The Unit Of Life Cell : What Is a Cell? | Learn Science


Unicellular organisms are capable of
(i) independent existence and
(ii) performing the essential functions of life. Anything less than a complete
structure of a cell does not ensure independent living. Hence, the cell is the
fundamental structural and functional unit of all living organisms.
Anton Von Leeuwenhoek first saw and described a live cell. Robert
Brown later discovered the nucleus. The invention of the microscope and
its improvement leading to the electron microscope revealed all the
structural details of the cell.
                                    In 1838, Matthias Schleiden, a German botanist, examined a large number of plants and observed that all plants are composed of different kinds of cells which form the tissues of the plant. At about the same time, Theodore Schwann (1839), a British Zoologist, studied different types of animal cells

and reported that cells had a thin outer layer which is today known as the
‘plasma membrane’. He also concluded, based on his studies on plant
tissues, that the presence of cell walls is a unique character of the plant
cells. On the basis of this, Schwann proposed the hypothesis that the bodies
of animals and plants are composed of cells and products of cells.
Schleiden and Schwann together formulated the cell theory.

This theory, however, did not explain how new cells were formed. Rudolf Virchow (1855) first explained that cells divided and new cells are formed from
pre-existing cells (Omnis cellula-e cellula). He modified the hypothesis of
Schleiden and Schwann give the cell theory a final shape. Cell theory
as understood today is:
(i) all living organisms are composed of cells and products of cells.
(ii) all cells arise from pre-existing cells.

                                                You have earlier observed cells in an onion peel and/or human cheek
cells under the microscope. Let us recollect their structure. The onion cell which is a typical plant cell has a distinct cell wall as its outer boundary, and just within it is the cell membrane. The cells of the human cheek
have an outer membrane as the delimiting structure of the cell. Inside each cell is a dense membrane-bound structure called the nucleus.

This nucleus contains the chromosomes which in turn contain the genetic material, DNA. Cells that have membrane-bound nuclei are called eukaryotic whereas cells that lack a membrane-bound nucleus are prokaryotic. In both prokaryotic and eukaryotic cells, a semi-fluid matrix
called cytoplasm occupies the volume of the cell. The cytoplasm is the main arena of cellular activities in both plant and animal cells.

Various chemical reactions occur in it to keep the cell in its ‘living state’. Besides the nucleus, the eukaryotic cells have other membrane-bound distinct structures called organelles like the endoplasmic reticulum (ER),
the Golgi complex, lysosomes, mitochondria, microbodies, and vacuoles. The prokaryotic cells lack such membrane-bound organelles. Ribosomes are non-membrane bound organelles found in all cells –
both eukaryotic as well as prokaryotic.

Within the cell, ribosomes are found not only in the cytoplasm but also within the two organelles –
chloroplasts (in plants) and mitochondria and on rough ER. Animal cells contain another non-membrane bound organelle called centrosome which helps in cell division.

Cells differ greatly in size, shape, and activities. For example, Mycoplasmas, the smallest cells, are only 0.3 μm in length while bacteria could be 3 to 5 μm. The largest isolated single cell is the egg of an ostrich.
Among multicellular organisms, human red blood cells are about 7.0 μm in diameter. Nerve cells are some of the longest cells. Cells also vary greatly in their shape. They may be disc-like, polygonal, columnar, cuboid,
thread-like, or even irregular. The shape of the cell may vary with the
function they perform.

                                              The prokaryotic cells are represented by bacteria, blue-green algae,

mycoplasma, and PPLO (Pleuro Pneumonia Like Organisms). They are
generally smaller and multiply more rapidly than eukaryotic cells. They may vary greatly in shape and size. The four basic
shapes of bacteria are bacillus (rod-like), coccus (spherical), vibrio (comma-shaped), and spirillum (spiral).
The organization of the prokaryotic cell is fundamentally similar even
though prokaryotes exhibit a wide variety of shapes and functions. All
prokaryotes have a cell wall surrounding the
cell membrane except in mycoplasma. The fluid matrix filling the cell is the cytoplasm. There is
no well-defined nucleus. The genetic material is basically naked, not enveloped by a nuclear
membrane. In addition to the genomic DNA (the single chromosome/circular DNA), many
bacteria have small circular DNA outside the genomic DNA. This smaller DNA are called
plasmids. The plasmid DNA confers certain unique phenotypic characters to such bacteria.
One such characteristic is resistance to antibiotics.

In higher classes, you will learn that this plasmid DNA is used to monitor bacterial transformation with foreign DNA. The nuclear membrane is found
in eukaryotes. No organelles, like the ones in eukaryotes, are found in prokaryotic cells except for ribosomes. Prokaryotes have something
unique in the form of inclusions.

A specialized differentiated form of the cell membrane called mesosome is the characteristic
of prokaryotes. They are essentially infoldings of cell membrane

The eukaryotes include all protists, plants, animals, and fungi. In eukaryotic cells, there is extensive compartmentalization of cytoplasm through the presence of membrane-bound organelles.

Eukaryotic cells possess an organized nucleus with a nuclear envelope. In addition,
eukaryotic cells have a variety of complex locomotory and cytoskeletal structures. Their genetic material is organized into chromosomes.

All eukaryotic cells are not identical. Plant and animal cells are different as the former possess cell walls, plastids, and a large central vacuole which are absent in animal cells. On the other hand, animal cells have centrioles which are absent in almost all plant cells Let us now look at individual cell organelles to understand their structure and functions

Cell Membrane
                              The detailed structure of the membrane was studied only after the advent

of the electron microscope in the 1950s. Meanwhile, chemical studies on the cell membrane, especially in human red blood cells (RBCs), enabled scientists to deduce the possible structure of the plasma membrane.

These studies showed that the cell membrane is mainly composed of lipids and proteins. The major lipids are phospholipids that are arranged in a bilayer. Also, the lipids are arranged within the membrane with the polar head towards the outer sides and the hydrophobic tails towards the inner part. This ensures that the nonpolar tail of saturated hydrocarbons is protected from the aqueous environment In addition to phospholipids membrane also contains cholesterol.

The lipid component of the membrane mainly consists of phosphoglycerides. Later, biochemical investigation clearly revealed that the cell membranes also possess protein and carbohydrates. The ratio of protein and lipid varies considerably in different cell types. In human beings, the membrane of the erythrocyte has approximately 52 percent protein and 40 percent lipids.

Depending on the ease of extraction, membrane proteins can be classified as integral and peripheral. Peripheral proteins lie on the surface of the membrane while the integral proteins are partially or totally buried in the membrane.

An improved model of the structure of cell membrane was proposed by Singer and Nicolson (1972) widely accepted as the fluid mosaic model  According to this, the quasi-fluid nature of lipids enables lateral movement of proteins within the overall bilayer. This ability to move
within the membrane is measured as its fluidity.

The fluid nature of the membrane is also important from the point of view of functions like cell growth, formation of intercellular junctions, secretion, endocytosis, cell division, etc.
One of the most important functions of the plasma membrane is the transport of molecules across it. The membrane is selectively permeable to some molecules present on either side of it.

Many molecules can move briefly across the membrane without any requirement of energy and this is called passive transport. Neutral solutes may move across the membrane by the process of simple diffusion along the concentration gradient, i.e., from higher concentration to lower.

Water may also move across this membrane from higher to lower concentrations. The movement of water by diffusion is called osmosis. As the polar molecules cannot pass through the nonpolar lipid bilayer, they require a carrier protein of the membrane to facilitate their transport across the membrane.

A few ions or molecules are transported across the membrane against their concentration gradient, i.e., from lower to higher concentration. Such transport is an energy-dependent process, in which ATP is utilized and is called active transport, e.g., Na+/K+ Pump.

Cell Wall
                  As you may recall, a non-living rigid structure called the cell wall forms an outer covering for the plasma membrane of fungi and plants. The cell wall not only gives shape to the cell and protects the cell from mechanical damage and infection, but it also helps in cell-to-cell interaction and provides a barrier to undesirable macromolecules. Algae have a cell wall, made of cellulose,

Galatians, mannans, and minerals like calcium carbonate, while in other plants it consists of cellulose, hemicellulose, pectins, and proteins. In the cell wall of a young plant cell, the primary wall is capable of growth, which gradually diminishes as the cell matures and the secondary wall is formed on the inner (towards membrane) side of the cell.

The middle lamella is a layer mainly of calcium pectate which holds or glues the different neighboring cells together. The cell wall and middle lamellae may be traversed by plasmodesmata which connect the cytoplasm of neighboring cells.

                The nucleus as a cell organelle was first described by Robert Brown as early as 1831. Later the material of the nucleus stained by the basic dyes was given the name chromatin by Flemming.

The interphase nucleus (nucleus of a cell when it is not dividing) has highly
extended and elaborate nucleoprotein fibers called chromatin, a nuclear matrix
and one or more spherical bodies called nucleoli (sing.: nucleolus) (Figure 8.11).
Electron microscopy has revealed that the nuclear envelope, which consists of two
parallel membranes with a space between (10 to 50 nm) called the perinuclear
space, forms a barrier between the materials present inside the nucleus and
that of the cytoplasm.

The outer membrane usually remains continuous
with the endoplasmic reticulum and also bears ribosomes on it. In a number of
places, the nuclear envelope is interrupted by minute pores, which are formed by the fusion of its two membranes. These nuclear pores are the passages through which the movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm. Normally,

there is only one nucleus per cell, variations in the number of nuclei are also frequently observed. Can you recollect the names of organisms that have more than one nucleus per cell? Some mature cells even lack nuclei, e.g., erythrocytes of many mammals and sieve tube cells of vascular plants. Would you consider these cells as ‘living’? The nuclear matrix or the nucleoplasm contains nucleolus and chromatin.

The nucleoli are spherical structures present in the nucleoplasm. The content of the nucleolus is continuous with the rest of the nucleoplasm as it is not a membrane-bound structure. It is a site for active ribosomal RNA synthesis. Larger and more numerous nucleoli are present in cells actively carrying out protein synthesis.

You may recall that the interphase nucleus has a loose and indistinct network of nucleoprotein fibers called chromatin. But during different stages of cell division, cells
show structured chromosomes in place of the nucleus.

Chromatin contains DNA and some basic proteins called histones, some non-histone proteins, and also RNA. A single human cell has approximately two meter-long thread of DNA distributed among its forty-six (twenty-three pairs) chromosomes. You will study the details of DNA packaging in the form of a chromosome in class XII. Every chromosome (visible only in dividing cells) essentially has a primary constriction or the centromere
on the sides of which disc-shaped structures called kinetochores are present (Figure 8.12). Centromere holds two chromatids of a chromosome. Based on the position
of the centromere,

the chromosomes can be classified into four types. The metacentric chromosome has a middle centromere forming two equal arms of the chromosome. The sub-metacentric chromosome has a centromere slightly away from the middle of the chromosome resulting in one shorter arm and one long arm.

In the case of the acrocentric chromosome, the centromere is situated close to its end forming one
extremely short and one very long arm, whereas the telocentric chromosome has a terminal centromere

                             Mitochondria (sing.: mitochondrion), unless specifically stained, are not

easily visible under the microscope. The number of mitochondria per cell is variable depending on the physiological activity of the cells. In terms of shape and size also, a considerable degree of variability is observed.

Typically it is sausage-shaped or cylindrical having a diameter of 0.2-1.0μm (average
0.5μm) and length 1.0-4.1μm. Each mitochondrion is a double membrane-bound structure with the outer membrane and the inner membrane dividing its lumen distinctly into two aqueous compartments,

i.e., the outer compartment and the inner compartment. The inner compartment is filled with a dense homogeneous substance called the matrix. The outer membrane forms the continuous limiting boundary of the organelle. The inner membrane forms a number of infoldings called the cristae (sing.: crista) towards the matrix. The cristae increase the surface area. The two membranes have their own specific enzymes associated with mitochondrial function.

Mitochondria are the sites of aerobic respiration. They produce cellular energy in the form of ATP, hence they are called ‘power houses’ of the cell. The matrix also possesses a single circular DNA molecule, a few RNA molecules, ribosomes (the 70S), and the components required for the synthesis of proteins. The mitochondria divide by fission.

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