Lipids and Membranes

Question 1

What is the plasma membrane?

Answer 1

The plasma membrane is a complex mixture of lipid, protein and carbohydrate which is the boundary that separates the living cell from its surroundings

Question 2

The plasma membrane exhibits selective permeability, what does this mean?

Answer 2

It allows some substances to cross it more easily than others

Question 3

What are often responsible for controlling passage across cellular membranes

Answer 3

Transport proteins

Question 4

Fats are constructed from two types of smaller molecules, what are they?

Answer 4

Glycerol and Fatty acids

Question 5

What is glycerol?

Answer 5

Glycerol is a three-carbon alcohol with a hydroxyl group attached to each carbon

Question 6

What is a fatty acid?

Answer 6

A fatty acid consists of a carboxyl group attached to a long carbon skeleton

Question 7

Why are fats hydrophobic?

Answer 7

Fats separate from water because water molecules hydrogen-bond to each other and exclude the fats

Question 8

What determines the solubility of fatty acids?

Answer 8

Fatty acids have a polar end (the carboxylic acid group) and a non-polar hydrocarbon chain. The ratio of the polar group to the non-polar group is the factor which determines water solubility.

Question 9

How can fatty acids vary?

Answer 9

Fatty acids vary in length (number of carbons) and in the number and locations of double bonds

Question 10

What are saturated fatty acids?

Answer 10

Saturated fatty acids have the maximum number of hydrogen atoms possible and no double bonds

Question 11

What are unsaturated fatty acids?

Answer 11

Unsaturated fatty acids have one or more double bonds

Question 12

What happens when phospholipids are in water?

Answer 12

When phospholipids are added to water, they self-assemble into double-layered sheetscalled bilayers. At the surface of a cell, phospholipids are also arranged in a bilayer, with the hydrophobic tails pointing toward the interior

Question 13

What is the most abundant lipid in the plasma membrane?

Answer 13

Phospholipids are the most abundant lipid in the plasma membrane

Question 14

What biomolecule increases stability of plasma membrane?

Answer 14

Cholesterol

Question 15

Though cholesterol is present in plants, what do they use to stabilise the membrane?

Answer 15

They use related steroid lipids to buffer membrane fluidity

Question 16

As temperatures cool, how do membranes change?

Answer 16

As temperatures cool, membranes switch from a fluid state to a solid state

Question 17

The temperature at which a membrane solidifies depends on the types of lipids, how so?

Answer 17

Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids

Question 18

What factors influence bilayer fluidity?

Answer 18

1. The length of the fatty acid tail
2. Temperature
3. Cholesterol content of the bilayer
4. The degree of saturation of fatty acids tails

Question 19

How does the length of the fatty acid tail contribute to the membrane’s fluidity?

Answer 19

The length of the fatty acid tail impacts the fluidity of the membrane. This is because the intermolecular interactions between the phospholipid tails add rigidity to the membrane. As a result, the longer the phospholipid tails, the more interactions between the tails are possible and the less fluid the membrane will be.

Question 20

How does the temperature contribute to the membrane’s fluidity?

Answer 20

As temperature increases, so does phospholipid bilayer fluidity. At lower temperatures, phospholipids in the bilayer do not have as much kinetic energy and they cluster together more closely, increasing intermolecular interactions and decreasing membrane fluidity. At high temperatures the opposite process occurs, phospholipids have enough kinetic energy to overcome the intermolecular forces holding the membrane together, which increases membrane fluidity.

Question 21

How does the cholesterol contribute to the membrane’s fluidity?

Answer 21

Cholesterol has a somewhat more complicated relationship with membrane fluidity. You can think of it is a buffer that helps keep membrane fluidity from getting too high or too low at high and low temperatures.
At low temperatures, phospholipids tend to cluster together, but steroids in the phospholipid bilayer fill in between the phospholipids, disrupting their intermolecular interactions and increasing fluidity.
At high temperatures, the phospholipids are further apart. In this case, cholesterol in the membrane has the opposite effect and pulls phospholipids together, increasing intermolecular forces and decreasing fluidity.

Question 22

How does the degree of saturation of fatty acids tails contribute to membrane’s fluidity?

Answer 22

Phospholipid tails can be saturated or unsaturated. The terms saturated and unsaturated refer to whether or not double bonds are present between the carbons in the fatty acid tails. Saturated tails have no double bonds and as a result have straight, unkinked tails. Unsaturated tails have double bonds and, as a result, have crooked, kinked tails

Question 23

Variations in lipid composition of cell membranes of many species appear to be what?

Answer 23

Adaptations to specific environmental conditions. Ability to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary

Question 24

What are Peripheral proteins?

Answer 24

Peripheral proteins are bound to the surface of the membrane. The ability to attach to the membrane but not be locked to it allows peripheral proteins to work on the surface of the cell membrane

Question 25

Give examples of peripheral proteins

Answer 25

Some act as receptors, some as enzymes, catalyzing the reaction and since many are cytoskeletons, they give a cell its shape, offers support, and facilitates movement through three main components: micro filaments, intermediate filaments, and micro tubule

Question 26

What are Integral proteins?

Answer 26

Integral proteins penetrate the hydrophobic core

Question 27

What is the function of Integral proteins?

Answer 27

Integral membrane proteins function as transporters, channels (see Potassium Channel), linkers, receptors, proteins involved in accumulation energy, and proteins responsible for cell adhesion. Examples, insulin receptors, Integrins, Cadherins.

Question 28

What are transmembrane proteins?

Answer 28

Integral proteins that span the membrane

Question 29

What is the function of transmembrane proteins?

Answer 29

Many transmembrane proteins function as gateways to permit the transport of specific substances across the membrane

Question 30

The hydrophobic regions of an integral protein consist of what?

Answer 30

One or more stretches of nonpolar amino acids, often coiled into α helices

Question 31

What is the fluid mosaic model?

Answer 31

In the fluid mosaic model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids

Question 32

How are proteins held in the membrane?

Answer 32

Proteins are not randomly distributed in the membrane | Membranes are held together mainly by weak hydrophobic interactions

Question 33

How do lipids and proteins move within the membrane

Answer 33

Most of the lipids and some proteins can move sideways (laterally) within the membrane

Question 34

What determines most of the membrane’s functions?

Answer 34

Proteins

Question 35

What functions can the membrane carry out?

Answer 35

i. Transport ii. Enzymatic activity iii. Signal transduction iv. Cell-cell recognition v. Intercellular joining vi. Attachment to the cytoskeleton and extracellular matrix (ECM)

Question 36

What determines the structure of the membrane?

Answer 36

The lipid bilayer

Question 37

How are cell-surface proteins important in the medical field?

Answer 37

HIV must bind to the immune cell-surface protein CD4 and a “co-receptor” CCR5 in order to infect a cell HIV cannot enter the cells of resistant individuals who lack CCR5 Drugs are now being developed to mask the CCR5 protein

Question 38

What are the three major components of cell membranes?

Answer 38

Lipids, proteins, and carbohydrates.

Question 39

Membranes have distinct inside and outside faces, how?

Answer 39

When vesicles fuse with the plasma membrane, and becomes continuous with the cytoplasmic layer of the plasma membrane (opposite). Molecules that begin on the inside face of the ER end on the outside face of the plasma membrane

Question 40

What controlls the cell exchange with it's surroundings

Answer 40

Controlled by the plasma membrane (selectively permeable, regulating the cell’s molecular traffic)

Question 41

How does the plasma membrane regulate what comes in and out

Answer 41

Hydrophobic (nonpolar) molecules, such as hydrocarbons or oxygen and carbon dioxide, can dissolve in the lipid bilayer and pass through the membrane rapidly Hydrophilic molecules including ions and polar molecules do not cross the membrane easily

Question 42

How do proteins built into the membrane play key roles in regulating transport?

Answer 42

Transport proteins allow passage of hydrophilic substances across the membrane Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel

Question 43

What are channel proteins and what are their function?

Answer 43

Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel

Question 44

What is the function of channel proteins called aquaporins?

Answer 44

Greatly facilitate the passage of water molecules

Question 45

What are carrier proteins?

Answer 45

Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle

Question 46

What is dynamic equilibrium involving molecules passing through the membrane

Answer 46

At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other

Question 47

What kind of transport is the diffusion of a substance across a biological membrane?

Answer 47

Passive transport because no energy is expended by the cell

Question 48

Water diffuses (osmosis) until when?

Answer 48

Until the solute concentration is equal on both sides

Question 49

What is tonicity?

Answer 49

Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water

Question 50

The tonicity of a solution depends on what?

Answer 50

Its concentration of solutes that cannot cross the membrane relative to that inside the cell

Question 51

What is an isotonic solution?

Answer 51

Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane

Question 52

What is a Hypertonic solution?

Answer 52

Solute concentration is greater than inside the cell; cell loses water

Question 53

What is a Hypotonic solution?

Answer 53

Solute concentration is less than inside the cell; cell gains water

Question 54

What happens to cells without cell walls in a hypertonic solution?

Answer 54

Shrivel in hypertonic solution

Question 55

What happens to cells without cell walls in a hypotonic solution?

Answer 55

Lyse (burst) in a hypotonic solution

Question 56

What is a turgid cell?

Answer 56

A plant cell in a hypotonic solution swells until the wall opposes uptake

Question 57

What is a ''flaccid cell'' (limp)?

Answer 57

If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell

Question 58

What is plasmolysis?

Answer 58

In a hypertonic environment, plant cells lose water | The membrane pulls away from the cell wall, causing the plant to wilt, a potentially lethal effect called plasmolysis

Question 59

What is facilitated diffusion?

Answer 59

Where transport proteins (Transport proteins include channel proteins and carrier proteins) speed the passive movement of molecules across the plasma membrane

Question 60

How do channel proteins faciliate transport across membranes?

Answer 60

Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane

Question 61

How do ion proteins faciliate transport across membranes?

Answer 61

Ion channels facilitate the transport of ions

Question 62

Give an example of an ion channel and what it does

Answer 62

Some ion channels, called gated channels, open or close in response to a stimulus. For example, in nerve cells, ion channels open in response to electrical stimulus

Question 63

Is faciliated diffusion passive or active?

Answer 63

Facilitated diffusion is still passive because the solute moves down its concentration gradient i.e. no energy required

Question 64

What is active transport?

Answer 64

Active transport requires energy, usually in the form of ATP hydrolysis, to move substances against their concentration gradients

Question 65

What are the proteins involved in active transport?

Answer 65

All proteins involved in active transport are carrier proteins

Question 66

Give an example of active transport, how its controlled and its function

Answer 66

Active transport allows cells to maintain concentration gradients that differ from their surroundings For example, an animal cell has a much higher potassium (K+) and a much lower sodium (Na+) concentration compared to its surroundings This is controlled by the sodium-potassium pump, a transport protein that is energized by transfer of a phosphate group from the hydrolysis of ATP

Question 67

Describe the sodium-potassium pump: a specific case of active transport

Answer 67

i. Cytoplasmic Na+ binds to the sodium-potassium pump. The affinity for Na+ is high when the protein has this shape. ii. Na+ binding stimulates phosphorylation by ATP. iii. Phosphorylation leads to a change in protein shape, reducing its affinity for Na+, which is released outside. iv. The new shape has a high affinity for K+, which binds on the extracellular side and triggers release of the phosphate group. v. Loss of the phosphate group restores the protein’s original shape, which has a lower affinity for K+. vi. K+ is released; affinity for Na+ is high again, and the cycle repeats.

Question 68

What is membrane potential?

Answer 68

It is the voltage across a membrane

Question 69

How is the voltage created in the membrane potential?

Answer 69

Voltage is created by differences in the distribution of positive and negative ions across a membrane

Question 70

What is the electrochemical gradient?

Answer 70

Two combined forces drive the diffusion of ions across a membrane A chemical force (the ion’s concentration gradient) An electrical force (the effect of the membrane potential on the ion’s movement)

Question 71

What is an electrogenic pump?

Answer 71

An electrogenic pump is a transport protein that generates voltage across a membrane.

Question 72

What is the mahor electrogenic pump of animal cells?

Answer 72

The sodium-potassium pump is the major electrogenic pump of animal cells

Question 73

What is the mahor electrogenic pump of plants, fungi, and bacteria?

Answer 73

A proton pump, which actively transports hydrogen ions (H+) out of the cell

Question 74

What is cotransport?

Answer 74

Cotransport occurs when active transport of a solute indirectly drives transport of other substances The diffusion of an actively transported solute down its concentration gradient is coupled with the transport of a second substance against its own concentration gradient (an energetically favorable transport is coupled to an energetically unfavorable transport to move molecules across membranes) Glucose will not easily move across the cell membrane. But the cell wants glucose and also has a lot of glucose within it already. So even if you have a simple passive transporter for glucose on the membrane, there is not much glucose that will get in.

Question 75

How do small molecules and water enter or leave the cell?

Answer 75

through the lipid bilayer or via transport proteins

Question 76

How do large molecules enter or leave the cell?

Answer 76

Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles (requires energy)

Question 77

What are the three types of endocytosis?

Answer 77

i. Phagocytosis-cellular eating ii. Pinocytosis-cellular drinking iii. Receptor-mediated endocytosis

Question 78

What is Phagocytosis-cellular eating and give an example?

Answer 78

Phagocytosis, or “cell eating”, is the process by which a cell engulfs a particle and digests it. Cells in the immune systems of organisms use phagocytosis to devour bodily intruders such as bacteria, and they also engulf and get rid of cell debris.

Question 79

What is Pinocytosis?

Answer 79

Pinocytosis is a cellular process by which fluids and nutrients are ingested by cells. Also called cell drinking, pinocytosis is a type of endocytosis that involves the inward folding of the cell membrane (plasma membrane) and the formation of membrane-bound, fluid-filled vesicles

Question 80

What is Receptor-mediated endocytosis?

Answer 80

binding of specific solutes to receptors triggers vesicle formation

Question 81

Give an example of receptor mediated endocytosis

Answer 81

Cholesterol intake - Human cells use receptor-mediated endocytosis to take in cholesterol, which is carried in particles called low-density lipoproteins (LDLs) Individuals with the disease familial hypercholesterolemia have missing or defective LDL receptor proteins

Question 82

Which makes up more of the membranes in a cell - internal or plasma membranes?

Answer 82

The internal membranes make up more of the membranes in a cell than the plasma membranes

Question 83

What are the compartmentalization functions of membranes?

Answer 83

Separate outside from inside as well as separating within the cell, allow localization of enzymes, allow filtration to occur (selective permeability)

Question 84

How do membranes function in storing energy?

Answer 84

Across a membrane you can build up a concentration gradient and harness the potential energy created by the gradient

Question 85

fats and oils (are/are not) in membranes

Answer 85

are not

Question 86

Membrane proteins that do not directly cross the cell membrane but rather are on the surface or associated with another protein are what?

Answer 86

Peripheral

Question 87

How do peripheral proteins attach?

Answer 87

Electrostatic interactions and hydrogen bonding interactions

Question 88

Is removing peripheral proteins easy or hard and why?

Answer 88

Fairly easy since high salt solutions or extreme pH's will disrupt their electrostatic interactions

Question 89

Is removing integral proteins easy or hard and why?

Answer 89

Hard because you cannot remove an integral membrane protein without destroying membrane integrity with strong detergents of organic solvents, often denaturing the protein as well

Question 90

What is the statement that all components of a membrane are not in a set position but more relative is what?

Answer 90

The fluid mosaic model

Question 91

The temperature that defines when the bilayer will lose fluidity is known as?

Answer 91

The transition temperature

Question 92

Why does the type and amount of phospholipids affect fluidity of the membrane?

Answer 92

Each different phospholipid has a different transition state (sphingomyelin generally makes the membrane less fluid)

Question 93

Why do fish make oils?

Answer 93

To keep their membranes fluid instead of freezing up

Question 94

Cholesterol is only present in what kind of cells?

Answer 94

Animal cells

Question 95

Besides functioning to affect membrane fluidity, cholesterol also

Answer 95

May decrease the permeability of the membrane

Question 96

Which molecule will diffuse most quickly across a lipid bilayer membrane? a) H2O b) O2 c) H2PO4– d) glucose e) Na+

Answer 96

B)Polar molecules and large ions dissolved in water cannot diffuse freely across the plasma membrane due to the hydrophobic nature of the fatty acid tails of the phospholipids that make up the lipid bilayer. Only small, non-polar molecules, such as oxygen and carbon dioxide, can diffuse easily across the membrane.

Question 97

Singer and Nicolson's fluid mosaic model of the membrane proposed that A) membranes are a phospholipid bilayer. B) membranes are a phospholipid bilayer between two layers of hydrophilic proteins. C) membranes are a single layer of phospholipids and proteins. D) membranes consist of protein molecules embedded in a fluid bilayer of phospholipids. E) membranes consist of a mosaic of polysaccharides and proteins.

Answer 97

D)

Question 98

An animal cell membrane will be more fluid at room temperature if it contains A) more cholesterol. B) longer chain fatty acids. C) more cis-unsaturated and polyunsaturated fatty acids. D) more trans-unsaturated fatty acids. E) any of the above

Answer 98

C)

Question 99

``` Which of these processes can move a solute against its concentration gradient? A) osmosis B) passive transport C) facilitated diffusion D) active transport ```

Answer 99

D)

Question 100

``` Which of the following proteins is not necessarily associated with the plasma membrane? A) recognition protein B) plasma protein C) receptor protein D) channel protein ```

Answer 100

B) (synthesised in liver)

Question 101

Which of the following affects the rate of diffusion through a selectively permeable membrane? I. concentration gradient II. temperature III. molecular size A).I only B) II only C) I and II D) I, II, and III

Answer 101

D) I, II, III

Question 102

The movement of potassium into an animal cell requires A) low cellular concentrations of sodium. B) high cellular concentrations of potassium. C) an energy source such as ATP. D) a cotransport protein. E) a potassium channel protein.

Answer 102

C) an energy source such as ATP.

Question 103

Which best describes a biological membrane? a) two layers of phospholipids with proteins embedded between the two layers b) a mixture of covalently linked phospholipids and proteins that determines which solutes can cross the membrane and which cannot c) two layers of phospholipids with proteins either spanning the layers or on the surface of the layers d) a fluid structure in which phospholipids and proteins move freely between sides of the membrane e) two layers of phospholipids (with opposite orientations of the phospholipids in each layer) with each layer covered on the outside with proteins

Answer 103

c) two layers of phospholipids with proteins either spanning the layers or on the surface of the layers

Question 104

Which amino acid would most likely be present in the outer side of a transmembrane domain of an integral membrane protein? a) a charged amino acid like lysine b) a polar amino acid like serine c) a special amino acid like proline d) a hydrophobic amino acid like valine e) any of the above, with no preference

Answer 104

d) Transmembrane domains need hydrophobic amino acids to interact with the hydrophobic interior of a membrane

Question 105

Based on our model of the membrane, which statement would lose points on an exam? a) Glycoproteins tend to have oligosaccharides on their outward-facing side. b) Transmembrane proteins often bind with cytoplasmic proteins, but not with extracellular molecules. c) The combinations of phospholipids in the two faces of the membrane often differ. d) Phospholipids tend to move faster laterally along the membrane than do the proteins. e) Some transmembrane proteins function as active transport systems.

Answer 105

b) Transmembrane proteins often bind with cytoplasmic proteins, but not with extracellular molecules.

Question 106

A correct distinction between facilitated diffusion and active transport is that a) active transport requires conformational changes in the transport protein associated with the transport process, and facilitated diffusion does not. b) active transport requires an integral membrane protein to carry out the transport, and facilitated diffusion does not. c) facilitated diffusion requires a protein-lined pore in the membrane, and active transport does not. d) facilitated diffusion depends on an existing energy gradient acting on the transported substance, while active transport makes such a gradient. e) facilitated diffusion requires cellular energy (often from ATP hydrolysis), but active transport does not.

Answer 106

d) facilitated diffusion depends on an existing energy gradient acting on the transported substance, while active transport makes such a gradient.

Question 107

Which part of the cell is most crucial for getting membrane proteins to their proper location? a) plasma membrane b) mitochondrion c) chloroplast d) Golgi apparatus e) lysosome

Answer 107

d) Golgi apparatus