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Cell Biology > LECTURE 12: Membranes and The Plasma Membrane > Flashcards

LECTURE 12: Membranes and The Plasma Membrane Flashcards

1
Q

plasma membrane

A

encloses contents of the cell
It’s the presence of the PM that delineates the stuff of life that occurs on the inside of cells and the outside environment (in single celled organisms)

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2
Q

List some of the Internal membranes

A 
• nuclear envelope
• mitochondrial membranes
• chloroplast membranes
• lysosomal membrane
• endoplasmic reticulum
• Other compartments of the endomembrane
system

Only in eukaryotic cells, of course…

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3
Q

Describe why biological membranes are selectively permeable barriers

A

Biological membranes serve as selectively permeable barriers. Each ‘allows’ select molecules to cross them, thus the contents are different in different cellular compartments and compared to the external environment (cellular homeostasis)

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4
Q

Membranes

A

are continuous, unbroken sheets of lipids that enclose the cell and intercellular compartments
- Only about 5 nm (50 atoms) thick… it’s amazing they keep the cell intact!

Dynamic structures capable of fusing without losing continuity, they can ‘self heal’ if punctured
- also can self assemble

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5
Q

List the functions of biological membranes

A
  • Compartmentalization
  • Scaffold for biochemical activities
  • Selectively permeable barrier
  • Transporting solutes
  • Responding to external signals
  • Intercellular interaction
  • Energy transduction
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6
Q

Compartmentalization

A

Membranes form continuous sheets that enclose intracellular compartments.

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7
Q

Scaffold for biochemical activities

A

Membranes provide a framework that organizes enzymes for effective interaction.

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8
Q

Selectively permeable barrier

A

Membranes allow regulated exchange of substances between compartments.

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9
Q

Transporting solutes

A

Membrane proteins facilitate the movement of substances between compartments.

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10
Q

Responding to external signals

A

Membrane receptors transduce signals from outside the cell in response to specific ligands.
- bind extracellular signals and react

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11
Q

Intercellular interaction

A

Membranes mediate recognition and interaction between adjacent cells.

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12
Q

Energy transduction

A

Membranes transduce photosynthetic energy, convert chemical energy to ATP, and store energy.

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13
Q

amphipathic

A

(with polar parts and non-polar
parts)
- all membrane lipids are this

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14
Q

Describe the molecular makeup of phospholipids

A

all contain phosphate group (except for glycolipids and cholesterol) – hence, they are called phospholipids
• phospholipids have a glycerol backbone and are sometimes
called phosphoglycerides

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15
Q

Name the 3 different lipids in cell membrane

A

:▪Phosphoglycerides (phospholipids) ▪Sphingolipids & Glycolipids ▪Cholesterol

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16
Q

Phospholipids

A

Glycerol backbone

  • Negatively charged phosphate group (purple or yellow) and a water soluble domain (blue).
  • Usually have one saturated and one unsaturated fatty (kinks) acid bound to a glycerol backbone

• Third glycerol OH group is coupled to a phosphate plus either:
• Phosphatidyl choline (PC) Phosphatidyl ethanolamine (PE)
• Phosphatidyl serine (PS)
Phosphatidyl inositol (PI

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17
Q

Membrane structures

A

self-organize

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18
Q

The lipid bilayer

A

– The most energetically favored orientation for the polar head groups is facing the aqueous compartments outside of the bilayer

– Hydrophobic tails interact via van der Waal’s forces, hydrogen bonding between polar heads and water

– Biological membranes also have proteins and carbohydrates attached. Protein/lipid ratios vary among membrane types (e.g. PM vs mitochondrial) and membrane sides (‘leaflets’)

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19
Q

Explain why phospholipids self-organize into liposomes

A

in a planar phospholipid bilayer the hydrophobic tails are exposed to water along the edges
formation of a sealed compartment shields hydrophobic tails from water
-> energetically favourable

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20
Q

What do phospholipids do in water?

A

Pure phospholipids will spontaneously form liposomes in water

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21
Q

a. describe how we can use liposomes to deliver drugs (and mRNA vaccines!) to cells

A
  • phosphoplipid + water in a test tube -> lipososomes
  • In lab-made liposomes, drugs or nucleic acids can be transported inside the liposome bilayer.
  • antibody proteins can help target to specific tissues.
  • PEG provides protection of the body’s immune system (stealth).
22
Q

What is the difference between a micelle and a liposome?

A

Micelles: single layer of fatty acids, no space in between them
Liposomes: double layer of phospholipids, aqueous space in middle

23
Q

Sphingolipids

A

These are sphingosine-based lipids. Sphingosine is an amino alcohol with a long hydrocarbon chain.
Ceramide = sphingosine + fatty acid (attached to N)

24
Q

Describe what roles sphingolipids play in cell membrane

A

a) membrane structure: decreasing membrane fluidity, becoming more gel-like rather than fluid (and therefore resistance to some stresses)
b) signaling to other cells – usually because of the sugar groups attached to them, allow other cell receptors to bind

25
Q

glycosphingolipids (a type of glycolipid)

A

sphingolipids with sugars attached
the ABO blood type oligosaccharides are attached to
sphingolipids!

26
Q

Glycolipids play key roles in:

A

cell membrane recognition and inter-cell communication
• electrical conduction (e.g. myelin around nerve cells)
Myelin sheath cells around nerve cells have membranes rich in glycolipids
• binding bacterial toxins (eg. botulism) and some viruses
– permitting their entry into the cell

27
Q

fluidity/viscosity

A

is a measure of freedom of movement within membranes.

28
Q

Describe the effect of the fatty acid (CH2)n chain on fluidity:

A

longer chains have less fluidity
presence of double bonds (unsaturated FA) increases “kinks” and hence increase fluidity
Sphingolipids are more rigid than phosphoglycerides (decrease fluidity)

Fluidity is dependent on types of lipids and temperature – high T, more fluid

29
Q

Describe why cells must keep membranes at a specific fluidity,

A

fluidity is important to balance mobility and organization

Biological membranes are heterogeneous, local patches of membrane may have unique fluidity,
e.g. sphingolipid-rich patches or “rafts” Certain proteins (esp. those involved in cell signaling) are preferentially found in lipid rafts

30
Q

Cells are able to ____________ to maintain_____

A

Cells are able to modify membrane chemistry to maintain correct fluidity

31
Q

outline all the ways the fluidity can be affected

A

a) remodeling enzymes for short-term response
- increased numbers of double-bonds by desaturase, alters the fatty acid chains (e.g. exposure of bacteria to cold can induce desaturase activity)
- shuffling of the fatty acid chains on phospholipids
• this is accomplished by phospholipase (cuts fatty acids from glycerol backbone), and acyltransferase (moves fatty acids between glycerol backbones)

b) over longer term, different phosphoglycerides (with more or less unsaturated FAs) are made by the cell

32
Q

Does remodelling occur in both prokaryotes and eukaryotes none or one?

A

Remodeling occurs in prokaryotes and eukaryotes, including animals, plants depending on the temperature they live at; most mammals regulate internal temperature with a few exceptions: extremities in hibernating animals often show changes in lipids

33
Q

Cholesterol

A

Animal cells need cholesterol!

Plant cells don’t have cholesterol, but they do have similar molecules called phytosterols.

34
Q

What’s the point to cholesterol of the cell?

A

• Can account for 50% of the PM of certain animal cells
• Gives membranes stability especially during changes in
temperature

35
Q

What 2 things does cholesterol do?

A

High temps can make membranes too fluid

  • cholesterol reduces this fluidity
  • cells adapt to high temp by producing more cholesterol

Low temps makes membrane more gel like
- Cholesterol prevents phospholipid tails from ‘crystallizing’, thus (counterintuitively) maintaining fluidity!!

36
Q

i

A

So cholesterol helps buffer membrane fluidity at BOTH hot and cold temps.

37
Q

Describe the experiment that demonstrated membrane fluidity

A

Take two cells, each treated antibodies that recognize either mouse or human membrane proteins. The different chemical labels (rhodamine or fluorescein) were covalently attacked to the antibodies.
Make the cells fuse together. Over time the labels mixed… therefore membrane fluidity!

38
Q

FRAP

A

Membrane fluidity can be measured through FRAP
FRAP = Fluorescence recovery after photobleaching

Label many many proteins on the membrane surface with a fluorescent marker (again, GFP will work here!) ‘Bleach’ a patch by illuminating it with a laser.
Measure how long it takes for other fluorescent proteins to diffuse into the bleached patch.

only tells us how much fluidity there is overall, and can’t trace movement of individual proteins. Single particle tracking microscopy can tell you how much an individual protein moves in a membrane

39
Q

The fluid mosaic model of membranes

A

a. lipid bilayer is core of membrane
b. lipid molecules are present in a fluid state capable of rotating and moving laterally within membrane.
c. proteins occur as “mosaic” of discontinuous particles.
d. some proteins penetrate deeply into, and even through lipid bilayer.
e. membranes are dynamic structures in which components are mobile

. f. components come together to engage in various transient interactions.

40
Q

what do we mean when we say that membranes are ‘sided’?

A

The PM is highly “sided”: each protein has specific internal/external orientation. E.g.:
– hormone receptors on outside
– signal proteins on the cytoplasmic side (G-proteins, protein kinases)

41
Q

Three classes of membrane proteins

A
  1. Integral proteins, (trans-membrane proteins), lipid soluble portion, cytosolic portion and exterior portion.
  2. peripheral proteins, inside or outside the membrane, associated by non-covalent bonds
  3. lipid-anchored proteins, inside or outside, covalently linked to a lipid.
42
Q

Integral membrane proteins

A 
penetrate into lipid bilayer
• usually pass right through membrane
• have segments protruding into
extracellular space.
• have segments protruding into
cytoplasm.
  • transmembrane segments
  • pass through lipid bilayer
  • usually consist mostly of hydrophobic (or neutral) amino acids (green dots on figure) organized in an alpha-helical conformation
43
Q

• hydrophobicity plot

A

show which amino acids are likely to be found within membrane (above red line)

44
Q

Describe the structure of integral membrane proteins

A

Many integral membrane proteins have multiple alpha-helical transmembrane regions.
These can come together to form an aqueous pore that passes through the membrane and allow the passage of ions and small molecules through them

45
Q

How do the integral membrane proteins differ in prokaryotes

A

many integral membrane proteins have ‘beta-barrel’ structures through which things can pass.’

Fun fact: Mitochondria and chloroplasts also have beta-barrel integral membrane proteins because they used to be prokaryotes

46
Q

Peripheral membrane proteins

A
  • located entirely outside of lipid bilayer.
  • either on extracellular or cytoplasmic surface.
  • associated with membrane surface by noncovalent bonds.
  • weak electrostatic bonds to hydrophilic head groups of phospholipids or hydrophilic portion of integral proteins.
  • Some peripheral proteins on cytoplasmic surface function in transmembrane signal transduction.
47
Q

Lipid-anchored membrane proteins

A
  • Protein is covalently linked to lipid molecules within bilayer.
48
Q

two types of lipid anchors

A

a. an anchor comprised of a lipid + carbohydrate chain, usually in
outer leaflet (e.g. glycosyl-phosphatidylinositol linked protein) .
The carbohydrates can be cleaved to liberate the protein so it can then go and do things.
GPI is another example of a glycolipid BTW

b. an anchor comprised only of a long hydrocarbon chain (added post- translationally), usually in inner leaflet
- both are often involved in cell signaling or adhesion

49
Q

Membrane carbohydrates

A
  • Most common in the plasma membrane - all of the membrane carbohydrates face
    outward into extracellular space.
  • covalently linked to either protein or lipid
50
Q

Glycoproteins

A
  • carbohydrate is present in short, branched oligosaccharides (<15 sugar monomers per chain)
  • may be attached to R group of:
    1. amino group of asparagine (N-linked)
    2. hydroxyl group of serine or threonine (O-linked)
  • play roles in cell interactions.

antibodies

51
Q
  1. Glycolipids
A
  • may play role in certain infectious diseases.
  • Many of them are glycosylated shpingolipids
  • carbohydrates of glycolipids determine blood type (A, B, AB
    or O)
  • antigens

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