SCIENCE

CELLS

WHAT IS CELL?Cell is

structural ,Functional and

basic 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.

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. 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 is enclosed in a membranous casing and is filled with a liquid substance which is called the cytoplasm. There are many cell organelles in a typical cell. Some of the main structures of a cell are as follows:

STRUCTURE OF CELL

 Cell wall is made of cellulose. It is somewhat hard but permeable to most of the substances. Cell wall is available in plant cells and in cells of bacteria and fungi.

CELL WALL

 Plasma membrane is a semi-permeable membrane. It is composed of bilayer of lipid and protein.

Plasma membrane provides a container to the cytoplasm. It facilitates passage of various substances in and out of the cell.

PLASMA MEMBRANE

FUNCTIONS OF PLASMA MEMBRANE

 Nucleus is covered by double membrane; called nuclear membrane. The fluid which is inside the nucleus is called nucleoplasm. Nucleus contains chromosomes which are important for the functioning of a cell. Chromosomes contain genes which are the carriers of genetic information. Nucleus plays an important role during cell division. Nucleus controls all the functions of the cell.

NUCLEUS

WHO DISCOVER CELL?

ROBERT HOOKE

The cell was first discovered by Robert Hooke in 1665, which can be found to be described in his book Micrographia. In this book, he gave 60 ‘observations’ in detail of various objects under a coarse, compound microscope.One observation was from very thin slices of bottle cork. Hooke discovered a multitude of tiny pores that he named "cells". This came from the Latin word Cella, meaning ‘a small room’ like monks lived in and also Cellulae, which meant the six sided cell of a honeycomb. However, Hooke did not know their real structure or function. What Hooke had thought were cells, were actually empty cell walls of plant tissues.

DISCOVERY OF CELL

Drawing of the structure of cork byRobert Hooke that appeared in Micrographia

With microscopes during this time having a low magnification, Hooke was unable to see that there were other internal components to the cells he was observing. Therefore, he did not think the "cellulae" were alive.[5] His cell observations gave no indication of the nucleus and other organelles found in most living cells. In Micrographia, Hooke also observed mould, bluish in color, found on leather. After studying it under his microscope, he was unable to observe “seeds” that would have indicated how the mould was multiplying in quantity. This led to Hooke suggesting that spontaneous generation, from either natural or artificial heat, was the cause. Since this was an old Aristotelian theory still accepted at the time, others did not reject it and was not disproved until Leeuwenhoek later discovers generation is achieved otherwise.

Anton van Leeuwenhoek is another scientist who saw these cells soon after Hooke did. He made use of a microscope containing improved lenses that could magnify objects almost 300-fold, or 270x.[5] Under these microscopes, Leeuwenhoek found motile objects. In a letter toThe Royal Society on October 9, 1676, he states that motility is a quality of life therefore these were living organisms. Over time, he wrote many more papers in which described many specific forms of microorganisms. Leeuwenhoek named these “animalcules,” which includedprotozoa and other unicellular organisms, like bacteria.[3] Though he had did not have much formal education, was able to identify the first accurate description of red blood cells and discovered bacteria after gaining interest in the sense of taste that resulted in Leeuwenhoek to observe the tongue of an ox, then leading him to study "pepper water" in 1676. He also found for the first time the sperm cells of animals and humans. Once discovering these types of cells, Leeuwenhoek saw that the fertilization process requires the sperm cell to enter theegg cell. This put an end to the previous theory of spontaneous generation. After reading letters by Leeuwenhoek, Hooke was the first to confirm his observations that were thought to be unlikely by other contemporaries.

The cells in animal tissues were observed after plants were because the tissues were so fragile and susceptible to tearing, it was difficult for such thin slices to be prepared for studying. Biologists believed that there was a fundamental unit to life, but were unsure what this was. It would not be until over a hundred years later that this fundamental unit was connected to cellular structure and existence of cells in animals or plants. This conclusion was not made until Henri Dutrochet. Besides stating “the cell is the fundamental element of organization”,  Dutrochet also claimed that cells were not just a structural unit, but also a physiological unit.

In 1804, Karl Rudolphi and J.H.F. Link were awarded the prize for "solving the problem of the nature of cells", meaning they were the first to prove that cells had independent cell walls by the Königliche Societät der Wissenschaft (Royal Society of Science), Göttingen. Before, it had been thought that cells shared walls and the fluid passed between them this way.

 Based on the level of organization of nuclear material, a cell can be categorized as prokaryote or eukaryote. In some organisms, the true nucleus is absent. This means that the DNA and RNA are not bound by a membrane. Bacteria are examples of prokaryotes. When genetic materials are bound by a membrane, it is termed as true nucleus. In this case, the cell is called eukaryotic. Organisms other than bacteria are eukaryotes. Eukaryotes are considered as more advanced than prokaryotes.

PROKARYOTES AND EUKARYOTES

Prokaryote cell. Eukaryote cell.

WHO PROPOUSED CELL THEORY?

SCHWANN AND SCHLEIDEN

Matthias Jakob Schleiden (1804–1881)

Theodor Schwann (1810–1882)

Credit for developing cell theory is usually given to two scientists: Theodor Schwann and Matthias Jakob Schleiden. While Rudolf Virchowcontributed to the theory, he is not as credited for his attributions toward it. In 1838, Schleiden suggested that every structural part of a plant was made up of cells or the result of cells. He also suggested that cells were made by a crystallization process either within other cells or from the outside. However, this was not an original idea of Schlieden. He claimed this theory as his own, though Barthelemy Dumortierhad stated it years before him. This crystallization process is no longer accepted with modern cell theory. In 1839, Theodor Schwannstates that along with plants, animals are composed of cells or the product of cells in their structures. This was a major advancement in the field of biology since little was known about animal structure up to this point compared to plants. From these conclusions about plants and animals, two of the three tenets of cell theory were postulated.

All living organisms are composed of one or more cells

The cell is the most basic unit of life

Mitochondrion is a capsule-like structure. It is a double membrane structure. Its inner membrane is projected into numerous finger-like structures; called cristae. Mitochondria are the sites of cellular respiration. After cellular respiration, energy is stored in the form of ATP (Adenosine triphosphate); in mitochondria. Mitochondria have their own DNA and ribosomes and hence mitochondria can produce their own protein. 

Cellular respiration; due to this, mitochondria are also known as the ‘powerhouse of the cell’.

MITOCHONDRIAF

FUNCTION OF MITOCHONDRIAF

 Endoplasmic reticulum is a mesh-like structure which is composed of numerous tubes. It extends from the plasma membrane to the nuclear membrane. There are two kinds of endoplasmic reticulum, viz. smooth ER and rough ER. Rough ER has ribosomes on its surface which give it the rough appearance.

ENDOPLASMIC RETICULUM

 It serves as the transport channel in the cell. Substances are transported from cell membrane to cytoplasm and to nucleus and vice-versa. ER also serves the role of packaging many substances in the cell.

FUNCTION OF ER

 Golgi Complex was discovered by Camillo Golgi. It is composed of many sac-like structures which are stacked one above another.

GOLGI COMPLEX

Golgi complex is responsible for packaging of various substances in the cell . 

Lysosome are small sac-like structures and they are derived from Golgi complex. Lysosome contains digestive enzymes.

FUNCTION OF GOLI COMPLEX

LYSOSOME

The enzymes in the lysosome digest foreign particles and thus destroy them. Sometimes, the lysosome may burst open and its content ends up digesting the contents of the cell. The cell gets killed in the process. Due to this, lysosome is also called the ‘suicide bag of the cell’.

FUNCTION OF LYSOSOME

 These are tiny dot like structures interspersed in the cytoplasm and also on the surface of Rough ER. Ribosome is responsible for protein synthesis. 

These are somewhat similar to mitochondria; in appearance. Plastids are found in plant cells. They are of two types, chromoplast and leucoplast. Colourful plastids are called chromomplast and colourless plastids are called leucoplast. Chloroplast is green in colour and is found in green parts of plants. Plastids too have their own DNA and ribosome.

RIBOSOME

PLASTIDS

 Leucoplasts are responsible for storing food; such as carbohydrates, protein and lipid. Chromoplasts impart various colours to the plant parts. A leaf of a plant is green in colour because of chloroplast. Chloroplast is the site of photosynthesis.

FUNCTIONS OF PLASTIDS

 These are fluid filled chambers and are often seen in many cells. Vacuoles are very large in plant cells. A plant cell usually has single but large vacuole. Such a vacuole fills almost the entire space inside the cell. Vacuoles are much smaller and very few in animal cells.

VACUOLES

PLANT CELLVS

ANIMAL CELL

• 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.

HISTORY OF DISCOVERY OF CELL

Anton van Leeuwenhoek's microscope from the 17th century with a magnification of 270x.

Robert Hooke's microscope