cell,subcellular organelles,and transport
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cell,subcellular organelles,and transportTRANSCRIPT
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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
