Cell and cell organelles

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CELL AND CELL ORGANELLES Alfna Rahman Assistant lecturer Biochemistry

What is a Cell???All organisms are built from cells.Cell is the functional unit of life.Cells make up all living matter.All cells arise from other cells.Genetic information required during the

maintenance of existing cells and the production of new cells passes from one generation to the next.

Anabolism and catabolism takes place in the cell.

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Types of Cells1. The prokaryotic cells,2. The eukaryotic cells.1) Prokaryotic cells:• Includes bacteria and cyanobacteria.• CHARACTERISTICS:• Has minimum of internal organization and

smaller in size.•No membrane bound organnelles present

•Genetic material not enclosed by a nuclear membrane.•Histones are not present in

prokaryotes.•Respiratory system closely associated

with its plasma membrane.• Sexual reproduction does not involve

mitosis or meiosis.

2)Eukaryotic cells:• Includes protists,fungi,plants and animals

including humans.• Cells are larger in size.• CHARACTERISTICS:•Many internal distinctive membrane bound

structures having specific functions.• Nucleus is the site for informational

components collectively called chromatin.

• Sexual reproduction involves both mitosis and meiosis.•Respiratory site is the mitochondria.• In plant cells,chloroplast converts the

radiant energy into chemical energy.

Cell Organelles1. NUCLEUS2. MITOCHONDRIA3. ENDOPLASMIC RETICULUM4. GOLGI COMPLEXES5. LYSOSOMES6. PEROXISOMES7. CYTOSKELETON8. CYTOPLASM

NUCLEUS• Contains more than 95% of cells DNA and

is the control centre of the eukaryotic cell.i. Nuclear envelope:double membrane

envelope separates nucleus from cytosol.

ii. Nuclear pore complexes:embedded in the nuclear envelope.these control the movement of proteins and the RNAs across the nuclear envelope.

i. Chromatin:DNA in the nucleus is coiled into a dense mass called chromatin,so named because it is darkly stained with certain dyes.

ii. Nucleolus:a second dense mass closely associated with the inner nuclear envelope.

iii. Nucleoplasm:contains various enzymes such as DNA polymerase,RNA polymerase.

Functions of nucleus• DNA replication and RNA transcription takes

place in nucleus• Transcription is the first step in the expression

of genetic information and is the major metabolic activity.• Nucleolus is non membrane bound and

contains RNA polymerase,RNAase,ATPase,and other enzymes,but no DNA polymerase.• Ribosome subunits assemble in nucleolus.

Mitochondrion• Power house of cell.• Number:it varies.some algae contain only

one,whereas protozoan chaos contain half a million.• Size: Vary greatly in size. Diameter of 0.2-

0.8 and a length of 0.5-1.0 seen in a typical mammalian liver cell.• Shape:sausage shaped.

Structure and functions• Bounded by two concentric membranes

which have markedly different properties and biological functions.• Outer mitochondrial membrane:• Consists mainly of phospholipids and

contains little amount of cholesterol.• Also contain many copies of protein called

“PORIN”.

Functions of porins and other proteins

a) These proteins form channels that permit substances with mw less than 10,000 to diffuse freely across the outer mitochondrial membrane.

b) Other proteins carry out various reactions in fatty acid and phospholipid biosynthesis and responsible for some oxidation reactions.

• Inner mitochondrial membrane:•Rich in proteins and the ratio of lipid

to protein is only 0.27:1 by weight.•Contains high proportion of

cardiolipin.• It is impermeable to polar and ionic

substances.

Cristae:

• Inner mitochondrial membrane that is highly folded.• Tightly packed inward folds are called cristae.INNER MEMBRANE SPACE:• Space between outer and inner membrane.MITOCHONDRIAL MATRIX:• Region enclosed by inner membrane.• Contain enzymes needed for citric acid cycle

and fatty acid oxidation,ribosomes,DNA strands.

Functions:•Many enzymes are located here.•Metabolism of

carbohydrate,fattyacid,nitrogen.• ETC and oxidative phosphorylation

enzymes.• It is specialized for the rapid oxidation of

NADH and FAD.• Energy produced is stored as ATP for

further use of energy in the body.

Endoplasmic reticulum• Eukaryotic cells are characterized by several membrane

complexes that are interconnected by separate organelles.• These organelles are involved in protein

synthesis,transport ,modification,storage and secretion.• Varying in size,shape,and amount the ER extends from

cell membrane,coats the nucleus,surrounds the mitochondria and appears to connect directly to golgi apparataus.• These membranes and the aqueous channels they

enclose are called”CISTERNAE”.

TYPES:2

• ROUGH surfaced ER:also known as “ergastoplasm”.they are coated with ribosomes.near the nucleus,this type of ER merges with the outer membrane of the nuclear envelope.• SMOOTH surfaced ER:They donot have

attatched ribosomes.

Functions

a) Functions of Rough ER:• Synthesize membrane lipids and secretory

proteins.• These proteins are inserted through the ER

membrane into the lumen of cisternae where they are modified and transported through the cell.

b) Functions of smooth :• Involved in lipid synthesis.•Modification and transport of proteins

synthesized in the rough ER.

GOLGI APPARATUS

• Also called dictyosomes.• Unique stack of smooth surfaced

compartments or cisternae that make up the golgi complex.• ER is usually closely associated with the golgi

complexes,which contain flattened fluid filled golgi sacs.• It has a “proximal” or “cis” compartment,a

“medial” and a “distal” or “trans” compartment.

Funtions• On the proximal or cis side:golgi complex receive

newly synthesized proteins by ER via tranfer vesicles.• The post translational modifications takes place in

golgi lumen,where the carbohydrates and lipid precursors are added to proteins to form glycoproteins and lipoproteins respectively.• On the distal or trans side they release proteins via

modified membranes called secretory vesicles.• These secretory vescicles move to and fuse with the

plasma membrane where the contents may be expelled by a process called “exocytosis”.

LYSOSOMES• Contains packet of enzymes.• Discovered by belgian biochemist de Duve in 1955.• SIZE:Mean diameter is approximately 0.4

micrometer.surrounded by lipoprotein membrane.• Found in all animal cells,except erythrocytes in

varying numbers and types.• pH: Inside lysosome is less than that of

cytosol.optimum is around 5.Acid phosphatase is a marker enzyme of this organelle.

Lysosomal enzymes1. Proteolytic

enzymes2. Nucleic acid

hydrolysing enzymes

3. Lipid hydrolyzing enzymes

4. Carbohydrate splitting enzymes

• Cathepsins,elastase,Collagenase.• Ribonuclease,DNA

ase• Lipase,fattyacyl

esterases.• Alpha-

glucosidase,hyaluronidase.

PEROXISOMES•Also called microbodies.•Approx:0.5 micron in diameter.•Probably formed by budding from

smooth ER.

Functions:i. They carryout oxidation reactions in which

toxic hydrogen peroxide is produced,which is destroyed by the enzyme catalase.

ii. Liver peroxisomes have an unusually active beta oxidation system capable of oxidising long chain fattyacids.

iii. In plants,peroxisomes present in seeds rich in lipids are called”glyoxysomes”,where fattyacids are degraded to succinate by glyoxylate pathway.

CYTOSKELETON• Comprises of a complex network of fine structures

called:microtubules,microfilaments,microtrabeculae.• Microtubules:long unbranched slender cylindrical

structures made by self-assembly of the heterodimer”tubulin”having mw 50,000.• Assembly and diassembly of the spindle structures

during mitosis,helps in maintaining shape of the cell.• May be involved in transmembrane signals.

•Micro filaments:more slender cylinder like structure made up of contractile protein”actin”.they are linked to inner face of plasma membrane.• Involved in the generation of forces for

internal cell motion.•Microtrabeculae:very fragile tubes that

form a transient network in the cytosol.

CYTOPLASM• Simplest structure of cell.• Organelles freesap is called as cytosol.• No specific structure for cytosol.• High protein content.• Support synthesis of proteins on the RER by supplying

co factors and energy.• Contain free ribosomes in the polysome form.• Contain different types of proteins and ribosomal RNA

or r-RNA.• Exists as 2 subunits and act as the site of protein

synthesis.

Cell and cell organelles

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