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B - JC > Biological Membranes > Flashcards

Biological Membranes Flashcards

1
Q

Roles of membranes

A
  • Partially permeable barriers between the cell and its environment, between organelles and the cytoplasm, and within organelles
  • Sites of chemical reactions
  • Sites of cell communication (cell signalling)
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2
Q

How do substances get through the membrane

A
  • Very small molecules diffuse through a cell membrane in between structural molecules
  • Dissolve in the lipid layer and past through
  • Passed through special protein channels or carried by carrier proteins
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3
Q

What are the roles of membranes at the cell surface

A
  1. Separates cell’s components from external environment
  2. Regulates transport of materials in/out of cell
  3. Contain enzymes for special metabolic pathways
  4. Contains antigens to recognise cell as ‘its own’
  5. Release chemicals that signal to other cells
  6. Contains receptors for chemical signals, hormones + drugs may bind these, site for cell communication
  7. Site of chemical reactions
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4
Q

What are the 2 roles of membranes within cells

A
  • Separation of contents
  • Metabolic processes
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5
Q

How does the membrane separate contents within cells

A

Separate organelle contents from cell cytoplasm so each organelle is a discrete entity and can perform its functions

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6
Q

How does membrane help metabolic processes - mitochrondria

A

Folded inner membrane (cristae), large SA for aerobic respiration, localises enzymes needed for respiration

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7
Q

How does membrane help metabolic processes - chloroplasts

A

Inner membranes have chlorophyll (thylakoid) for photosynthesis

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8
Q

How does membrane help metabolic processes - intestinal cells

A

Digestive enzymes on plasma membrane of epithelial cells in small intestine, these catalyse final stages of sugar breakdown

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9
Q

What is cell signalling

A

Communication between cells for cell recognition/identification. Cells work together/coordination between action of different cells to trigger a response/reaction inside the cell

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10
Q

Explain the role of membrane-bound receptors

A

Sites where hormones and drugs can bind.
E.g. Glycoprotein can act as a receptor as they have complementary-shaped signalling chemicals, such as hormones on its surface.

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11
Q

The fluid mosaic model is composed of

A

Phospholipids, cholesterol, proteins, carbohydrates (glycolipids, glycoproteins)

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12
Q

A phospholipid is composed of

A

Glycerol with 2 fatty acids and a phosphate
(unlike a lipid molecule, a triglyceride, which has 3 fatty acids)

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13
Q

What is the phospholipid bilayer

A

Phospholipids are polar molecules with a positively charged hydrophilic ‘head’ (phosphate) and hydrophobic fatty acid tails. They are arranged into layers with the heads on the outside (facing fluid) and tails inside.

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14
Q

Do all cell membranes gave the same composition

A

No - different tissues have specific combos - e.g. more protein rector molecules

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15
Q

Function of phospholipid

A
  • Forms the phospholipid bilayer
  • All other molecules are embedded in it
  • Waterproof barrier to water soluble ions and charged molecules
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16
Q

Function of Cholesterol

A
  • Gives mechanical stability and flexibility
  • Helps regulate fluidity of the membrane
  • Resists the effects of temp changes on membrane structure; prevents too stiff at low or too fluid at high temp
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17
Q

Function of Glycolipid

A
  • A carbohydrate chain attached to a lipid.
  • Acts as receptors and involved in cell recognition
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18
Q

Function of Glycoprotein

A
  • A carbohydrate chain attached to a protein.
  • Acts as receptors and involved in cell recognition
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19
Q

Function of Channel Protein

A
  • Allow hydrophilic/polar/charged substances to pass through water
  • Can be open all the time or can open/close
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20
Q

Function of extrinsic protein

A
  • Embedded in a single layer of phospholipid
  • May be an enzyme, antigens or involved in transport
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21
Q

Function of intrinsic protein

A
  • Spans whole lipid bilayer
  • Involved in transport across the membrane
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22
Q

Function of Glycocalyx

A
  • Formed by the interactions of all the carbohydrates chains attached to either lipids or protein
  • They are hydrophilic allowing the cell to interact with its watery environments and obtain dissolved substances
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23
Q

What factors affect membrane structure and permeability

A
  • Temperature
  • Solvents
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24
Q

How does temp increase affect phospholipids

A
  1. Phospholipids gain more KE
  2. Move more/quicker in random directions
  3. Molecules vibrate
  4. Bonds break
  5. Increase membrane fluidity and permeability
  6. Cholesterol molecules buffer effect as it reduces the increase in membrane fluidity
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25
Q

How does temp increase affect cell

A
  • Affects function/position of membrane embedded proteins e.g. if enzyme drifts sideways = slower rate of reaction is catalysed
  • Increase in membrane fluidity may change ability of cells to signals to other cells by releasing chemicals, often by exocytosis
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26
Q

How does temp decrease affect phospholipids

A
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27
Q

What determines fluidity at cold temps

A

Proportion of saturated and unsaturated fatty acids within a membrane

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28
Q

Effect of temp on proteins

A
  • High temp causes atoms to vibrate, breaks H and ionic bonds that hold structure together
  • Tertiary structure changes/unfolds = denatures
  • Cytoskeletal threads under plasma membrane are made of proteins. If both membrane-embedded proteins + cytoskeletal threads denature, plasma membrane will fall apart - more permeable + pores
  • Active site changes or if enzyme moves in membrane = ror catalysed decreases
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29
Q

Effect of solvents on phospholipids

A

Phospholipids are soluble in alcohol, membrane breaks down

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30
Q

What is a saturated phospholipid

A

No double bonds and as a result straight, unkinked tails

31
Q

What is an unsaturated phospholipid

A

Contains double bonds, therefore kinks in the tail

32
Q

Passive movement of molecules

A

Diffusion, facilitated diffusion

33
Q

Active movement of molecules

A

Active transport, endocytosis, exocytosis
Requires ATP

34
Q

Simple diffusion definition

A

Movement of molecules from an areas of highER conc of that molecule to an area of lowER conc, it may be across a membrane - doesn’t involve metabolic energy (ATP)

35
Q

What factors speed up diffusion

A
  1. Temperature
  2. Diffusion distance
  3. Surface area
  4. Size of diffusing molecule
  5. Concentration gradient
36
Q

How does temperature affect diffusion

A

As temp increases, molecules have more KE, so ror increases. As they lose heat, ror slows

37
Q

How does diffusion distance affect diffusion

A

The thicker the membrane across which molecules have to diffuse, the slower of ror

38
Q

How does SA affect diffusion

A

More diffusion can take place across a larger SA. Cells specialised for absorption have extension to their cell surface membrane - increase SA

39
Q

How does size of diffusing molecule affect diffusion

A

Smaller ions/molecules diffuse more rapidly than large molecules

40
Q

How does conc gradient affect diffusion

A

Steeper (more molecules on one side of membrane) = faster the diffusion to the side with fewer molecules.

41
Q

How is concentration gradient maintained

A

Molecules entering cells pass into organelles and are used for metabolic reactions.
E.g. oxygen diffusing in the cytoplasm of respiring cells then diffuses into mitochondria and is used for aerobic respiration, more replace it

42
Q

How do small molecules move through membrane

A

O2, CO2 - can pass through cell membranes by simple diffusion

43
Q

How do fat soluble molecules move through membrane

A

Steroid hormones - can diffuse through cell membrane as they dissolve in the lipid bilayer. Move down conc gradient

44
Q

How does water move through membrane

A
  • Polar + insoluble subs cannot pass the phospholipid bilayer
  • Direct diffusion happens with very large water concs
  • In membranes where very high rate of water movement is required, aquaporins allow water to cross without moving through lipid environment
45
Q

What are aquaporins

A

specific water channel proteins

46
Q

Facilitated diffusion definition

A

Movement of molecules from an area of high concentration to an area of low concentration, across a partially permeable membrane via protein channels or carriers. It does not involve metabolic energy ATP.

47
Q

How does small polar molecules move through a membrane by FD

A

E.g. ions with electrical charge
Insoluble in lipids because they can’t interact with hydrophobic tails - diffuse through water-filled protein channels pores in membranes around 0.8 nm diameter

48
Q

How does small water-soluble molecules move through a membrane by FD

A

Cholesterol molecules within membranes reduce permeability of the membrane

49
Q

How does glucose molecules move through a membrane by FD

A

Turn large to diffuse through water food protein channels. They bind to a transmembrane carrier protein, which opens to allow the glucose to pass across the membrane.

50
Q

How do different cell types structure differ

A

Different cell types have different proportions of transmembrane protein channels and transmembrane protein carriers according to their function.

51
Q

What is ATP

A

adenosine triphosphate

52
Q

Active transport definition

A

Movement of substance from a region of lower conc to a region of higher conc (against conc gradient)

53
Q

What substances are often moved in active transport

A

Large, charged particles

54
Q

What does active transport require

A

Energy supplied in the form ATP - this is released in the process. It involves protein carriers in the membrane

55
Q

Role of carrier proteins in active transport

A

The energy carrying molecule ATP enables the protein to change shape - conformational change. This allows the molecule to be moved through the membrane

56
Q

Sodium Potassium pump

A

Pumps out 3Na+ and pumps in 2K+

57
Q

Examples of active transport

A
  • Root hair cells to absorb of ions from soil
  • guard cells to transport potassium ions
  • digestion of food in the small intestine into bloodstream
58
Q

Exocytosis definition

A

Bulk transport of molecules too large to pass through a cell membrane (even via channel or carrier proteins) OUT of a cell

59
Q

Process of exocytosis

A

Membrane-bound vesicles containing the large molecule is moved towards and fuses with the plasma membrane. Fused site opens, releasing the contents of the secretary vesicle.

ATP is needed to fuse the membrane together and move the vesicles. A molecule of ATP is hydrolysed for every step that a motor protein takes on the cytoskeleton thread, as it drags the vesicle.

60
Q

Exocytosis at synapses

A

Chemicals in vesicles moved, by motor proteins moving along cytoskeleton threads, to the presynaptic membrane. Here the vehicle membrane and plasma membrane fuse and neurotransmitter chemicals are released into the synaptic clef.

61
Q

Endocytosis definition

A

Bulk transport of molecules too large to pass through a cell membrane (even via channel or carrier proteins) INTO a cell

62
Q

Endocytosis process

A

A segment of the plasma membrane surrounds and encloses the particle and brings it into the cell, enclosed in a vesicle.

ATP is needed to provide energy to form the vehicle and move them, using molecular motor proteins, alongside a skeleton threads into the cell interior.

63
Q

Endocytosis example

A

Phagocytosis: eating by cells, refers to intake of cell matter.
- Phagocytic cell approaches a bacteria cell -> extends to surround bacteria -> bacteria is now enclosed within a phagocytic vesicle (phagosome)

Pino(endo)cytosis: ingestion of liquid by cell

64
Q

Osmosis definition

A

Net movement of water from an area of higher water potential to an area of lower water potential across a partially permeable membrane. It is a passive process

65
Q

What is water potential

A

Measure of the tendency of water molecules to diffuse from one region to another. Measured in kilopascals (kPa). Pure water has a potential of 0 (impure will be < 0)

66
Q

When a solute is added to water why does the water potential decrease

A

Smelled proportion of water molecules - The more concentrated the solution, the lower the water po

67
Q

Hypertonic

A

External solution has a lower water potential (higher concentration of solute) than molecule - water moves OUT

68
Q

Isotonic

A

External solution has the same water potential as molecule

69
Q

Hypotonic

A

External solution has a higher water potential (lower concentration of solute) than molecule - water moves IN

70
Q

Hypertonic in animal vs plant cells

A

Animal: water moves out -> cell shrivels up -> crenation

Plant: water moves out -> membrane separates from cell wall (goes inwards) -> cytoplasm shrinks -> plasmolysis
The area between cell membrane and cell wall is filled with solution

71
Q

Isotonic in animal vs plant cells

A

Animal: normal, equilibrium
Plant: cell is flaccid, incipient plasmolysis

72
Q

Hypotonic in animal vs plant cells

A

Animal: water moves in -> plasma membrane breaks -> cell lysis/cytolysis

Plant: water moves in -> cell wall prevents cell lysis -> vacuole gets bigger (takes in water) -> slight increase in size -> turgid

73
Q

Why don’t plant cells burst in hypotonic solutions

A

Cellulose cell wall resist further swell as its cell content push out against the cell wall - exerts pressure

74
Q

What is the difference between flaccid and plasmolysed

A

Plasmolysed refers to a single cell.
Flaccid refers to a tissue which is softer when all its cells are plasmolysed

As all metabolic reactions occur in the water (enzyme catalysed reactions must be in solution), plant tissue which is dehydrated in this way is at risk of death.

B - JC (4 decks)

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