Biochemistry Lecture 1.0...
1.01 JUNE 14, 2013
Cell and Cell Membrane: A Biochemical Approach Allan L. Hilario, M.D.
Key Concept: Organisms can be classified according to their nutritional pattern – their energy source and source of carbon. Thus, there is metabolic diversity among organisms.
Outline I. Introduction Cell Phylogeny Classification Types of Cells (Comparisons): o Prokaryotes vs. Eukaryotes o Animal vs. Plant Eukaryotic cells Method of Cell Organelle Identification II. The Prokaryotic Cell Bacteria Structure Components Biochemical Reactions o Metabolism o Metabolic terms, functions and locations III. The Eukaryotic Cell Cytoplasm o Cytosol o Organelles Cytoskeleton Microfilaments Intermediate filaments Microtubules/Tubulin Nucleus Mitochondria Endoplastic Reticulum Smooth ER Rough ER Golgi Apparatus Vesicular Organelles Lysosomes Peroxisomes
IV. The Cell Membrane Structure: Fluid Mosaic Model Functions Composition Membrane Fluidity o Lipid movement o Self-sealing o Selective permeability Membrane Proteins o Integral proteins o Peripheral proteins Membrane Microdomains o Lipid raft o Caveola Cell Membrane Transport o Passive transport Simple diffusion Facilitated diffusion - Transporters: Transporters vs. Channels Ionotropic channels vs. Ionophores o Active transport Primary Transport Secondary Transport o Diseases Involving Defects in Membranes Artificial Membranes (Liposomes) V. Review Questions VI. Answer to Review Questions
Classification According to Nutritional Patterns
Nutritional Classification of Organisms: a. Phototrophs- energy from light Two kinds of phototrophs: 1. Autotrophs- carbon from carbon dioxide Ex. cyanobacteria, plants 2. Heterotrophs- carbon from organic compounds Ex. purple and green bacteria b.
Chemotrophs- energy from chemical compounds 1. Lithotrophs- energy from inorganic compounds Ex. sulfur bacteria, hydrogen bacteria 2. Organotrophs- energy from organic compounds Ex. most prokaryotes, all nonphototrophic eukaryotes *Note: Heterotrophs can also be chemotrophs.Lithotrophs and Organotrophs are also examples of heterotrophs.
Types of Cells A. Comparison of Prokaryotes and Eukaryotes Key Concept: In general, prokaryotes are structurally simpler and smaller than eukaryotes. Eukaryotes on the other hand are structurally larger and more complex than prokaryotes.
Cell Phylogeny Key Concept: Grouping organisms according to common properties implies that a group of organisms evolved from a common ancestor.Based on the similarities in ribosomal RNA, living organisms are classified into 3 domains.
Three Domains of Life: 1. Archaebacteria- prokaryotes that do not contain peptidoglycan in their cell wall. E.g.extreme halophiles, methanogens and extreme thermophiles 2. Eubacteria- prokaryotes with peptidoglycan in their cell wall E.g. purple bacteria,cyanobacteria, flavobacteria, gram positive and gram negative bacteria, and thermotoga 3. Eukaryotes- consist the animals, ciliates, fungi, plants, flagellates and microsporidia
Fig. 1.1 Phylogenic Tree of the Three Domains of Life
Fig1.2. Comparisons of Prokaryotes and Eukaryotes B. Comparison of Eukaryotic Animal and Plant Cells Key Concept: There are unique features that distinguish plant cells from animal cells. Animal Cell Plant Cell Lysosomes Cell Wall Mitochondria Glyoxysome Centrosome Plasmodesma Vacuole (smaller) Vacuole(large) Thylakoids Chloroplast Starch Granule Plastids
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Cell and Cell Membrane: A Biochemical Approach
Fig. 1.3. Isolation of cell organelles The large and small particles in the suspension can be separated by centrifugation at different speed Particles of different density can be separated by isopycnic centrifugation
THE PROKARYOTIC CELL
Metabolism – the entire set of enzyme-catalyzed transformations of organic molecules of living cells. It is the sum of anabolism and catabolism. Anabolism – the phase of intermediary metabolism concerned with the energy-requiring biosynthesis of cell components from smaller precursors Catabolism – the phase of intermediary metabolism concerned with the energy – yielding degradation of nutrient molecules Metabolic Terms Glycolysis -
Cellular Location Cytoplasm of cells
Cytoplasm of hepatocytes, renal cells and in special condition the intestinal cells
Hexose Monophosphate Shunt Glycogenesis
Cytoplasm of all cell Cytoplasm of hepatocytes and muscles
Tricarboxylic Acid Cycle or TCA or Kreb’s Cycle Oxidative phosphorylation and the electron transport chain Lipolysis (BetaOxidation) Lipogenesis
Mitochondria in the cytoplasm
The plasma membrane and the layers outside it constitute the cell envelope The nucleoid contains a single, circular molecule of DNA, and the cytoplasm contains one or smaller, circular segments of DNA called plasmids
Mitochondria Cytoplasm and SER of hepatocytes and adipocytes
Alpha and BetaOxidation Omegaoxidation
Nucleus, most rRNA in the nucleoulus Rough endoplasmic Reticulum and cytoplasm
Mitochondria in the cytoplasm
Smooth Endoplasmic Reticulum and Golgi apparatus
Protein Sorting Ketogenesis
Golgi apparatus Matrix of the mitochondia of hepatocytes, and in special condition, renal cells
Mitochondria in the cytoplasmof the hepatocytes
Amino Acid Synthesis
Cytoplasm of all nucleated cells
Amino Acid Catabolism
Cytoplasm of all cells
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BIOCHEMISTRY THE EUKARYOTIC CELL -
Cytoplasm portion of the cell enclosed by the cell membrane and outside the nucleus where the cellular organelles are found. Most of the biochemical reactions happens in the cytoplasm: Glycolysis Gluconeogenesis Fatty acid synthesis Protein synthesis Part of urea cycle Purine metabolism Amino acid synthesis
a. b. c. d. e. f. g.
Cell and Cell Membrane: A Biochemical Approach As the ionic strength increases, G actin aggregates reversibly to form F actin, a helical homopolymer. G actincarries a firmly bound ATP molecule that is slowly hydrolyzed in F actin to form ADP. Actin therefore also has an enzyme property (ATPase activity). Myosin is the globular protein that interacts with actin in myocytes for muscle contraction. b. -
MAJOR PARTS OF THE CYTOPLASM: Cytosol fluid portion of the cytoplasm Biochemical reactions in the cytosol: NADPH production (pentose phosphate pathway; malic enzyme) [NADPH]/[NADP+] high Isoprenoid and sterol synthesis Fatty acid synthesis
o a. b. c. d. o -
Organelles tiny structures that perform different functions in the cell. specialized structures within the cell that have characteristic shapes they perform specific functions in cellular growth, maintenance, and reproduction. linked together by cytoskeleton of the cell.
Cytoskeleton network of protein scafolding which mainatains cell’s shape and serves as tracts for organellar movement. assembles as polymers from protein subunits. 3 types of protein filaments: Microfilaments (6–8 nm) Actin is the protein component of the microfilaments. 2 forms of actin: G-actin (globular) Monomolecular form asymmetrical molecule with a mass of 42 kDa, consisting of two domains. F-actin (filamentous) Polymer form
Intermediate filaments (10 nm) belongs to five related protein family with cell type-specificity. These includes: 1) Cytokeratins (ectoderm) 2) Desmin (ectoderm) 3) Vimentin (mesoderm) 4) Glial fibrillary acidic protein (GFAP) (endoderm) 5) Neurofilament (endoderm) These proteins have a rod-shape structure at the center called super helix. The intermediate filament is produced from 8 protofilaments. Microtubules (25 nm) Basic components: α– and β–tubulin (53 and 55 kDa). Thirteen protofilaments form a ringshape and polymerize to form a long tube. Important during mitosis.
Plant alkaloids: Vinblastine- vinca alkaloid; acts in G & S phases by inhibiting microtubule formation, inhibits DNA/RNA synthesis; antineoplastic Vincristine- vinca alkaloid; acts in M & S phases by inhibiting microtubule formation, inhibits DNA/RNA synthesis; antineoplastic. Colchicine- disruption of cytoskeletal functions through inhibition of β-tubulin polymerization into microtubules which prevents activation, degranulation, and migration of neutrophils thought to mediate some gout symptoms; antigout(acute gout). Paclitaxel (Taxol)- natural taxane, prevents depolymerization of cellular microtubules, which results in DNA, RNA, and protein synthesis inhibition’ antineoplastic
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Nucleus largest organelle of the eukaryotic cell. repository and cellular localization of storage, replication and expression of genetic information. contains almost 99% of the cell DNA except for the 1% present in the mitochondria. Biochemical reactions inside the nucleus: Replication (replication also occurrs in the mitochondria) Synthesis of rRNA by the nucleolus (dense part of the nucleus). Transcription of the tRNA, mRNA, and other types of RNA (occurs in the euchromatin part of the nucleoplasm). Biosynthesis of NAD+
a. b. c.
Cell and Cell Membrane: A Biochemical Approach Rough endoplasmic reticulum (rER) -coated with ribosomes Function: site of protein synthesis Smooth endoplasmic reticulum (sER) -without ribosomes Functions: site of carbohydrate and lipid synthesis helps to detoxify certain compounds
Transport from the ER through the Golgi apparatus -receives (on the cis-side) many of the transport vesicles produced in the rough ER -consists of flattened membranous sacs called the cisternae -exports many substances (from trans-side) in transport vesicles - compose the 1% DNA of the cell - evolved from the “Endosymbiosis Theory” Function: Production of energy (ATP) Some processes occurring within the mitochondria: Electron transport chain and oxidative phosphorylation Urea cycle Tricarboxylic acid cycle/Citric acid cycle/Kreb's cycle ß oxidation in animal cells ketogenesis Endosymbiosis Theory Key concept: The endosymbiosis theory explains the origin of mitochondria and chloroplasts and their double membranes. This concept postulates that chloroplasts and mitochondria are result of years of evolution initiated by the endocytosis of an aerobic bacteria and blue-green algae. With this theory, an accepted mechanism on how eukaryotic cells evolved from prokaryotic cells is explained.
rERcis-Golgi network cell exterior
Function: modification, sorting, and packaging of proteins and other materials
Vesicular organelles Lysosomes -common in animal cells but rare in plant cells Functions: contain hydrolytic enzymes necessary for intracellular digestion get rid virus and bacteria, digest food particles and other damaged organelles Peroxisomes Functions: involve in the breakdown of very long chain fatty acids contain the enzyme catalase which decomposes the toxic hydrogen peroxide Secretory vesicles Function: transport substances to the cell surface for release
THE CELL MEMBRANE Key Concept: The central architectural feature of biological membranes is a double layer of lipids, which acts as barrier to the passage of polar molecules and ions.
Endoplasmic Reticulum network of tubules and flattened sacs that serve a variety of functions in the cell
Cell Membrane Fluid Mosaic Model (Singer and Nicolson) – according to this model, the molecular arrangement of plasma membrane resembles a continually moving sea of fluid lipids that contains a mosaic of many different proteins.
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Cell and Cell Membrane: A Biochemical Approach group at the aqueous interface and the remainder of the molecule within the leaflet. Note: Cholesterol is distributed in both leaflets of the lipid bilayer. B. Proteins 1. Integral proteins 2. Peripheral proteins C. Carbohydrates 1. Glycolipids 2. Glycoproteins
Cell Membrane Composition The basic structural framework of the plasma membrane is the lipid bilayer.
Cell Membrane Functions 1. Boundary 2. Controlled metabolite transport 3. Signal reception and transmission 4. Enzymatic reactions 5. Contact with other cells 6. Anchor for cytoskeleton
A. Lipid bilayer 1. Phospholipids – lipids with phosphate groups a. Phosphoglycerides – most common phospholipid; consists Cell Membrane fluidity of glycerol backbone to which are attached two fatty acids in ester linkage and a phosphorylated alcohol (e.g. ethanolamine, Membrane Fluidity refers to the viscosity of the lipid bilayer or the degree of resistance of membrane components to move. choline, serine, glycerol or inositol) Several factors affect the fluidity of the membrane. • •
b. Sphingomyelin – contains a sphingosine backbone instead of glycerol: Ceramide = sphingosine + fatty acid
Glycosphingolipids (Glycolipids) – sugar containing lipids built on a backbone of ceramide a. Cerebrosides b. Gangliosides Note: The outer leaflet consists predominantly of phosphatidylcholine, sphingomyelin, and glycolipids, whereas the inner leaflet contains phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol.
Sterols – steroidal alcohols The most common of which is cholesterol, it intercalates among the phospholipids of the membrane, with its hydroxyl
Temperature (Higher temp., more fluid) Content of unsaturated FA (higher content, more fluid Double bonds produce kink structures. These structures are hard to pack, making disorganization in the membrane. Content of cholesterol (paradoxical- when cholesterol interacts with unsaturated/ short-chain fatty acyl PL results to less fluidity while when cholesterol interacts with sphingolipids and long-chain fatty acyl PL tends to make it more fluid. Cholesterol and temperature: when amphipatic cholesterol is placed in a very fluid membrane because of high temperature, it can penetrate deep into the membrane. low temperature makes the membrane less fluid, cholesterol can’t penetrate into the first layer of the membrane thus making it relatively more fluid. [emphasized in the lecture] “Cholesterol inserts into bilayer membranes with its hydroxyl group oriented toward the aqueous phase and its hydrophobic ring system adjacent to fatty acid tails of phospholipids. The hydroxyl group of cholesterol forms hydrogen bonds with polar phospholipid head groups”.[1D trans, 2012] “Part of the steroid ring (the four hydrocarbon rings in between the hydroxyl group and the hydrocarbon "tail") is closely attracted to part of the fatty acid chain on the nearest phospholipid. o This helps slightly immobilize the outer surface of the membrane and make it less soluble to very small water-soluble molecules that could otherwise pass through more easily. o Without cholesterol, cell membranes would be too fluid, not firm enough, and too permeable to some molecules.In other words, it keeps the membranefrom turning to mush.”[1D trans 2012]
Cell Membrane Lipid Membrane Movement Membrane Fluidity allows various components of the membrane to move in different directions. The lipid component may exhibit the following movement:
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Cell and Cell Membrane: A Biochemical Approach Scramblases are proteins that move any membrane phospholipid across the bilayer down its concentration gradient [from leaflet that has higher concentration to the leaflet with lower concentration] activity is NOT ATP-dependent. Cell Membrane Self-sealing The lipid bilayer of the cell membrane has the ability to reseal after a small disruption through lateral diffusion of its lipid component. Larger tear due to mechanical stress is an energy-requiring process through Ca2+- dependent process similar to exocytosis-like process. [emphasized in the lecture] Cell Membrane Selective Permeability
Figure 2.1. Uncatalyzed lateral 6iffusion
The hydrophobic hydrocarbon chains in lipid bilayer provide an impervious barrier for ionic and polar substances. Specific proteins regulate the passage of these substances in and out of the cell. Aquaporins for water. Polar substances should shed its hydration shell in order to pass the hydrophobic barrier.
Figure 2.2 Uncatalyzedtransbilayer diffusion At physiological temperatures, transbilayer diffusion [flip-flop] of a lipid molecule from one leaflet to the other occurs very slow [figure 1x]. Lateral diffusion IN THE PLANE of the bilayer is very rapid [figure 2x].
Cell Membrane Asymmetry The cell membrane has different composition of lipid and protein in its outer and inner leaflet of the lipid bilayer.
Figure 2.3 Three types of phospholipid translocatorsin the plasma membrane [Nelson and Cox, 2010] Flippase catalyze translocation of the aminophospholipids [phosphatidylethanolamine and phosphatidylserine] from the extracellular to the cytosolic leaflet. Flippases consume 1 ATP per molecule of phospholipid Keeping phosphatidylserine out of the extracellular leaflet is important: its exposure on the outer surface triggers apoptosis and engulfment by macrophages that carry phosphatidylserine receptors Floppase move plasma membrane phospholipids from the cystolic to the extracellular leaflet.like Flippases, they are ATP-dependent. They are members of ABC transporter family which actively transport hydrophobic substrates outward across the membrane.
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Cell and Cell Membrane: A Biochemical Approach
The lipid composition of the bilayer is asymmetric, with a higher content of phosphatidylcholine and sphingomyelin in the outer leaflet and a higher content of phosphatidylserine and phosphatidylethanolamine in the inner leaflet. Phosphatidylinositol which can function in the transfer of information from hormones and neurotransmitters is also only found in the inner leaflet. [Lieberman and Marks, 2009] CELL MEMBRANE PROTEINS
Types of Membrane Proteins Integral proteins Have alpha-helical structure (with 15-20 amino acids that have bulky side-chains) Interact intensively with the phospholipids (require the use of detergents of solubilisation) Removable only by agents (e.g. detergents, organic solvents) that interfere with hydrophobic interactions
Peripheral proteins Have amphiphatic alpha-helical structure Do not interact directly with the hydrophobic cores of the phospholipids in the bilayer Removed by relatively mild treatments that interfere with electrostatic interactions or break hydrogen bonds (e.g. carbonate at high pH)
Caveolae – invaginations of lipid rafts by the action of caveolin. Important in membrane breakage and sealing.
CELL MEMBRANE TRANSPORT - the transfer of solutes and information across membranes
TYPES OF CELL MEMBRANE TRANSPORT a. Cross-membrane movement of small molecules a. Diffusion (Passive and Facilitated) b. Active Transport b. Cross-membrane movement of large molecules (Endocytosis and Exocytosis) c. Signal transmission across membrane a. Cell surface receptors 1) Signal transduction (e.g. glucagon ->cAMP) 2) Signal internalization (coupled with endocytosis eg LDL receptors, Insulin and Glut transporters) b. Movement to intracellular receptors (steroid hormones, thyroid hormones, retinoids, Vit. D) d. Intracellular contact of Communication A. PASSIVE TRANSPORT - transport that do not require energy -movement of solute from an area of HIGH CONCENTRATION to an area of LOW CONCENTRATION -depends on concentration gradient; creation depends on 1. chemical gradient -difference of solute concentration or its ratio C 2 /C1 2. transmembrane electrical gradient (Vm in millivolts). -Solutes follow the 2nd law of thermodynamics where it tends to assume spontaneously the greatest randomness and the lowest energy 1. Simple diffusion - without membrane protein movement of solutes down its electrochemical gradient due torandom thermal movement or simply because the solute is permeable through the lipid bilayer because it is small enough and/or hydrophobic. 2. Facilitated diffusion movement of solutes down its electrochemical gradient through either transporters or ion channels (membrane proteins). Transporters allows the passage of hydrophobic solute by LOWERINGthe energy of activation of the solute
Hydropathy plot- the specific number of a transmembrane segment based on its amino acid sequence. CELL DOMAINS
Figure1. Lowering of Activation energy with the use of transporters
These are specialized features of plasma membrane. 1.
Lipid rafts – thickened portion of the bilayer involved in signal transduction and anchorage. It is where GPI-anchors and interacting peripheral proteins can be found.
Figure 2. Illustration of Passive and Active Transport
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Cell and Cell Membrane: A Biochemical Approach
Transporters vs. Channels
-Bind molecules and ions with high specificity, undergo conformational changes allowing the transport of molecules and ions across membranes
They show some specificity but does not act like an enzyme and only forms pores which open or close with much conformational changes
-Catalyze transport at rates well below the limits of free diffusion -saturable in the same sense as as enzymes so that futher increase in substrate conc. -does not provide a greater transport which SLOWER THAN with channel
- does not act like an enzyme, it is not saturable with ion substrate (in contrast with saturation kinetics seen in transporters) -transport is FASTER THANtransporter which may reach the limit of unhindered diffusion
Involve in passive (facilitated diffusion) and active transport
Mostly involve in facilitated diffusion
Types of Transport Systems based on Direction of Movement
*Acetylcholine- Na and Ca + *Serotonin and Glutamate- K , + 2+ Na , Ca *Glycine- Cl Specific channels
*Valinomycin- K channels + *Monensin- Na channels *Gramicidin- folding creates hollow channels
*Dendrotoxin (mamba snake)- K + *Tetrodotoxin (puffer fish)- Na *Cobrotoxin and alpha conotoxin- Acetylcholine receptor ion channel
*Diptheria toxin and activated complement- create large cellular pores causing lysis *Alpha-hemolysis (Strep.)- leak out ATP
Type of cell membrane transport that requires energy expenditure Active transport is the diffusion of molecules and ions against its electrochemical gradient with the use of energy which is mostly supplied by ATP. o Its protein transporter has an ATPase activity hydrolyzing ATP to ADP producing the needed energy.
Primary Active Transport - solute accumulation coupled directly to an exergonic chemical reaction such as breakdown of ATP Secondary Active transport - occurs when endergonic (uphill) transport of one solute is coupled to an exergonic (downhill) flow of a different solute that was originally pumped uphill by a primary active transport.
TYPES OF ACTIVE TRANSPORT
Uniport system- moves one type of molecule bidirectionally (egused in transporting glucose in the cell through the influence of insulin) Co-transport system- transfer of one solute depends upon the stoichiometric simultaneous or sequential transfer of another solute. >>2 types Symport- moves these solutes in the same direction (eg Glucose – sodium transport) Antiport-move two molecules in opposite directions + 2+ (eg, Na in and Ca out; Chloride-bicarbonate exchanger of the RBC membrane)
Figure 4.1. Illustrative concept of the types of active transport.
Aquaporins • water channels that only allow the passage of water molecules in the membranes of RBC and cells of the collecting ductules of the kidney. • composed of tetramerictransmembrane proteins • Mutation in some of these channel (AP-2) proteins may cause nephrogenic Diabetes Insipidus
Ionotropic Channels vsIonophores Ionotropic Channels
Receptors that are themselves acts as channels and DOES NOT NEED a secondary messenger
Non-receptor compounds that produce channels or pores in the cell. -If it produces pores or channels in bacteria, they serve as antibacterial. -if they cause production of pores or channels on cell membranes of human cells, they cause injury or disease.
Figure 4.2 A concept of how a transport ATPase works; here, the binding of Phosphate from ATP to the receptor found on the Transport ATPase would facilitate a conformational change to allow the material to go into the cell.
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Cell and Cell Membrane: A Biochemical Approach Figures 4.4 & 4.5 Some diseases associated with abnormalities in the cell membrane and in ion channels. What is important to note here is the general concept that: Genetic Mutations genes encoding for ion channels are therefore abnormal Ion channel mutations abnormalities in ion transport diseases d/t channelopathies.
ARTIFICIAL MEMBRANES & THEIR IMPORTANCE -
Drug delivery where liposome being hydrophobic can easily enter the cell, cancer cell and bacterial cells(Liposomal Doxurubicin or ticarcillin and tobramycin). Gene therapy- liposome can easily penetrate the nucleus that gene insert may be easily incorporated. • The PL may be attached to an antibody where drug delivery may be target-oriented.
LIPOSOMES Liposome-like structures underlie such things as LDL-particles and are being used in medicine among other areas. Figure 4.3 Taken directly from the lecture slide; here are some types of ATP-driven Active transporters. When ATP-driven is mentioned, it means that the channel requires the use of Adenosine Triphosphate (ATP) as energy in order to establish the transport of the cellular material. NOTE: The discussion on Nerve Impulses involving ion channels and pumps is extensively discussed in Cell and Muscle Physiology; you may refer to your Physiology trans or book for an easier concept.
Liposomes are bilayered lipid vesicles Form by sonicating lipids in aqueous solution Vehicles for drug, nucleic acid, Ab delivery Used in cosmetics Liposomes can be filled with drugs, and used to deliver drugs for cancer and other diseases.
**** Review Questions: 1. These interactions are the main driving force for the formation of lipid bilayers. a. Hydrophilic interactions b. Hydrophobic interactions c. Hydrogen bonding d. Ionic bonding 2. Which of the following characterstics is shared by simple and facilitated diffusion of glucose? a. Occurs down an electrochemical gradient b. Is saturable c. Requires metabolic energy d. Is inhibited by the presence of galactose + e. Requires a Na gradient 3. The following biochemical reactions occur in the cytoplasm, EXCEPT: a. Glycolysis b. Glycogenesis c. Kreb’s cycle d. Amino acid synthesis 4. Prokaryotic cells, but not eukaryotic cells have: a. Endoplasmic reticulum
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Cell and Cell Membrane: A Biochemical Approach
b. Histones c. Nucleoid d. Nucleus e. Plasma membrane 5. Mitochondria is associated with all of the following, EXCEPT: a. ATP synthesis b. DNA synthesis c. Protein synthesis d. Hydrolysis of various macromolecules at low pH e. Two different membranes 6. Which of the following can transport a solute against its concentration gradient? a. Primary active transport b. Facilitated transport c. Simple diffusion d. None of the above 7. According to the fluid mosaic model of a membrane: a. Proteins are always completely embedded in the lipid bilayer b. Transverse movement (flip-flop) of a protein in the membrane is thermodynamically favorable c. The transmembrane domain has largely hydrophobic amino acids d. Proteins are distributed symmetrically in the membrane e. Peripheral proteins are attached to the membrane only by noncovalent forces 8. The ion channel that is affected in cystic fibrosis. a. Na+ + b. K c. Ca2+ d. Cl9. The most abundant type of phospholipid in cell membranes. a. Sphingomyelin b. Phosphoglycerides c. Glycolipids d. Cholesterol 10. Which of the following will specifically increase the fluidity of the cell membrane? a. Decrease in temperature b. Increase the unsaturated fatty acids c. Increase the saturated fatty acids d. Removal of cholesterol Answers
1.B 2.A 3.C 4.C 5.D 6.A 7.C 8.D 9.B 10.B Page 10 of 10 Transcribers: Aclan, J.V., Advento, V., Bolos, C.,Cabiscuelas, Y.N., Cuaderno, C., Gamboa, K.A., Javier, K., Laurilla, L.B., Rodenas, E., Roxas, F.