CELL STRUCTURE and FUNCTION

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)

CELL THEORY

• 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.

SHAPE

• 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

SIZE

• 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.

CELL WALL

• Outermost layer, non living ,rigid
• Found in bacterial cells, fungal cells and plant cells.
• Permeable
• Made up of cellulose (in bacteria- peptidoglycans, in fungus- Chitin)

FUNCTION

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
• Semi-permeable
• Made up of phospholipids, proteins, carbohydrates and Cholesterol

FUNCTION :

It allows outward and inward movement of molecules across it like diffusion, osmosis, active transport, phagocytosis and pinocytosis

PROTOPLASM

• According to Huxley , protoplasm is “physical basis of life”
• Includes organic and inorganic molecules

CYTOPLASM

Components of a typical animal cell:

1. Nucleolus
2. Nucleus
3. Ribosome (dots as part of 5)
4. Vesicle
5. Rough endoplasmic reticulum
6. Golgi apparatus (or, Golgi body)
7. Cytoskeleton
8. Smooth endoplasmic reticulum
9. Mitochondrion
10. Vacuole
11. Cytosol (fluid that contains organelles; with which, comprises cytoplasm)
12. Lysosome
13. Centrosome
14. 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

NUCLEUS

STRUCTURE

• 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

FUNCTION

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

Endoplasmic Reticulum

• 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.

FUNCTION

• 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

GOLGI COMPLEX

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;

6. Macromolecules;

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

FUNCTION

• 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
  called Cristae
• 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

FUNCTION

• Synthesizes and stores energy rich molecules ATP(Adenosine Tri phosphate) during aerobic respiration

PLASTIDS

• 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

FUNCTION

Helps in photosynthesis

VACUOLES

• 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

FUNCTION

• 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 .

FUNCTION

These are sites of protein synthesis

CENTROSOMES

• 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

FUNCTION

Helps in cell division.

NON LIVING CELL INCLUSIONS

• Ergastic substances
• Cytoskeleton

All cell is regarded as a true biological atom.