Unit 2 (Week 4)

Question 1

What is any membrane made by living cells; can be the plasma membrane or an internal membrane that surrounds an organelle?

Answer 1

Biological membrane (all 5-10 nm) 5,000-10,000 of these stacked on top of each other to be as thick as paper.

Question 2

What are the two primary components of a membrane?

Answer 2

Phospholipids that form the basic matrix of the membrane and proteins which are embedded in the membrane or loosely attached to its surface.

Carbohydrates may or may not be attached to the membrane lipids and proteins.

Question 3

What is the phospholipid bilayer?

Answer 3

The basic framework of a biological membrane, consisting of two layers of phospholipids.

Question 4

What is a leaflet and what are the two in the phospholipid bilayer?

Answer 4

Leaflets are one half of the bilayer and faces different regions. There are two which are called the cytosolic leaflet and the extracellular leaflet.

Question 5

T/F The bilayers are symmetrical.

Answer 5

False.

Lipid composition may be more abundant in one leaflet compared to the other. Glycolipids are found primarily in the extracellular leaflet of the plasma membrane. The carbohydrate portion protrudes into the extracellular medium.

Question 6

What is a protein that has one or more regions that are physically embedded in the hydrophobic region of a membrane’s phospholipid bilayer?

Answer 6

Transmembrane protein

They are folded into “a helix.” The nonpolar amino acids interact favorably with the nonpolar lipid tails.

Question 7

What is a type of integral membrane protein that is attached to the membrane via a lipid molecule?

Answer 7

Lipid-anchored protein

Question 8

What are proteins such as a transmembrane protein or a lipid-anchored protein that cannot be released from the membrane unless the membrane is dissolved with an organic solvent or detergent called?

Answer 8

Integral membrane proteins

Question 9

What is a protein that is noncovalently bound to a region of an integral membrane protein that projects out from the membrane or noncovalently bound to the polar head group of a phospholipids?

Answer 9

Peripheral membrane proteins (hydrogen and/or ionic bonds)

Question 10

Roughly, how many medications today exert their effects by binding to membrane proteins?

Answer 10

70%

Question 11

What is a property of biological membranes in which individual molecules remain in close association yet have the ability to move rotationally or laterally within the plane of the membrane? Membranes are semifluid.

Answer 11

Fluidity

Question 12

What is a property of biological membranes in which movement of membrane components occurs only in two dimensions?

Answer 12

Semifluid

Fluid would be movement in 3 dimensions.

Question 13

What enzyme allows the polar region of a phospholipid to flip to the other leaflet and does not happen spontaneously like rotational or lateral movements?

Answer 13

Flippase (energetically unfavorable since energy input in the form of ATP is needed)

Question 14

How fast can a lipid traverse a bacterial cell? An animal cell?

Answer 14

1 second and 10 to 20 seconds.

Question 15

What is in a membrane, a group of lipids, sometimes including associated proteins, that float together as a unit in a larger sea of lipids?

Answer 15

Lipid raft

Typically have a high amount of cholesterol and/or unique set of transmembrane and lipid-anchored proteins.

Question 16

What is a key property that affects the fluidity of the phospholipid bilayer?

Answer 16

The length of lipid tails. Ranges from 14-24 carbon atoms with 16-18 being the most common. Shorter tails are less likely to interact with each other, which makes the membrane more fluid.

Question 17

What is another property in phospholipid tails that help it remain more fluid in the context of carbon-hydrogen relationship?

Answer 17

The double bonds between carbon and hydrogen. (Remember: saturated fats are solid at room temperature versus olive oil that remains more fluid. Olive oil have double bonds while saturated fat is saturated with hydrogen bonds.

Question 18

T/F Cholesterol is found in plant and animal cells.

Answer 18

False.

Plants have phytosterols that resembles cholesterol’s chemical structure.

Question 19

What is the third factor that helps plasma membrane’s retain their fluidity and why?

Answer 19

Cholesterol, a short and rigid molecule.

It stabilizes cell membranes based off of lipid composition and temperature.

Question 20

How does the change in temperature affect cholesterol in the membrane?

Answer 20

At high temperatures, cholesterol makes the membrane less fluid.

At lower temperatures, the opposite happens and prevents it from freezing.

This phenomenon helps animals cope with changing temperatures to make sure their cells operate optimally.

Question 21

Why might transmembrane proteins move around the phospholipid bilayer?

Answer 21

They could be bound to components of the cytoskeleton or attached to molecules outside the cell like the extracellular matrix (ECM) of animal cells

Question 22

Discuss how transmembrane proteins are important in the binding of cells to each other and the binding of cells to the extracellular matrix.

Answer 22

Transmembrane proteins called cell adhesion molecules bind to each other to promote cell-to-cell adhesion. In addition, they can bind to filaments in the extracellular matrix, such as collagen fibers, thereby causing a cell to adhere to the extracellular matrix.

Question 23

Where does the synthesis of most lipids by the cytosol and endomembrane system occur?

Answer 23

The cytosolic leaflet of the smooth ER membrane.

Question 24

[Synthesis of membrane phospholipids at the ER membrane] What are the steps needed for fatty acids to enter the ER? (5)

Answer 24

1. Fatty acids are activated with the attachment of a CoA molecule.
2. This fatty acids are then bonded to glycerol-phosphate and inserted into the cytosolic leaflet of the ER membrane via acyl transferase.
3. The phosphate is removed by phosphatase enzyme.
4. A choline already linked to phosphate is attached via choline phosphotransferase.
5. Flippases transfer some of the phospholipids to the other leaflet into the ER lumen.

Question 25

What are the variety of mechanisms where lipids made in the ER membrane are transferred to other membranes?

Answer 25

1. Lipids are transported via vesicles.
2. Phospholipids can diffuse laterally around the membrane.
3. Via lipid-exchange proteins where a protein extracts a lipid from one membrane, diffuses through the cell, and inserts the lipid into another membrane.

Question 26

What do most transmembrane proteins contain that direct them to the ER membrane?

Answer 26

ER signal sequence

Question 27

What conditions of the protein must be met in order for a region to become a transmembrane segment?

Answer 27

If the polypeptide contains a stretch of amino acids that are mostly hydrophobic and form “a helix”

Question 28

What are the steps for the transmembrane protein to enter the plasma membrane of the ER Lumen?

Answer 28

1. The protein is synthesized by a ribosome and begins synthesis into the ER. The ER signal sequence is cleaved by signal peptidase.
2. Synthesis continues and a hydrophobic transmembrane segment is made as the polypeptide is threaded through the channel.
3. Synthesis is complete and the transmembrane segment remains in the hydrophobic region of the phospholipid bilayer.

Question 29

What structural feature of a polypeptide causes a region of it to form a transmembrane segment?

Answer 29

The most common feature causing a transmembrane segment to form is a stretch (about 20) of amino acids that mostly have hydrophobic (nonpolar) side chains.

Question 30

What is the covalent attachment of a carbohydrate to a protein or lipid, producing a glycoprotein or glycolipid, respectively?

Answer 30

Glycosylation (glycolipids and glycoproteins)

Question 31

What is thought of as the reason why carbohydrates are attached to lipids and proteins on the extracellular side of a plasma membrane?

Answer 31

Proper migration of individual cells and cell layers relies on the recognition of cell types via the carbohydrates on their cell surfaces.

They also have a protection effect.

Question 32

What two types of protein glycosylation occur within eukaryotes?

Answer 32

N-linked and O-linked

Question 33

What is N-linked glycosylation?

Answer 33

The carbohydrate is attached to a nitrogen atom of the asparagine side chain.

This involves 14 sugar molecules, called a carbohydrate tree, is first built onto a lipid found in the ER membrane.

An enzyme in the ER, oligosaccharide transferase, transfers the carbohydrate tree from the lipid to an asparagine in the polypeptide. This commonly occurs on the membrane proteins that are transported to the cell surface.

Question 34

What is O-linked glycosylation?

Answer 34

This only occurs in the Golgi apparatus.

This involves a string of sugars to the oxygen atom of a serine or threonine side chain of a polypeptide.

This is important in animals for the production of proteglycans, which are highly glycosylated proteins that are secreted from cells and help organize the extracellular matrix that surrounds the cells.

Proteoglycans are also a component of mucus, a slimy material that coats many cell surfaces and is secreted into fluids such as saliva.

High concentrations of carbohydrates give mucus its slimy texture.

Question 35

What is the movement of ions or molecules across a biological membrane?

Answer 35

Membrane transport

Question 36

What is the property of membranes that allows the passage of certain ions or molecules but not others?

Answer 36

Selective permeability

Question 37

What are the three ways substances can move directly across a membrane?

Answer 37

Simple diffusion, facilitated diffusion, and active transport.

Question 38

What is when a substance moves across a membrane from a region of high concentration to a region of lower concentration by passing directly through the phospholipid bilayer?

Answer 38

Simple diffusion

This is an example of passive transport. The diffusion of a solute across a membrane in a process that is energetically favorable and does not require an input of energy.

Question 39

What is a mechanism of passive transport in which a transport protein provides a passageway for a substance to cross a membrane from an area of higher concentration to one of lower concentration?

Answer 39

Facilitated diffusion.

Another example of passive transport.

Question 40

What is the transport of a substance across a membrane from an area of low concentration to one of higher concentration with the aid of a transport protein; requires an input of energy?

Answer 40

Active Transport (against the gradient)

Question 41

What do you call ions and polor molecules in relation to water?

Answer 41

Solutes, because they dissolve in water which then makes a solute; which is what water is when solvents are dissolved by it. A solution is the final product.

Question 42

What are the four factors that affect the ability of solutes to pass through a phospholipid bilayer?

Answer 42

Size, polarity, charge, and concentration.

Question 43

What does transmembrane gradient or concentration gradient refer to (definition)?

Answer 43

A situation in which the concentration of a solute is higher on one side of a membrane than on the other.

A universal feature of all living cells. For example, right after you eat a meal containing carbohydrates, a higher concentration of glucose is found outside your cells than inside; this is an example of a chemical gradient.

Question 44

What are the two components involving ions?

Answer 44

Electrical and chemical

Question 45

What is the combined effect of both an electrical and a chemical gradient across a membrane; this determines the direction in which ions will move?

Answer 45

Electrochemical gradient

For example, a net positive charge outside the cell and larger amount of Na+ outside the cell compared to within a cell creates a Na+ electrochemical gradient.

Question 46

What type of transport typically dissolves a pre-existing gradient?

Answer 46

Passive transport

Question 47

What is the condition in which the solute concentrations on both sides of a plasma membrane are equal, which does not cause a cell to shrink or swell?

Answer 47

Isotonic

Question 48

What is the difference between hypertonic and hypotonic in relation to outside the cell?

Answer 48

Outside the cell becomes hypertonic when the gradient is greater than the cytosolic side of a cell.

Hypotonic condition exists outside the cell when its gradient is lower than the inside of the cell

Question 49

If solutes cannot readily move across the membrane, what happens in order to balance solute concentrations?

Answer 49

Water will move across the membrane from the hypotonic compartment (with a lower solute concentration) into the hypertonic compartment (with a higher solute concentration)

This process is called OSMOSIS.

Question 50

What senses changes in cell volume and allows for necessary movements across the membrane to prevent osmotic changes and maintain normal cell shape?

Answer 50

Transport proteins

Question 51

What phenomenon occurs when cells, in extreme cases, takes up too much water and ruptures? (when placed in a hypotonic solution)

Answer 51

Osmotic lysis

Question 52

What is the process called when a cell is placed into a hypertonic solution where the cell will exert water and shrink in an effort to stabilize solute concentrations?

Answer 52

Crenation

Question 53

Let’s suppose the inside of a cell has a solute concentration of 0.3 M and the outside has a concentration of 0.2 M. If the membrane is impermeable to solutes, in which direction will water move?

Answer 53

Water will move from outside to inside, from the lower to the higher concentration.

Question 54

What prevents organisms like bacteria, fungi, algae, and plant cells from osmotic lysis?

Answer 54

The rigidity of the cell wall prevents the cell from taking up more water than it can.

Question 55

What is the shrinkage of algal or plant cytoplasm that occurs when water leaves the cell by osmosis, with the result that the plasma membrane no longer presses on the cell wall?

Answer 55

Plasmolysis

Question 56

What do contractile vacuoles do in microorganisms, like amoebae and paramecia, that live in extracellular environments that are extremely hypotonic?

Answer 56

Water has a great tendency to move into these cells via osmosis, and when the water does, one or more contractile vacuoles prevent osmotic lysis by taking up water and then periodically discharging it by fusing with the plasma membrane.

Question 57

What is a transmembrane protein that provides a passageway for the movement of ions and hydrophilic molecules across the phospholipid bilayer?

Answer 57

Transport proteins

These play a central role in the selective permeability of biological membranes.

Question 58

What are the two categories of transport proteins that move solutes across the membrane?

Answer 58

Channels and transporters.

Question 59

What is a transmembrane protein that forms an open passageway for the facilitated diffusion of ions or molecules across a membrane?

Answer 59

A channel

When this channel is open, the movement of solutes can be extremely quick, up to 100 million ions or molecules per second.

Question 60

What is the property of many channels that allows them to open and close to control the movement of solutes across a membrane?

Answer 60

Gated

Question 61

How are these gated channels controlled?

Answer 61

The noncovalent bonding of small molecules such as ligands.

Question 62

What are ligands?

Answer 62

Ligands are molecules such as hormones and neurotransmitters.

They are often important in the cell transmission of signals between neurons and muscle cells or between two neurons.

Question 63

How did Peter Ager first prepare his experiment to test if protein, CHIP28, was responsible for the channel within red blood and liver cells? Try to remember the other steps as well. (3) Steps Total

Answer 63

1. They isolated the gene or nucleotides of the CHIP28 in a test tube and added an enzyme, RNA polymerase, to synthesize many copies of CHIP28 mRNA.
2. CHIP28 mRNA was inserted into frog eggs where its ribosomes create CHIP28 proteins. The CHIP28 proteins then are inserted into the plasma membrane to become a transmembrane protein.
3. As a control measure, frog eggs without the CHIP28 protein are measured under microscopy as well as the experimental group while in a hypotonic medium.

Result: The experimental group ruptured via osmotic lysis

Question 64

BONUS: What does CHIP28 stand for?

Answer 64

Channel-forming integral membrane protein with a molecular mass of 28,000 daltons. It was renamed aquaporin to indicate its newly identified function of allowing water to diffuse through a channel in the membrane.

Question 65

What observations about particular cell types in the human body led to the experimental strategy of Peter Ager’s experiment?

Answer 65

Most cells allow movement of water across the cell membrane by passive diffusion. However, it was noted that certain cell types had a much higher rate of water movement, indicating that something different was occurring in these cells. (Red blood cells; kidney and bladder cells that regulate water balance in animals)

Question 66

What were the characteristics of CHIP28 that made Agre and associates speculate that it may transport water? In your own words, briefly explain how they tested the hypothesis that CHIP28 has this function.

Answer 66

The researchers identified water channels by characterizing proteins that are present in red blood cells and kidney cells but not other types of cells. Red blood cells and kidney cells have a faster rate of water movement across the membrane than other cell types. These cells are more likely to have water channels. By identifying proteins that are found in both of these types of cells but not in other cells, the researchers were identifying possible candidate proteins that function as water channels. In addition, CHIP28 had a structure that resembled other known channel proteins. Agre and his associates experimentally created multiple copies of the gene that produces the CHIP28 protein and then artificially transcribed the genes to produce many mRNAs. The mRNAs were injected into frog oocytes where they could be translated to make the CHIP28 proteins. After altering the frog oocytes by introducing the CHIP28 mRNAs, the researchers compared the rate of water transport in the altered oocytes versus normal frog oocytes. This procedure allowed them to introduce the candidate protein into a cell type that normally does not have the protein present.

Question 67

Explain how the results of the experiment support the proposed hypothesis.

Answer 67

After artificially introducing the candidate protein into the frog oocytes, the researchers found that the experimental oocytes took up water at a much faster rate in a hypotonic solution as compared to the control oocytes. The results indicated that the presence of the CHIP28 protein did increase water transport into cells.

Question 68

What is a transmembrane protein that binds a solute and undergoes a conformational change to allow the movement of the solute across a membrane; also called a carrier?

Answer 68

Transporters

They are bound in a hydrophilic pocket where the change switches exposure of the pocket from one side of the membrane to the other side.

Question 69

Since providing the principle pathway for cellular uptake, what does transporters help the cells accomplish?

Answer 69

Uptake of organic molecules such as sugars, amino acids, and nucleotides to include some hormones and neurotransmitters.

In addition to the above, they also help in exporting waste. Ex. transporter remove lactic acid, a by-product of muscle cells during exercise.

Others regulate pH level and controlling cell volume.

Question 70

T/F Transporters are much faster than channels.

Answer 70

False. They’re rate of transport is typically 100 to 1,99

Question 71

How are transporters named?

Answer 71

The number of solutes they bind and the direction in which they transport those solutes.

Question 72

Three Types of Transporters

What is a type of transporter that binds a single ion or molecule and transports it across the membrane?

What is a type of transporter that binds two or more ions or molecules and transports them in the same direction across a membrane; also called a cotransporter?

What is a type of transporter that binds two or more ions or molecules and transports them in opposite directions across a membrane?

Answer 72

Uniporter

Symporter

Antiporter

Question 73

What is the difference between a channel and a transporter?

Answer 73

When its gate is open, a channel provides a direct passageway for the movement of a solute across a membrane.

A transporter does not provide a direct passageway for such movement. Instead, a solute must first enter a hydrophilic pocket on one side of the membrane.

The transporter then undergoes a conformational change that exposes the pocket on the other side of the membrane, where the solute is released.

Question 74

What is the movement from a region of lower concentration to one of higher concentration? (in other words)

Answer 74

The movement of a solute across a membrane against its concentration gradient, which is what active transport accomplishes.

This process is energetically unfavorable and requires an input of energy.

Question 75

What is a type of transport that involves pumps that directly use energy to transport a solute against a gradient?

Answer 75

Primary active transport

Question 76

What is a transporter that directly couples its conformational changes to an energy source, such as ATP hydrolysis?

Answer 76

A pump (aka primary active transport)

Question 77

What is a type of membrane transport that involves the utilization of a pre-existing gradient to drive the active transport of another solute?

Answer 77

Secondary active transport

Ex. In animals, Na+/solute symporters are prevalent in animal cells. The Na+ electrochemical gradient functions as normal, while the another solute “hitches” a ride with the Na+. In this case, the solute is the only actively transported substance.

H+/sucrose symporters are more common in bacteria, fungi, algae, and plant cells.

Question 78

What was discovered about the phenomenon of active transport in the 1940s, where it was a study conducted of the transport of sodium ions (Na+) and potassium ions (K+)?

Answer 78

In animal cells, the concentration of Na+ is lower inside the cell than outside of the cell.

Whereas K+ is higher inside the cell than outside.

After analyzing the movement of these ions across the plasma membrane of muscle cells, neurons, and red blood cells, researchers determined that the export of Na+ is coupled to the import of K+.

Question 79

What was the pump pertaining to Na+/K+ named?

Answer 79

Na+/Ka+ - ATPase.

Question 80

Every time ATP is hydrolyzed for a Na+/K+ - ATPase pump, how many ions of Na+ move into the extracellular environment and how man K+ ions move into the cytosol?

Answer 80

3 Na+ ions and 2 K+ ions

Question 81

What attachment and detachment of a molecules switches a Na+/K+ - ATPase antiporter from the E2 (transport of Na+) protein conformation to the E1 (transport of K+) protein conformation?

Answer 81

A phosphate molecule created from the hydrolysis of ATP

ATP —-> ADP + Phosphate

Question 82

What is a pump that generates an electrical gradient across a membrane?

Answer 82

Electrogenic pump

Ex. [Na+/K+ - ATPase antiporter pump, since one cycle of pumping results in the net export of one positive charge, the Na+/K+ - ATPase also produces an electrical gradient across the membrane.

Against the gradient = low to high concentration

Question 83

What happens to the protein (for E1 and E2 conformations) when phosphate attaches to the transmembrane pump?

Answer 83

It changes the structure of the protein which also changes its affinity towards certain ions like Na+ and K+

Question 84

What do you call the mechanism when phosphate is bound to a pump covalently?

Answer 84

Phosphorylation

Question 85

[Begin Exocytosis and Endocytosis] What are the two other mechanisms in which eukaryotes transport larger molecules, like proteins and polysaccharides across a plasma membrane?

Answer 85

Exocytosis and endocytosis

Question 86

What are two examples of exocytosis?

Answer 86

Secretion of hormones (polypeptides) such as insulin from beta cells of the pancreas

Digestive enzymes from the exocrine cells in the lumen of the small intestine are secreted for digestion.

Question 87

What are three examples of endocytosis?

Answer 87

Insoluble important nutrients are bound to proteins in the blood like lipids and iron and taken into cells.

Macrophages, cells of the immune system, engulf and destroy bacteria via phagocytosis

Question 88

What is a process in which material inside a cell is packaged into vesicles and excreted into the extracellular environment? (typically by the Golgi apparatus)

Answer 88

Exocytosis

Question 89

What is the purpose of the protein coat in exocytosis?

Answer 89

The coat allows the budding process at the surface of the Golgi membrane to form a vesicle.

Question 90

What is a process in which the plasma membrane invaginates, or folds inward, to form a vesicle that brings substances into the cell?

Answer 90

Endocytosis

Question 91

What are the three types of endocytosis?

Answer 91

Receptor-mediated endocytosis

Pinocytosis

Phagocytosis

Question 92

What is a common type of endocytosis in which a receptor in the membrane is specific for a given cargo?

Answer 92

Receptor-mediated endocytosis

Question 93

What is a type of endocytosis that involves the formation of membrane vesicles from the plasma membrane as a way for cells to internalize the extracellular fluid?

Answer 93

Pinocytosis (from the Greek word meaning, cell drinking) (if the material is a liquid)

This allows a cell to sample the extracellular solutes. Particularly important with cells actively involved in nutrient absorption.

Question 94

What is a type of endocytosis that involves the formation of a membrane vesicle, called a phagosome, or phagocytic vacuole, which engulfs a particle such as a bacterium?

Answer 94

Phagocytosis. (particulate, like a bacterial cell or organic fragment)

Macrophages kill bacteria this way. Once inside the Macrophage, the phagosome (cargo) fuses with a lysosome, and the digestive enzymes within the lysosome destroy the bacterium.