THE CELL: DEFINTION, STRUCTURE, FUNCTIONS CELL THEORY
DEFINITION AND FORMS OF CELLULAR EXISTENCE
The cell is the simplest, smallest, basic, structural and functional unit of life. The cell can carry out all life activities such as growth, respiration e. t. c.
Cells can exist in various forms which include
- As a single cell (unicellular forms) g. Amoeba, Chlamydomonas, Chlorella, Euglena
- Chlamydomonas(Chlamydomonas rivalries)
Chlamydomonas is a unicellular plant like protist, motile using its flagellum and has a cup shaped chloroplast. It is sensitive to light using its eye spot. In the presence of light, starch grains appear in the cytoplasm. These disappear in a dark environment.
Chlamydomonas cells through its flagella (whip like) respond to changes in the surrounding such as moving from a dark region to a region of light. This organism disposes off its excess water using its contractile vacuoles. It reproduces asexually and sexually when it reaches its maximum size.
Amoeba (Amoeba proteus)
Amoeba is a unicellular animal like protest and the simplest organism with heterotrophic mode of feeding. It is shapeless. It digests its complex food with its food vacuole. The undigested food materials are got rid of with excess water through its contractile vacuole. Amoeba moves about and captures its food with the aid of its pseudopodia (‘’false feet’’). At maximum growth, amoeba reproduces asexually by dividing into two daughter cells (binary fission)
Paramecium is a more complex unicellular organism compared to Amoeba. It is slipper- shaped and moves with the aid of its cilia, which also directs food into its gullet. The undigested food material is removed through the anal pore. Excess water is got rid of through the anterior and posterior contractile vacuoles. Two types of nucleus are visible in this organism, the micronucleus and mega nucleus
Euglena (Euglena viridis)
a protist that has both plant and animal features.
As a plant, it has chloroplasts (star shaped) for photosynthesis, pyrenoid for starch storage and paramylum granules (form in which starch is stored).
As an animal, it has no cellulose cell wall, but a plasma membrane called pellicle, a gullet, eye spot, contractile vacuole and flagellum for movement.
- In colonial forms g. Volvox, Sponges, Pandorina
Volvox is made up of severalchlamydomonas- like cells which are arranged in angular layer to form a hollow ball-like structure. The cells are connected by cytoplasm strands.
The cell of Volvox moves about through the beating of the flagella. Most cells of Volvox lose their ability to reproduce. Only a few of them can reproduce to form two daughter cells
- In filamentous forms g. Spirogyra, Oscillateria, Oedogonium
Spirogyra is a filament of identical cells which are joined end to end without branching. Each cell of spirogyra carries out its function independent of another cell. Spirogyra has spirally arranged chloroplast for photosynthesis.
The filament grows indefinitely. The break-off cells from the filament have the ability to reproduce asexually to form a new filament. Spirogyra also has ability to reproduce sexually.
- As a part of multicellular organisms
A multicellular organism is made up of interdependent cells carrying out several special functions. The cells of the multicellular organisms are arranged to perform similar functions and then form tissues. A group of specialized tissues performing similar function forms an organ. Organs are further organized to carry out special functions in form of system or organ system. Ultimately, the interdependent biological system results in the complex multicellular organisms.
CELL STRUCTURES AND THEIR FUNCTIONS
The study of a cell is called cytology. A cell consists of a living material called protoplasm, which is surrounded by a membrane called plasma membrane. In plants, a cell wall (non-living part) is also present surrounding the plasma membrane. The protoplasm is made up of two major parts: a. cytoplasm b. Nucleus
The cytoplasm contains cell organelles or inclusions, which are cellular structures with specific functions. The cells which contain a nucleus and organelles are called eukaryotic cells. These show a high level of cellular organization. However, some cells lack definite nucleus and organelles. In such cells (prokaryotic cells e.g. bacteria), materials containing instructions for growth and development are found in a particular part of the cell and so are not enclosed in any membrane.
Nucleus is the largest cellular organelle enclosed by a double-layered nuclear membrane with many pores. It contains chromosomes and nucleolus which are rich in proteins and nucleic acids.
The nucleus performs the following functions:
- Exchange of materials between the nucleus and the cytoplasm through the nuclear pores.
- Storage of hereditary information by the chromosomes
- Control centre of cellular activities
Structure of a typical animal cell
CELL COMPONENTS AND THEIR FUNCTIONS
|i. Mitochondrion||Site of cellular respiration for energy production. It consists of enzymes and DNA. The enzymes convert ADP to ATP.|
|ii. Vacuole||Surrounded by a membrane called tonoplast. It contains cell sap mineral salt and sugar. It act as an osmoregulator (removing excess water).|
|iii. Endoplasmic reticulum (ER)||Aids transport of materials within the cytoplasm. Rough endoplasmic reticulum(RER) (having ribosome attached) is for protein synthesis and transportation. Smooth endoplasmic reticulum (SER) is for lipids and steroids synthesis and transportion.|
|iv. Golgi bodies/apparatus||For synthesis, packaging and distribution of materials.|
|v. Chloroplast||Contains chlorophyll for photosynthesis in green plants|
|vi. Lysosomes||Site for production of enzymes for respiration and destruction of worn out cells.|
|vii. Ribosome||For protein synthesis|
|viii. Centrioles||Active in cell division by producing spindle fibres to which chromosomes are attached. (present only in animal cell).|
|ix. Cell plasma) membrane||This is flexible and selectively permeable. It controls the materials entering or leaving the cell|
|x. Cell wall||It protects the cell, thus providing mechanical support in plants.|
Structure of a typical plant cell
SIMILARITIES BETWEEN PLANT AND ANIMAL CELLS
Both plant and animal cells have the following in common
(a)Nucleus (b) golgi bodies (c) mitochondria (d) cytoplasm (e) chromosomes (f) endoplasmic reticulum (g) nucleolus (h) ribosomes (i) lysosomes (j) cell membrane
DIFFERENCES BETWEEN PLANT AND ANIMAL CELL
|PLANT CELL||ANIMAL CELL|
|1||Has chloroplast||No chloroplast|
|2||Rectangular and definite in shape||Usually spherical or shapeless|
|3||Has rigid cellulose cell wall||No cell wall|
|4||Centrioles are absent||Centrioles are present for cell division|
|5||Cytoplasm is less dense||Cytoplasm is dense granular|
|6||Starch granules present||Glycogen granules present|
|7||Presence of large central permanent vacuole||Vacuoles are absent or small and temporary when presnt|
This is the summary of research efforts of many scientist like Robert Hooke, Matthias Schleidein and Theodore Schwann in 1838 and 1839 likewise, Rudolf Virchow (1855) etc.
The cell theory states that:
- All living organisms are composed of one or more cells.
- The cell is the structural and functional unit of all living organisms i.e. the basic organizational unit of life.
- All existing cells come from pre-existing ones.
- A cell contains information for its structural and functional development in its nucleic acids. This information is passed down from the parents to the off spring
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