1. Structure and Functions of the Cell
A cell is the basic living structural and functional unit of the body.
Cytology: It is a branch of science concerned with the study of cells.
Cell theory explains:
a)
All living
organisms are composed of cells and cell products.
b)
The cell is the
basic unit of structure & function of all living organisms.
c)
All cells come
from the division of a pre-existing cell.
d) An organism as a whole can be understood through the collective activities & interactions of its cells.
We can divide the cell into four principal parts:
- Plasma (cell) membrane: it is the outer
lining, a limiting membrane
separating the cell’s internal parts from extracellular materials & the external environment.
- Cytoplasm: cytoplasm is the substance that surrounds
organelles and is located between the nucleus and plasma membrane
- Organelles: These are permanent structures with
characteristic morphology that are highly specialized in specific cellular
activity.
- Inclusions: they are the secretions and storage
products of cells.
Fig. 1: Cell
1.1 Cell membrane (Plasma membrane)
· Each cell has a limiting boundary: the cell membrane, plasma membrane, or plasmalemma.
· It is a living membrane, the outermost in animal cells.
· The plasma membrane maintains the integrity of the cell.
· It keeps the cell and its contents separate and distinct from the surroundings.
· It is a double-layered membrane measuring about 4.5 nm and made of phospholipids, cholesterol, glycolipids, & carbohydrates (oligosaccharides).
· The bilayer is self-sealing. If a needle is inserted and pulled out, it automatically seals.
· The plasma membrane is made of proteins and lipids, and several models were proposed regarding the arrangement of proteins and lipids. The fluid mosaic model proposed by Singer and Nicholson (1972) is widely accepted.
According to the fluid mosaic model,
- The plasma membrane is composed of a lipid bilayer of phospholipid molecules into which a variety of globular proteins are embedded.
- Each phospholipid molecule has two ends: an outer head (hydrophilic, i.e., water-attracting) and an inner tail (pointing centrally hydrophobic, i.e., water-repelling).
- The protein molecules are arranged in two different ways:
· Peripheral proteins or extrinsic proteins: these proteins are present on the outer and inner surfaces of the lipid bilayer.
· Integral proteins or intrinsic proteins: These proteins penetrate the lipid bilayer partially or wholly.
Fig. 2
1.1.1 Functions
· The plasma membrane encloses the cell contents.
· It provides cell shape (in animal cells), e.g., the characteristic shape of red blood cells, nerve cells, bone cells, etc
· It allows transport of certain substances into and out of the cell but not all substances, so it is termed selectively permeable.
· Separates cells from one another.
· Provides an abundant surface on which chemical reactions can occur.
1.2 The Cytoplasm
1.2.1 The Cytoplasm
·
Cytoplasm is a
matrix or ground substance in which various cellular components are found.
·
It is a thick,
semi-transparent, elastic fluid containing suspended particles and a series of
minute tubules and filaments that form the cytoskeleton.
·
Water
constitutes 75-90% of the cytoplasm.
·
It also
contains solid components, proteins, carbohydrates, lipids, and inorganic
substances.
·
The inorganic components
exist as solutions because they are soluble in water.
·
The majority of
organic substances, however, are found as colloids. Colloids are particles that
remain suspended in the surrounding medium.
The cytoplasm contains many cell organelles, of
which we shall learn about:
·
those that trap
and release energy, e.g., mitochondria;
·
those that are
secretory or involved in synthesis and transport, e.g., Golgi, ribosomes, and
endoplasmic reticulum
·
the suicidal
bags, i.e., lysosomes
·
the nucleus,
which controls all activities of the cell and carries the hereditary material.
1.3 CELL ORGANELLES
Organelles are specialized structures with
characteristic shapes that have specific functions.
1.3.1 Mitochondria
Structure
·
Also called the
powerhouse of the cell.
·
Appear as tiny thread-like
structures under a light microscope. Approximately 0.5 – 1.00 µm (micrometer)
·
The number is usually
a few hundred to a few thousand per cell.
·
It is a small,
spherical, rod-shaped, or filamentous structure.
·
It generates
energy.
·
Each mitochondrion
possesses two membranes; one is a smooth (outer) membrane, and the other is
arranged in a series of folds called cristae.
· The central cavity of a mitochondrion enclosed by the inner membrane is the matrix.
· Wall made of a double membrane.
· It contains its own DNA (circular), RNAs, ribosomes, and enzymes.
Function
· Oxidises pyruvic acid (breakdown product of glucose) to release energy, which gets stored in the form of ATP for ready use. This process is also called cellular respiration.
·
The most
important function of the mitochondria is to produce energy.
·
Mitochondria
help the cells to maintain proper concentration of calcium ions within the
compartments of the cell.
·
The
mitochondria also help in building certain parts of blood and hormones like
testosterone and estrogen.
·
The liver cells’
mitochondria have enzymes that detoxify ammonia.
·
The
mitochondria also play an important role in the process of apoptosis, or
programmed cell death.
·
Play an
important role in fat metabolism.
Fig. 3: Structure of Mitochondria
1.3.2 Endoplasmic Reticulum (ER)
Structure
·
It is a network of membranes that forms
flattened, interconnected sacs called as Cisternae are tubular in
structure and forms a three-dimensional polygonal network.
·
They are frequently associated with
ribosomes and special proteins called translocons that are necessary for
protein translation within the RER.
·
The phospholipid membrane encloses a
space, the lumen, from the cytosol, which is continuous with the perinuclear
space.
·
The surface of the rough endoplasmic
reticulum is studded with the protein-manufacturing ribosomes, which gives it a
rough appearance. Hence, it is referred to as the rough endoplasmic
reticulum.
·
The smooth endoplasmic reticulum consists of
tubules, which are located near the cell periphery. This network increases the
surface area for the storage of key enzymes and the products of these enzymes.
·
Rough endoplasmic reticulum synthesizes
proteins, while smooth endoplasmic reticulum synthesizes lipids and steroids.
It also metabolizes carbohydrates and regulates calcium concentration, drug
detoxification, and attachment of receptors on cell membrane proteins.
·
Endoplasmic reticulum varies in extent,
extending from the cell membrane through the cytoplasm and forming a continuous
connection with the nuclear envelope.
Functions
·
They play a
vital role in the synthesis of proteins, lipids, glycogen, and other steroids
like cholesterol, progesterone, testosterone, etc.
·
They provide the increased surface area
for cellular reactions.
·
They help in the formation of the nuclear
membrane during cell division.
·
They play a vital role in the formation
of the skeletal framework.
Fig. 4: Endoplasmic Reticulum
1.3.3 Golgi Body or Golgi Apparatus or Golgi Complex
·
The Golgi
apparatus is a series of flat membrane-bound sacs called cisternae.
·
It has two
faces: the convex entry cis face (faces the rough ER) and the exit or
trans face (faces the plasma membrane).
·
The number of
Golgi apparatus differs according to the cell’s secretion activity.
·
The Golgi
apparatus is specialized for receiving the molecules of substances secreted by
the endoplasmic reticulum across a group of transporting vesicles.
·
Then, it
classifies and modifies these vesicles and distributes them to the places where
they are used in the cell.
1.3.4 Lysosomes (lysis = breaking down; soma =
body)
·
Lysosomes are
small, round, membranous vesicles formed by Golgi bodies.
·
They contain a
group of digestive and hydrolytic enzymes (60 kinds).
·
The lysosomes’
function is to get rid of worn-out cells and organelles that no longer have
benefits, a process called autophagy.
·
Furthermore,
lysosomes digest the large molecules of nutrients engulfed by the cell and
change them into structurally simpler substances to enable the cell to benefit
from them.
·
For example,
white blood cells use the digestive enzymes present inside the lysosomes to
digest and destroy the pathogens that invade the cell.
·
The cell is not
affected by the lysosome enzymes because these enzymes are surrounded by a
membrane,
·
Lysosomes are
called “suicidal bags,” as enzymes contained in them can digest the cell’s own
material when damaged or dead, called autolysis.
1.3.5 Ribosome
Structure
·
Situated in two areas of the cytoplasm;
few are seen scattered in the cytoplasm, and a few are connected to the
endoplasmic reticulum.
·
80S ribosomes,
respectively comprising little (40S) and substantial (60S) subunits.
·
Composed of RNA
(rRNA) and ribosomal proteins.
·
When joined to the ER, they are
called the rough endoplasmic reticulum.
·
The free and the bound ribosomes are
very much alike in structure and are associated with protein synthesis.
Fig. 5: Ribosome
Functions
·
They assemble amino acids to form
specific proteins; proteins are essential to carry out cellular activities.
·
The proteins that are synthesized by
the ribosomes present in the cytoplasm are used in the cytoplasm itself. The
proteins produced by the bound ribosomes are transported outside the cell.
1.3.6 Centriole
It is present in all animal cells, located just
outside the nucleus. It is cylindrical, 0.5 µm in length, and without a
membrane. It has 9 sets of peripheral tubules but none in the center. Each set
has three tubules arranged at definite angles. It has its own DNA and RNA, and
therefore it is self-replicating.
Function
Centrioles are involved in cell division. They give
orientation to the ‘mitotic spindle,’ which forms during cell division.
1.3.7 Peroxisomes
·
They are
similar to lysosomes but are smaller and contain several oxidases.
Functions
·
They oxidise
amino acids and fatty acids.
·
They oxidise
toxic substances, such as alcohol in liver cells.
1.3.8 Proteasomes
·
Tiny barrel-shaped
structures.
·
Found in the nucleus
and cytoplasm.
·
Contains
proteases (enzymes that cut the protein into smaller peptides).
Functions
·
They degrade
unneeded, damaged, or faulty proteins into smaller peptides.
1.3.9 The Cytoskeleton
·
It is the
network of three types of protein filaments, i.e., microfilaments, intermediate
filaments, and microtubules.
·
Microfilaments
o
Thin, composed
of the protein actin.
o
Help generate
movement and provide mechanical support.
o
They are
involved in muscle contraction, cell division, and cell locomotion.
o
They anchor the
cytoskeleton to integral proteins in the plasma membrane.
·
Intermediate
Filaments
o
They are
thicker than microfilaments.
o
They provide tensile
strength for the cell.
·
Microtubules
o
These largest cytoskeletal components
are long, composed of the protein tubulin.
o
They help to
determine cell shape.
o
Microtubules also form the spindle
fibers for separating chromosomes during mitosis.
o
When arranged in geometric patterns
inside flagella and cilia, they are used for locomotion.
1.4 nucleus (the
hereditary organelle)
Structure
·
Nucleus is a membrane-bound structure that
contains the cell’s hereditary information and controls the cell’s growth and
reproduction.
·
The nucleus is present in all eukaryotic
cells, they may be absent in few cells, like mammalian RBCs.
·
The shape of the nucleus is mostly
round; it may be oval or disc-shaped, disc shaped depending on the type of cell.
·
The nuclear envelope is a double
membrane that separates the nucleus from the cytoplasm.
·
All traffic into and out of the nucleus
passes through nuclear pores that bridge the double membranes.
·
The nuclear envelope consists of
phospholipids that form a lipid bilayer.
·
The nuclear envelope is perforated with
numerous pores called nuclear pores.
·
The envelope helps to maintain the
shape of the nucleus and assists in regulating the flow of molecules into and
out of the nucleus through nuclear pores.
·
The nuclear envelope is connected with
the endoplasmic reticulum (ER) in such a way that the internal compartment of
the nuclear envelope is continuous with the lumen of the ER.
·
Chromosomes consist of DNA,
which contains hereditary information and instructions for cell growth,
development, and reproduction.
·
When a cell is “resting,” i.e. not
dividing, the chromosomes are organized into long entangled structures
called chromatin and not into individual chromosomes.
·
Nucleoplasm is the
gelatinous substance within the nuclear envelope.
·
The nucleolus is not
surrounded by a membrane, it is a densely stained structure found in the
nucleus.
Figure 6- A Nucleus
Functions of Nucleus
·
It controls the hereditary characteristics of an organism.
·
It main cellular metabolism through
the synthesis of particular enzymes.
·
It is responsible for protein
synthesis, cell division, growth, and differentiation.
·
Stores hereditary material in the form of
deoxyribonucleic acid (DNA) strands. Also stores proteins and ribonucleic
acid (RNA) in the nucleolus.
·
It is a site for the transcription process
in which messenger RNA (mRNA) is produced for protein synthesis.
·
It helps in the exchange of DNA and RNA
(hereditary materials) between the nucleus and the rest of the cell.
·
The nucleolus produces ribosomes and is known as a protein factory.
·
It also regulates the integrity of
genes and gene expression.
1.4 Cytoplasmic
inclusions
Cytoplasmic
inclusions are diverse intracellular non-living substances that are not
bound by membranes. Inclusions are stored nutrients, secretory products,
and pigment granules.
Examples of
inclusions are glycogen granules in the liver and muscle cells, lipid droplets
in fat cells, pigment granules in certain cells of skin and hair, and crystals
of various types.
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