Cell Membrane

CELL MEMBRANE(Dr. I Dewa Ayu Susilawati, drg. M. Kes)

TujuanSetelah mempelajari bab ini, anda diharapkan mampu:1. Menjelaskan struktur membran2. Menjelaskan transport mll membran3. Menjelaskan signaling sel

PLASMA MEMBRANE FUCTION

Struktur membran• The cell membrane is a

biological membrane that separates the interior of all cells from the outside environment

• It consists of the phospholipid bilayer with embedded proteins, which are involved in a variety of cellular processes such as cell adhesion, ion conductivity and cell signaling

• The biological membrane is a collage of many different proteins embedded in the fluid matrix of the lipid bilayer.

• The lipid bilayer is the main fabric of the membrane, and its structure creates a semi-permeable membrane. The hydrophobic core impedes the difusion of hydrophilic structures, such as ions and polar molecules but allows hydrophobic molecules, which can dissolve in the membrane, cross it with ease.

• Proteins determine most of the membrane's specific functions. The plasma membrane and the membranes of the various organelles each have unique collections of proteins. For example, to date more than 50 kinds of proteins have been found in the plasma membrane of red blood cells.

Lipids membran

• The cell membrane consists of three classes of amphipathic lipids: phospholipids, glycolipids, and cholesterols.

• The amount of each depends upon the type of cell, but in the majority of cases phospholipids are the most abundant. In RBC studies, 30% of the plasma membrane is lipid

Plasma membrane

Lipid E. coli Erthrocyte Rough endoplasmic reticulum

Outer mitochondrial membranes

Phosphatidylcholine 0 17 55 50

Phosphatidylserine 0 6 3 2

Phosphatidylethanolamine

80 16 16 23

Sphingomyelin 0 17 3 5

Glycolipids 0 2 0 0

Cholesterol 0 45 6 <5

Lipid bilayer

• The cell membrane consists primarily of a thin layer of [amphipathic] phospholipids which spontaneously arrange so that the hydrophobic "tail" regions are shielded from the surrounding polar fluid, causing the more hydrophilic "head" regions to associate with the cytosolic and extracellular faces of the resulting bilayer. This forms a continuous, spherical lipid bilayer

Phospholipid cell membrane

fosfolipid

• The fatty chains in phospholipids and glycolipids usually contain an even number of carbon atoms, typically between 16 and 20. The 16- and 18-carbon fatty acids are the most common. Fatty acids may be saturated or unsaturated. The length and the degree of unsaturation of fatty acid chains have a profound effect on membrane fluidity[6] as unsaturated lipids create a kink, preventing the fatty acids from packing together as tightly, thus decreasing the melting temperature (increasing the fluidity) of the membrane.

MEMBRANE FATTY ACIDS

MEMBRANE FATTY ACIDS

Polyunsaturated fatty acids (PUFA)

MEMBRANE FATTY ACIDS

Kolesterol membran

Carbohydrates• Plasma membranes also contain

carbohydrates, predominantly glycoproteins, but with some glycolipids (cerebrosides and gangliosides).

• For the most part, no glycosylation occurs on membranes within the cell; rather generally glycosylation occurs on the extracellular surface of the plasma membrane (hanya dipermukaan ekstraselular)

Proteins membran

• Integral (transmembran protein ):Protein kanal, reseptor, pompa, karier

• Protein tertanam dalam lipid: protein G (berperan pd signaling)

• Protein periferal: enzim, hormon

• The entire membrane is held together via non-covalent interaction of hydrophobic tails, however the structure is quite fluid and not fixed rigidly in place. Under physiological conditions phospholipid molecules in the cell membrane are in the liquid crystalline state. It means the lipid molecules are free to diffuse and exhibit rapid lateral diffusion along the layer in which they are present. However, the exchange of phospholipid molecules between intracellular and extracellular leaflets of the bilayer is a very slow process

• The arrangement of hydrophilic heads and hydrophobic tails of the lipid bilayer prevent polar solutes (e.g. amino acids, nucleic acids, carbohydrates, proteins, and ions) from diffusing across the membrane, but generally allows for the passive diffusion of hydrophobic molecules. This affords the cell the ability to control the movement of these substances via transmembrane protein complexes such as pores and gates.

The Fluid Quality of Membranes

• The cell membrane must be a dynamic structure if the cell is to grow and respond to environmental changes. To keep the membrane fluid at physiological temperatures the cell alters the composition of the phospholipids.

• The right ratio of saturated to unsaturated fatty acids keeps the membrane fluid at any temperature conducive to life. For example winter wheat responds to decreasing temperatures by increasing the amount of unsaturated fatty acids in cell membranes.

• In animal cells cholesterol helps to prevent the packing of fatty acid tails and thus lowers the requirement of unsaturated fatty acids. This helps maintain the fluid nature of the cell membrane without it becoming too liquid at body temperature

Membranes are asymmetric

Fluid mosaic model

• According to the fluid mosaic model of S. J. Singer and Garth Nicolson 1972, the biological membranes can be considered as a two-dimensional liquid where all lipid and protein molecules diffuse more or less easily

TRANSPORT MELALUI MEMBRAN

• membrane transport refers to the collection of mechanisms that regulate the passage of solutes such as ions and small molecules through biological membranes namely lipid bilayers that contain proteins embedded in them.

• The regulation of passage through the membrane is due to selective membrane permeability

Membrane Permeability

• The permeability of a membrane is the ease of molecules to pass through it.

• Permeability depends mainly on the electric charge of the molecule and to a lesser extent the molar mass of the molecule.

• Electrically neutral and small molecules pass the membrane easier than charged, large ones.

• Thermodynamically the flow of substances from one compartment to another can occur in the direction of a concentration or electrochemical gradient or against it.

• Transport against the gradient, will require the input of energy.

Relative permeability of a phospholipid bilayer to various substances

Type of substance Examples Behaviour

Gases CO2, N2, O2 Permeable

Small uncharged polar molecules

Urea, water, ethanol Permeable, totally or partially

Large uncharged polar molecules

Glucosa, fructose Not permeable

Ions K+, Na+, Cl-, HCO3- Not permeable

Charged polar molecules ATP, amino acid, glucose-6 phosphate

Not permeable

DIFFUSION• The nature of biological membranes, especially that of

its lipids, is amphiphilic, as they form bilayers that contain an internal hydrophobic layer and an external hydrophilic layer. This structure makes transport possible by simple or passive diffusion, which consists of the diffusion of substances through the membrane without expending metabolic energy and without the aid of transport proteins.

• If the transported substance has a net electrical charge, it will move not only in response to a concentration gradient, but also to an electrochemical gradient due to the membrane potential. dibahas di biolistrik

Passive diffusion

* Passive diffusion is a spontaneous phenomenon that increases the entropy of a system * Membrane proteins are not involved in passive diffusion

The diffusion velocity of a pure phospholipid membrane will depend on:

• concentration gradient,• hydrophobicity,• size,• charge, if the molecule has a net charge.

Permeability of phospholipid bilayers. Small uncharged molecules can diffuse freely through a phospholipid bilayer. However, the bilayer is impermeable to

larger polar molecules (such as glucose and amino acids) and to ions.

Channel and carrier proteins(A) Channel proteins form open pores through which molecules of the appropriate size (e.g., ions) can cross the membrane. (B) Carrier proteins selectively bind the small molecule to be transported and then undergo a conformational change to release the molecule on the other side of the membrane.

Model of active transportEnergy derived from the hydrolysis of ATP is used to transport H+ against the electrochemical gradient (from low to high H+

concentration). Binding of H+ is accompanied by phosphorylation of the carrier protein, which induces a conformational change that drives H+ transport against the

electrochemical gradient. Release of H+ and hydrolysis of the bound phosphate group then restore the carrier to its original conformation.

Facilitated diffusionthe transport is facilitated by the presence of channel

proteins, which facilitate the transport of, in this instance, water or certain hydrophilic ions and

molecules

Active transport

• In active transport a solute is moved against a concentration or electrochemical gradient, in doing so the transport proteins involved consume metabolic energy, usually ATP

Co transport A transport protein can move various ions and

molecules, they are distinguished according to their directionality

antiporter: (also called exchanger or counter-transporter) transport proteins which transport a molecule against its gradient and at the same time displaces one or more ions along its gradient, both gradients being opposite,

symporter: transport proteins which move a molecule against its gradient while displacing one or more different ions along their gradient which is in the same direction as that of the transported molecule

Transport partikel

SIGNALING

• Koordinasi berbagai fungsi jaringan pada organisme multiselular dimungkinkan karena sel-sel mengambangkan sistem komunikasi. Sel yg letaknya berjauhan berkomunikasi dengan mengirimkan signal kimia, berupa molekul-molekul yang diekskresikan oleh suatu sel, yg kemudian terikat ke membran plasma atau masuk ke dalam sitoplasma sel lain.