Biochemistry II Lesson 6: Cell Membranes

What does it mean to say that the Cell Membrane is "semi-permeable"?

To say that the Cell Membrane is "semi-permeable" is to say that some things can enter while some things cannot.

CRB True or false? Because the Cell Membrane is Semi-Permeable, it is not able to create separate internal and external environments.

False. One of the key things about Cell Membranes are that they can create distinct Internal and External Environments! Cell membranes are considered "Selectively Permeable." They cannot control the movement of some particles, though.

The ______________ of a Phospholipid is what gives it is hydrophobic characteristics. The ______________ of a Phospholipid is what gives it is hydrophillic characteristics.

(A) Phosphate Head, Fatty Acid Tails

(B) Fatty Acid Tails, Phosphate Head

(C) Glycerol Backbone, Fatty Acid Tails

(D) Fatty Acid Tails, Glycerol Backbone

(B) Fatty Acid Tails, Phosphate Head

The Fatty Acid Tails of a Phospholipid is what gives it is hydrophobic characteristics. The Phosphate Head of a Phospholipid is what gives it is hydrophillic characteristics.

CRB In the cell membrane, there are also the closely related Sphingolipids. Which of the following statements about Sphingolipids are true?

I. Sphingolipids lack Glycerol.

II. All Sphingolipids have a hydrophilic region and two fatty-acid like tails that form a hydrophobic region.

III. Some common Sphingolipids are Ceramide, Sphingomyelins, Gangliosides and Cerebrosides.

(A) II only

(B) I and II only

(C) I and III only

(D) I, II and III

(C) I and III only

Each of the following statements about Sphingolipids are true:

I. Sphingolipids lack Glycerol.

II. Sphingolipids have a hydrophilic region and can have either one or two fatty-acid like tails that form a hydrophobic region. Most have two, but not all do.

III. Some common Sphingolipids are Ceramide, Sphingomyelins, Gangliosides and Cerebrosides.

A molecule that has both hydrophobic and hydrophillic characteristics, such as a Phospholipid, is said to be:

(A) Amphiprotic

(B) Amphipathic

(C) Dipolar

(D) Dihydrous

(B) Amphipathic

A molecule that has both hydrophobic and hydrophillic characteristics, such as a Phospholipid, is said to be Amphipathic.

CRB True or false? Because phospholipids are Amphipathic with distinct Hydrophilic and Hydrophobic regions, they naturally assemble Micelles or Liposomes in aqueous solutions.

True. Because phospholipids are Amphipathic with distinct Hydrophilic and Hydrophobic regions, they naturally assemble Micelles or Liposomes in aqueous solutions.

CRB Which type of interaction is driving this Micelle and Liposome formation?

(A) Van der Waals Interactions

(B) Hydrophobic Interactions

(C) Atmospheric Pressure

(D) Condensation Interactions

(B) Hydrophobic Interactions

Hydrophobic Interactions are driving the Micelle and Liposome formation.

Describe whether or not each compound is able to pass through the Cell Membrane via Passive Diffusion:

(1) Gases

(2) Small, Polar Compounds

(3) Large, Non-polar Compounds

(4) Large, Polar Compounds

(5) Charged Compounds

(1) Gases - Yes, quickly.

(2) Small, Polar Compounds - Yes, slowly.

(3) Large, Non-polar Compounds - Yes, slowly.

(4) Large, Polar Compounds - No.

(5) Charged Compounds - No.

Which of the following are unable to pass through the Cell Membrane via Passive Diffusion?

I. Cholesterol

II. Na+

III. Glucose

(A) I Only

(B) I and II Only

(C) II and III Only

(D) I and III Only

(C) II and III Only

Cholesterol is a large, non-polar molecule, allowing it to pass through the Cell Membrane via Passive Diffusion at a slow rate.

Na+ is a charged compound, preventing it from utilizing Passive Diffusion.

Glucose is a large, polar compound, preventing it from utilizing Passive Diffusion.

Draw a Saturated Fatty Acid versus a Unsaturated Fatty Acid.

Phospholipids vary in that they often have different groups attached to which part of a Fatty Acid?

(A) Glycerol

(B) Alkyl Group

(C) Phosphate Group

(D) Diol Group

(C) Phosphate Group

Phospholipids vary in that they often have different groups attached to the Phosphate group of the Fatty Acid via a Phosphoester Bond.

Draw a cis-unsaturated Fatty Acid versus a trans-?

CRB True or false? Part of why Trans-Fatty Acids have been so heavily regulated in foods are because they can dangerously increase the fluidity of the cell membrane.

False. Part of why Trans-Fatty Acids have been so heavily regulated in foods are because they can dangerously Decrease the fluidity of the cell membrane.

CRB But hey, I thought that Saturated Fatty Acids decrease the fluidity of the membrane and Unsaturated Fatty Acids increase the fluidity! What gives?

The Unsaturated Fatty Acid tails that increase fluidity have "kinks" in their tails, pushing away adjacent Phospholipids away. Trans fatty acids do not have these kinks and therefore act more like Saturated Fatty Acids.

True or False? Cholesterol can be extremely dangerous if it begins to infest the plasma membrane of your cells.

False. Cholesterol plays an essential and important role within the cell membrane.

As temperatures increase, cholesterol will __________ the fluidity of the cell membrane. As temperatures decrease, cholesterol will __________ the fluidity of the cell membrane.

(A) increase, increase

(B) increase, decrease

(C) decrease, decrease

(D) decrease, increase

(D) decrease, increase

As temperatures increase, cholesterol will decrease the fluidity of the cell membrane. As temperatures decrease, cholesterol will increase the fluidity of the cell membrane.

Cholesterol does this by increasing the distance between phospholipids at low temperatures and decreasing the distance at high temperatures. Think of cholesterol acting as a buffer for membrane fluidity.

Compare Integral and Transmembrane Proteins.

Integral and Transmembrane proteins are the same exact thing! They both span the entire cell membrane from intracellular to extracellular.

CRB As compared to Integral Proteins, where would you find Embedded Proteins?

(A) In the hydrophobic tail region only.

(B) In the Cytoplasmic Surface of the Cell Membrane only.

(C) In the Extracellular Surface of the Cell Membrane only.

(D) In either the Cytoplasmic or Extracellular Surface of the Cell Membrane only.

(D) In either the Cytoplasmic or Extracellular Surface of the Cell Membrane only.

CRB True or false? Because there are clear preferences for where some molecules like to exist in the cell membrane, the cell membrane has a Polarity.

True. Because there are clear preferences for where some molecules like to exist in the cell membrane, the cell membrane has a Polarity.

Which of the following are roles that proteins play within a cell membrane?

I. Transport molecules

II. Transmit signals

III. Maintain the structural integrity of the cell membrane

(A) I Only

(B) II Only

(C) I and II Only

(D) I and III Only

(C) I and II Only

Proteins transport molecules across a cell membrane. They may also help transmit signals across a membrane.

Which of the following exist within the cell membrane to help with cell-cell communication?

(A) Glycocholesterols

(B) Glycoacids

(C) Glycoproteins

(D) Glycosugars

(C) Glycoproteins

Glycoproteins and Glycolipids play a role in the cell membrane.

CRB Define and draw a Glycoprotein.

Glycoprotiens are membrane-bound proteins that have an associated carbohydrate.

In this picture, the red is the membrane-bound protein.

Why do we sometimes call our representation of the cell membrane the "Fluid Mosaic Model?

We call it the "Fluid Mosaic Model" because it is not stagnant but is actually fluid. Fatty acids, Cholesterol, and proteins are moving around.

A protein that is stuck within the cell membrane and is not exposed to either the extracellular or intracellular area is known as a:

(A) lipidbound protein

(B) peripheral protein

(C) channel protein

(D) carrier protein

(A) lipidbound protein

A protein that is stuck within the cell membrane and is not exposed to either the extracellular or intracellular area is known as a lipidbound protein.

Jack is studying a protein in his biochemistry class. He learns that this protein is located on the intracellular side of the cell membrane and converts one molecule into another molecule. This protein is known as a:

(A) lipidbound protein

(B) peripheral protein

(C) channel protein

(D) carrier protein

(B) peripheral protein

Peripheral proteins are on either the extracellular or intracellular side of the cell membrane but not both.

Mike is observing a protein in the lab. He notices that the protein spans the entire membrane and allows potassium to flow from one side of the cell membrane to the other. He doesn't notice any ATP production. This protein is known as a:

(A) lipidbound protein

(B) peripheral protein

(C) channel protein

(D) carrier protein

(C) channel protein

Mike is observing a protein in the lab. He notices that the protein spans the entire membrane and allows potassium to flow from one side of the cell membrane to the other. He doesn't notice any ATP production. This protein is known as a channel protein

Sarah is also observing a protein in the lab. She notices that the protein spans the entire membrane and allows potassium to flow from one side of the cell membrane to the other. She notices ATP production. This protein is known as a:

(A) lipidbound protein

(B) peripheral protein

(C) channel protein

(D) carrier protein

(D) carrier protein

Sarah is also observing a protein in the lab. She notices that the protein spans the entire membrane and allows potassium to flow from one side of the cell membrane to the other. She notices ATP production. This protein is known as a carrier protein

At low temperatures, phospholipids become "crystalized." What does this mean?

At low temperatures, phospholipids will huddle close together and exhibit relatively little movement.

Addition of saturated fats will _________ the fluidity of the cell membrane. Addition of unsaturated fats will _________ the fluidity of the cell membrane.

(A) increase, increase

(B) increase, decrease

(C) decrease, decrease

(D) decrease, increase

(D) decrease, increase

Addition of saturated fats will decrease the fluidity of the cell membrane. Addition of unsaturated fats will increase the fluidity of the cell membrane.

Compare Transbilayer Diffusion to Lateral Diffusion in terms of direction of movement, speed, and energy usage.

Neither Transbilayer Diffusion nor Lateral Diffusion require energy.

Transbilayer Diffusion moves a lipid from one leaflet of the cell membrane to the other. This is a slow process.

Lateral diffusion moves a lipid from side to side. This is a fast process.

Compare the action of Flippase, Floppase, and Scramblase in terms of direction of movement, speed, and energy usage.

All three enzymes catalyze reactions that are fast. Flippase and Floppase both require ATP as an energy source, whereas Scramblase does not require an outside energy source.

Flippase moves a lipid from the outer leafleat to the inner leaflet (think I-nside).

Floppase moves a lipid from the inner leaflet to the outer leaflet (think O-utside).

Scramblase moves one lipid to one side while moving another one back. Basically it causes two lipids to switch places.

Your bladder needs to contain water without it leaking out into the rest of your body's tissues. The cells in your bladder most likely utilize which cell junction to allow for this ability?

(A) Synapse Junctions

(B) Gap Junctions

(C) Desmosomes

(D) Tight Junctions

(D) Tight Junctions

Tight Junctions are waterproof seals. They are common in areas that contain large amounts of water such as the bladder, intestines, and the Kidney.

Your skin experiences a high amount of stress and needs cell junctions that allow for a little bit of wiggle room without losing cells. The cells in your skin most likely utilize which cell junction to allow for this ability?

(A) Synapse Junctions

(B) Gap Junctions

(C) Desmosomes

(D) Tight Junctions

(C) Desmosomes

Desmosomes are like "spot welds." They link two cells together but still allow ions and water to pass through. They are common in places with high amounts of stress such as the skin or intestines.

CRB Which of the following structures would you expect attaching epithelial cells to the underlying basement membrane?

(A) Hemidesmosomes

(B) Gap Junctions

(C) Desmosomes

(D) Tight Junctions

(A) Hemidesmosomes

Hemidesomosomes connect Epithelial Cells to the underlying Basement Membrane.

Cardiac muscle cells need to allow an electrical signal to pass from one cell to another in order to allow for a coordinated heart beat. These cells most likely utilize which cell junction to allow for this ability?

(A) Synapse Junctions

(B) Gap Junctions

(C) Desmosomes

(D) Tight Junctions

(B) Gap Junctions

Gap Junctions are like tunnels that allow for substances and electrical signals to quickly pass from one cell to another.

CRB Which of the following statements about Gap Junctions are true?

I. Gap Junctions can also be called Connexons.

II. The pores of Gap Junctions are formed by pores in interacting Connexin molecules.

III. Molecules that flow between cells at Gap Junctions are said to follow a Paracellular Route.

(A) II only

(B) I and II only

(C) I and III only

(D) I, II and III

(B) I and II only

The Paracellular route is related to Tight Junctions. Each of the following statements are true about Gap Junctions:

I. Gap Junctions can also be called Connexons.

II. The pores of Gap Junctions are formed by pores in interacting Connexin molecules.

CRB Many plant cells have both a plasma membrane and a cell wall. Which of the following is the Major difference between the composition of the two?

(A) The Cell Wall has more Lipid Rafts.

(B) The Cell Wall has more Carbohydrates.

(C) The Cell Wall is more permeable.

(D) The Cell Wall is more rigid.

(B) The Cell Wall has more Carbohydrates than the Plasma Membrane.

CRB Which of the following statements about Waxes and Plasma Membranes are true?

I. Waxes are typically found in animal, but not plant, cell membranes.

II. Waxes are very hydrophobic, having a low melting point.

III. Waxes contribute to both stability and rigidity in the cell membrane.

(A) III only

(B) I and II only

(C) II and III only

(D) I, II and III

(A) III only

Each of the following statements about Waxes and Plasma Membranes are true:

I. Waxes are typically found in plant, but not animal, cell membranes.

II. Waxes are very hydrophobic, having a high melting point.

III. Waxes contribute to both stability and rigidity in the cell membrane.

CRB Which are able to move faster in the Plasma Membrane, Lipid Rafts or Phospholipids, and why?

(A) Phospholipids, because they are diffusing from high concentration to low concentration.

(B) Lipid Rafts, because they are diffusing from high concentration to low concentration.

(C) Phospholipids, because they are smaller than Lipid Rafts.

(D) Lipid Rafts, because they are only on the surface of the Plasma Membrane, whereas Phospholipids have tails entering the middle of the membrane.

(C) Phospholipids, because they are smaller than Lipid Rafts.

CRB Various Peptide Hormones and Neurotransmitters may take a while to have an effect on cells if they have been absent for a long period of time. Which of the following is the best explanation?

(A) The Cell Membrane has forgotten the structure of these over time, and needs to alter its semi-permeable nature to become permeable to these signaling molecules again.

(B) The Cell Membrane has stopped creating and seen a decrease in channels for these signaling molecules to enter through, so new ones must be transcribed.

(C) The Cell Membrane has stopped creating and seen a decrease in receptors for these extracellular signaling molecules, and must upregulate these.

(D) None of the above.

(C) The Cell Membrane has stopped creating and seen a decrease in receptors for these extracellular signaling molecules, and must upregulate these receptors.