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1 Cell and Molecular Biology Behrouz Mahmoudi Cell Organelles-1 1

2 Eukaryotic cells contain well defined cellular organelles such as:  Nucleus  Mitochondria  Endoplasmic reticulum  Golgi apparatus  Peroxisomes  lysosomes 2

3 MITOCHONDRIA Mitochondria has got an inner membrane and an outer membrane. The space between these two is called intermembranous space. Inner membrane convolutes into cristae and this increases its surface area. Both the membranes have different appearance and biochemical functions: 3

4 Inner membrane: It surrounds the matrix. It contains components of electron transport system. It is impermeable to most ions including H, Na, ATP, GTP, CTP etc and to large molecules. For the transport special carriers are present e.g. adenine nucleotide carrier(ATP –ADP transport). Complex II i.e. Succinate dehydrogenase. Complex V i.e. ATP synthase complex. Outer membrane: It is permeable to most ions and molecules which can move from the cytosol to inter membranous space. 4

5 Three Stages of Aerobic Cellular Respiration: 1-Glycolysis in cytosol 2-Kreb’s cycle in mitochondrial matrix 3-Electron Transport Chain at inner membrane of mitochondria C 6 H 12 0 6 + 6O 2  6CO 2 +6H 2 0 + ENERGY 5

6 Glycolysis 6

7 Krebs Cycle Each pyruvate (carbohydrate) molecule is completely oxidized into carbon dioxide Energy released from these reactions results in the formation of 1 ATP molecule but 3 NADH molecules 7

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9 Electron Transport Chain During the electron transport chain, H + is moved against a gradient. The energy needed to do this is supplied by electrons carried by NADH 9

10 What happens along the inner membrane of the mitochondria? The loss of electrons from NADH result in the addition of energy to protein pumps in the memebrane resulting in a H + being moved from the inside to the outside of the inner membrane This happens many times creating an imbalance (gradient) of H +. Oxygen pulls electrons to keep them moving. 10

11 What happens along the inner membrane of the mitochondria? ATP is made as H+ ions are allowed back into the matrix of the mitochondria by a different protein (ATP synthase). The energy released by the “rush” of H + is used by this enzyme to make ATP (kind of like a rush of water in a stream being used to turn a water wheel). Ultimately, aerobic respiration produces ~36 ATP molecules from each individual glucose molecule. 11

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13 ENDOPLASMIC RETICULUM Cytoplasm of eukaryotic cells contain a network of interconnecting membranes. This extensive structure is called endoplasmic reticulum. It consists of membranes with smooth appearance in some areas and rough appearance in some areas- Smooth endoplasmic reticulum and rough endoplasmic reticulum. 13

14 Rough Endoplasmic Reticulum These membranes enclose a lumen. In this lumen newly synthesized proteins are modified. Rough appearance is due to the presence of ribosomes attached on its cytosolic side(outer side). These ribosomes are involved in the biosynthesis of proteins. 14

15 These proteins are either incorporated into the membranes or into the organelles. Special proteins are present that are called CHAPERONES. Theses proteins play a role in proper folding of proteins. Protein glycosylation also occurs in ER i.e. the carbohydrates are attached to the newly synthesized proteins. 15

16 Smooth Endoplasmic Reticulum Smooth endoplasmic reticulum is involved in lipid synthesis. Cholesterol synthesis Steroid hormones synthesis. Detoxification of endogenous and exogenous substances. The enzyme system involved in detoxification is called Microsomal Cytochrome P450 monooxygenase system(xenobiotic metabolism). 16

17 ER along with Golgi apparatus is involved in the synthesis of other organelles –lysosomes & Peroxisomes. Elongation of fatty acids e.g. Palmitic acid 16 C- Stearic acid 18 C. Desaturation of fatty acids. Omega oxidation of fatty acids. 17

18 A signal sequence of approximately 20 amino acids and rich with hydrophobic amino acids is often located at the N-terminus. Since the ribosome masks about 30 amino acids, the signal sequence isn’t fully exposed until the nascent polypeptide is about 50 amino acids long. SRP-ribosome attaches to SRP receptor and then docks on a protein translocator. SRP and receptor dissociate. Translation and translocation proceed in unison - co-translational transport. The energy for transport is provided by the translation process - as the polypeptide grows, it is pushed through the protein translocator. SRP: signal-recognition particle SRP receptor 18

19 The signal sequence of secreted proteins is cleaved by a signal peptidase. In the literature, the signal sequence of secreted proteins is often called a “leader peptide”. Translocation of protein across the ER membrane 19

20 A single-pass membrane protein In this case, a start transfer sequence followed by a stop transfer sequence yields a 2-pass transmembrane protein with the N and C-terminus in the cytosol. 20

21 N-linked glycosylation of ER proteins. Most of the glycosylation associated with proteins outside the cell begins in the ER. A large preformed oligosaccharide is transferred from a lipid called dolichol to the side-chain of asparagine. Because the covalent linkage is to the nitrogen of the asparagine side-chain, this is called N-linked glycosylation. In the ER, N-linked oligosaccharides are modified and the modifications are used as signals to distinguish properly folded from unfolded proteins. ABO blood type is determined by two glycosyltransferases 21

22 Calnexin is a chaperone, characterized by assisting protein folding and quality control, ensuring that only properly folded and assembled proteins proceed 22

23 If a protein doesn’t achieve a properly folded state, it gets exported from the ER into the cytosol where it is degraded by the proteasome. 23

24 Phospholipids are synthesized in the cytoplasmic leaflet of the ER. 24

25 Phospholipid translocators flip-flop the phospholipids. 25

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