Molec Cell exam 2

What does the plasma membrane do?

It acts as a selective barrier surrounding the cells.

• Controls the boundary btwn cell and its environment

Membrane associated proteins allow cels to

-receive info

-Import/export molecules

-Move

What is a cell membrane made of

Lipid Bilayer+ Proteins

-These proteins can either be embedded or associated w membrane

Phospholipids

-Polar hydrophilic head group+ Non polar hydrophobic tail

-Amphipathic

What are other membrane lipids that are amphipathic?

-Cholesterol and Glycolipids (contain sugar)

Why does a lipid bilayer form?

Hydrophilic heads can interact with the water while the hydrophobic tails hide from it.

-This lipid bilayer is the basis of all biological membranes

Hydrophobic tails are ______

Energetically unfavorable

Membrane is a _________

Flexible two-dimensional fluid

Things membrane component can and can’t do.

Can: Lateral diffusion(moving sideways) ,Rotation, Flex fatty acid tails

Can’t: Flip-flop in which a lipid spontaneously switch from one monolayer to the other.

Function of cholesterol in membranes

-Controls membrane fluidity

-Keep membrane fluidity from going either direction (kinda like homeostasis)

Cholesterol during high temps

At high temp cholesterol restrains the movement of fatty acids (maintaining stability)

Cholesterol during low temp

At low temperatures, cholesterol prevents tight packing of fatty acids (maintaining fluidity)

Where are membranes made?

• Begins in the ER, specifically in the cytoplasmic side of the ER

Where are phospholipids produced in the ER membrane

On the cytoplasmic side

Scramblases

Randomly transfer phospholipids from one monolayer to the other, transfers phospholipids on the cytoplasmic side to the cytoplasmic side.

Flip passes

• In the golgi

• Flippases transfer specific phospholipids to particular sides of the membrane

• SPECIFIC NOT RANDOM

Membranes are _______

Assymetrical

For ex: 1. (-)Charged phospholipids are maintained on the cytoplasmic side

2. Lipids that are glycosylated in the golgi remain on cytoplasmic side

Glycosylation

Adding a sugar/cabohydrate group

What happens to membrane orientation when a Golgi vesicle fuses wth the plasma membrane

The side that faced the Golgi lumen becomes extracellular side of the plasma membrane. Therefore glycosylated lipids//proteins that faced the Golgi lumen are now facing outside the cell

Types of membrane proteins

1. Transporters: carrier proteins and ion channels

2. Anchors: for extracellular matrix and/or cytoskeletal proteins integrins)

3. Surface receptors: Growth factor receptors

4. Enzymatic proteins: Adenylyl cyclase

Integral proteins vs peripheral proteins.

Integral proteins are physically integrated into the membrane in some way while peripheral proteins are associated w membrane indirectly

How does a polypeptide usually cross as a lipid bilayer

-As an α-helix

-The α-helix structure allows hydrophobic R-groups to be exposed while the backbone is shielded from the membrane

-The C-N backbone of polypeptides is hydrophilic

What is a B-barrel and how are R groups arranged in it.

-It is when B sheets are arranged to form a pore across the membrane

-The Hydrophobic R groups associate with the membrane, while the hydrophilic R groups line the channel across the membrane.

How can membrane proteins be solubilized and why?

With detergents.

Detergents are amphipathic so they can interact w membrane proteins and membrane lipids.

Why are membrane proteins isolated

-to examine protein function

-Determine the 3D shape of the protein

How do most membrane proteins move within the membrane

Most membrane proteins have free diffusion throughout the membrane

How can the movement of membrane proteins be restricted?

-Attachment to cytoplasmic proteins

-Attachment to extracellular matrix proteins

-Attachment to transmembrane proteins on a neighboring cell

-Restriction via tight junctions or other protein barrier

Why is the movement of some membrane proteins restricted?

Restriction of protein movement allows for directionality/sidedness of cells, keeping specific proteins in specific regions of the membrane.

What are the functions of glycoproteins and proteoglycans?

• Form a protective carbohydrate layer

• Act as cell identity markers

How do membrane carbohydrates allow cells to recognize one another?

Specific oligosaccharide side chains on glycolipids and glycoproteins can be recognized by other cells

What does it mean that biological membranes are selectively permeable

• Small nonpolar molecules and small uncharged polar molecules can diffuse across the membrane, although often very slowly.

• Large polar molecules and charged molecules cannot freely diffuse and need a hydrophilic channel or carrier protein.

How does the concentration of the molecule affect the movement across the membrane?

• Molecules and ions naturally diffuse down their concentration gradient → from high concentration → low concentration.

What is an electrochemical gradient

-Established by the (-) charged phospholipids and ion concentrations inside and outside the cell

Passive Transport

-Passive transport: moves substances down their gradient; no energy input required.

Active transport

-Active transport requires energy and can move substances against their gradient.

Input of energy can come from: Gradient of another molecule, ATP, or light

How are transport proteins, channels, and pumps specific?

• They are specific to their cargo, ranging from whole families of molecules to specific individual ions.

• Transport proteins are also specific to their membrane location.

• This allows cells to control what crosses a membrane and where it crosses.

How do transport/carrier proteins move molecules across membranes?

1. Binds its cargo.

2. Binding causes a shape/conformational change.

3. The protein exposes the cargo to the other side.

4. The cargo is released.

Driven by concentration of the molecule

What does the Na⁺/K⁺ pump do and why is it important?

• Uses about 25% of ALL cellular ATP.

• Establishes high Na⁺ outside the cell and high K⁺ inside.

• These concentration gradients store energy that can be used for many other cellular activities

How does the Na⁺/K⁺ pump work?

The pump undergoes conformational changes involving phosphorylation through ATP use.

For each ATP hydrolyzed:

• 3 Na⁺ exit the cell.

• 2 K⁺ enter the cell.

Each cycle contributes to the cell being negative inside

What does the Ca²⁺ pump in the ER do?

• It is an ATP-driven pump.

• Uses ATP to move Ca²⁺ into the ER.

• In muscle cells, the specialized ER is called the sarcoplasmic reticulum.

How can one molecule's gradient provide energy to transport another molecule?

ATP- and light-driven pumps establish concentration gradients.

These gradients can then power gradient-driven pumps:

• One molecule moves down its gradient.

• The energy released drives another molecule against its gradient

Na+/Glucose symporter

Imports glucose into intestinal epithelial cells against the gradient of glucose, with the gradient of NA+

How is glucose moved directionally from the intestine into the bloodstream?

The polarization of intestinal epithelial cells keeps different membrane proteins in specific locations:

• Na⁺/glucose symporter takes glucose in from the gut.

• Glucose can then be passively released toward the blood.

• The Na⁺/K⁺ pump establishes the Na⁺ gradient that ultimately drives this process.

Passive ions are often __________

Selective for a specific ion

What determines which ion can pass through a passive ion channel?

-A selectivity filter is the portion of the channel protein whose chemistry allows selection of the specific ion.

Why can't Na⁺ pass through a K⁺ channel even though Na⁺ is smaller?

In a K⁺ channel:

• Na⁺ with H₂O attached is too big to enter.

• Without H₂O, Na⁺ is too small to interact properly with the selectivity filter.

Ion channels opened and closed?

-Ion channels are constantly fluctuating btwn open and closed

-Regulated by interacting w particular ligands

What do K⁺ leak channels do?

K⁺ leak channels contribute significantly to the resting membrane potential.

-K+ exit from cells, down its gradient, is limited by the (+) charge on the outside

of the membrane.

-Changes to the electrical gradient across the membrane can cause

significant changes in movement of ions across the membrane.

Ion channels are regulated in three known ways

1. Mechanically/Physically gated channels

2. Ligand-gated channels

3. Voltage-gated channels.

How can physical triggering of ion channels cause leaves to close?

Physical triggering of ion channels → change in membrane voltage → triggers voltage-gated channels → large H₂O changes in cells at the base of the leaf → leaves close.

How do mechanically gated channels allow sound to be detected?

• Stereocilia of auditory hair cells are attached to the tectorial membrane

which vibrates due to sound waves

• Movement of the stereo cillia open mechanically gated ion channels

resulting in an influx of (+) charged ions

• This activates the underlying auditory nerves

What roles do mechanically gated and voltage-gated channels have in electrical signaling?

In both plants and animals mechanically gated and voltage-gated ion channels underlie the ability to generate an electric impulse

Why are voltage-gated ion channels necessary for an action potential?

-Experiments using squid axons showed that without voltage-gated ion channels, action potentials cannot be reached.

-Voltage-gated channels open in response to depolarization of the surrounding membrane.

What happens to a voltage-gated Na⁺ channel during an action potential?

Closed → Open → Inactivated → Recovered/Closed

• Depolarization opens the channel.

• It then becomes inactivated.

• It must recover before it can open again.

Why is Na⁺ channel inactivation important for action-potential propagation?

Inactivation prevents the channel from responding again until the initial signal has moved down the cell membrane.

This helps the electrical signal progress along the membrane rather than immediately reactivating the region behind it.

What roles do Na⁺ and K⁺ channels play during an action potential?

• Opening voltage-gated Na⁺ channels → Na⁺ influx → membrane depolarization.

• Na⁺ influx triggers voltage-gated K⁺ channels.

• K⁺ channels help re-establish the resting membrane potential.

How do neurons communicate w target cells at synapses

• Neurons interact with target cells at synapses.

• Presynaptic cell = releases neurotransmitters.

• Postsynaptic cell = receives the neurotransmitters.

How does an action potential cause neurotransmitter release and postsynaptic depolarization?

• Action potential reaches the end of the presynaptic cell.

• Triggers voltage-gated Ca²⁺ channels → Ca²⁺ enters.

• Ca²⁺ influx triggers release of synaptic vesicles containing neurotransmitters.

• Neurotransmitters trigger ligand-gated ion channels in the postsynaptic membrane.

• This results in membrane depolarization

How can signaling at a synapse lead to muscle contraction?

• The postsynaptic cell can be a muscle cell.

• Na⁺ influx ultimately triggers Ca²⁺ release from the sarcoplasmic reticulum.

• The released Ca²⁺ leads to muscle contraction.