cell,subcellular organelles,and transport

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cell,subcellular organelles,and transport

Transcript of cell,subcellular organelles,and transport

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Cell and

Sub-cellular Structures Dr.GANESH

CellCell is the Basic Structural and Functional Unit of all Living Organisms

• Therefore, Evolution of cell is a crucial milestone in theevolution of life.

Cell structure• A eukaryotic cell contains a)plasma membrane, b)cytosol and c)subcellular organelles.

• Cell is a closed compartment containing aqueous fluid called –

cytosol surrounded by cell membrane called –

plasma membrane

Cell• Subcellular organelles are bathed by cytosol and include –

nucleus, mitochondria, endoplasmic reticulum, ribosomes golgi apparatus (golgi complex), lysosomes, peroxisomes, and cytoskeleton.

Cell

• Cell = Plasma membrane + cytoplasm• Cytoplasm = Cytosol + Subcellular organelles

CellAll subcellular organelles, except

ribosomes and cytoskeleton, are compartments within the cell,surrounded by cell membrane and containing their own aqueous fluid

Cell• Diversity of cell types serves the function of

the particular tissues and organs in which they are present.

• Depending on the function, different cell types differ in their organelle content, or their organelles may contain different amounts of particular enzymes or structural molecules.

Cell Membrane

Contents:

• Structure • Function• Transport Across Cell Membrane

Cell Membrane • thin hydrophobic sheet

in fluid state which envelopes the cell• made up of lipid bilayer (two layers)

• containing also proteins• Lipid and Protein molecules are bound to

each other by non-covalent bonds• Carbohydrates, found in lesser amounts

are bound to lipid and protein molecules by covalent bonds

Cell MembraneMembranes

• Define the external boundaries of cells and organelles

• Maintain their integrity and

• Serve to compartmentalize functions within the cells.

Salient features of Cell Membrane• Membranes are

flexible (because they are fluid), elastic and self-sealing

• flexibility permits the shape changes that accompany cell

growth and movement of cells, gives stabilityAlso enables the cell to perform exocytosis and

endocytosis

• Membranes are selectively permeable to molecules.

-Being hydrophobic membranes are permeable to only lipid soluble/hydrophobic substances and impermeable to hydrophilic/polar substances.

-However, membranes have transport systems (made of proteins) to permit and regulate the movement of polar compounds across its thickness.

Functions of Cell Membrane1. Cell Membranes maintain the shape and size

of the cells.2. Protects the cytoplasm and the cell

organelles from the external environment3. The intracellular membranes serve to

compartmentalize functions within the cells.4. Membranes regulate the transport of

substances like nutrients, ions, gases, water, various products, wastes into and out of cells and their organelles.

5.Membranes bound enzymes carry out metabolic reactions near the inner surface of the cell membrane. Egs:Succinate dehydrogenase .

6.Membranes are involved in signal transduction; i.e. proteins in membranes detect specific signals

transmit such signals to the cell interior by specific

chemical events.7. Membrane mediates cell-to-cell communication

between adjacent cells by gap-junctions.8. Membrane regulates the flow of information

between cell and its environment

Structure of Cell MembraneMembranes are

sheet-like complex structures composed of -• Lipids• Proteins

and• Carbohydrates

Structure of Cell Membrane• Lipid bilayer conformation

is the basic structure of all biological membranes.

• lipid bilayer is made up of amphipathic lipid molecules

(having both hydrophilic or polar part and a hydrophobic or non-polar part)

crucial in the formation of membrane structure

Structure of Cell MembraneFluid Mosaic Model proposed by Singer and Nicolsonto explain the structure of cell membraneAccording to this, • membrane is a

fluid lipid bilayer witha mosaic of embedded proteins

Phospholipids

Cholesterol

Proteins (peripheral and integral)

Carbohydrates

Membrane Components

AP Biology

In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer

It’s like a fluid…It’s like a mosaic…

It’s the Fluid Mosaic Model!

Structure of Cell Membrane

In other words, the model is compared to icebergs (membrane proteins) floating in a sea ( predominantlyphospholipid molecules)

Structure of Cell MembraneMembranes are 5-8 nm thick and appear trilaminar when viewed through an electronmicroscope

Structure of Cell Membrane• Different membranes within the cell

and between cells have different compositions

• This difference reflects the diversity

of biological roles of these membranes

Structure of Cell MembraneFor e.g. • myelin sheath of neurons, which acts as an electrical insulator, is rich in lipids whereas • inner mitochondrial membrane in which many enzyme-catalyzed processes take placecontains more proteins than lipids

Lipids

All Lipids present in cell membrane are –Amphipathic Lipids

Compound Lipids and Cholesterol (Free Cholesterol)

Phospholipids Glycolipids conceived as having a –polar head and a non-polar tail.

Amphipathic Lipid

Polar Head

Non-polar Tails

LipidsAmphipathic lipids self-assemble in aqueous medium into bilayer sheets (lipid bilayer) with their hydrophobic parts (non-polar tails) of eachlayer facing and interacting with each other forming ahydrophobic membrane core and hydrophilic parts (polar heads) facing towards the two surfaces interacting with the aqueous medium

Lipid Monolayer

Non-polar Medium

Aqueous Medium

Lipid BilayerAqueous Medium

Aqueous Medium

Hydrophobic Core

Lipids of cell membrane

Ampipathic lipids such as• phospholipids (e.g. lecithin,cephalin,sphingomyelin) • glycolipids and • cholesterol.

Lipids

Specific type of lipidmay be present in particular tissues Example: • Nerve tissues

have large quantity of glycolipids and sphingomyelins.

• Mitochondrial membrane rich in cardiolipin.

Proteins

• Make up about 50% of total membrane massIn a typical cell

• Distributed Asymmetrically in the lipid bilayer

Proteins• Membrane proteins are of 2 types.1.Peripheral membrane proteins attached to the lipid bilayer on either surface E.g. succinate dehydrogenase (TCA Cycle), endoplasmic reticulum enzymes, etc2. Integral membrane proteins

deeply embedded in the lipid bilayer. – Some integral membrane proteins may completely

span the lipid bilayer – –transmembrane proteins

e.g. receptor proteins, transport proteins, channel proteins, etc).

Peripheral membrane proteins

Integral membrane proteins

Transmembrane proteins

Cell Membrane proteins

Examples Integral membrane proteins •Receptor proteins• Transport proteins• Channel proteins

AP Biology

Many Functions of Membrane Proteins

Outside

Plasmamembrane

InsideTransporter Cell surface

receptorEnzymeactivity

Cell surface identity marker

Attachment to thecytoskeleton

Cell adhesion

Membrane Carbohydrates

• relatively minor components 5-8% of the total membrane mass.

• covalently linked to lipids and proteins as glycolipids and glycoproteins.

• Located on the extra cellular side of the membrane, which forms a loose outer carbohydrate coat called Glycocalyx

Functions of Glycocalyx

• Gives a net negative surface charge and repel from other electrically negative particles.

• Cell to cell attachment is possible.• Part of receptor substances for binding

hormones such as insulin• Some of the carbohydrate moieties

enter into immune reaction.

Glycoprotein Glycolipid

Glyccalyx

Cell Membrane Carbohydrates

Oligosaccharide

Glycoprotein

Peripheral Membrane Protein

Integral Membrane Proteins

Glycolipid

Glycoprotein Glycolipid

LipidBilayer

Polar head Non polar tail

Cell Membrane Structure

Integral membrane proteins

Transmembrane proteins Peripheral membrane proteins

Phospholipids

Cholesterol

Proteins (peripheral and integral)

Carbohydrates

Membrane Components

Fluidity of MembranesMembrane consists of

a mosaic of lipids and proteins that can move laterally (so fluid) in the plane of the membrane.

fluidity makes the membrane • flexible (which in turn permits the shape changes that

accompany cell growth and cell movements), • increases permeability ,gives stability

and enables them to

• invaginate or evaginate allowing them to ingest or to expel materials.

Factors Affecting the Fluidity

• Unsaturated cis-fatty acids • Short chain fatty acids and

• High temperature Increase the membrane fluidity.

Whereas,Cholesterol decreases the

membrane fluidity

Fluid_mosiac_model.mp4

Specialised Membrane Structures• Tight Junction• seen in epithelial cells, where the lateral membrane

of a cell is fused with lateral membrane of adjacent cell. This prevents the movement of molecules through the gap between the cells. This ensures that, molecules move only through the luminal side to the serosal side.

E.g.: Seen in gastrointestinal epithelial cells.• Myelin SheathSpecialized structure for the conduction of nerve

impulse, rich in lipids.

Specialised Membrane Structures• Synaptic membranes: Cell membranes associated with synapses. Required for the release or reception of

neurotransmitters.• Microvilli: Hair like projections produced by the membrane

evagination, which increases absorptive surface area.

Eg: intestinal epithelial cells.

Specialized Membrane Structures

Tight Junction

Synaptic membrane

Microvilli

Tight Junction

Eg: gastrointestinal epithelial cells Eg: gastrointestinal epithelial cells

Eg: neurons

For cell to cell communication Enhance absorption of food

Transmit information between neurones

Myelin Sheath

Myelin sheath

Eg: neuronsFor conduction of nerve impulse

AP Biology

Any Questions??

AP Biology

Cell membrane is the boundary between inside & outside… separates cell from its environment

INfoodcarbohydratessugars, proteinsamino acidslipidssalts, O2, H2O

OUTwasteammoniasaltsCO2

H2O products

cell needs materials in & products or waste out

IN

OUT

Can it be an impenetrable boundary? NO!

Transport Across Cell Membrane

• Membranes act as effective barrier for the passage of molecules, thereby keeping some substances inside the cell and others out.

• Yet they also contain transport systemswhich confer on membranes the important property of selective permeability by allowing specific molecules to be taken up and unwanted compounds to be removed from the cell

Transport Across Cell Membrane

• As the membrane core is hydrophobic in naturehydrophobic molecules move more readily across the membrane than hydrophilic ones.

• As the membrane fluidity increases, permeability to hydrophilic substances also increases

AP Biology

Transport across cell membrane What molecules can get through directly?

fats & other lipids

inside cell

outside cell

lipid

salt

aa H2Osugar

NH3

What molecules can NOT get through directly?

polar molecules H2O

ions salts, ammonia

large molecules starches, proteins

AP Biology

Transport across cell membrane Membrane becomes semi-permeable

with protein channels specific channels allow specific material

across cell membrane

inside cell

outside cell

sugaraaH2O

saltNH3

Classification of Transport Across Cell Membrane

Small Molecules Macromolecules & Particles

Passive transport(Energy independent)

Endocytosis

Membrane Transport

Simple Diffusion Facilitated Transport (Carrier mediated)

Ion-channels

Active transport(Energy dependent, Carrier mediated)

Eocytosis

Facilitated Transport(Passive transport)

Transport of Small Molecules

Simple Diffusion

Active transport(Energy dependent,)

Non-mediated transport (no carrier proteins)

Carrier mediated

Ion-channels

Another Way to Classify

Contents

•Transport of Small Moleculeso Non-mediated transport (no carrier proteins)

-- Simple Diffusion-- Ion-channels

o Carrier mediated-- Facilitated Transport(Passive)

-- Active transport(Energy dependent) •Macromolecules & Particles

Non-mediated Transport (no carrier proteins)

Simple Diffusion- Very Small molecules (like water) and gases (CO2,O2)

enter the cell by this method.- It is a very slow process.- Doesn’t require energy (energy independent/passive).- It is a non-mediated transport

(no carrier proteins involved).- Molecules diffuse from a region of higher concentration to a region of lower concentration (down the concentration gradient)- diffusion occurs through a membrane opening or through intermolecular spaces.

Simple diffusion

Eg: a) Respiratory exchange of gases between pulmonary

alveolar membrane and tissue capillary wall

b) Intestinal absorption of pentoses, some mineral ions and water-soluble vitamins

and c) renal reabsorption of urea

Ion-channels

• specialized protein molecules that span the membranes & permit the rapid transport of ions such asNa+, K+, Cl-.

• The channels generally remain closed but in response to stimulus, open allowing rapid flux of ions down the gradient

Carrier Mediated Transport

• specific carrier molecules are required• protein in nature.• Have specific binding sites

for the molecules to be transported• Transport is dependent on

availability of free binding sites on the carrier protein

• more rapid than simple diffusion.

Classification of Carrier Mediated Transport

There are 2 Ways of Classification1. Depending upon Number of Molecules Transported and Direction of Transport2. Depending upon Whether Energy is Required or not

Classification of Carrier Mediated Transport

1. • Uniport• Co-transport

- Symportand - Antiport

2.• Facilitated Transport(Passive)• Active transport(Energy dependent)

Carrier Mediated Transport• Uniport

Movement of one molecule from one side to anotherE.g.: movement of glucose from the cells of GIT to ECF.

• Co-transport Movement of one molecule depends on simultaneous or sequential transfer of another moleculeCo-transport may be- Symport Two molecules move in the same directionE.g.: Na+/Glucose transport.-Anti-portTwo molecules move in opposite directionsE.g.: Cl- – HCO3

- exchange in RBCs

end

end

end

Carrier Mediated Transport

• Based on energy need, carrier-mediated transport are of two types:

--Facilitated Transport (energy independent/passive) and

-- Active transport (energy independent/active)

Facilitated Transport• It is passive transport and carrier mediated. • Here transport is down the concentration gradient.• Once the molecule binds to the biding site,

a conformational change occurs in the carrier making the binding site exposed to the opposite direction

Now the molecule is released from the carrier. Another conformational change in the carrier

leads to the exposure of the binding site to the region where free molecules to be transported are present.

• Structurally similar solutes can inhibit the entry of one another by competitive inhibition.

E.g. There are four different facilitated carrier systems for carbohydrates and five for amino acids.

AP Biology

Facilitated transport Diffusion through protein channels

channels move specific molecules across cell membrane

no energy needed

“The Bouncer”“The Bouncer”

open channel = fast transport

facilitated = with help

high

low

facilitated diffusion.mp4

Mechanism : Ping pong model

Transport Across Cell Membrane

Facilitated transport

Active Transport• Transport is

carrier mediated against the concentration gradient and hence energy-dependent.

• Transport occurs only in one way, against the concentration gradient.

• The energy comes usually from hydrolysis of ATP molecules

• About 40% of the total energy of the cell is used for the active transport.

AP Biology

Active Transport

“The Doorman”“The Doorman”

conformational change

Cells may need to move molecules against concentration gradient shape change transports solute from

one side of membrane to other protein “pump” “costs” energy = ATP

ATP

low

high

Active TransportE.g. • Na+–K+ pump or Na+-K+ ATPase is the best example for active transportbecause virtually all cells have it. Other examples • Ca+–dependent ATPase (in sarcoplasmic reticulum of skeletal muscles), • H+–dependent ATPase located in the membrane of epithelial cell lining the

stomach and has the function of acid (H+) secretion

Na+-K+ ATPase

• Na+-K+ ATPase establishes and maintains a high intracellular K+ concentration and a low Na+ concentration compared to their concentrations in ECF.

• The Na+ – K+ ATPase, expels 3 Na+ ions and brings 2K+ ions from outside to inside with a concomitant hydrolysis of ATP. Drugs like digitalis (a cardiac glycoside) and ouabain inhibit Na+ – K+ ATPase.

ADP + Pi

2 K+ 2 K+

3 Na+ 3 Na+

ATP

Active Transport

Na+-K+ ATPase

Sodium pump or Na+-K+ ATPase

Sodium Potassium Pump.mp4

Classification of active transport

• based on the source of energy 1. Primary active transport - Transport of

molecules is directly linked to the hydrolysis of ATP, which provides energy.

E.g. Na+–K+ pump or Na+-K+ ATPase 2.Secondary active transport- Transport of

molecules is indirectly linked to the hydrolysis of ATP. Eg: Glucose and galactose are absorbed from the intestine

by secondary active transport. Concentration gradient of Na+ is maintained by Na+ – K+ ATPase.

Physiological importance of active transport

• -Responsible for the generation of the resting membrane potential, basis for excitability in nerve and transmission of nerve impulse

• -Na+ pump is driving force for several secondary active transport of nutrients into the cell. For example, glucose is co-transported with sodium into the cell

• -Calcium pump (Ca++ dependent ATPase): found in sarcoplasmic reticulum of skeletal muscles. It transports calcium from the cytosol to the sarcoplasmic reticulum. It regulates muscle contraction

• -Proton pump (H+ dependent ATPase): located in the parietal cells of the stomach . It is responsible for the secretion of Hcl into stomach lumen, to maintain the highly acidic pH essential for gastric digestion.

• Clinical application: Cardiotonic rugs like digitalis (a

cardiac glycoside) and ouabain inhibit Na+ – K+ ATPase. They are used in treatment of heart failure.

Transport of Macromolecules & Particles

-transported by Endocytosis and Exocytosis-Macromolecules such as proteins,

polysaccharides, hormones and particles like viruses, bacteria etc are transported by these mechanisms.

Endocytosis-internalize extra-cellular macromolecules byinvagination of cell membrane

- Endocytosis may be either Phagocytosis or Pinocytosis.

Endocytosis: Process by which cells take up the large molecules

Transport Across Cell Membrane

endocytosis animation2.avi.mp4

Phagocytosis (Gk: Phagein = to eat)• Occurs in specialized cells such as macrophages • and • granulocytes.• ingestion of large particles such as viruses, bacteria, cells or debris.• endocytic vesicle (phagosome)

fuses with the lysosome.hydrolytic enzymes of lysosomes break down the macromolecular contents released in to the cytosol reused or further catabolized

Pinocytosis(cell drinking)

• Cellular uptake of fluid and fluid contents containing small particles.

E.g.: -Intake of chylomicron by the hepatocytes; -internalization of LDL by LDL receptor

Exocytosisextrusion of particulate or macromolecular materials,

which can’t pass out through the intact membrane.• secretory vesicle is pinched off from the Golgi

apparatus; • moves towards and fuses with the plasma

membrane.• E.g. a) Release of Trypsinogen by pancreatic

acinar cells. b) Release of Insulin by -cells of Langerhans.• c) Release of acetylcholine by pre-synaptic

cholinergic nerves.

Exocytosis: Process of extrusion of a macromolecule from the cell

Transport Across Cell Membrane

exocytosis animation.avi.mp4

Disorders of Membrane Structure and Transport

Abnormality in membrane structure or transport can cause diseases. • Respiratory distress syndromeDefect in biosynthesis of dipalmitoyl lecithin(lung surfactant)• Familial hypercholesterolemia Mutations in the gene encoding LDL receptorCystic fibrosisMutations in the gene encoding Cl- transporter

Sub-cellular Organellesinclude -- nucleus, Ribosomesendoplasmic reticulum, golgi apparatus/complex,mitochondria, lysosomes,peroxisomes and cytoskeleton.

Sub-cellular Organelles

All eukaryotic cells contain all these organelles RBC is not a true cell and contain only the plasma membrane and cytoskeleton

Nucleus• largest sub-cellular organelle.• double membrane –

nuclear membrane, surrounds it.

• At intervals nuclear membrane has nuclear pores, permit the passage of molecules in and out of the nucleus.

• nucleus of eukaryotic cell contains a dense body known as nucleolusrich in rRNA.

Nucleus• Nucleoplasm

– ground material of nucleus

rich in enzymes such as, DNA polymerases, RNA polymerases, etc.

• Nucleus of an interphase (non-dividing) cell filled with a diffuse material – chromatin. – During the cell division, chromatin condenses to form

chromosomes. – Humans have 23 pairs of chromosomes compactly

packed in the nucleus.

Nucleus –Functions• Nuclear DNA --the repository of genetic information

serves two purposes --i) By DNA replication

provides genetic information to offspring or daughter cellsduring cell division., thus it is blue print of life.

ii) By transcription (RNA synthesis) provides information for the synthesis of all protein molecules of the cell.

Both replication and transcription take place in the nucleus.

Function of nucleolus: -Synthesis of rRNA and ribosomes

Mitochondriaspherical, oval or rod like bodies.

have two membranes – outer and inner membrane. outer membrane is smooth while the inner membrane is for folded to form cristae components of electron transport chain (ETC) and oxidative phosphorylation buried in the inner mitochondrial membrane.

Mitochondria-structure

Mitochondria• The central cavity of the mitochondrion

contains the matrix

• Matrix contains enzymes and chemical intermediates of --TCA cycle

Heme synthesis Urea cycle, etc. Also present in the matrix are, mitochondrial

DNA, RNA and ribosomes.

Functions• ETC and oxidative phosphorylation-- situated in

inner mitochondrial membrane are involved in ATP synthesis, hence mitochondria are regarded as ‘powerhouse of the cell’

• Some of the major pathways operate in the mitochondria. They are, TCA cycle, -Oxidation of fatty acid, ketone bodies formation, gluconeogenesis (partly), urea cycle (partly), heme synthesis (partly), pyrimidine synthesis (partly) .

• Mitochondrial DNA codes for some of the mitochondrial proteins involved in oxidative phosphorylation

Endoplasmic Reticulum (ER)

network of membrane-enclosed spaces extends throughout the cytoplasm.• classified into

rough and smooth ER

rough appearance (when observed under electron microscope) is due to ribosomes attached to the cytoplasmic side of the membrane.

smooth ER does not have ribosomes.

Functions of ER

• Rough ER : involved in synthesis of proteins (lipoproteins, glycoproteins)

• Smooth ER: I. Metabolism of drugs and toxic compounds (cyt

P450 monooxygenases are present in liver cell smooth ER)

II. Synthesis of lipids (TAG, phospholipids, cholesterol) and

III. Ca2+ storage in skeletal and cardiac muscle.(note- sarcoplasmic reticulum of muscle is a modified ER)

Golgi Complex/Golgi Apparatus

• group of membrane bound flattened tubes or sacs placed one over another

in a pile or stack.

Golgi Apparatus - Functions

Main functions of Golgi apparatus are protein sorting, packaging and secretion.• newly synthesized proteins are

handed over to the Golgi apparatus, which catalyze the addition of carbohydrates, lipids or sulfate moieties to the proteins.

Lysosomes

membrane bound vesicle containing various hydrolytic enzymes (hydrolases.• Lysosomal enzymes

are capable of digesting proteins, carbohydrates, lipids and nucleic acids

• pH inside the lysosomes is less than that of cytosol necessary for its digestivse function

Lysosomes -Functions• hydrolases breakdown complex molecules

brought into the cell by endocytosis, phagocytosis or worn-out organelles from the cells own cytoplasm. Lysosomes - termed as ‘suicide-bags’ as their lysis can lesad to digestion and death of the cell

Sphingolipidosis – group of disorders in which excess of sphingolipids accumulates in lysosomes

Peroxisomesmall spherical or oval membranous bodies• contain enzymes --

peroxidases and catalase

Peroxisome - FunctionsFree radicals formed by peroxidation of PUFA capable of damaging cell membranes, tissues, and genes Such reactions are implicated in inflammatory diseases, ageing process and

malignant transformation.

• Catalase and peroxidase enzymes destroy such unwanted peroxides and other free radicals

Ribosomes:

nucleoproteins present either freely in cytosol or bound to ER• Function:provide necessary infrastructure for mRNA, tRNA & amino acid to interact with each other for translation process.

Cyto skeletonMade up of microtubules and actin filaments role in maintaining the cellular structure, mobility and cell division. Hereditary spherocytosis due to mutations in genes encoding spectrin or other structural proteins in red blood cell membrane, leading to excessive hemolysis

Organelle Function

Nucleus Provides genetic information to offspringRNA transcription, directs protein

synthesis

Mitochondria Energy production from the oxidation of food substances and the release of adenosine triphosphate

Endoplasmic reticulum

Translation and folding of new proteins (rough endoplasmic reticulum), synthesis of lipids (smooth endoplasmic reticulum)

summary

Golgi appartus

Sorting, packaging, and modification of proteins

Endoplasmic reticulum

Translation and folding of new proteins (rough endoplasmic reticulum), synthesis of lipids (smooth endoplasmic reticulum)

Lysosome Breakdown of large molecules

Peroxisome breakdown of metabolic hydrogen peroxide and free radicals

Organelle Function

Ribosome Translation of RNA to form proteins

Cytoskeleton Maintaining the cellular, shape, motility and cell division.

Sub-Cellular Fractionationisolation of an organelle in a relatively pure formin order to study its functions

Cell membrane is disrupted usually by mechanical means called homogenization• subcellular organelles

can then be separated from the homogenate by differential centrifugation using the instrument ultracentrifuge

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