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Cell Structure and Function
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Celly Basic living unit of body is the cell.y Cells are specially adapted to perform different
functions.y Major parts nucleus and cytoplasm.y N ucleus separated from cytoplasm by nuclear
membrane.y Cytoplasm separated from extracellular
environment by cell membrane (plasmamembrane).
y Cellular contents are collectively called theprotoplasm includes water (70 to 85%),
electrolytes, proteins, lipids, carbohydrates.
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Nucleusy Control center of the cell.
y Membrane bound organelle that contains most of the cells D NA , which is organized into
chromosomes.y Communicates with the rest of the cell through
small nuclear pores.y Main functions are regulation of gene expression
and control of DNA
replication and cell division.y The nucleolus is a sub-organelle within the
nucleus that functions in ribosomal R NA synthesisand ribosome assembly.
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DNA
y G enes/D NA (Deoxyribonucleic acid) controlscell function and reproduction.
y Basic building blocks of D NA are phosphoric acid,
deoxyribose and four nitrogenous bases adenine,guanine, thymine and cytosine.y N ucleotide is made up of phosphoric acid,
deoxyribose and one of the four bases.y G
enetic code consists of successive triplets of bases which determine sequence of amino acidsthat will form proteins within the cell.
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DNA
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Cell growth and reproductiony G enes also control cell growth and reproduction.y Cells go through phases of the cell cycle G1 , S,
G2 , M.
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E ndoplasmic reticulum ( E R)y N etwork of tubular and flat vesicular
structures that is continuous withmembrane of nucleus.
y Walls composed of a lipid bilayer withlarge amounts of protein.
y Space inside the tubules is filled with a
watery medium called the endoplasmicmatrix ; this space is continuous with thespace between the two nuclear membranes.
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Functions of E Ry Rough E R
y Protein synthesisy Synthesis of new
membrane
y Smooth E Ry Transporty
Synthesis of lipidsy Detoxificationy Storage of calcium ions
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G olgi apparatusy Consists of four or more layers of membrane-
bound stacks called cisternae located betweenthe E R and cell surface.
y
Located on side of the cell from which substancesare secreted.y Has two main functional regions cis Golgi
(closest to nucleus) and trans-Golgi .y
Substances fromE
R are transported to theG
olginetwork, where they are processed andpackaged as lysosomes, secretory vesicles or other cytoplasmic components and delivered todifferent parts of the cell.
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Mitochondriay Has two membranes ; an outer membrane and an
inner membrane with many infoldings calledcristae , to which oxidative enzymes are attached.
y
Inner cavity is called the matrix ; contains DNA
,ribosomes, small granules and dissolvedenzymes.
y Called the powerhouse of the cell because it isthe major source of energy for the cell.
y A lso plays a role in apoptosis, regulation of membrane potential, regulation of metabolismand cell proliferation.
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Lysosomes and peroxisomesy Lysosomes
y Vesicular organelles that contain digestiveenzymes ; membrane composed of a lipid bilayer.
y
Break off fromG
olgi apparatus and are distributedthroughout the cell.
y Peroxisomesy
Physically similar to lysosomes but are formed fromthe E R.y Contain oxidative enzymes that are involved in
detoxification and breakdown of fatty acids.
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Cytoskeletony System of fibers that maintains the structure of
the cell.y Composed of microtubules , microfilaments and
intermediate filaments .y Functions include
y Maintaining cell shapey Providing mechanical strengthy O rganelle transporty Chromosome separation in mitosis
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Microtubulesy 24 nm in diameter ; composed of - and - tubulin
dimers that polymerize end to end and bundletogether to form hollow cylindrical structures.
y Assembly occurs at + end and disassemblyoccurs at end.
y Cilia and flagella are made up of microtubules involved in cell movement.
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y Microfilaments (actin filaments)y 4 -6 nm in diameter ; actin filaments polymerize to
form long thin fibers.y Interact with myosin in muscle fibers ; also play a
role in mitosis.
y Intermediate filamentsy 8-14 nm in diameter ; made up of various sub-units.y Form flexible scaffolding for cell to resist external
pressure ; connect nuclear membrane to cellmembrane.
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Cell membraney Permeability barrier ; structure is called the fluid
mosaic model.y Maintains composition of cytoplasm which is
different from extracellular fluid.y Composition of cell membrane
y Protein 55%y Phospholipids 2 5%y Cholesterol 13 %y O ther lipids 4 %y Carbohydrates 3 %
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Phospholipidsy Form lipid bilayer composed of two layers of
phopholipid molecules ; has hydrophobic andhydrophilic component.
y Hydrophobic end non-polar end ; forms middleof membrane.
y H ydrophilic end polar end ; forms outer surfaces of membrane.
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Proteins
y Two main typesy Integral (intrinsic)y Peripheral (extrinsic)
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Integral proteinsy E mbedded in the membrane. Two types:
y T ransmembrane proteins span the entiremembrane.
y L ipid anchored proteins anchored to membraneleaflet by covalent bonds.
y Contain hydrophobic side chains that interact withthe phospholipids, anchoring the protein to themembrane.
y Can only be removed by disrupting the lipidbilayer using detergents or denaturing agents.
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Integral proteins functionsy Provide structural channels (pores) for transport
of water and water-soluble substancesy A ct as carrier proteinsy A cts as receptors for water-soluble chemicalsy A lso function in cell-cell communication and
adhesion
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Peripheral proteinsy A ttached to lipid bilayer or to integral proteins by
ionic and hydrogen bonds ; do not usuallypenetrate lipid bilayer.
y
Can be removed by anything that disrupts ion or hydrogen interaction such as changes in pH or asolution with high salt concentration.
y Functionsy E nzymesy Transporters of small molecules through membrane
pores
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Functions of carbohydratesy Usually negatively charged provide overall
negative charge for cell membrane.y Receptorsy Involved in identification and recognition
(glycocalyx)y Cell-cell adhesion
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Cholesteroly A nother type of lipid found in membranes.y Cholesterol molecule inserts itself in the same
orientation as the phospholipid molecules.y
A mount found varies with type of membrane.y Plasma membranes have about one cholesterol
per phospholipid molecule.y O thers such as bacterial membranes have no
cholesterol.
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Function of cholesterol in cellmembranes
y Immobilize first few hydrocarbon groups of phospholipid molecules maintains firmness andintegrity of membrane.
y Prevents crystallization and phase shift of membrane maintains fluidity.
y Interact with proteins to form lipid rafts specialized membrane domains that may beinvolved in cell signaling eg. caveolae found inendothelial cells and adipocytes.
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Cell membrane functionsy Maintaining cell shape and structurey Transporty Intercellular communicationy Intercellular adhesiony Movement
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Intercellular communicationy Cell communication directs activity of the cell.
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Intercellular communicationy Cells respond to signals through receptors.y Receptors can be membrane-bound or
intracellular .y
Membrane-bound receptorsy G -protein linked receptorsy Tyrosine kinase receptorsy Ion channel receptors
y Cell converts signal into response by way of signal transduction pathways.
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Cell junctions
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Protein synthesis
DNA RNA Protein
Transcription Translation
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Protein synthesis
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Transcriptiony Transcription is formation of mR NA from D NA .y RNA polymerase II and other proteins
(transcription factors) bind to promoter site (T AT A sequence) near transcription start site.
y A fter transcription, the 5` end is capped and apoly( A ) tail is added to 3 ` end.
y A pre-mR NA is formed then introns are removedduring post-transcriptional processing.
y mR NA then moves to the cytoplasm andassociates with ribosomes to form protein.
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Transcription
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Translationy Formation of protein from mR NA .y Ribosome binds to mR NA and moves
downstream until it reaches the start codon ( A UG which codes for methionine).
y A mino acids (aas) within the cytoplasm areactivated by enzymes and combine with tR NA .
y tR NA -aa-enzyme complex attaches to mR NA andthe ribosome complex moves along the mR NA using it as a template for protein synthesis.
y Translation stops at one of three stop codons(UGA , U AA or U AG ).
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Posttranslational modificationy O nce the protein is formed it is modified through
a number of processes including phosphorylation,hydroxylation, carboxylation and glycosylation.
y There is also protein folding and that determinesfinal structure of protein.
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Regulation of gene expressiony There is spatial and temporal regulation of gene
expression.y Regulatory elements D NA sequences where
regulatory proteins can bind and affecttranscription.y Silencer sequencesy E nhancer sequences
y T ranscription factors proteins that bind tospecific D NA sequences.y A ctivators bind to D NA and enhance or promote
gene expression.y
Repressors bind to D NA and inhibit gene
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Regulation of gene expressiony mRNA splicing removal of introns ; this is achieved through
the use of spliceosomes that cleave the intron sequences and join the exons.
y Alternative splicing combination of different exons to producedifferent proteins.
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Regulation of gene expressiony miRNA (microRNA) regulate translation of
mR NA by inhibiting translation or destroyingmR NA ; thought to bind to 3 ` UTR region of mR NA .
y siRNA (small interfering RNA or shortinterfering RNA) small double stranded R NA that also interfere with mR NA translation.
y snRNA (small nucleolar RNA) components of the spliceosome.
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Regulation of gene expressiony P bodies (processing bodies) proteins found
within the cytoplasm that sequester mR NA s anddegrade them byy Removing the 5` cap and poly ( A ) taily Degrading the message
y Exosomes hollow structures that take in R NA molecules and degrade them.
y Riboswitch part of an mR NA molecule that hasa specific binding site for a particular molecule ; once the target molecule binds it preventstranslation of the mR NA .
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Cell cycle
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DNA replicationy Replication is initiated by the O RC ( O rigin
recognition complex) that bind at points along theDNA called origins.
y O rigins are rich in A -T pairs.y O RC consists of initiator proteins and helicase
enzyme that help to unwind D NA helix.y RNA primase inserts primer sequence at initiation
point.y DNA polymerase moves along the D NA strand
which acts as a template and reads thenucleotide sequence.
y DNA ligase binds successive nucleotides to form
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Mitosisy Process by which a cell splits into two daughter
cells.y Phases prophase, metaphase, anaphase and
telophase.
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Cell cycle checkpointsy Two main check points that exist are
y G1 /S checkpointy G2 /M checkpoint