Question

Discuss the elements of cellular structures that make the cell viable and able to withstand changes. Add reference

Answer 1

The cell is the smallest structural and functional unit of the
body. The human body contains about 100 trillion cells.
Different types of cells of the body possess features which
distinguish one type from the other and are specially adapted to perform particular functions, e.g. the red blood cells transport oxygen from lungs to the tissues, muscle cell is specialised for the function of contraction.
A typical cell, as seen by the light microscope, consists of
three basic components:
1.Cell membrane,
2.Cytoplasm and
3.Nucleus.
1.CELL MEMBRANE
Cell membrane or the plasma membrane is the protective
sheath, enveloping the cell body. It separates the contents of
cell from the external environment and controls exchange
of materials between the fluid outside the cell (extracellular
fluid) and the fluid inside the cell (intracellular fluid). A
detailed knowledge of its structure is essential
for the understanding of cell functions. Therefore, it will be
discussed separately.
2.CYTOPLASM
Cytoplasm is an aqueous substance (cytosol) containing a
variety of cell organelles and other structures. The structures
dispersed in the cytoplasm can be broadly divided into
three groups: organelles, inclusion bodies and cytoskeleton.
A. ORGANELLES
The organelles are the permanent components of the cells
which are bounded by limiting membrane and contain
enzymes hence participate in the cellular metabolic activity.
These include:
1. Mitochondria
Mitochondria are the major sites for aerobic respiration.
These are oval structures and more numerous in metaboli-
cally active cells.Functions. In addition to their role as power generating units, the mitochondria may have a role in synthesizing membrane bound proteins since they also possess deoxyribonucleic acid (DNA) and ribosomes.   
2. Endoplasmic reticulum
Endoplasmic reticulum (ER) is a system of flattened
membrane-bound vesicles and tubules called cisternae.It is continuous with the outer membrane of the nuclear envelop, Golgi apparatus and possibly with the cell membrane. Morphologically, two types of endoplasmic
reticulum can be identified: rough or granular and smooth
or agranular.
(i) Rough endoplasmic reticulum.
(ii) Smooth endoplasmic reticulum.
Functions. Its main functions are:
-Synthesis of carbohydrates and complex proteins.
-Packaging of proteins synthesized in the rough ER into
vesicles.
-Site of formation of lysosomal enzymes.
-Transport of the material to the other parts of cell or to
the cell surface membrane and secretion.
- Glycosylation of proteins to form glycoproteins.
4. Ribosomes
Ribosomes are spherical particles which contain 80–85%
of the cell’s ribonucleic acid (RNA). They may be present in
the cytosol as free (unattached) or in bound form (attached
to the membrane of endoplasmic reticulum). Slightly
smaller form of ribosomes is also found in mitochondria.
Functions. They are the site of protein synthesis. They
synthesize  all transmembrane proteins, secreted proteins and most proteins that are stored in the Golgi apparatus, lysosomes and endosomes.
5. Lysosomes
Lysosomes are rounded to oval membrane bound organ-
elles containing powerful lysosomal digestive (hydrolytic)
enzymes. They are formed by the Golgi apparatus. As many as 40 different lysosomal enzymes have been synthesized. Lysosomes are particularly abundant in cells involved in phagocytic activity, e.g. neutrophils and macrophages.
There are three forms of lysosomes:
Primary lysosomes or storage vacuoles are formed from the various hydrolytic enzymes synthesized by rough ER and packaged in the Golgi apparatus.
Secondary lysosomes or autophagic vacuoles are formed by fusion of primary lysosomes with parts of damaged or worn out cell components.
Residual bodies are undigestible materials in the lysosomes.
6. Peroxisomes
Peroxisomes, also known as microbodies, are spherical
structures enclosed by a single layer of unit membrane.
These are predominantly present in hepatocytes and tubu-
lar epithelial cells.
Functions. They essentially contain two types of enzymes:
Oxidases which are active in oxidation of lipid and
Catalases which act on hydrogen peroxide to liberate
oxygen.
7. Centrosome
The centrosome consists of two short cylindrical structures
called centrioles.It is situated near the centre of the cell close to the nucleus. The centrioles are responsible for movement of chromosomes during cell division.
2. CYTOPLASMIC INCLUSIONS
The cytoplasmic inclusions are the temporary components
of certain cells. These may or may not be enclosed in the
membrane. A few examples of cytoplasmic inclusions are:
Lipid droplets. These are seen in the cells of adipose
tissue, liver and adrenal cortex.
Glycogen. It is seen in the cells of liver and skeletal
muscles.
Proteins as secretory granules are seen in the secretory
glandular cells .
Melanin pigment is seen in the cells of epidermis, retina
and basal ganglia.
Lipofuscin. It is a yellow brown pigment believed to be
derived from secondary lysosomes and is seen in the
cardiac muscle and brain cells of elderly people.
C. CYTOSKELETON
The cytoskeleton is a complex network of fibres that main-
tains the structure of the cell and allows it to change shape
and move. It primarily consists of.
microtubules.
intermediate filaments.
microfilaments.
. NUCLEUS
Nucleus is present in all the eukaryotic cells. It controls all
the cellular activities including reproduction of the cell.
Most of the cells are uninucleated except few types of cells like skeletal muscle cells which are multinucleated. The nucleus consists of nuclear membrane, nucleoplasm and nucleolus.