Cell Membrane:

• cell membrane is 8-10nm thick.
• it is a lipid bilayer in which protein molecules are embedded
• bilayer of phospholipid molecules that are amphipathic, i.e. they consist of a polar, hydrophilic (water-loving) head and a non-polar, hydrophobic (water-hating) tail.

Components of cell membrane
1)Lipid component consists of four phopholipids:phosphatidylcholine (lecithin), phosphatidylethanolamine (cephalin),sphingomyelin and phosphatidylserine.cholesterol and glycolipids are also present.

• lipids that constitute bilayer are amphiphilic because of their hydrophobic (nonpolar) chains directed toward the center of the membrane and their hydrophilic (charged) heads directed outward.

• lipid components exhibit asymmetry in which  phosphatidylcholine (lecithin) and  sphingomyelin are located in outer leaflet; phosphatidylserine and  phosphatidylethanolamine (cephalin) are located in inner leaflet.
• cholesterol molecules are intercalated among the phospholipids of the membrane with its hydroxyl group at aqueous interface and remainder of the molecule within the leaflet.

2)Protein component consists of integral and peripheral protein

• peripheral proteins exhibit a looser association with membrane surfaces and can be easily disassociated from the lipid bilayer by changes in ionic strength and ph.

• Integral proteins are directly incorporated within the lipid bilayer.transmembrane proteins are integral proteinsthat span the lipid bilayer,exposing the protein to both extracellular space and cytoplasm.Many transmembrane proteins are also known as receptor proteins.

Receptor Proteins
a)Ion channel linked receptors:they include voltage gated ion channels,mechanical gated ion channels,and neurotransmitter gated ion channel.
b)G protein linked receptors:theses channels work through cAMP pathway or Calcium ion pathway
c)Enzyme linked receptors.

Cytology

Nucleus:
>site of DNA replication and transcription of DNA.
>contains all of the enzymes required for replication,repair for DNA,transcription and processing of precursor RNA molecules.

>enclosed by NUCLEAR ENVELOP and contains NUCLEAR LAMINA,NUCLEOLUS and CHROMATIN.

Nuclear Envelop:
>double membrane containing pores(90nm.
>outer membrane is continuous with endoplasmic reticulum.


Nuclear Lamina:
>lattice network of proteins lamins.
>lamins attach chromatin to inner membrane of nuclear envelop.
>lamins attach chromatin to inner membrane of nuclear envelop.
>participate in breakdown and reformation of  nuclear envelop during cell cycle.
>phosphorylation of the lamin during prophase of mitosis initiates nuclear disassembly into small vesicles.


Nucleolus:
>responsible for rRNA synthesis and ribosome assembly.
>has three zones:
Granular zone-found at periphery,contains ribosomal precursor particles.
Fibrillar zone-centrally located.contains ribonuclear protein fibrils.
Fibrillar center-contains DNA that is not transcribed.

Chromatin:
>complex of histones proteins,DNA,nonhistone proteins.
Forms of chromatin;
1)Heterochromatin:highly condensed and transcriptionally inactive.in a typical eukaryotic cell ~10% of the chromatin is heterochromatin.e.g barr body.
2)Euchromatin:extended form of DNA and transcriptionlly active.in atypical cell ~90% of total chromatin.

Organelles

Ribosomes:
>made of rRNA and protein.
>consists of large(60Svedberg) that is synthesized in nucleolus and small(40Svedberg) that is synthesized in nucleus.
>assembled in nucleus and transported to cytoplasm.

Polysomes: ribosomes often form polysomes which consist of a single mRNA that is being translated by several ribosomes at same time.

Types of Ribosomes
1)Free polysomes:-the sites for the synthesis for protein destined for the nucleus,peroxisomes,or mitochondria.
2)Membrane associated polysomes:-site of synthesis of secretory proteins,membrane protein and lysosomal enzymes.

Endoplasmic Reticulum:
>two forms smooth endoplasmic reticulum(SER) and rough endoplasmic reticulum(RER).

1)Smooth endoplasmic reticulum(SER):
>it's a network of membranous sacs.vesicles and tubules continuous with RER.
>contains enzymes involved in the biosyntheseis of triglycerides,phospholipids and sterols.
Functions of SER:
>detoxification reaction, making componds water solubles by steroids synthesis,hydroxylation reactions and conjugation reaction.
>glycogen degradation and gluconeogenesis:removal of phosphate group from glucose-6-phosphate by enzyme glucose-6-phosphatase.
>reaction in lipid metabolism.
>sequestration and release of clcium reaction.

2)Rough endoplasmic reticulum(RER):
>lipid bilayer continuous with outer nuclear membrane.
>cisternae have studded with ribsomes on the cytoplasmic side.
>RER ssynthesizes proteins so are very prominent in cells that are specialized in synthesis of proteins.

Golgi Apparatus:

>consists of disc-shaped smooth cisternae that are assembled In stacks (dictyosomes).
>have diameter of approximately 1nmicrometer and associated with numerous small
membrane-bound vesicles.
>Golgi apparatus has two distinct faces: . The cis (forming) face is associatedwith the RER. . The trans (maturing) face is often oriented toward the plasma membrane. The transmost region is a network of tubular structures known as the trans-Golgi network.

Functions of Golgi apparatus:
>Proteins and Lipids-The Golgi apparatus is the site of post-translational modification and sorting of newly synthesized proteins and lipids.
>Glycoproteins-Further modification of the carbohydrate moiety of glycoproteins produces complex and
hybrid oligosaccharide chains. This determines which proteins remain in the Golgi apparatus
or leave the Golgi apparatus to become secretory proteins, lysosomal proteins, or part of the
plasma membrane. Two diseasesare caused by a breakdown in this process,I-cell disease and
hyperproinsulinemia.

Lysosomes:

>spherical membrane-enclosed organelles that contain enzymes required for intracellular digestion.

>Lysosomes consist of two forms:
1)Primary Iysosomes:
>formed by budding from the trans side of the Golgi apparatus.
2)Secondary Iysosomes are formed by the fusion of the primary lysosome with the substrate
to be degraded.
>Lysosomes contain approximately 60 hydrolytic enzymes.
These include nucleases for degrading DNA and RNA, lipases for degrading lipids, glycosidases for degrading glycoconjugates (glycoproteins,proteoglycans, and glycolipids), proteases and peptidases for degrading proteins, and a variety of phosphatases.All lysosomal enzymes are acid hydrolases, with optimal activity at a pH of approximately 5.0.The synthesis of the lysosomal hydrolases occurs in the RER; the hydrolases are transferred to the Golgi apparatus, where they are modified and packaged into lysosomes.
Diseases caused by lysosomes are:

Peroxisomes(microbodies):

>single and spherical membrane-limited organelles(0.15-0.5 μm in diameter).
>contains a number of enzymes that form hydrogen peroxide(can damage to cell) which is immediately broken down by catalase.that oxidizes various potentially toxic molecules as well as prescription drugs, particularly in the large and abundant peroxisomes of liver and kidney cells.

>functions of peroxisomes:

• synthesis and degradation of hydrogen peroxide
• ß -oxidation of long-chain fatty acids 
• bile acid and cholesterol synthesis

diseases associated with peroxisomes include:

• zellweger syndrome
• X-chromosome-linked adrenoleukodystrophy
• hyperpipecolatemia.

Mitochondria:

>a double membrane enclosed organelles that have enzymes for ATP production

>0.5–1 μm in diameter and 1-10μm in length

>it contains its own DNA (double stranded circular), and make some of their own protein

Outer membrane:

it is seive like membrane that contains many transmembrane protein Porins that form channels through which small molecules (<5000 daltons) readily pass to enter the intermembrane space from the cytoplasm.

Inner membrane:

it has numerous infoldings called cristea that increase the total surface area.they contain the enzymes for electron transport and oxidative phosphorylation.the number of mitochondria and the number of cristea per mitochondria are proportional to the metabolic activity activity of the cells in which they reside.

Matrix is enclosed by inner membrane that contains

• dehydrogenases
• a double stranded DNA genome
• RNA, proteins and ribosomes
• intramitochondrial granules.

Cytoskeleton


It's a complex network of microtubules, actin filaments (microfilaments), and intermediate filaments.It provides the shape to the cells and also play a role in movement of organelles and movement of entire cell.

Microtubules:

• 25nm in diameter
• Found in all eukaryotic cells
• tubulin polymers-rapid assembly and disassembly of  α- and β-tubulin
• defective in Chediak-Hegashi syndrome
• important in spindle formation during meiosis and mitosis
• involved in intracellular vesicle and organelle transport
• kinesin and dynein-ATPase responsible for generating force that drives
• CLINICAL:The antimitoticalkaloids are useful tools in cell biology (eg, colchicineis used to arrest chromosomes in metaphase and to prepare karyotypes) and in cancer chemotherapy(eg, vinblastine, vincristine, and taxolare used to arrest cell proliferation in tumors).

Intermediate filaments:

• 10nm-12nm in diameter
• they play role in cytoplasmic link between extracellular matrix,cytoplasm and nucleus
• contain several tissue specific proteins like cytokeratins,desmin,vimentin,neurofilaments and glial fibrillary acidic protein

Microfilaments:

• 6nm in diameter
• composed of F-actin and G-actin
• Twotypes of movement:

1. local movement(e.g endocytosis,phagocytosis,cytokinesis and amoeboid movement)-inhibitid by cytochalasin B
2. sliding filament movement actin in almost every cell.