cell and its functions

8/9/2019 Cell and Its Functions

1/57

Cell Structure and Function


2/57

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.


3/57

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.


4/57

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.


5/57

DNA


6/57

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.


7/57


8/57


9/57

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.


10/57


11/57

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


12/57

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.


13/57


14/57

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.


15/57


16/57

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.


17/57

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


18/57

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.


19/57

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.


20/57


21/57

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 %


22/57

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.


23/57


24/57

Proteins

y Two main typesy Integral (intrinsic)y Peripheral (extrinsic)


25/57


26/57

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.


27/57

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


28/57

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


29/57


30/57


31/57

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


32/57

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.


33/57


34/57

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.


35/57

Cell membrane functionsy Maintaining cell shape and structurey Transporty Intercellular communicationy Intercellular adhesiony Movement


36/57

Intercellular communicationy Cell communication directs activity of the cell.


37/57

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.


38/57


39/57

Cell junctions


40/57


41/57

Protein synthesis

DNA RNA Protein

Transcription Translation


42/57

Protein synthesis


43/57

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.


44/57

Transcription


45/57

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 ).


46/57


47/57

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.


48/57

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


49/57

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.


50/57

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.


51/57

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 .


52/57

Cell cycle


53/57


54/57

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


55/57

Mitosisy Process by which a cell splits into two daughter

cells.y Phases prophase, metaphase, anaphase and

telophase.


56/57


57/57

Cell cycle checkpointsy Two main check points that exist are

y G1 /S checkpointy G2 /M checkpoint

cell and its functions

Documents