Week 6 Recall Questions

Question 1

What are the key parts of a phospholipid and why is it amphipathic?

What are the hydrophobic and hydrophilic parts?

Answer 1

B/c both philic and phobic.
- bilayer effect caused by hydrophobic effect and clumping AA together and slightly attracted to 1 another.

Heads are philic = outside
- stable

Tails are phobic = inside
- stable

Van der waals interactions between lipids inside dictate physical state (solid or fluid)

Question 2

What are the key elements of the plasma membrane?

Answer 2

• Selectively permeable, maintains structural integrity of cell.
- barrier between inside cell, cytoplasm and outside environment.

• regulates movement of ions and molecules (into and out of cell. permeability barrier, regulation of transport)

• cell-cell recognition, communication between cells and connecting cells to form tissues and organs (detection of signals, intercellular communication)

Quick cheat sheet:
• detection of signals
• intercellular communication
• Selectively permeable
- regulation of transport
- acts as barrier
- maintains shape

Question 3

How or why is a plasma membrane selectively permeable?

Answer 3

• the phospholipid bilayer prevents the diffusion of most molecules across the membrane, due to the hydrophobic nature of the tails.

• inside of cytoplasm can have a different concentration of molecules than outside.
= some molecules can cross, others can’t, and some need a “window or door to get through”

• regulates movement of ions and molecules (into and out of cell. permeability barrier, regulation of transport)
- pH
-temperature

Question 4

What was a key finding of the experiment by Frye and Edidin?

What were the variables for the experiment?

Answer 4

• wondered if membrane was static and solid or dynamic and fluid.

• if static (not moving): then the different dyed would stay in respective parts

• if dynamic (fluid): then dyed proteins would mix.

• provided evidence of idea vicious or Liquid membrane (fluid).
• phospholipids, and proteins move laterally (side to side)
• rarely flip-flop

• 1970
• fused mouse and human cells
• dyed the proteins and membranes and forced them to fuse
• 40 minutes of incubation time for the dyed proteins to mix.
• 70°C
• IV: (x-axis- time measured in minutes)
• DV: (y-axis- amount of mixing. Double stained cells)
• control variables: temperature, experimental control: unmixed cells in same conditions to show cells done spontaneous change colours)

• calculate a rate of mixing, based on this rate, bilayer is about as liquid as olive oil for light machine oil.

• another experiment they looked at different temp affects
- IV: temp
- DV: time

Question 5

What are the key elements of the fluid mosaic model of membrane?

Answer 5

• the plasma membrane, is a mosaic of components— primarily, phospholipid, cholesterol, and proteins— that move freely and fluently in the plane off the membrane.

• mechanism for the fluidity is that the phospholipids move side to side. (10^7 times per second)

• phospholipids flip-flip rarely (once per month)
- this is an unfavourable condition b/c charged polar head that needs to go through hydrophobic layer.
- ions also don’t cross hydrophobic core

Question 6

What factors are influencing membrane fluidity?

Answer 6

1. Temperature:
   - ⬆️ temperature = ⬆️ Kinetic energy = ⬆️ phospholipids laterally movement even more = ⬆️ gaps and leakages of ions and molecules from cytoplasm into cells (or other way around) = ⬆️ fluidity
   • Membrane becomes more permeable

• ⬇️temperature = ⬇️ kinetic energy = ⬇️ phospholipids laterally movement = ⬆️ density of packed phospholipids = ⬆️ membrane becomes more viscous (possibly even solid) = ⬇️ fluidity
  • Membrane solidifies

• too fluid membrane: molecules that shouldn’t cross the membrane, cross.
• not fluid enough membrane: nothing can cross.
• optimal functioning membranes: all have very similar fluidity and viscosity. (Similar to olive oil).
- important for: transport of molecules, movement of peripheral proteins electron transport chain, no ion leakage in or out.

2. Type of phospholipids (fatty acids)
3. Cholesterol

Question 7

How do cells regulate membrane fluidity?

Answer 7

• Organisms can regulate membrane fluidity by changing the degree of fatty acid unsaturation

• Enzymes (desaturases) remove 2 H from saturated fatty acids and introduce double bonds or insert sterols. = changes saturated fatty acid in phospholipid to unsaturated fatty acid
- can activate or deactivate desaturases

• Fig(not in this): as temperatures get higher, amount of desaturases declines.
- as temperature goes low = ⬆️ amount of desaturases enzyme = means cell has more unsaturated fatty acids in membrane. = keeping fluidity similar. Even tho temp went down
- as temp changes, cell can regulate enzyme activity.

Quick cheat sheet:
• ⬆️ temp = ⬇️ desaturases (enzyme) activity = ⬇️ unsaturated fatty acids
• ⬇️ temp = ⬆️ desaturases (enzyme) activity = ⬆️ unsaturated fatty acids
• how do regulate? = by changing relative proportion of unsaturated fatty acids through desaturases

Question 8

What is the role of saturated and unsaturated fats and cholesterol in the phospholipid bilayer?

Answer 8

To regulate the fluidity or viscosity of the membrane.

Fats:
• Saturated fatty acids = straight = ⬆️ packing or stacking of phospholipids more tightly = ⬇️ less space between phospholipids = ⬆️ viscosity of membrane = ⬇️ (less) fluidity

• Unsaturated fatty acids more fluid than saturated ones.
• Kinks from double bonds = ⬆️ spaces between phospholipids = ⬆️ increase freedom of movement = ⬆️ fluidity

— Shorter fatty acids more fluid than longer fatty acids
• more carbon chains ⬆️ fluidity
• not as important as saturation level of fatty acids

Cholesterol:
- Large planar molecule (hydrogen bonds to polar head)
- Regulates fluidity in animal membranes
- At low temp prevents close packing of phospholipids therefore hinders solidification. = retains fluidity
- At high temp restricts lateral movement in membrane therefore decreases disruption = limits extreme fluidity
- Decreases permeability to small ions and small polar molecules
- keeps layers organized
- adding sterols or cholesterol acts as buffer

Question 9

Where in the membrane are membrane proteins located and what terms are used to describe the location?

Answer 9

Proteins are embedded in or attached to fluid phospholipid bilayer (mosaic part):

•Peripheral proteins
- Proteins associated with outside part of membrane = edge of periphery = peripheral
- include receptor proteins for hormones, matrix of structural proteins that attach to membrane and provide shape, etc.

• Integral proteins
- When span whole membrane = philic & phobic = full integrated = integral
- Include transport proteins (permeases).

Question 10

What are 4 key functions of proteins?

Answer 10

1. Transport (most substances do not diffuse freely)
   —windows and doors into or out of cell
2. Enzymatic activity (e.g. ETC)
   —catalyze rxn
3. Signal transduction (e.g. for hormones)
   — receive and transmit signals
4. Attachment/recognition (e.g. to cytoskeleton (inside), cell to cell (outside))
   — provide stability
   — recog = attach to “friends” —> important for multicellular organisms

Question 11

What are the differences between peripheral and integral membrane proteins?

Answer 11

Peripheral proteins:
• Hydrophilic (interact with water based solutions = cytoplasm or outside environment

• loosely bound to side of membrane through lipid or protein attachment (non-covalent).

• peripheral proteins assist transmembrane proteins with their function, e.g. electron transport chains

• non-polar functional group —> associated with hydrophobic core = extend through phospholipid bilayer = little more attached.

• if mostly hydrophilic and only attached to phospholipids = little less attached.
~
~
Integral membrane proteins:

• Span both sides of membrane = transmembrane proteins

• Are amphipathic
— in order to be long & anchored membrane —> transmem pros reflect structure of membrane = amphipathic

• Interact with water based cytoplasm and outside environment

• Pass through hydrophobic core of the lipid bilayer
- have hydrophobic domains (17- 20 nonpolar AA inside membrane) and polar or charged (AA) domains are exposed (outside or inside (cyto) of membrane)

Question 12

On which side of the membrane would you find carbohydrates/glycoproteins?

Answer 12

The outside.

Question 13

What is a glycoprotein and a glycolipid?

Answer 13

• Carbohydrates are attached to proteins = glycoprotein

• Carbohydrates are attached to lipids = glycolipids

Question 14

How is a membrane (or the bilayer) asymmetrical?

Answer 14

• not organized in same way from outside to inside. = outside environment is different from cytoplasm.

1. Membrane layers (called leaflet) (outer and inner) have different lipid compositions
   — diff fatty acids in upper leaflet & lower
2. Proteins have a specific orientation
3. Outside face: Carbohydrates are attached to lipids (= glycolipids) and proteins (= glycoproteins)

• diff sides of membrane have diff peripheral proteins and diff densities of them. = v

4. Outside face: Proteins anchor membrane to fibres of extracellular matrix (ECM)
5. Inside face: integral and peripheral Proteins anchor membrane to diff components of cytoskeleton

Question 15

Why is transport of substances across the membrane necessary?

Answer 15

The exchanges between the membrane is necessary to maintain function.

• What we eat needs to transported to each and every cell of our body so that energy could be made out of it.
• protein, hormone, enzymes needs to be transported to target cells or target organs.

Question 16

What kind of substances are repelled by the hydrophobic interior of the plasma membrane?

Answer 16

• impermeable to large, polar, and/or charged molecules (hydrophilic) + water
- Can’t pass through membrane’s hydrophobic core
- Require membrane proteins for transport

• permeable to small, non-polar molecules
- e.g. O2 and CO2 (gases), small hydrocarbons (hydrophobic)
- Diffuse through the bilayer between the lipids

Question 17

What kind of molecules move by simple diffusion versus facilitated diffusion (Fig. 4.13)?

Answer 17

Simple Diffusion:
Gases- CO2, N2, O,
Small uncharged polar molecules- Ethanol
Water and Urea

Facilitated Diffusion:
Water and Urea

Large uncharged polar molecules- Glucose
Ions- K^+, mg^2+, Ca^2+, Cl^-, HCO3^-, HPO4^2-.
Charged polar molecules- amino acids, ATP, glucose-6-phosphate

Question 18

How do you tell/measure the difference between simple and facilitated diffusion (Fig 4.15)?

Answer 18

Passive Simple Diffusion: The tendency of molecules to move down a concentration gradient,

- Move from high conc. to low conc.
- direction and speed are not random
- Releases energy = system becomes less organized
- if Two or more different solutes = move independently from each other
- overall concentration of solutes doesn’t matter, only concentration difference of a specific solute or molecule.
- passive transport can’t establish [] gradient = only dissipate one
- rate is dependent on the concentration gradient and reaches equilibrium

Facilitated diffusion: Diffusion of substances aided by membrane transport proteins (= act like doors) (“accelerated diffusion”)

- Movement AGAINST conc. gradient
- Proteins used to move molecules across membrane
- Rate dependent more on amount of transport proteins than on concentration gradient, (e.g. saturation: all proteins are being used)
- more transport proteins = faster rate
- no energy required as along as have transport proteins or the “door is open
- rate is dependent on concentration gradient AND the number of proteins, maximum rate can be reached before equilibrium,

Question 19

If a cell reaches diffusion equilibrium do the molecules crossing the membrane still move?

Answer 19

• At equilibrium, movement of molecules does not stop.
• At equilibrium, there is equal movement of materials in both directions

Question 20

What is the definition of osmosis?

Answer 20

Osmosis: Diffusion of water across a selectively permeable membrane

• simulation osmosis
• high [] to low []
  — high [] of solutes has low [] of moving/available H2O molecules. = To lower [] area of solutes or higher [] of H2O
• rate is dependent on the gradient (of H2O vs solutes), reaches equilibrium.
• Water can cross membrane, BUT solutes (polar ions) cannot = [] of solutes/ions of each side of membrane can be diff = diff results in measurable force/potential.
• Can be measured as a force/osmotic potential

Question 21

What are the two ways water moves across the plasma membrane?

Answer 21

1. Osmosis which is passive diffusion
2. Aquaporin which is active transport.

Question 22

For osmosis to take place, what must the selectively permeable membrane allow to cross versus what must the membrane hold back?

Answer 22

Selectively permeable to water and impermeable to the solute.

And concentration of the solute must be different on the two sides of the membrane.

Question 23

What is meant by hyper-, hypo- and isotonic?

What happens to a cell in a hyper- or hypotonic environment (relative to cytoplasm)?

Answer 23

• Hyper-, hypo-, and isotonic relate to solute concentrations relative to the cell

hypertonic: more free H2O molecules outside cell and more solutes inside cell than outside.
• net movement of water- into cell
• cell is- increasing
• solution is- hypotonic
• hyper = “more or lots”

hypotonic: more free H2O molecules inside cell and less solutes inside than outside.
• net movement of water- out of cell
• cell is- shrinking. Loses volume.
• solution is- hypertonic
• hypo = “ less or low”

isotonic: amount of free water on both sides of membrane is the equal and there is no net movement of H2O molecules
• net movement of water- balanced
• cell is- stays the same
• Iso = “ equal or the same”

Question 24

What are the differences between a channel protein and a carrier protein? Give examples.

Answer 24

Channel proteins:
- Form open hydrophilic channels that allow specific molecules to pass, e.g. ion channels, aquaporins.
- molecules can move in both directions of the protein = movement determined by [] gradient.
- Can be gated, e.g. K+ voltage gated channel: gate opens in response to a voltage change across the membrane
- open channel still selective for molecular properties like polarity, charge, hydrophobic/philic, size,

Ex: aquaporin allows polar water through but hydrogen protons that are smaller but charged can’t go through b/c in the protein channel there are some positive charges that repeal hydrogen protons.

Carrier proteins
• Change shape when specific molecules bind to carry them across membrane
- b/c of binding = carrier proteins highly specific
• Direction of transport follows concentration gradient
• High specificity e.g. Glucose transporter

Question 25

What are key differences between passive and active transport?

Answer 25

1.Passive
- Movement across a membrane that does not require input of energy from surroundings (no ATP)
- Types: Diffusion (including osmosis, facilitated diffusion (accelerated diffusion)
— all concentration gradients
- only applies to molecules that can easily pass directly through membrane.

2. Active
   - Movement across a membrane that does require additional energy from the surroundings
   - ATP through phosphorylation (primary active transport)
   - Concentration gradients (secondary active transport)
   - Membrane proteins that transport specific molecules
   - go against concentration gradient
   — release follows long diffusion movement but building up of [] needs E. = 2-step process
   -“ water dam being opened and a lot of water flowing out and that flow can be used for energy”

Question 26

Does Active Transport use carrier molecules?

Answer 26

Yes, Carrier proteins allow chemicals to cross membranes against a concentration gradient or when the phospholipid bilayer of the membrane is impermeable to a chemical.

Question 27

What energy carrier do cells most frequently use to drive active transport?

What kind of transport does the Sodium/Potassium pump represent?

Answer 27

• Sodium-Potassium pump.

• active transport
• Utilizes the energy released from ATP hydrolysis to move ions against their concentration gradients across a membrane barrier.
• mostly in animal cells

Question 28

The sodium potassium pump creates an electrochemical gradient across the membrane.

What can this gradient be used for?

Answer 28

• Transports 3 Na^+ ions out of cell into extracellular fluid then transports 2 K^+ ions into cell.
• 1 more charge transported out = builds up/contributes to electro-chemical gradient with more positive charges outside cell than in.
• Requires ATP and ATP phosphorylates (protein that lead to conformation change)
• ADP + P is result of hydrolysis

Gradient is used for:
• maintain osmotic equilibrium
• Maintain membrane potential in cells

Question 29

What is an electrochemical gradient/potential, what are its key features?

Answer 29

• the difference of [] gradients of ions with the same charge across a membrane

• consists of two parts:
1. Chemical gradient/difference in solute concentration across a membrane.
2. Electrical gradient/difference in charge across a membrane.

• can power things “like a battery”

• way to store energy in the cell

Question 30

What is a membrane potential? Where are the positive and negative charges located?

Answer 30

• Cells have a separation of charge across their membrane due to the active transport of ions across the membrane.
— “is a build up of Electrical charge difference across membrane.”

• based on active transport

Hydrogen pump:

• active transport, direct
• Creates a voltage due to uneven distribution of ions, animal cells have a normal potential - 50 to -70 mV.

• more positive ions (+ charge) than negative ions is located outside

• more negative ions (- charge) than positive ions is located inside

Question 31

What is a proton pump? How is it similar or dissimilar to other transport proteins?

Answer 31

•Primary Active Transport (moves protons)
- b/c only 1 step of energy transfer involved (phosphorylated)

• Actively transport protons (H*) out of the cell against concentration gradient

• builds up Electrical charge difference across membrane = membrane potential

• Energy comes from ATP hydrolysis
- Phosphate from ATP is attached to pump protein Phosphate released after protons are exported.

Question 32

What is secondary or indirect active transport and what is an example?

Answer 32

Secondary active transport uses the energy stored in the gradients set up by primary active transport, to move other substances against their own gradients.

Example~

Sucrose-H^+ cotransporter

• indirect active transport: energy comes from a [] gradient.

• secondary active transport: 2 step process.
- Step 1: ATP used to establish [] gradient
- Step 2: energy is released when co-transporter is powered by this gradient and Carries sucrose with H ion into cell

• Symport: H^+ and sucrose must move together through transport protein in same direction.
- How cell gets energy In form of carbohydrates into the cell
• sucrose accumulates in the cell
• Power comes from [] gradient of H protons.
• Proton gradient established by proton pump, NOT directly from АТР = then used for for active transport of sucrose

• proton pump not part of co-transporter = is separate protein

Question 33

What are examples of a uniport, symport or antiport transport protein?

Answer 33

• uniport protein transport 1 kind of molecule in 1 direction.
Ex: integral membrane protein, such as ion channel or carrier protein. Hydrogen pump

• symport protein transport 2 kinds of molecules in 1 direction.
Ex: Sucrose-H^+ cotransporter

• antiport transport 2 kinds of molecule in opposite directions at same time.
Ex: sodium-potassium pump

Question 34

How would you describe exocytosis and endocytosis?

Answer 34

Exocytosis
- Cells secrete proteins and other molecules by the fusion of vesicles with the plasma membrane

• “Export”
• produces vesicle inside cell = membrane proteins ensure vesicle fuses to membranes perfectly = no gaps, no holes, no leakage = meaning integrity of cell membrane upheld all the time = membrane of vesicle becomes part of cell membrane.
• Each vesicle that secretes molecules = ⬆️ surface area and volume

Ex: secretory cells = vesicles filled with mucus, hormones, digestive enzymes = vesicles deliver to cell membrane = then to outside of cell
- in human body: secrete into bloodstream or digestive tract.

Endocytosis
- Cells take in materials by forming new vesicles from the plasma membrane, (phagocytosis, pinocytosis, receptor mediated endocytosis)

• “import”
• Inside of vesicles is functionally the same as extracellular environment = engulfs randomly what’s on outside of cell = becomes full contents = vesicle closes off = vesicle taken inside cell and processed

Question 35

What are the differences between phagocytosis, pinocytosis and receptor mediated endocytosis?

What’s the difference in their functions?

Answer 35

Phagocytosis:
• cell engulfs particles
- large particles engulfed = Large vesicle = large food vacuole formed = moves into cell = particles then digested
• Used for eating and defence
• “Cell eating”
• Possible b/c fluid membrane = able to be pinched without making holes/gaps
• Cells with solid walls not able to perform this

Ex: can observe in amoeba or tetrahymena
- In human body: white blood cells engulf, bacteria and viruses to control infection

Pinocytosis:
• cell non-specifically engulf extracellular fluid. Rather than large particles
• “Cell drinking”
• Not intended to take up H2O but it’s the dissolved solutes in H2O
- liquid engulfed = cell pinched off vesicle filled with liquid = transport into cell where ever they need to go

Receptor-Mediated endocytosis
• used for uptake of high [] of specific ligands (subtrates)
• Uses receptor proteins
- Want a specific molecule/ligand = integral membrane proteins bind to specific ligand = Binding of ligand to receptor on membrane enough times = causing vesicle to form = transport vesicle into cell
• good for when cells have a low [] of a molecule

Question 36

How is membrane flow affected by endo- or exocytosis?

What happens if one of the processes dominates?

Answer 36

Membrane flow
• Constant replacement of membrane (turnover) = constant exchange of membrane pieces between inner membrane system of cell and plasma membrane

• Turnover ≠ fluidity of membrane

• Through exocytosis = vesicles and vacuoles carrying what cells want to secrete = extra membrane pieces

• Membrane is synthesized in ER, flows though Golgi vesicles, new membrane is added to cell membrane

• Membrane is pinched off in endocytosis, forms vesicles,

• Maintaining a constant cell surface (size) requires constant adjustment, adding and subtracting membrane (through exo and endo processes)
- way for cell to take damaged parts of membrane inside = phospholipids/ membrane proteins can be recycled

Question 37

How can archaeal phospholipid differ from bacterial or eukaryotic phospholipids?

Answer 37

The “fatty acid bridge” in Archaea is a stereoisomer (enantiomer) (L-glycerol) of the one in Bacteria/Eukaryotes (D-glycerol), has ether linkages instead of ester linkages between glycerol and lipid, has branched isoprene chains (can be joined) not just saturated or unsaturated.

Quick cheat sheet:
• enantiomer (L-glycerol) of (D-glycerol)
• ether linkage
• branched

Question 38

How is a phospholipid in a monolayer different from one in a bilayer?

Answer 38

• it is a single phospholipid layer

• have phosphate groups at either end of fatty acid chain
• long fatty acids branched in between chain.

• fatty acid composition: variation is based on saturation, length and branching

• membrane still
- asymmetric
- fluid to certain extent
- has embedded membrane proteins that have diff functions = regulates ion and solute [] inside cell = facilitate cells interaction with environment

Question 39

How does a phospholipid monolayer or having branched hydrocarbon affect membrane fluidity and permeability?

Answer 39

• phospholipid monolayers are less fluid and permeable for protons and other ions
• Bi-layers with branched fatty acids are more fluid and permeable than monolayers but still less so than phospholipid with unbranched fatty acids

• Benefit is that maybe when archaeans live in extreme environments having a more stable, less permeable membrane makes extremophile lifestyle possible.