Membranous System Of Cell

CELL STRUCTURES INVOLVED IN MANUFACTURING AND BREAKDOWN

This term refers to the organelles that are connected together by phospholipid membranes.Prokaryotes dont have an endomembrane systemThe endomembrane system includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, vacuoles, and the plasma membraneThe Endomembrane System

PropertiesTransport system for moving molecules through interior of the cell. Lipid bi-layer Composition with proteins attached to either side or transversing them. Divides cell into organelles.

Cell Membraneseparates living cell from nonliving surroundings thin barrier = 8nm thick Controls traffic in & out of the cell selectively permeable allows some substances to cross more easily than others hydrophobic vs hydrophilic Made of phospholipids, proteins & other macromolecules

PhospholipidsFatty acid tailshydrophobicPhosphate group head hydrophilicArranged as a bi layer

PhosphateFatty acidPhospholipid bilayer

PolarhydrophilicheadsNonpolarhydrophobictailsPolarhydrophilicheads

GlycoproteinPhospholipidsExtracellular fluidGlycolipidCholesterolTrans membraneproteinsFilaments ofcytoskeletonCytoplasmPeripheralproteinMembrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayerMembrane fat compositionFat composition affects flexibilitymembrane must be fluid & flexible% unsaturated fatty acids in phospholipids keep membrane less viscouscold-adapted organisms, like winter wheat increase % in autumn Cholesterol in membrane

Membrane ProteinsProteins determine membranes specific functionsPeripheral proteins loosely bound to surface of membranecell surface identity marker (antigens)Integral proteins penetrate lipid bi layer, usually across whole membrane Trans membrane proteinTransport proteinsChannels, Permeases (pumps)

Examples

NH2H+COOHCytoplasmRetinalchromophoreNonpolar(hydrophobic)a-helices in thecell membraneH+

Porin monomerb-pleated sheetsBacterialoutermembraneProton pump channel in photosynthetic acteriaWater Channels in Bacteria Function through conformational changeFunctions of Membrane Proteins

Outside Plasma membraneInsideTransporterEnzyme ActivityCell surface ReceptorCell surface Identity markerCell AdhesionAttachment to CytoskeletonMembrane carbohydrates Play a key role in cell-cell recognitionimportant in organ & tissue developmentbasis for rejection of foreign cells by immune system

Movement across the Cell Membrane

Passive TransportSimple diffusiondiffusion of non polar, hydrophobic moleculeslipidshigh low concentration gradientFacilitated transportdiffusion of polar, hydrophilic moleculesthrough a protein channelhigh low concentration gradientActive transportdiffusion against concentration gradientlow highuses a protein pumprequires ATPGetting through cell membraneATP

ATPGetting through cell membraneMembranes provide compartmentalization, which allows different places in the cell to have different environments.

With this arrangement, the cell can build in some areas, break down in others, and reserve other areas for storage or other functions.

Why an Endomembrane System?Lets Follow a Protein

From the start of its existence through a few possible paths in the endomembrane system.

Protein structure is determined in the cells DNA, which is in the NUCLEUS.The nucleus controls the cells activities and is responsible for inheritanceInside is a complex of proteins and DNA called chromatin, which makes up the cells chromosomesWhen its time to make a protein, the appropriate DNA is read and converted into RNA (transctiption) within the nucleus.

Once a chain of RNA gets made, it exits the nucleus via pores in the nuclear envelope.

The nuclear envelope is a double membrane with pores that allow material to flow in and out of the nucleusIt is attached to a network of cellular membranes called the endoplasmic reticulum.

Two membranes ofnuclear envelope

Nucleus

Nucleolus

Chromatin

Pore

Endoplasmicreticulum

Ribosomes

See where the pores are? See how the nuclear envelope hooks up to the ER like a big maze?

Ribosomes make proteins for use in the cell and exportRibosomes are involved in the cells protein synthesisRibosomes are synthesized in the nucleolus, which is found in the nucleusCells that must synthesize large amounts of protein have a large number of ribosomes

Can you name a few cells that would have a lot of ribosomes?

Some ribosomes are free ; others are boundFree ribosomes are hanging out in the cytoplasmBound ribosomes are attached to the endoplasmic reticulum (ER) which is connected with the nuclear envelopeRibosomes are NOT made of membranes! They are made of protein and RNA.They are associated with the endomembrane system.

Ribosomes

Cytoplasm

ER

Largesubunit

Smallsubunit

Diagram of a ribosome

TEM showing ERand ribosomes

Free ribosomesBound ribosomesEndoplasmic reticulum (ER)

Diagram of a ribosome

Many cell organelles are connected through the Endomembrane systemSome components of the endomembrane system communicate with others via the formation and transfer of small membrane balloons called vesiclesOne important result of communication is the synthesis, storage, and export of molecules

Proteins get created in The endoplasmic reticulum A biosynthetic factory

There are two kinds of endoplasmic reticulumsmooth and roughSmooth ER has no attached ribosomesRough ER is studded with attached ribosomesThey differ in structure and function, but they are connected

Nuclearenvelope

Ribosomes

Rough ER

Smooth ER

The endoplasmic reticulum is a Bio-factory

Smooth ER is involved in a bunch of diverse metabolic processes For example, enzymes produced by the smooth ER are involved in synthesizing lipids, oils, phospholipids, and steroids.ROUGH ER USES LIPIDS FROM THE SMOOTH ER TO BUILD MEMBRANES.

Rough ER makes additional membrane for itself It also makes all proteins destined for secretion (export)Once proteins are synthesized, they are transported in vesicles to other parts of the Endomembrane system.

Transport vesiclebuds off

Ribosome making protein

Sugar chain added to a protein (glycosylation)

Polypeptide

Glycoprotein

1

2

3

4

Transport vesicleWith secretoryProtein inside

Rough ER

Next Stop for our vesicle with protein inside: THE GOLGI. The Golgi apparatus finishes, sorts, and ships cell products

The Golgi apparatus alongside the ER by modifying products of the ERProducts travel in transport vesicles from the ER to the Golgi apparatusOne side of the Golgi apparatus functions as a receiving dock for the product and the other as a shipping dockProducts are modified as they go from one side of the Golgi apparatus to the other.After going through the Golgi, the proteins (or other stuff) is shipped out in a new set of vesicles.

Receiving side ofGolgi apparatus

Transportvesiclefrom ER

New vesicleforming

Shipping sideof Golgi apparatus

Transportvesicle fromthe Golgi

Golgi apparatus

Golgi apparatus

Where are the Proteins going?

A lysosome is a membranous sac containing digestive enzymesThe enzymes and membrane are produced by the ER and transferred to the Golgi apparatus for processingThe membrane serves to safely isolate these potent enzymes from the rest of the cellIf it werent there, the cell would digest itself.If the proteins are digestive enzymes, they may go to LYSOSOMES.Lysosomes are digestive compartments within a cell

One of the several functions of lysosomes is to remove or recycle damaged parts of a cellThe damaged organelle is first enclosed in a membrane vesicleThen a lysosome fuses with the vesicle, dismantling its contents and breaking down the damaged organelle

Digestiveenzymes

Lysosome

Plasmamembrane

Digestiveenzymes

Lysosome

Plasmamembrane

Food vacuole

Digestiveenzymes

Lysosome

Plasmamembrane

Food vacuole

Digestion

Food vacuole

Plasmamembrane

Lysosome

Digestiveenzymes

Lysosome

Vesicle containingdamaged mitochondrion

Vesicle containingdamaged mitochondrion

Lysosome

Lysosome

Vesicle containingdamaged mitochondrion

Digestion

Some proteins go to assist in the function of VACUOLES, which do maintenance jobs in the cellVacuoles are membranous sacs that are found in a variety of cells and possess an assortment of functionsExamples: central vacuole in plants with hydrolytic (water-breaking) functions, pigment vacuoles in plants to provide color to flowers, and contractile vacuoles in some protists to expel water from the cell

Contractilevacuoles

Chloroplast

Nucleus

Nucleus

Nucleus

Smooth ER

Nuclearmembrane

Rough ER

Lysosome

Transportvesicle

Golgiapparatus

Transportvesicle

Vacuole

Plasma membrane

Proteins can be exported out of the cell by EXOCYTOSIS

A cell uses two mechanisms for moving large molecules across membranesExocytosis is used to export bulky molecules, such as proteins or polysaccharidesEndocytosis is used to import substances useful to the livelihood of the cellIn both cases, material to be transported is packaged within a vesicle that fuses with the plasma membraneExocytosis and endocytosis transport large molecules across membranes

There are three kinds of endocytosisPhagocytosis is engulfment of a particle by wrapping cell membrane around it, forming a vacuolePinocytosis is the same thing except that fluids are taken into small vesiclesReceptor-mediated endocytosis is where receptors in a receptor-coated pit interact with a specific protein, initiating formation of a vesicle

Foodbeingingested

CYTOPLASM

Foodvacuole

Food orother particle

Pseudopodium

EXTRACELLULARFLUID

Phagocytosis

Phagocytosis

Plasmamembrane

Vesicle

Plasma membrane

PinocytosisPinocytosis

Material boundto receptor proteins

Coatedpit

Plasma membrane

Coat protein

Coatedvesicle

Coatedpit

Specificmolecule

Receptor-mediated endocytosisReceptorsCoated pit