2nd Edition 1
Chapter 2:
Cell membrane
cell surfaceand
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Outline
2.1 Components and structure of cell membrane
2.2 Transmembrane transport
2.3 Cell adhesion molecules and cell junction
2.4 Extracellular matrix and cell wall
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2.1 Components and structure of cell membrane
• All cells are surrounded by a layer of membrane;
• In eukaryote cell, membrane compartmentalizes the
cell into sub-compartments termed organelles;
• Prokaryote cell lacks sub-compartment.
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eukaryote and prokaryote cell(See Chapter 1)
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Common functions of plasma membrane
Act as permeability barrier
Intimately engaged in the assembly of cell walls
Form specific junctions between cells
Anchor components of the extracellular matrix
Contain receptor proteins that bind specific signaling
molecules
Take part in the compartmentalization of cell
Energy transduction
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The structure of plasma membrane
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Basic compositions
lipids
proteins
saccharide
2.1 Components and structure of cell membrane
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The basic compositions of some bio-membranes
Membrane Proteins (%) Lipids (%) Saccharide (%)Plasma membrane Red blood cell 49 43 8 Myelin membrane 18 79 3 Liver cell 54 36 10Nucleus membrane 66 32 2Golgi body 64 26 10Endoplasmic reticulum 62 27 10Mitochondrion Outside membrane 55 45 trace Inside membrane 78 22 -Chloroplast 70 30 -
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2.1.1 Lipids in biomembrane
main types of membrane lipids:
Phospholipid
• Phosphoglycerides
• Sphingolipids
Cholesterol (steroids)
amphipathic molecules
hydrophilic head group + hydrophobic tail group
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Phosphoglycerides
PC: phosphatidylcholine X=cholinePE: phosphatidylethanolamine X=ethanolaminePS: phosphatidylserine X=serine
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phosphatidylcholine
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The class of sphingolipids
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Sphingomyelin
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Structure of major phospholipid molecules
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Structure of glycolipid molecules in plasma membrane
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Cholesterol
Cholesterol is smaller than the other lipids of the
membrane and less amphipathic.
Cholesterol is absent from the plasma membranes
of most plant.
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2.1.2 Proteins in biomembrane
Three forms of proteins link to membrane
• Integral proteins (Transmembrane proteins)
• Lipid-anchored membrane proteins
• Peripheral membrane proteins
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Proteins associated with the lipid bilayer
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proteins on cell membrane can be classed to :
• Channel proteins: to form pores for the free transport of small
molecules and ions across the membrane;
• Carrier proteins: to facilitated diffusion and active transport of
molecules and ions across the membrane;
• Cell recognition proteins: to identifie a particular cell;
• Receptor proteins: to bind specific molecules, such as hormones
and cytokines, and mediate signal transduction;
• Enzymatic proteins: that catalyze specific chemical reactions.
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Integral proteins
Cytosolic domain
Exoplasmic domain
Transmembrane domain
hydrophilic surfaces studded in membrane
interact with the aqueous solutions
interact with the hydrocarbon core of the phospholipid bilayer
bind to other molecules or ions
anchoring cytoskeletal proteins
triggering intracellular signaling pathways
form channels and pores
glycosylated
localized to the exoplasmic domains
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Structural basis of integral proteins
(A) α-helix model of bacteriorhodopsin
(B): -barrel model of one subunit of OmpX
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Lipid-anchored membrane proteins
covalent bound to
lipid molecules of
the phospholipid
bilayer.
polypeptide chain
does not enter the
phospholipid bilayer.
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Peripheral membrane proteins
bound to the membrane indirectly by interactions
with integral membrane proteins or directly by
interactions with lipid head groups.
localized to either the cytosolic or the exoplasmic
face of the plasma membrane.
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2.1.3 Membrane carbohydrate
• 2%~10% of membrane content depending on
cell types;
• covalently bound to membrane proteins and
lipids to form glycoproteins or glycolipids;
• all membrane carbohydrate pitch on the
outside of plasma membrane.
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Structure of glycolipid molecules in plasma membrane
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Function of membrane carbohydrate
• Protect cells against mechanical and chemical
damage;
• Preventing unwanted protein-protein interactions;
• Help membrane proteins to form correct three-
dimensional configures ;
• Help to transfer of new proteins to correct position;
• Cell recognition, cell adhension and cell junction.
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2.1.4 structure characters of plasma membrane
Fluid mosaic model
Lipid raft model
Membrane fluidity
Membrane asymmetry
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Arrangement of lipid molecules in an aqueous environment
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Fluid mosaic model
S.J. Singer and G.L. Nicolson in1972
•membranes as dynamic structures in which lipids and proteins are mobile
•lipid bilayers form the basis of the membranes
•proteins either span the bilayer or are attached to either side of the lipid membrane;
•the membranes are asymmetrical
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Lipid rafts model
a complementation for the fluid mosaic model
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Membrane fluidity
The possible movements of phospholipids in a membrane
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The physical state of the lipid of a membrane
• phase transition
liquid-like state frozen crystalline gel
• transition temperature:
the temperature point when the lipid phase
transition appears.
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Factors influence bilayer fluidity
• unsaturation state of the fatty acids in the bilayer;
• the length of the hydrocarbon chains of a lipid;
• cholesterol molecules:
Decrease bilayer fluidity
above the transition
temperature
increase bilayer fluidity
below transition temperature
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Membrane fluidity
Cell fusion technique reveals membrane protein mobility
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Membrane fluidity
Membrane protein mobility revealed by FRAP technique
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Factors influence membrane protein mobility
• Integral proteins
• Membrane lipid fluidity
• ECM
• Cell junctions
• Ligand, antibody and
drug molecules
Restriction on membrane protein mobility by ECM
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Membrane Asymmetry
• The two halves of the bilayer often contain different types of
phospholipids and glycolipids.
• The proteins embedded in the bilayer have a specific orientation
Freeze-fracture replication
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• Lipid-digesting
enzymes that cannot
penetrate the plasma
membrane and are
subsequently only
able to digest lipids
that reside in the
external monolayer of
the bilayer.
Membrane Asymmetry
SM, sphingomyelin; PC, phosphatidylcholine; PS, phosphatidylserine; PE, phosphatidylethanolamine; PI, phosphatidylinositol; Cl, cholesterol
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2.2 Transmembrane Transport
a pure phospholipid
bilayer
• Plasma membrane is semipermeable
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2.2.1 Overview of trans-membrane transport
Property Passive Diffusion Facilitated Diffusion Active Transport Cotransport
requiring specific transport protein No Yes Yes Yes
Solute transported against its gradient
No No Yes Yes
Coupled to ATP hydrolysis
No No Yes No
Driven by movement of a ion down its gradient
No No No Yes
ExamplesO2, CO2, steroid hormones, many drugs
Glucose and amino acids (uniporters); ions and water (channels)
Ions, small hydrophilic molecules, lipids (ATP- powered pumps)
Glucose and amino acids (symporters); various ions and sucrose (antiporters)
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Types of trans-membrane transport Active transport Passive transport
transport proteins channel proteinstransporters
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Three types of transporters
Uniport symport antiport
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2.2.2 Passive transport
• no metabolic energy is expended;
• no specific transport proteins needed;
• molecules move down its chemical concentration
gradient.• Diffusion rate is determined by:
– concentration gradient across the layer– hydrophobicity– size – electric potential across the membrane
Passive diffusion (simple diffusion)
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• polar molecules, ions and water, transport across
membrane by a protein-mediated movement
• exhibits the following distinguishing properties from
passive diffusion:
− The rate is far higher than passive diffusion
− The partition coefficient K is irrelevant
− Occurs via a limited number of uniporter molecules
− Transport is specific.
Facilitated Diffusion
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A typical example of facilitated diffusion
Uniporter mediates passive movement of a glucose solute.
GLUT1 facilitates the unidirectional transport of glucose down its concentration gradient
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• assist diffusion
• water, ions and hydrophilic small molecules
• down concentration or electric potential gradients
• form a hydrophilic passageway across the
membrane
Ion channel
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Ion channel
The structure and ion selectivity of a bacteria K+ channel.
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Ion channel
Channel proteins • nongated channels• gated channels
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Some examples of ion channels Channel Location FunctionsK+ leakage channel The plasma membrane of
most animalsKeep resting potential
Voltage-gated Na+ channel The plasma membrane of neural axon
Produce action potenpial
Voltage-gated K+ channel The plasma membrane of neural axon
Resume resting potential after starting action potenpial
Voltage-gated Ca2+ channel The plasma membrane of nerve terminal
Activate releasing of nerve transmitter
Acetylcholine acceptor Acetylcholine-gated Na+ and Ca2+ channel)
The plasma membrane of muscle cells (The link-end of nerve and muscle)
Excitable synaptic transmission of signals (transform chemical signal to electric one in target cells)
GABA acceptor (GABA- gated Cl- channel)
The plasma membrane of many nerve cells (at synapse)
Inhibitory synaptic transmission of signals
Stress-gated positive ion channel
Auditory hair cells in inner ear
Detect the jutter of voice
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2.2.3 Active transport • mediated by a specific membrane proteins• against their concentration gradient • need the energy supply
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• Four classes: P, V, F and ABC (ATP-binding cassette transporter)
• Energy supply is coupled to the hydrolysis of ATP
ATP-driven pumps
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• Examples of four classes of ATP pumps Class Examples
P-Class Plasma membrane of plants, fungi, bacteria (H+ pump), Plasma membrane of higher eukaryotes (Na+/K+ pump), Apical plasma membrane of mammalian stomach (H+/K+ pump), Plasma membrane of all eukaryotic cells (Ca2+ pump), Sarcoplasmic reticulum membrane in muscle cells (Ca2+ pump)
V-Class Vacuolar membranes in plants, yeast, other fungi, Endosomal and lysosmal membranes in animal cells, Plasma membrane of osteoclasts and some kidney tubule cells.
F-Class Bacterial plasma membrane, Inner mitochondrial membrane, Thylakoid membrane of chloroplast
ABC-Class
Bacterial plasma membranes (amino acid, sugar, and peptide permeases), Mammalian plasma membranes (transporters of phospholipids, small lipophilic drugs, cholesterol, other small molecules)
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• two types: symporter and antiporter
• coupled to an energetically favorable reaction
• use the energy stored in an electrochemical
gradient
• Establishment of these electrochemical
gradients is energy consuming
• secondary active transport.
Cotransporters
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Two types of carriers enable gut epithelial cells to transfer glucose and amino acid across the gut lining
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Comparative of symports in animal and plant cells
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2.3 Cell adhesion molecules and cell junction
• Cell–cell adhesion
• Cell-matrix adhesion
• Cell adhesion molecules (CAMs)
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Three models of cell adhesion
23/5/14 58Major families of CAMs
2.3.1 CAMs
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Cell Adhesion Molecules Family Ligands recognized Stable cell junction
Cadherins Homophilic interactions Adherens junctions and desmosomes
IntegrinsExtracellular matrix Focal adhesions and
hemidesmosomes
Members of Ig superfamily No
Selectins Carbohydrates No
Ig superfamilyIntegrins No
Homophilic interactions No
Cadherin
Homophilic interactions
Selectins
Recognition of specific carbohydrates Heterophilic interactions
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Function of lectins in cell adhesion
Carbohydrate chain recognitionCAMs adhesion
Ig superfamily
Homophilic /heterophilic interactions
• Receptor for ECM• Extracellular domain
interacts with ECM protein
• Intracellular tail interacts with actin
Heterophilic interactions
integrin
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2.3.2 Cell Junctions
Three types of cell junctions:
Tight junction
Anchoring junction
Gap junction
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Cell Junctions
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Tight junction
A current model of a tight junction
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Function of tight junction
• form seals that prevent the free passage of molecules between the
cells of epithelia;
• prevent leakage of molecules across the epithelium though the
gaps between cells;
• separate the apical and basolateral domains of plasma membrane
by preventing the free diffusion of lipids and proteins between them
and help establish and maintain cell polarity.
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Tight junction and cell polarity
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• Cell-cell anchoring junction
●Adherens junction
●Desmosome
• Cell-Matrix anchoring junction
●Focal adhesion
●Hemidesmosome
Anchoring junction
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Adherens Junction
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Desmosomes
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Focal adhesion
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Hemidesmosomes
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Function of anchoring junction
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An overview of the types of interactions involving the cell surface.
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A summary of junctional and nonjunctional adhesive mechanisms
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• In animal
●connexon:
Six connexins assemble to form a connexon
with an open hydrophilic pore in its center;
Connexon in one cell aligns with the connexon
of adjacent cell forming a channel.
• In plant
●plasmodesmata
Gap junction
Function of gap junction
• Mechanical connection;• Electrical coupling :
• Matebolic coupling: (<1000 Dolton)
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Gap junction in animals
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Gap junction in plants
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2.4 Extracellular Matrix and Cell Wall
An overview of cells interact with their environment
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2.4.1 Extracellular Matrix
The extracellular matrix (ECM) is a complex meshwork of
proteins and polysaccharides secreted by cells into the
spaces between them. The ECM plays important roles in
cell-cell signaling, wound repair, cell adhesion and tissue
function.
Three major component of ECM
proteoglycan
structure protein
adhesive protein
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proteoglycan: matrix of ECMcore protein + glycosaminoglycans (GAGs)
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Glycosaminoglycans
• a repeating disaccharide
with a -A-B-A-B-A-
structure
• disaccharides include∶chondroitin sulfate
hyaluronic acid
keratan sulfate
heparan sulfate
…..
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hyaluronic acida nonsulfated GAG, assemble proteoglycans into huge complexes by linkage of the core proteins.
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Collagens and elastins: structure protein of ECMCollagen
•water-insoluble fibrous glycoproteins, the backbone
proteins for ECM.
•most abundant protein in the human body (> 25 percent of
all protein) with high tensile strength:
collagen fiber (Φ1 mm) suspending 10 kg.
•produced primarily by fibroblasts, and also by smooth
muscle cells and epithelial cells.
•tropocollagen consisting of three polypeptide chains
(Gly-X-Y)n
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Structure of Collagens
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Structural features of collagens
• All collagen molecules are trimers consisting of three polypeptide chains, called chains.
• Along at least part of their length, the three polypeptide chains of a collagen molecule are wound around each other to form a unique, rod-like triple helix
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◆Assembling of collagen
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Structure of elastin
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adhesive proteins
Fibronectin (FN)
RGD motif (for integrin binding) mediate cell adhesion to ECM.
Laminin (LN)
Key structural component of basal lamina, a specialized ECM
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Fibronectin (FN)
Laminin (LN)
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2.4.2 Connecting Cells to the ECM
The components of the ECM, such as fibronectin, laminin, proteoglycans, and collagen are capable of binding to receptors situated on the cell surface.
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2.4.2 Connecting Cells to the ECM
• The ECM interacts with the surface of the cell
through fibronectin
• Cells attach to the ECM by means of integrins
• Integrins are receptor proteins which are of
crucial importance
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the integrins binding to RGD motif (Arg-Gly-Asp) in Fibronectin
The most important family of receptors that attach cells to their extracellular microenvironment is the integrins.
Basement membrane (Basal lamina)
Structural components:
•laminin : main component, organizer
• Ⅳ type collagen
•entactin
•perlecan
a specialized ECM structure underlies the epithelium,
which lines the cavities and surfaces of organs.
Structure of basement membrane
• Cellulos• Hemicellulose• Pectin• Lignin• glycoprotein
2.4.3 CELL WALLS structural components of plant cell wall:
90% sugar
CELL WALLS
Layers of plant cell walls
structural components of bacterial cell wall:murein: peptidoglycan
? Function of Penicillin Gram-positive vs Gram-negative
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bacterial cell wall and cytoplasmic membrane
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