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Cell And Molecular Biology > Eukaryotic Membranes And Organelles > Flashcards

Eukaryotic Membranes And Organelles Flashcards

1
Q

Main roles of the plasma membrane

A
  1. Compartmentalisation
  2. Transport
  3. Signal transduction
  4. Intracellular junctions
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2
Q

What is the plasma membrane

A

• The plasma membrane is the boundary that separates the living cell from its surroundings.It exhibits selective permeability

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3
Q

Phospholipid bilateralne

A

• Phospholipids are the most abundant lipid in the plasma membrane
• Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions
• A phospholipid bilayer can exist as a stable boundary between two aqueous compartments

-The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it

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4
Q

Membrane proteins and functions in the cell memabrnes

A

• Mosaic–collection/clusteringof proteins
• Membraneproteinsspecifictocell type
• Receptors can often infer cell/organ function
• Key roles in cellular homeostasis

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5
Q

Examples of membrane proteins(types of them)

A

Transport
Enzyme activity
Signal transduction
Cell-cell recognition
Intercellular joining
Attachment to cytoskeleton and extra cellular matrix

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6
Q

Factors which influence membrane fluidity

A

• Fluidityaffectedbytemperature
• Steroids(cholesterol)
• Impaired fluidity – loss of
membrane function/integrity
• Influence cell/organelle function

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7
Q

Osmosis

A

-Movement of water particles from low to High concentration/water potential
-in most cells water tends to move inward

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8
Q

Types of solutions

A

• Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane
• Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water
• Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water

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9
Q

Effect of hypotonic solution

A

Lysis of the cell

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10
Q

Effect of hyper tonic solution

A

Shrivelling

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11
Q

Facilitated diffusion

A

• Most substances in the cell are too large or too polar to cross membranes by simple diffusion
• These can only move in and out of cells with the assistance of transport proteins
• This is called facilitated diffusion; the solute diffuses as dictated by its concentration gradient

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12
Q

Transport proteins

A

are large, integral membrane proteins with multiple transmembrane segments

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13
Q

Channel proteins

A

form hydrophilic channels through the membrane to provide a passage route for solutes

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14
Q

Carrier proteins

A

(transporters or permeases) bind solute molecules on one side of a membrane, undergo a conformation change, and release the solute on the other side of the membrane

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15
Q

Aquaporins

A

• Movementofwateracrosscellmembranesin some tissues is faster than expected given the polarity of the water molecule
• Aquaporin (AQP) was discovered only in 1992
• Aquaporins allow rapid passage of water through membranes of erythrocytes and kidney cells

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16
Q

Ion Channels

A

• Ion channels allow the diffusion of ions such as K+ and Na+ through a pore
• Most ion channels are ‘gated’ which means that they only allow flow following certain stimuli
• They often maintain the resting membrane potential of the cell

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17
Q

Carrier proteins

A

• Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane
• This conformational change is cause by binding or release of the solute

• Facilitated diffusion is still passive because the solute moves down its concentration gradient, and the transport requires no energy
• Some transport proteins, however, can move solutes against their concentration gradients

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18
Q

Active transport

A

• Moves substances against their concentration gradients
• Requires energy, usually in the form of ATP
• Is performed by specific proteins embedded in the membranes
• Allows cells to maintain concentration gradients that differ from their surroundings
• The sodium-potassium pump is one type of active transport system

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19
Q

Resting k+ channel(ion Channels)

A

Always open
Allows passage of potassium molecules

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20
Q

Voltage gated channel(ion channel)

A

Opens transiently in response to change in the membrane potential

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21
Q

Ligand gated channel(ion channels)

A

Opens(closes)in responses to a specific extra cellular neurotransmitter

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22
Q

Opens(closes) in response to a specific intracellular molecule

A
23
Q

Sodium potassium pump

A

Cytoplasmic Na+ binds to the sodium- potassium pump. The affinity for Na+ is high when the protein
has this shape.

Na binding stimulates phosphorylation by atp

Phosphorylation leads to a change in protein shape, reducing its affinity for Na+, which is released outside
The new shape has a high affinity for potassium,which binds to extra cellular side and triggers release of the phosphate group

24
Q

Co-transport

A

• Cotransport occurs when active transport of a solute indirectly drives transport of other substances (e.g. Sucrose-H+ co-transporter)

25
Q

Bulk transport

A

• Small molecules and water enter or leave the cell through the lipid bilayer or via transport proteins
• Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles
• Bulk transport requires energy
• The two mechanisms are exocytosis and endocytosis

26
Q

Exocytosis

A

• In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell
• A membrane bound spherical vesicle buds from the golgi apparatus and moves along a microtubule to the plasma membrane
• The membrane of the vesicle fuses with the plasma membrane, allowing the contents to spill out into the extracellular space

27
Q

Ca2+ independent exocytosis

A

• Ca2+ independent secretion happens continuously
• An example is the delivery of newly synthesized membrane proteins
• Ca2+ dependent secretion requires the influx of calcium to occur
• An example of this is insulin secretion

28
Q

Endocytosis

A

• In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane
• Endocytosis is a reversal of exocytosis, involving different proteins
• There are three types of endocytosis • Phagocytosis (“cellular eating”)
• Pinocytosis (“cellular drinking”)
• Receptor-mediated endocytosis

29
Q

Endocytosis: Phagocytosis

A

• In phagocytosis, the cell forms a ‘pseudopodia’ around an extracellular particle, surrounding it in a large vacuole.
• This vacuole will then fuse with a lysosome, forming a ‘phagolysosome’ to digest the engulfed contents
• An example is the engulfment of pathogens by immune cells such as macrophages

30
Q

Endocytosis: Pinocytosis

A

• In pinocytosis, small droplets are taken up when extracellular fluid is “gulped” into tiny vesicles
• This is a non-specific process
• Epithelial cells in capillaries use pinocytosis to engulf the liquid portion of blood (plasma) at the capillary surface. The resulting vesicles travel across the capillary cells and release their contents to surrounding tissues

31
Q

Receptor mediated endocytosis

A
  1. The ligand binds to the membrane bound receptor
  2. This signals for membrane clatharin coating and invagination
  3. The clatharin coated vesicle is formed with both ligand and receptor contained within
  4. The vesicle becomes an early endosome, the receptor is released and recycled back to the cell surface
    An example is the uptake of LDL
32
Q

What is the Endomembrane System?

A

• Compilation of membranous organelles
• Locatedwithinthecytoplasm • Structuraldivide
• Functionaldiversity
• Evolvedfrommitochondrion

33
Q

Relationships among organelles of the endomembrane system.

A

1)Nuclear envelope is connected to rough ER, which is also continous with smoothER.
2 Membranes and proteins produced by the ER flow in the form of transport vesicles to the Golgi.
3) Golgi pinches of transport vesicles and other vesicles that give rise to lysosomes, ohter types of specialized vesicles, andvacuoles.
4
5

6

34
Q

Nucleus and its role

A

-Central hub for cellular information
-Nuclear envelope –double membrane (bi-layers)
-Envelope contains perforations or pores (100nm in diameter)
-Pores contain pore complexes – regulate entry and exit of RNA and proteins
-Inner surface of the envelope has a lining – mechanical stability of nuclei structure
-DNAorganisedinto chromosomes
-Chromatin – structure for DNA coiling
-Nucleolus – rRNA synthesis – Ribosome subunit assembly and export

35
Q

Role of ribosomes

A

• Protein Factories
• ERlocalisedorfreeinthecytosol
• Synthesisedwithinthenucleolus
• Secretory cells rich in ribosomes bound to ER
• Cytosolicribosomesgeneratelocalacting enzymes
• ERboundribosomessynthesisesecreted proteins

36
Q

Ribosomes in translation

A

• The genetic code will then command the translation of each codon into an amino acid, this process is called the translation.
• It occurs in the cell cytoplasm and the resulting proteins are assembled by the ribosomes (complexes of rRNA and protein).

37
Q

What is the role of the Endoplasmic Reticulum?

A

• Biosynthetic hub
• Half of all cellular membranes are found in the ER
• Smooth/Rough ER – distinct but connected

• Smooth ER
• Lipid/steroid synthesis
• Drug detoxification – CYP450
enzymes (hydroxylation)
• Drugs induce ER proliferation
• Rich Ca2+ store

38
Q

Er function

A

• Musclecontraction
• ER – known as the sarcoplasmic
reticulum (SR) in muscle
• Rich in Ca2+ - released in response
to action potential
• Ca2+allowsactin:myosin interaction to generate force

39
Q

Rough ER

A

-RoughER
-Ribosome rich – polypeptide synthesis
-Folding of the polypeptide chain to active conformation
-Secretedproteins–Insulin– β-cells
-Folded proteins trafficked to golgi via transport vesicles
-Membraneproteinsynthesis

40
Q

Protein Folding within the ER

A

Thiol-disulphideexchange
mechanism
ER lumen is an oxidising cellular environment
Proteinfoldingrequiresthe formation of disulphide bridges (Cys-Cys)
Forms active protein structure
ER stress pathway to deal with misfolded proteins

41
Q

Golgi apparatus

A

• Cellularwarehouse–proteinsorting, shipping, modification, manufacture
Structure:
• Flattened membranous sacs – cisternae
• Cisternaeareenclosedstructures
• Up to 100 stacks of cisternae
• Golgimorenumerousandlarger in cells with a high secretory function

Role:
-Golgi receives folded proteins from the ER in transport vesicles
-Enter via cis region
- Modification in transit to the
trans region of the Golgi
- Carbohydrate/phospholipid modification
-Cargo packaged for transport
-Polysaccharide synthesisinthe golgi – (pectin in plants)

42
Q

Vesicular transport from the Golgi

A

Constitutive secretory vesicles= Does not require stimuli – e.g. antibodies (activated B Cell)

Regulated secretory vesicles= Requires stimuli – e.g. cytokines/neurotransmitters/insulin

Lysosomal vesicles=
Enzymes involved in proteolytic degradation pathways

43
Q

What is the role of Lysosomes?

A

• A digestive hub of the cell Nucleus
• Specialised acidic environment for hydrolytic enzymes
• RoleinPhagocytosisand Autophagy
• Macrophagesingestvirusesparticles, bacteria, or apoptotic (dying) cells
►
• Phagocyticvesiclesthenfusewith lysosomes and digestion take place

44
Q

Autophagy

A

• Cellsurvivalmechanism–recycle damaged or dysfunctional cellular components
• Upper (TEM) shows a vesicle containing two disabled organelles (a mitochondrion and a peroxisome*)
*peroxisomes contain toxic peroxides
• Lower diagram shows such a vesicle fusing with a lysosome and digestion taking place

45
Q

Mitochondria

A
  • Powerhouse of the cell
  • ATP generation via oxidative phosphorylation
    -Site of Kreb’s Cycle
    -mtDNA & ribosome rich
    -Cell fate decisions (apoptosis)
46
Q

Mitochondrial inheritance

A

-Mutations in the mitochondrial DNA (mtDNA) that affect mitochondria function.
-Unique characteristics due to maternal inheritance
mitochondria are critical to cell function.
-The subclass of these diseases that have neuromuscular disease symptoms are often called a mitochondrial myopathy.

47
Q

Mitochondrial structure

A

• Two membranes
• smooth outer membrane
• highly folded inner membrane (cristae)
• Internal fluid-filled space
• mitochondrial matrix
• DNA, ribosomes & enzymes

48
Q

Mitochondrial Reticula

A

• Mitochondria form a ribbon-like network or reticula
• Join together and break apart in relation to a variety of stimuli
• Dynamic – movement – cytoskeleton

49
Q

Dynamics of mitochondria

A

• Mitochondria are not static organelles
• Responsive to both inter- / intracellular stimuli
• Constant state of flux
• Receives significant communication from ER

50
Q

Mitochondria – Energy generation

A

• During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis
• Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food
• These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation

51
Q

ATP Production – Cellular Respiration

A

• During cellular respiration, most energy flows in this sequence:
glucose → NADH → electron transport chain → proton-motive force → ATP
• About 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 38 ATP

52
Q

Citric cycle

A

• The citric acid cycle completes the energy-yielding oxidation of organic molecules
• In the presence of O2, pyruvate enters the mitochondrion
• Before the citric acid cycle can begin, pyruvate must be converted to acetyl CoA, which links the cycle to glycolysis

53
Q

Chemiosmosis

A

• Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space
• H+ then moves back across the membrane, passing through channels in ATP synthase
• ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP
• This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work

Cell And Molecular Biology (20 decks)

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