Cellular & Molecular Biology - Chapter 12

What is responsible for transferring small water-soluble molecules across the membrane?

Specialized membrane transport proteins

What does diffusion depend on?

Size and solubility

Molecules can diffuse rapidly across bilayer if _____ enough.

Small

Why do many of the molecules that cells uses as nutrients have difficulty passing through a pure lipid bilayer?

Too large and polar

Lipid membrane is impermeable to ________. (No matter how small!)

Ions and charged molecules

What are the properties inside of the lipid bilayer?

Hydrophobic and uncharged (nonpolar)

Does it have to be uncharged in all ways, shapes, and forms in order to get through the membrane?

Yes

Can easily diffuse by themselves (Can go across membrane on their own, fast!)

Small nonpolar molecules

O₂, CO₂, N₂, steroid hormones

Small nonpolar molecules

• Even if small, polarity makes it irrelevant and doesn’t let it go through at an appreciable level

• Does not move fast enough for life

• Can sneak through but very rare

• Can NOT diffuse by themselves (unless very rarely)

Small uncharged polar molecules

H₂O, ethanol, glycerol

Small uncharged polar molecules

Can NOT diffuse by themselves (unless very rarely)

Larger uncharged polar molecules

Amino acids, glucose, nucleosides

Larger uncharged polar molecules

Charged and small (tiniest thing)

Ions

H⁺, Na⁺, K⁺, Ca²⁺, Cl^-, Mg²⁺, HCO₃^-

Ions

What can NEVER diffuse?

Ions

Can something with a significant amount of charge sneak through the membrane on its own?

No

What are the two classes of membrane transport proteins?

Transporter and channel

Transporter or Channel?

Allows passage only to those molecules or ions that fit into a binding site on the protein

Transporter

Transporter or Channel?

Transfers the molecules one at a time by changing own conformation (Lets one molecule through as you change the shape)

Transporter

Transporter or Channel?

Turnstile (Doors to library!) (Proteins changes shape like an “airlock” system)

Transporter

Transporter or Channel?

Binds with great specificity - very selective (Only brings in a few molecules)

Transporter

Transporter or Channel?

Discriminate on basis of size and electrical charge

Channel

Transporter or Channel?

If _______ is open, an ion or small enough molecule with appropriate charge can go through

Channel

Transporter or Channel?

Gateway

(need to open door once to get everyone through)

(Proteins changes shape like a “gateway” system)

Channel

Transporter or Channel?

When it changes shape, you can get a ton of stuff through with just one shape change. (Brings in many molecules)

Channel

Both transporter and channel proteins have _______ that transverse the lipid bilayer multiple times.

Polypeptide chains

What do transport proteins and channel proteins have in common?

Transmembrane proteins (amphipathic) and very specific (can only move one thing)

What is the direction of movement in a concentration gradient?

High to low concentration

What is the energy source for passive transport?

Concentration gradient

Molecules will spontaneously flow “downhill” from a region of high concentration to a region of low concentration — if a pathway exists

Passive transport

No other driving force needed

Passive transport

ALL ion channels and many transporters

Passive transport

Something is starting at high concentration, moves to low concentration, that’s providing the energy and that’s making things move

Concentration gradient

What could mediate the passive transport of a solute?

Conformational change in transporter

What is an example of passive transport?

Glucose transporter

What happens in a Glucose Transporter?

• Glucose fits in the transporter’s binding site

• Transport changes shape and allows the glucose to follow its gradient

What is the energy source of a Glucose Transporter?

Concentration gradient; concentration gradient pushing to other side where it’s low concentration

Move a solute against its concentration gradient

Active transport

Drive the flow “uphill” by coupling to process that provides energy

Active transport

Does moving against a concentration gradient require energy?

Yes (doesn’t occur naturally)

What are the three main ways a cell carries out active transport?

Coupled pump, ATP-driven pump, and light-driven pump

What is a coupled reaction?

Pairing two reactions, uses energy from favorable reaction to help unfavorable reaction happen; both happening in exact same time and space; as one happens, other one has to happen

Couple downhill (high to low) transports of one molecule to provide energy for an uphill (low to high) transport of another

Coupled transporter

Is going from low to high favorable or unfavorable?

Unfavorable

• Energy from thing that’s favorable is providing the energy for the thing that’s unfavorable

• One going down concentration gradient and shape change makes other one happen

Coupled transporter

Use energy carriers, like ATP

ATP-driven pump

ATP, an energy carrier, does what?

Provides energy

Couple uphill transport to an input of energy from light (Requires light to hit its proteins)

Light driven transporters

What transporter is found mainly in bacterial cells?

Light driven transporters

Each cell membrane has own ________ set of ________.

Characteristic, transporters

The driving force that moves an ion across the membrane is the sum of the concentration gradient and the voltage across the membrane

Electrochemical gradient

What does a positive charge want to be with?

Negative charge (opposites attract!)

What is the charge outside of the cell?

Positive

What is the charge inside of the cell?

Negative

Explain an electrochemical gradient when voltage and concentration gradients work in same direction.

Chemically, it goes from outside (high) to inside where concentration is lower. Electrically, the positive goes to negative. Both want to move ion inside the cell!

Electrochemical gradient when voltage and concentration gradients work in the same direction

What is an example of electrochemical gradient when voltage and concentration gradients work in the same direction?

Na+ (sodium)

Steep electrochemical gradient (Huge driving force, rushes into cells!)

Electrochemical gradient when voltage and concentration gradients work in the same direction

Enters cells at every opportunity

Electrochemical gradient when voltage and concentration gradients work in the same direction

Explain sodium regarding the electrochemical gradient.

High outside cells and low inside cells, wants to go into cells every chance it gets, crosses very fast

Explain an electrochemical gradient when voltage and concentration gradients work in opposite directions.

Chemically, always goes from high to low (inside to outside). Electrically, positive charge goes outside (positive).

Electrochemical gradient when voltage and concentration gradients work in opposite directions

What is an example of electrochemical gradient when voltage and concentration gradients work in opposite directions?

K+ (potassium)

Small electrochemical gradient (driving force is small!)

Electrochemical gradient when voltage and concentration gradients work in opposite directions

Little net movement of K+ across membrane

Electrochemical gradient when voltage and concentration gradients work in opposite directions

Explain potassium regarding the electrochemical gradient.

High inside cells and low outside cells, channels open and flows from inside to outside, doesn’t go very fast

A difference in the electrical potential on each side of the membrane

Membrane potential

Intracellular/extracellular concentrations are measured in ________.

mM (millimolar)

SODIUM/Na+:

• ____ outside cells

• _____ inside cells

High, low

POTASSIUM/K+:

• ____ outside cells

• _____ inside cells

Low, high

Transports Na+ out of cell and K+ into cell

Na+-K+ pump (Sodium potassium pump)

What is creating the electrochemical gradient?

Sodium potassium pump

Sodium potassium pump: Both _____ and _____ move against the concentration gradient, which means energy.

Sodium and potassium

What energy does the sodium potassium pump utilize?

ATP

Explain the sodium potassium pump.

Transporter transmembrane protein that’s going to take potassium from where it’s low and pump it where it’s high and does the opposite for sodium.

The sodium-potassium pump cycle takes how long?

10 milliseconds

Sodium-potassium pump:

• Making _________

• Going _____ concentration gradient

• Needs _______

• Creating _______ gradient

Sodium and potassium, against, energy, electrochemical

What is the energy source of the glucose-sodium symport?

Electrochemical gradient

What is the glucose-sodium symport an example of?

Coupled reaction

Explain the glucose-sodium symport.

It is a transporter (transmembrane protein) which fits glucose and wants to move it to the other side. Glucose goes from low concentration to high concentration, which is not favorable. Because it isn’t favorable, we need to couple it to something that is favorable, which is the electrochemical gradient. Sodium is going down electrochemical gradient (high to low concentration). Power of sodium will cause change in shape and now they’re both inside!

Glucose-Sodium Symport:

1. It is a transporter (transmembrane protein) which fits glucose and wants to move it to the other side. Glucose goes from low concentration to high concentration, which is not _______.

2. Because it isn’t favorable, we need to couple it to something that is favorable, which is the ______________.

3. ______ is going down electrochemical gradient (high to low concentration).

4. Power of sodium will cause change in _______ and now they’re both inside!

Favorable, electrochemical gradient, sodium, shape

For glucose transporters in the gut, what is the part facing inside of the body going to do?

Push glucose out

For glucose transporters in the gut, why is it important to keep proteins separated into different locations on the membrane?

On one side you’re taking glucose to a cell from the gut and the other side you’re pushing it out into the blood where the body can use it

For glucose transporters in the gut, you can’t let that get messed up why?

The cell would stop working and we would starve to death

What is the most tightly controlled/regulated ion in the cell?

Calcium (Ca²⁺)

Is calcium low or high inside of the cell?

Low

What does the Ca²⁺ pump do?

Pumps calcium outside of the cell (cytoplasm), pumping against electrochemical gradient and does require ATP

Calcium pump: How do we keep calcium low inside of cells utilizing the Ca²⁺ (Calcium) pump?

The Ca2+ pump pumps calcium out of the cell and goes against the concentration gradient

What is the energy source for the Ca²⁺ (Calcium) pump?

ATP

Ion channels are ______!

Selective

K⁺ channel has a narrow selectivity filter that is lined with carbonyl oxygens that bear a partial ____ charge.

Negative

In ion channels, what do we need for it to be just right?

Size and charge of channel

Ion channels are ______ proteins.

Transmembrane

NOT continuously open

Ion channels are gated (Gated-ion channels)

When an ion channel is open, what happens?

Many ions can get through

What are the three different types of stimuli gated ion channels respond to?

Membrane potential, ligand binding channel, and mechanical force applied to channel

Channels that require ligands to bind to them so they can change shape

Ligand gated ion channels

______ control whether they open or close ion channels.

Ligands

Small molecule

Ligand