lecture 9- transport across cell membranes

What are the four major mechanisms of membrane transport?

Passive transport, active transport, endocytosis, and exocytosis

What factors most strongly determine membrane permeability?

Lipid solubility, molecular size, polarity, and charge

Which molecules diffuse most easily through membranes?

Small hydrophobic molecules such as O₂, CO₂, and hydrocarbons

Why do hydrophobic molecules cross membranes easily?

They dissolve in the lipid bilayer and diffuse directly through it

How does molecular size affect membrane permeability?

Smaller molecules diffuse more easily than larger molecules

How does polarity affect membrane permeability?

Polar molecules are less permeable because they interact strongly with water

Why are charged molecules poorly permeable through membranes?

Their hydration shell increases their effective size and prevents passage through the hydrophobic bilayer

What is passive transport?

Movement of molecules across a membrane without energy input

What drives passive transport?

A concentration gradient or electrochemical gradient

What is diffusion?

Net movement of molecules from high concentration to low concentration

When does diffusion stop?

At equilibrium when concentrations are equal and there is no net movement

Does diffusion require ATP?

No, diffusion uses the kinetic energy of random molecular motion

What is filtration?

Movement of water and solutes driven by pressure differences across a membrane

What is osmosis?

Diffusion of water across a selectively permeable membrane

In which direction does water move during osmosis?

From higher water concentration to lower water concentration

What is hydrostatic pressure?

Mechanical pressure generated by water movement across a membrane

What causes hydrostatic pressure during osmosis?

Accumulation of water on one side of a semipermeable membrane

What is a hypotonic solution?

A solution with lower solute concentration than the cell

What happens to a cell in a hypotonic solution?

Water enters the cell causing swelling

What is an isotonic solution?

A solution with equal solute concentration inside and outside the cell

What happens to a cell in an isotonic solution?

No net movement of water occurs

What is a hypertonic solution?

A solution with higher solute concentration than the cell

What happens to a cell in a hypertonic solution?

Water leaves the cell causing shrinkage

What is the Donnan equilibrium?

Distribution of ions across a semipermeable membrane caused by impermeable charged molecules

What is the most abundant intracellular cation?

Potassium K⁺

What is the most abundant extracellular cation?

Sodium Na⁺

What is the most abundant extracellular anion?

Chloride Cl⁻

How do freshwater organisms deal with osmotic challenges?

They regulate excess water entering their cells

How do marine organisms deal with osmotic challenges?

They regulate excess salt entering their bodies

What is isosmotic regulation?

Maintaining body fluids at the same osmotic concentration as the environment

How do plant cells maintain rigidity?

Through turgor pressure pressing the membrane against the cell wall

What is turgor pressure?

Pressure generated when water enters plant cells and pushes against the cell wall

What are the major ways molecules cross membranes?

Simple diffusion, facilitated transport, channel-mediated transport, and active transport

What is simple diffusion?

Direct movement through the lipid bilayer without proteins

Which molecules use simple diffusion?

Small nonpolar molecules and some small uncharged molecules

What is facilitated diffusion?

Passive transport aided by membrane proteins

Does facilitated diffusion require ATP?

No, it moves molecules down their concentration gradient

What proteins mediate facilitated diffusion?

Carrier proteins and channel proteins

What is the Gibbs free energy relationship for spontaneous transport?

ΔG < 0

What does a negative ΔG indicate?

Transport is energetically favorable and spontaneous

What does a positive ΔG indicate?

Energy input is required for transport

What is active transport?

Movement of molecules against their gradient using energy

What is the source of energy for most active transport?

ATP hydrolysis

What happens when a concentration gradient is established?

Energy is stored in the gradient

What happens when a concentration gradient collapses?

Stored energy is released

What drives diffusion of uncharged molecules?

The concentration gradient

When is diffusion into a cell favorable?

When concentration outside the cell exceeds concentration inside

What drives diffusion of ions?

The electrochemical gradient

What is an electrochemical gradient?

The combined effects of concentration difference and electrical charge difference

What is membrane potential?

Voltage difference across a membrane

How is membrane potential measured?

As the voltage inside relative to outside the cell

What is the typical membrane potential range?

Approximately -20 mV to -200 mV

Why is the inside of most cells negative?

Organic anions remain inside while K⁺ diffuses outward

What is resting potential?

The membrane potential of an excitable cell at rest

Which ion contributes most to resting membrane potential in animal cells?

Potassium K⁺

What are K⁺ leak channels?

Channels that randomly open and allow K⁺ diffusion out of the cell

Why does K⁺ diffusion eventually stop?

The electrical gradient balances the concentration gradient

What is electrochemical equilibrium?

The point where electrical and concentration forces are equal and opposite

What equation predicts ion equilibrium across membranes?

The Nernst equation

What factors increase diffusion directly through a lipid bilayer?

Greater lipid solubility, smaller size, and lower charge

Which molecules diffuse directly through membranes most readily?

O₂, CO₂, and H₂O

Why can ions not diffuse through lipid bilayers?

Their charge prevents passage through the hydrophobic core

What is a semipermeable membrane?

A membrane that allows some substances to pass while restricting others

Why do sugars require transport proteins?

They are large hydrophilic molecules that cannot diffuse freely through the bilayer

What determines the direction of movement for each molecule?

Its own concentration gradient

What determines ion movement across membranes?

Concentration gradient and membrane potential

What are channel proteins?

Membrane proteins that form aqueous pores for diffusion

What are carrier proteins?

Membrane proteins that bind solutes and change conformation to move them across the membrane

How do carrier proteins transport molecules?

By alternating exposure of a binding site to opposite sides of the membrane

What is the glucose transporter an example of?

Facilitated diffusion through a carrier protein

How does a glucose carrier work?

It binds glucose on one side and releases it on the other after a conformational change

Is facilitated diffusion unidirectional or bidirectional?

Bidirectional, depending on the concentration gradient

What does saturation mean in facilitated diffusion?

All transporters are occupied and transport reaches a maximum rate

Why does facilitated diffusion show saturation?

Transport is limited by the number of available carriers

Why does increasing substrate concentration eventually stop increasing transport rate?

All carriers become occupied

What are the key properties of carrier proteins?

Specificity, saturation, and transport down a concentration gradient

Why do diabetics sometimes lose glucose in urine?

Glucose transporters become saturated when blood glucose levels are extremely high

How do liver cells maintain glucose uptake?

Incoming glucose is rapidly phosphorylated

Why does glucose phosphorylation promote glucose entry?

It keeps intracellular glucose concentration low

Why can't phosphorylated glucose leave the cell easily?

The phosphate group prevents transport through glucose carriers

How is glycogen related to glucose storage?

Glucose is converted into glycogen for storage

How do liver cells release glucose?

Glycogen is broken down into glucose which diffuses out

What is an ion channel?

A membrane protein forming an aqueous pore for ion movement

How do ions move through channels?

By diffusion down their electrochemical gradient

Are ion channels selective?

Yes, they contain selectivity filters based on size and charge

What is a selectivity filter?

A narrow region that allows only specific ions to pass

Do ion channels require ATP?

No, transport through channels is passive

Are ion channels always open?

No, most are gated

What does it mean for a channel to be gated?

It can switch between open and closed conformations

What determines ion flow through a channel?

Ion concentration, membrane potential, and whether the channel is open

What is a voltage-gated channel?

A channel that opens or closes in response to membrane potential changes

What is the function of voltage-gated channels?

Electrical signaling in nerves, muscles, and other excitable cells

What is a ligand-gated channel?

A channel that opens or closes when a chemical ligand binds

What are common ligands for ligand-gated channels?

Neurotransmitters

What is a mechanically gated channel?

A channel that responds to physical force or stretching

What sensory processes use mechanically gated channels?

Hearing and touch

How do auditory cells detect sound?

Vibrations open mechanically gated ion channels

How do touch receptors function?

Mechanical pressure opens gated ion channels

Channel proteins vs Carrier proteins

Channels form pores allowing rapid diffusion while carriers bind solutes and undergo conformational changes

Simple diffusion vs Facilitated diffusion

Simple diffusion occurs directly through the bilayer while facilitated diffusion requires membrane proteins