Ultrastructure of cell and cell organelles

Ultrastructure of cell and cell organelles:-

General Introduction:-

·  Plant cells are eukaryotic cells that vary in several fundamental factors from other eukaryotic organisms.

·  Both plant and animal cells contain nucleus along with similar organelles.

·  One of the distinctive aspects of a plant cell is the presence of a cell wall outside the cell membrane.

·  The plant cell is rectangular and comparatively larger than the animal cell.

·  Even though plant and animal cells are eukaryotic and share a few cell organelles, plant cells are quite distinct when compared to animal cells as they perform different functions.

Ultrastructure of Cell Organelles:- Just like different organs within the body, plant cell structure includes various components known as cell organelles that perform different functions to sustain itself.  These organelles include:-

1. Cell Wall:-

·  This is a hard and dead covering of the cell.

·  It is a rigid layer which is composed of cellulose, glycoproteins, lignin, pectin and hemicellulose.

·  It is located outside the cell membrane.

·  It comprises proteins, polysaccharides and cellulose.

·  The primary function of the cell wall is to protect and provide structural support to the cell.

·  The plant cell wall is also involved in protecting the cell against mechanical stress and to provide form and structure to the cell.

·  Protects the cell from mechanical damage and infection.

·  It also filters the molecules passing in and out of the cell.

·  The formation of the cell wall is guided by microtubules.

·  It consists of three layers, namely, primary, secondary and the middle lamella.

·  Plasmodesmata:- It makes connections between cells.

·  The middle lamella is made up of Ca-pectate which acts as a gum that joint cells together.

·  The primary cell wall is formed by cellulose laid down by enzymes.

2. Plasma membrane:- 

·  It is the semi-permeable membrane that is present within the cell wall.

·  It is a differentialy permeable, electrically charged and selective membrane.

·  It is composed of a thin layer of protein and lipid.

·  Singer and Nicholson (1972):- He gave the Fluid mosaic model which is a universal theory.

Ø 75A ° thick.

Ø There are 2 layers of phospholipid.

Ø Proteins are arranged in 2 ways:-

i. Extrinsic proteins (Peripheral proteins):- They are situated on both the surfaces.

ii. Interinsic Proteins (Integrated Proteins):- They are situated in the middle of the lipid bilayer. With their help, the plasma membrane controls the movement of substances.

·  The cell membrane plays an important role in regulating the entry and exit of specific substances within the cell.

·  For instance, cell membrane keeps toxins from entering inside, while nutrients and essential minerals are transported across.

·  Functions:- Shape, protection, transport (diffusion, osmosis, active transport)

·  In 1838, the term 'Protoplasm' was given by Purkinje. Protoplasm is called the physical basis of life.

Protoplasm = cytoplasm + nucleoplasm

3. Golgi complex:-

·  They are found in all eukaryotic cells.

·  They are involved in distributing synthesized macromolecules to various parts of the cell.

·  Its main function is glycosylation of proteins and packing in vesicles.

4. Endoplasmic Reticulum:-

·  It is continuous with the outer membrane of the nucleus.

·  It is of two types -

i. Rough Endoplasmic Reticulum (RER):- It is rough due to the presence of ribosomes on its outer surface. Its work is protein formation.

ii. Smooth Endoplasmic Reticulum (SER):- Ribosomes are not found on its outer surface due to which it is smooth. It performs the function of lipid formation.

5. Lysosomes:- 

·  Lysosomes are called suicidal bags as they hold digestive enzymes in an enclosed membrane.

·  They perform the function of cellular waste disposal by digesting worn-out organelles, food particles and foreign bodies in the cell.

·  Lysosomes are predominantly found in eukaryotic animal cells and are responsible for breaking down cellular debris.

·  In plants, the role of lysosomes is undertaken by the vacuoles as traditional cell biology dictates.

6. Ribosomes:-

·  They were discovered by George Pallade. That is why they are also called pallade granules.

·  They are the smallest membrane-bound organelles which comprise RNA and protein.

·  They are the sites for protein synthesis, hence, also referred to as the protein factories of the cell.

·  These are of two types -

i. 80S:- They are made up of 60S and 40S sub-units. These are found in cytoplasm, on RER and the outer nuclear membrane.

ii. 70S:- They are made up of 50S and 30S sub-units. They are present in mitochondria and chloroplasts.

7. Peroxisomes:-

·  Peroxisomes are small vesicles, single membrane-bound organelles found in the eukaryotic cells. 

·  They contain digestive enzymes for breaking down toxic materials in the cell and oxidative enzymes for metabolic activity.

·  Peroxisomes play an important role in lipid production and are also involved in the conversion of reactive oxygen species such as hydrogen peroxide into safer molecules like water and oxygen by the enzyme catalase.

·  In plants, peroxisomes facilitate photosynthesis and seed germination. They prevent loss of energy during photosynthesis carbon fixation.

8. Vacuoles:-

·  Central vacuole occupies around 30% of the cell’s volume in a mature plant cell.

·  Tonoplast is a membrane that surrounds central vacuole.

·  The vital function of central vacuole apart from storage is to sustain turgid pressure against the cell wall.

·  The central vacuole consists of cell sap. It is a mixture of salts, enzymes and other substances.

·  The cell excrete its extra waste material into the central vacuole.

9. Mitochondria:-

·  They are the double-membraned organelles found in the cytoplasm of all eukaryotic cells.

·  These are the main sites of aerobic respiration.

·  They provide energy by breaking down carbohydrate and sugar molecules, hence they are also referred to as the “Powerhouse of the cell.”

·  A circular DNA is found in its matrix.

·  It is an semi - autonomous organelle.

10. Chloroplasts:- 

ØIt is an elongated organelle enclosed by phospholipid membrane.

ØThe chloroplast is shaped like a disc and the stroma is the fluid within the chloroplast that comprises a circular DNA.

ØIt is an semi - autonomous organelle.

ØEach chloroplast contains a green coloured pigment called chlorophyll required for the process of photosynthesis.

ØThe chlorophyll absorbs light energy from the sun and uses it to transform carbon dioxide and water into glucose.

11. Nucleus:- 

·  The nucleus was discovered by Robert Brown.

·  The nucleus is a double membrane-bound structure that is present only in eukaryotic cells.

·  The vital function of a nucleus is to store DNA or hereditary information required for cell division, metabolism and growth.

·  Nucleolus:- It manufactures cell’s protein-producing structures and ribosomes.

·  Nucleopore:- Nuclear membrane is perforated with holes called nucleopore that allows proteins and nucleic acids to pass through.

·  Chromatin:- DNA is found inside the nucleus as chromatin. It is of 2 types -

i. Euchromatin:- It contains active genes.

ii. Heterochromatin:- It contains inactive genes.