The Fundamental Unit of Life : If we think about how different living organisms are formed and what is the basic unit of life. We will arrive at a term, “Cell”. Cell is the building block of every living organism. It is the smallest possible part of a living body. Cell also contains different parts and organelles, which perform specific functions.
Discovery of Cell:
- Cell was discovered by Robert Hooke in 1665.
- Robert Hooke was an English scientist and architect.
- Cell is a Latin word for “a little room”.
- He observed slice of cork under microscope, he saw that the structure is similar to the structure of honeycomb.
- The cell is the basic structural and functional unit of living organisms.
- It is called the functional unit of life as all the functions of the body takes place at cellular level in cell.
- Cells are present in all organisms and there is no organism who does not have one cell.
- The branch of science which study the function, structure and chemistry of cell is known as cytology.
- Cell Theory was proposed by biologists Matthias Jakob Schleiden and Theodor Schwann.
- It was formulated in 1839.
- Later, it was extended by Rudolf Virchow.
- The Cell Theory states that:
- All living organisms are composed of one or more cells.
- The cell is the basic unit of structure and organization in organisms.
- Cells arise from pre-existing cells.
Classification of organisms on basis of number of cells:
|Unicellular Organisms||Multicellular Organisms|
|These are made up of only one cell .||These are made up of many cells.|
|All functions of organism are performed by one cell.||Different cells perform their respective functions.|
|No division of labour.||There is division of labour.|
|In the process of reproduction whole cell is involved.||In the process of reproduction only some cells are involved which are called germ cells.|
|Life of organism is short.||Life of organism is long.|
|E.g., Bacteria, Amoeba, Paramecium, and Yeast.||E.g., Animals, Plants, and Fungi .|
Cellular organisms are again divided into two main types:
- These organisms have primitive and incomplete cells.
- They contain prokaryotic cells.
- Prokaryotic cell lacks a nuclear membrane.
- Nuclear material of prokaryotic cell has a single chromosome which is in direct contact with cytoplasm.
- The undefined nuclear region in the cytoplasm is called nucleoid.
- Mitochondria is absent but ribosomes are present.
- E.g., archaebacterial, bacteria, and cyanobacteria.
- These organisms have complete cells.
- These organism contains eukaryotic cells.
- Eukaryotic cells are membrane bound cells.
- Eukaryotic cells contain plasma membrane, nucleus, DNA and cytoplasm with ribosomes, mitochondria.
- E.g., Plants and Animals.
Difference between Prokaryotic cell and Eukaryotic cell
|S. No.||Prokaryotic cell||Eukaryotic cell|
|1.||Size of cell is generally small (1–10nm).||Size of cell is generally large (5-100nm).|
|2.||Nucleus is absent.||Nucleus is present.|
|3.||It contains a single chromosome.||It contains more than one chromosome.|
|4.||Nucleolus is absent.||Nucleolus is present.|
|5.||Cell wall is absent.||Cell wall is present.|
|5.||Cell organelles are absent.||Cell organelles (such as mitochondria, plastids, endoplasmic reticulum, Golgi apparatus, and lysosomes) are present.|
|6.||Cell division takes place by fission or budding.||Cell division takes place by mitotic or meiotic cell division.|
- For a eukaryotic cell, the basic cell shape is spherical but the shape is ultimately decided by the function of the cell.
- The shape may be variable or fixed.
- Irregular shape occurs in Amoeba and Leucocytes.
- Fixed cell shape occurs in Plants, animals and paramecium.
- Cells may have different shapes-polyhedral, spherical, spindle-shaped, elongated, branched, discoidal etc.
- The size of cells varies from small cells (in bacteria) to very large cells (in ovule of Cycas).
- Nerve cells of human can be a metre long.
- Some cells are visible to naked eyes and many can be seen through a microscope.
- The size of prokaryotic cell is in between 1 to 10 µm. The size of eukaryotic cells is in between 10-100 µm.
- The largest cell in human body is the Female ovum or egg.
Structure of Cell
All the cells contain three major functional regions: cell membrane, nucleus and cytoplasm.
- It is the outermost covering of cell that separates the contents of the cell from the external environment.
- The plasma membrane only allows the movement of some selective substances in and out of the cell. It is also known as Selectively Permeable Membrane.
- The movement of gases like carbon dioxide and oxygen through the plasma membrane occurs through diffusion process.
- The spontaneous movement of substances from the region of high concentration to the region of low concentration is termed as diffusion.
- The movement of water molecules through permeable membrane is called osmosis.
- The plasma membrane is flexible and made up of organic molecules proteins and lipids.
- Flexibility of cell membrane also allows it to engulf food and other materials from external environment, such processes are known as endocytosis.
- A rigid outer covering which is present outside the plasma membrane is cell wall.
- In plant cell, cell wall is present whereas it is absent in animal cell.
- Plant cell wall is composed of cellulose. It is a complex substance and provides structural strength to the plants.
- When a living plant cell loses water through osmosis, there is shrinkage and contraction of cell contents away from the cell wall. This phenomenon is called plasmolysis.
- It is a large, centrally located spherical component.
- It is bounded by two nuclear membranes.
- Nuclear envelope is a pocket which encloses all the components of the nucleus. Nuclear envelope separates the nucleus from cytoplasm.
- Nuclear membrane contains many tiny pores which allow the movement of substances between nucleoplasm and cytoplasm.
- Nucleoplasm contain two type of nuclear structure- nucleolus and chromatin material. Nucleolus is the factory for the production of ribosomes.
- Ribosome are required for protein synthesis.
- Chromatin material contains genetic materials such as DNA and proteins. The functional segment present in a DNA molecule is known as gene.
- A chromosome is a long DNA molecule with part or all of the genetic material of an organism.
- Nucleus plays a central role in cellular reproduction.
- Cytoplasm is the fluid content present inside the plasma membrane.
- Cytoplasm contains many specialized cell organelles.
- A cell organelle is a specialised subunit that has a specific function, usually within a cell.
- E.g., endoplasmic reticulum, golgi apparatus, lysosomes, mitochondria, plastids and vacuoles.
- It is a membranous network of tube like structures extending from the nuclear membrane to plasma membrane.
- There are two types of endoplasmic reticulum:
- Rough Endoplasmic Reticulum (RER): In RER ribosomes are present on the surface for the synthesis of proteins. RER helps in the synthesis and transport of proteins.
- Smooth Endoplasmic Reticulum (SER): In SER ribosomes are absent and SER’s function is to secrete lipids. SER helps in the synthesis of lipids and fat molecules.
- These proteins and lipids molecules help in the formation of cell membrane. This process is known as membrane biogenesis.
- The Golgi Apparatus is named after Camillo Golgi.
- Golgi developed the process of staining individual nerve and cell structures using a weak solution of silver nitrate. This process is called Black Reaction.
- It consists of a set of membrane bound, fluid filled vesicles, vacuoles and flattened cisternae (closed sacks).
- The cisternae are usually arranged parallel to each other.
- It is responsible for packaging proteins into vesicles prior to secretion and therefore plays a key role in the secretory pathway.
- It is also involved in the synthesis of cell wall, plasma membrane, and lysosomes.
- It is the waste disposal system of the cell.
- Small, spherical, sac-like structures which contain several digestive enzymes enclosed in a membrane.
- It helps in digestion of foreign substances and worn-out cell organelles.
- It helps to keep the cell clean.
- During any disruption in cellular metabolism, for example, when the cell gets damaged, lysosomes may burst and the enzymes digest their own cell. Therefore, lysosomes are also known as suicide bags of a cell.
- It is also known as the powerhouse of the cell.
- The energy required for any chemical process inside the cell is released by mitochondria in the form of ATP molecules.
- Mitochondria have two membrane covering. The outer covering is very porous while inner is deeply folded.
- Mitochondria have their own DNA and ribosomes.
- Plastids are present only in plant cells.
- Different types of plastids are:
1. Chloroplasts: These are the green coloured plastids containing chlorophyll. Chloroplasts help in the manufacture food for plants by the process of photosynthesis.
2. Chromoplasts: These are the colourful plastids, that come in many colours except green. Chromoplasts impart various colours to flowers to attract insects for pollination.
- Leucoplasts: These are colourless plastids. Lecuoplasts help in the storage of food in the form of starch, proteins and fats.
- These are the storage sacs for solid or liquid contents.
- Plants cells have very large vacuoles while animal cells have small vacuole.
- They help to maintain the osmotic pressure in a cell.
- They provide turgidity and rigidity to plant cells.
Differences Between Animal Cell and Plant Cell:
|S. No.||Animal Cells||Plant Cells|
|1.||Animal cells are generally small in size.||Plant cells are larger than animal cells.|
|2.||Cell wall is absent.||Plasma membrane of plant cell is surrounded by a rigid cell wall of cellulose.|
|3||Plastids are absent, except in case of protozoan Euglena.||Plastids are present.|
|4.||Vacuoles are many, small, and temporary.||Vacuoles are permanent and large.|
|5.||They have centrosomes and centrioles.||They lack centrosomes and centrioles.|
The Fundamental Unit of Life
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