Saturday, April 18, 2020

=》Organization of Cell
• Contains two parts
i) Nucleus: Separated from cytoplasm by nuclear membrane
ii) Cytoplasm: Separated from surrounding fluid by plasma membrane
• Protoplasm:- Substances that make up cell..
e.g. i) Water, ii) Electrolytes, iii) Lipids, iv) Proteins, v) Carbohydrates

1)=> Water
• Principal fluid medium of cell
• Present in most cells except Fat Cells
• 70-85% of cell
• Contains dissolved chemicals (take part in reactions) & suspended particles (places of reactions)

2)=> Ions
• Inorganic chemicals for cellular reactions
• Necessary for cellular control mechanisms
e.g. Transmit impulses in nerves and muscle fibers

3)=> Proteins
• Abundant in cell after water
• 10-20% of cell
• Two types
• i) Structural Proteins
->Long filaments & polymers
->Form microtubules, which provide cytoskeleton of
-Cilia
-Nerve Axons
-Mitotic Spindles (Cell Division)
-Tangled Mass (holding cytoplasm & nucleoplasm in their compartments)
->Fibrillar Proteins
-Found outside cell as collagen & elastin of blood vessel wall, tendon, ligaments,etc.
• ii) Functional Proteins
-> Tubular-globular
-> Mainly enzymes
-> Often mobile
-> Adherent to membranous structures
-> Come in contact with substances & catalyze
-> Split glucose into its constituents
-> Provide energy for cellular functions

4)=> Lipids
• Heterogenous group of compounds
• Soluble in fat solvents
• Phospholipids & Cholestrol
-> 2% of total cell mass
-> Due to water insolubility form membranes
• Triglycerides
-> Neutral fats
-> 95% of fat cells (store-house of energy giving nutrients)

=> Carbohydrates
• Minor structural function (Glycoprotein) but major nutritive function (Glucose).
• %age
-> Average cell (1% of total mass)
-> Muscle cell (3% of total mass)
-> Liver cell (6% of total mass)
• Readily available in ECF as Glucose
• Stored as Glycogen in cells
-> A glucose polymer
-> Depolymerizes when needed for energy

PHYSICAL STRUCTURE OF THE CELL
=> Introduction
• Organelles
-> Organized physical structures
-> Important to cell as Mitochondrial absence results in cessation of 95% of cell's energy releasing from nutrients.

=> MEMBRANES OF CELL
• Composed mainly of Lipid & Proteins
• Include membranes of
-> Cell
-> Nucleus
-> Endoplasmic Reticulum
-> Mitochondria
-> Lysosomes
-> Golgi apparatus.
• Lipid
-> Prevents passage of water & water-soluble substances

• Proteins (Penetrating Membrane)
-> Pathways/Pores for passage of specific substances
-> Catalyse (enzyme) chemical reactions

=> Cell Membrane (Plasma Membrane)
• Envelops cell
• Thin, pliable & elastic
• 7.5-10nm thick
• Composed mainly of Lipid & Proteins
• Composition
-> Proteins 55%
-> Phospholipids 25%
-> Cholesterol 13%
-> Other lipids 4%
-> Carbohydrates 3%

• Lipid Barrier
->Lipid bilayer
>Two layered lipid film
>Each layer = 1 molecule thick
>Continuous on cell surface
>Large globular proteins interdigitate
->Composed of
1) Phospholipids
> Abundant lipids
>Two ends
- Hydrophilic end
_Water soluble
_Fat insoluble
_Phosphate portion
_Constitute two surfaces, one faces intracellular water and other faces extracellular water.
- Hydrophobic end
_Water insoluble
_Fat soluble
_Fatty acid portion
_Repelled by water but attracted to each other in middle of membrane
_Impermeable to water-soluble substances, such as ions, glucose, and urea.
_Permeable to fat-soluble substances, such as oxygen, carbon dioxide, and alcohol.

2) Sphingolipids
> Derived from amino alcohol sphingosine
> Have hydrophobic and hydrophilic groups
> Found in less quantity especially in nerve cell membranes
> Functions
-Protection from harmful environmental factors
-Signal transmission
-Adhesion sites for extracellular proteins

3) Cholesterol
> Contains fat soluble steroid nuclei
> Dissolved in bilayer
> Determines degree of permeability (Or Impermeablility) of water-soluble substances
> Controls fluidity of membrane

• Proteins (of Membrane)
-> Globular masses floating in lipid bilayer
-> Mainly glycoproteins
-> Two types
1) Integral Proteins
> Protrude all way through membrane
> Act as structural channels (or pores)
-Through which water and water-soluble substances (e.g.ions) diffuse
-Allow selective diffusion
>Act as carrier proteins
- Transport substances (Which don't otherwise/in any other way cross lipid bilayer)
- Transport substances opposite to their diffusing gradients (Active Transport)
> Act as Enzymes
> Act as Receptors
- For water-soluble chemicals like peptide hormones
- Ligands bind to receptor causing conformational changes in receptor. This induces interactions between receptor and cytoplasmic proteins (second messengers) that transmit signals from cell's exterior to interior.
2) Peripheral Proteins
> Attached to integral proteins on one surface of membrane
> Act as enzymes or transport controllers

• Carbohydrates (of Membrane)
-> Exist as glycoproteins (with most of integral proteins) or glycolipids (with 1/10 of Lipids) or Proteoglycan's (CHO+Protein Core). Glyco portions protrude outside and swing loosely.
-> Outside surface of cell with a loose carbohydrate coat is called glycocalyx.
-> Functions
1. Have -ve charge & repel other -vely charged substances
2. Attach cells to one another
3. Act as receptors for binding hormones,
(e.g. Insulin).
- Binding activates internal proteins that activate intracellular enzaymes
- Some carbohydrate moieties enter into immune reactions


=> CYTOPLASM AND ITS ORGANELLES
• Contains minute and large particles, and organelles
• Contains
-> Neutral Fat Globules, Glycogen granules, Ribosomes, Secretory Vesicles.
-> Five important organelles; Endoplasmic reticulum, Golgi apparatus, Mitochondria
• Cytosol
-> Jelly-like fluid portion of cytoplasm with dispersed particles
-> Contains dissolved proteins,
electrolytes, and glucose.

• Endoplasmic Reticulum
-> Network of tubular and flat vesicular
structures
-> Processes molecules made by cell and transports them to their specific destinations inside or outside cell.
-> Tubules and vesicles
> Interconnected
> Their walls' composition resembles cell membrane but 30-40 times greater in total surface in liver cells.
> Their inner space
- contains endoplasmic matrix, a watery medium, and continuous with space between two layers of nuclear membrane.
- Allows entry of cellular chemicals and directs them to specific areas
> Attached enzyme systems provide machinery for metabolic cellular functions
-> Granular Endoplasmic Reticulum
> Ribosomes
- Minute granular particles attached to outer surfaces
- Composed of RNA and proteins
- Involved in protein synthesis
> ER with Ribosomes is termed as GER.
-> Agranular Endoplasmic Reticulum
> ER without Ribosomes is termed as AER.
> Involved in synthesis of lipids and in processes promoted by intrareticular enzymes

• Golgi Apparatus
-> Closely related to ER
-> Membrane resembles AER
-> Four or more stacked layers of thin, flat, enclosed vesicles lying near one side of nucleus.
-> Prominent in secretory cells and located on their secretory portion.
-> ER vesicles continually pinch off
from ER then release substances by fusing with Golgi apparatus.
-> Substances are processed to form lysosomes, secretory vesicles, and other
cytoplasmic components.

• Lysosomes
-> Vesicular organelles
-> Form by breaking off from Golgi Apparatus then disperse in cytoplasm.
-> Provide an intracellular digestive system that digests
(1) Damaged cellular structures
(2) Ingested food particles
(3) Unwanted matter such as bacteria
-> 250-750 nm in diameter (nm D)
-> Surrounded by lipid bilayer membrane
-> Contains small granules (5-8nm D) having 40 different hydrolase (digestive) enzymes.
-> Hydrolytic Enzymes
> Split organic molecules into two or more parts by combining H and OH portions of H2O with different parts of a compound. e.g.
- Protein --------》Amino acids
- Glycogen --------》 Glucose
- Lipids --------》 Fatty acids & Glycerol.
> Concentrated in Lysosomes and prevented by membrane from digesting cellular structures.
> When membrane breaks then come in contact with organic substances and split them to small & diffusable parts like Glucose and Amino Acids

• Peroxisomes
-> Physically resemble lysosomes but different in two ways:
1) Formed by self replication or by budding from AER (SER) not Golgi Apparatus
2) Contain Oxidases not Hydrolases
-> Oxidases combine O2 with H ions to form H2O2 (An oxidizing substance) used with Catalase (An oxidizing enzyme).
- Oxidize substances that might be poisonous to cell
-> Half of drunk alcohol is detoxified into acetaldehyde by peroxisomes of liver cells
-> Catabolize long chain fatty acids
• Secretory Vesicles
-> Involved in secretion of substances
-> Formed by ER-Golgi apparatus system then released into cytoplasm from Golgi Apparatus as storage vesicles (Secretory Vesicles / Secretory Granules).
-> Store proenzymes (Inactivated enzymes) in pancreatic acinar cells.
> Secreted by outer cell membrane into pancreatic duct then into duodenum then become activated and digest food.

• Mitochondria
-> Extract energy from nutrients for cellular functions, so termed as Powerhouses of cell.
-> Number varies from less than a hundred to several thousand as per cell physiology e.g.
Cardiomyocytes > Adipocytes
-> Located in active portions of cell
-> Variable in size and shape e.g.
From few hundred nm D & globular to 1 ųm D & 7 ųm length
-> Composed of two lipid bilayer membranes
1) Outer membrane
2) Inner membrane
-> Cristae
> Tubular structures formed by infoldings of inner membrane
> Sites
- For attachement of oxidative enzymes
- For occurrence of chemical reactions
-> Matrix
> Occupies inner cavity of mitochondrion.
> Contains enzymes that extract energy from nutrients.
-> Cristae and Matrix enzymes together cause oxidation of nutrients by forming CO2+H2O and releasing energy. This energy is used to form ATP. ATP diffuses through out cell and releases energy for cellular functions.
-> Self-replicative i,e; one can form another as per ATP need (as per cell physiology). Because of presence of DNA.


• Cell Cytoskeleton
-> Network of fibrillar proteins organized into filaments or tubules
-> Ribosomes synthesize precursor proteins that polymerize to form filaments. e.g.
~ Ectoplasm
> Actin filaments in outer zone of cytoplasm
> Elastic support for cell membrane

~ Muscle Contractile Machine
> Formed by actin & myosin organization
> Cause muscle contraction

~ Microtubules.
> Composed of polymerized tubulin
> Strong filaments
> Found in flagellum of sperm, center of cilium, centrioles and mitotic spindle.
-> Functions
> Provides rigid physical structures for cell
> Determines cell shape & participates in cell division
> Allows cells to move
> Provides a track for movement of organelles within cell

• Nucleus
-> Controls center of cell
-> Sends messages to cell to grow and mature, to replicate, or to die
-> Contains DNA, comprising of genes.
-> Genes
> Determine characteristics of proteins like structural proteins and enzymes that
control cytoplasmic and nuclear activities.
> Control and promote reproduction of cell
> Mechanism
- Firstly two identical genes are formed. Mitosis forms two daughter cells each of which carries one set of DNA genes.
-> Chromatin Material
> Darkly staining during interphase
> Organized into Chromosomes during mitosis
-> Nuclear Membrane (Nuclear Envelope)
> Composed of two lipid bilayer membranes
1) Outer membrane
- continuous with outer membrane of ER
2) Inner membrane
> The space b/w nuclear membranes is continuous with space b/w ER membranes.
> Contains thousands of nuclear pores
- Protein substances attach to pores and decrease diameter to 9nm.
- Molecules of 44,000 MW can pass

-> Nucleoli
> Staining structures in nuclei of cells
> Lacks a membrane
> Contain RNA & ribosomal proteins
> Enlarge during protein synthesis
> Form within nucleus whereas Ribosomes form within cytoplasm
> DNA genes synthesize RNA, some of which are stored in nucleoli but mostly transported to cytoplasm
> Here along with specific proteins assemble mature Ribosomes for forming cytoplasmic proteins.


=> COMPARISON OF ANIMAL CELL
WITH PRECELLULAR FORMS OF LIFE
• It is believed that animal cell has evolved (Million years back) from a virus like simple organism.
-> 15 nm: Small virus
-> 150 nm: Large virus
-> 350 nm: Rickettsia
-> 1 µm: Bacterium
-> 5-10 µm+: Cell
• (1) Small Virus
-> 15 nm
-> Nucleic acid
> Embedded in protein coat
> Composed of DNA & RNA
> Self-Replicative
> Makes virus a living structure by propagating its lineage.
• (2) a Large Virus
-> 150 nm
• (3) a Rickettsia
-> 350 nm
• (4) a Bacterium
-> 1 µm
• (5) a Nucleated Cell
-> 5-10 µm+
-> 1000 times diameter of small virus
-> 1 Billion times volume of small virus
• Evolution of Life
~ As life evolved
-> More chemicals and specialized functions developed
-> Formation of membrane with a fluid matrix
-> Specialized chemicals developed
-> Enzymes appearance with catalysis
-> Organelles developed
> Rickettsial and Bacterial stages
> More efficient than dispersed chemicals
-> Complex organelles developed
> Nucleated cell stage
> Like Nucleus
- Distinguishes animal cell from lower forms of life
- Control center for cellular activities
- Provides reproduction of new cells


=> FUNCTIONAL SYSTEMS OF THE CELL
• ENDOCYTOSIS
~ "INGESTION BY CELL"
~ "Large particles enter cell by specialized function of cell membrane"

-> The life, growth and reproduction of cell need nutrients that pass membrane by diffusion or active transport.
-> Diffusion
> "Movement of molecules from area of high conc. to area of low conc. due to random molecular motion."
> Through Channels/Pores (Water-soluble Substances )
> Through Lipid matrix (Fat-soluble Substances)
-> Active Transport
> Carrying of a substance through membrane by carrier type of integral proteins (that protrudes all the way through membrane).
~/-> Two forms of Endocytosis

1) Pinocytosis (Cell Drinking)
> Ingestion of minute particles
> Vesicles of extracellular fluid
> Occurs continually
> Occurs in most cells but rapid in some cells e.g.
-In macrophages 3% of membrane is engulfed as vesicles/minute
> Small vesicles
- 100-200nm D
- Seen only by EM
> Pinocytosis = Macromolecule Attachment
> Way of entry of large protein molecules
> Mechanism:
- Specific protein molecules attach to specific receptors (specialized proteins) in small pits (coated pits) on membrane.
- Inside of pits contains Clathrin (Fibrillar Protein), Actin and Myosin (Contractile Protein Filaments).
- After binding of protein molecules with receptors, pit invaginates with fibrillar proteins closing the borders over attached proteins and small ECF fluid.
- Invaginated portion breaks away and forms pinocytotic vesicle.
- Exact cause of formation is not yet clear.
- Energy is supplied by ATP
- Calcium ions of ECF react with Contractile Protein Filaments that provide force for pinching the vesicle.

2) Phagocytosis (Cell Eating)
> Ingestion of large particles (Like bacteria, cells, or portions of degenerating tissue)
> Occurs in same way as pinocytosis except involving large particles than molecules
> Occurs in few cells like Macrophages and WBCs
> Initiates due to binding of a bacterium, dead cell or tissue debris on surface receptors of phagocyte
In case of a bacterium binding occurs between phagocyte receptors and antibody already attached to bacterium for purpose of dragging it to phagocyte receptors ( Termed as Opsonization).
> Mechanism:
- Membrane receptors attach to surface ligands of particle.
- Membrane evaginates around particle with receptors attaching to more surface ligands of particle then closed phagocytic vesicle is formed
- Contractile fibrils contract around outer edge of phagocytic vesicle then pinch and push vesicle to cell interior.

• Lysosomes Digest Endocytosed Foreign Particle
-> Digestive Vesicle
> (Digestive organs of cell)
> Lysosomes attach to endocytosed vesicle and release acid hydrolases forming a Digestive Vesicle.
> Hydrolyses CPLN into AA, FA and Glucose that diffuse into cytoplasm
> Residual Body
- Digestive vesicle with indigestible substances.
- Excreted by cell through exocytosis

-> Regression of Tissue
> Process of reduction of tissue mass
e.g.
- In uterus after pregnancy
- In muscles during long periods of inactivity
- In mammary glands at end of lactation
> Due to lysosomal activity

-> Autolysis of Cell
> Lysosomes remove
- Damaged cell
- Damaged portions of cell
> Cell damage (due to heat, cold, trauma & chemicals) ruptures lysosomes
> Hydrolases digest organic substances
> In slight damage cell portion is removed and repaired
> In severe damage whole cell is removed (Autolysis) and replaced by new cell formed by mitotic division of adjacent cell.

-> Lysosomes Bactericidal Action
> Contain bactericidal agents that kill phagocytized bacteria
> Two types of agents
1) Lysozyme :-
- Dissolves bacterial cell membrane.
2) Lysoferrin :-
- Binds iron and other substances before they can promote bacterial growth.
3) Acid with 5.0 pH
- Activates hydrolases and inactivates bacterial metabolic systems

-> Autophagy
> "Recycling of Cell Organelles"
> Literally means, “to eat oneself.”
> Process in which obsolete organelles and proteins degrade and recycle
> Autophagosomes
- Transfer worn-out cell organelles to lysosomes
- Double membrane structures
- Formed in cytosol
> Mechanism:-
Lysosomal membrane invaginates and forms vesicles for taking organelles into lysosomal lumen where get digested and nutrients are reused.
> Importance:-
- Contributes to routine turnover of cytoplasmic components
- Develops tissue and gives survival to cells during nutrient scarce conditions
- Maintains homeostasis
- In liver cells, Mitochondrion has life span of 10 days

=> Functions of Endoplasmic Reticulum
• Contains lipid bilayer membrane with protein enzymes that catalyze substance synthesis
• Synthesis begins in ER then products pass to Golgi apparatus where undergo processing and release into cytoplasm
• GER forms proteins
-> Contains ribosomes on outer surface
-> Ribosomes synthesize protein molecules and extrude them to cytosol and endoplasmic matrix.
• SER forms lipids
-> Synthesizes lipids like Phospholipids and Cholesterols which incorporate into & extend lipid bilayer of ER
-> ER growth remains in limit by continuous breaking of vesicles
• Other Functions of ER (Mostly of SER)
1. It provides enzymes that control glycogen breakdown when glycogen is used for energy.
2. It provides enzymes that detoxify drugs by coagulation, oxidation, hydrolysis, and conjugation with glycuronic acid.

=> Functions of Golgi Apparatus
• Synthesizes carbohydrate polymers not synthesized by ER like Hyaluronic acid and Chondroitin sulfate which are Involved in migration and proliferation of cells and are major components of:
-> Proteoglycans secreted in mucus and other glandular secretions
-> Ground substance, acting as fillers between collagen fibers and cells
-> Organic matrix in both cartilage and bone
• Processing and Vesicle Formation :-
-> Major function of ER and Golgi Apparatus
-> Substances (e.g. Proteins) form in GER then move to SER where pinch off to form vesicles that diffuse into deepest layer of Golgi apparatus and release substances.
-> Substances
> Receive carbohydrate moieties
> Compact into highly concentrated packets
> Pass to outer layers undergoing compaction and processing
> Finally pinch off as Golgi vesicles and diffuse throughout cell
-> Timing of these processes :-
> A glandular cell is bathed in
radioactive amino acids
> Radioactive protein molecules are detected in
- GER within 3-5 min.
- Golgi apparatus within 20 min
- Cell surface secretions within 1-2 hours

• Types of Vesicles :-
-> 2-types
1) Secretory Vesicles
2) Lysosomes
-> Some vesicles are destined for intracellular use.
-> In secretory cells, Golgi apparatus mainly forms secretory vesicles.
> First diffuse to cell membrane
> Then after calcium ions interaction, fuse with cell membrane and release substances to exterior by exocytosis
- Opening of membrane's outer surface for extrusion of contents

( Way of calcium ions interaction with vesicular membrane to cause fusion is not yet known.)
• Intracellular Vesicles Replenish Cellular Membranes
-> Golgi vesicles fuse with cell and organelles' membranes like mitochondria, ER, and extend them. Thus, replenish them as they're used in endocytotic vesicles.
-> So, Golgi apparatus and ER form intracellular structures and secretory substances.

=> MITOCHONDRIA
(EXTRACT ENERGY FROM NUTRIENTS)

• Cells extract energy from foodstuffs (like Glucose, AA, FA) after reacting with oxygen
• GIT and Liver convert organic nutrients into their monomers before undergoing cellular metabolism e.g.
-> Carbohydrates ----> Glucose
-> Proteins ----> Amino Acids (AA)
-> Fats/Lipids ----> Fatty Acids (FA)
• Mechanism:-
-> Foodstuffs
> Enter cell
> React with oxygen
-> Enzymes control reaction and channel released energy in proper direction
-> Reactions occur in mitochondria
-> Released energy is used to form ATP
-> ATP is used to energize intracellular metabolic reactions.

=> Functional Characteristics of ATP
• A nucleotide
• Composed of
1) Nitrogenous base adenine
2) Pentose sugar ribose
3) Three phosphate radicals
• Last two phosphate radicals are connected by high energy phosphate bonds
-> Represented by ~
-> Contain 12,000 cal./mole of ATP
-> Very labile and splits instantly on demand
• Mechanism:-
-> ATP releases energy by splitting into phosphate radical (Pi) and ADP
-> Released energy is used to energize cellular functions like synthesis of substances and muscular contraction.
-> Phosphate radical (Pi) and ADP gain energy from cellular nutrients by recombining into ATP
-> And Entire process is repeated
-> So, ATP is energy currency of cell because it can be spent and remade continually with turnover of few minutes.

=> Chemical Processes in Formation of ATP (Role of Mitochondria)

• After entering cell, Glucose is converted into Pyruvic Acid through Glycolysis
-> Few ADP convert to ATP
-> 5% of cell energy metabolism
-> 95% of ATP forms in Mitochondria

• Pyruvic acid (from carbohydrates), fatty acids (from lipids), and amino acids (from proteins) convert into acetyl-coenzyme A (CoA) in mitochondrial matrix.
-> Undergoes dissolution through Citric Acid Cycle (Krebs Cycle) for extracting energy.
> acetyl-CoA splits into H atoms and CO2
> CO2 diffuses out of mitochondria then out of cell and finally excreted through lungs
> H atom combines with oxygen giving tremendous release of energy that forms ATP
Mechanism
- Protein enzymes (ATP Synthetase) are integral part of cristae that protrude into mitochondrial matrix
- Firstly an electeon is removed from H atom, converting it into hydrogen ion
- Lastly hydrogen ions combine with oxygen to form water+tremendous release of energy to ATP synthetase. Which uses energy to form ATP from ADP.
- ATP is transported out of mitochondria into cell cytoplasm and nucleoplasm, where energizes cell function.
- This whole process is called chemiosmosis (ATP formation).

• Uses of ATP in Cellular Function
~ATP is used mainly in three functions
(1) Transport e.g.
-> Na, K, Other ions and organic substances transport through membrane
-> Accounts for 80% of ATP in renal tubular cells

(2) Synthesis e.g.
-> Protein ( by ribosomes), Phospholipids, cholesterol, purines, pyrimidines, etc.
-> Protein is composed of amino acids linked by peptide linkages. One such linkage requires breakdown of 4 high energy bonds.
-> Accounts for 75% of energy in some cells especially during growth phase.

(3) Mechanical work e.g.
-> Muscle contraction, ciliary and ameboid motion.
• ATP in a Nut Shell (Summary)
-> Always available to release energy rapidly when needed
-> Alternative chemical reactions are slower
-> Formed by breakdown of CPLN
-> More than 95% is formed by Mitochondria so termed as PowerHouses of Cell

=> LOCOMOTION OF CELLS
• Muscle cells constitute 50% of body mass and have most obvious movement.
• Two other movements include
-> Ameboid Locomotion
-> Ciliary Movement
• AMEBOID MOVEMENT
-> Cell movement in relation to surroundings e.g. WBCs movement through tissues.
-> Resembles amebae movement
-> Pseudopodium
> Protrudes from one end
> Projects away from cell body & secures new tissue area
> Now cell remainder is pulled to pseudopodium
-> Mechanism
> At leading edge of pseudopodium new membrane is formed continuously by fusion of exocytotic vesicles
> In mid and rear portions of cell old membrane is absorbed continuously by formation of endocytotic vesicles
> Exocytotic vesicles are lined by receptors proteins. After fusion, Proteins attach to ligands in surrounding tissue and pull cell remainder to pseudopodium.
> At opposite end of cell, receptors pull away from ligands and form new endocytotic vesicles which move to pseudopodial end and form new membrane for pseudopodium
> Ameboid cells contain an actin protein in cytoplasm. Actin polymerizes to form filamentous network in enlarging pseudopodium and contracts after binding to myosin by utilizing ATP.
> Contraction also occurs in ectoplasm of cell body due to presence of an actin network.

-> Types of Cells exhibiting ameboid locomotion
> When WBCs move from blood to tissues to form macrophages
> Fibroblasts move to damaged area for repairing.
> Germinal cells of skin, ordinarily sessile, move to cut area for repairing the opening
> Embryonic cells move long distances from origin sites to new areas


-> Control of Ameboid Locomotion
(Chemotaxis)

> Chemotaxis:-
"The process which initiates ameboid locomotion due to appearance of chemical substances in tissues is called Chemotaxis."
> Chemotactic Substance:-
"The chemical causing chemotaxis is called chemotactic substance."
> Positive Chemotaxis:-
- When movement occurs towards source of chemotactic substance.
- Towards chemical gradient
> Negative Chemotaxis:-
- When movement occurs away from source of chemotactic substance.
- Away from chemical gradient
> Exact mechanism of control by chemotaxis is not yet known:-
- However, known facts elaborate that Cell side exposed to chemotactic substance develops membrane changes that cause pseudopodial protrusion.


=> CILIA AND CILIARY MOVEMENTS
• Whip like cilia movement on cell surface
• Occurs in two places
1) On surfaces of respiratory airways
2) On inside surfaces of uterine tubes
• In nasal cavity and lower respiratory airways moves mucus layer at 1 cm/min toward pharynx for removing trapped particles
• In uterine tube, moves fluid from uterine tube ostium to uterus cavity for transporting ovum
• Cilium
-> Sharply pointed straight or curve hair projecting 2-4 ųm from cell surface.
-> Many cilia project from a single cell
e.g. 200 cilia on epithelial cell in respiratory airway.
-> Covered by cell membrane protrusion
-> Supported by
- 11 microtubules & 9 double tubules around periphery of cilium
- 2 single tubules in center
-> An outgrowth of its basal body that lies beneath cell membrane
-> Moves forward with a sudden bending at cell surface (Rapid whiplike stroke 10 to 20 times per second.).
-> Then moves backward slowly to initial position.
-> Rapid forward-thrusting pushes fluid
in forward direction
-> Slow backward movement has almost no effect on fluid movement.
-> Fluid continually propels in fast stroke direction. Because cells have many cilia with same direction.
• Sperm Flagellum
~ Mostly similar to cilium except
-> Longer and moves in quasi-sinusoidal waves instead of whiplike movements

• Mechanism of Ciliary Movement:-
(All aspects of ciliary movement are not known)
1]-> Nine double tubules and two single tubules are linked by protein cross-linkages; collectively called axoneme.
2]-> Cilium can beat even when membrane+other elements, exceptp axoneme, are removed.
3]-> Two conditions for axoneme continued beating after removal of other elements
i) ATP
ii) Appropriate ionic conditions like Mg & Ca
4]-> During forward stroke, front edge double tubules slide outward to cilium tip
while those on back edge remain in place
5]-> Multiple protein arms composed of protein dynein (ATPase) project from each double tubule toward an adjacent double tubule
-> ATP releases energy to ATPase dynein arms causing their heads to crawl along adjacent double tubule.
-> Front tubules crawl outward while back tubules remain stationary and result in bending.
Cilia contraction control is not understood
->Cilia without central tubules fail to beat. So, A signal might be transmitted along these to activate dynein arms.


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