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Year One: First Things First > Membrane Structure and Functions > Flashcards

Membrane Structure and Functions Flashcards

1
Q

How can membranes be described as?(4)

A

· Flexible

· Self-sealing

· Selectively permeable

· They define the external boundaries
Divide the internal space

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2
Q

What 4 ways are used to describe membrane molecular function?

A

· Transport

· Cell recognition

· Cellular communications

· metabolic regulation

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3
Q

Why do lipid proportions vary within different types of cells and give example?

A

The membrane compositions are different due to their function for example the myelin sheath is made up of lipids because they are electrical insulators and enable saltatory conduction, other membranes do not need insulation, so their lipid content is much less

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4
Q

What are the compositions of the phospholipid membrane?(5)

A 
· Phospholipid 
· Cholesterol- provide fluidity 
· sphingosine- ceramide 
· Glycolipids- Ceramide combined with carbohydrate group 
· Sphingomyelin
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5
Q

What is meant by amphipathic regarding composition of the membrane?

A

Sterols, phospholipids and glycolipids are a class of lipids called amphipathic which means they have hydrophilic and hydrophobic components within them

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6
Q

Recall examples of the polar head group

A
  1. Choline
  2. Serine
  3. Ethanolamine
  4. glycerol
  5. inositol
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7
Q

How are the names of the phsopholipid decided?

A

Names of hydrophilic head is incorporated into the phospholipid name eg if choline head, then the name is phosphatidyl choline

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8
Q

What are sphingolipids?

A

Glycolipids and sphingomyelin

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9
Q

What is the role of the hydrophobic middle layer?

A

The hydrophobic middle layer acts as a barrier to other molecules which isolate the two sides of the membrane.

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10
Q

Is the membrane symmetrical or asymmetrical?

A

asymmetry. If you ‘cut’ the membrane in a transverse way the two sides (the upper side and lower side are different)

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11
Q

Describe membrane synthesis

A

Synthesis takes place in the sER and transported to the GA
· The enzymes are present in the cytosol where synthesis takes place

· To distribute the glycolipids, they need to be transported from side of the membrane to the other

· ABC transporters which enable the highly charged group to move across the hydrophobic tails

· This requires particular enzymes- floppase, fippase, scramblase. Floppase and fippase require ATP to move.

· It is energetically unfavourable because the highly charged hydrophilic heads must pass through the highly hydrophobic tails

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12
Q

Why is the cystolic side of the membrane bigger the other?(2)

A

because the new lipids are inserted into the cytosolic side, so scramblases transfer random phospholipids from the cytosolic side to the other side so the bilayer evens out, there are the same number of lipids on the same side.

  1. Flippases transfer specific phospholipids to the cytosolic side which generates the asymmetry.
    “
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13
Q

Summarise the three ABC transporters(The ATP-binding cassettetransporters)”

A

· Floppase- moves phospholipids from the inner to the outer leafletand to maintain the charge.It is found on the inner surface of the membrane.

· Flippase- moves phospholipids from the outer to the inner leaflet- they also mediate transport of cholesterol from the intracellular monolayer to the extracellular monolayer.

3.Scramblase- bidirectional movement.

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14
Q

How does scramblase regulate apoptosis and what is the advantage?

A

They predominantly transportphosphatidylserine which is recognized as an apoptosis signal, it

  1. Phosphatidylserine is usually found on the inner leaflet of the bilayer
    Advantage to apoptosis is turning over old cells without causing inflammation.
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15
Q

Describe the two types of movement of lipids in the membrane

A

· Lateral movement of lipids in the membrane is rapid-gives fluidity to the membrane, they appear to ‘hop’ this is because the phospholipids are linked to the actin cytoskeleton.

· Passing the hydrophilic molecule through a hydrophobic layer is energetically unfavourable.

· Transverse movement is slow and requires the action of three enzymes- flippase, floppase, scramblase.

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16
Q

Why is membrane asymmetry important and how is it maintained?

A

Critical for the membranes to function, and transverse movement is what allows the membrane to maintain that asymmetry,

  1. this is possible via an uncatalyzed route however it is slow and unreliable so the asymmetry equilibrium would not be maintained.
  2. Allows themembraneto be rigid and allows the cell to have a different intracellular environment from the existing extracellular environment
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17
Q

Recall the factors that affect membrane fluidity

A

Temperature
Degree of saturation
Cholestrol content
Chain length

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18
Q

How does tempearture affect membrane fluidity?

A

· As temp increase, the energy associated with the molecules also increase so there is more movement and
the distance increases and fluidity increases.

At low temperatures, cholesterol inserts itself between some phospholipids. It also prevents crystallization between phospholipids”

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19
Q

How does chain length affect membrane fluidity?

A

· Longer chain length means more rigidity because there are more interactions between the chains.
“

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20
Q

How does degree of saturation affect membrane fluidity?

A

More saturation results in the phospholipids being packed closer together.
The non-covalent bonds between adjacent ones is strong and the accumulation is high. Reduces fluidity.

Unsaturation will have kinks so they can’t pack together.

Distance between phospholipids increases so there is more fluidity

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21
Q

How does cholesterol affect membrane fluidity at low and high temperatures?

A

· At low temp, molecules pack close together. As cholesterol increases, interaction between the lipids decrease.

Increases fluidity.

At high temperatures molecules have more energy, cholesterol stabilises the membrane by maintaining the interaction between lipids.

Fluidity decreases.

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22
Q

How does spur cell anaemia occur?

A

· Cholesterol content increased by 25-65% leading to decreased membrane fluidity. Spurs form in the membrane.

· Cells get trapped and they rapture leading to anaemia.

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23
Q

Describe the structure of integral proteins(4)

A

· Strong non covalent bonds

· Trans-membrane domain often a-helix with the R groups facing outwards (hydrophobic)

· Can be predicted from sequence
There two types: single or multi pass

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24
Q

Describe the association between integral proteins and lipid bilayer

A

strong because there are large numbers of non covalent interactions between the membrane lipid and the protein generally hydrophobic in nature but there will be some hydrophilic at the external surface

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25
Q

Describe the structure of peripheral proteins

A

Located on the extra cellular or cytosolic membrane

Covalent in nature

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26
Q

Summarise the three types of interactions between peripheral proteins and the bilayer

A

o interactions between the phospholipid or and the protein component of the integral protein.

o interactions between integral
o interaction between membrane lipids only. 1 and 3 are more transient- easier to remove”

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27
Q

What is the difference between peripheral and integral proteins?

A

Peripheral proteins are easier to remove- more transient

28
Q

Describe lipid anchored membrane proteins(3)

A

”
· Covalently linked to a lipid molecule such as glycerol- phosphatidylinositol.

· Anchored by carbohydrate bridge or fatty acid.

· Most glycolipids are externally facing.
“

29
Q

Describe carbohydrates in the membrane

A

”
· Carbohydrates are associated with both membrane lipids and proteins

· Forms 2-10% of the membrane weight

· Carbohydrate on all membranes faces away from the cytosol

· They are often involved in cell-cell interactions or cellular recognition.
“

30
Q

Give an example of carbohydrates in the cell membrane

A

The blood group antigens are glycolipids

31
Q

In RBCs, what proportion of the weight is carbohydrates?

A

8%

32
Q

What are selectins?

A

Important group of membrane glycoproteins.

They are involved in immune response including lymphocyte glycoproteins.

33
Q

What nature are the R- groups of integral proteins that interact with the bilayer?

A

Hydrophobic in nature

34
Q

Summarise the different types of membranes and their major function(7)

A
  1. plasma membrane - barrier, transport
  2. mitochondria(inner and outer)- energy transduction, barrier
  3. ER(rough, smooth)- translation protein, synthesis of complex lipids
  4. Golgi- post translational modification, processing for secretion
  5. Nuclear membrane- attachment of chromatin
  6. Lysosomes- hydrolytic enzymes
  7. Peroxisomes- fatty acid oxidation
35
Q

Why do myelin sheaths have high lipid composition?

A

they are electrical insulators and enable saltatory conduction

36
Q

Which molecules can do simple diffusion?

A

small uncharged polar:H2O and CO2, glycerol and urea

hydrophobic: N2, O2

37
Q

What are the two types of simple diffusion?

A
  • Non mediated: simple diffusion or diffusion through a pore

* Mediated: carrier mediated, a uniport

38
Q

What is the difference between symport and antiport?

A
  • Symport : moves two molecules in the same direction across the membrane.
  • Antiport : transports two molecules in different directions.
39
Q

What are the similarities between transporters and enzyme kinetics?(3)

A
  1. there is a maximum rate at which molecules can be transported, the limiting factor in carrier mediated diffusion is the number of carriers. saturable
  2. • Enzymes are specific to their substrate in the same way that carrier proteins are specific to their molecules.
  3. The equivalent of Km is Kt and there is also Vmax and ½ V max.
40
Q

Why is carrier mediated diffusion more efficient?

A

• Main reason for transport is efficient even if it Is driven by concentration gradient

41
Q

What is Kt and what does high Kt mean?

A
  • Kt is the transport constant- affinity for transporter for glucose like Km in enzyme
  • The lower the Kt the higher the affinity for the molecules by the transporters
42
Q

Explain the difference between glucose transporters GLUT1 and GLUT2

A

GLUT 1 has a relatively low Kt (1mM) which means the transporter is going to be saturated at relatively low glucose concentrations so the transporter has a high affinity for the molecule (less of it is needed to bind ‘sticky’).

• GLUT2 in comparison it needs a much higher concentration, so it has a low affinity for the substrate which means it is only active at very high concentrations of glucose.

43
Q

What is the normal resting glucose concentration?

A

4-5mM

44
Q

Recall the different types of glucose transporters and the tissues they work on and their normal saturable concentrations

A

GLUT1 → all mammalian tissues, responsible for basal glucose uptake → 1mM

GLUT2 → liver and pancreatic β cells,
in the pancreas, plays a role in regulation of insulin
→ in the liver, removes excess glucose from the blood → 15-20mM

GLUT3 → all mammalian tissues, responsible for basal glucose uptake → 1mM

GLUT4 → muscle and fat cells, the amount in muscle plasma membranes increases with endurance training → 5mM

GLUT5 → small intestine. primarily a fructose transporter → -

45
Q

Describe the difference between glucose uptake in the liver and the brain

A

if the GLUT2 transporter was very active at low levels of glucose (if it had a low Kt and a high affinity) it would uptake glucose into the liver all the time and the body would not have enough glucose free in the blood.

• The brain has a high Kt glucose transporter because it needs a low glucose environment to work. The type of transporter is important to a whole organ.

46
Q

Describe the structure of GLUT2

A
  1. very large and has 12 transmembrane helices domains.

2. both the amino and carboxyl termini exposed on the cytoplasmic side of the plasma membrane

47
Q

Describe the process of glucose being taken into the cell(4)

A
  1. The glucose binds to the transporter which undergoes a conformational change which opens the transporter to the other side of the membrane
  2. • When glucose is taken into the cell it is phosphorylated to give glucose-6-phosphate
  3. • It keeps glucose in the cell by effectively removing “pure” glucose- glucose phosphate no longer fits the membrane carrier
  4. it “activates” glucose for biosynthesis reactions
48
Q

What is the nature of glucose uptake? (2)

A
  • Driven by concentration gradient. Gradient maintained by phosphorylation
  • Bidirectional transport
49
Q

Why is glucose converted to G6P once its in the cell?

A

there are no transporters for G6P on the cell membrane. so it wont leave the cell

50
Q

How is glucose uptake regulated?

A
  1. you can transport D-glucose but you cannot transport L-glucose.
  2. regulating the number of glucose transporters on the cell surface which happens with GLUT4- expressed on adipocytes and muscle, the expression can be increased by the presence of insulin
  3. within those cells there are reserves of GLUT4 transporters within the cell that can be recruited in the presence of insulin
51
Q

What are the concentrations of NA+ outside and inside the cell?

A

Concentration of Na+ outside cell is 143mM

Concentration inside cell is 14mM

52
Q

How are the concentrations of Na outside and inside the cells maintained?(3)

A
  1. the sodium/potassium ATPase which is co-transport and it is an antiport system, to maintain a low concentration of sodium within the cell.
  2. ATPase uses ATP as its energy source which is used to transport the sodium out against the concentration gradient of the cell in exchange for potassium,
  3. there is an electrochemical gradient and a concentration gradient.
53
Q

How does foxglove alleviate symptoms of congestive heart disease?(4)

A
  1. The active ingredient is digitalis which is a cardiotonic steroid it inhibits the sodium potassium ATPase pump.
  2. • If the pump is inhibited the intracellular concentration of sodium increases which stimulates a second transporter called the sodium calcium exchanger (antiport system)
  3. sodium is pumped out and calcium is pumped in,
  4. calcium in the heart is important to maintain the contraction of the heart so digitalis improves the rate of contraction of the heart.
54
Q

What is the role of SGLUT-1 and 2?

A

Found in the gut
this transports glucose from area of lower concentration to an area of higher concentration using the concentration gradient of Na+.
Symport

55
Q

How does the Na/K ATPase maintain glucose levels in the cell?(5)

A

1) The sodium potassium ATPase continually pumps sodium out of the cell and potassium ions into the cell using the energy from ATP hydrolysis to do so
2. This creates a concentration gradient, as there is now a higher concentration of sodium ions in the small intestine lumen

  1. This causes sodium ions to diffuse from the small intestine lumen into the cell down their concentration gradient via a sodium-glucose co-transporter protein, which brings glucose into the cell at the same time and it gets phosphorylated. This causes the glucose concentration in the cell to increase
  2. the glucose diffuses through the epithelial cell and into the tissue fluid by facilitated diffusion, through a glucose carrier protein that is only found on the inner surface of the epithelial cell.
  3. Na+ is pumped actively out by the Na+/K+ ATPase in order to maintain the Na+ concentration gradient.
56
Q

How does cellular asymmetry arise?

A

tight junctions, which act as ‘fences’ against transmembrane diffusion, lock the asymmetry in place.

57
Q

What is rehydration therapy?(3)

A

• The treatment is ORT (oral rehydration therapy) which consists of a solution of glucose and NaCl.

  1. If sodium and glucose both bind to the co-transporter proteins it will carry them into the cell down their concentration gradients
  2. this lowers the water potential in the cell so water diffuses from the lumen back into cells.
58
Q

How does V. cholerae cause cholera?(2)

A

activating chloride ion channels in the cell membrane which causes chloride ions to diffuse out into the lumen

  1. lowers the water potential so water is lost from cells into the lumen
59
Q

What is the main defect in cystic fibrosis?(2)

A
  • Defect in chloride ion channel transporter

* Single deletion at position 508

60
Q

Why is compartmentalisation important?(3)

A
  • Separates reactions within membranes
  • Enables the local environment to be regulated for instance pH
  • Brings reactants together can change microenvironment of the enzymes eg pH
61
Q

How does I-cell disease arise compared to normal lysosome storage?(4)

A
  1. I cell- lysosome storage disease.
  2. Proteins normally formed transported to Golgi and attached to mannose signal and targeted to lysosome and which is acidic.
  3. Acidity is maintained by proton pump.
  4. In I cell disease, the mannose is phosphorylated so it is not target to lysosome. So, lysosome is packed with substrate.
62
Q

Describe the relationship between membranes and their environments(3)

A

• Cell communication- receptors hormonal and neurotransmitters, and growth factors

  • Membrane carbohydrates are involved in cell recognition eg blood antigens and lymphocyte trafficking.
  • Lymphocytes recognise carbohydrates expressed by cells especially in disease or infection
63
Q

why is glucose transport carrier-mediated?

A

glucose transport is carrier-mediated; it cannot get through quick enough by simple diffusion.

→It can be passive (by facilitated diffusion) or active (by sodium symporter).

→ if there was no carrier, glucose transport would be incredibly slow.

→The carrier-mediated facilitated diffusion speeds up the rate of glucose transport greatly.

64
Q

what kind of kinetics do glucose transporters show?

A

similar to enzyme kinetics.

→They exhibit saturation-type kinetics (ie. starts to plateau) and can be regulated.

→The kinetics are also specific to the molecules they transport.

65
Q

how can glucose uptake be increased in cells using insulin?

A

When there is no insulin present, the transporters are brought inside the cell via a vesicle (ie. away from the membrane).

→This stops the transporters from functioning.

→When insulin is present, the vesicle with the transporters fuses with the membrane to express them on the membrane.

→This allows the transporters to function.

66
Q

where is the SGLUT-2 found?

A

→On the other side of the cell (facing the extracellular fluid, the basal domain) is the GLUT -2 which transports glucose down its concentration gradient (from higher in the cell to lower in ECF). The Na+/K+ ATPase pump also works down here

Year One: First Things First (27 decks)

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