Cell as the Basic Unit of Life
- The cell is the basic structural and functional unit of all living organisms.
- It is the smallest part of the body of an organism, is capable of independent existence and is able to perform the essential functions of life.
Discovery of the Cell
- Robert Hooke observed cork cells under a simple microscope designed by him.
- He observed compartments resembling honeycombs and each compartment was separated by a wall.
- He termed each compartment as a ‘cell’.
- It was the first indication that living organisms are made of cells.
- In 1838, Matthias Schleiden and Theodor Schwann proposed the basic cell theory. In 1858, another scientist Virchow made an addition to the existing cell theory.
- The postulates of the modern cell theory are
- The cell is the smallest unit of structure of all living things.
- The cell is the unit of function of all living things.
- All cells arise from pre-existing cells.
Variety in Cells
On the basis of the number of cells
|Organisms made of a single cell. They are called unicellular organisms. Examples: Bacteria, Yeast, Chlamydomonas, Amoeba, Paramoecium ||Organisms made of a few hundred to few thousand cells.Examples: Spirogyra, Volvox||Organisms made of millions to billions of cells. They are called multicellular organisms.Examples: Man, cow, mango tree, crow|
On the basis of size of cells
- Smallest cell: Examples: Bacteria (0.3–5.0 µm), red blood cells (7 µm)
- Longest cell: Example: Nerve cell in the neck of a giraffe (˃3 m long)
- Largest cell: Example: Ostrich egg (170 mm × 130 mm)
On the basis of shape of cells
- Columnar: Epithelial cells
- Spherical: Human ovum
- Oval: Fat cells
- Spherical, biconcave: Red blood cells
- Rectangular: Spirogyra
- Spiral: Sperm cell
- Rod-shaped: Bacteria
- C-shaped: Cartilage cells
- Cylindrical: Striated muscle fibre cells
- Branched: Nerve cells
- Spindle-shaped: Smooth muscle cells
- Bean-shaped: Guard cell from a plant leaf
Structural Organisation of a Cell
Plasma membrane/Cell membrane
- Forms the outermost covering in animal cells
- Lies next to the cell wall in plant cells
- Separates cellular material from its surroundings.
- Acts as an effective barrier and regulates the entry of substances in and out of the cell
Cell wall (in plant cells only)
- Found in plant cells
- Situated just outside the plasma membrane
- Mainly composed of cellulose
- Provides protection
- Gives rigidity and shape to plant cells
- Transparent jelly-like material
- Centre of all metabolic activities
- Different organelles contained in it perform different functions
- Largest cell organelle
- Mostly spherical and dense
- Nuclear membrane with pores, which allow substances to enter and leave the nucleus
- Regulates cell functions
- Contains chromosomes, made of genes, which control hereditary characteristics
- Embedded within the nucleus of the cell.
- One or more in number
- Produces ribosomes
- Participates in protein synthesis by forming and storing RNA
- Network of thread-like structures made of DNA
- Chromosomes carry hereditary information or Genes
- Fluid-filled membrane bound spaces
- Larger and permanent in plant cells
- Small and temporary in animal cells
- Storage of water and other substances, food, pigments and waste products
- Provides turgidity to the cells
Study of Plant and Animal Cells
Differences between Plant and Animal Cells
|PLANT CELL||FEATURE||ANIMAL CELL|
| ||Structural differences|
|Presence of a definite cell wall made of cellulose||Cell wall||Absence of cell wall|
|Present internal to the cell wall||Cell membrane||Forms the boundary of the cell|
|Presence of one or more prominent vacuoles||Vacuoles ||Presence of small and temporary vacuoles|
|Presence of plastids||Plastids ||Absence of plastids|
| ||Functional differences|
|Usually larger with distinct outlines||Size ||Usually smaller with less distinct boundaries|
|Not so dense ||Cytoplasm ||Denser and more granular|
|Only a thin lining of cytoplasm, which is mostly pushed to the periphery||Arrangement of cytoplasm ||Cytoplasm fills up almost the entire cell|
| ||Other differences|
|Rectangular ||Shape ||Spherical |
|Starch ||Storage material||Glycogen |
Prokaryotic and Eukaryotic Cells
|PROKARYOTIC CELL ||EUKARYOTIC CELL |
|Absence of well-defined nucleus ||Presence of well-defined nucleus with a nuclear membrane|
|Absent ||Present |
|Presence of a single length of only DNA ||Presence of several lengths of DNA wound around certain proteins|
|Presence of smaller ribosomes||Presence of larger ribosomes|
|Absence of other cell organelles||Presence of several other cell organelles such as mitochondria, ER, chloroplasts etc.|
|Cell division occurs by fission or budding but not by mitosis ||Cell division occurs by mitosis or meiosis|
|Bacteria, blue green algae||Euglena, Amoeba, plants, animals|