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Biological Membranes and Transport - Coggle Diagram
Biological Membranes and Transport
Three major structures
micelles
have larger, more polar head than tail
兩親分子
fatty acids
sodium dodecyl sulfate
a few thousand lipid molecules
bilayers
two leaflets (e.g., layers) of lipid monolayers
form in aqueous solution
phospholipids
sphingolipids
polar head groups and more than one lipid tail
liposomes
spherical vesicle
合成囊泡膜可以在體外製造，並且可以包含人工插入的蛋白質。
The central aqueous cavity can enclose dissolved molecules.
useful artificial carriers of molecules (e.g., drugs)
囊泡很容易與細胞膜或彼此融合。
Functions of Membranes
Define the boundaries of the cell
import and export
Selective import of nutrients (e.g. lactose)
Selective export of waste and toxins (e.g. antibiotics)
將代謝物和離子保留在細胞內
感知外部信號並將信息傳輸到細胞中
在細胞內提供分隔
separate energy-producing reactions from energy-consuming ones
keep proteolytic enzymes away from important cellular proteins
產生和傳遞神經信號
將能量存儲為質子梯度
Support synthesis of ATP
Common Features of Membranes
30–100Å
two leaflets of lipids
stabilized by noncovalent forces
Asymmetric
Carbohydrate moieties are attached on the outer leaflet.
它們可以被電極化
Two leaflets have different lipid compositions
The outer leaflet is often more positively charged.
Phosphatidylserine outside has a special meaning
platelets
blood clotting
other cells
marks the cell for destruction
The Composition of Membranes
Lipid composition of membranes varies by
organisms
tissues
organelles
脂質與蛋白質的比例不同
type of phospholipid varies
abundance and type of sterols varies
lack of sterols in prokaryotes
膽固醇在質膜中占主導地位，在線粒體中幾乎不存在
galactolipids abundant in plant chloroplasts but almost absent in animals
Archaea Have Multiple Unique Membrane Constituents
Unique glycerol chirality in phospholipids
L-glycerol in archaea
D-glycerol in bacteria
Unique fatty acids
branched isoprene chains in archaea
unbranched fatty acid chains in bacteria
Unique linkages
ether linkages in archaea
ester linkages in bacteria
Membrane topology
monolayer in some archaea
Lipid Monolayer in Archaea
Functions of Proteins in Membranes
Receptors: detecting signals from outside
light
hormones
neurotransmitters
pheromones
Channels, gates, pumps
nutrients (maltoporin)
ions (K-channel)
neurotransmitters (serotonin reuptake protein)
Enzymes
lipid biosynthesis (some acyltransferases)
ATP synthesis
Membrane Proteins
Peripheral Membrane Proteins
Associate with the polar head groups of membranes
與膜的結合相對鬆散
通過與整合膜蛋白的脂質或水性結構域的離子相互作用
通過使用高鹽或改變 pH 值破壞離子相互作用來去除
Purified peripheral membrane proteins are no longer associated with any lipids.
Amphitrophic Membrane Proteins
attached to the membrane by covalent interaction with lipids or carbohydrates attached to lipids.
生物調節導致脂質附著或裂解
Lipid Anchors
lipoproteins
contain a covalently linked lipid molecule
long-chain fatty acids
isoprenoids
sterols
glycosylated phosphatidylinositol (GPI)
The lipid part can become part of the membrane.
The protein is now anchored to the membrane.
reversible process
Farnesylation of Proteins
Proteins can be targeted to the inner leaflet of the plasma membrane by farnesylation.
catalyzed by farnesyl transferase
Integral Membrane Proteins
Span the entire membrane
asymmetry
different domains in different compartments
Tightly associated with membrane
Removed by detergents that disrupt the membrane
Purified integral membrane proteins still have phospholipids associated with them.
Membrane Proteins Also Contain β-Sheets
Physical Properties of Membranes
Not permeable to large polar solutes and ions
Permeable to small polar solutes and nonpolar compounds
Permeability can be artificially increased by chemical treatment.
when we want to get DNA into the cell
Membrane Phases
Depending on their composition and the temperature, the lipid bilayer can be in gel or fluid phase
liquid-ordered state (i.e., “gel phase”)
individual molecules do not move around
liquid-disordered state (i.e., “fluid phase”)
individual molecules can move around
Under physiological conditions, membranes are more fluid-like than gel-like.
Organisms Can Adjust the Membrane Composition
Membrane fluidity is determined mainly by the fatty acid composition and melting point.
More fluid membranes require shorter and more unsaturated fatty acids.
Sterols and Hopanols Increase Membrane Rigidity and Permeability
Cell membranes of many eukaryotes contain sterols
cholesterol in animals
phytosterols in plants
ergosterol in fungi
Cell membranes of aerobic prokaryotes contain hopanols.
Membrane Dynamics
Lateral Diffusion
fast up to 1 μm/sec
Fluorescence recovery after photobleaching (FRAP)
通過監測螢光返回率來監測側向脂質擴散。
Step 1
Label Outside of Cell
Step 2
Photobleaching
Step 3
Measure Recovery
Transverse Diffusion
非常慢 t1/2 in days
需要催化
Flippases,Flopase,Scramblase
需要耗能
Membrane Rafts
Cholesterol
GPI-linked protein
Acyl groups
Caveolin
Doubly acylated protein
Neurotransmitter Release Is Mediated by SNARE-Type Proteins
T-SNARE
target membrane
V-SNARE
vesicle membrane
Q-SNARE (e.g., SNAP-25)
regulatory proteins that are Ca2+ induced