The Structure of the Cell

The cell as a fundamental unit structure and

function of living things is responsible for
initiating, regulating and directing the life-
sustaining reactions.

Cell biology is a branch of biology that studies the

structure, function, and behavior of cells.
 Parts of a Cell
Eukaryotic cells have three main parts namely: cell
membrane, cytoplasm and
nucleus.
 1. Cell Membrane
The cell membrane ( also known as
plasma membrane ) is a double-layered
membrane that encloses the cell
the boundary between the cell and its external
environment;
as a structure that controls the movement of substances
in and out of the cell. It is a
semipermeable membrane
 1. Cell Membrane

Cell membranes are represented according to a

fluid-mosaic model, due to the fact that they are:
•Fluid – the phospholipid bilayer is viscous and
individual phospholipids can move position
•Mosaic – the phospholipid bilayer is embedded
with proteins, resulting in a mosaic of components
Phospholipids – Form a bilayer with phosphate heads facing outwards and fatty acid tails
facing inwards
Cholesterol – Found in animal cell membranes and functions to improve stability and
reduce fluidity
Proteins – May be either integral (transmembrane) or peripheral and serve a variety of
roles
 Protoplasm and Its Organelles
Protoplasm is considered as the physical basis of life. The protoplasm of a
cell consists of a nucleus, cell membrane, and the cytoplasm.
 Protoplasm and Its Organelles
The plasma or cell membrane
encloses a mass of jellylike material
called protoplasm. The protoplasm
is a colorless or somewhat grayish,
translucent, viscous substance that
is capable of flowing. It is a colloid,
can change its state from a
semisolid gel to a semiliquid
solution. Its texture also varies. It is
composed of 20 % carbon, 10 %
hydrogen, 62 percent oxygen, 3 %
nitrogen, and 5 % trace elements
 Cytoplasm and Karyoplasm (Nucleoplasm)
If a nucleus is present in a c ell, the protoplasm is
differentiated into two:

(1) cytoplasm, where the protoplasm is lying outside

the nucleus; and
(2) karyoplasm or nucleoplasm, the protoplasm inside
the nucleus
 Cytoplasm and Karyoplasm (Nucleoplasm)

Cytoplasm is the gelatinous

liquid that fills the inside of a
cell. It is composed of water,
salts, and various organic
molecules. Some intracellular
organelles, such the nucleus and
mitochondria, are enclosed by
membranes that separate them
from the cytoplasm.
 Cytoplasm and Karyoplasm (Nucleoplasm)

The nucleoplasm, also known as

karyoplasm, is the type of
protoplasm that makes up the cell
nucleus, the most prominent
organelle of the eukaryotic cell. It
is enclosed by the nuclear
envelope, also known as the
nuclear membrane..
 The Nucleus: Control Center of the Cell

The nucleus is the first organelle

that biologists observe in a cell. It
is a spherical structure that is
usually located near the center
of the cell.
Robert Brown discovered the
nucleus in the cell in the year
1831. The nucleus is located in
the eukaryotic cells.
 The Nucleus: Control Center of the Cell

The nucleus is the control center

of the cell since it contains the
genetic material, DNA, which
determines the characteristics of
a cell and directs the production
of proteins.
They act as spools around
which DNA winds to create
structural units called
nucleosomes
 The Nucleus: Control Center of the Cell

The DNA molecules are found

either free in the protoplasm or
combined with protein
( histones) in a definite structure
called chromosomes, which are
separated from the rest of the
cell by a double membrane
called the nuclear membrane
 The Nucleus: Control Center of the Cell

One part of the chromatin is

condensed into a darker
area called the nucleolus.
It’s primary function is to
produce and assemble the
cell's ribosomes.

ribosome biogenesis
 RIBOSOMES (Protein Factories of the Cell )

Ribosomes are small

granular structures made
up of RNA and proteins. The
RNA found in ribosomes is
produced in the nucleus
and enters the cytoplasm
through the nuclear pores.

Protein Synthesis
 Endoplasmic Reticulum: Manufacturers and
Shippers of the Cel

Endoplasmic reticulum
(ER) is a membranous
structure forming a
network of canals where
proteins and other
molecules are being
transported.
 Endoplasmic Reticulum: Manufacturers and
Shippers of the Cel

The rough endoplasmic

reticulum (rER) is an
organelle where most of the
ribosomes are attached.
Proteins produced in the
ribosomes need to be
transported through the
rER.
 Endoplasmic Reticulum: Manufacturers and
Shippers of the Cel
Smooth endoplasmic
reticulum (sER) is a branching
canal (without ribosomes) that
serves for the transport of large
molecules inside the cell. Some
steroids are produced in the
sER. The sER also functions as a
storage site of calcium ions in
muscle cells.
 Golgi Bodies: Packaging Counters of the Cell

A Golgi body, also known as a

Golgi apparatus, is a cell
organelle that helps process
and package proteins and
lipid molecules, especially
proteins destined to be
exported from the cell. Named
after its discoverer, Camillo
Golgi, the Golgi body appears
as a series of stacked
membranes..
 Golgi Bodies: Packaging Counters of the Cell

These biochemicals
leave the golgi
by exocytosis before
being delivered to
different
intracellular or
extracellular targets.
 Mitochondria : Powerhouses of the Cell
Mitochondria (singular, mitochondrion) are
microscopic rod-shaped bodies in the cytoplasm.
They are the centers of cellular respiration, a
process in which energy-rich adenosine
triphosphate or ATP, a high energy storage and
transfer, is produced.
 Lysosome: Suicide Bags of the Cell
Lyso means „dissolving power‟ and
some means „body‟. found to be rich
in strong hydrolytic enzymes which
are kept intact by a thick lysosomal
membrane. Once the membrane is
damaged, the lysosome releases its
content into the cytoplasm and may
destroy the entire cell. The lysosome
also makes possible the destruction
of microorganisms such as bacteria
and other harmful foreign bodies
taken in by the cell through its
membrane
 Vacuoles: Storage Tanks of the Cell

Vacuoles are membrane-bound organelles for the

storage of food, enzymes, and other materials needed
by the cell. Some vacuoles also store waste products.
 Cytoskeleton : Framework of the Cell

Cytoskeleton is a network
of thin and fibrous
elements serving as the
“bones and muscles” of
cells. contractile protein
called actin

Microfilaments are
cytoskeletons attached to
the cell membrane
responsible for the shape
of the cell.
 Cytoskeleton : Framework of the Cell
Microtubules are cytoskeletons that are not connected to the cell
membrane but they serve as anchorage of organelles in the cell. The
centrioles and spindle fibers, which are involved in cell division, are
composed of microtubules
AMOEBA

FLAGELLUM

PARAMECIUM
 The Cell Wall

The cells of plants, fungi

and some single-celled
organisms are protected
and supported by a rigid
cell wall, which lies outside
the plasma membrane and
is composed mainly of
cellulose, except in the
case of fungi and bacteria.
aminopolysaccharide polymer
Both the primary and secondary
walls are composed mainly of
polysaccharide cellulose

Two primary cell walls of adjacent

cells are separated by another
layer, the middle lamella, which is
composed of a jellylike
polysaccharide called pectin.

Aside from cellulose, the

secondary walls usually contain
lignin, a complex organic
compound that adds hardness and
rigidity to cell walls
 PLASTIDS
Plastids are double-membrane organelles which are found in the
cells of plants and algae. Plastids are responsible for
manufacturing and storing of food. These often contain
pigments that are used in photosynthesis and different types of
pigments that can change the colour of the cell.

Leucoplasts are plastids which store foods of plants such as fats,

oils, starch, proteins, etc. Chloroplasts are the photosynthetic
organelles of plants. They contain chlorophylls (green colour
pigments). Chromoplasts are different coloured pigment
containing plastids of plants.
 Chloroplasts
Chloroplasts are the
chromoplasts containing
the green pigment called
chlorophyll. Chlorophyll is
very essential in the
manufacture of food by
green plants. This process
will be discussed in
succeeding chapters.