suspended particles and a series of minute o Containing granule and involving in
tubules and filaments that form cytoskeleton. synthesis of protein.
Water constitutes 75-90% of the cytoplasm. It 2. Agranular E.R.
also contains solid components, proteins, o It synthesize lipid & involves in
carbohydrates, lipids and inorganic detoxification.
substances. The inorganic components exist
as solutions because they are soluble in
water. The majority of organic substances
d. Golgi Complex Cell inclusions
- It is located near to the nucleus.
- Large and diverse group of chemicals, which
- It consist 4-8 membranous sacs.
are produced by cells, are cell inclusions.
- It process, sort, pack & deliver protein to
- It is mainly organic and includes melanin,
various parts of the cell.
glycogen & Lipids.
e. Mitochondria
CANCER CELLS
- It is a small, spherical, rod shaped or
- Cells out of control
filamentous structure.
- Normal human body cells usually divide at a
- It generates energy.
controlled rate required to replace the dying
- Each mitochondria possess two membrane,
ones and for growth. Cancer cells are
one is smooth (upper) membrane and the
different. They lack the controlling
other is arranged with series of folds called
mechanism.
cristae. The central cavity of a mitochondrion
- Cancer occurs when cells grows and divide at
enclosed by the inner membrane is the
abnormal rate & then spread beyond the
matrix.
original site. Some of the risk factors for
f. Lysosomes cancer occurrence are radiation, chemicals,
extreme pressure and hormonal therapy.
- It appear as membrane enclosed spheres.
- They are formed from Golgi complexes & Two stages of the CELL LIFE CYCLE
have single membrane.
1. Interphase
- They contain powerful digestive (hydrolytic
- It is the period between cell divisions, is the
enzyme) –capable of breaking down many
time of DNA replication.
kinds of molecules.
- During replication, DNA unwinds, and each
- The lysosomal enzyme believed to be
strand produces a new DNA molecule.
synthesized in the granular endoplasmic
2. Cell Division
reticulum and Golgi complex.
- Cell division includes nuclear division and
g. Cyto-skeleton
cytoplasmic division.
- the cytoplasm has a complex internal
a. Mitosis
structure consisting of a series of exceedingly
- It is the replication of the cell’s nucleus, and
small microfilaments, microtubule &
cytokinesis is division of the cell’s cytoplasm.
intermediate filaments together referred to as
- Mitosis is a continuous process divided into
the cyto-skeleton.
four phases.
h. Centrosome 1. Prophase
o Chromatin condenses to become
- It is a dense area of cytoplasm generally
visible as chromosomes. Each
spherical and located near the nucleus it
chromosome consists of two
contain centrioles.
chromatids joined at the centromere.
- It also contains DNA that controls their
Centrioles move to opposite poles of
replication.
the cell, and astral fibers and spindle
- Centrosomes are made of microtubules,
fibers form. Nucleoli disappear, and
which seam drinking straws.
the nuclear envelope degenerates.
- They are Involved in the movement of
2. Metaphase
chromosome during cell division.
o Chromosomes align at the center of
i. Cilia/flagella the cell.

- It is a thread like appendages, which are made 3. Anaphase

of microtubules. When they are beating forms
o The chromatids of each chromosome
rhythmic movement. They are found in
separate at the centromere. Each
female reproductive organ and upper
chromatid is then called a
respiratory tube.
chromosome. The chromosomes
migrate to opposite poles.
 has a basement membrane, and does not
4. Telophase have blood vessels.
o Chromosomes unravel to become - The basement membrane is secreted by the
chromatin. The nuclear envelope and epithelial cells and attaches the epithelium to
nucleoli reappear. the underlying tissues.
b. Cytokinesis
Functions of Epithelial Tissues
- It begins with the formation of the cleavage
furrow during anaphase. - Protect underlying structures
- It is complete when the plasma membrane - Act as barriers
comes together at the equator, producing two - Permit some substances to pass through
new daughter cells. epithelial layers
- Secrete substances, and absorb substances

Glands

- Glands are organs that secrete.

1. Exocrine glands –secrete through ducts
2. Endocrine glands –release hormones that
are absorbed directly into the blood.
- Glands are classified as unicellular or
multicellular. Goblet cells are unicellular
glands. Multicellular exocrine glands have
ducts, which are simple or compound. The
ducts can be tubular or end in small sacs (acini
or alveoli). Tubular glands can be straight or
coiled.
- Glands are classified according to their mode
of secretion.
o Merocrine glands (pancreas) secrete
substances as they are produced,
o Apocrine glands (mammary glands)
accumulate secretions that are
released when a portion of the cell
pinches off.
o Holocrine glands (sebaceous glands)
TISSUES accumulate secretions that are
- These are collections of similar cells and the released when the cell ruptures and
extracellular substances surrounding them. dies.
- The four primary tissue types are Epithelial, Connective tissue
Connective, Muscle, and Nervous Tissues.
- It is distinguished by its extracellular matrix.
Histology
Functions of Connective Tissue
- It is the microscopic study of tissues.
- Connective tissues enclose and separate
Embryonic Tissue organs and tissues
- All four of the primary tissue types are derived - Connect tissues to one another; help support
from each of the three germ layers and move body parts
(mesoderm, ectoderm, and endoderm). - Store compounds
- Cushion and insulate the body
Epithelial Tissue - Transport substances
- Protect against toxins and injury
- Epithelium consists of cells with little
extracellular matrix. It covers surfaces, usually
 - Connective tissue is classified according to the
type and proportions of cells and extracellular
matrix fibers, ground substance, and fluid.
Cells of Connective Tissue
Embryonic connective tissue
- The extracellular matrix results from the
- It is called mesenchyme, consists of irregularly
activity of specialized connective tissue cells;
shaped cells and abundant matrix, and gives
in general, blast cells form the matrix, cyte
rise to adult connective tissue.
cells maintain it, and clast cells break it down.
Fibroblasts form protein fibers of many Adult connective tissue consists of connective tissue
connective tissues, osteoblasts form bone, proper, supporting connective tissue, and fluid
and chondroblasts form cartilage. connective tissue.

Connective tissue commonly contains: Connective Tissue Proper

- adipose cells 1. Loose connective tissue

- mast cells
- Areolar connective tissue has many different
- white blood cells
cell types and a random arrangement of
- macrophages
protein fibers with space between the fibers.
- mesenchymal cells (stem cells).
- This tissue fills spaces around the organs and
Extracellular Matrix attaches the skin to underlying tissues.

Major components of the extracellular matrix of ■ Adipose tissue

connective tissue are
- It has adipocytes filled with lipid and very little
1. Protein fibers extracellular matrix (a few reticular fibers). It
2. Ground substance functions in
3. Fluid 1. Energy storage
2. Insulation
Protein fibers of the matrix have the following
3. Protection
characteristics:
- Adipose tissue can be yellow or brown.
o Tropocollagens –are linked together to form - Brown adipose is specialized for generating
collagen fibrils, which are joined to form heat.
collagen fibers. The collagen fibers resemble
■ Reticular tissue
ropes. They are strong and flexible but resist
stretching. - It is a network of reticular fibers; it forms the
o Reticular fibers –are fine collagen fibers that framework of lymphatic tissue, bone marrow,
form a branching network that supports other and the liver.
cells and tissues.
2. Dense connective tissue
o Elastic fibers –have a structure similar to that
of a spring. After being stretched, they tend to ■ Dense regular connective tissue
return to their original shape.
- It is composed of fibers arranged in one
Ground substance has the following major direction, which provides strength in a
components: direction parallel to the fiber orientation.

o Hyaluronic acid –makes fluids slippery. Two types of dense regular connective tissue exist:
o Proteoglycan –aggregates trap water, which
1. Collagenous (tendons and most
gives tissues the capacity to return to their
ligaments)
original shape when compressed
2. Elastic (ligaments of vertebrae).
ordeformed.
o Adhesive molecules –hold proteoglycans ■ Dense irregular connective tissue
together and to plasma membranes.

Connective Tissue Classifications

 - It has fibers organized in many directions,
which produces strength in different
directions.

Muscle Tissue
Two types of dense irregular connective tissue exist:
- It has the ability to contract.
1. Collagenous (capsules of organs and
1. Skeletal (striated voluntary) muscle attaches
dermis of skin)
to bone and is responsible for body
2. Elastic (large arteries).
movement. Skeletal muscle cells are long and
Supporting Connective Tissue cylindrically shaped with many peripherally
located nuclei.
1.Cartilage
2. Cardiac (striated involuntary) muscle cells are
- It has a relatively rigid matrix composed of cylindrical, branching cells with a single,
protein fibers and proteoglycan aggregates. central nucleus. Cardiac muscle is found in the
The major cell type is the chondrocyte, which heart and is responsible for pumping blood
is located within lacunae. through the circulatory system.
3. Smooth (non-striated involuntary) muscle
■ Hyaline cartilage forms the walls of hollow organs, the iris of
- It has evenly dispersed collagen fibers that the eye, and other structures. Its cells are
provide rigidity with some flexibility. spindle-shaped with a single, central nucleus.
Examples include the costal cartilage, the Nervous tissue
covering over the ends of bones in joints, the
growing portion of long bones, and the - It is able to conduct electrical impulses and is
embryonic skeleton. composed of:
1. Neurons (conductive cells)
■ Fibrocartilage 2. Neuroglia (support cells)
- It has collagen fibers arranged in thick Neurons
bundles; it can withstand great pressure, and
it is found between vertebrae, in the jaw, and - Have cell processes called dendrites and
in the knee. axons. Dendrites receive electrical impulses,
and axons conduct them.
■ Elastic cartilage - Neurons can be:
- It is similar to hyaline cartilage, but it contains 1. Multipolar (several dendrites and an
elastin. It is more flexible than hyaline axon)
cartilage and is found in the external ear. 2. Bipolar (one dendrite and one axon
3. Pseudo-unipolar (one axon)
2. Bone
Tissue Membranes
- Bone cells, or osteocytes, are located in
lacunae surrounded by a mineralized matrix 1. Mucous membranes
(hydroxyapatite) that makes bone very hard.  It is consist of epithelial cells,
Spongy bone has spaces between bony their basement membrane, the
trabeculae; compact bone is more solid. lamina propria, and sometimes
smooth muscle cells.
Fluid Connective Tissue  They line cavities that open to
1. Blood the outside and often contain
- suspended in a fluid matrix mucous glands, which secrete
2. Hemopoietic tissue mucus.
- forms blood cells 2. Serous membranes
 These are line cavities that do not
open to the exterior and do not
 contain glands but do secrete
serous fluid.
3. Synovial membranes
 These are formed by connective
tissue, line joint cavities, and
secrete a lubricating fluid.