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Biology A-Level > Cell Membranes (Chapter 4) > Flashcards

Cell Membranes (Chapter 4) Flashcards

1
Q

What is the main function of the cell membrane?

A

Controlling the exchange of materials such as nutrients and waste products between the cell and its environment

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2
Q

What are other functions of membranes?

A 
Cell recognition (antigens)
Cell communication (protein receptors)
Within cells, internal membranes created compartments enabling specific metabolic reactions to occur in discrete locations
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3
Q

What are characteristics of biological membranes?

A

Very thin sheet-like structures
The components associate through non-covalent interactions
Fluid structures
Highly selective permeability barriers
Consist mainly of proteins and lipids
Asymmetric - the two faces of the membrane are different

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4
Q

What affects how a molecule is transported across a membrane?

A

Its size, solubility and charge

In general, the smaller and more hydrophobic it is, the more rapidly it will diffuse

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5
Q

What molecules cannot diffuse regardless of size?

A

Charged molecules and ions - they must travel through a membrane protein

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6
Q

LEARN THE FLUID MOSAIC MODEL OF THE CELL SURFACE MEMBRANE

A

OK

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7
Q

What is a phospholipid?

A

A molecule with a hydrophilic head (phosphate group) and two hydrophobic tails

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8
Q

What is the role of phospholipids in the cell surface membrane structure?

A

They form the phospholipid bilayer - the basic structure of the membrane

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9
Q

What is the function of cholesterol?

A

At low temperatures, cholesterol increases the fluidity of membranes, preventing it from becoming too fluid
At high temperatures, the interactions of the phospholipid tails with the cholesterol molecules helps to stabilise cells

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10
Q

Why does my heart flip every time I see you smile?

A

Beta-galactimase

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11
Q

What happens to phospholipids on the surface of water?

A

They form a single layer with their heads in the water (because these are hydrophilic/polar) and their tails projecting out of the water (because these are hydrophobic/non-polar)

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12
Q

What happens when phospholipids are shaken up with water?

A

They can form stable ball-like structures called micelles

Or two-layered structures called bilayers, which form in sheets

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13
Q

What is the basic structure of cell membranes?

A

The phospholipid bilayer (which also contains proteins)

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14
Q

What is the model for the structure of cell membranes called?

A

The fluid mosaic model

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15
Q

Why is the name fluid mosaic model used?

A

‘fluid’ because both the phospholipids and proteins can move about by diffusion
‘mosaic’ describes the pattern produced by the scattered protein molecules

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16
Q

Describe the basic structure of cell membranes

A

A bilayer of phospholipid molecules

  • the tails point inwards, facing each other and forming a non-polar hydrophobic interior
  • the heads face the aqueous medium surrounding the membrane
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17
Q

What factors affect fluidity of the phospholipids (membranes) and how?

A

1) Saturation - the more unsaturated, the more fluid the membrane because the fatty acid tails are bent and so fit together more loosely
2) Length of tail - longer tail, less fluid
3) Temperature - lower temp, less fluid

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18
Q

What are the two types of protein in a membrane?

A

Intrinsic/integral proteins

Extrinsic/peripheral proteins

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19
Q

What are intrinsic proteins?

A

Proteins that are found embedded within the membrane

- they may only be found in the inner layer, outer layer, or spanning the whole membrane (transmembrane proteins)

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20
Q

Describe the structure of intrinsic proteins

A

Hydrophobic regions - repelled by aqueous environment
Hydrophilic regions - repelled by hydrophobic interior and therefore face the aqueous environment or have hydrophilic pores

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21
Q

What are extrinsic proteins?

A

They are found on the inner or outer surface of the membrane
- many are bound to intrinsic proteins but some are bound in other ways

22
Q

What is the total thickness of the membrane on average?

A

7nm

23
Q

Describe the functions of phospholipids

A

1) They form the bilayer
2) Because the tails are non-polar, it is difficult for polar molecules/ions to pass through - so they act as a barrier to most water-soluble molecules e.g preventing sugars and amino acids from leaking out
3) Some can be modified chemically to act as signalling molecules
4) Can be hydrolysed to release small, water-soluble glycerol-related molecules which diffuse through the cytoplasm and bind to specific receptors

24
Q

What is cholesterol?

A

A relatively small molecule with a hydrophobic head and hydrophilic tail

25
Q

Where is cholesterol found?

A

Lots in the animal CSM
Less in plant CSM
Absent in prokaryotes

26
Q

What does cholesterol do at low temperatures?

A

It increase the fluidity of the membrane, preventing it from becoming too rigid by preventing the close packing of phospholipid tails
- this means that cells can survive colder temps

27
Q

What does cholesterol do at high temperatures?

A

The interaction of the phospholipid tails with the cholesterol molecules helps to stabilise cells when the membrane could otherwise become too fluid

28
Q

What is cholesterol important for?

A

1) The mechanical stability of membranes - w/o it, membrane quickly break and burst open
2) Hydrophobic regions of cholesterol molecules help to prevent ions/polar molecules from passing through the membrane - v important in nerve cells

29
Q

What are glycolipids?

A

Lipid molecules with short carb chains attached

30
Q

What are glycoproteins?

A

Protein molecules with short carb chains attached

31
Q

Describe the functions of the carbohydrate chains on glycolipids and glycoproteins

A

1) They project like antennae into the watery fluids surrounding the cell, where they form H-bonds with the water molecules, helping to stabilise the membrane structure
2) They help the GL+GP to act as receptor molecules, which bind with particular substances at the cell surface

32
Q

What are the three types of receptor?

A

1) Signalling receptors
2) Receptors involved in endocytosis
3) Receptors involved in binding cells to other cells (cell adhesion) in tissues and organs of animals

33
Q

What do signalling receptors do?

A

They are part of a signalling system that coordinates the activities of cells

  • the receptors recognise messenger molecules
  • when the messenger molecule binds to the receptor, a series of chemical reactions is triggered inside the cell
34
Q

What do receptors do in endocytosis?

A

They bind to molecules that are parts of the structures to be engulfed by the CSM

35
Q

What do some types of glycolipid/glycoprotein act as?

A 
Cell markers (antigens) allowing cell-cell recognition
- each type of cell has its own type of antigen
36
Q

What are the roles of proteins in the membrane?

A

1) Transport proteins
2) Enzymes
3) Attached to the cytoskeleton - help to maintain, decide and change the shape of the cell
4) In membranes of organelles - e.g. involved in respiration and photosynthesis in mitochondria and chloroplasts

37
Q

What are the two types of transport protein?

A

Channel proteins and carrier proteins

38
Q

Describe the functions of transport proteins

A

1) Provide hydrophilic channels for ions/polar molecules to pass through the membrane
2) Have roles in facilitated diffusion and active transport
3) Each one is specific for a particular kind of ion/molecules - therefore the types of substances that enter/leave the cell can be controlled

39
Q

Why is cell signalling important?

A

Helps to explain how living organisms control and coordinate their bodies

40
Q

What is signalling?

A

Getting a message from one place to another

41
Q

Why do living organisms need signalling?

A

Because all cells and organisms must be able to respond appropriately to their environments

42
Q

How is signalling made possible in organisms?

A

By means of a complex range of signalling pathways which coordinate the activities of cells, even if they are large distances apart

43
Q

Describe the signalling pathway

A 
Stimulus/signal 
Receptor
Transmission of signal (involves crossing barriers)
Target/effector
Appropriate response
44
Q

Why is the CSM a critical component of most signalling pathways?

A

Because it is a barrier to the movement of molecules, controlling what moves between the external and internal environments of the cell

45
Q

Describe signalling molecules?

A

Very small for easy transport

Hydrophobic or water-soluble

46
Q

What does the signal do when it gets to the receptor?

A
  • It brings about change in the shape of the receptor which allows the receptor to act with the next component in the pathway
  • The next component is a ‘G protein’ which acts as a switch to bring about the release of a’second messenger’ (a small molecule which diffuses through the cell, relaying the message)
47
Q

How do hydrophobic signalling molecules work?

A

They can diffuse directly across the CSM and bind to receptors in the cytoplasm/nucleus

48
Q

How do water-soluble signalling molecules work?

A

They arrive at a protein receptor in the CSM

The receptor is a specific shape which recognises the signal

49
Q

How many second messenger molecules can be made in response to one receptor molecule being stimulated and what does this represent?

A

Many - this represents an amplification of the original signal (a key feature)

50
Q

What happens after the second messenger molecule is released?

A
  • The molecule activates an enzyme, which activates more enzymes, increasing the amplification at each stage
  • Finally, an enzyme is produced which brings about the required change in cell metabolism/movement/secretion
51
Q

What other ways can a receptor alter the activity of a cell?

A

1) Opening an ion channel, resulting in a change of membrane potential
2) Acting directly as a membrane-bound enzyme
3) Acting as an intracellular receptor when the initial signal passes straight through the CSM

52
Q

Apart from the secretion of chemical signals, what is another mechanism of signalling?

A

Direct cell-cell contact

Biology A-Level (44 decks)

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