Chapter 1
Membranes
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Phospholipid Bilayer
Phospholipid
2 fatty-acid chains
Glycerol + Phosphate + nitrogenous compound
One end strongly non-polar
Other end strongly polar
Polar head Hydrophilic
Nonpolar tails Hydrophobic
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Cell Membrane Structure
Davson-Danielli Model (1935)—
phopholipid bilayer sandwiched between 2 protein layers.
Singer-Nicolson, Fluid Mosaic Model (1972)—Membrane proteins
embedded in phospholipid bilayer. Proteins free to move in the
lipid bilayer.
Cell membrane is selectively permeable
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Components of Cell Membrane
Eukaryotic cells have many membranes. All share similar structure
4 components:
Phospholipid bilayer—
All other components embedded in bilayer. Barrier to watersoluble molecules
Transmembrane proteins—
Span width of bilayer. Involved in transport. Can move within
bilayer
Interior protein network—
Membranes supported by intracellular proteins. Other proteins
allow for movement of transmembrane proteins within bilayer.
Cell surface markers—
Sections of cell membrane have different types of glycolipids
and glycoproteins. Act as cell identity markers.
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5
Types of Membrane Proteins
Transporters- Involved in the movement of substances.
Enzymes- Allow chemical reactions on the interior of membrane
Cell surface receptors- Detect chemical messages
Cell surface identity markers- different cell types have unique
markers
Cell adhesion proteins- cause cells to stick to one another
Attachments to cytoskeleton- proteins attach to cytoskeleton
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Membrane Protein Structure
•
•
Anchoring proteins to the bilayerModified lipids link to membrane phospholipid and bond directly
to protein. Protein tethered to cell membrane
Transmembrane proteinsnonpolar region of protein in bilayer
polar regions extend outward
Different types
–
Single-pass anchors- pass through bilayer only once
 Ex hormone receptors
–
Multiple-pass channels and carriers- multiple alpha helix
sections pass through membrane (move ions and water
soluble molecules)
–
Pores- beta sheet sections of protein form large opening that
allow water and other material to move through membrane
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Movement of materials across cell membrane
Passive Transport —Does NOT require energy
Diffusion
Facilitated Diffusion
Osmosis
Active Transport —Requires the expenditure of energy
Sodium-Potassium Pump
Coupled Transport
Other means
Endocytosis—Phagocytosis and Pinocytosis
Receptor-mediated endocytosis
Exocytosis
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Passive Transport
Molecules and Ions move about constantly and randomly
•
Diffusion
– Net movement from areas of high concentration to areas
of low concentration.
Equilibrium
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Cell Membrane Is Selectively Permeable
Many molecules needed by cells are polar
Polar molecules cannot cross non-polar Phospholipid barrier
Polar molecules diffuse through channel proteins
Different types of channels assist specific molecules
Ions diffuse through Ion Channels
Cations and Anions cannot move
through phospholipid layer
Interior of channel proteins is polar
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Facilitated Diffusion
Carrier Proteins transport ions and other solutes (ex. glucose and
amino acids) across membrane down concentration gradient
Facilitate movement by physically binding molecules on one side of
membrane then releasing them on the other (configuration change)
Characteristics of Facilitated diffusion
Specific- each carrier type
transports specific molecules
Saturates- if all carriers in use,
Increase in gradient does not
Increase rate of transport
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Osmosis
Diffusion of water across a selectively permeable membrane
Cytoplasm of a cell is an Aqueous Solution lots of substances
(solute) dissolved in water (solvent).
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Osmosis—Cont.
Osmotic concentration - concentration of all solutes in an Aqueous
solution
Isosmotic –
solute concentrations are equal
Hyperosmotic –
solution with the higher solute
concentration
Hyposmotic –
solution with the lower solute concentration
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Active Transport
Active transport requires energy (ATP’s) to move a substance against
its concentration gradient.
Involves highly selective protein carriers within the membrane
Allows cells to maintain specific ions or molecules at cytoplasmic
concentration that are higher or lower than in the extra-cellular fluid.
Example of an active transport mechanism
Sodium-potassium pump
1/3 of all ATP’s used by a cell fuels this pump
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•
•
Endocytosis - enveloping food
– phagocytosis - material taken in is in
particulate form
– pinocytosis - material taken in is in liquid
form
Exocytosis - discharge of material from
vesicles at the cell surface
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1.
Apa perbedaan plasmolisis, krenasi
dan lisis?
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a. Lisis : hancurnya sel karena rusaknya atau
robeknya membran plasma.
b. Krenasi : sel berkeriput karena kekurangan air
c. Plasmolisis : peristiwa lepasnya membran plasma
dari dinding sel
d. Osmosis adalah perpindahan ion atau molekul air
(pelarut) dari kerapatan tinggi ke kerapatan rendah
dengan melewati suatu membran (disebut juga
difusi lewat membran)
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2. Mengapa peristiwa pompa Na+ dan K+
termasuk transpor aktif? Jelaskan!
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Mekanisme pompa Na+ - K+
1. ATPase mengikat Na+ pada permukaan sitoplasmik ®
mengakibatkan ATPase menghidrolisa ATP ® ADP + P + E.
2. ATPase mengalami fosforilasi.
3. Terjadi perubahan konfigurasi dari ATPase ® Na+ ditranspor
keluar sel.
4. ATPase mengikat K+ pada permukaan ekstra seluler.
5. ATPase mengalami defosforilasi.
6.
ATPase kembali pada posisi semula. K+ ditranspor ke dalam
sel.
2. Pompa Na+ dan K+ termasuk transpor aktif sebab:
- terjadi perpindahan zat dari konsentrasi rendah ke konsentrasi
tinggi
- perpindahan zat melawan gradien konsentrasi membuthkan
energi
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EXPERIMENTAL
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Diffusion, Osmosis, Plasmolisis, Crenation
Materials
Allium cepa
(bawang merah)
Solanum tuberosum
(kentang)
Pheretima sp.
(cacing tanah)
Rhoe discolor
(adam hawa)
Ink (tinta)
Bottle
Sugar
Silet
Sum
2
2
2
1
1 bottle
2
insufficient
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