CELL: THE UNIT OF LIFE
- What is a cell?
- Cell theory
- An overview of a cell
WHAT IS A CELL?
- Cell is the fundamental, structural and functional unit of all living organisms
- Robert Hooke (1665) – an English scientist who observed honeycomb like dead cells and coined the term CELL
- Antonie van Leeuwenhoek first described a living cell (1667)
- Robert Browne discovered nucleus (1833)
- Mathias J Schleiden (1838) ; a German botanist and
- Theodore Schwann (1839) ; a British Zoologist proposed cell theory.
- All living organisms are composed of cells and product of cells
- All cells arise from pre existing cells through the process of cell division
- The body of living organisms is made up of one or more cells
CELL NUMBER SHAPE AND SIZE
- Unicellular organisms – Organisms with single cell, capable of independent existence and carries all functions like digestion, excretion ,respiration ,growth & reproduction (Acellular). Examples , Amoeba, Euglena
- Multicellular organisms – Organisms with more than one cell
- Cells in multicellular organisms vary in size & shape depending on function.
- Parenchyma – Polyhedral cells performs storage.
- Sclerenchyma – spindle shaped cells & provides mechanical support,
- Nerve cells- long and branched cells conducting nerve impulses
- RBC -Biconcave & helps in carrying oxygen
- Muscle cells- cylindrical or spindle shaped concerned with the movement of body parts
- varies from few microns (1cm= 10mm; 1mm=1000µm) to few cms
- Smallest living cell is PPLO ( Pleuro Pneumonia Like Organism) – 0.1µm
- Largest living cell is Egg of an Ostrich , 170 to 180 mm in diameter.
- Bacteria – 0.1 to 0.5 µm
- Sclerenchyma fibre upto 60cms in length
CELL STRUCTURE AND FUNCTION
- Cell has non living outer layer called CELL WALL found only in plant cells
- Below cell wall is CELL MEMBRANE
- CELL MEMBRANE encloses PROTOPLASM
- PROTOPLASM has semi fluid matrix called CYTOPLASM and large membrane bound structure called NUCLEUS
- CYTOPLASM has many membrane bound organelles like Endoplasmic reticulum , Golgi Bodies Mitochondria ,Plastids and vacuoles.
- They also have non membrane bound structures called Ribosomes and Centrosomes
- Cytoplasm without Cell organelles are called Cytosol.
- Outermost layer, non living ,rigid
- Found in bacterial cells, fungal cells and plant cells.
- Made up of cellulose (in bacteria- peptidoglycans, in fungus- Chitin)
Rigidity, mechanical support and protection
CELL MEMBRANE (Plasma membrane )
- Present in all cells, just below the cell wall in plant cells, outermost membrane in animal cells
- Made up of phospholipids, proteins, carbohydrates and Cholesterol
It allows outward and inward movement of molecules across it like diffusion, osmosis, active transport, phagocytosis and pinocytosis
- According to Huxley , protoplasm is “physical basis of life”
- Includes organic and inorganic molecules
Components of a typical animal cell:
- Ribosome (dots as part of 5)
- Rough endoplasmic reticulum
- Golgi apparatus (or, Golgi body)
- Smooth endoplasmic reticulum
- Cytosol (fluid that contains organelles; with which, comprises cytoplasm)
- Cell membrane
- Semi fluid matrix present between cell membrane and nuclear membrane
- It has various living cell inclusions called cell organelles and non living substances called Ergastic substances
- Largest cell organelle present in eukaryotic cells
- It is usually spherical
- It has double layer nuclear membrane with nuclear pores
- It has transparent granular matrix called nucleoplasm, chromatin network composed of DNA and histone proteins
- It also has a spherical body called Nucleolus
It is the control centre of the cell.
It contains genetic material DNA which regulates
all metabolic activities of the body
CHROMOSOME ( vehicles of heredity )
- Nucleus of a non dividing cell has network of fibres called chromatin.
- During cell division, chromatin condenses to form distinct chromosomes.
- Chromosomes help in transmission of characters or genes
- Chromosome has centromere at the centre & arms on either sides called chromatids
- Chromatid- Thread like chromonema
MEMBRANE BOUND CELL ORGANELLES
- ER is a network of membrane bound tubular structures in cytoplasm
- It extends from cell membrane to nuclear membrane
- it exists as flattened sacks called Cisternae, unbranched tubules and oval vesicles
- There are two types of ER, ROUGH ER and SMOOTH ER.
- Helps in intracellular transportation
- It provides mechanical support to cytoplasmic matrix
- It helps in the formation of micro bodies, nuclear membrane and golgi complex.
- It helps in detoxification of metabolic wastes
Diagram of secretory process from endoplasmic reticulum (orange) to Golgi apparatus (magenta).
1. Nuclear membrane;
2. Nuclear pore;
3. Rough endoplasmic reticulum (RER);
4. Smooth endoplasmic reticulum (SER);
5. Ribosome attached to RER;
7. Transport vesicles;
8. Golgi apparatus;
9. Cis face of Golgi apparatus;
10. Trans face of Golgi apparatus;
11. Cisternae of the Golgi Apparatus
- It has a group of curved, flattened plate like compartments like Cisternae.
- The cisternae produce a network of tubules from the periphery
- These tubules end in spherical enzyme filled vesicles.
- Commonly called packaging centres of the cell
- They store the product of ER
- They produce Lysosomes
- They secrete various Enzymes, hormones and cell wall material
MITOCHONDRIA (power house of the cell )
- Spherical or rod shaped
- It has two membranes, outer membrane is smooth, inner membrane produces finger like infoldings
- Inner membrane also has stalked particles called Racker’s particles or Oxysomes
- The mitochondria is filled with granular mitochondrial matrix
- Matrix has circular mitochondrial DNA, RNA, 70s Ribosomes, proteins, Enzymes and lipids
- Synthesizes and stores energy rich molecules ATP(Adenosine Tri phosphate) during aerobic respiration
- Present in plant cells, photosynthetic bacteria and Euglena (bacteria can be chemosynthetic also)
- 3 types- Chromoplast (different colored plastids), Leucoplast ( Colourless) and Chloroplast( Green colored plastids)
CHLOROPLAST: (kitchen of the cell)
- Contains green colour pigment called chlorophylls
- Chloroplast has double membrane
- Matrix is called Stroma
- Stroma has membranous sacks called Thylakoids
- Thylakoids are arranged one above the other to form granum
- Grana are interconnected by Frets
Helps in photosynthesis
- Single membrane bound sack like vesicles
- Absent in animal cells
- Plant cells have large vacuoles- distinct character
- Also present in lower organisms
- The membrane of vacuole is called tonoplast
- Vacuole is filled with watery fluid called cell sap which has dissolved salts, sugars, enzymes etc
LYSOSOMES (Suicidal bag of cell )
- Single membrane bound vesicles filled with hydrolytic enzymes found only in animal cells.
- Produced from golgi complex
- 4 types- Primary, secondary, residual & auto lysosomes
- Intracellular digestion
- Destroy old and non functional cells
- Recycles worn out cells
NON-MEMBRANOUS CELL ORGANELLES
RIBOSOMES (Protein factories of the cell )
- Present in cytoplasm, mitochondria, chloroplast & also found attached to rough ER & nuclear membrane
- They are made up of r RNA and proteins
- Prokaryotes have 70s ribosomes, Eukaryotes have 80s ribosomes .
These are sites of protein synthesis
- Found in animal cells & in motile algae.
- It has two cylindrical structures called centrioles surrounded by centrosphere.
- Centrioles are arranged at right angles
- They are made up of micro tubules
Helps in cell division.
NON LIVING CELL INCLUSIONS
- Ergastic substances
All cell is regarded as a true biological atom.