Cells: Fundamental Units of Life
Introduction:
In 1838, Matthias Schleiden, a German botanist, first proposed the idea that all plants consist of cells. A year later, Theodor Schwann, a German zoologist, independently asserted that all plants and animals are made up of cells, thus extending the theory to animal life. This joint finding forms the basis of the cell theory.
The cell is an accumulation of living substances (protoplasm) which is limited by an outer membrane (plasma membrane) and possesses a nucleus.
A cell, is therefore, defined as a mass of protoplasm having a distinct nucleus and limited by a plasma membrane.
All cells are either simple or complex:
- Simple cells, which make up the Bacteria and Blue-Green Algae do not have a nucleus and are called 'prokaryotes', meaning 'before nucleus'.
- A complex cell has a nucleus that contains DNA, a nucleic acid that stores genetic information. These cells are called 'eukaryotes' meaning 'true nucleus'.
The Cell: Shape, Size and Structure:
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| Different types of cells |
Size: The size of the cell ranges from 1/10th to 1/1000th of a millimetre. Cells are generally microscopic, but certain cells, like the nerve cell in elephant, has been found to measure upto 1 metre. The smallest cells are those of Mycoplasma gallisepticum, an organism intermediate between viruses and bacteria. Their size is about 1/10000th of a millimetre!
Structure: The cell, seen under a light microscope, reveals the following main components:
- A cell wall-> is absent in animal cells and plasma membrane which encloses the mass of protoplasm.
- A cytoplasm-> in which living and non-living inclusions are present. The living inclusions are called cell organelles.
- A nucleus-> is also present. It contains one or more dense bodies called nucleoli and a network of thread-like structures called chromatin.
Cell wall and Plasma Membrane:
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| Layers of The Cell Wall |
The cell wall is entirely lacking in animal cells. It is a semi-rigid, freely permeable, non-living part of the cell. It is made up of a complex chemical substance called cellulose. The cell wall gives strength and rigidity to the cell.
Cell Membrane or Plasma Membrane is present in both plants and animals. It is a living, elastic, differentially permeable membrane covering the cell. It gives external form to the protoplasm and also provides mechanical support. Being selectively permeable, it allows only certain molecules to move in and out of the cell.
Protoplasm:
The protoplasm is a jelly-like, transparent, granular substance that occurs everywhere in the cell. It responds to stimuli like pin-pricks, electric shocks, chemicals and sudden variations in light and temperature. If it is removed from the cell, the cell dies. There are two parts of the protoplasm:
- Cytoplasm: It is the protoplasm that occurs between the plasma membrane and the nucleus.
- Nucleoplasm: It is the portion of protoplasm that occurs in the nucleus and is limited by a nuclear membrane.
Cytoplasm:
The part of protoplasm which occurs between the plasma membrane and the nuclear envelope is called the cytoplasm.
The cytoplasm is composed of two distinct types of structures:
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| The Cytoplasm |
- The Matrix: It is a transperent homogeneous semi-fluid substance.
- The Organelles: Organelles are living parts of the cells having definite shape, structure and function. They are found embeded in the cytoplasm.
Pictures of Cells:
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| A generalised Plant Cell |
Charecteristics of Plant cell:
- Cellulose Cell Wall present.
- Plastids are present.
- Vacuoles are large and prominent.
- Centrosome is absent.
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| A generalised animal cell |
Charecteristics of Animal Cell:
- Cellulose Cell Wall is absent.
- Plastids are absent.
- Vacuoles are usually absent. If present, they are large and prominent.
- Centrosome is present.
Cell Organelles:
Cell organelles are living parts of the cell having definite shape, structure and function.
The cell organelles are:
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| The Endoplasmic Reticulum |
1)Endoplasmic Reticulum: It is a complex tubular membrane system which almost fills up the intracellular cavity. Some tubes are studded with rounded bodies called ribosomes.
One one end, it is connected to the outer membrane of the nucleus, and on the other to the plasma membrane. It forms the supporting skeletal framework of the cell. It also provides a pathway for the distribution of nuclear material from one cell to the other.
2)Ribosomes: Ribosomes are dense, spherical and granular particles which occur freely in the matrix or remain attached to the endoplasmic reticulum.
Ribosomes are sites for protein synthesis.
3)Mitochondria: The Mitochondria (singular: mitochondrion) are tiny bodies of varying shapes distributed in the cytoplasm. Each is bounded by a double membrane envelope. The inner membrane is thrown into folds known as cristae.
Mitochondria are the sites of cellular respiration. Respiration releases energy. Hence, the mitochondria are also known as the power house of the cell.
There is a greater concentration of mitochondria in animal cells than plant cells. In plants, the chloroplasts, by a process called photosynthesis capture light energy and partly convert it into chemical energy.
4)Golgi Complex: The Golgi Complex, also named Golgi bodies or Golgi Apparatus, occurs in all cells with a few exceptions.
The main function of the Golgi body, is secretory. The synthetic products of the cell, i.e., proteins, hormones and enzymes are secreted by The Golgi bodies.
5)Lysosomes: Lysosomes are tiny, spherical sac-like structures evenly distributed in the cytoplasm. They occur mostly in animal cells, but have been located in certain plant cells as well.
The chief function of lysosomes is digestion. The enzymes present in the Lysosomes are capable of breaking a wide variety of food particles.
Lysosomes can also be dangerous if by any chance, it explodes and then the enzymes would digest the cell itself and the cell would die!
6)Centrosome and Centrioles: Just outside the nucleus of the animal cell, there is a small clear area of cytoplasm without any granules. In this portion of the cytoplasm, which is called the centrosome, one or two tiny micro-tubular structures develop. These are called centrioles.
The centriole is concerned with cell division. It initiates cell division in animal cells, only!
7)Plastids: Plastids occur in most plant cells and are absent in animal cells.The types of plastids are:
8)Nucleus: The nucleus is perhaps the most important part of a cell or the 'brain' of a cell. The nucleus regulates all metabolic and hereditary activities of the cell. It is composed of the following structures:
2)Ribosomes: Ribosomes are dense, spherical and granular particles which occur freely in the matrix or remain attached to the endoplasmic reticulum.
Ribosomes are sites for protein synthesis.
3)Mitochondria: The Mitochondria (singular: mitochondrion) are tiny bodies of varying shapes distributed in the cytoplasm. Each is bounded by a double membrane envelope. The inner membrane is thrown into folds known as cristae.
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| Mitochondria |
There is a greater concentration of mitochondria in animal cells than plant cells. In plants, the chloroplasts, by a process called photosynthesis capture light energy and partly convert it into chemical energy.
4)Golgi Complex: The Golgi Complex, also named Golgi bodies or Golgi Apparatus, occurs in all cells with a few exceptions.
The main function of the Golgi body, is secretory. The synthetic products of the cell, i.e., proteins, hormones and enzymes are secreted by The Golgi bodies.
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| A Lysosome |
The chief function of lysosomes is digestion. The enzymes present in the Lysosomes are capable of breaking a wide variety of food particles.
Lysosomes can also be dangerous if by any chance, it explodes and then the enzymes would digest the cell itself and the cell would die!
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| Centrosomes and Centrioles |
6)Centrosome and Centrioles: Just outside the nucleus of the animal cell, there is a small clear area of cytoplasm without any granules. In this portion of the cytoplasm, which is called the centrosome, one or two tiny micro-tubular structures develop. These are called centrioles.
The centriole is concerned with cell division. It initiates cell division in animal cells, only!
7)Plastids: Plastids occur in most plant cells and are absent in animal cells.The types of plastids are:
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| A chloroplast |
- Chloroplasts-> They contain the green pigment called chlorophyll which trap solar energy and utilise it to manufacture food.
- Chromoplasts-> They contain pigments which give red and yellow colors to flowers and fruits. By imparting color to flowers, they attract insects for pollination.
- Leucoplasts-> These are colorless pigments which store food in the form of carbohydrates (starch), fats and proteins.
8)Nucleus: The nucleus is perhaps the most important part of a cell or the 'brain' of a cell. The nucleus regulates all metabolic and hereditary activities of the cell. It is composed of the following structures:
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| A nucleus of an animal cell |
- Nuclear Membrane: It is an envelope which separates the nucleoplasm from the cytoplasm.
- The Nucleoplasm: The space within the nucleus is filled with ground substances or matrix called the nucleoplasm. The nucleolus and the chromatin threads remain suspended in it.
- Chromatin: These are tiny and thin thread-like structures containing chromosomes which in turn stores DNA which contains genetic information.
The Major differences between Animal and Plant Cells:
Animal Cell
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Plant Cell
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1) Cell
wall absent. Cellulose in any form is also absent.
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1) Cellulose
Cell wall is present.
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2)
Cytoplasm is denser, more granular and
occupies most of the space in a cell.
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2) Cytoplasm is pushed to the periphery and
forms a thin lining against the cell wall.
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3) Vacuoles absent. If present, they are small
and temporary.
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3)
Vacuoles are large and prominent.
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4) Plastids are absent.
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4) Plastids are absent.
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5) Centrosome is present.
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5) Centrosome is absent.
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6) Metabolic rate is
fast.
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6) Metabolic rate is slow.
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NOTE: Some cells like the mature red blood cells of mammals and the sieve tubes of plants donot contain a nucleus!
Cell Inclusions:
As a result of diverse metabolic activities of the cell organelles, a variety of soluble and insoluble chemicals products accumulate within the cell. These are called cell inclusions. These are generally found in vacuoles and granules.
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| A vacuole of a plant cell |
1)Vacuoles: Vacuoles are clear spaces present in the cytoplasm enclosed by a membrane. These contain fluid called cell-sap in which soluble substances are found. In animal cells, vacuoles, if present, are small and temporary. They store glycogen and proteins.
In plant cells, vacuoles are large and permanent. They generally contain water, pigments, alkaloids, fats, sugar and proteins. Presence of water in vacuoles make the cells more turgid.
2)Granules: These are tiny bodies found in the cytoplasm of the plant and animal cells. They are not bounded by the membrane. Sometimes pigments may occur as granules in special cells, glycogen or starch granules may be present as a reserve food, or secretory granules may accumulate in gland cells.
Depending upon their nature, cell inclusions are classified as reserve, secretory or waste products:
- Reserve Product:- Carbohydrates, Fats, Oils, Yolk (in animal eggs) and proteins.
- Secretory Product:- Enzymes, Pigments, Nectar (in plants).
- Waste Product:- Mineral crystals, Gums and resins, Latex, Alkaloids, Tannin and essential oils.
THANK YOU
By SPANDAN
From Anita Prasad's Textbook for Biology
From Anita Prasad's Textbook for Biology
