CELL MEMBRANE, CHANNELS AND TRANSPORTER

MEMBRAN : membatasi lingk intrasel dg ekstrasel LO Memahami membran sel permeabilitas sel Imemahami lingkungan intrasel MEMBRAN SEL MEMBRAN : membatasi lingk intrasel dg ekstrasel terdiri atas : LIPID dan PROTEIN,KH

MEMBRAN SEL * Tebal : 7,5 – 10 nm * 3 garis paralel - Electron dense layer : 2 garis tebal, hitam lapisan padat electron : 2,5 – 3 nm - Electron-lucent layer : 1 lapisan diantaranya jernih : 3,5– 4 nm

Komposisi membran sel 1. Lipid (40%) : - Phospholipid : * Phosphatidylcholine (lecithin) * Phosphatidylethanolamine (cephalin) - Sphingomyelin - Cholesterol Proteins (55%) : a. Integral proteins  usually they are insoluble in water solutions, are associated with lipids, this proteins may be attached to oligosaccharides  forming glycoproteins b. Peripheral proteins are soluble in aqueous solutions, and are usually free of lipids 3. Carbohydrates (5%) → oligosaccharide : - glycoproteins - glycolipids - glycocalyx

Gambaran struktur membran sel

Molekul protein – lemak penyusun membran Struktur membran Molekul protein – lemak penyusun membran

Molekul fosfolipid pada Membran Gambaran molekul fofsfolipid yang merupakan komponen penting dari membran sel dan menentukan bentuk membran sel

Struktur fosfolipid

Protein Protein integral ; menembus dan tertanam dlm lapisan ganda, terikat pd bagian ekor nonpolar a. Protein transmembran; membentuk saluran (pori-pori) → transport zat yg melewati membran b. peranannya: ● Berikatan dng karbohidrat dpt membentuk sisi reseptor untuk menerima pesan kimia dr sel lain : kelenjar endokrin ● Sebagai pemberi tanda, atau antigen, yg menjadi identitas jenis sel

2. Protein perifer : terikat pada permukaan membran, dapat dng mudah terlepas dr membran. Fungsi : - Protein ini mungkin terlibat dlm struktur pendukung dan perubahan bentuk membran saat pembelahan atau pergerakan sel

Karbohidrat Karbohidrat juga berkaitan dng molekul lipid dan protein. dalam bentuk: * Glikolipid dan glikoprotein → mrpk sisi pengenal permukaan untuk interaksi antar sel mempertahankan sel-sel darah merah : - agar tetap terpisah - atau memungkinkan penggabungan sel-sel yg sama untuk membentuk sebuah jaringan.

Fungsi membran sel: Protective function: The cell membrane protects the cytoplasm and the organelles of the cytoplasm. As a semipermeable membrane: The cell membrane permits only some selective substances to pass through it and acts as a barrier for other substances. Absorptive function: The nutrients are absorbed into the cell through the cell membrane.

Excretory function: The metabolites and other waste products from the cell are excreted out through the cell membrane. Exchange of gases: Oxygen enters the cell from the blood and carbon dioxide leaves the cell and enters the blood trough the cell membrane. Maintenance of shape and size of the cell: The cell membrane is responsible for the maintenance of shape and size of the cell.

Sitoskeleton membran Cytoskeletal network

Type of transport

Transpor melalui membran sel

Pergerakan materi menembus membran sel Prinsip dasar : * TRANSPORT PASIF * TRANSPORT AKTIF * Mekanisme transport pasif : ● difusi bebas ● osmosis ● difusi terfasilitasi ● filtrasi * Mekanisme transport aktif : - memerlukan energi, - enzim

Mekanisme transport aktif 1. Transport aktif diperantarai carrier Carrier : protein integral yg disebut pompa → pompa ion natrium/kalium yg aktif dlm sel hidup. → Pompa kalsium : - kontraksi otot 2. Transport massa berukuran besar : vesikel (vakuola) → * Fagositosis * Pinositosis * Endositosis * Eksositosis

Transport protein, mekanisme pembawa, dan poros tetap Transport molekul-molekul berbeda melewati membran memperlihatkan spesifitas tinggi Permeabilitas molekul berkaitan dengan susuna kimianya. Macam selektifitas ini berkaitan dengan transport protein → carrier / pembawa Ada 2 hipotesis : 1. mekanisme carrier 2. mekanisme poros tetap

Mekanisme poros tetap Mekanisme Carrier

Difusi Water* Gases - CO2 - N2 - O2 Small uncharged polar molecules - ethanol - urea* Hydrophobic molecules - steroid hormones

Large uncharged polar molecules - glucose Charged polar molecules Must be transported ! Ions - K+, Na+, Mg2+, Ca2+, Cl-, HCO3-, HPO42- Large uncharged polar molecules - glucose Charged polar molecules - amino acids - ATP

Hypothetical diagram of simple diffusion through the cell membrane Hypothetical diagram of simple diffusion through the cell membrane. A = Diffusion through lipid layer. B = Diffusion through ungated channel. C - Diffusion through gated channel.

Transport of Small Molecules selectively permeable to small molecules. melalui phospholipid bilayer, membran plasma membentuk barrier yang menghalangi pertukaran molekul antarastiplasma dg lingk eksternal sel Specific transport proteins (carrier proteins and channel proteins) then mediate the selective passage of small molecules across the membrane, allowing the cell to control the composition of its cytoplasm.

Mobility of phospholipids in a membrane Individual phospholipids can rotate and move laterally within a bilayer.

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.

Permeability of phospholipid bilayers : Gases, hydrophobic molecules, and small polar can diffuse through phospholipid bilayers. Larger polar molecules and charged molecules cannot

Hypothetical diagram of facilitated diffusion from higher concentration (ECF) to lower concentration (ICF). through the cell membrane. Stage 1. Glucose binds with carrier protein. Stage 2. Conformational change occurs in the carrier protein and glucose is released into ICF.

Ion channels Conduct ions 10 ions/sec Recognize & select specific ions 8 Conduct ions 10 ions/sec Recognize & select specific ions Open and close in response to specific signals

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 transport Energy 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.

Transporter Uniporter - The facilitated diffusion of glucose is an example of uniport, the transport of only a single molecule. Symporter - The coordinate uptake of glucose and Na+ is an example of symport, the transport of two molecules in the same direction. Antiporter - Active transport can also take place by antiport, in which two molecules are transported in opposite directions.

Model of an ion channel In the closed conformation, the flow of ions is blocked by a gate. Opening of the gate allows ions to flow rapidly through the channel. The channel contains a narrow pore that restricts passage to ions of the appropriate size and charge.

Examples of uniport: Model for the facilitated diffusion of glucose The glucose transporter alternates between two conformations in which a glucose-binding site is alternately exposed on the outside and the inside of the cell. In the first conformation shown (A), glucose binds to a site exposed on the outside of the plasma membrane. The transporter then undergoes a conformational change such that the glucose-binding site faces the inside of the cell and glucose is released into the cytosol (B). The transporter then returns to its original conformation (C).

Examples of symport :  Glucose transport by intestinal epithelial cells The glucose transporter alternates between two conformations in which a glucose-binding site is alternately exposed on the outside and the inside of the cell. In the first conformation shown (A), glucose binds to a site exposed on the outside of the plasma membrane. The transporter then undergoes a conformational change such that the glucose-binding site faces the inside of the cell and glucose is released into the cytosol (B). The transporter then returns to its original conformation (C).

Examples of antiport Ca2+ and H+ are exported from cells by antiporters, which couple to their export to the energetically favorable import of Na+.

Summary Membrane Structure Membrane models have evolved to fit new data: science as a prosses A membrane is a fluid mosaic of lipids, protein, and carbohydrates Osmosis in the passive transport of water Cell survival depends on balancing water uptake and loss Specific protein facilitate the passive transport of selective solutes

Summary Traffic Across Membranes Active transport is the pumping of solute against their gradiens Some of ion pumps generate voltage across membranes In cotransport, a membrane protein couples the transport of one solute to another Exocytosis and endocytosis transport large molecules

NOTES:

Overview of lipids and proteins. ■ The cell membrane is mainly composed of lipids and proteins. ■ Its frame work consists of a double layer of phospholipids. ■ There are two major types of proteins : tightly coiled, rod –shaped, fibrous proteins and the more compact, globular – shaped integral proteins and peripheral proteins.

These are the different types of molecules of the cell membrane. Fibrous protein Phospholipid Examples of globular – shaped proteins H2O Molecule Na+ Glycoprotein Pore protein Channel protein Glycoprotein Pore protein Channel protein

■ Two layer of phospholipids molecules self – assemble so that their water soluble (hydrophilic) heads form the surface and interior of the membrane, and the water insoluble (hydrophobic) tails face each other. Hydrophobic tails Hydrophilic heads

■ The fibrous proteins may span the entire membrane and serve as receptors for the cell

■ One type of globular protein form “pores” to allow lipid insoluble water molecules to pass through. H2O Molecule

■ Other integral proteins serve as channel proteins and selectively transport ions for the cell. H2O Molecule Na+

■ Globular proteins, which are peripheral (associated with the surface of the cell), may be enzymes or glycoproteins (proteins with carbohydrate associations that identify the cell. H2O Molecule Na+

■ Cholesterol molecules are embedded in animal cell. Membranes but not in plant cell membranes, they help make the membrane (along with the phospholipids) impermeable to water- soluble substances. Cholesterol also stabilized themembrane. H2O Molecule Na+

● EXOCYTOSIS ● FILTRATION ● OSMOSIS ● ENDOCYTOSIS SPECIAL CATEGORIES OF TRANSPORT ● ENDOCYTOSIS ● EXOCYTOSIS ● FILTRATION ● OSMOSIS

to a dilute solution through a cell membrane.     Osmosis is the movement of water from a concentrated solution to a dilute solution through a cell membrane.

Permeable Permeable is having pores or openings that permit liquids or gases to pass through, or that something is capable of being permeated.

Vacuole A vacuole is found in both plant and animal cells, but in the plant cells there are fewer of them yet they are larger and in the animal cells the are small. They have fluid filled sacs and store food, water and waste (plants need to store large amounts of food). Vacuoles have membrane-bound sacs for storage, digestion and waste removal. They also contain water solutions.

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