Cell membrane or Plasma membrane:-
Membrane
is the covering of the animal cell. In plant cell there is another layer over
the cell membrane known as the cell wall.
Cell
membrane is flexible and is made up of mainly lipids and proteins. The cell
membrane is the outermost layer of the cell that separates cytoplasm from the
external environment. This is also known as the plasma membrane. The cell
membrane defines the shape and size of the cell, encloses the cytoplasm and protects
it from the external environment.
The
cell wall is a tough but flexible porous layer leads a definite shape and to
the cell protection.
Robert
Brown in the year 1831 Brown had no idea about its function. This is one of the
most important organelles of the cell. This is also known as the cell’s control
room. The nucleus is the largest and most distinct of all cell organelles.
Schleiden, who was one of the proponents of cell theory, thought that new cells
were created from the nucleus and he called it as cytoblast.
Barring a few exceptions, almost all
eukaryotic cells have a nucleus. Red blood cells in some mammals and phloem
sieve tube in plants are examples of cells that do not have a nucleus. Even
these cells do have. nuclei in the beginning, but it is later thrown out of the
cells and destroyed.
The nucleus regulates and controls
all the functions of a cell and determines the characteristics of the organism.
It consists of all generic information. The nucleus is also closely involved in
the process of cell division.
Nucleus:-
The
membranes that encloses the nucleus and separates it from contents of nucleus
and separate it from contents of cytoplasm is known as the nuclear membrane.
This is very similar to the cell membrane. Almost the entire genetic material
of the cells is found in the nucleus.
On
the basis of the presence or absence Of the organised nucleus, cells are
categorized into two types, i.e. Prokaryotic cell (without organised nucleus)
and Eukaryotic cell (with organised nucleus).
Cytoplasm:-
The
cytoplasm is the fluid content bounded by the plasma membrane.
Cell organelles:-
Now
let us discuss some important cell organelles.
1.
Mitochondria, 2. Plastids,
3.
Ribosomes, 4. Endoplasmic reticulum, 5.Golgi apparatus, 6.Lysosomes,
7.
Vacuoles.
They
are important because they carry out very crucial functions in the cells.
Endoplasmic reticulum (ER):-
Lysosome:-
Lysosome
were discovered as tiny partic1es in the cytoplasm. It was found that they
contained the destructive enzymes. These enzymes normally do not come in
contact with the rest of the cell. The materials that need to be destroyed are
transported to the lysosomes. Then the lysosomes get them digested at times,
the lysosomes burst and the enzymes are released to digest the cell. Hence,
lysosomes are also known as the suicidal bags of the cell.
Mitochondria:-
There
are about 100-150 mitochondria in each cell. When seen under the compound
microscope, the mitochondria appear as oval or cylindrical dots in the cell.
Mitochondria are made up of a double-membrane wall. The inner membrane of the
wall protrudes into the interior in folds and forms structures called cristae;
the space between cristae known as the matrix.
Mitochondria
are responsible for cellular respiration, a process through which the cell
derives its energy to do work. Because of this, mitochondria are also known as
the powerhouse of the cell.
Ribosomes:-
There
are small granule like structures in the cytoplasm of the cell. They are called
ribosomes. They are two types. Free ribosomes are scattered in cytoplasm.
Attached ribosomes are on the surface of rough endoplasmic reticulum. Ribosomes
are the sites for protein synthesis.
Chloroplast:-
Chloroplast
is a type of plastids in Green colour. Plastids are present only in plant
cells. Plastids are mainly of two types: chromoplasts (colored) and leucoplasts
(colorless).
Chloroplasts
are of different shapes disc, oval etc. In algae, these can be found as
ladders, stars, spirals or reticulate. The diameter of chloroplasts in higher
plants can vary between 4 to 10 micron. The primary function of chloroplasts is
to trap the energy from sunlight and transform it to chemical energy, thus
helping to carry out photosynthesis.
The
large empty spaces present in the cell are vacuoles. These are fluid-filled
saclike structures. In animal cells vacuoles are small in size while in plant
cells they are Large. In mature plant cells they might occupy almost the entire
cell space. Vacuoles maintain turgur pressure within the cell. They export
unwanted substances from the cell.
Scihleiden
and Schwann together formu1ated the cell theory. Rudolf Carl Virchow (1855) first explained
that the new cells could be formed only by the division of the pre-existing
cells.
(i)
All living organisms are composed of cells and products of cells.
(ii)All
cells arise from pre-existing cells.
Meristematic
tissues at the growing tip that bring about growth in length are called as
apical meristematic tissues.
Tissues
present around the edges in a lateral manner and giving rise to growth in the
girth of stem are called lateral meristematic tissues.
Areas
from which branching takes place Or a leaf or a flower stalk grows, we find a
kind of meristematic tissue called intercalary meristematic (also called
Cambium) tissue.
Dermal tissue:-
Dermal
tissue (Dermis) usually consists of a single layer of tissues showing variations,
in the types of cells on the basis of their functions and location. These tissues
are divided into three different types. They are epidermis (outer most layer),
mesodermis (The middle layer) and endodermis (the innermost layer).
The
walls of the cells of dermal tissue are comparatively thicker than the cells of
meristematic tissues. In desert plants it may be even more thick and waxy.
Small pores are seen in the epidermis of the leaf, called stomata. They are
enclosed by two kidney shaped cells, called guard cells. Cells of the roots
have long-hair like parts, called root hairs.
Stomata
and root hair are also dermal tissues that are essential for gaseous exchange,
transpiration as well as absorption of water and minerals Photosynthesis is
also carried out by certain cells of this tissue.
Ground Tissue:-
The
cells of the ground tissue. It forms the bulk of the plant body. The ground
tissue is useful for storing food and providing physical support to the plant
body. There are mainly three types of ground tissues. They are parenchyma
collenchyma and sclerenchyma.
Vascular Tissue:-
We
know that roots can absorb water from the soil and send it to other parts of
the plant. The leaves and other green parts prepare food and supply it to all
the parts of the plant. Let us study the tissues involved in transportation. We
have done an experiment on transportation in class VII, in the chapter on plant
nutrition. We have observed that when a plant is kept in red colored water.
Some of the pans of the plant turned red.
Do
the same experiment again by keeping a small plant (with roots) in red colored
water. Leave it for two hours. Now cut a T.S. of the stem and observe it under
the microscope.
•
Which portion of the plant is responsible for this transport?
•
Draw a rough sketch of that portion and mark the portion that appears red.
Xylem
is responsible for the transportation of materials like water and salts from
the root. And phloem helps in the transportation of the food material prepared
by photosynthesis to the other parts of the plants. Hence they are known as
conducting or vascular tissues. Xylem and Phloem together form the vascular
bundles.
Difference between Prokaryotic &
Eukaryotic cell:-
