Membran sel

The Cell Membrane

Cell Membrane:

At Very High Magnification & in color

Membrane Structurehttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CellMembranes.html

Membran sel.. Fungsi:Membatasi isi sel dari lingkungannyaMengatur permeabilitas terhadap senyawa-senyawa / ion-ion yg melewatinya diatur oleh protein integralProtein selaput bfgs sbg protein pengenal (reseptor) molekul2 khusus (hormon, antigen, metabolit) & agensia khas (bakteri, virus etc)Protein selaput bfgs sbg enzim khusus, co. pd selaput mitokondria, kloroplas, RE, badan golgi, membran plasmaSebagai reseptor thdp perubahan lingkungan co. suhu, intensitas cahaya etc.

Membran sel. Cont.Selaput plasma memp. Sifat hidrofobik di bag. tengah & hidrofilik di permukaan luar maupun di permukaan sitosolik.

Selaput plasma tdr dr senyawa2 lipid (memberi sifat hidrofobik-menentukan struktur), protein menentukan fgs membran, & KH (memberi sifat hidrofilik).

Komposisi penyusun membran bervariasi sesuai dg macam selaputnya & dpt berubah sesuai tingkat perkenmbangan sel, umur dan lingkungan

www.biologie.uni-hamburg.de/b-online/e22/22.htm

Membran sel . Cont.

Fluid like compositionslike soap bubbles:2 lapis lipidProtein tertanam pd lap lipid (dsbt protein transmembrane)Protein yg mengapung diantara lipid (dsbt protein integral)Proteins yg tdpt pd bag luar lipid bilayer (peripheral).

Lipid membranSebagian besar tssn atas phospholipidPhospholipid tssn atas glycerol & 2asam lemak + PO4 groupP-Lipids merupakan molekul polar P-Lipids are represented like thisText pg. 81

Lipid membranbentuk BilayerOutside layerInside Layer

Model molekul phospholipid amphipatic glycerol

asam lemak (hydrophobic) - tailphosphate (hydrophilic) -head

Protein membranIntegral: embedded within bilayerPeripheral: reside outside hydrophobic region of lipids

Protein membran. cont.Text pg 80

Protein membran integral

Protein membran peripheralIntegral

Model membranFluid Mosaic Model 1972 Singer & Nicolson : lipid diatur dlm struktur 2 lapis dg protein yg berbeda2 tertanam / menempel pada lipid (mosaik), dimana protein & fosfolipid dpt bergerak bebas pada membran (fluid).

Fluid Mosaic MembraneText pg 80

Fluid Mosaic. cont.Components of a phospholipid bilayer.1.phospholipids2.proteins - enzymes, receptors, transport.3.glycolipids4.glycoproteins5.carbohydrates6.cholesterol

Membrane Functionsallows for different conditions between inside and outside of cellsubdivides cell into compartments with different internal conditions allows release of substances from cell via vesicle fusion with outer membrane: http://www.emc.maricopa.edu/faculty/farabee/BIOBK/exocyt.gif

Fluiditas membranBag mana dr membran yg memberikan ciri khas fluid / cair?

Membrane hrs tetap dlm keadaan cair utk fgs perakitan & fgs biologis

Bakteri & yeast beradaptasi pd temperatur yg bervariasi dg merubah pjg & ketidakjenuhan ekor hidrofobiknya, makin tdk jenuh = makin cair

Sel hwn mengatur fluiditas dg cholesterol

Cholesterol membuat lipid bilayer kurang berbentuk & menurunkan permeabilitas thdp molekul kecil larut air dg imobilisasi parsial molekul fosfolipid pd msg2 molekul kolesterol.

Tanpa cholesterol, sel hwn memerlukan ddg sel.

Permeabilitas membranMembranes are impermeable to certain molecules.

How do these molecules enter the cell?

How to get other molecules across membranes??Transport proteins Transports molecules or ions across biological membranes3 types of transport proteins:1.uniport

2.symport

3.antiport

Uniport Transport Protein

Carries a single solute across the membrane.

Symport Transport ProteinTranslocate 2 different solutes simultaneously in same direction.

Antiport Transport ProteinExchanges 2 solutes by transporting them in opposite directions.

There are two ways that the molecules typically move through the membrane: passive transport and active transport

Active transport requires that the cell use energy that it has obtained from food to move the molecules (or larger particles) through the cell membrane.

Passive transport does not require such an energy expenditure, and occurs spontaneously.

Mekanisme transport membranI. Passive TransportDifusi- perpindahan substansi (molekul) dg turunnya gradien konsentrasi dari daerah dg konsentrasi molekul tinggi ke daerah dg konsentrasi molekul rendah.

Facilitated diffusion- tipe passive transport yg menggunakan bantuan transport proteins.

Osmosis

OsmosisPerpindahan air melewati membran semi-permeable. Air bergerak dr konsentrasi air tinggi ke konsentrasi air rendah.

Example: Salt in water, cell membrane is barrier. Salt will NOT move across membrane, water will.

Question:Whats in a Solution?Answer:

solute+solventsolution

Molekul terlarut + pelarut larutan

NaCl+H20saltwater

Osmosis pada media Hypertonic (larutan dg konsentrasi zat terlarut tinggi)

Hypertonic solutions- shrink cells

Osmosis pada media Hypotonic (larutan dg konsentrasi zat terlarut rendah)

Hypotonic solutions- swell cells

Movement of H2OWater will ALWAYS diffuses down a concentration gradient from a HYPOTONIC solution to a HYPERTONIC solution.

ALWAYS REMEMBERHYPOTONICHYPERTONIC

Sel hewanSel hewan diletakkan ke dalam larutan hipotonik akan mengalami HEMOLYSIS (sel pecah).Sel hewan diletakkan ke dalamin larutan hipertonik akan mengkerut.

hemolisismengkerut

Sel tumbuhanFirmness or tension (vacuole full) that is found in plant cells (cell wall) that are in a hypotonic environment is called TURGID.This process is called TURGOR PRESSURE.

WaterWaterWaterWater

When the plasma membrane pulls away from the cell wall (vacuole empty) in a hypertonic environment (loss of water) is called PLASMOLYSIS

Diffusion via carrier proteins ( facilitated diffusion )The solute molecules are bound to a specific protein carrier (permease), which carries them across

is driven by potential energy of a concentration gradient.

e.g. movement pf glucose into red blood cells, which is not inhibited by respiratory inhibitors

Glucose Transporter:How it works..glucose binds to outside of transporter (exterior side with higher glucose conc.)glucose binding causes a conform. change in proteinglucose drops off inside cellprotein reassumes 1st configuration

Types of Protein Transporters: Ion Channelswork by facilitated diffusion No Energy! deal with small molecules... ionsopen pores are gated- Can change shape.important in cell communication

Ion ChannelsWork fast: No conform. changes neededNot simple pores in membrane: specific to different ions (Na, K, Ca...)

Toxins, drugs may affect channelssaxitoxin, tetrodotoxincystic fibrosis

Toxinshow they work

Membrane Transport MechanismsII. Active TransportActive transport- The movement of molecules (small or large) across the plasma membrane & against concentration gradient in which energy (ATP, PEP, light energy, electron transport) is required.

Examples:

1.Sodium (Na) - Potassium (K) Pump2.Exocytosis3.Endocytosis

Sodium-Potassium PumpThe mechanism that uses energy (active transport) released from splitting ATP to transport Sodium (Na+) out of and Potassium (K+) into cells.

Sodium-Potassium Pump. cont.http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/eustruct/sppump.html

Na-K Pump Model: Part I3 Na+ bind to inner region of proteinNa+ binding triggers phosphorylation of protein. ATP ADP + PiPhosphorylation causes conformation change and Na+ binding site faces outside3 Na+ released to outside

Na-K Active Pump: Part II2 K+ ions on outside are able to bindK+ binding causes dephosphorylation and new conformation change2K+ ions exposed to inside and releasedCyclic process uses ATP energy to drive Na & K ion transport against conc. Gradient

Question: How are large molecules transported into and out of the plasma membranes?

Answer: Exocytosis and Endocytosis

EndocytosisTransports macromolecules and large particles into the cell.

Part of the membrane engulfs the particle and folds inward to bud off.

The energy requiring movement of particles (foreign or natural) into the cell.

3 types of endocytosis:A.PhagocytosisB.PinocytosisC.Receptor-mediated endocytosis

Putting Out the GarbageVesicles (lysosomes, other secretory vesicles) can fuse with the membrane and open up the the outside

A. PhagocytosisCell eating: cells engulf particles with pseudopodia and pinches off a food vacuole.

Two examples:1.White Blood Cell2.Amoeba

Phagocytosis. cont.Invagination of membrane surrounding the particles.depression then pinched off to form phagocytic vacuole. Lysosomes release their enzymes Useful products are absorbed and undigested matter is expelled by exocytosis.

B. PinocytosisCell drinking: droplets of extracellular fluid are absorbed into the cell by small vesicles.

Example:1.Fungi

Pinocytosis.... cont.

C. Receptor-Mediated EndocytosisImporting specific macromolecules (hormones) into the cell by the inward budding of vesicles formed from coated pits (receptors).

ExocytosisCell secretes macromolecules (proteins and other biochemicals) out of cell

Part of the Endomembrane System: the fusion of transport vesicles with plasma membrane.

replace the plasma membrane that have been removed by endocytosis or to add new membrane

Exocytosis (Cellular Secretion)

Membrane Permeability1) lipid soluble solutes go through fastersmaller molecules go faster1) uncharged & weakly charged go faster

2) Channels or pores may also exist in membrane to allow transport12

Cellular MembranesREVIEWImportance of MembranesMembrane StructureProteinsFluid Mosaic modelPermeabilityTypes of Transport Passive and Active

Membrane Structure

The cell is highly organized with many functional units or organelles inside. Most of these units are limited by one or more membranes. To perform the functions of an organelle, the membrane is specialized in that it contains specific proteins and lipid components that enable it to perform its unique roles. In essence membranes are essential for the integrity and function of the cell. Membrane functions:be protective regulate transport in and out of cell or organelleallow selective receptivity and signal transduction by providing transmembrane receptors that bind signaling moleculesallow cell recognitionprovide anchoring sites for cytoskeletal components. This allows the cell to maintain its shape and perhaps move to distant sites.provide a stable site for the binding and catalysis of enzymes.regulate the fusion of the membrane with other membranes in the cell via specialized junctions provide a passageway across the membrane for certain moleculesallow directed cell or organelle motility