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FCP-1: Cell Biology 1 st contact session: cell membranes, cytoplasmic organelles, the cytoskeleton, intercellular connections, cell adhesion molecules,

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Presentation on theme: "FCP-1: Cell Biology 1 st contact session: cell membranes, cytoplasmic organelles, the cytoskeleton, intercellular connections, cell adhesion molecules,"— Presentation transcript:

1 FCP-1: Cell Biology 1 st contact session: cell membranes, cytoplasmic organelles, the cytoskeleton, intercellular connections, cell adhesion molecules, transport across cell membranes, ATP production

2 Part 1: intracellular structures and organelles

3 Simplified depiction of a cell

4 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

5 Mitochondria (1) Main function: energy production through oxidative phosphorylation

6 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

7 Mitochondria (3) Electron transport chain (oxidative phosphorylation, generation of ATP/energy): Later…

8 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

9 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 H 2 O 2, used to oxidize other substrates, neutralizing H 2 O 2 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)

10 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)

11 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

12 Ribosomes: protein assembly lines

13 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

14 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

15 Cytoskeleton: intracellular highways Maintains structure, helps to move and change shape Also moves proteins and organelles around

16 Molecular motors to move cargo Kinesin, dynein, myosin: all use ATP (energy)

17 Part 2: Intercellular connections

18 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

19 Holding cells together: zonula adherens

20 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

21 Attaching cells to the basal lamina: hemidesmosomes and focal adhesions

22 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)

23 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

24 Part 3: transport across cell membranes

25 Exo- and endocytosis Note that the cytoplasmic side of the membrane always remains the cytoplasmic side

26 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)

27 How do molecules move across the cell membrane? Small non-polar and neutral polar molecules diffuse directly across (O 2, N 2 CO 2 ) 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

28 How do molecules move across the cell membrane? Some channels are gated (opened upon a particular stimulus):

29 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

30 Ion channels Possible configurations:

31 Part 4: Energy (ATP) production

32 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

33 Main site of ATP production: the citric acid cycle cytoplasm mitochondria But before we get to this point…..

34 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

35 Or….Glycogen breakdown Net gain from 1 mol glucose-6-phosphate: 4 ATP – 1 ATP = 3 ATP; 2 pyruvate; 2 NADH

36 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

37 Main site of ATP production: the citric acid cycle cytoplasm mitochondria

38 From NADH/FADH 2 to ATP

39 ATP production: adding it up 1 pyruvate generates 4 NADH, 1 FADH 2 and 1 GTP (ATP) 1 NADH = 3 ATP, 1 FADH 2 = 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

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