Cell Bio VCU Exam 3

3 Main Functions of Membrane

1. Receiving information

2. Import/export of molecules

3. Capacity for movement and expansion (cytoskeleton)

Why are organelles enclosed in membranes?

To keep chemical reactions separate

Which organelles have two membranes?

Mitochondria, Chloroplast, & Nucleus

Define fluid mosaic model

Membrane proteins are mobile within a sea of phospholipids & cholesterol

Define hydrophobicity of membrane lipids

"Amphipathic"

Possessing hydrophilic heads & hydrophobic tails

Most common phospholipid in cell membrane?

Phosphatidylcholine (Choline & Phosphate head; connected via Glycerol to Hydrocarbon tails)

What about membranes is fundamental to the creation of a living cell?

That phospholipid bilayers spontaneously close into sealed compartments because it is energetically favorable

What are liposomes?

Pure phospholipids added to water (artificial bilayers that can be created in lab)

What 4 ways can phospholipids move within the membrane?

1. Lateral diffusions (side to side)

2. Rotation of tails

3. Flexion of tails

4. Flip-Flop (very rare, proteins are required to do this active movement)

What two factors influence fluidity of membrane?

1. Length of hydrocarbon tails (shorter tail= more fluid)

2. Number of double bonds in tails (More unsaturated= more double bonds= more fluid)

Define Lipid Rafts

Specialized, tightly packed micro-domains that serve as organization centers of cholesterol and transmembrane proteins

Functions of Lipid Rafts

1. Assembly of signal molecule

2. Membrane protein trafficking

3. Regulating neurotransmission

4. Receptor trafficking

5. Influencing membrane fluidity

Cholesterol & Membrane Fluidity

Stiffens the bilayer and makes it more rigid and less permeable (Fills in the space between kinks in hydrocarbon tails with double bonds and makes up 20% of the membrane)

4 Reasons Why Membrane Fluidity is Important

1.Enables membrane proteins to diffuse rapidly and interact w/ one another

2. Membrane lipids and proteins can diffuse from where they entered the bilayer

3. Membranes can fuse and mix molecules

4. Ensures membrane are distributed evenly between daughter cells

Phosphatidylcholine

Structural phospholipids (mainly on extracellular face of membrane)

Sphingomyelin

Phospholipids with myelin sheathing (SIGNALING and mainly on extracellular face of membrane)

Phosphatidylserine

Phospholipids on the cytosolic face that flip to outside during apoptosis (cell death)

Phosphatidylethemolamine

Phospholipid on cytosolic face of membrane that helps in membrane fusion

Glycolipids

Phospholipids that provide energy to cell and act as surface markers

Inositol Phospholipids

Important signaling molecules

Where Does Bilayer Synthesis Occur?

Endoplasmic Reticulum (Then exported to other membranes by budding and fusion)

Flippase

Catalyzes the transfer of phospholipid molecules form one monolayer to the other

4 Types of Membrane Proteins

Transporters, Anchors, Receptors, Enzymes

How much of the plasma membrane is made up of proteins?

50%

Define Integral Membrane Proteins

Proteins that are firmly embedded within the cell membrane.

3 Types of Integral Membrane Proteins

Transmembrane, Monolayer Alpha-Helix, Lipid-Linked

Transmembrane Proteins

- Integral

- Extend through entire bilayer

- Hydrophobic interiors & Hydrophilic exposed heads

- Alpha helix & Beta barrel

Monolayer Proteins

- Integral

- Located entirely within cytosolic face of membrane

- Alpha helix

Lipid-linked Proteins

- Integral proteins

- Lie entirely on outside of bilayer

- One or more covalently bound lipid groups

Protein Attached Proteins

- Peripheral proteins

- Bound indirectly through interactions with other membrane proteins

-Lie entirely on one side of membrane or the other

What form do most polypeptide chains take to cross the membrane?

Alpha helix

Describe the hydrophobicity of Alpha helices and why they must be in that conformation to cross the membrane.

The external amino acids are HYDROPHOBIC --> So they interact with the hydrophobic tails of membrane phospholipids

The polypeptide backbone is polar and therefore HYDROPHILLIC --> So they allow hydrophilic (water soluble) molecules

What are cannabinoids?

An example of transmembrane proteins that act as receptors to receive signals and relay them to other systems in the cell

Define Aqua Pores

Water filled pores formed by alpha helices(most commonly) or beta sheets

**Tight, can be opened or closed

Which is more versatile, alpha helix or beta barrel?

Beta barrel, cannot curve into tight spaces, only make wide channels

Define Detergents

Soapy substances that are used to destroy lipid bilayers to release the membrane proteins within (For scientific study)

What is the structure of a detergent?

Amphipathic, lipid like molecules with only one tail

What specific lab procedure uses detergents?

SDS-PAGE in breaking open membranes to make a cell lysate

What's a micelle?

The circle formed when detergent molecules aggregate (combine into a whole) in water

Define Cell Cortex

The meshwork of fibrous proteins attached to the cytosolic face of the plasma membrane

What is spectrin?

The main component of cell cortex in red blood cells (Mutations in spectrin=anemia)

Define Membrane Domains

Functionally specialized regions on the cell or organelle surface Lipid Rafts!

Four Methods of Restricting Protein Movement Around Membrane

1. Tether proteins to cell cortex inside cell

2. Tether proteins to extracellular matrix molecules outside cell

3. Tether proteins to other proteins from another, adjacent cell's membrane

4. Diffusion barriers caused by tight junction with other cells

Where are three areas in the body that tight junctions exist?

Blood brain barrier, Gut lining, Lungs

What is the Carbohydrate Layer?

"Sugar coating" around extracellular side of plasma membrane caused by sugar chains linked to membrane proteins

Glycoproteins

Membrane proteins connected to short chains of sugars called "oligosaccharides"

Proteoglycans

Membran proteins connected to one or more long polysaccharide chains

What are the functions of the Carbohydrate Layer?

-Protect cell surface

-Absorb water to help with motility and avoid sticking together

-Cell to cell recognition

Liposome

Vesicles with no transmembrane proteins, therefore impermeable

What are the two types of transmembrane proteins?

Transporters & Channels (both transverse the membrane with polypeptide chains many times)

How do transporter transmembrane proteins work?

Using their moving parts to shift molecules from one side of the membrane to the other

--> Only allow molecules that fit to its BINDING SITE, very specific!

How do channel transmembrane proteins work?

They are tiny hydrophilic pores that allow ions to pass via diffusion (PASSIVE TRANSPORT)

--> Discriminate based on size and electrical charge

What are the most plentiful ions in the cell's environment?

Na+, K+, Ca2+, Cl-, & H+

What are the 3 types of active transporters?

1. Coupled Pump

2. ATP-driven Pump

3. Light-driven Pump (bacteria)

How do transporters serve to specialize the cell?

Cell signaling tells which transporters to work, therefore specializing the characteristic set that is functioning at certain times

Define Electrochemical Gradient

The net driving force of PASSIVE ion movement across the membrane.

It is the sum of the CONCENTRATION GRADIENT and the VOLTAGE across the membrane. These can either work in unison (Lot of movement) or opposition (Little net movement).

Define Membrane Potential

The difference in electrical potential (charge) on each side of the membrane

What is the resting membrane potential of a cell?

Negative, around -60mV

What is the result of the Sodium/Potassium pump?

Keeps Na concentration lower in the cell & K concentration higher by pumping 3 sodium ions out for every 2 potassium ions pumped in

What are the steps of the Na+/K+ pump?

1. Na+ binds to pump

2. Pump is phosphorylated and conformation changes

3. Na+ is ejected out of cell and K+ binds to pump

4. Pump is dephosphorylated and conformation reverts

5. K+ is ejected into cell

What is the result of the Calcium pump?

Returns Ca2+ to sarcoplasmic reticulum from cytosol to allow for relaxation of muscle cell

How does the Calcium pump work?

Requires ATP to actively change shape to pump 2 Ca2+ at a time back into the sarcoplasmic reticulum

What are Major Intrinsic Proteins?

Integral membrane proteins that form pores in the cell membrane

What are the 2 types of coupled transporters?

Symport: Allow for a molecule going down its gradient to provide energy for another molecule going the same direction

Anitport: Allow for a molecule going down it gradient to provide energy for another molecule going the opposite direction

What is the function of the Glucose/Sodium Symport?

Allows glucose to pass through the membrane UP its concentration gradient using the energy from sodium passing DOWN its electrochemical gradient

Where is the Glucose/Sodium symport usually found?

The lining of the gut

Ion Channels

VERY selective, depends on DIAMETER and SHAPE of ion channel along with the distribution of CHARGED AA lining it

Describe a K+ Channel

-Narrow selectivity filter

-Lined with carbonyl oxygens

-Partial negative charge

Are ion channels continously open?

NO, they are gated

What are the 3 types of ion channels?

Voltage Gated

Ligand-Binding

Stress Gated

Define Resting Membrane Potential

Voltage difference across plasma membrane when a cell is not stimulated

Define Action Potential

Spike in electrical activity propagated rapidly along a membrane causing depolarization as it goes

Describe Depolarization

Na+ channels open due to stimulus and the positive ions flood in, making the membrane less negative

Describe Repolarization

Return of cell membrane potential to usual negative state as Na+ channels return to closed state

Where is the electrical signal from the membrane converted into a chemical signal?

The axon nerve terminal

Where is the chemical signal from neurotransmitters converted back into an electrical signal?

The transmitter-gated ion channels

Define Excitatory Neurotransmitters

Neurotransmitters that allow the influx of Na+, therefore increasing the likelihood of firing an action potential

Define Inhibitory Neurotransmitters

Neurotransmitters that allow the influx of Cl-, therefore decreasing the likelihood of firing an action potential

Describe Electrophysiology

Glass electrode placed on living cell to measure voltage changes

*Allows us to see action potentials firing

What is Ohm's Law?

Voltage = Current x Resistance

Describe Patch Clamp Electrophysiology

Microelectrode removes "patch" of membrane and runs current through the ion channels

Where is it possible that organelles evolved membranes from?

Invagination of the plasma membrane (explains the double membranes on Nucleus, Mitochondria, & Chloroplasts)

Define what the Endomembrane System is and its organelles.

Interconnected network of membrane enclosed organelles; ER, Golgi apparatus, lysosomes, peroxisomes, and endosomes

Where are 4 places proteins go directly from the cytosol?

Nucleus, Chloroplasts, Mitochondria, and ER

Where can proteins go from the ER?

Golgi apparatus to plasma membrane, lysosomes, or endosomes

How do proteins pass into the nucleus?

Through nuclear pores that penetrate the inner and outer membranes

Describe the structure and function of nuclear pores

- Made up of 30 different proteins

- Jumbled meshwork of proteins inside inhibit large molecules from diffusing through but allow small water-soluble molecules

-Protein fibrils extend on both sides and form a basket on nuclear side

Nuclear Localization Sequence

1 or 2 short sequences of positively charged lysines or arginines that direct a protein to the nucleus

What are Nuclear Transport Receptors?

NTRs, receptors that grab onto repeated AA sequences within the pore's tangle of proteins to carry the new protein cargo inside

Signal Sequences

Conserved amino acid sequences that act as a molecular "address" telling the cell where this protein needs to go live in the cell

What type of energy is used for nuclear protein import?

Ran-GTP Hydrolysis

What conformation are proteins in when imported into the Nucleus?

Fully folded

What conformation are proteins in when imported into Mitochondria & Chloroplasts?

They are unfolded as they are transported through both membranes and refolded upon entry

Where are the signal sequences for most Mitochondrial & Chloroplast bound proteins?

The N-terminus

What is the most extensive membrane system in a eukaryotic cell?

The endoplasmic reticulum

Once entered in ER, do proteins reenter the cytosol?

Only for transport in vesicles

Which 2 kinds of proteins can be transferred into the ER?

Water-soluble proteins & Prospective transmembrane proteins

Where are Water-Soluble Proteins entering the ER destined to go?

Secretion or the lumen of another organelle

Where are Prospective Transmembrane Proteins entering the ER destined to go?

Stay in ER membrane, membrane of another organelle, or the plasma membrane

**Directed to ER via an 8 hydrophobic amino acid sequence

At what point during synthesis do portends begin entering the ER?

While still attached to ribosomes, undergoing polypeptide chain synthesis