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A Level Biology: 2.1.5 Biological Membranes > 2.1.5a Plasma Membrane > Flashcards

2.1.5a Plasma Membrane Flashcards

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1
Q

Appearance of cell surface membrane

A

Seen using a light microscope, the cell membrane appears as a THIN LINE
but with an electron microscope, it appears as a DOUBLE LINE

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2
Q

What is compartmentalisation

A

Membranes formed from phospholipid bilayers help to compartmentalise different regions within the cell, as well as forming the cell surface membrane

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3
Q

Importance of compartmentalisation

A

This is vital as it allows different parts of the cell to carry out specific functions under optimum conditions

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4
Q

4 points

Role of membranes (part 1)

A

Membranes are vital structures found in all cells
- The cell surface membrane creates an enclosed space separating the internal cell environment from the external env
- Intracellular membranes (internal membranes) form compartments within the cell, sa organelles (including the nucleus, mitochondria, RER) and vacuoles
- Membranes not only separate different areas but also control the exchange of materials passing through them; they are partially permeable
- sites of chemical reactions

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5
Q

3 points

Role of membranes (part 2)

A
  • Membranes form partially permeable barriers between the cell & its environment, between cytoplasm & organelles and also within organelles
  • Substances can cross membranes by diffusion, facilitated diffusion, osmosis & active transport
  • Membranes play a role in cell signalling by acting as an interface for communication between cells
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6
Q

Example of a membrane-bound organelle & why it needs to be kept compartmentalised

A

An eg of a membrane-bound organelle is the lysosome (animal cells), each containing many hydrolytic enzymes that can break down many diff kinds of biomolecule.
These enzymes need to be kept compartmentalised otherwise they would breakdown most of the cellular components

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7
Q

see slide 8-10 for structure of a phospholipid dia

A

10 in particular

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8
Q

Structure of a phospholipid

A

Has a hydrophilic (phosphate) head

And hydrophobic (lipid) tails

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9
Q

What does hydrophilic mean

A

Water loving - attracts water

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10
Q

What does hydrophobic mean

A

Water hating - repels water

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11
Q

Relationship between water and lipids

A

These two substances do not mix

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12
Q

Why do water & lipids not mix

A
  • Water is a polar molecule (the oxygen end is slightly -ve and the hydrogen end slightly +ve)
  • Fats are non-polar & do not form hydrogen bonds w water
  • Fats are said to be hydrophobic & lie on the surface of the water to reduce the SA in contact between the fat & the water
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13
Q

What happens when phospholipids are exposed to water

A

When exposed to water, phospholipids form one of two structures: a micelle or a bilayer
(see slide 12 for dia)

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14
Q

Structure of the head and tails of phospholipids

A

In both a micelle and a bilayer, the hydrophilic heads face the water & the hydrophobic tails point inwards away from the water

This behaviour is key to the role that phospholipids play in membranes

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15
Q

How do phospholipids form a barrier to dissolved substances

A
  • Phospholipids have a ‘head’ & ‘tail’
  • The head is hydrophilic & the tail is hydrophobic
  • The molecules automatically arrange themselves into a bilayer - the heads face out towards the water on either side of the membrane (see slide 13)
  • The centre of the bilayer is hydrophobic so the membrane doesnt allow water-soluble substances (like ions) through it - it acts as a barrier to these dissolved substances
  • (But fat-soluble substances, eg fat-soluble vitamins, can dissolve in the bilayer & pass directly through the membrane)
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16
Q

What is the cell surface membrane

A

A phospholipid bilayer that contains intrinsic and extrinsic proteins

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17
Q

Who came up with the ‘fluid mosaic model’

A

Singer and Nicholson, 1972

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18
Q

What does the ‘fluid mosaic model’ help explain

A
  • Passive and active movement between cells & their surroundings
  • Cell-to-cell interactions
  • Cell signalling
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19
Q

Why does the ‘fluid mosaic model’ describe cell membranes as ‘fluid’

A
  • The phospholipids & proteins can move around via diffusion
  • The phospholipids mainly move sideways, within their own layers
  • The many different types of proteins interspersed throughout the bilayer move about within it (a bit like icebergs in the sea) although some may be fixed in position
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20
Q

Why does the ‘fluid mosaic model’ describe cell membranes as ‘mosaics’

A
  • The scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above
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21
Q

What are the 4 main component the ‘fluid mosaic model of membranes’ include

A
  • Phospholipids
  • Cholesterol
  • Glycoproteins & glycolipids
  • Transport proteins
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22
Q

Structure of the bilayer

A

The cell surface membrane is a bilayer of phospholipids with embedded proteins
(slide 16-17)

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23
Q

What actually is a bilayer

A

2 layers of phospholipid molecules

24
Q

4 points

Functions of the cell surface membrane

A
  • Partially permeable barriers between the cell & its outside env, between organelles and the cytoplasm & within organelles
  • Controls which substances enter & leave the cells
  • Membranes allow recognition by other cells (eg. cells of the immune system)
  • Sites of cell communication (‘cell signalling’)
25
Q

9 components

Components of the bilayer

A
  • Phospholipid bilayer
  • Various protein molecules (channel proteins, carrier proteins)
  • Intrinsic proteins (completely embedded in bilayer)
  • Extrinsic proteins (partially embedded in bilayer)
  • Glycoproteins (carbohydrate attached to protein)
  • Glycolipids (carbohydrate attached to phospholipid)
  • Cholesterol (between fatty acid tails)
  • Receptor sites (for chemical signals)
  • Enzymes & coenzymes
26
Q

see slide 20-22 for components of the bilayer dia

A
27
Q

Structure of phospholipids

A
  • The tails form a hydrophobic core comprising the innermost part of both the outer & inner layer of the membrane
28
Q

What are the 2 main types of protein in the plasma membrane

A

INTRINSIC PROTEINS - channel proteins, carrier proteins, glycoproteins

EXTRINSIC PROTEINS (peripheral proteins)

29
Q

What type of protein is a glycoprotein

A

An intrinsic protein
(see slide 26 for dia)

30
Q

What are Glyoclipids

A

Lipids with an attached carbohydrate (sugar) chain

31
Q

Structure of extrinsic proteins

A
  • Normally have hydrophilic R groups which interact with polar heads of phospholipids or intrinsic proteins
32
Q

What do extrinsic proteins on the extracellular side of the membrane act as

A
  • Receptors for hormones or neurotransmitters
  • Cell recognition

Many are glycoproteins

33
Q

What are extrinsic proteins on the cytosolic involved in

A
  • Cell signalling
  • Chemical reactions

They can dissociate from the membrane & move into the cytoplasm

34
Q

Where is cholesterol found

A
  • Located randomly in the plasma membrane between the tails of the phospholipid molecules
35
Q

What is cholesterol

A

A type of lipid which the molecular formula C27 H46 O
(slide 30-31)

36
Q

Where are extrinsic proteins found

A
  • Found outside the cell surface membrane or bound to an intrinsic protein
37
Q

What do glycolipids act as

A
  • Act as receptor molecules
  • They also act as cell markers or antigens
  • Can be recognised by the cells of the immune system as self (cells of the organism) or non-self (cells from another organism)
    (see slide 27)
38
Q

Functions of glycoproteins

A
  • Involved in cell adhesion (cells joining tg to form tissues)
  • Act as receptors for cell signalling (chemical binds to receptor & causes a response)
  • Act as binding sites for drugs
  • Stabilise the membrane by forming hydrogen bonds w surrounding water molecules
39
Q

Where are glycoproteins located

A
  • Embedded in the cell surface membrane w attached carbohydrate (sugar) chains of varying length/shape
40
Q

How can phospholipids be chemically modified to act as signalling molecules

A

Phospholipids can be chemically modified to act as signalling molecules by:
- moving within the bilayer to active other molecules (enzymes)
- being hydrolysed, which releases smaller water-soluble molecules that bind to specific receptors in the cytoplasm

41
Q

Purpose of phospholipids

A
  • Phospholipids form the basic structure of the membrane (the phospholipid bilayer)
  • Phospholipids bilayers act as a barrier to most water-soluble substances (the non-polar fatty acid tails prevent polar molecules or ions from passing across the membrane)
  • This ensures water-soluble molecules sa sugars, amino acids, proteins cannot leak out of the cell & unwanted water-soluble molecules cannot get it
42
Q

Why is cholesterol important for fluidity of the membrane

A
  • Very important in controlling membrane fluidity.
    Cholesterol increases the fluidity of the membrane, stopping it from becoming too rigid at low temps (allowing cells to survive at lower temps)
  • This occurs bc cholesterol stops the phospholipid tails packing too closely tg.
43
Q

Why is cholesterol important for stabilising the membrane

A
  • Interaction between cholesterol & phospholipid tails also stabilises the membrane at higher temps by stopping the membrane from becoming too fluid
    • cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them & causing phospholipids to pack more closely tg
    • the impermeability of the membrane to ions is also affected by cholesterol
44
Q

What would happen to membranes without cholesterol

A

Cholesterol increases the mechanical strength & stability of membranes - w/o it, membranes would break down & cells would burst

45
Q

What type of protein are channel proteins

A

Intrinsic proteins

46
Q

What are channel proteins

A

Channel proteins are water-filled (hydrophilic) pores
(see slide 32 for dia)

47
Q

Purpose of channel proteins

A
  • They allow charged substances (eg. ions) to diffuse through the cell membrane
  • The diffusion of these ions does not occur freely, most channel proteins are ‘gated’, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore
  • This allows the channel protein to control the exchange of ions
48
Q

What type of protein are carrier proteins

A

Intrinsic proteins

49
Q

Structure of carrier proteins

A
  • Unlike channel proteins which have a fixed shape, carrier proteins can switch between 2 shapes
  • This causes the binding site of the carrier protein to be open to one side of the membrane first, & then open to the other side of the membrane when the carrier protein switches shape
50
Q

Diffusion in carrier proteins

A
  • The direction of movement of molecules diffusing across the membrane depends on their relative concentration on each side of the membrane
  • Net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient (from an area containing many of that specific molecule to an area containing less of that molecule)
    (see slide 33)
51
Q

see slide 34 for dia of all the proteins in the membrane

A
52
Q

Site of chemical reactions

A
  • Like enzymes, proteins in the membranes forming organelle, have to be in particular positions for chemical reactions to take place.
    FOR EG. the electron carriers & the enzyme ATP synthase have to be in the correct positions within the cristae (inner membrane of mitochondrion) for the production of ATP in respiration.
  • The enzymes of photosynthesis are found on the membrane stacks within the chloroplasts
53
Q

EXAM Q: Alcohol, caffeine and nicotine are all lipid-soluble molecules. They have an almost instant and widespread effect on the body. Explain why (2)

A

Lipid-soluble molecules can pass through membranes by simple diffusion so diffuse quickly through the whole Boyd

54
Q

EXAM Q: Membranes, particularly those present within mitochondria, are often highly folded. Suggest what advantages this folding provides (6)

A

Processes occur across membranes.
These processes are enzyme controlled
Folding gives increased SA
so more enzymes, increasing rate of reaction
therefore an increased rate of ATP production

55
Q

EXAM Q: Explain why the protein, phospholipid, cholesterol & carbohydrate composition of different cell types varies

A

Composition depends on cell function -
- if cells need to transport specific types of molecules across their membrane, they will have more proteins for channels & carriers.
- If cells have enzymes embedded in their membranes, they will have a greater proportion of protein
- If cells have recognition or signalling molecules, they will have more glycoproteins

56
Q
A

A Level Biology: 2.1.5 Biological Membranes (2 decks)

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