Living organisms made up of Cell
Living things are made up of the most fundamental unit called cell. this is
the smallest unit that a living thing is made up of. When we peel an onion
and put it under a microscope we can draw a structure what we see through
the microscope, those are the cells of the onion.
Cell
All the cell in a particular area looks similar in size and structure.
These small structures are the basic building unit of an organism. The
shape and size of a particular cell is related to the specific function
they perform. Some cells like Amoeba change shape for movement. Each cell
in an organism have the capacity to perform certain basic functions. A
particular cell has different components known as cell organelles. Each
kind of cell organelle perform a specific function and due to this cell
organelles a cell can live independently by performing all the basic
functions.
Unicellular organism
A single cell may constitute a whole organism like amoeba, Chlamydomonas,
bacteria, etc. such organisms made up of only one cell are known as
unicellular organism.
Multicellular organism
When many cell groups together in a single body and assume different
function in it to from various body parts then the organism is called as a
multicellular organism. Fungi, plants and animals are some examples of
multicellular organism. Every multicellular organism has evolved from a
single cell. A single cell divides to produce cells of their own kind.
Structure of cell
Each cell has different organelles like the plasma membrane, nucleus and
the cytoplasm. Due to these contents of a cell all activities inside the
cell and interactions with environment is possible.
Cell membrane
Cell membrane is often called as a plasma membrane. This is the outermost
covering of the cell that separates the cell from its external environment.
The cell membrane performs the function of allowing things and the movement
of some other material inside the cell or outside the cell.
Some substances like carbon dioxide and oxygen can move across the cell
membrane by a process called diffusion. Water also obey the law of
diffusion. Cell membrane is a selectively permeable membrane
which allows diffusion occurring by the process of osmosis.
If we drop a particular plant or animal cell into a solution of sugar
in water then the following three things could happen:
If the solution outside is very dilute then the cell will gain water by
osmosis, such a solution is known as hypotonic solution.
If solution outside is as concentrated as the content inside the cell then
there is no net movement of water across the cell membrane and the solution
is known as isotonic solution.
If the solution outside is very concentrated then the cell will lose water
by osmosis add the solution is known as a hypertonic solution.
Unicellular fresh water organisms tends to gain water through osmosis.
absorption of water by plant root is an example of osmosis.
Endocytosis
Plasma membrane is also very flexible consisting of organic molecules like
lipids and proteins. The flexibility of a cell membrane is used to engulf
in food and other materials from the external environment and such process
is known as endocytosis used by a unicellular organism amoeba.
Cell wall
In addition to the plasma membrane another rigid outer covering of the cell
is called as the cell wall. The cell wall lies outside the plasma membrane.
Cell wall in plants is mainly composed of cellulose, cellulose is a complex
substance and provide structural strength to plants. When solution is
dilute outside a cell it survives without bursting because cell wall exerts
equal pressure against the swollen cell.
Nucleus
The nucleus has a double layered covering called nuclear membrane that has
spores which allows transfer of material from inside the nucleus to its
outside that is to the cytoplasm. The nucleus contains chromosomes which
contains the information for Inheritance of features from parents to the
next generation in the form of DNA molecules (Deoxyribonucleic Acid).
Nucleus plays a central role in cellular reproduction, the process by which
a single cell divides and form into two new cells. In some organisms like
bacteria has a defined region containing nucleic acids and is called as
nucleoid.
Prokaryotes and eukaryotes
Organisms which lack nucleus membranous are called prokaryotes. Prokaryotes
also lack most of other cytoplasmic organelles.
Organisms having nuclear membrane and other cell organelles are called
eukaryotes.
Cytoplasm
The cytoplasm is the fluid contained inside the plasma membrane, it also
contains very specialized cell organelles and each of the organelles
perform a specific function for the cell.
Cell organelles
Large and complex cell including cells of multicellular organism needs a
lot of chemical activity to support their complicated structure and
function, to support this function there are separate structures bounded by
a membrane. Some important cell organelles are endoplasmic reticulum, Golgi
apparatus, lysosome, mitochondria, plastids and vacuoles.
Endoplasmic reticulum
It is a large network of membrane bound tubes and sheets looks like tubules
or round or oblong bags. Endoplasmic reticulum serves as channel of
transport of material between various regions of the cytoplasm or between
the cytoplasm and the nucleus. there are two types of Endoplasmic
reticulum:
The rough endoplasmic reticulum (RER). It looks rough under microscope
because it has a particular organelle called ribosome attached to its
surface.
Smooth endoplasmic reticulum (SER) helps in the manufacturing of fat
molecules and lipids important for cell function. Some of this protein and
lipids help building the cell membrane and this process is known as
biogenesis.
Ribosomes
Ribosomes in an active cell are sites of protein manufacturing and then it
is used in various places in the cell according to the need.
Golgi apparatus
Consists of a system of membrane bounded vesicles arranged approximately
parallel to each other in stacks called cisterns. the different function of
the Golgi apparatus includes storage, modification and Packaging of
products in vesicles preferably the materials synthesized near the
endoplasmic reticulum. it also helps in the formation of lysosomes in a
cell.
Lysosomes
A kind of waste disposal system in the cell helps keeping the cell clean by
digesting any foreign material as well as worn out cell organelles.
lysosomes are able to do this because they contain a powerful digestive
enzyme capable of breaking down all organic materials. Lysosomes are also
known as suicide bag because they may burst and spill the enzyme into the
cytoplasm which digest their own cell.
Mitochondria
It is known as the powerhouse of the cell. The energy required for various
chemical activity needed for life is released by mitochondria in the form
of adenosine triphosphate (ATP) molecules. Mitochondria has two membrane
covering outer one is very porous and the inner one is deeply folded.
Mitochondria are strange organelles as they have their own DNA and
ribosomes. It is also capable of making some of their own proteins.
Plastids
Plastids are only present in plant cells. Two types of plastids are there
namely chromoplast and leucoplast. Chromoplast are coloured plastids and
leucoplast are white or colourless plastids. Plastids contain a pigment
chlorophyll known as chloroplast. Chloroplast is important for the
photosynthesis in a plant.
The internal organisation of plastids consists of numerous membrane layer
embedded in a material called the stroma. Like mitochondria, plastid also
have their own DNA and ribosomes.
Vacuoles
Vacuoles are store house for solid or liquid content in a cell. They are
very small in animals while plants have very large vacuoles. The central
vacuole of some plant up to 90% of the cell volume.
Plant cell vacuoles are full of cell sap and provides turgidity and
rigidity to the cell. Vacuoles store amino acids, sugar, various organic
acid and some proteins. In some unicellular organism like amoeba, vacuoles
play an important role in expelling excess water and wastes from the cell.