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Cell membrane and transport of substances across membranes
The cell membrane is mainly composed of lipids and proteins, of which the lipids are mainly phospholipids, which form the basic skeleton of the cell membrane. Proteins are embedded in or embedded in the phospholipid bilayer and perform a variety of functions.
Edited at 2024-10-31 08:47:18
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Cell membrane and transport of substances across membranes
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Cell membrane and transport of substances across membranes
The cell membrane (plasma membrane) is a thin film covering the surface of the cytoplasm
Plasma membrane and biofilm systems are collectively referred to as biofilms
Intracellular membrane system: a general term for model structures within cells that have certain connections.
Section 1 Chemical composition and biological characteristics of cell membrane
membrane lipids
Phospholipids
Glycerophospholipid
Phosphatidylcholine (Lecithin)
Phosphatidylethanolamine. cephalin
Phosphatidylserine
Phosphatidylinositol, located in the inner layer of the plasma membrane, is less abundant in the membrane structure but plays an important role in cell signaling.
With glycerol as the skeleton, the hydroxyl groups at positions 1 and 2 of the glycerol molecule form ester bonds with fatty acids, and the hydroxyl group at position 3 forms ester bonds with the phosphate group. Phospholipid molecules are called amphipathic molecules or amphoteric molecules because they have a hydrophilic head and a hydrophobic tail.
The small hydrophilic group and the phosphate group are highly polar and are called the head group. The hydrophilic head group
Fatty acid chains are hydrophobic, non-polar, and are called hydrophobic tails
Sphingomyelin
The only phospholipid on the cell membrane that does not use glycerol as the backbone, the membrane content is less, and the neuron cell membrane content is more.
Sphingosine is used instead of glycerol. The long-chain unsaturated fatty acid is bonded to the amino group. One hydroxyl group at the end of the molecule is bonded to choline phosphate, and the other free hydroxyl group forms hydrogen with the polar head of adjacent lipid molecules, water molecules, and membrane proteins. key
cholesterol
amphipathic molecule
The polar head is a hydroxyl group, and the middle is a sterol ring, connected to a short hydrophobic hydrocarbon chain tail.
The hydroxyl group is close to the polar head of the phospholipid molecule, the sterol ring is fixed on the hydrocarbon chain near the head, and the tail of the hydrocarbon chain is buried in the center of the lipid bilayer.
Cholesterol tends to be evenly distributed between lipid bilayers
Glycolipids
Composed of lipids and oligosaccharides, bacterial and plant cells are derivatives of glycerophospholipids (mostly phospholipid choline), and animal cell membranes are almost all derivatives of sphingosine, called glycosphingolipids.
鞘糖脂特点：糖基取代了磷脂酰胆碱作为极性的头部。
Cerebroside, the simplest, major glycolipid in myelin
Gangliosides, most abundant in the plasma membrane of neurons
They are all located on the non-cytoplasmic side of the plasma membrane, and the sugar groups are exposed on the cell surface.
membrane lipids
Existence form in experiment
1. Globular molecular cluster
2. Bilayer
Features as an ideal structure for biofilms
1. Form a barrier that separates two water-soluble environments
2. Continuous, with the tendency to fuse with itself to form a closed cavity.
3. Soft and deformable
membrane protein
Intrinsic membrane protein, also known as transmembrane protein, amphipathic molecule
In a single membrane crossing, the alpha helix conformation crosses the hydrophobic region of the lipid bilayer. Generally, the N segment of the peptide chain is located on the outside of the cell membrane.
multiple membrane piercings
multi-subunit transmembrane protein
a helix conformation
Beta tubes penetrate membranes and are mainly found in mitochondria, chloroplast outer membranes, and bacterial plasma membranes.
There are at least 8 and up to 22 beta chains surrounding the tube, and they are linked by hydrogen bonds.
Extrinsic membrane proteins, also known as peripheral proteins, are proteins that are loosely bound to the cell membrane and do not insert into the double lipid layer. They are distributed on the cytoplasmic side or extracellular side of the plasma membrane.
Some through non-covalent bonds (such as weak electrostatic interactions) the polar region of the lipid molecule head or the hydrophilic side of the membrane-penetrating protein
Some peripheral proteins are located on the cytoplasmic side of the membrane and are bound to the membrane by interacting with the monolayer of the cytoplasmic side of the lipid bilayer through the hydrophobic face of the α-helix exposed on the protein surface.
Lipid-anchored proteins, adiponectins, can be located on both sides of the membrane and bind covalently to lipid molecules in the lipid bilayer.
Located on the cytoplasmic side of the plasma membrane, it is directly anchored by forming covalent bonds with certain amyl chains or isoprenyl groups in the lipid bilayer.
Proteins located outside the plasma membrane are anchored to the plasma membrane by covalently bonding to oligosaccharide chains connected to phosphatidylinositol molecules in the outer layer of the lipid bilayer. They are also called glycosylphosphatidylinositol-anchored proteins.
Detergent
SDS ion type
The hydrophobic region of the detergent molecule replaces the phospholipid molecule and binds to the hydrophobic region of the membrane-penetrating protein. It also binds to the hydrophobic tail of the phospholipid molecule, thereby separating the membrane-penetrating protein from the phospholipid molecule.
TritonX-100 non-ionic detergent
Its polar segment is uncharged and acts similarly to SDS, but is milder.
membrane sugar
93% of sugars are combined with membrane proteins in the form of oligosaccharide or polysaccharide chains to form glycoproteins. Glycosylation mainly occurs on asparagine, followed by serine and threonine residues, and often at several positions. Glycosylation occurs simultaneously.
Most membrane proteins carry multiple oligosaccharide chains
7% of membrane sugars are covalently bound to membrane lipids as oligosaccharide chains to form glycolipids.
Each glycolipid molecule carries only one oligosaccharide chain
Cell coat or glycocalyx
A carbohydrate-rich peripheral zone on the surface of most eukaryotic cells
Nowadays, the extracellular coat is generally used to refer to the carbohydrate substances connected to the plasma membrane, that is, the oligosaccharide chain portion extended by the glycoproteins and glycolipids in the plasma membrane to the outer surface. Therefore, the extracellular coat is essentially a plasma membrane structure. part. The extracellular covering that is not connected to the plasma membrane is called extracellular material or extracellular structure.
The basic function is to protect cells from various physical and chemical damages. It can also establish a water-salt balanced microenvironment around cells and participate in cell recognition, adhesion, and migration.
The biological properties of cell membranes are asymmetry and fluidity
(1) Membrane asymmetry determines the directionality of membrane function
Membrane lipid asymmetry
Distributed in lipid bilayer
Outer layer
Sphingomyelin and phosphatidylcholine
inner layer
Phosphatidylethanolamine
Phosphatidylserine
Phosphatidylinositol
Membrane protein asymmetry
Membrane sugar asymmetry
Cell membrane glycolipids and glycoprotein oligosaccharide chains are only distributed on the outer surface of the plasma membrane. In the inner membrane system, oligosaccharide chains are distributed on the inner surface of the membrane cavity.
It is closely related to the directionality and asymmetry of membrane function, ensuring its high degree of order.
(2) Membrane fluidity is the guarantee of membrane functional activities
The lipid bilayer is a two-dimensional liquid crystalline fluid
phase change
phase change temperature
How membrane lipid molecules move
Lateral diffusion, main mode of movement
flipping motion
rotational motion
bending motion
Factors affecting membrane fluidity
The degree of saturation of fatty acid chain
Unsaturated is called linear type and has the greatest aggregation tendency and is closely arranged into a gel state.
Unsaturation bends at the double bond and is loosely arranged, increasing fluidity.
The more unsaturation, the lower the phase transition temperature.
The length of the fatty acid chain
The shorter the length, the greater the fluidity and the lower the phase transition temperature.
Dual regulatory effects of cholesterol
Not only stabilizes the plasma membrane, but also prevents sudden decrease in fluidity
Lecithin to sphingomyelin ratio
High sphingomyelin content reduces fluidity
Effect of membrane proteins
interface grease
Fat-rich area
Mercury of membrane protein
lateral diffusion
rotational motion
3. Multimolecular structural model of cell membrane
1) Sheet structure
Two layers of phospholipid molecules, with the inner and outer surfaces covered by a layer of globular protein molecules, forming a protein-phospholipid-protein triple-plywood structure.
2) Unit membrane model
It is believed that the proteins on both sides of the lipid bilayer molecular layer are not spherical but are single peptide chain proteins in the form of b-sheets that bind to the polar ends of phospholipids through electrostatic interactions.
3) Flow mosaic model
The lipid bilayer in the membrane constitutes the coherent main body of the membrane, which has the orderliness of crystalline molecular arrangement and the fluidity of a liquid. Proteins in the membrane associate with the lipid bilayer in different ways. It is a dynamic, asymmetric and fluid structure
lattice tessellation model
The mosaic protein and surrounding lipid molecules form a crystalline part of the membrane that is fluid.
plate mosaic model
In the flowing lipid bilayer, there are lipid plates of different sizes and greater rigidity that can move independently.
4) Lipid raft model
The membranous bilayer contains microdomains composed of specialized lipids and proteins, rich in cholesterol and sphingolipids, and are called lipid rafts.
The fat raft forms an effective platform, with two characteristics: many proteins gather in the fat raft to facilitate interaction; provide an environment that is conducive to protein transformation and form an effective conformity.
Functions, involved in signaling, receptor-mediated endocytosis, and cholesterol metabolic transport.
Section 2 Transport of small molecules and ions across membranes
1. Simple diffusion of substances depends on the permeability selectivity of the membrane
The smaller the molecular weight, the stronger the lipid solubility, and the faster it passes through the lipid bilayer.
subtopic
An uncharged polar small molecule that rapidly
An uncharged polar small molecule that rapidly
It is highly impermeable to charged
Non-polar small molecules, such as oxygen and other gases. fast
Larger molecules, such as glycerol. Slow, barely enough glucose
Simple diffusion, also called passive diffusion
2. Membrane transport proteins mediate the transport of substances across membranes
membrane transport protein
carrier protein
Can mediate both passive and active transport
channel protein
Passive transport
Along the concentration gradient
along the electrochemical gradient
1) Facilitated diffusion is passive transport mediated by carrier proteins
Highly specific, example: glucose.
The two conformations of the carrier protein are alternately changed to complete the process of penetrating the membrane protein multiple times.
The rate of facilitated diffusion is proportional to the concentration difference of the solute within a certain limit. When the diffusion rate reaches a certain level, it is no longer affected by the concentration. The rate of simple diffusion is always proportional to the concentration difference.
2) Active transportation is an energy -consuming transportation of the carrier protein inverse gradient gradient
ATP-driven pumps are all transmembrane proteins
P-type ion pump
To transport cations, they all have two large subunits (a subunits), and most have two small b subunits. At least one a catalytic subunit undergoes phosphorylation and dephosphorylation reactions.
1) Na-K pump
The molecular weight of subunit A is 120KD. It is an integral protein that penetrates the membrane multiple times and has ATPase activity. The B subunit has a molecular weight of 50KD and is a tissue-specific glycoprotein. It does not directly participate in the transmembrane transport of ions, but it can help the newly synthesized A subunit of the endoplasmic reticulum to fold. When the two separate, the A subunit loss of enzyme activity.
Aspartate residues covalently bind to phosphorylate
3 Na are released and 2 K are obtained
Na-K pump inhibitor ouabainin
2) Ca pump
Hydrolyze one ATP and transport 2 Ca into it against the concentration
V-type proton pump
Membrane acidic compartment of eukaryotic cells
Transport H ions
Requires ATP function but does not form phosphorylation intermediates
F-type proton pump
Bacterial plasma membrane, mitochondrial inner membrane, chloroplast membrane
Learn about proton transport and ATP synthesis
ABC transporter
transport protein
Excretion of toxins, xenobiotics, and metabolites into urine, bile, and intestinal lumen
2 transmembrane domains T and 2 cytoplasmic ATP-binding domains
flippase model
co-transportation
Co -transportation
Two solute molecules are transported across the membrane in the same direction
Opposite transport
A process in which the same membrane protein transports two different ions or molecules across the membrane in opposite directions.
Features: 1. Small molecule substances are transported across the membrane against concentration or electrochemical gradient. 2. It requires energy consumption. ATP can be directly hydrolyzed or the energy from the ion electrochemical gradient is used. 3. It needs to be mediated by a specific carrier protein on the membrane.
3) Ion channels efficiently transport various ions
channel protein, also called ion channel
Features: 1. Passive transport, the channel is bidirectional, the net flux of ions depends on the electrochemical gradient, and is not combined with solute molecules during the process. 2. It is highly selective for the size and charge of the transported ions. 3. High transport rate. 4. Most ion channels are not continuously open
type
1. Ligand-gated channels
acetylcholine receptor
2. Voltage gated channel
Changes in membrane potential are the direct factor controlling the opening and closing of voltage-gated channels.
1) Structure of K ion channel
A single channel is composed of four identical a subunits. S4 is a voltage-sensitive segment. The N segment of the A subunit forms a spherical region in the cytoplasm. S5S6 is connected to the polypeptide segment of H5. The H5 fragment sticks into the center of the channel, forming a sufficiently large loop of residues.
2) Switching mechanism of K channel
Ball and chain model
3) Selectivity of K channel
Inverted tapered pore, selective filter
Electronegative oxygen atoms interact to replace water molecules in the hydration layer, allowing energy to be compensated. Utilizing electrostatic repulsion, K can quickly pass through the pore along the electrochemical gradient.
3 mechanical gated channels
The channel protein senses external forces acting on the cell outer membrane and undergoes conformational changes.
Ion transport across membranes and membrane potential
1) Resting potential
polarization
Determinants of resting potential
One is the diffusion potential caused by the electroosmosis phenomenon of ions along the concentration gradient.
The other is the membrane potential difference caused by the Na-K pump
2) Mediation of action potential
Since the decrease in membrane potential results in a decrease in polarity on both sides of the membrane, this phenomenon is called depolarization.
exceeds threshold
action potential
hyperpolarization
4) Water Channel (AQP)
1. Classification of water channels
Can only cross water 1, 2, 4, 5, 6, 8
Water, glycerin and urea, etc. - water glycerol channel 3, 7, 9, 10
11, 12 The third type of AQP subfamily
2. Structure of aquaporin
A tetramer surrounded by four symmetrically arranged cylindrical subunits. There is a central pore in the center of each subunit that only allows water molecules to pass through. 6 long A helices, 2 short helices, NPA motif.
3. The screening mechanism of water molecules by water channels
1. The diameter of the central pore channel of AQP 1 limits the passage of molecules larger than water molecules.
2. Control of solute binding sites in the central pore of AQP1. The energy in the dehydration process is compensated.
It is generally believed that water channels are proteins that are in a continuously open state.
Section 3 Transport of macromolecules and substances across membranes
1. Endocytosis
1) Phagocytosis is the process of ingestion of particulate matter by phagocytes
The membrane vesicles formed by phagocytosis are called phagosomes or phagosomes
2) Pinocytosis is the process by which cells engulf liquids and soluble substances
The process of non-specific uptake of extracellular fluid by cells. When a certain soluble substance around the cells reaches a certain level, pinocytosis can occur.
Pinocytosome
Liquid phase swallows each other
nonspecific intrinsic endocytosis
adsorption endocytosis
Large molecules or small particles are first adsorbed on the cell surface in some way
After entering the cell, it fuses with endosomes or lysosomes and is degraded.
3) Receptor-mediated endocytosis improves the efficiency of uptake of specific substances
Specific uptake of components with very low extracellular content
1. The formation of tegument cells and vesicles
Receptors are concentrated in specific areas of the plasma membrane called vesicles. There are fossae that act as selective receptors. The inner surface of the plasma membrane in the depression is covered with a spiky layer of electron-dense material, including clathrin and adapter proteins.
Extracellular solutes (ligands) bind to receptors at coated pits, and clathrin accumulates on the cytoplasmic side of coated pits, further invaginates, separates from the plasma membrane, and forms coated vesicles to enter the cell. The outer surface is covered with vesicles and is composed of a cage-like basketry structure assembled from clathrin.
Clathrin consists of 3 heavy chains and 3 light chains. ——Trileg protein complex.
The main function is to pull the plasma membrane inward and indent it
adapter protein
Participate in the formation of the envelope and serve as a link. Can specifically bind to different types of receptors.
Clathrin has no specificity and its specificity is regulated by adapter proteins.
2. No vesicles are formed and fused with endosomes
Clathrin plays a role in pit formation
Involvement of the small molecule GTP-binding protein dynamin.
Fall off and become no vesicles
Binds to early endosomes
Endosome is a membrane-surrounded organelle formed in animal cytoplasm through endocytosis. Its function is to transport newly ingested substances through endocytosis for lysosomal degradation.