Lectures 4-6

Question 1

What are the interactions between phospholipids and proteins in membranes? Why?

Answer 1

We can deduce that these forces are van der Waals.

This is because the inside of the phospholipid bilayer is hydrophobic (non-polar) due to the abundance of C-H bonds.

We can conclude from this that if proteins are to stay in membranes, their outer layers must be hydrophobic too, and forces between two hydrophobic objects are van der Waals/hydrophobic exclusion.

Question 2

Why are membranes termed fluid mosaic models?

Answer 2

Because they are fluid, meaning that they are dynamic. Apart from proteins anchored to the cytoskeleton, all things present in the membrane mix and move.

Question 3

What are the four types of phospholipid motions from least common to most common?

Answer 3

Transversal diffusion, lateral diffusion, rotational movement, and swinging movement.

Question 4

What makes the kinks in fatty acids?

Answer 4

Cis double bonds.

Question 5

What determines the fluidity of the membrane?

Answer 5

The saturation of the fatty acids that make the membrane.

Question 6

In animals, what fills up the gaps left by kinks in phospholipids?
How does this affect membrane fluidity?

Answer 6

Cholesterol. This reduces the fluidity of the membrane, making it more rigid.

Question 7

How do living organisms without cholesterol control membrane fluidity?

Answer 7

Usually bacteria, they possess fatty acid desaturases (enzymes) that introduce double bonds in fatty acids.

This makes the membrane more fluid, which introduces more movement in the membrane, effectively reducing the freezing point.

Question 8

What are the four components of biological membranes?

Answer 8

• The phospholipid bilayer,
• The interior protein network,
• Cell-surface markers,
• Transmembrane proteins.

Question 9

What is the function of the phospholipid bilayer?

Answer 9

It selects what goes in and out of the cell. Its hydrophobic inner membrane filters out polar molecules such as water, ions, etc. from entering.

It does allow oxygen and carbon to pass through it though.

Question 10

What is the interior protein network and what is its function?

Answer 10

It is a network of proteins inside the membrane, also called the peripheral membrane proteins. It is used to reinforce the membrane’s shape.

Question 11

What are cell-surface markers? How are they made and what is their functions?

Answer 11

Cell-surface markers are made by adding polysaccharides to lipids and proteins in the ER. It makes glycoproteins and glycolipids. These are transported to the membrane by the Golgi.

Cell-surface markers act in cell to cell adhesion. It helps determine cell type and foreign organisms (that don’t have the same markers).

Question 12

What are transmembrane proteins and what is their functions?

Answer 12

These are proteins that are embedded directly into the membrane. They can act as carriers or channels involved in protein transport. They can also be receptors involved with cell signaling.

Question 13

How are transmembrane proteins anchored inside the membranes?

Answer 13

There are two possible mechanisms.
Either they are anchored to a lipid that is inserted into the membrane, or they span the membrane (have parts of the protein go through the actual membrane).

Question 14

What are lipid anchors and how are they produced/attached?

Answer 14

Lipid anchors are produced in the rough endoplasmic reticulum. They are a hydrophobic region of the lipid that inserts into the membrane and is covalently bonded to the protein.

They anchor the protein to the membrane.

Question 15

What are integral membrane proteins and how do they stay in the membrane?

Answer 15

Integral membrane proteins are usually made up of alpha-helices. They have both hydrophobic and hydrophilic regions. This allows them to stay in the membrane, as the hydrophobic region binds to the membrane using van der Waals, and the hydrophilic region does not want to go inside the membrane.

Question 16

What are protein channels?

Answer 16

Usually, they are proteins containing beta-barrels that make a pore/hole in the membrane to allow some molecules to enter.

Question 17

Much of the transport of molecules through the membrane is controlled by proteins. What are the 6 functions of transmembrane proteins?

Answer 17

• Transporters
• Enzymes
• Cell surface receptors
• Cell-surface identity marker
• Cell to cell adhesion
• Attachment to cytoskeleton

Question 18

What are the two types of transporter proteins and what do they do?

Answer 18

Transporter proteins will only let specific molecules into or out of the cell. The two types are channels (passive transport) or carriers (active transport).

Question 19

What do enzymes do?

Answer 19

They carry out specific reactions while being embedded inside the membrane.

Question 20

What do cell-surface receptors do?

Answer 20

They bind to molecules on the outside of the cell and transmit the signal to the inside of the cell. For example, a hormone can bind to a signal receptor and the signal receptor will transfer the message to produce X molecule.

Question 21

What do cell-surface identity markers do?

Answer 21

They help identify the cell within the body. There exist Natural Killer MHC cells that detect cells with the wrong identity markers and destroy them.

Question 22

What do cell to cell adhesion molecules do?

Answer 22

They are used to attach to other cells, either for short or longer periods of time.

Question 23

What is the difference between active and passive transport?

Answer 23

Active transport requires energy while passive transport does not.

This is because in passive transport, the molecules travel with the concentration gradient (from low concentration to high concentration) until equilibrium. This happens spontaneously.

In active transport, the molecules travel against the concentration gradient (from low to high) this requires energy.

Question 24

What is the difference between simple and facilitated diffusion?

Answer 24

Simple diffusion is when the molecules travel through the phospholipid part of the membrane, this occurs with lipid-soluble (non-H2O soluble) molecules, and without the use of a channel or carrier.

Facilitated diffusion is when molecules go through the membrane with the help of a channel or a carrier.

Question 25

What are examples of molecules that can and cannot pass through the membrane using simple diffusion?

Answer 25

Can: gaseous oxygen and carbon dioxide (when breathing).

Cannot: Charged molecules or polar molecules, such as Cl- and glucose.

Question 26

What do hypertonic and isotonic mean?

Answer 26

Hypertonic: area of high concentration.

Isotonic: when equilibrium is reached on both sides of the membrane.

Question 27

What does hypotonic mean?

Answer 27

Hypotonic: area of low concentration.

Question 28

What do channel proteins usually carry across membranes?

What are the two types of channel proteins?

Answer 28

Channel proteins, also called ion channels, usually carry ions (charged molecules) across a membrane.

There are gated and non-gated channels.

Question 29

What are gated channels? What determines if they are opened or closed?

Answer 29

Gated channels are protein channels that can open and close.

They can open and close in response to stimuli (ex. neurotransmitters).

Question 30

What controls movement through gated channels? (3 things)

Answer 30

• Membrane potential
• Concentration gradient
• Whether or not the channel is open

Question 31

What is particular about the interiors of channels?

Answer 31

They are hydrated to facilitate the passage of hydrophilic molecules (charged).

Question 32

What is membrane potential? Is the cell positively or negatively charged?

Answer 32

The difference in electrical potential on either side of a membrane.

Usually cells are negatively charged (70mV).

Question 33

Why is membrane potential important?

Answer 33

For the transmission of nervous signals, muscle contractions, and the opening and closing of gated channels.

Question 34

What do carrier proteins carry across membranes?

What is required for them to carry molecules across?

Answer 34

Charged ions, amino acids, sugars and other nutrients.

A conformational (shape) change is required.

Question 35

What is osmosis?

Answer 35

The passage of water across a membrane.

Question 36

How does water pass across a membrane?

Answer 36

It uses a channel protein called aquaporin (facilitated diffusion).

Question 37

Why does water go in and out of the cell so much?

Answer 37

To regulate the concentration of solutes. As water is the solvent for many molecules in the cell, it goes in and out to manage the concentration of these molecules.

When water goes out of the cell, it increases the concentration of solute (same solute but in less solvent).

When water goes in the cell, it dilutes the solute, reducing its concentration.

Question 38

What is hydrostatic pressure? What happens if there is too much of it?

Answer 38

The pressure of the cytoplasm against the cell membrane. If hydrostatic pressure is too high, the cell bursts.

Question 39

What is osmotic pressure? What happens if it is too high?

Answer 39

Osmotic pressure is the pressure of the outside water on the cell. If it is too high, the cell will shrink.

Question 40

What happens to a normal cell in a hypertonic solution? Why?

Answer 40

The cell will shrink or shrivel. This is because the osmotic (outside) pressure of the solution on the cell will be too great for the hydrostatic (inside) pressure to manage.

Question 41

What happens to a normal cell in a hypotonic solution?

Answer 41

The osmotic (outside) pressure is not great enough to counter the hydrostatic (inside) pressure, the cell bursts.

Question 42

Cells are often found in environments where osmotic and hydrostatic pressures are not in equilibrium, how does the cell regulate osmosis?

Answer 42

Extrusion: single-celled organisms, like Paramecium, will physically pump out water from their cell to maintain osmotic and hydrostatic pressures.

Isosmotic regulation: organisms regulate the concentration of solute on the outside of their cell to match the outside environment so that osmosis does not occur.

Turgor pressure: Cell walls in plants allow them to have hypertonic levels of solutes in the central vacuole, which presses against the cell wall.

Question 43

What form of energy is required for active transport?

Answer 43

Energy is provided by the hydrolysis of ATP to make ADP. This releases energy.

Question 44

What is phosphorylation?

Answer 44

The addition of a phosphate group to a protein. This phosphate group comes from the removal of a phosphate group from ATP to make ADP + Energy.

Question 45

Why does phosphorylation give rise to changes in protein shape?

Answer 45

Adding a phosphate group changes the tertiary organization of the protein (as it is very charged).

Question 46

What is the difference between uniporters, symporters and antiporters?

Answer 46

These types of carrier proteins are different in the number and direction of the molecules they carry.

Uniporters: one molecule, one direction.

Symporters: two molecules, one direction.

Antiporters: two molecules, two directions (in and out).

Question 47

The sodium-potassium pump is an example of what type of carrier protein?

Answer 47

An antiporter carrier protein, but also a coupled transport.

Question 48

What is coupled transport?

Answer 48

Coupled transport is when a molecule uses the energy released from active transport to move against its own concentration gradient. This happens in the sodium potassium pump.

Question 49

What is endocytosis and what are its different sub-categories.

Answer 49

Endocytosis is a form of bulk transport where molecules enter the cell in large quantities.

Phagocytosis: solid matter.

Pinocytosis: liquid matter.

Receptor mediated endocytosis: specific molecules bind to cell surface receptors to be ingested by the cell.

Question 50

What is exocytosis?

Answer 50

The bulk transport of molecules out of the cell, using vesicles.

Examples are the release of hormones or neurotransmitters.