Cell bio 2

What is the fluid mosaic model of membrane fluidity? What evidence

supports this model?

When a human cell and a mouse cell were placed together, they ended up forming a hybrid cell and after an hour the proteins that were mouse and human were mixed up. When cutting the plasma membrane, there is a notable difference between the extracellular (more smooth) and the cytoplasmic layer (bumpy).

Why is important for a plasma membrane to be fluid and flexible?

It is important because it allows it to change shape for movement and growth. It allows molecular transport, cell division and it ensures that membrane proteins and lipids can diffuse laterally to maintain structural integrity.

What functions does the plasma membrane play in a cell?

The plasma membrane acts as a semi-permeable barrier, molecular transport, responds to signals, cell movement, and cellular compartmentalization.

Why do phospholipids spontaneously form bilayers and liposomes in

water?

Because the are amphipathic which means they have a hydrophilic head and a hydrophobic tail. When placed in water, they tend to form the most energetically favorable shape. Water molecules organize in a way that maximizes hydrogen bonding.

What is meant by “amphipathic”? What is the significance of amphipathic

molecules for the plasma membrane?

This means they have a hydrophilic head and a hydrophobic tail. They are able to spontaneously form stable, selective, fluid bilayer in water which allows them to create a barrier.

What factors influence membrane fluidity? How would you expect a

plasma membrane with lots of saturated fatty acids to compare with a

plasma membrane with lots of unsaturated fatty acids? Why

Membrane fluidity depends on its composition. More saturated tails means less fluid while more unsaturated tails means a more fluid membrane. If the fatty acids have a longer tail, they are less fluid and with a shorter tail, they are more fluid. Cholesterol acts as a buffer to membrane fluidity at different temperatures. It decreases membrane fluidity at high temperatures and increases fluidity at low temperatures.

What does it mean that membranes are asymmetric? How is such

asymmetry generated?

Membrane assembly begins in the ER, new phospholidpids are deposited only to the cytosolic half of the bilayer. Phospholidpid asymmetries arise in the golgi apparatus. Flippases are proteins that catalyze the flip flopping of specific phospholipids from one half of the membrane to the other half. The cytosolic and non-cytosolic halves have different phospholipid compositions.Scramlase randomly moves phospholipids from the cytosolic half to the non-cytosolic half.

How can proteins associate with the cell membrane? What distinguishes

these types of proteins from one another?

Proteins associate with the lipid bilayer in multiple ways. Integral proteins span membrane, attached to one half of the lipid bilayer, and are covalently bound to lipid. Peripheral proteins are associated with integral proteins. Transmembrane, monolayer associated, and lipid linked are integral membrane proteins. Protein attached are peripheral membrane proteins.

What types of structures are typically observed spanning the plasma

membrane? Why are these structures formed and where are the

hydrophobic and hydrophilic regions of the protein located relative to the

lipid tails of the plasma membrane?

Proteins usually cross membranes as an alpha helix. Some proteins cross plasma membrane multiple times- eg. G-coupled protein receptors and aquaporins. The hydrophobic part is inside and the hydrophilic part is outside. Lipids and proteins are both within the plasma membrane, the phospholipids hydrophobic tails point away from the water and the hydrophilic heads point towards the water. Transmembrane proteins span the plasma membrane as alpha helices. The hydrophobic regions interact with lipid tails and the hydrophilic regions face the extracellular or cytosol space.

What are the functions of glycocalyx (glycolipids + glycoproteins +

proteoglycans)

Glycocalyx is carbohydrate layer from glycosylated proteins and lipids. It protects the cell inside of it and aids in adherence to its environment

What is the cell cortex and what is its function?

The cell cortex is a meshwork of filamentous proteins attached to the inner leaflet of the plasma membrane (cytosolic). Which strengthens and supports plasma membrane. Actin and myosin make up much of cell cortex in many animal cells. It reinforces the underlying cell cortex in animal cells.

Why is it important that not all of the membrane is fluid? (I.e. why is it

important that some proteins are anchored?)

Some proteins are anchored in the plasma membrane. They set up membrane domains; specialized regions along the cell membrane.

Imagine you are studying the high mountain meadow ecosystems of

the Rocky Mountains (9800 ft and higher). Do you think the pikas that

live there would have more or less cholesterol in the plasma

membranes of their cells than a kangaroo rat living in a desert steppe

community? Why

The Pikas that live there would have more cholesterol in their cell plasma membrane because in low temperatures cholesterol increases membrane fluidity which ensures the membrane is not too rigid.

Now imagine you were studying the Rocky Mountain glacier lily. What

would it have instead of cholesterol to ”buffer” membrane fluidity?

How would you expect the chemical composition of the plasma

membranes of the glacier lily compare with something like a cholla

cactus (a desert plant)?

Instead of cholesterol, the glacier lily would use other phytosterols to buffer. The glacier lily would have more unsaturated fatty acids to maintain fluidity in cold environments than a cactus.

Imagine you find a mutant fruit fly that has defective flippase. What do you

think would be the consequence for membranes in this fly line? Why?

It would exhibit severe membrane instability and lipid asymmetry because the flippase is necessary to move phospholipids from the cytosol side to the non-cytosol side.

You are a graduate student and you isolate a new pore protein from the

plasma membrane of a human kidney cell. What structures might you

expect to find in this pore forming protein? Where would the hydrophilic

and hydrophobic parts of the protein be localized in the membrane? What

types of techniques did you need to use to isolate this protein? Why?

Alpha helices or a beta-barrel structure (less likely). Hydrophibic region within the lipid bilayer and the hydrophilic regions will face the aqueous pore lumen or the extracellular/cytosolic sides. Detergents and chromatography are necessary for isolation due to the proteins hydrophobic nature.

You are a doctor and you have a patient come to you with a rare form of

anemia. You discover that this patient has a mutation in their spectrin

gene. What type of anemia do they have? What do their red blood cells

look like? Why?

Herediraty spherocytosis, a congenital hemolytic anemia. Their red blood cells appear spherical rather than biconcave. This is because spectrin is crucial for structural stability of the RBC membrane.

Why can large non-polar molecules, like steroid hormones, rapidly diffuse

across the plasma membrane whereas small, charged molecules, like Na+

cannot?

Membrane are semi-permeable. The lipid bilayer is impermeable to most uncharged polar molecules and all charged ions. Small nonpolar molecules and large nonpolar molecules are able to diffuse while no charged ions like Na+ are able to.

Know the concentration gradients, whether each ion has a higher

concentration inside or outside of the cell for Na+, K+, Ca2+, Cl-, and glucose.

Know the electrical gradient that gives cells a membrane potential as well

K+ has a higher concentration inside the cell. Na+ has a higher concentration outside, Ca2+ is higher outside, and Cl- is higher outside. Glucose is higher outside than inside. The cytoplasm is more negative and the extracellular space is more positive.

What are the defining characteristics of active and passive transport?

Passive transport requires no ATP, it goes downhill and is energetically favorable, molecules move down their concentration gradients and/or their electrical gradients, and it may occur through simple diffusion (no proteins needed) or facilitated diffusion with a protein transporter. Active transport requires energy, moves uphill (energetically unfavorable), moves molecules against their electrochemical gradients, and it requires proteins

We talked about three types of passive transport. Be prepared to describe

each type.

Diffusion, Channel facilitated diffusion, and transporter facilitated diffusion. Simple diffusion needs to proteins and molecules move down their concentration/electrical gradient. Channel proteins let only ions of correct size and charge into cell when open. Transporter proteins bind only specific molecules and ions; high specificity binding induces conformational change.

How does the K+ channel demonstrate selectivity for K+ as compared with

Na+ or Cl-?

Ion channels are selective, they only allow ions of correct size and charge to pass, alpha helices often form subunits to build a channel across the membrane.

Many ion channels are gated. What does this mean and how can channels

be opened?

Gated channels are opened by a stimulus, they open fast when stimulated. There are ligand-gated, voltage-gated, stress-gated/mechanically-gated, and temperature gated.

Provide an example of a ligand gated, stress/mechanically gated, and a

temperature-gated channel. How do each of these work? What do they

have in common?

How are transporter proteins different than channel proteins?

Very specific binding between solute (substrate) and binding site on transporter protein (induces conformational change).

Describe glucose transporters. How do glucose transporters demonstrate

characteristic features of passive transporter proteins?

There are 14 different GLUT transporter types in human body, all have 12 membrane spanning domains. GLUT 1 is in most cells and it helps in basal glucose uptake, GLUT 3 is in the neurons, placenta, testes, and brain and it helps with basal glucose uptake. GLUT 4 is in skeletal and cardiac muscle and fat and its activity increased by insulin. It allows for passive movement of glucose into cell; upregulated in response to insulin.

What is osmosis? How does osmosis relate to cell biology? (What happens

to cells in isotonic, hypotonic, and hypertonic solutions?)

Osmosis is the water moving from area of low concentration to high concentration. Hypotonic is a lower solute concentration outside of cell relative to inside. Hypertonic is a higher solute concentration outside than inside. Isotonic is equal solute concentration outside and inside.

What are aquaporins and what do they do in a cell? How might a mutation

in AVPR2 or AQP2 affect kidney cells? How would a mutation in one of

these genes affect the physiology of an affected person?

Aquaporins are channel proteins that lead to rapid diffusion of water. They are proteins with 6 membrane spannind domains. There are seven different kinds of aquaporins expressed in the kidney. nephrogenic diabetes insipidus is a mutation in AVPR2 or AQP2. This leads to an excessive urine production, patients must drink as much water as they excrete leading to excessive thirst.

What are some examples of ATPase pumps in the cell? (Know the Na+/K+

pump and a few other examples.)

Na+/K+ ATPase is an enzyme and transport protein, it requires ATP hydrolysis, maintains gradients across cell membrane - can be used to drive other solute transport (coupled transport). Sodium and potassium are both going against their gradient → 3 sodium out; 2 potassium in. The Na+-driven glucose pump which is located in the apical plasma membrane of kidney and intestinal cells and uses the Na+ gradient functions to active import of glucose. The Na+-H+ exchanger which is located in the plasma membrane of animal cells and uses the Na+ gradient, functions to active export of H+ ions, pH regulation.

How are the concentration gradients (which are often established by

ATPase pumps) used to actively transport other molecules? What is an

example of an antiporter and a symporter?

A symporter

How can light be used for active transport?

We have looked at glucose transport as both a passive process and an

active process. Describe how these two processes happen in an intestinal

cell. Where is the active transport and where is the passive transport taking

place? How is cell polarity (and distinct membrane domains) important for

glucose to be transported from the lumen of the intestine, through the

epithelial cells, and to the bloodstream to be carried to the liver?

What are the two conditions that determine whether an ion, like K+, moves out of

the cell through open leak channels?

What are the different parts of a neuron? What roles do they play in electrical

signaling? What are the roles of ligand-gated (neurotransmitter) and voltage-

gated channels in neuron action potential and neurotransmitter release?

What are the steps in an action potential? Why don’t action potential travel back

up the axon from the direction they came? (Hint: be specific at the cell level,

what is happening with ion channels during the falling and undershoot phases?

What are the three conformations of the voltage gated Na+ channels and how do

these conformations matter for an action potential?)

What factors contribute to rapid electrical signaling in neurons?

How do Ca2+ voltage gated channels contribute to neuronal signaling?

Why are neurotransmitters rapidly degraded or reabsorbed following

neurotransmission?

There are a large number of neurotoxins that act on a neuron’s ability

to generate an action potential, release neurotransmitters, degrade

neurotransmitters, inhibit a neurotransmitter receptor, or cause a

massive release of a neurotransmitter. Why do you think there are so

many naturally occurring toxins that inhibit neurotransmission? How

would each be expected to alter ion flow across the membrane and

how would this impact neuronal function?

What are the three stages involved in the breakdown of food? Where does

each stage occur within a cell?

What are activated carriers? Where is the energy stored in these? Which

activated carriers play a role (i.e. are used or made) during glycolysis? The

transfer of pyruvate into the mitochondrial matrix? The Citric acid/Kreb’s

cycle?

Know the overall formula for cellular respiration. Which molecule is oxidized

and which is reduced? How do you know?

What is meant by the energy “investment” and ”payoff” phases of glycolysis?

What are the net products of glycolysis?

Table 13-1 lists four different types of enzymes involved in glycolysis. Know

an example of each type of enzyme, what it does in general, and the specific

role it plays in glycolysis.

Does glycolysis require oxygen? Does it make CO2

Why is fermentation necessary under anaerobic conditions? What

type of fermentation would one of your muscle cells likely undergo

when you are running as fast as you can?

Why is it important for a cell to tightly regulate metabolic pathways?

Maintain homeostasis, ensure efficient resources utilized, balance energy supply with demand

Step 1 glycolysis

Glucose → Hexokinase → glucose 6-phosphate

Step 3 glycolysis

Fructose 6-phosphate → phosphofructo kinase → fructose 1, 6 bispohosphate

Step 4 glycolysis

Fructose 1, 6 bisphosphate → aldose → Dihydroxyacetone phosphate AND Glyceraldehyde 3 - phosphate

Glycolysis step 4.2

Dihydroxyacetone phosphate → Triose phosphate isomerase ← Glyceraldehyde 3-phosphate

Glycolysis step 6

Glyceraldehyde 3-phosphate → glyceraldehyde 3-phosphate dehydrogenase → 1.3 -Bisphosphoglycerate

Step 7 glycolysis

1.3 Bisphosphoglycerate → phosphoglycerate kinase → 3-phosphoglycerate

Step 10 glycolysis

Phosphoenolpyruvate (PEP) → pyruvate kinase → pyruvate