Membrane Transport: Diffusion, Proteins, and Ion Gradients
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59 Terms
How do small nonpolar molecules cross the membrane?
Simple diffusion directly through the lipid bilayer.
What are examples of molecules that diffuse easily across membranes?
O₂, CO₂, steroid hormones.
Why can't most molecules cross the lipid bilayer?
The hydrophobic interior repels polar molecules and ions.
What is facilitated diffusion?
Movement down the concentration gradient using a membrane protein.
Does facilitated diffusion require energy?
No.
What is the difference between simple diffusion and facilitated diffusion?
Simple diffusion does not use a protein, while facilitated diffusion uses a protein.
What is the function of transport proteins?
To move hydrophilic molecules across the membrane without touching the lipid interior.
What are the two types of transport proteins?
Channels and transporters.
What do channel proteins form?
Hydrophilic pores through the membrane.
What determines channel selectivity?
Size and charge.
How do transporters move molecules?
By binding the molecule and then changing shape.
Which is faster, channels or transporters?
Channels are faster; transporters are slower.
What is an example of a transporter?
Glucose transporter.
What is a multipass transmembrane protein?
A protein that crosses the membrane multiple times.
Why do proteins cross the membrane multiple times?
To create a pathway through the hydrophobic membrane.
What are the ion concentrations inside vs outside the cell?
K⁺ is high inside; Na⁺ is high outside.
What is membrane potential?
The voltage difference across the membrane caused by unequal ion distribution.
What is the typical resting membrane potential in neurons?
About negative seventy millivolts.
Why is the inside of the cell negative?
Proteins and nucleic acids carry negative charges.
What is the state of resting membrane potential?
Stable but not zero.
What is a concentration gradient?
Movement from high concentration to low concentration.
What is passive transport?
Movement down the gradient without energy.
What is active transport?
Movement against the gradient requiring energy.
What are examples of passive transport?
Simple diffusion, facilitated diffusion, osmosis.
What is an electrochemical gradient?
The combined effect of the concentration gradient and membrane potential.
What affects the movement of charged solutes?
The electrochemical gradient.
What is an example of an ion with a steep electrochemical gradient?
Na⁺.
What is osmosis?
Movement of water down its concentration gradient.
What is osmolarity?
Total solute concentration in a solution.
Where does water move toward?
The solution with higher solute concentration.
What is the function of aquaporins?
Water channels that increase water transport.
Why do cells swell in water?
Higher solute concentration inside the cell draws water inward.
What are ways cells prevent swelling?
Eject water, pump solutes out, or use cell wall pressure.
What is primary active transport?
Uses ATP directly to move molecules against the gradient.
What is an example of a primary pump?
Na⁺ K⁺ pump.
What is the function of the Na⁺ K⁺ pump?
Pumps three Na⁺ out and two K⁺ into the cell.
What is the energy source for the Na⁺ K⁺ pump?
ATP.
What is the purpose of the Na⁺ gradient?
Stores energy for other transport processes.
What is the function of the Ca²⁺ pump?
Keeps cytosolic Ca²⁺ levels low.
Where do Ca²⁺ pumps move calcium?
Out of cytosol into the ER or outside the cell.
Why must Ca²⁺ levels stay low?
Ca²⁺ acts as an intracellular signal.
What is secondary active transport?
Uses ion gradient energy instead of ATP directly.
What is an example of secondary transport?
Na⁺ glucose symporter.
How does the Na⁺ glucose symporter work?
Na⁺ moves down the gradient, pulling glucose into the cell.
Why is the Na⁺ gradient important?
It provides energy for many transport systems.
What is a symporter?
A transporter moving two molecules in the same direction.
What is an antiporter?
A transporter moving molecules in opposite directions.
What is a uniporter?
A transporter moving one molecule type.
What is the energy source for a gradient-driven pump?
Ion gradient such as Na⁺ gradient.
What is the energy source for an ATP-driven pump?
ATP hydrolysis.
What is the energy source for a light-driven pump?
Light energy.
What happens to glucose transport after a meal?
High glucose outside the cell allows glucose to enter via facilitated diffusion.
What happens to glucose transport during fasting?
Glucose leaves liver cells down the gradient.
What happens if the Na⁺ K⁺ pump stops working?
The Na⁺ gradient collapses and secondary transport stops.
What happens if the membrane becomes too fluid?
Increase saturated fatty acids or cholesterol.
What happens if a cell is placed in a hypotonic solution?
Water enters the cell causing swelling.
What happens if a cell is placed in a hypertonic solution?
Water leaves the cell causing shrinkage.
Why do ion channels open and close?
To regulate ion flow across the membrane.
What is an example of a cell using many aquaporins?
Kidney cells.