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Cytology > Plasma membrane & Junctions > Flashcards

Plasma membrane & Junctions Flashcards

1
Q

3 main functions of plasma membrane

A
  1. substance import/export (acts as a selective barrier)
  2. compartmentalisation
  3. allows for movements (eg. pseudopods)
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2
Q

plasma membrane components

A
  1. phospholipid bilayer
  2. cholesterol
  3. proteins
  4. glycolipids/proteins
  5. cytoskeleton connections on intracellular compartment
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3
Q

structure of phospholipid

A

-phosphate polar head (hydrophilic): heads can either contain serine, inositol or choline alcohol residues which impact membrane fluidity and properties

-2 hydrocarbon lipid tails of different lengths and saturation that are non polar and hydrophobic

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4
Q

different families of phospholipids

A
  1. Phosphoglycerides: most abundant (in all cell membranes) and have glycerol backbone
  2. Sphingolipids have an apolar and polar side and their backbone is made of the amino-alcohol sphingosine
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5
Q

what arrangement do phospholipids take when exposed to water and why

A

Spontaneous closure into either a bilayer, lysosome or micelle. Aggregation to arrange hydrophobic tails internally and hydrophilic heads externally –> avoids exposure of lipid tails to water which is energetically favourable

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6
Q

thickness of plasma membrane and thickness of internal membrane (organelles)

A

plasma membrane: 7.5 nm

internal compartment membrane: 6nm

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7
Q

Evolution of theories regarding plasma membrane

A
  1. Charles Overton (1890s): stated that cells possess a lipid ‘coat’ on the outside
  2. Gorter-Grendel (1925): observed that phospholipids have a bilayer arrangement
  3. Davson-Danielli (1935): ‘sandwich model’ where proteins covered inner and outer environment (only surface, no integral proteins)
  4. Singer-Nicolson (1972): fluid mosaic model where proteins laterally move in bilayer and can be integral
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8
Q

Factors affecting the fluidity of plasma membrane (5)

A
  • temperature (+)
  • length of fatty acid tails (-)
  • abundance of integral proteins (-)
  • cholesterol (+/- depending on fatty acid tail chemistry)
  • % of unsaturated fatty acid tails (kinks) (+)
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9
Q

How does the presence of cholesterol affect the plasma membrane

A

ROLE: interacts with fatty acid tails to influence fluidity (either positively or negatively)

  1. for SATURATED fatty acid tails (no double bonds): increase fluidity by preventing fatty acids from packing too close together.
  2. for UNSATURATED fatty acid tails (double bonds): decrease fluidity by filling in gaps in membrane caused by kinks, anchoring the tails.
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10
Q

How is movement of phospholipids within membrane enabled?

A

MOVEMENT VIA DIFFUSION:
-lateral drift in the plane of membrane

-RARE: switching of phospholipids between inner/outer leaflets due to flip/flopase enzyme activity (eg. important in apoptosis)

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11
Q

3 main properties of the plasma membrane

A
  1. fluidity (lateral movement of proteins and phospholipids is allowed)
  2. discontinuity (structure is interrupted by integral proteins)
  3. asymmetry (disequilibrium of inner and outer leaflet)
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12
Q

2 factors maintaining membrane asymmetry

A
  1. TRANSVERSE asymmetry: structural differences through thickness of membrane
    - PC (choline phospholipids) mainly on outer leaflet
    -PS (serine phospholipids) mainly inner leaflet
    -Glycolipids exclusively on outer layer
    -Carbs only attached to outer membrane proteins
    -glycocalyx only on outer layer
  2. REGIONAL asymmetry: specialisation of membrane at different sites (cell polarity) - eg, different morphologies of lateral/apical/basal cell surfaces
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13
Q

what is the freeze fracture technique?

A

-allows visualisation of plasma membrane by splitting it into 2 layers to observe proteins that are completely embedded
PROCESS:
-tissue is frozen and cut along the hydrophobic plane in the middle of the bilayer
-creation of E-face (backed by extracellular portion) and P-face (backed by cytoplasmic/protoplasm portion)
-TEM used to visualise each face

!!! usually P face contains more proteins/particles than E face

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14
Q

2 protein location types on plasma membrane

A
  1. integral proteins: found within bilayer
  2. extrinsic proteins: non covalently associated with either inner or outer leaflet of membrane
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15
Q

6 types of membrane proteins and basic functions

A
  1. pumps: active ion transport
  2. channels: passive diffusion along conc grad
  3. receptor proteins: localisation of ligands for cell signalling pathways
  4. linker proteins: anchor extracellular matrix with the intracellular cytoskeleton
  5. enzymes: ATPase and digestive enzymes
  6. structural proteins: form junctions with surrounding cells
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16
Q

Lipid raft definition

A

Microdomains of the plasma membrane which are dynamic (control movement and distribution of proteins in bilayer), and enriched with sterol and sphingolipid domains.

Due to presence of cholesterol/highly saturated fatty acid tails these regions show less fluidity and are thicker

17
Q

function of lipid rafts

A

mainly caveolin and flotillin proteins: recruit signalling molecules so that they serve as centers for assembly of signalling receptors (GPCRs, RTKs)

18
Q

2 types of lipid rafts

A
  1. Planar lipid rafts: found in neurons, run in line with the plane of the membrane and are enriched in the protein flotillin (scaffolding protein)
  2. Caveolae: flask shaped invaginations of membrane containing caveolin proteins (prominent in smooth muscle cells)
19
Q

Relevance of lipid rafts in infection

A

Lipid rafts are often the location of initial contact between microorganism and cell:
-bacteria hijack rafts and attach their own receptors
-use of rafts to avoid phagocytosis or degradation via lysosome
-use of raft receptors to create vehicles transporting bacteria into cell without their breakdown

20
Q

Glycocalyx structure and function

A

-STRUCTURE: carbohydrate coat (containing some proteins) covering outer leaflet (not present on inner membrane)

-FUNCTION: filtering material, regulating close cell interactions, involved in cell adhesion mechanism, contains enzymes and antigens

21
Q

Types of transport across membrane

A

Membrane possesses selective permeability depending on type of substance:

  1. SIMPLE DIFFUSION: small/non polar molecules
  2. FACILITATED DIFFUSION: small polar molecules either via carrier or channel proteins
  3. ACTIVE TRANSPORT: ions and larger charged molecules via pumps (active carrier proteins) using ENERGY
  4. BULK TRANSPORT: transport of hormones/ steroids via vesicular transfer: endocytosis and exocytosis using ENERGY
22
Q

components and function of endomembrane system

A

CONTAINS: RER/SER/lysosomes and vesicles /nuclear envelope/Golgi apparatus

FUNCTION: system of lipoprotein membranes that form intracellular packaging and transport network

23
Q

Types of junctions (5)

A

OCCLUDING: tight

ANCHORING: adherens, desmosome, hemidesmosome

COMMUNICATING: gap

24
Q

structure and function of tight junctions

A

-point to point fusion of adjacent cells for localised sealing
1. acts as a diffusional blocker: materials can’t pass from lumen to extracellular space
2. maintains cell polarity: blocks movement of apical specific structures so that they are not lost to the lateral surface

STRUCTURE:
-claudin, occludins and tricellulin proteins joining in intercellular space
- connection to zona occludins attached to actin filaments on the intracellular portion of membranes
-intracellular connection to ZO1/2/3 (Zonula Occludens 1/2/3)
-connection to JAM (junction adhesion molecules) acting as signalling molecules

!!! found in blood-brain barrier and GI tract, Sertoli cells for blood-tests barrier in seminiferous tubule

25
Q

structure and function of adherens junctions

A

-provide adhesion between adjacent cells to provide resistance to shearing/abrasive forces

STRUCTURE:
-E cadherins joined with Ca2+ in intercellular space
-connection to beta and alpha catenins on plasma membrane
-connection to vinculin proteins that join to actin filaments

!! Found mainly on the same tissues as tight junctions (but are found slightly more basally)

26
Q

pathology resulting from malfunctioning adherens junctions

A

carcinomas occur with the down regulation of e-cadherins

27
Q

structure and function of desmosomes

A

-similar to adherens junctions but STRONGER resistance to shearing forces, and tensile strength is provided

STRUCTURE:
-interlocking cadherin proteins in intercellular space
2 types: desmoglein (1-4) and desmocollin (1-3)
-Ca2+ keeping proteins interlocking
-connection to plaque proteins (desmoplakin and plakoglobins) which link to intermediate filaments containing keratin

!!! prominent in cardiac myocytes (intercalated discs), oral mucosa layer

28
Q

structure and function of hemidesmosomes

A

-connect cell membrane to basal lamina (form basement membrane)

STRUCTURE:
-integrin proteins connecting cell and lamina
-connect to desmoplakin proteins
-link to intercellular intermediate filaments like keratin

!! prominent in the skin/epithelial tissue

29
Q

structure and function of gap junctions

A

-used for cell to cell communication by allowing passage of ions/small molecules through adjacent cells
-this is important in homeostatic control of cell proliferation: apoptosis

STRUCTURE:
-made of 2 connexons linked together between membranes
-each made up of 6 connexins

!! prominent in cardiomyocytes (intercalated discs), electrical synapses of neurons, epithelial cells

30
Q

Pathologies arising from malfunctioning gap junctions

A

Connexins: tumour suppressors

transformed cancer cells usually show mutations in connexins/connexons which breaks gap junctions and affects communication between cells
-throws cell proliferation: apoptotic ratio in disequilibrium –> tumorous growth

31
Q

How are channels of gap junctions regulated

A

certain substances induce opening or closing od channels:

  1. Ca2+ binding opens channels
  2. phosphorylation of connexins (closed when phosphorylated)
32
Q

Types of active transport

A
  1. symport systems: two substances moved across a membrane in the same direction
  2. antiport systems: two substances are moved across a membrane in opposite directions

!! all need ATP

33
Q

primary vs secondary active transport definition

A

PRIMARY: uses energy in the form of ATPdirectly to move solutes against conc gradient

SECONDARY (cotransport): does NOT utilize ATP directly. Instead, it uses one ion going down its concentration gradient to drive the movement of another ion going against its concentration gradient.

Cytology (8 decks)

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