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CH. 7: Membrane Structure and Function - Coggle Diagram
CH. 7: Membrane Structure and Function
Passive Transport
Diffusion (movement of particles into available space) = result of thermal energy
Substance diffuses from area of high concentration to a low concentration area
Diffuses down concentration gradient (region along which density increases/decreases)
Diffusion = spontaneous
Molecules move randomly, but movement of population of molecules may be directional
Passive transport = diffusion of substance across biological membrane; requires no energy input
Effects of Osmosis on Water Balance
Osmosis = diffusion of free water across selectively permeable membrane
Water moves from low solute concentration to high
Water balance in cells w/out cell walls
Tonicity = ability of surrounding solution to cause cell to gain/lose water
Isotonic
Equilibrium; no net movement; transfer at equal rate
Hypertonic (more solute in surrounding solution)
Cell loses water, shrivels, likely dies
Hypotonic (less solute in surrounding solution)
Water enters cell, cells lyse (burst), swell up
osmoregulation = control of solute concentrations + water balance
Animals must adapt osmoregulation methods when in hyper/hypotonic solutions
Water balance in cells w/ cell walls
Includes plants, prokaryotes, fungi, some protists
Cell wall helps maintain water balance
healthy plant cells = turgid (very firm) due to turgor pressure
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Facilitated Diffusion
Passive diffusion via transport proteins
Channel proteins
Ion channels = channel proteins that transport ions
Many are gated channels (open/close in response to stimulus)
Stimulus examples: electrical, chemical
Carrier Proteins
Seem to change shape to allow substances through
May be triggered by binding + release of transported molecule
The Fluid Mosaic Model
Membrane staples = lipids, proteins
most abundant = phospholipids
Phospholipids = amphipathic (has hydrophobic and hydrophilic regions)
most membrane proteins = amphipathic
Fluid mosaic model = proteins suspended in fluid phospholipid layer
Fluidity of Membranes
Structure
Mostly held together by hydrophobic interactions
Lipids and some proteins shift sideways
Move fluidly past each other, may flip to other side of layer
Properties
Movement = very fast
adjacent phospholipids switch positions ~10^7 times a second
proteins move slower (much larger)
may seem immobile, drift, or move in highly directed ways
Temperature
As temp drops, membrane solidifes
May remain fluid if contains phospholipids with unsaturated tails
Kinks in the hydrocarbon tails prevent tight packing, results in more fluid membrane
Fluidity
Must be fluid to work properly
Fluidity affects permeability and membrane protein movement
If solidified, enzymatic proteins may become inactive
If too fluid, protein function not supported
Membranes usually about as fluid as olive oil
Cholesterol
Exists between phospholipid molecules
affects membrane fluidity, depends on temperature
High temps
Makes less fluid, restricts phospholipid movement
Lowers temp required to solidify; "fluidity buffer"
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Evolution of Differences in Lipid Composition
Lipid variations appear to have evolutionary purposes
Examples
Fish in extreme cold
Have fatty acids with unsaturated hydrocarbon tails
Enable fluidity of membranes even during cold
Bacteria + Archaea in extreme heat
Have strange lipids that may prevent excessive fluidity
Ability to change composition
Property that evolved in organisms where temperature varies
Examples
Plants that tolerate cold
Unsaturated phospholipid concentration increases in Autumn
Bacteria + Archaea
Unsaturated phospholipid concentration varies by temp
Membrane Proteins + Functions
Proteins determine most of membrane's functions
Different cells have different membrane proteins
2 major populations
Integral proteins
Penetrate hydrophobic interior of bilayer
Most = transmembrane proteins
Transmembrane proteins span membrane (instead of extending partway through)
peripheral proteins
Bound on surface; not embedded in bilayer
Important in medical field
Example
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Functions
Intercellular joining
Proteins of adjacent cells may hook together in junctions
Cell-cell recognition
Some glycoproteins = tags recognized by membrane proteins of other cells
Signal transduction
Receptor proteins bind with chemical messengers to relay information through cell by changing shape
Attachment to cytoskeleton and ECM
Microfilaments + other cytoskeleton components may noncovalently bond w/ membrane proteins
Helps maintain cell shape, stabilizes protein locations
These proteins help coordinate extra/intracellular exchanges
Enzymatic activity
Enzymes embedded with active sites exposed to solution, part of metabolic processes
Transport
Transport proteins span membrane, allows for substance transport
Roles of Membrane Carbs
Carbs participate in cell-cell recognition
cell-cell recognition = cells ability to tell one neighboring cell from another
cell-cell functions
basis for immune system's rejecting foreign cells
Binding to molecules on the extracellular plasma membrane surface
Extracellular carbs vary from species to species and cell to cell
Example: variations in carbs result in different blood types
important to tissue + organ cell sorting in embryos
membrane carb structure
Usually short, branched, less than 15 sugar units
Some covalently bonded to lipids
Forms glycolipids
Most covalently bonded to proteins
Forms glycoproteins
Membrane Synthesis and Sidedness
Membranes have distinct outer + inner faces
may differ in lipid concentration
Proteins have directional orientation in membrane
Asymmetricity
proteins, lipids, carbs arranged asymmetrically
Determined as membrane is built
Selectively Permeable Membrane Structure
Selective permeability = some substances cross easier than others
Permeability of Lipid Bilayer
Nonpolar molecules = hydrophobic (e.g. hydrocarbons, CO2, etc.)
Can cross easily w/out membrane proteins
Hydrophilic (polar) molecules + ions can't pass due to hydrophobic interior
Includes water, glucose + other sugars, charged atoms, etc.
Membrane proteins play key role in transport regulation
Transport Proteins
Types
Channel Protein
Has hydrophilic channel
Lets certain molecules + ions pass through
Example: aquaporins let water through
Aquaporin structure = four polypeptide subunits that form channels
Carrier Protein
hold onto "passengers"
Change shape to facilitate movement through membrane
Functions
Lets hydrophilic substances avoid lipid bilayer contact
Specific to each substance
Example: glucose carrier protein in red blood cells only transports glucose
Active Transport
Ion Pumps + Maintenance
Cells have voltages across plasma membranes
Voltage = electrical potential energy; separation of opposite charges
Cytoplasmic membrane side = negative, extracellular side = positive
Membrane potential = voltage across membrane = energy source, affects traffic of charged substances
passive transport
Cations into cell
Anions out of the cell
Electrochemical gradient = combo of chemical + electrical forces acting on ion
Ions diffuse down electrochemical gradients
Active transport maybe necessary when gradients oppose each other
Electrogenic pump
transport protein, generates voltage across a membrane
Example: proton pump pumps hydrogen ions out of cell
Help store energy that can be used for cellular work
Cotransport
Solutes can do work as they diffuse across membrane (e.g. water doing work flowing downhill)
cotransport = transport protein (cotransporter) couples downhill + uphill diffusion of 2 substances
uphill substance moves across its own gradient
Examples
IN animal cells, cotransporter transports glucose with downhill movement of Na+
plant cells use h+ gradient to drive active transport of amino acids, sugars, etc.
sucrose transport coupled with return of H+
Need for Energy in Active Transport
Active transport = transport of solute against its concentration gradient; requires energy
All transport proteins involved = carrier proteins
Allows for maintenance of solute concentrations in cell
ATP hydrolysis supplies energy for most active transport
Example: terminal phosphate group transferred directly to transport protein
Protein changes shape
Sodium-potassium pump works this way
Pump exchanges Na+ for K+ across animal cell plasma membranes
Bulk Transport
Large molecules transported in bulk in/out cell via vesicles
Includes proteins + polysaccharides
Exocytosis
Cell secretes molecules vesicles fuse with cell membrane
Method
Transport vesicle buds from Golgi apparatus
Moves along microtubule to cell membrane
vesicle and cell membranes fuse, contents spill out of cell
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Functions
Product exportation
Example: Release of neurotransmitters to by nerve cells
Example: Protein + carb delivery in cell wall production
Example: Insulin secretion in pancreas
Endocytosis
Substances taken in by forming new vesicles from plasma membrane
Method
Area of membrane sinks in, pinches to form vesicle
Types
Pinocytosis
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Receptor-mediated
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Phagocytosis
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Functions
Receptor-mediated for cholesterol intake in humans
Used for membrane + steroid synthesis
Cholesterol travels via low-density lipoproteins (LDLs)
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Used along with exocytosis rejuvenating + remodeling plasma membrane
Process occurs continuously in most eukaryotic cells