By: Mrs. Kalaivani SathishM.Pharmacy

Cell is the structural and funtional unit of all living organism.bacteria are unicellular (consist of single cell),human beings are multicellular (may have upto 10 cells.

Each cell take nutrients and converts energy.a cell also carried out specialized funtions and reproduces and stores its own set of instructions for carrying out these activities.

There are two types of cells called prokaryote and eukaryote cells.procaryote cells are usually independent while eukaryote cells are found in multicellular organisms.

The simplest form and the first type of cells to evolve are prokaryote cells. Prokaryotes are unicellular organisms that do not developed or differentiate into multicellular forms .some bacteria grow in filaments or masses of cells but each cell in the colony is identical and capable of independent existence. they may adjacent to one another because they do not separate after cell division though there is no continuity or communication between them.

Besides lack of nuclear membrane, a prokaryote cell also lacks intracellular organelles. Prokaryotic cells have three architectural region that includes appendages called flagella and vili, cell envelope or cell wall consisting of a capsule and a cytoplasmic region.

Enclosing a cell, there is cell envelope ,which generally consists of a cell wall, covering a plasma membrane .some bacteria have a further covering layer, called capsule.

A cell wall consists of peptidoglycans in bacteria and act as a barrier against the external forces .it prevent the cell from expanding and bursting insides the cell is cytoplasmic region that contains cell genome (DNA), ribosomes and various other inclusion.

Prokaryotes carry chromosomal DNA elements called plasmids, it function such as antibiotics resistance.

Characteristics Prokaryotic Cell Eukaryotic Cell

Size Small (1 – 10 micro meter)

Large (1 – 100 micro meter)

Cell membrane Cell is enveloped by a rigid cell wall.

Cell is enveloped by a flexible plasma membrane.

Sub Cellular Organelles Absent Distinct organelles are found(e.g. Mitochondria, nucleus, lysosomes)

Nucleus Not well defined, DNA is found as nucleoid, histones are absent.

Nucleus is well defined, surronded by a membrane. DNA is associated with histones.

Characteristics Prokaryotic Cell Eukaryotic Cell

Energy Metabolism Mitochondria absent, enzymes of energy metabolism bound to membrane.

Enzymes of energy metabolism are located in mitochondria.

Cell Division Usually fission and no mitosis.

Mitosis

Cytoplasm Organelles and Cytoskeleton Absent

Contains organelles and cytoskeleton.

Eukaryote cells include fungi, animal cells, plant cells as well as some unicellular organisms. it 10 times larger than a prokaryotic cell and can be as much as 1000 times more in volume.

Composition and functions Human body contains different organs such as the

heart, lung and kidneys each one of which performs different funtions.EC also have a set of intracellular components called sub cellular organelles some of it surrounded by a protective membrane. Composition and functions of organelles are described below:

Plasma membraneThe outer lining of a eukaryotic cell is called plasma

membrane. this serves to separate and protect cell from its surrounding environment. it is made up of a double layer of proteins and lipids.

CytoskeletonCytoskeleton is a complex and dynamic component that

acts to organise and maintain cell shape, anchor organells in their place, helps during endocytosis (uptake of material by the cell) and moves intracellular organelles of the cell during the processes of growth and motility. There are a number of proteins associated with cytoskeleton.

CytoplasmInside the cell there is a large fluid –filled space called

cytoplasm. It is like soup within which all the cell organelles .cytoplasm is also a home for cytoskeleton.it contains several salts and is an excellent conductor of electricity ,creating environment for the mechanics of the cell.

Nucleus Nucleus ,also called cells information centre, is the most

conspicuous organelle found in a eukaryotic cell.

It houses cells chromosomes and is a place where almost all the DNA replication and RNA synthesis occur. Nucleus is spheroidal in shape and is separated from cytoplasm by a membrane called nuclear envelope.

Eukaryotic genetic material is more complex and is divided into discrete units called genes. human genetic material is made up of two distinct components called nuclear genome and mitochondrial genome.

RibosomeRibosome is a large complex composed of many

molecules, including RNA and proteins. these are responsible for the processing of genetic instruction carried by mRNA. the process of converting mRNA codon into the sequence of amino acids that make up a protein is called translation. some of the ribosome float free in the cytoplasm, called free ribosome while others are bound to endoplasmic reticulum.

Mitochondria and chloroplastsThese are power generator and self-replicating

organelles. they occur in various numbers, shapes and sizes in the cytoplasm of all eukaryotic cells. Both the organelles are surrounded by a double membrane with an intermembrane space. they have many folding, filling their inner space. they have two functionally distinct membrane systems, inward folds are called cristae.

Chloroplast are similar to mitochondria but are found in plants where they convert light energy (from the sun) into ATP through the process called photosynthesis.

Endoplasmic recticulam and golgi apparatusER is the transport network for molecules. It is present

in two forms called rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER) .

Proteins that are to be exported out of the cell are passes to the Golgi apparatus, also called golgi bodies or Golgi complex .

Lysosomes and peroxisomesThey are also referred as garbage disposal system of the

cell.both are spherical ,bound by a single membrane and are rich in digestive enzymes for degrading proteins, nucleic acids and polysaccharides that work as low pH. an important function of lysosome is to digest foreign bacteria that invade the cell.

They also help in recycling of receptor proteins and other membrane components , help in repair of the damage to plasma membrane.

Peroxisomes function to get the body of toxic substances such as hydrogen peroxide or other metabolites, and contain enzyme for enzyme utilization. higher number of peroxisomes can be found in liver where toxic by-products are known to accumulate. it resemble lysosomes.peroxisomes are self-replicating whereas lysosomes are formed in Golgi complex.

Centrosome Centrosome called cytoskeleton organizer,

produce microtubules of a cell, which is the key component of cytoskeleton .cetrosomes are composed of two centrioles. a single centrosome is present in animal cells. it found in some fungi and algae.

Vacuoles Vacuoles store food and waste products. some

vacuoles also store water and are described as liquid –filled spaces. they are surrounded by a membrane.

Cell membrane It refers to the outer covering of the cell and is

found in all living cells. it is a highly viscous structure.

Fluid-mosaic model of cell membrane Biological membranes have such a structure

where some proteins span a lipid bilayer while others are partially immersed. this is called Fluid-mosaic model as a membrane consists of mosaic of proteins and lipids.

1.cell membrane may be associated with several enzymes e.g. Phospholipase are used as markers of cell membrane.

2. Membranes also contain specific receptors for external stimuli such as hormones. after a hormone binds to its receptor, it generates signals and regulates intracellular activities.

3.carbohydrates components of glycoproteins in a membrane act as recognition sites for several substance such as antibodies.

4. A membrane also acts as a barrier thereby loss of useful substances while permitting entry of nutrients, thus it is important in transport of substances.

Tight junctions are specialized sites of contact that block solutes from diffusing between the cells in an epithelium.

A simple epithelium like the lining of the intestine or the lungs is composed of a layer of cells that adhere tightly to one another to form a thin cellular sheet.

Solutes cannot diffuse freely through this layer because of the specialized contacts referred to as called tight junctions.

Tight junction strands are composed of proteins called occludin and claudins.

Tight junction serves as a barrier to the free diffusion of water and solutes from the extra-cellular compartment to the other side.

some tight junctions are permeable to specific ions or solutes, to which other tight junctions are impermeale.

TJ are also present between the endothelial cells that line walls of the capillaries. it form blood brain barrier, which prevent substance from passing from the bloodstream into the brain. small ions and water may not penetrate BBB.

Cytoskeleton is a system of protein fibbers that pervades cytoplasm. It is composed of three distinct types of fibrous structures called microtubules, intermediate filaments and microfilaments.

Functions: 1.it provides structural support that determines

shape of the cell . 2.it functions as an internal frame work which is

responsible for the positioning of various organelles within the cell.

3.it functions as a force –generating apparatus that moves cells from one place to another e.g .locomotion of sperm, white blood cells.

MICROTUBULES : Microtubules are stiff ,hollow and tubular structures

that are assembled from protein tubulin .these are arranged in longitudinal rows and are called proto-filaments.

They help in determining shape of the cell and maintaining the position of the membranous organelles such as Endoplasmic Reticulum and Golgi Complex.

Intermediate FilamentsThey are strong rope like fibbers, Depending on the

cell type they are composed of a variety of different globular proteins sub units. Examples: Keratin filaments, Neuro filaments.

MicrofilamentsThey are composed of a double helical polymer of the

protein actin, which is one of the major proteins of the muscle cells.

They play a key role in all type of contractility and motility with in cells.

Transport of various substances across a plasma membrane may occur by several mechanisms, which includes, diffusion (passive diffusion), facilitated diffusion and active transport.

Diffusion of a substance may occur either passively (called passive diffusion) or may required some carrier protein, which facilitates diffusion (Called Facilitated diffusion).

Diffusion or passive diffusion refers to a process in which a solute moves down an electro chemical gradient from one side of the membrane to the other.

It is an energy independent process, which takes place by the dissipating its free energy stored in the gradient.

Water moves readily through a semi permeable membrane from a region of lower solute concentration to a region of higher solute concentration. This process is called as osmosis.

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Large polar molecules such as glucose and amino acids, cannot diffuse across the phospholipid bilayer. Also ions such as Na+ or Cl- cannot pass.

These molecules pass through protein channels instead. Diffusion through these channels is called FACILITATED DIFFUSION.

Movement of molecules is still PASSIVE just like ordinary diffusion, the only difference is, the molecules go through a protein channel instead of passing between the phospholipids.

To move substances against a concentration or electrochemical gradient, a cell must use energy. Active transport mechanisms do just this, expending energy (often in the form of ATP) to maintain the right concentrations of ions and molecules in living cells

Primary Active Transport – these are found in all living organisms and require an energy source such as ATP for the translocation of the cations.

These transporters since utilize ATP are also referred to as ATPases.

Secondary Active Transport – they utilize a secondary source of energy such as trans – membrane electro chemical gradient of either Na+ or H + to translocate small organic molecules e.g. Glucose and Amino Acids etc.

When membrane is freely permeable to ions (Na+, Cl-) and if the concentration of ions on both the sides is different, the ions freely diffuse to attain equal concentration.

Gibbs –Donnam observed that the presence of a non-diffusible ion on one side of the membrane alters the diffusion of the diffusible ions.

In Sodium Proteinate (Na+Pr-) the protein (Pr-) ion is non-diffusable through the membrane.

In a compartment separated by a membrane. Initially, sodium proteinate is on side – 1 while sodium chloride is on side – II.

Diffusible ions (Na+ Cl-) can freely pass through the membrane, on side 1. Na + ions will balance the incoming Cl- ions besides pr- ions, while on side – II Na + ions have to balance only cl- ions.

Therefore the concentration of Na+ on side 1 is greater than on side – II. However, from the thermodynamical point of view , at equilibrium, the concentration of Na+Cl- on both sides should be the same.

Thus Na+ Cl- 1 = Na+ Cl- Side – II Na+ 1 > Na+ Side – II Cl- 1 < Cl Side – IIConsequently, the concentration of Cl- ions

should be greater on side – II. Further, the total concentration of ions on side 1 is higher than on side II,

The presence of a non-diffusible ion influences the concentration of diffusible ions across the membrane.

The concentration of oppositely charged (Na+) is greater on the side of membrane containing non diffusible ions (Pr-)

The concentration of simillarly charged ions (Cl-) is higher on the side of the membrane not containing non – diffusible ions (Pr-).

The net concentration of total ions will be greater on the side of the membrane containing non – diffusible ions. This leads to a difference in the osmotic pressure on either side of the membrane.