prof. dr. mohammad ahmed azmi , ph.d

Prof. Dr. Mohammad Ahmed Azmi, Ph.D

A BASIC CELL

Eukaryotic cell Prokaryotic cell

BASIC STRUCTURE OF CELL

A typical cell has two main components

Cytoplasm

Nucleus

PROTOPLASM

Definition:

The fluid living content of the cell.OR

The different substances that make up the cells are collectively called as

PROTOPLASM.

COMPOSITION OF PROTOPLASM

The protoplasm is composed of five basic substances:

WaterElectrolytes Proteins LipidsCarbohydrates

Water:Principal fluid medium of the cell.

Concentration …….. (65% to 80%).

Many cellular chemicals are dissolved in water.

Others are suspended in membranous form.

ELECTROLYTESThe most important electrolytes are: K+ , Mg + + , PO4--, SO4-, HCO3- Little amount are: Na+, Ca++, Cl-Provide in organic chemicals for cellular

reactions Important for the transmission of electro-

chemical impulses in nerve and muscle fibers Intra cellular electrolytes determine the

activity of enzyme-catalyzed reactions

PROTEINSComplex organic nitrogenous compounds

Consisting of polymers of amino acids

Constitute 10-20 % of the cell mass

Types of proteins

Structural proteins

Globular proteins

Nucleoproteins

Structural Proteins

Present in the form of long thin filamentsoProvide contractile mechanism of all muscles

Also organized into microtubulesoProvide cytoskeleton of many organelles

e.g., cilia, nerve axons, mitotic spindles

Extracellularly, fibrillar proteins are found in the form of collagen & elastin fibres of connective tissue, blood vessels, tendons, ligaments e.t.c.

Globular proteinsPresent in the globular form Act as enzymes and participate in chemical

reactions e.g. chemical reaction that splits glucose into the cell providing energy for cellular reactions.

NucleoproteinsFound in the nucleus in the form of RNA &

DNA

LipidsMost important are:

PHOSPHOLIPIDS CHOLESTEROL

Constitute about 2-3 % of cell mass Mainly in soluble in water In addition some cells also contain TRIGLYCERIDES also called neutral fats

Carbohydrates

Constitutes only about 1% of cell mass

Have little structural function in the cell

Stored in ECF in the form of glucose

Small amount are stored in the form of glycogen (an insoluble polymer of glucose supplying energy to cell

Cytoplasm:This is a portion of cytoplasm which surrounds the nucleus

It is peripherally covered by a membrane called cell membrane

Zones of Cytoplasm:Ectoplasm(Cortex)Endoplasm

Ectoplasm (Cortex)Present just beneath the cell membrane.Contains microfilaments mainly composed of ACTIN

Endoplasm:Contains clear liquid portion of cytoplasm

present between the cortex and nuclear membrane

This liquid portion is called CYTOSOL in which inclusions / particles and organelles are

present

Cell membrane / Plasma membrane

Outer covering which envelops the cell

Thin and elastic membrane

Its structure cannot be seen easily under light microscope

About 8-10 nm in thickness

Composition of Cell membrane

StructureThe basic structure is LIPIDLipid molecules form a bi-layer

Lipid bi-layer is composed of:PHOSPHOLIPIDCHOLESTROL

One part of both PL and chol molecule is water soluble……HydrophilicOther part is fat soluble……Hydrophobic

The PO4 radical of PL is hydrophilic

& fatty acid radical is hydrophobic .

The hydroxyl (OH) radical of cholesterol is hydrophilic

The steroid nucleus is hydrophobic

The fatty portions (Hydrophobic) occupying the centre of the membrane

The hydrophilic portions are directed towards the inner and outer surfaces of the lipid bi-layer

Membrane Proteins

Integral proteins

These protrude all the way through the membrane.

Many integral proteins provide structural channels or pores through which water-soluble substances especially ions can diffuse between ECF & ICF.

Others act as carrier proteins specially transporting substances in and out of the cell.

Peripheral Proteins

Do not penetrate the membrane but attached only to the surface on the membrane .

These mainly act as enzymes.

Membrane CarbohydrateThese occur in the form of glyco-lipids and glyco-proteins

The entire surface has a loose layer of carbohydrate called GLYCOCALYX . These have several functions.

Many are negatively charged that provides over all negative charged surface that repel other negative objects

Many cells are attached to each other by these carbohydrate moieties

Many act as receptor substances for binding hormones

Some are involved in immune system

Presence of Channels and Pumps

Na+ - K+ leak channelThe cell membrane contains protein channels, called leak channels

They allow Na+ or K+ to leak down their concentration gradients INTO or OUT OF the cell.

Cell membranes are considerably more permeable to K+ than to Na+ because they have many more K+ leak channels than Na+ leak channels.

Na+ - K+ Electrogenic Pump

The Na+/K+ Pump creates a concentration gradient by moving 3 Na+ out of the cell and 2 K+ into the cell.

The movement of ions occur against concentration gradient.

This pump requires energy in the form of ATP during the movement of ions.

Gating of Protein ChannelsVoltage gated - Na+ channel

At – 90mv the gate remain closed from outside.When potential changes from -90 to +35mv, conformational changes suddenly occurs and the gate opens

During this time large amount of Na enters in to the cell. This causes depolarization

Voltage gated – K+ channel

At-90mv the gate remain closed from inside of the cell.

When potential changes from -90 to +35mv, the gate opens slowly to cause the out flux of K+

to the exterior of the cell.

Carrier Protein Channel

A substance cannot pass through the membrane whiteout specific binding with the receptor site.

After binding conformational changes occur and the channel open and the substances in to the cell

Ligand Gated ChannelChannels open by binding of an other molecule with the protein.

Conformational changes occur in the protein molecule that opens or closes the gate. This is called ligand gatingsubstance that binds with the protein is called ligand Best example is the effect of acetylcholine also called acetylcholine channel.

Plays an important role in the transmission of signals from :

nerve cell …………………………………… nerve cell

nerve cell …………………………………… muscle cell

Ca++ Pump

Calcium ions play a crucial role in the metabolism and physiology of eukaryotes.

Normally maintained at very low concentration in the intra cellular cytosol Two calcium pumps are involved

Present in the cell membrane …………….. Pumps calcium to the out side of the cell

Pumps calcium into one or more intra vesicular organelles of the cell

In both cases carrier protein penetrates the membrane

It acts as ATP-ase that transports ions across the membranes using energy obtained from the hydrolysis of ATP.

Cell Organelles

Prof. Dr. Mohammad Ahmed Azmi, Ph.D.

Membranous OrganellesEndoplasmic Reticulum (ER)

• Network of tubular and vesicular structures which are interconnected with each

other

•It is covered by limiting membrane composed of protein & lipid bi-layer

•Lumen of ER contains a fluid called endoplasmic matrix

•ER forms a link between nucleus and cell membrane

•OR

•Forms connection of cell membrane with the nuclear membrane

•Granular or Rough A granular or Smooth ER

• Ribosomes are attached on the surface of ER

•Synthesis of proteins

•Role in the degradation of worn out cytoplasmic

organelles like mitochondria

• Donot have ribosomes

•Synthesize lipids, steroid hormones

•Also involved in the detoxification of some

drugs

Granular or Rough ER A granular or Smooth ER

Mitochondria• rod-shaped or oval shaped

•Covered by lipid bi-layer membrane

•Outer membrane is irregular & folded forming shelves – cristae

•Inner space of cristae – called mitochondrial matrix

•Matrix contains many enzymes and protein molecules for involved in respiration &

formation of ATP

Power house of cell

Golgi Apparatus• Consists of 5 to 8 membranous sacs usually flattened in structure called cisternae

•Situated near the nucleus

•Present in all cells but larger in those that synthesize & export proteins.

•Proteins move from rough ER to the gogli apparatus where they are packaged into

secretory vesicles called secretory granules

•The seceretory vesicles are stored & when proteins are needed they move to the cell

membrane & so proteins are exported.

Lysosomes• They are oval or spherical membrane bound vesicular organelle

•Found throughout the cytoplasm

•Enzymes formed in rough ER are processed and packed in the form of small

vesicles in the golgi apparatus

•These vesicles are then moved out from golgi apparatus and become the

lysosomes.

Lysosomes

Primary lysosomes

Inactive in nature containing

Hydrolytic enzyme

Formed from Golgi apparatus

Secondary lysosomes

Active containing lysosomal enzymes

proteases, lipases amylases

nucleases

Peroxisomes • Are microvesicular structures like lysosomes.

• They are pinched off from ER & not from golgi apparatus.

• Contain some oxidative enzymes in the phagocytosis of foreign particles

Centrosome and Centrioles

• Membranous bound cellular organelle, cylindrical in shape

•Situated in the centre of cell and close to nucleus

•Responsible for the movement of chromosomes during cell division.

Non-membranous OrganellesRibosomes

• Non-limiting cellular organelle

•Granular and small dot-like structure

•Made up of proteins (35%) and RNA (65%)

•RNA present in ribosomes are called r-RNA

•Ribosomes are responsible for protein synthesis

Attached to ER Free ribosome in the cytoplasm

Responsible for protein of Hb, peroxisome & mitochondria

Protein formation

Cytoskeleton: Consists of:

-Microtubules

-Microfilaments

•Cell organelles that determine the shape of cell & support

•Also responsible for cellular movement and response of cell to external stimuli

Microtubuls

•Spindle fibers

•Cilia

•Flagella

•Actin & myosin