Cellular level of organization: Structure and Functions of Cell.
Introduction:
Cell is defined as a basic structural and functional unit of the body.
Cytology is a branch of science that deals with the study of cells.
The animal cell is divided into two major parts as,
Plasma Membrane.
Subcellular organelles.
Nucleus.
Nucleolus
Mitochondria.
Golgi complex.
Ribosomes.
Endoplasmic reticulum.
Lysosomes.
Centrioles.
Subcellular organelles:
Cells contain many minute structures in it which perform their own functions in the cell.
The cell hence can be viewed as an industrial unit where smaller departments work together to produce goods.
The important cell organelles are as follows,
Cytoplasm
Nucleus.
Nucleolus
Mitochondria.
Golgi complex.
Ribosomes.
Endoplasmic reticulum.
Lysosomes.
Centrioles.
Cytoplasm:
It is a gel like substance which is enclosed by the plasma membrane and external to the nucleus of the cell.
The semifluid portion of the cytoplasm in which the cell organelles are suspended is called “Cytosol or Intracellular fluid.”
The cell organelles are embedded in the cytosol.
Cytosol is a clear viscous fluid containing almost 75-90% of water with substances like proteins, fats, lipids, inorganic substances either dissolved or suspended.
Nucleus:
It is the largest structure in the cell and spherical or oval in shape.
The cytoplasm in the nucleus is called “nucleoplasm”.
Chromatins are found dispersed in the nucleoplasm.
The nucleus is covered by a double membrane made up of phospholipids and called “Nuclear envelope or Nuclear membrane”.
Externally the nuclear membrane is continuous with endoplasmic reticulum.
The nuclear membrane contains pores where the external and internal nuclear membranes are fused, these pores are called “Nuclear Pores”.
The nuclear pores act as a channel between the nucleus and the cytoplasm.
Nucleus contains a dense spherical body called “Nucleolus”.
Nucleolus contains proteins, DNA and RNA.
Mitochondria:
This special cell organelle is involved in cellular respiration and generates ATP hence called “Powerhouse of the Cell”.
The number of mitochondria in the cell varies as per nature of the cell.
Shape varies, but usually ellipsoidal.
Mitochondria contains two membranes made up of lipoproteins.
Outer membrane is intact and covers the entire structure of the mitochondria.
Inner membrane is folded and called “Cristae” while a single fold is called “Crista”.
The space between two membranes is called intermembrane space.
The center cavity of the mitochondria is filled with fluid called “Mitochondrial Matrix”.
Mitochondrial matrix contains various enzymes needed for cellular respiration.
Mitochondria also contains DNA, which is received from the mother of the individual i.e. Mitochondrial DNA is maternally inherited.
Functions:
ATP synthesis.
Mitochondria are involved in apoptosis, cancer, aging, and pathologies like Parkinson disease and diabetes.
Mitochondria also plays a role in cell signalling.
Golgi complex:
Also called “Golgi Apparatus”.
It is present near the nucleus.
It consists of 4 to 6 flat sacs called “Cisterns” stacked upon each other in bent semicircular shape.
Each cistern is divided into three prominent regions called,
Cis
Intermediate.
Trans.
Cis region is receiving point from where substances enter the golgi apparatus.
Intermediate region deals with modification and storage of substances.
Trans region is a region from where substances leave the golgi apparatus.
Golgi apparatus specifically deals with modification and storage of chemicals synthesized in the cells.
The substances formed or modified in the apparatus are enclosed in vesicles which are then used or released from the cells.
The cells having secretary functions show well developed golgi apparatus.
Ribosomes:
These are the sites of protein synthesis in the cell.
They are made up of two subunits
Smaller subunit (40S):
Lager Subunit (60S):
Ribosomes receive a message of protein synthesis in the form of mRNA from the nucleus and accordingly they join amino acids to form the proteins.
They are of two types,
Membrane bound: Bound to endoplasmic reticulum.
Free: Found free in cytoplasm.
Endoplasmic reticulum:
It is the transport system of the cell.
It is made up of membrane bound channels called “Cisterns” of different shapes.
It is an interconnected network of internal membranes.
It is of two types,
Rough Endoplasmic Reticulum. (RER)
Smooth Endoplasmic Reticulum. (SER)
Rough Endoplasmic Reticulum:
Ribosomes are attached to the external surface and hence they appear granular.
These are responsible for synthesis of secretory proteins and membranes.
Smooth Endoplasmic Reticulum:
Ribosomes are not attached to the external surface and hence appear smooth.
They are involved in fatty acids, phospholipids and sterol synthesis.
These are also involved in inactivation of certain chemicals like alcohol, carcinogens, pesticides etc.
Lysosomes:
They are vesicles containing many hydrolytic enzymes.
They are formed by the Golgi Complex.
They contain about 60 different hydrolytic enzymes.
As the hydrolytic enzymes are capable of digesting all cellular components when released the Lysosomes are also called Suicidal Bags.
Centrioles:
A centriole is a cylindrical organelle composed mainly of a protein called tubulin.
Centrioles are found in most eukaryotic cells.
A bound pair of centrioles, surrounded by a highly ordered mass of dense material, called the pericentriolar material makes up a structure called a centrosome.
They play a major role in cell division.
Peroxisomes:
These are similar to lysosomes but are much smaller in size.
These are smaller vesicles containing many oxidase enzymes capable of causing oxidation of many substances like amino acids, fatty acids, uric acid etc.
The oxidation of fatty acids is a major source for metabolic energy, it produces Hydrogen Peroxide as a byproduct which is harmful to the cell.
Peroxisomes contain one enzyme called “Catalase” that causes deactivation of the formed hydrogen peroxide hence protecting the cell.
Functions of Cell:
Movement of substances across the cell membrane,
cell division to make new cells, and
protein synthesis.
Phagocytosis for protection.
Secretion of enzymes / hormones.
Commonly Asked Questions
What is a cell? Explain using a well labelled diagram.
Define Cell, Enlist different cell organelles.
Draw a well labelled diagram of Mitochondria and give its functions.
Why Lysosomes are called as suicidal bags of the cell.
Why is mitochondria called a Powerhouse of the cell.
Draw a well labelled diagram of Golgi complex and give its functions.