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OCR A A-Level Biology > Unit 5 - Plasma Membranes > Flashcards

Unit 5 - Plasma Membranes Flashcards

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1
Q

What components make up cell membranes?

A

Phospholipids arranged in a bilayer, cholesterol and protein molecules.

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2
Q

What are extrinsic proteins?

A

Proteins attached to only the inner or outer surface of the membrane.

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3
Q

What are intrinsic proteins?

A

Proteins which span the whole width of the membrane.

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4
Q

What are the functions of intrinsic proteins?

A

As carrier/channel proteins (for transport), receptors and enzymes.

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5
Q

How do channel proteins help transport substances across the cell membrane?

A

Channel proteins provide a hydrophilic channel allowing passive movement of polar molecules and ions down a concentration gradient across the membrane.

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6
Q

How do carrier proteins help transport substances across the cell membrane?

A

Carrier proteins play a role in both passive and active transport, often involving the protein’s shape changing.

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7
Q

How are channel proteins held in position?

A

Channel proteins are held in position by interactions between the hydrophobic core of the membrane and the hydrophobic R groups on the outside of the proteins.

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8
Q

What are glycoproteins?

A

Glycoproteins are intrinsic proteins embedded in the membrane with attached carbohydrate chains.

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9
Q

What are the functions of glycoproteins?

A
  • Cell adhesion
  • As receptors for chemical signals (cell signalling)
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10
Q

Give an example of a glycoprotein involved in cell signalling.

A

One of:
- receptors for neurotransmitters
- receptors for peptide hormones (insulin and glucagon)
- receptors which bind to drugs

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11
Q

What are glycolipids?

A

Lipids embedded in the cell membrane with attached carbohydrate chains.

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12
Q

What is the function of glycolipids?

A

As cell markers/antigens so the immune system can recognise the cell as self or non-self.

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13
Q

How does cholesterol add stability to the membrane?

A

The hydrophobic and hydrophilic ends interact with the phospholipids, pulling them closer.

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14
Q

How does cholesterol increase membrane fluidity and prevent the membrane from becoming too rigid?

A

The cholesterol molecules are positioned between the phospholipids, preventing them from packing too closely and crystallising.

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15
Q

Will small non-polar molecules diffuse across a plasma membrane?

A

Yes - they will diffuse rapidly.

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16
Q

Will small polar molecules diffuse across a plasma membrane?

A

Yes - they will diffuse slowly (compared to non-polar molecules).

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17
Q

Will charged particles diffuse across a plasma membrane?

A

No - they are unlikely to diffuse across.

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18
Q

Why is membrane fluidity important?

A
  • So substances can diffuse across
  • So membranes can fuse, e.g. in exocytosis
  • So cells can move and change shape
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19
Q

At temperatures below freezing, how permeable is a plasma membrane?

A

The membrane typically has low permeability as the phospholipids are packed rigidly and have low energy. However, if ice crystals pierce the membrane, the permeability will increase significantly.

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20
Q

As the temperature increases (before 45 degrees), how does membrane structure and permeability change?

A

The kinetic energy of the phospholipids increases and they move more, creating gaps in the bilayer. This increases the permeability of the membrane.

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21
Q

At temperatures above 45 degrees, how does membrane structure and permeability increase?

A

The permeability will increase. This is because firstly, the phospholipid bilayer may start to melt, and secondly, channel/carrier proteins may denature.

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22
Q

Why is water important for membrane stability?

A

Because it interacts with the hydrophilic heads of the phospholipid bilayer.

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23
Q

What effect will organic solvents have on the membrane?

A

Organic solvents will disrupt/dissolve the membrane. Non-polar alcohols can enter the cell membrane but will disrupt the membrane, increasing permeability. Strong organic solvents will dissolve membranes.

24
Q

Why are pure or very strong alcohol solutions toxic?

A

They dissolve cell-surface membranes, destroying cells.

25
Q

What are the four ways that molecules can cross a plasma membrane?

A
  • simple diffusion
  • facilitated diffusion
  • osmosis
  • active transport
26
Q

What is diffusion?

A

Diffusion is the net movement of particles down a concentration gradient.

27
Q

Is diffusion an active or passive method of transport and why?

A

Diffusion is passive because no energy from an external source is required to move the particles.

28
Q

What is Fick’s law?

A

Rate of diffusion is proportional to:
(SA x conc. diff.) / length of diffusion path

29
Q

Will increasing the surface area across which particles diffuse increase or decrease the rate of diffusion?

A

Increase because there is more space for molecules to diffuse.

30
Q

Will increasing the size of the concentration gradient increase or decrease the rate of diffusion?

A

Increase.

31
Q

Will increasing the thickness of the membrane or the distance over which diffusion occurs increase or decrease the rate of diffusion?

A

Decrease because it takes longer for molecules to pass through.

32
Q

Will increasing the temperature increase or decrease the rate of diffusion?

A

Increase because the molecules will have more kinetic energy, meaning they move across the membrane faster.

33
Q

What is facilitated diffusion?

A

When carrier/channel proteins help polar/very large molecules diffuse across a membrane.

34
Q

How do charged molecules like ions diffuse?

A

With the help of channel proteins, which provide a hydrophilic channel so they don’t have to pass directly through the phospholipid bilayer.

35
Q

How do large molecules like glucose and amino acids diffuse?

A

Using carrier proteins.

36
Q

What happens in active transport?

A
  • Molecule binds to receptors on carrier protein
  • ATP binds to carrier protein on inside and is hydrolysed
  • Protein changes shape and molecule is released inside
37
Q

Give two examples of active transport.

A

Two of:
- Glucose and amino acids in small intestine
- Mineral ions in plant roots
- Excretion of hydrogen ions and urea by kidneys
- Sodium and potassium ions in neurones and muscle cells

38
Q

What is bulk transport?

A

An active process used to move extremely large substances into cells.

39
Q

What is endocytosis?

A

Bulk transport of material into the cell.

40
Q

What are the three types of endocytosis?

A

Phagocytosis (solids), pinocytosis (liquids) and receptor-mediated endocytosis (uses receptor proteins).

40
Q

What happens in endocytosis?

A

Cell-surface membrane invaginates and fuses to form a vesicle which pinches off and transfers the material into the cell.

41
Q

What is exocytosis?

A

Bulk transport of material outside of the cell.

42
Q

What happens in exocytosis?

A

A vesicle containing the material fuses with the cell-surface membrane and releases the material outside of the cell.

43
Q

What is ATP required for in bulk transport?

A
  • Movement of vesicles
  • Changing of the cell-surface membrane’s shape
  • Fusion of the membrane
44
Q

Define osmosis.

A

The net movement of solvent molecules from a region of their high concentration to one of their low concentration across a partially permeable membrane.

45
Q

What is water potential?

A

The tendency of water molecules in a system to move.

46
Q

What is water potential measured in?

A

kiloPascals.

47
Q

What is the water potential of pure water?

A

0 kPa.

48
Q

Which direction does water move in (concerning water potential)?

A

From higher water potential to lower water potential.

49
Q

Is solute potential a negative or positive value?

A

Negative.

50
Q

Is pressure potential a negative or positive value?

A

Positive.

51
Q

What is the equation for water potential?

A

Solute potential + Pressure potential

52
Q

What is osmotic potential?

A

The amount of dissolved solutes in a solution.

53
Q

What is an isotonic solution?

A

A solution with the same osmotic concentration as the cell’s cytoplasm.

54
Q

What is a hypertonic solution?

A

A solution with a higher osmotic concentration than the cell’s cytoplasm.

55
Q

What is a hypotonic solution?

A

A solution with a lower osmotic concentration than the cell’s cytoplasm.

OCR A A-Level Biology (5 decks)

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