BASIC STRUCTURE AND FUNCTION OF CELL AND ITS ORGANELLES

It is the presence of cell that differentiates the living from non-living. Cell is thus rightly known as the basic unit of life.

DEFINITION OF CELL – Cell is the fundamental structural, functional and biological unit of all living organisms.

CELL THEORY – Originally formulated by Schleiden and Schwann in 1839 and modified by Rudolf Virchow in 1855. It is stated as:

• All living organisms are composed of cells and products of cells.
• All cells arise from pre-existing cells.

TYPES OF CELLS

PROKARYOTIC CELLS – These include bacteria, blue green algae, mycoplasma and PPLO (pleuro pneumonia like organisms).

They have no well defined nucleus and no membrane bound organelles. They possess inclusion bodies (that store reserve material) and mesosomes (infoldings of cell membrane).

EUKARYOTIC CELLS – Protists, plants, animals and fungi are included in eukaryotes.

These have well defined membrane bound nucleus and possess membrane bound organelles.

CELL MEMBRANE

Cell membrane forms the outermost covering of the cell (except plants and fungi where cell wall is the outermost layer).

STRUCTURE –

Cell membrane consists of a bilayer of phospholipids arranged in such a manner that their hydrophilic heads containing phosphate groups lie outside in the aqueous environment and their hydrophobic fatty acid tails face towards inner side.

Cholesterol and glycolipids are other membrane lipids. Cell membrane also possesses proteins and carbohydrates.

Two types of protein are present in the cell membrane – Integral or transmembrane proteins that are partially or totally embedded in the membrane and Peripheral proteins that lie on the surface of the membrane. A peripheral protein typically works together with an integral protein.

Cell membrane is selectively permeable.

FUNCTION –

• Transport of molecules across the cell.
• As receptors, they receive signals from outside the cell.
• Act as ion channels or transport proteins to facilitate transport of molecules.
• It helps cells adhere with each other and to extracellular matrix.
• Transmembrane glycoproteins act as surface antigens which determines cell’s identity.
• Catalyze some cellular reactions.

CELL WALL

STRUCTURE –

This is a non living and rigid structure that makes the outermost covering of fungi and plants. In algae, it is made of cellulose, galactans, mannans and calcium carbonate. In other plants, it is composed of cellulose, hemicellulose, pectins and proteins.

FUNCTION –

• Gives shape to the cell.
• Protects the cell from injury and infection.
• Helps in cell to cell interaction.
• Forms a barrier to undesirable substances.

CYTOPLASM

This is a jelly like material that fills the cell. Various cell organelles float within the cytoplasm.

FUNCTION –

• Provides structural support to cell.
• Site of chemical reactions.
• Helps dissolve solutes.

ENDOPLASMIC RETICULUM

STRUCTURE –

It is a network of tiny tubular structures scattered in the cytoplasm. Its membrane is continuous with the outer nuclear membrane.

Endoplasmic reticulum (ER) divides the intracellular space into two compartments – luminal (inside ER) and extra-luminal (cytoplasmic).

TYPES –

ROUGH ER – They bear ribosomes on their surface. They are concerned with protein synthesis.

SMOOTH ER – They do not have ribosomes. They are major site for protein synthesis.

FUNCTIONS –

• Synthesis of proteins (in rough ER).
• Synthesis of lipids (in smooth ER).
• Metabolism and storage of calcium.

As soon as protein emerges from the ribosome, a signal sequence in the amino terminus of protein targets the RNA-ribosome complex to the ER membrane where translation continues.

Transmembrane proteins stay in ER membrane while secretory proteins are released into the ER lumen.

Membrane proteins, lipids and secretory proteins are then packaged into vesicles that are transported to golgi body where post-translational modification takes place.

Vesicles then separate from ER membrane, travels to golgi body, fuses with it and release their content.

GOLGI APPARATUS

STRUCTURE –

They consist of numerous flat, disc shaped sacs or cisternae arranged parallel to each other. (Size – 0.5 to 1 micron). Cisternae have convex cis (forming face) and concave trans (maturing face).

FUNCTION –

• Performs packaging of materials to be delivered to either inside or outside the cell.
• Post-translational modification of proteins takes place in golgi apparatus.

LYSOSOMES

STRUCTURE –

Membrane bound vesicular structure.

FUNCTION –

• Contain hydrolytic enzymes that digest carbohydrates, proteins, lipids and nucleic acids at acidic PH.
• In white blood cells, they play a role in immune response by digesting phagocytized bacteria.
• Perform autolysis of a dying cell.

MITOCHONDRIA

STRUCTURE –

A sausage shaped, double membrane bound organelle. The inner membrane has infoldings called cristae. The inner compartment which is enclosed within the inner membrane is called matrix. The space enclosed between inner and outer membrane is called outer compartment. The number of mitochondrion varies per cell depending on physical activity of cell. Matrix also contains a single circular DNA, few RNA and ribosomes (70S). Mitochondria are capable of synthesizing proteins and dividing by fission.

FUNCTION –

• Site of aerobic respiration.
• Produce energy in the form of ATP. (Therefore called ‘powerhouse of cell’).

ENDOSYMBIOSIS THEORY – This theory states that mitochondria were once free living organisms that later on became part of modern cells.

RIBOSOMES

STRUCTURE –

Granular structures that lack a membrane. Ribosomes are synthesized in the nucleus. Both eukaryotic ribosomes (80 S) and prokaryotic ribosomes (70 S) have two subunits.

FUNCTION –

• They manufacture proteins.

VACUOLE

STRUCTURE –

The vacuole is a membrane bound space found in the cytoplasm. It is bound by a single membrane called tonoplast. Plants possess a single large central vacuole whereas animal cells have several small vacuoles.

FUNCTION –

• They store food, water, waste and other materials that are not useful for the cell.
• In amoeba the contractile vacuole is important for excretion.

PLASTIDS

Plastids are found in plant cells and in euglenoids.

FUNCTION –

• They contain pigments that impart specific color to plants.
• CHLOROPLASTS – Contain chlorophyll and carotenoid. They perform photosynthesis.
• CHROMOPLASTS – Contain carotenoids. They impart yellow, orange or red color to plants.
• LEUCOPLASTS – Are colorless. They store nutrients. (Amyloplasts – store starch, Elaioplasts – store oils and fats, Aleuroplasts – store proteins).

ENDOSYMBIOSIS THEORY – States that plastids were once free living organisms which later became part of modern cells.

NUCLEUS

STRUCTURE –

The nucleus is surrounded by two membranes – inner and outer, each of which is a phospholipid bilayer. Nuclear pores on the nuclear membrane provide controlled passage between the nucleus and cytoplasm.

Most cells have one nucleus ( exception – red blood cells have none, sieve tube cells of vascular plants have none, some other cells have multiple nuclei). Nucleus contains chromatin which are long strands of DNA. DNA is the genetic material holds instructions to make proteins.

Chromosomes are strands of DNA wrapped around proteins. They can be seen under a microscope only during cell division when they become highly condensed.

Each nucleus has a nucleolus which is responsible for manufacturing ribosomes. Nucleolus is not covered in a membrane. The matrix of nucleus is called nucleoplasm.

FUNCTIONS –

• Controls cell activity (known as ‘headquarters of a cell‘).
• Site of DNA replication and transcription.
• Nucleolus synthesizes ribosomes.

CYTOSKELETON

STRUCTURE –

A network of filamentous structure of proteins present in the cytoplasm.

TYPES –

• Microfilaments
• Intermediate filaments
• Microtubules

FUNCTION –

• Provides shape and support to cell.
• Helps in motility of cell.

CENTROSOME AND CENTRIOLES

STRUCTURE –

A centrosome is composed of two cylindrical structures called centrioles. The two centrioles are arranged perpendicular to each other in a cartwheel like manner. Centrioles consist of nine evenly spaced peripheral fibrils made of tubulin protein. Each peripheral fibril is a triplet.

FUNCTION –

• Centrioles form the basal body of cilia and flagella which facilitate cell movement.
• They form basal body of spindle fibers that give rise to spindle apparatus during cell division.