Transcript Cell Organisation

FCP-1: Cell Biology
1st contact session: cell membranes, cytoplasmic
organelles, the cytoskeleton, intercellular
connections, cell adhesion molecules, transport
across cell membranes, ATP production
Part 1: intracellular
structures and organelles
Simplified depiction of a cell
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
Mitochondria (1)
Main function: energy production through
oxidative phosphorylation
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
Mitochondria (3)
Electron transport chain (oxidative phosphorylation,
generation of ATP/energy):
Later…
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
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)
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)
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
Ribosomes: protein assembly lines
Endoplasmic reticulum: processing
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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
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
Cytoskeleton: intracellular
highways
• Maintains structure, helps to move and change shape
• Also moves proteins and organelles around
Molecular motors to move cargo
Kinesin, dynein, myosin: all use ATP (energy)
Part 2: Intercellular
connections
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
Holding cells together:
zonula adherens
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
Attaching cells to the basal lamina:
hemidesmosomes and
focal adhesions
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)
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
Part 3: transport across
cell membranes
Exo- and endocytosis
Note that the cytoplasmic side of the membrane always
remains the cytoplasmic side
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)
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
How do molecules move across the
cell membrane?
• Some channels are gated
(opened upon a particular
stimulus):
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
Ion channels
Possible configurations:
Part 4: Energy (ATP) production
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
Main site of ATP production:
the citric acid cycle
cytoplasm
mitochondria
But before we get to this point…..
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
Or….Glycogen breakdown
Net gain from 1 mol glucose-6-phosphate: 4 ATP – 1 ATP = 3 ATP;
2 pyruvate; 2 NADH
Or…Beta-oxidation of fatty acids
- Takes place in mitochondria: long-chain fatty acids
transported in by carnitine
- 18-C fatty acid generates 8 acetyl-CoA
Main site of ATP production:
the citric acid cycle
cytoplasm
mitochondria
From NADH/FADH2 to ATP
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