HISTORY OF DISCOVERY OF
CELLS
• Robert Hooke was the
first to discover cell (1665).
• Leeuwenhoek was the
first to discover free living cells in pond water (1674).
• Robert Brown discovered
the nucleus (1831).
• Purkinje coined the
term ‘protoplasm (1839).
• Schleiden (1838) and
Schwann (1839) proposed the Cell Theory. Virchow (1855) made further addition
to the cell theory.
• The discovery of
electron microscope (1940) made it possible to study the structures of cell
organelles.
Cell: Cell is called the
fundamental unit of life.
A cell is capable of
independent existence and can carry out all the functions which are necessary
for a living being. A cell carries out nutrition, respiration, excretion, transportation
and reproduction; the way an individual organism does. Unicellular organisms
are capable of independent existence which shows a cell’s capability to exist
independently. Due to this, a cell is called the fundamental and structural
unit of life. All living beings are composed of the basic unit of life, i.e.
cell.
CELL THEORY (Schleiden, Schwann and Virchow):
• All living organisms
are composed of one or more cells.
• The cell is the basic
unit of structure, function, and organization in all organisms.
• All cells come from
preexisting, living cells.
STRUCTURE OF CELL
Shape and Size of Cells:- Cells come in all shapes and sizes. While most of the cells
are spherical in shape, cells of various other shapes are also found. Most of
the cells are microscopic in size, i.e. it is impossible to see them with naked
eyes. Cells can only be seen with a
microscope.
Some cells are fairly large, e.g.
- a neuron in human body can be as long as 1 meter.
- The egg of an ostrich is the largest known cell of a living animal and an average egg is 15 cm long and 13 cm wide.
A cell comprises of
- membranous casing (Plasma Membrane)
- Cytoplasm: liquid substance filled inside. Cytoplasm contains many cell organelles in a typical cell.
Some
of the main structures of a cell are as follows:
Plasma Membrane:
Definition
Plasma membrane can be
defined as a biological membrane which is composed of two layers of lipids; and
proteins. It is a thin semi permeable membrane layer, which surrounds the
cytoplasm and other constituents of the cell.
Structure:
Function
- It separates the contents of the cell from its outside environment and it regulates what enters and exits the cell.
- Plasma membrane plays a vital role in protecting the integrity of the interior of the cell by allowing only selected substances into the cell and keeping other substances out.
- The lipid bilayer is semi-permeable, which allows only selected molecules to diffuse across the membrane.
Movement Across Plasma Membrane
Water moves readily
across cell membranes through. If the total concentration of all dissolved
solutes is not equal on both sides, there will be net movement of water
molecules into or out of the cell. Whether there is net movement of water into
or out of the cell and which direction it moves depends on whether the cell’s
environment is isotonic, hypotonic, or hypertonic.
Hypertonic
The word "HYPER" means more, so hypertonicity refers to a solution that has a higher or more of a concentration to it's external environment. The cell has a higher number of particles (solutes) dissolved in it than the solution outside of the cell membrane. When a cell’s cytoplasm is bathed in a hypertonic solution the water will be drawn into the solution and out of the cell by osmosis. This causes water to move out of the cell, it shrivels up and shrinks.
Check the gif in the link for understanding
Hypotonic
The word "HYPO"
means less, in this case there are less solute molecules outside the cell.
Cells hypotonic to their surrounding solutions cause water to move into the
cell and cause it to expand. The cell has a smaller number of solutes than the
solution outside of the cell membrane. A hypotonic solution is a solution
having a lesser solute concentration than the cytosol.
When a cell’s cytoplasm
is bathed in a hypotonic solution the water will be drawn out of the solution
and into the cell by osmosis. If water molecules continue to diffuse into the
cell, it will cause the cell to swell or become tugid. The opposite of a hypertonic
environment is a hypotonic one, where the net movement of water is into the
cell.
Isotonic
A cell in an isotonic
environment is in a state
of equilibrium with its
surroundings. When the amount of solute is the same on the inside and outside
of the cell, osmotic pressure becomes equal. "ISO"
means the “same” meaning that the osmotic pressure and concentration of solutes
is the same in both the internal and external environments of the cell. Cells
isotonic to their surrounding solutions have an equal concentration of solutes
in and out of the cell membrane. This creates an equilibrium that maintains the
status of the cell. No
change will occur in the cell.
Cell wall: The cells of
plants, fungi and bacteria have a rigid non-living, and outer covering
called cell wall. It is composed mainly of cellulose. It lays outside the cell
membrane.
Functions:
1. It
provides mechanical strength to cell.
2. It helps
the cell to tolerate huge changes in surrounding medium.
Nucleus: Spherical and centrally
located in the cell. It is bound by a double layered nuclear envelope.
Contains nucleolus and
chromosomes.
Chromosomes are
made of DNA[1] and
protein. DNA molecules form the genes. Genes carry hereditary information from
one generation to next.
Because of presence of
genetic material, nucleus controls all metabolic functions of the cell. Thus
nucleus is extremely important.
In a non-dividing cell,
chromosome remains elongated and thread like. They are known as chromatin.
In a dividing cell they
condense to form rod like structures known as chromosomes.
Depending on the presence
or absence of nucleus, the cell can be prokaryotic or eukaryotic.
Prokaryotic
vs Eukaryotic Cells
.PNG)
If these differences aren't enough, check this interesting gif on Pro vs Eu
Cytoplasm: It is the fluid content of the
cell. It occurs between nucleus and cell membrane. It stores many vital
chemicals. Many important metabolic processes such as glycolysis occur in
cytoplasm.
Cytoplasm contains many specialized cell organelles. They are
as follows:
I.
Endoplasmic Reticulum: An extensive, interconnected,
membrane bound network of tubes and sheets.
Depending on presence or absence of ribosome,
ER is of two types:
· RER (Rough endoplasmic reticulum):
contains ribosome
· SER (Smooth endoplasmic reticulum: Ribosomes
absent.
Functions of ER:
1. Proteins (RER) and lipids (SER)
synthesis
2. Provides for transport of materials
within the cell.
3. In liver cells SER helps in removal
of toxic compounds.
II.
Golgi Apparatus: It is a network of stacked, flattened
membrane bound sacs, vesicles and tubules.
Discovered by Camillo Golgi.
Functions:
1.
Storage,
modifications and packaging of proteins, lipids and carbohydrates synthesized
inside the cells.
2.
Formation
of lysosomes.
III.
Mitochondria: Covered by two membranes. Contain
their own DNA and ribosomes.
Functions:
1.
Major
source of energy for the cell – hence known as power house of cell.
2.
The
three steps of energy creation: Glycolysis, Krebs Cycle and Electron Transport;
occur in mitochondria.
Energy is released in the form of ATP (adenosine triphosphate)
IV.
Plastids: Have double membrane. Contain their
own DNA and ribosomes.
Figure 1:
Types of Plastids
Functions of Plastids:
1.
Chloroplasts contain chlorophyll which is important for
photosynthesis. Chloroplasts known as ‘granaries of cell’
2.
Chromoplasts give colour to plant parts such as
flowers, fruits etc.
3.
Leucoplasts: Storage
Amyloplasts store starch
Elaioplasts store lipids
Etioplasts store proteins
V.
Lysosomes: Lysosomes
are membrane-enclosed organelles that contain an array of enzymes capable of
breaking down all types of biological molecules—proteins, nucleic acids,
carbohydrates, and lipids. Known as ‘suicidal bags’ or ‘scavengers of cell’.
Functions:
1. Since
lysosomes are little digestion machines, they go to work when the cell absorbs
or eats some food. Once the material is inside the cell, the lysosomes attach
and release their enzymes. The enzymes break down complex molecules that can
include complex sugars and proteins.
2. If
food is scarce and the cell is starving, the lysosomes digest the cell
organelles for nutrients.
3. Completely break-down cells that have died (autolysis)
VI.
Vacuole: Vacuole is a sac like structure
filled with cell sap. It is bound by a single membrane known as tonoplast.
Functions:
1.
In
plant cells, it stores solids such as amino acids, sugars etc; and some
important liquids.
2. It provides rigidity to cells.
3. In unicellular organisms, it helps in
nutrition and osmoregulation.
Check link below to view
VII.
Ribosomes: Sites of protein synthesis
VIII.
Centrosomes: Present only in animal cells. It is made of 2 centrioles.
Function: helps in cell division
Cellular Organization
Structural organization of cell helps it to perform its
unction
