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Notes on Membrane Structure and Function
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The plasma membrane separates the living cell from its nonliving surroundings
• this thin barrier controls traffic into and out of the cell
• like other membranes, the plasma membrane is selectively permeable, allowing some substances to cross more easily than others
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Membrane Structure
• the main macromolecules in membranes are lipids and proteins, but include some carbohydrates
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• the most abundant lipids are phospholipids
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• the phospholipids and proteins in membranes are described by the fluid mosaic model
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• the molecules in the bilayer are arranged such that the hydrophobic fatty acid tails are sheltered from water while the hydrophilic phosphate groups interact with water
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Phospholipids and most other membrane constituents are amphipathic molecules which have both hydrophilic and hydrophobic regions
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Membranes are Fluid
• membrane molecules are held in place by relatively weak hydrophobic interactions
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• most of the lipids and some proteins can drift laterally in the plane of the membrane, but rarely flip-flop from one layer to the other
Lateral movement(107 times per second)
Flip-flop( once per month)
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Membranes are Mosaics of Structure and Function
• proteins determine most of the membrane’s specific functions
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Membrane carbohydrates are important for cell-cell recognition
• the membrane plays the key role in cell-cell recognition
• cell-cell recognition is the ability of a cell to distinguish one type of neighboring cell from another
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• this attribute is important in cell sorting and organization into tissues and organs during development
• it is also the basis for rejection of foreign cells by the immune system
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• cells recognize other cells by keying on surface molecules, often carbohydrates, on the plasma membrane
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Traffic Across Membranes
• a steady traffic of small molecules and ions moves across the plasma membrane in both directions
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for example, sugars, amino acids, and other nutrients enter a muscle cell and metabolic waste products leave
it also regulates concentrations of inorganic ions, like Na+, K+, Ca2+, and Cl-, by shuttling them across the membrane
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• however, substances do not move across the barrier indiscriminately; membranes are selectively permeable
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Permeability of a molecule through a membrane depends on the interaction of that molecule with the hydrophobic core of the membrane
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• hydrophobic molecules, like hydrocarbons, CO2, and O2, can dissolve in the lipid bilayer and cross easily
• ions and polar molecules pass through with difficulty
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• this includes small molecules, like water, and larger critical molecules, like glucose and other sugars
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Specific ions and polar molecules can cross the lipid bilayer by passing through transport proteins that span the membrane
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• some transport proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane
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• others bind to these molecules and carry their passengers across the membrane physically
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Each transport protein is specific as to the substances that it will translocate (move)
• for example, the glucose transport protein in the liver will carry glucose from the blood to the cytoplasm, but not fructose, its structural isomer
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Passive transport
Diffusion is the tendency of molecules of any substance to spread out in the available space
• diffusion is driven by the intrinsic kinetic energy (thermal motion or heat) of molecules
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In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated, down its concentration gradient
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• the diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen
• the concentration gradient represents potential energy and drives diffusion
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Osmosis is the passive transport of water
• the solution with the higher concentration of solutes is hypertonic
• the solution with the lower concentration of solutes is hypotonic
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Tap water is hypertonic compared to distilled water but hypotonic when compared to seawater
• solutions with equal solute concentrations are isotonic
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Imagine that two sugar solutions differing in concentration are separated by a membrane that will allow water through, but not sugar
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• the hypertonic solution has a lower water concentration than the hypotonic solution
• osmosis continues until the solutions are isotonic
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Cell survival depends on balancing water uptake and loss
An animal cell immersed in an isotonic environment experiences no net movement of water across its plasma membrane
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• water flows across the membrane, but at the same rate in both directions
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The same cell in a hypertonic environment will lose water, shrivel, and probably die
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A cell in a hypotonic solution will gain water, swell, and burst
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Organisms without rigid walls have osmotic problems in either a hypertonic or hypotonic environment and must have adaptations for osmoregulation to maintain their internal environment
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for example, Paramecium, a protist, is hypertonic when compared to the pond water in which it lives
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• in spite of a cell membrane that is less permeable to water than other cells, water still continually enters the Paramecium cell
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• to solve this problem, Paramecium have a specialized organelle, the contractile vacuole, that functions as a pump to force water out of the cell
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The cells of plants, prokaryotes, fungi, and some protists have walls that contribute to the cell’s water balance
• a plant cell in a hypotonic solution will swell until the elastic wall opposes further uptake
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• at this point the cell is turgid, a healthy state for most plant cells
Turgid cells contribute to the mechanical support of the plant
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If a cell and its surroundings are isotonic, there is no movement of water into the cell and the cell is flaccid and the plant may wilt
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In a hypertonic solution, a cell wall has no advantages
• as the plant cell loses water, its volume shrinks
• eventually, the plasma membrane pulls away from the wall
• this plasmolysis is usually lethal
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Specific proteins facilitate passive transport of water and selected solutes:
Many polar molecules and ions that are normally impeded by the lipid bilayer of the membrane diffuse passively with the help of transport proteins that span the membrane
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• the passive movement of molecules down its concentration gradient via a transport protein is called facilitated diffusion
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• many transport proteins simply provide corridors allowing a specific molecule or ion to cross the membrane
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Some channel proteins, gated channels, open or close depending on the presence or absence of a physical or chemical stimulus
for example, when neurotransmitters bind to specific gated channels on the receiving neuron, these channels open
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• this allows sodium ions into a nerve cell
• when the neurotransmitters are not present, the channels are closed
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Active Transport
Active transport is the pumping of solutes against their gradients
• some facilitated transport proteins can move solutes from the side where they are less concentrated to the side where they are more concentrated
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• this active transport requires the cell to expend its own metabolic energy
• active transport is performed by specific proteins embedded in the membranes
(ATP)
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The sodium-potassium pump actively maintains the gradient of sodium (Na+) and potassium ions (K+) across the membrane
• typically, an animal cell has higher concentrations of K+ and lower concentrations of Na+ inside the cell
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• the sodium-potassium pump uses the energy of one ATP to pump three Na+ ions out and two K+ ions in
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Some ion pumps generate voltage across membranes
• all cells maintain a voltage across their plasma membranes
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The cytoplasm of a cell is negative in charge compared to the extracellular fluid because of an unequal distribution of cations and anions on opposite sides of the membrane
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• two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane
• ions diffuse not simply down their concentration gradient, but diffuse down their electrochemical gradient
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For example, before stimulation there is a higher concentration of Na+ outside a resting nerve cell
• when stimulated, a gated channel opens and Na+ diffuses into the cell down the electrochemical gradient
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Special transport proteins, electrogenic pumps, generate the voltage gradients across a membrane
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• the sodium-potassium pump in animals restores the electrochemical gradient not only by the active transport of Na+ and K+, but because it pumps two K+ ions inside for every three Na+ ions that it moves out
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In plants, bacteria, and fungi, a proton pump is the major electrogenic pump, actively transporting H+ out of the cell
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• protons pumps in the cristae of mitochondria and the thylakoids of chloroplasts concentrate H+ behind membranes
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• these electrogenic pumps store energy that can be accessed for cellular work
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Two Types of Active Transport
1. Exocytosis
2. Endocytosis
Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins
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Large molecules, such as polysaccharides and proteins, cross the membrane via vesicles
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During exocytosis, a transport vesicle budded from the Golgi apparatus is moved by the cytoskeleton to the plasma membrane
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• when the two membranes come in contact, the bilayers fuse and spill the contents to the outside
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During endocytosis, a cell brings in macromolecules and particulate matter by forming new vesicles from the plasma membrane
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• endocytosis is a reversal of exocytosis
• a small area of the plasma membrane sinks inward to form a pocket
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• as the pocket into the plasma membrane deepens, it pinches in, forming a vesicle containing the material that had been outside the cell
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One type of endocytosis is phagocytosis, “cellular eating”
• in phagocytosis, the cell engulfs a particle by extending pseudopodia around it and packaging it in a large vacuole
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• the contents of the vacuole are digested when the vacuole fuses with a lysosome
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In pinocytosis, “cellular drinking,” a cell creates a vesicle around a droplet of extracellular fluid