FCP-1: Cell Biology

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FCP-1 Cell Biology

• 1st contact session cell membranes, cytoplasmic
  organelles, the cytoskeleton, intercellular
  connections, cell adhesion molecules, transport
  across cell membranes, ATP production

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Part 1 intracellular structures and organelles
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Simplified depiction of a cell
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Cell membrane components

• Main component phospholipids
• (hydrophilic outside, hydrophobic
• inside, spontaneous bi-layer)
• Selectively permeable
• Inner membranes have similar
• structure
• Proteins integral vs peripheral
• Modifications
• Anchors

Cell adhesion molecules, pumps, channels,
receptors, enzymes
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Mitochondria (1)
Main function energy production
through oxidative phosphorylation
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Mitochondria (2)

• Used to be free-living bacteria
• Contains the components of the electron transport
  chain
• (energy production) in the inner membrane
• Contains own genome (smaller than nucleus) and
  ribosomes (protein synthesis machinery)
• Zygote mitochondria come from the ovum maternal
  inheritance of mtDNA
• Very ineffective DNA repair leads to mistakes
  results in a large number of rare diseases
  associated with defects in energy metabolism

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Mitochondria (3)
Electron transport chain (oxidative
phosphorylation, generation of ATP/energy)
Later
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Lysosomes rubbish bins

• Large, irregular structures in the cytoplasm
• Acidic interior, digest endocytosed bacteria and
  discarded cell components
• Filled with acid hydrolases, cannot function at
  normal cellular pH, will not destroy other cell
  components
• Lysosomal storage diseases result from absence of
  enzyme, accumulation/engorgement of lysosomes

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Peroxisomes detox and more

• Catalyse various anabolic and catabolic
  reactions, e.g. breakdown of very long chain
  fatty acids, production of plasmalogen (myelin),
  production of bile acids
• Enzymes oxidize substrates, generating toxic
  H2O2, used to oxidize other substrates,
  neutralizing H2O2
• NB for the detox of ethanol
• PXR gene product is outer pxome receptor, PEX
  gene products import proteins into pxome, and
  enzymes are targeted into pxome by PTS signal
• Errors in pxome assembly result in Zellweger
  syndrome, neonatal adrenoleukodystrophy and
  infantile Refsum disease (lethal in infants)

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Nucleus command HQ

• Contains all of the DNA (nuclear genome)
  required for gene expression, in the form of
  chromatin
• Site of gene expression (DNA ? mRNA)

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Nucleus

• DNA (chromosomes) normally unravelled,
  disorganized chromatin
• Individual chromosomes condense before cell
  division
• Nucleolus contains RNA, proteins
• ribosome assembly
• Nuclear envelope a double-layer membrane
• Contains pore complexes for shuttling of
• proteins, ribosomes and RNA ribosomes and
  RNA produced in nucleus, must shuttle to
  cytoplasm for protein synthesis, some proteins
  (i.e. transcription factors) must shuttle back to
  nucleus

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Ribosomes protein assembly lines
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Endoplasmic reticulum processing

• Complex series of tubules in the cytoplasm
• Contiguous to the nuclear membrane
• Smooth ER steroid synthesis
• Rough ER covered with ribosomes,
• protein synthesis, folding and
• modification

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Golgi apparatus add some sugar

• Stacked membrane-enclosed sacs
• Proper glycosylation (sticking on
  carbohydrate/sugar chains) of lipids and proteins
• Directional (cis?trans)
• Vesicles shuttle from the ER, through the Golgi,
  out for secretion

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Cytoskeleton intracellular highways

• Maintains structure, helps to move and change
  shape
• Also moves proteins and organelles around

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Molecular motors to move cargo
Kinesin, dynein, myosin all use ATP (energy)
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Part 2 Intercellular connections
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Holding cells together Tight junctions

• Surround the outer layer of epithelial cells
  (intestinal mucosa, renal tubules, choroid plexus
  in brain)
• Also contribute to endothelial barrier function
• Totally obliterates the gap between cells,
  prevents protein
• leakage between cells

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Holding cells together zonula adherens
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Holding cells together desmosomes

• - Adhesion protein cadherin, helps to withstand
  shear stress in epithelium,
• particularly in epidermis
• - Defining feature dense plaques on cytoplasmic
  side, attached to
• cytoskeletal filaments
• Blistering diseases (Pemphigus) are auto-immune,
  attack desmogleins
• (cadherins), cause layers of skin to pull apart

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Attaching cells to the basal lamina
hemidesmosomes and focal adhesions
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Gap junctions intercellular communication

• 1 subunit connexin
• Pore with 6 connexins
• connexon
• permit passage of ions
• and small metabolites
• between cells
• highly selective (20 diff
• connexin genes, each for
• different flow-through)

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Cell adhesion molecules

• All intercellular connections consist of cell
  adhesion
• molecules (CAMs)
• 4 broad families integrins, cadherins,
  selectins and
• IgG adhesion molecules
• Not just for adhesion, but also for signalling
• cells that lose contact with other cells undergo
  
• dissociation-induced apoptosis (anoikis)
• collagen-integrin interaction essential for
  osteoblast
• differentiation

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Part 3 transport across cell membranes
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Exo- and endocytosis
Note that the cytoplasmic side of the membrane
always remains the cytoplasmic side
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Endocytosis continued

• Phagocytosis eating of bacteria, dead tissue by
  leukocytes
• Pinocytosis drinking of solutes
• Both processes involve invagination of the plasma
  membrane before pinching off vesicle inside the
  cell
• Clathrin-mediated endocytosis three-legged
  clathrin
• molecules cover endocytotic vesicle
• (NB for receptor internalization and
• synaptic function)

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How do molecules move across the cell membrane?

• Small non-polar and neutral polar molecules
  diffuse directly across (O2, N2 CO2)
• Everything else needs help!
• Transport proteins form channels for transport of
  various molecules
• Even water! (through aquaporins)
• Some are non-selective ion
• channels, some are very selective

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How do molecules move across the cell membrane?

• Some channels are gated
• (opened upon a particular
• stimulus)

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How do molecules move across the cell membrane?

• Carrier proteins transport molecules WITH a
  concentration or electrical gradient facilitated
  diffusion, does not require energy (example
  glucose)
• Other carriers transport molecules AGAINST a
  gradient active transport, requires energy
• Many carrier proteins are therefore ATPases
  hydrolyses ATP for energy for transport
• Secondary active transport transport of one
  molecule coupled to the transport of another
  (often Na)
• Symport two molecules moving in the same
  direction
• Antiport exchange of molecules in opposite
  directions

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Ion channels
Possible configurations
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Part 4 Energy (ATP) production
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ATP hydrolysis energy
ATP ? ADP Pi 30-50 kJ energy
Energetically unfavourable (unstable)
Energetically more stable
Interesting factoid 60 of energy goes towards
maintenance of body temp
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Main site of ATP production the citric acid
cycle
cytoplasm
mitochondria
But before we get to this point..
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Glycolysis (Embden-Meyerhof pathway)
1x 6-carbon
2x 3-carbon
Net gain (1 mol glucose) 4 ATP 2 ATP 2 ATP
2 pyruvate 2 NADH
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Or.Glycogen breakdown
Net gain from 1 mol glucose-6-phosphate 4 ATP
1 ATP 3 ATP 2 pyruvate 2 NADH
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OrBeta-oxidation of fatty acids

• Takes place in mitochondria long-chain fatty
  acids
• transported in by carnitine
• - 18-C fatty acid generates 8 acetyl-CoA

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Main site of ATP production the citric acid
cycle
cytoplasm
mitochondria
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From NADH/FADH2 to ATP
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ATP production adding it up

• 1 pyruvate generates 4 NADH, 1 FADH2 and 1 GTP
  (ATP)
• 1 NADH 3 ATP, 1 FADH2 2 ATP
• 1 pyruvate (4x3) (1x2) 1 15 ATP
• 1 glucose (2 ATP 2 pyruvate 2 NADH) 2
  (2x15) (2x3) 38 ATP
• 1 glucose-6-P (from glycogen) 39 ATP
• 1 18-C fatty acid 8 x 15 120 ATP
• 1 triglyceride 360 ATP