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CELL BIOLOGY

Plasma membrane is made of bilayer of phospholipids and the amount of specific phospholipids present in the inner and outer leaflet of the plasma membrane is different. Plasma membrane composition of phospholipids is- phosphatidyl choline, sphingomyelin, phosphatidyl inositol, phosphatidyl ethanolamine and phosphatidyl serine.

Plasma membrane/ cell membrane is a bilayer (two leaflets) of phospholipid membrane that functions to isolate a cell from the outside environment. According to fluid mosaic model (in the diagram) by Singer and Nicholson - plasma membrane structure includes an array of carbohydrates, a variety of proteins and cholesterol intersperse in the sea of 2 layers of phospholipids; these components are not rigid but are dynamic and show fluidity/ dynamic nature.

Different models were given to justify the nature of plasma membrane-
Overton- single lipid layer
Gorter and Grendel - two layers of phospholipid
Davson and Danielli- Protein associated with lipid bilayer
Robertson Model- Rail road track view/ unit membrane
Singer and Nicholson- Fluid Mosaic model

Plant cells are functional and structural units of all plant life under Kingdom Plantae. They are capable of synthesizing own food. Plant cell differ from animal cells in the presence of specialised structures- plastids, rigid cell wall, large vacuole, plasmodesmata and absence of centrosome, centrioles.

Peptidoglycan/ murein is a component of bacterial cell walls. Peptidoglycan structurally is a heteropolysachharide where carbohydrates (N-acetylglucosamine NAG and N-acetylmuramic acid NAM )are conjugated with amino acids (linked to NAM).

Peroxisomes are small, oxidative organelles found in eukaryotic cells. Their structure includes a single membrane of phospholipids enclosing a transient crystalline core. Functions of peroxisomes include- fatty acid oxidation, lipid biosynthesis, hydrogen peroxide metabolism.

N-linked glycosylation is a process that occurs in the lumen of endoplasmic reticulum. During N-glycosylation, a carbohydrate chain is added to the amino acid asparagine of the protein to form glycoprotein. Addition of carbohydrate moieties help in protein folding, protein targeting and during cell-cell interaction.

Passive transport also called passive diffusion is a type of transport where molecules move along their concentration gradient, i.e., from a region of their higher concentration to a region of their lower concentration. This transport does not use cell's energy. The different types of passive transport are- simple diffusion and facilitated diffusion (channel mediated diffusion, osmosis and carrier mediated diffusion).

Nucleus is another double membrane bound organelle which primarily houses the genetic material. As seen the diagram, the nucleus is enclosed by a nuclear envelope which has numerous "nuclear pores". Nucleus also contains, RNA, nucleolus, nucleoplasm, several enzymes and proteins.

Nuclear pore complex (NPC's) is an arrangement of proteins in the nuclear envelope,whose function is to they selectively regulate the passage of molecules into the nucleus from the the cytoplasm and vice-versa. NPC gives access to the nucleus and controls transport of molecules into and out of nucleus.

Cell organelle is cellular structures present in the cell that perform specialized functions. A represented by the chart, organelles maybe bound by a single phospholipid membrane (rough endoplasmic reticulum, golgi apparatus, peroxisome, vacuole, lysosome, smooth endoplasmic reticulum), a double phospholipid membrane (mitochondria, chloroplast, nucleus)or membrane might be absent (ribosomes, cytoskeletal elements, centrosome, centriole).

Nuclei of eukaryotic cells have nuclear compartmentalization of chromatin, RNA, transcription factors and enzymes. The different nuclear compartments are- replication factors/factories, Nuclear speckles, PML bodies, nucleolus and Cajal bodies.

Mitochondria have two membranes- outer mitochondrial membrane and inner mitochondrial membrane (as shown in the diagram).

Microtubules are are largest, cytoskeletal structures, with a hollow structure (25 nm in diameter), made up of dimers of tubulin proteins. Three monomers of tubulin proteins are present- alpha, beta and gamma. Functions of microtubules include- formation of centrioles, attachment to centromere of chromosomes during cell division, providing motility when present in flagella/ cilia and transport of substances from one part of cell to another.

In Eukaryotic cells, Mitochondria are a double membrane bound structures/ organelle and semi-autonomous organelle housing its own proteins, DNA, enzymes, ribosomes. The outer membrane is smooth, whereas the inner membrane is thrown into folds to increase surface area.

The mechanism of microtubule treadmilling is a phenomenon where tubulin subunits are added at the plus end (growing end) of a microtubule and are removed from the minus end (opposite end) simultaneously.

Lipid Rafts or Lipid microdomains are lipid platform with sphingolipids, cholesterol and proteins as its major components. They are highly dynamic and are formed on only outer leaflet of the plasma membrane.

The structural rearrangements- micelle and liposomes, though both spherical structures, have different arrangement of phospholipids.

The phospholipids and proteins present in the plasma membrane are free to move around laterally in that leaflet of the plasma membrane. Lateral diffusion of proteins, as illustrated by the diagram, can be seen by hybridizing a mouse cell and a human cell and then tracking them using specific fluorescent antibodies.

Lysosomes are nicknamed as 'suicide sacs, as they contain hydrolytic enzymes. the structure of lysosomes include single membrane enclosing various hydrolytic enzymes whose functions are- to break down damaged cell organelles, help in digesting material ingested via phagocytosis and to digest the cell during apoptosis.

Inclusion bodies / Cytoplasmic Inclusions/ Elementary Bodies are particles embedded in cytoplasm or freely floating in prokaryotic cells. The flowchart presents commonly found inclusion bodies in bacteria enlisting their functions.

In cell biology, Golgi Apparatus/ Golgi Body is an organelle bound by single membrane, present in eukaryotic cells. The structure of Golgi Apparatus includes a complex of vesicles as seen in the diagram. Golgi apparatus function is sorting, transportation, modification and packaging of proteins and synthesis and modification of lipids and polysaccharides.

Gram negative bacteria are prokaryotic organisms (example: E. coli, Shigella, Salmonella, Heliobacter, etc). the structure of gram negative bacteria is characterized by presence of: a thin peptidoglycan cell wall, outer membrane called Lipopolysaccharide (endotoxin), lipoteichoic acids in its cell wall. Gram negative bacteria appear pink after Gram staining.

Gram positive Bacteria are prokaryotic organisms (example: Lactobacillus, Staphylococus, Clostridiun, Mycoplasma, etc). Gram Positive Bacteria are structurally characterized by presence of a multilayered peptidoglycan cell wall and appears purple after Gram Staining.

Cell walls in bacteria is made up of peptidoglycan.
According to microscopic studies, structure of cell wall peptidoglycan is thin in Gram Negative bacteria (<15nm) because of presence of a single layer. Whereas in Gram Positive bacteria, several layers of peptidoglycan (15-80 nm) are present which make the cell wall thicker.
Gram Negative bacteria also have an outer membrane called LPS-Lipopolysaccharide or endotoxin which is absent in Gram Positive bacteria.

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