Mol Bio Exam 3

Study guide questions and quiz questions

What are the properties and characteristics of a plasma membrane?

A plasma membrane is a selective barrier, that enables cells to receive information, to export and import molecules and made up of a phospholipid bilayer and membrane proteins. it also allows for the movement of the cell.

What is the fluid mosaic model of the plasma membrane?

Describes the membrane as a two-dimensional fluid consisting of a continuous lipid bilayer with mobile embedded/associated proteins.

Describe the structure of a phospholipid.

Consists of a glycerol backbone, two hydrophobic fatty acid tails and polar hydrophilic head group containing phosphate linked to a small polar group like choline or serine. One tail is saturated while the other is unsaturated.

How do hydrophobic/hydrophilic molecules interact with water?

• Hydrophilic molecules interact with water by forming hydrogen bonds and electrostatic interactions with water molecules, this is energetically favorable.

• Hydrophobic molecules force adjacent water molecules to form highly ordered cage-like structures, which is energetically unfavorable.

Are triacylglycerols found in the plasma membrane?

triacylglycerols are purely hydrophobic fat molecules used for energy storage. Plasma membranes require amphipathic lipids (like phospholipids, glycolipids, and cholesterol).

Why do phospholipids form bilayers and sealed compartments?

Due to their amphipathic nature, hydrophobic tails aggregate inward away from water, while hydrophilic heads face the aqueous environment. This protects the hydrophobic tails from being exposed to the water on the outside.

What determines membrane fluidity?

• length of of hydrocarbon chains: Shorter chains reduce interactions between tails, making the membrane more fluid.

• Number of double bonds (unsaturation): Double bonds produce kinks in the tails, preventing tight packing and increasing fluidity.

• Cholesterol content: Modulates packing density.

Can phospholipids (and proteins) move within the membrane? 

Phospholipids can move via lateral diffusion (side to side), Rotation (rotating or spinning in place), flexion (flexing hydrophobic tails) or in rare cases flip-flopping across the membrane, which only happens with enzymes.

The experiments that detail this are cell fusion via hybrid cell formation (fusing human and mouse cells labeled w/ diff. fluoresent dyes mixes over time)

What experiments/techniques demonstrate/test phospholipid and protein movement w/in the cell membrane?

• The experiments that detail this are cell fusion via hybrid cell formation (fusing human and mouse cells labeled w/ diff. fluorescent dyes mixes over time)

• FRAP (Fluorescence Recovery After Photobleaching): Laser bleaches a small area of fluorescently labeled membrane; recovery of fluorescence measures lateral diffusion rate.

How does cholesterol affect the plasma membrane?

Cholesterol is short and rigid. It fills gaps between unsaturated fatty acid tails, stiffening the membrane region and making it less permeable and less fluid at body temperatures.

Describe how membranes are synthesized

• In the ER, new phospholipids are added to the cytosolic half of the ER membrane.

• Scramblase randomly flip’s lipids to ensure symmetric growth.

• At the Golgi, flippase selectively moves specific phospholipids to the cytosolic monolayer to establish asy mmetry, after the delivery of a new membrane from the ER.

• Vesicles transport the membrane while preserving cytosolic vs. non-cytosolic orientation.

How do proteins interact with the plasma membrane?

• Transmembrane: Crosses the entire lipid bilayer.

• Monolayer-associated alpha-helix: Anchored in the cytosolic monolayer by an amphipathic alpha-helix.

• Lipid-linked: Covalently attached to a lipid molecule embedded in either monolayer.

• Protein-attached (peripheral): Attached indirectly via non-covalent interactions with integral membrane proteins.

Why are transmembrane proteins made up of a-helices?  Are b-sheets found within transmembrane proteins?

The polypeptide backbone is hydrophilic. Forming an alpha-helix maximizes internal hydrogen bonding among backbone atoms while presenting hydrophobic amino acid side chains outward toward the lipid tails. B-sheets curve into closed b-barrels, making aqueous pores through the membrane.

What is the cell cortex?  How does it interact with the plasma membrane?

A meshwork of fibrous proteins (such as spectrin and actin) located directly beneath the cytosolic surface of the plasma membrane. It attaches via transmembrane proteins and attachment proteins to reinforce and maintain cell shape.

What limits protein movement within the plasma membrane?

Proteins tethered to the cell cortex, extracellular matrix components outside, surface proteins of adjacent cells, can all limit protein movement w/in the plasma membrane. as well as restriction by diffusion barriers such as tight junctions.

Where are sugars found on transmembrane proteins?  What do the sugar moieties do?

sugars are located on the non-cytosolic / extracellular face of the membrane. Sugar moieties Protect the cell surface from mechanical/chemical damage, lubricate the cell, and function in cell recognition.

What kinds of organic molecules can pass through a plasma membrane?  What kinds cannot?

The kinds of cells that can pass are small non-polar molecules and steroid hormones, along with small uncharged polar molecules (like water and glycerol).

The cells that cant pass are any large uncharged polar molecules and ALL CHARGED IONS.

What is osmosis?

The passive net movement of water across a semipermeable membrane from a region of low solute concentration to high solute concentration.

What is facilitated diffusion?  What two types of proteins are used for this purpose?

Passive transport of solutes down their concentration gradient using specific membrane proteins. These include transporters and channels.

How do transporters work (in general)?  How do ion channels work (in general)?

• Transporters (Carriers): Bind specific solutes and undergo a series of conformational changes to transfer the solute across.

• Ion Channels: Form continuous hydrophilic pores across the membrane allowing selective ions to pass rapidly down their gradient when open.

What is an electrochemical gradient?

The combined driving force moving a charged solute across a membrane, composed of:

1. The concentration gradient (chemical difference).

2. The voltage gradient / membrane potential (electrical difference).

What is active transport?  What three ways do cells power active transport?

Active transport is the movement of a solute against its electrochemical gradient, requiring energy. This happens through coupled pumps, ATP-pumps and light-driven pumps.

How does the Na⁺/K⁺ pump work?

• Uses ATP hydrolysis to pump 3 Na^++ out of the cell and put 2 K^+ in per cycle.

• Cytosolic Na^+ binds → pump auto phosphorylates via ATP → conformational change ejects Na^+ outside → extracellular K^+ binds → dephosphorylation triggers return to original shape, releasing K^+ inside.

How did nuclei evolve?

Evolved via invagination of the plasma membrane around genomic DNA.

How are proteins imported into organelles?

Imported one of three ways, transport through nuclear pores (gated transport of folded proteins), transportation across membranes (Unfolded proteins translocated across organelle membranes), Transport by vesicles (Soluble/membrane proteins moved between compartments in the endomembrane system.)

What is a signal sequence?

A continuous stretch of amino acids (typically 15–60 residues long) that acts as a molecular "address label" directing a protein to its specific organelle destination.

Describe the nuclear envelope and how materials cross it.

Describe the pool of ribosomes within a cell.

  How are proteins targeted to the ER?

How are vesicles formed?  What is clathrin?  What roles do Rab proteins and SNAREs play in vesicle targeting?

What role does the Golgi apparatus play in protein trafficking?

What is endocytosis?  What is receptor-mediated endocytosis?  What is exocytosis?

What is a lysosome?  How do materials get there?

What is transcytosis?

What is autophagy?

How do animal cells signal to one another?

What is contact-dependent signaling?

Where are communication receptors found in an animal cell?

Cell signaling can be fast or slow.  How do each work?  What are the results of each?

Describe molecular switches found within cells.

What are the three main classes of cell-surface receptors?

Describe how G-protein coupled signaling works to activate enzymes and also ion channels. 

How is cAMP formed and destroyed?  What does it do?

How is Phospholipase C (PLC) activated by G-protein coupled receptors?  What reaction does PLC catalyze?

What is calmodulin?

How do RTKs work?  What is an SH2 domain?

Uniport

Carries a single solute type passively or actively.

Symport

Coupled transporter moving two solutes in the same direction.

Antiport

Coupled transporter moving two solutes in opposite directions.

Mitochondria (Endosymbiont Theory):

Evolved from an aerobic prokaryote engulfed by an early anaerobic eukaryotic cell.