lecture 10- transport across cell membranes
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123 Terms
What is active transport?
Movement of substances across a membrane against their concentration or electrochemical gradient requiring energy
How does active transport differ from facilitated diffusion?
Active transport requires energy and can move substances against gradients while facilitated diffusion is passive and only moves substances down gradients
What is the Gibbs free energy change for active transport?
ΔG is greater than 0
Why does active transport require energy?
Molecules are moved from lower concentration to higher concentration or against an electrochemical gradient
What proteins are required for active transport?
Specific integral membrane transport proteins
Why does active transport show saturation kinetics?
The number of transport proteins is limited
What are the three major types of active transport?
ATP-driven pumps, coupled transport, and light-driven pumps
What is an ATP-driven pump?
A transporter that uses ATP hydrolysis to move molecules against their electrochemical gradient
What is coupled transport?
Transport in which movement of one solute is linked to movement of another solute
What is a symport?
Coupled transport in which two solutes move in the same direction
What is an antiport?
Coupled transport in which two solutes move in opposite directions
What is secondary active transport?
Transport powered indirectly by an ion gradient rather than direct ATP hydrolysis
Why is secondary active transport considered indirect?
ATP is required to establish the gradient that provides the energy
What is a light-driven pump?
A transporter that uses energy from absorbed light to move molecules against a gradient
Why is an electrochemical gradient considered stored energy?
Its collapse can perform work and drive transport processes
What are the major functions of the Na⁺/K⁺ pump?
Maintain high intracellular K⁺, low intracellular Na⁺, membrane potential, osmotic balance, and electrical signaling
Why does Na⁺ tend to enter cells spontaneously?
Both its concentration gradient and electrical gradient favor entry
Why does K⁺ tend to leave cells?
Its concentration gradient favors movement out of the cell
Why does K⁺ not rapidly diffuse out of cells?
The electrical gradient opposes K⁺ loss
Why must Na⁺ be pumped out of cells?
To maintain low intracellular sodium concentration
Why must K⁺ be pumped into cells?
To maintain high intracellular potassium concentration
What percentage of cellular ATP can be consumed by the Na⁺/K⁺ pump?
Up to about 30 percent
What is the transport ratio of the Na⁺/K⁺ pump?
3 Na⁺ out and 2 K⁺ in per ATP hydrolyzed
Is the Na⁺/K⁺ pump electrogenic?
Yes because it exports more positive charge than it imports
Why is the Na⁺/K⁺ pump considered electrogenic?
Three positive charges leave while only two positive charges enter
How does the Na⁺/K⁺ pump help maintain membrane potential?
It creates a net loss of positive charge from the cell
How does the Na⁺/K⁺ pump contribute to osmotic balance?
It prevents excessive accumulation of ions and water inside the cell
What would happen if the Na⁺/K⁺ pump stopped functioning?
Na⁺ and Cl⁻ would accumulate, water would enter, and the cell could burst
Describe the Na⁺/K⁺ ATPase cycle.
Three Na⁺ bind inside, ATP phosphorylates the pump, Na⁺ is released outside, two K⁺ bind outside, phosphate is removed, and K⁺ is released inside
What role does phosphorylation play in the Na⁺/K⁺ pump?
It causes a conformational change that releases Na⁺ outside the cell
What role does dephosphorylation play in the Na⁺/K⁺ pump?
It restores the original conformation and releases K⁺ inside the cell
What is energy coupling in the Na⁺/K⁺ pump?
ATP energy is temporarily stored in a phosphorylated protein intermediate
Why is the sodium gradient useful to cells?
It stores energy that can drive other transport processes
What happens when the sodium gradient collapses?
Stored energy is released and can perform work
What is the basis of secondary active transport?
Using the energy released by an ion moving down its electrochemical gradient
What powers Na⁺-glucose symport?
The Na⁺ electrochemical gradient
What is cooperative binding in Na⁺-glucose symport?
Na⁺ and glucose bind the transporter together
How does Na⁺-glucose symport move glucose against its gradient?
Energy from Na⁺ entering down its gradient drives glucose uptake
Does Na⁺-glucose symport directly use ATP?
No
Why is ATP still required for Na⁺-glucose symport?
ATP powers the Na⁺/K⁺ pump that maintains the sodium gradient
What type of transport is Na⁺-glucose symport?
Secondary active transport
How is glucose absorbed from the intestinal lumen?
Through Na⁺-glucose symporters in the apical membrane
Where is the Na⁺/K⁺ ATPase located in intestinal epithelial cells?
In the basolateral membrane
Why is intracellular Na⁺ kept low in intestinal epithelial cells?
The Na⁺/K⁺ pump continuously exports Na⁺
How does glucose leave intestinal epithelial cells?
Through glucose transporters in the basolateral membrane by facilitated diffusion
Why are intestinal epithelial cells considered polarized?
Different transport proteins are located on apical and basolateral surfaces
What is an H⁺ ATPase?
A proton pump that uses ATP to move H⁺ against its concentration gradient
What gradient does the H⁺ ATPase establish?
A proton concentration gradient and pH gradient
Where are H⁺ ATPases commonly found?
Bacteria, fungi, plants, lysosomes, and vacuoles
How do plants use proton gradients?
To power symport transport of sugars and amino acids
Why are lysosomes acidic?
H⁺ ATPases pump protons into them
Why is lysosomal acidity important?
Acidic conditions are required for degradative enzymes
What is the structure of a neuron?
Cell body, dendrites, and axon
What is the function of dendrites?
Receive signals from other cells
What is the function of the axon?
Transmit signals to target cells
What is myelin?
Multiple layers of membrane wrapped around an axon
Which cells form myelin in the peripheral nervous system?
Schwann cells
What is the function of myelin?
Insulate axons and increase conduction speed
What are Nodes of Ranvier?
Gaps between myelinated regions containing high densities of ion channels
Why are Nodes of Ranvier important?
Action potentials are regenerated at these sites
What is Multiple Sclerosis?
An autoimmune disease involving destruction of myelin
How does loss of myelin affect nerve function?
Signal transmission becomes slower and less efficient
What is the resting membrane potential of a neuron?
Approximately -70 mV
Why is the inside of a neuron negative at rest?
K⁺ leak channels allow positive charge to leave while Na⁺ channels remain closed
What generates resting membrane potential?
Differential permeability of the membrane to ions
What are K⁺ leak channels?
Channels that allow passive diffusion of K⁺ out of the cell
Why doesn't the Na⁺/K⁺ pump completely offset K⁺ leakage?
The pump cannot fully replace all K⁺ leaving through leak channels
What is an action potential?
A rapid, temporary reversal of membrane potential used for signaling
How long does a typical action potential last?
Approximately 5 milliseconds
What are the two major phases of an action potential?
Depolarization and repolarization
What is depolarization?
Membrane potential becomes less negative and often positive
What triggers depolarization?
Opening of voltage-gated Na⁺ channels
What is the threshold potential for triggering an action potential?
Approximately -50 mV
What happens if threshold is not reached?
The membrane repolarizes and no action potential occurs
Why does Na⁺ enter rapidly during depolarization?
Voltage-gated Na⁺ channels open
What happens to Na⁺ channels shortly after opening?
They become inactivated
What is the refractory period?
Brief interval during which Na⁺ channels cannot reopen
How long does the refractory period last?
Approximately 2–3 milliseconds
What causes repolarization?
Opening of voltage-gated K⁺ channels
Why does K⁺ exit during repolarization?
Its electrochemical gradient favors movement out of the cell
What restores resting membrane potential after repolarization?
Closure of voltage-gated K⁺ channels and continued K⁺ leak channel activity
How does an action potential propagate?
Depolarization at one region triggers Na⁺ channel opening in adjacent regions
Why does an action potential travel only in one direction?
Previously activated Na⁺ channels are in their refractory period
What is saltatory conduction?
Action potentials jumping from one Node of Ranvier to the next
Why does myelin increase conduction velocity?
Depolarization only occurs at nodes rather than across the entire membrane
What is a synapse?
Junction between a neuron and its target cell
What is the presynaptic cell?
The cell that sends the signal
What is the postsynaptic cell?
The cell that receives the signal
How is a signal transmitted across a chemical synapse?
Through release and detection of neurotransmitters
What triggers neurotransmitter release?
Entry of Ca²⁺ through voltage-gated calcium channels
Why does Ca²⁺ enter the presynaptic cell?
Its concentration is much higher outside the cell
What does Ca²⁺ do inside the presynaptic cell?
Triggers fusion of synaptic vesicles with the plasma membrane
What are synaptic vesicles?
Membrane-bound sacs containing neurotransmitter molecules
How do neurotransmitters reach the postsynaptic cell?
They diffuse across the synaptic cleft
What happens when neurotransmitters bind postsynaptic receptors?
Ligand-gated ion channels open
What is the acetylcholine receptor?
A ligand-gated ion channel activated by acetylcholine
What do excitatory neurotransmitters do?
Promote action potential generation
Which ion channel do most excitatory neurotransmitters open?
Ligand-gated Na⁺ channels
What effect does Na⁺ entry have on a postsynaptic cell?
Depolarizes the membrane
What do inhibitory neurotransmitters do?
Make action potentials less likely