Microscopic Structure of living organisms Flashcards
1
Q
Describe the Origin Of Life
A
- All organisms derive from a single primordial cell born more than 3 billion years ago.
- This cell, out-reproducing its competitors, took the lead in the process of cell division and evolution.
- 1.5 billion years ago, procaryotic cells to eukaryotic cells
2
Q
Describe Bacteria
A
- Bacteria are the simplest organisms found in most natural environments.
- They are spherical or rod-shaped cells, commonly several micrometers in linear dimension.
- They often possess a tough protective coat, called a cell wall, beneath which a plasma membrane encloses a single cytoplasmic compartment containing DNA, RNA, proteins, and small molecules.
3
Q
Bacteria; survival and reproduction
A
- Bacteria are small and can replicate quickly, simply dividing in two by binary fission.
- “Survival of the fittest” = survival of those that can divide the fastest. The ability to divide quickly enables populations of bacteria to adapt rapidly to changes in their environment
- Under optimal conditions a single procaryotic cell can divide every 20 minutes and thereby give rise to 5 billion cells in less than 11 hours.
4
Q
Bacteria; ecological niches
A
- Eubacteria: inhabit soil, water, and larger living organisms
- Archaebacteria: found in bogs, ocean depths, salt brines, and hot acid springs
5
Q
Characteristics of bacteria
A
- Some bacteria that can utilizes organic molecule such as food, including sugars, amino acids, fats, hydrocarbons, polypeptides, and polysaccharides.
- Some obtain their carbon atoms from CO2 and their nitrogen atoms from N2.
6
Q
Metabolic Reactions of Bacteria
Explain the importance of metabolic reactions before and after evolution began
A
- A bacterium growing in a salt solution containing a single type of carbon source, such as glucose, carries out reactions while deriving the chemical energy from glucose the chemical energy and using the carbon atoms of glucose to synthesize every type of organic molecule that the cell requires.
- Metabolic pathways: The reactions catalyzed by enzymes working in reaction “chains” so that the product of one reaction is the substrate for the next
- Originally, when life began on earth, there was no need for elaborate metabolic reactions as cells with relatively simple chemistry could survive and grow on the molecules in their surroundings.
- As evolution began, competition for these limited natural resources would have become more intense. Organisms that had developed enzymes to manufacture useful organic molecules more efficiently and in new ways would have had a strong selective advantage. In this way the complement of enzymes possessed by cells is thought to have gradually increased, generating the metabolic pathways of present organisms.
7
Q
Metabolic Pathways
A
- Glycolysis occurs in virtually every living cell and drives the formation of the compound adenosine triphosphate, or ATP, which is used by all cells as a source of chemical energy.
- Certain thioester compounds play a fundamental role in the energy-transfer reactions of glycolysis and in a host of other basic biochemical processes in which a thiol and a carboxylic acid are joined by a high-energy bond involving sulfur
- Some reactions involve the synthesis of small molecules which are utilized in further reactions to make the large polymers specific to the organism. Other reactions are used to degrade complex molecules, taken in as food, into simpler chemical units.
8
Q
Evolutionary Relationships and DNA Sequences
A
- The enzymes that catalyze metabolic reactions, while serving the same essential functions undergo modifications as organisms have evolved into divergent forms.
- For this reason the amino acid sequence of the same type of enzyme in different living species provides a valuable indication of the evolutionary relationship between these species.
- Highly conserved sequences reveal relationships between organisms that diverged long ago
- Rapidly evolving sequences can be used to determine how more closely related species evolved.
9
Q
Cyanobacteria fixing CO2 and N2
A
- A strong selective advantage would have been gained by any organisms able to utilize carbon and nitrogen atoms (in the form of CO2 and N2) directly from the atmosphere.
- CO2 and N2 are very stable while abundantly available.
- Requires a large amount of energy and chemical reactions to convert them to a usable form - that is, into organic molecules such as simple sugars.
- In the case of CO2; photosynthesis, in which energy captured from the sun drives the conversion of CO2 into organic compounds. The interaction of sunlight with chlorophyll excites an electron to a more highly energized state. As the electron drops back to a lower energy level, the energy it gives up drives chemical reactions that are facilitated and directed by protein molecules.
- One of the first sunlight-driven reactions was probably the generation of “reducing power.” The carbon and nitrogen atoms in atmospheric CO2 and N2 are in an oxidized and inert state. One way to make them more reactive, so that they participate in biosynthetic reactions, is to reduce them by give them a larger number of electrons.
- In the first step electrons are removed from a poor electron donor and transferred to a strong electron donor by chlorophyll in a reaction that is driven by sunlight. The strong electron donor is then used to reduce CO2 or N2.
- One of the first sources of electrons was H2S, from which the primary waste product would have been elemental sulfur. Later the more difficult but ultimately more rewarding process of obtaining electrons from H2O was accomplished, and O2 was released in large amounts as a waste product.
- Cyanobacteria (also known as blue-green algae and are self sufficient) are today a major route by which both carbon and nitrogen are converted into organic molecules and thus enter the biosphere.
- Able to live on water, air, and sunlight alone
10
Q
Bacteria and aerobic respiration of food molecules
A
- Oxygen is an extremely reactive chemical toxic to anaerobic bacteria.
- By using oxygen, organisms are able to oxidize more completely the molecules they ingest. For example, in the absence of oxygen, glucose can be broken down only to lactic acid or ethanol, the end products of anaerobic glycolysis. But in the presence of oxygen glucose can be completely degraded to CO2 and H2O. In this way much more energy can be derived from each gram of glucose.
- The energy released in respiration - the aerobic oxidation of food molecules - is used to drive the synthesis of ATP in much the same way that photosynthetic organisms produce ATP from the energy of sunlight.
- In both processes there is a series of electron-transfer reactions that generates an H+ gradient between the outside and inside of a membrane-bounded compartment; the H+ gradient then serves to drive the synthesis of the ATP.
11
Q
Eukaryotic cells and organelles
A
- Have a nucleus, which contains most of the cell’s DNA, enclosed by a double layer of membrane
- The DNA is thereby kept in the cytoplasm, where most of the cell’s metabolic reactions occur. .
- In the cytoplasm: chloroplasts and mitochondria and each have their own double layer of membrane. Have symbiotic origin.
- Mitochondria in plants, animals and fungi whereas chloroplasts in plants
12
Q
Eucaryotic Cells Depend on Mitochondria for Their Oxidative Metabolism
A
- Mitochondria resemble prokaryotic organisms like bacteria in size and shape, containing DNA, making proteins, and reproducing by dividing in two.
- Mitochondria are responsible for respiration.
- Without mitochondria the cells of animals and fungi would be anaerobic organisms, depending on the relatively inefficient and antique process of glycolysis for their energy.
- Comparative nucleotide sequence analyses have revealed that at least two groups of these organisms, the diplomonads and the microsporidia, diverged very early from the line leading to other eucaryotic cells
- Amoeba Pelomyxa palustris while lacking mitochondria, carries out oxidative metabolism by harboring aerobic bacteria in its cytoplasm in a permanent symbiotic relationship.
- The plasma membrane is heavily committed to energy metabolism in procaryotic cells but not in eucaryotic cells, where this crucial function has been relegated to the mitochondria.
- Because eucaryotic cells need not maintain a large H+ gradient across their plasma membrane, as required for ATP production in procaryotes, it became possible to use controlled changes in the ion permeability of the plasma membrane for cell-signaling purposes.
- Ion channels mediate the elaborate electrical signaling processes in higher organisms - notably in the nervous system -and they control much of the behavior of single-celled free-living eucaryotes such as protozoa
13
Q
Cholorplasts as Procaryotic Cells
A
- Chloroplasts and procaryotic cyanobacteria carry out photosynthesis by absorbing sunlight in the chlorophyll attached to their membranes. They are similar in size and chlorophyll bearing membranes are stacked in layers
- Chloroplasts reproduce by dividing, and they contain DNA Chloroplasts share a common ancestry with cyanobacteria and evolved from procaryotes that made their home inside eucaryotic cells.
- These procaryotes performed photosynthesis for their hosts, who sheltered and nourished them.
14
Q
In Eucaryotic Cells the Genetic Material Is Packaged in Complex Ways
A
- Eucaryotic cells contain DNA. In human cells, for example, there is about 1000 times more DNA than in typical bacteria.
- The length of DNA in eucaryotic cells is great so the risk of entanglement and breakage becomes severe which is why the histones have evolved to bind to the DNA and wrap it up into compact and manageable chromosomes
- Tight packaging of the DNA in chromosomes important for cell division in eucaryotes
- Most eucaryotes have histones bound to their DNA
- The membranes enclosing the nucleus;
- Further protect the structure of the DNA and machinery
- In gene expression; (1) DNA transcription-DNA to RNA sequences (2) RNA translation- RNA sequences to proteins.
- In procaryotic cells - the translation of RNA sequences into protein begins as soon as they are transcribed, even before their synthesis is completed.
- In eucaryotes, however (except in mitochondria and chloroplasts), the two steps in the path from gene to protein are kept strictly separate: transcription occurs in the nucleus, translation in the cytoplasm.
- The RNA has to leave the nucleus before it can be used to guide protein synthesis. While in the nucleus it undergoes elaborate changes in which some parts of the RNA molecule are discarded and other parts are modified (RNA processing).
15
Q
Protozoa Include the Most Complex Cells Known
A
- Protists; free living, single celled eukaryotes.
- Photosynthetic or carnivorous, motile or sedentary.
- They have structures like sensory bristles, photoreceptors, flagella, leglike appendages, mouth parts, stinging darts, and musclelike contractile bundles.
- Protozoa- larger and active protists .
- Didinium is a carnivorous ciliate swims around in the water at high speed through synchronous beating of its cilia. When it encounters a suitable prey, usually another type of protozoan, such as a Paramecium, it releases numerous small paralyzing darts from its snout region. Then the Didinium attaches to and devours the Paramecium, inverting like a hollow ball to engulf the other cell, which is as large as itself.
- Cytoskeletal structures beneath plasma membrane: control swimming, and paralyzing and capturing its prey -lying just beneath the plasma membrane.
- Cell cortex includes microtubules that form the core of cilim and enable it to beat
16
Q
Eucaryotic Cells Have a Cytoskeleton (with filaments)
A
- Cytoskeleton: gives cell its shape, its capacity to move, and its ability to arrange its organelles and transport them from one part of the cell to another.
- The cytoskeleton has 2 protein filaments; actin filaments and microtubules; involved in the generation of cellular movements and internal movements in cytoplasm
- Actin filaments enable individual eucaryotic cells to participate in the contraction of muscle in animals
- Microtubules are the main structural and force-generating elements in cilia and flagella - the long projections on some cell surfaces that beat like whips and serve as instruments of propulsion. Also it partitions DNA equally between the two daughter cells when a eucaryotic cell divides. Without microtubules, therefore, the eucaryotic cell could not reproduce.
17
Q
Eucaryotic Cells Contain a Rich Array of Internal Membranes
A
- A human cell contains about 1000 times as much DNA as a typical bacterium. This large size creates problems because of the raw materials for the biosynthetic reactions occurring in the interior of a cell must ultimately enter and leave by passing through the plasma membrane covering its surface.
- increase in cell volume= increase in cell surface because membrane is a site of many reactions
- Membranes surround the nucleus, the mitochondria, and (in plant cells) the chloroplasts and form many organelles
- Endoplasmic reticulum: where lipids and proteins of cell membranes, as well as materials destined for export from the cell, are synthesized.
- Golgi Apparatus: stacks of flattened sacs which is involved in the modification and transport of the molecules made in the ER. Rough ER contains studded ribosomes for protein synthesis and Smooth ER lacks ribosomes and helps with lipid metabolism
- Lysosomes: which contain stores of enzymes required for intracellular digestion and so prevent them from attacking the proteins and nucleic acids elsewhere in the cell.
- Peroxisomes: where dangerously reactive hydrogen peroxide is generated and degraded during the oxidation of various molecules by O2.
- Membranes also form small vesicles and, in plants, a large liquid-filled vacuole.
- Membrane-bounded structures correspond to distinct internal compartments within the cytoplasm. The organelles cover half of the cytoplasm and the remaining is referred to as the cytosol.
- Endocytocis: portions of the external surface membrane invaginate and pinch off to form membrane-bounded cytoplasmic vesicles that contain both substances present in the external medium and molecules previously adsorbed on the cell surface.
- Very large particles or even entire foreign cells can be taken up by phagocytosis - a special form of endocytosis.
- Exocytosis is the reverse process, whereby membrane-bounded vesicles inside the cell fuse with the plasma membrane and release their contents into the external medium. In this way membranes surrounding compartments deep inside the cell serve to increase the effective surface area of the cell for exchanges of matter with the external world.
- Plasma membrane: outer boundary of the cell, a sheet of phospholipid molecules in which proteins are embedded and some proteins act as pumps or channels for transporting specific molecules in and out of cell
- Cell wall: plant cells have a rigid cell wall made of cellulose in a matrix of other polysaccharides
- Chloroplasts: contains cholorophyll, found in plants only
18
Q
Procaryotes
A
- Prokaryotes:
- Bacteria and cyanobacteria
- anaerobic or aerobic
- few or no organelles
- circular DNA in cytoplasm
- RNA and protein synthesized in same compartment
- No cytoskeleton
- Cell division: chromosomes pulled apart by attachements to plasma membrane
- Mainly unicellular
19
Q
Eukaryotes
A
- protists, fungi, plants and animals
- aerobic
- organelles like nucleus, mitochondria, chloroplasts and ER
- very long and linear DNA molecules containing many noncoding regions bounded by nuclear envelope
- RNA synthesized and processed in nucleus, protein synthesied in cytoplasm
- cytoskeleton composed of protein filaments, cytoplasmic streaming, endocytosis and exocytosis
- cell division: chromosomes pulled apart by spindle fibers
- mainly multicellular
20
Q
Membrane Structures
A
- The plasma membrane encloses the cell, defines its boundaries, and maintains the essential differences between the cytosol and the extracellular environment.
- Inside the cell the membranes of membrane-bounded organelles in eucaryotic cells maintain the characteristic differences between the contents of each organelle and the cytosol.
- Ion gradients across membranes, established by the activities of specialized membrane proteins, can be used to synthesize ATP, to drive the transmembrane movement of selected solutes, or, in nerve and muscle cells, to produce and transmit electrical signals.
- Plasma membrane also contains proteins that act as sensors of external signals, allowing the cell to change its behavior in response to environmental cues; these protein sensors, or receptors, transfer information rather than ions or molecules across the membrane.
- All membranes have a common general structure: a very thin film of lipid and protein molecules, held together mainly by noncovalent interactions.
- Cell membranes are dynamic, fluid structures, and most of their molecules are able to move around
- The lipid layer provides the basic structure of the membrane and serves as an impermeable barrier to the passage of most water-soluble molecules.
- Protein molecules “dissolved” in the lipid bilayer: transporting specific molecules across it or catalyzing membrane associated reactions such as ATP synthesis.
- Other proteins serve as structural links that connect the membrane to the cytoskeleton and/or to either the extracellular matrix or an adjacent cell, while others serve as receptors to detect and transduce chemical signals in the cell’s environment.
- Cell membranes are asymmetrical structures: the lipid and protein compositions of the outside and inside faces differ from one another in ways that reflect the different functions performed at the two surfaces of the membrane.
21
Q
Glycolipid Molecules
A
- Galactocerebroside is called a neutral glycolipid because the sugar that forms its head group is uncharged.
- A ganglioside always contains one or more negatively charged sialic acid residues (also called N-acetylneuraminic acid, or NANA).
- Whereas in bacteria and plants almost all glycolipids are derived from glycerol, as are most phospholipids, in animal cells they are almost always produced from sphingosine, an amino alcohol derived from serine, as is the case for the phospholipid sphingomyelin
22
Q
Four major phospholipids in mammalian plasma membranes
A
- All of the lipid molecules are derived from glycerol except for sphingomyelin, which is derived from serine
- Phosphatidylethanolamine, Phosphatidylserine, Phosphatidylcholine, Sphingomyelin
23
Q
Influence of cis-double bonds in hydrocarbon chains.
A
The double bonds make it more difficult to pack the chains together and therefore make the lipid bilayer more difficult to freeze
24
Q
Black Memrane(Synthetic lipid bilayer)
A
This planar bilayer is formed across a small hole in a partition separating two aqueous compartments. Black membranes are used to measure the permeability properties of synthetic membranes
25
Q
Micelles and Bilayers
A
Lipid molecules form such structures spontaneously in water. The shape of the lipid molecule determines which of these structures is formed. Wedge-shaped lipid molecules form micelles, whereas cylindershaped phospholipid molecules form bilayers
26
Q
Describe Biological membranes, lipid bilayer, 3 major classes of membrane lipid molecules, and different mixtures of lipids
A
- Biological membranes: double layer of lipid molecules in which various membrane proteins are embedded.
- This lipid bilayer is fluid, with individual lipid molecules able to diffuse rapidly within their own monolayer.
- Most types of lipid molecules, however, very rarely flipflop spontaneously from one monolayer to the other.
- There are three major classes of membrane lipid molecules - phospholipids, cholesterol, and glycolipids - and the lipid compositions of the inner and outer monolayers are different, reflecting the different functions of the two faces of a cell membrane.
- Different mixtures of lipids are found in the membranes of cells of different types, as well as in the various membranes of a single eucaryotic cell. Some membrane-bound proteins require specific lipid head groups in order to function,
27
Q
Membrane Lipids Are Amphipathic Molecules, Most of Which Spontaneously Form Bilayers
A
- Lipid molecules are insoluble in water but dissolve readily in organic solvents. They constitute about 50% of the mass of most animal cell membranes
- Lipid molecules are amphipathic (or amphiphilic) that is, they have a hydrophilic(“water-loving,” or polar) end and a hydrophobic(“water-hating,” or nonpolar) end.
- Phospholipids are most abundant, these have a polar head group and two hydrophobic hydrocarbon tails.The tails are usually fatty acids, and they can differ in length ( 14 - 24 carbon atoms). One tail usually has one or more cis-double bonds (that is, it is unsaturated), while the other tail does not (that is, it is saturated).
- Differences in the length and saturation of the fatty acid tails are important because they influence the ability of phospholipid molecules to pack against one another, and for this reason they affect the fluidity of the membrane
- It is the shape and amphipathic nature of the lipid molecules that cause them to form bilayers spontaneously in aqueous solution.
- When lipid molecules are surrounded on all sides by water, they tend to aggregate so that their hydrophobic tails are buried in the interior and their hydrophilic heads are exposed to water.
- Depending on their shape, they can do this in either of two ways: they can form spherical micelles, with the tails inward, or they can form bimolecular sheets, or bilayers, with the hydrophobic tails sandwiched between the hydrophilic head groups
- Because of their cylindrical shape, membrane phospholipid molecules spontaneously form bilayers in aqueous environments and lipid bilayers tend to close on themselves to form sealed compartments, thereby eliminating free edges where the hydrophobic tails would be in contact with water.
- For the same reason compartments formed by lipid bilayers tend to reseal when they are torn. A lipid bilayer has other characteristics besides its self-sealing properties that make it an ideal structure for cell membranes. One of the most important of these is its fluidity, which is crucial to many membrane functions.
28
Q
Carrier Proteins
A
- Carrier proteins bind specific solutes and transfer them across the lipid bilayer by undergoing conformational changes that expose the solute binding site sequentially on one side of the membrane and then on the other.
- Some carrier proteins simply transport a single solute “downhill,” whereas others can act as pumps to transport a solute “uphill” against its electrochemical gradient, using energy provided by ATP hydrolysis or by a “downhill” flow of another solute (such as Na+) to drive the requisite series of conformational changes.
- Carrier proteins belong to a small number of families, each of which comprises proteins of similar amino acid sequence
- The family of cation transporting ATPases, which includes the ubiquitous Na+-K+ pump, is an important example; each of these ATPases contains a large catalytic subunit that is sequentially phosphorylated and dephosphorylated during the pumping cycle.
- The superfamily of ABC transporters is especially important clinically: it includes proteins that are responsible for cystic fibrosis, as well as for drug resistance in cancer cells and in malaria-causing parasites
- Carrier proteins functioning as uniports, symports, and antiports
29
Q
Kinetics of simple diffusion compared to carrier-mediated diffusion
A
- Whereas the rate of the former is always proportional to the solute concentration, the rate of the latter reaches a maximum (Vmax) when the carrier protein is saturated.
- The solute concentration when transport is at half its maximal value approximates the binding constant (KM) of the carrier for the solute and is analogous to the KM of an enzyme for its substrate. .
30
Q
Carrier Protein mediating Facilitated Diffusion of A Solute
A
- Conformational change in a carrier protein could mediate the facilitated diffusion of a solute.
- The carrier protein shown can exist in two conformational states:
- In state “pong” the binding sites for solute A are exposed on the outside of the bilayer;
- in state “ping” the same sites are exposed on the other side of the bilayer.
- Therefore, if the concentration of A is higher on the outside of the bilayer, more A will bind to the carrier protein in the pong conformation than in the ping conformation, and there will be a net transport of A down its electrochemical gradient.
31
Q
Pumping cycle of the Na+-K+ ATPase
A
- The binding of Na+ (1) and the subsequent phosphorylation by ATP of the cytoplasmic face of the ATPase (2) induce the protein to undergo a conformational change that transfers the Na+ across the membrane and releases it on the outside (3). Then the binding of K+on the extracellular surface (4) and the subsequent dephosphorylation (5) return the protein to its original conformation, which transfers the K+ across the membrane and releases it into the cytosol (6).
- These changes in conformation are analogous to the ping pong transitions except that here the Na+- dependent phosphorylation and the K+-dependent dephosphorylation of the protein cause the conforma-tional transitions to occur in an orderly manner, enabling the protein to do useful work. Although for simplicity only one Na+- and one K+-binding site are shown, in the real pump there are thought to be three Na+- and two K+-binding sites. Moreover, although the ATPase is shown as alternating between two conformational states, there is evidence that it goes through a more complex series of conformational changes during the actual pumping cycle.
32
Q
The Na+-K+ ATPase.
A
- This carrier protein actively pumps Na+ out of and K+ into a cell against their electrochemical gradients.
- For every molecule of ATP hydrolyzed inside the cell, three Na+ are pumped out and two K+ are pumped in.
- The specific pump inhibitor ouabain and K+ compete for the same site on the external side of the ATPase.
33
Q
Response of a human red blood cell to changes in osmolarity
A
- Response of a human red blood cell to changes in osmolarity (also called tonicity) of the extracellular fluid.
- Because the plasma membrane is freely permeable to water, water will move into or out of cells down its concentration gradient, a process called osmosis.
- If cells are placed in a hypotonic solution (i.e., a solution having a low solute concentration and therefore a high water concentration), there will be a net movement of water into the cells, causing them to swell and burst (Lyse)
- Hypertonic solution, they will shrink.
34
Q
Transcellular transport of glucose
A
- The transcellular transport of glucose across an intestinal epithelial cell depends on the asymmetrical distribution of transport proteins in the cell’s plasma membrane.
- Glucose is pumped into the cell through the apical domain of the membrane by a Na+-powered glucose symport, and glucose passes out of the cell (down its concentration gradient) by facilitated diffusion mediated by a different glucose carrier protein in the basal and lateral membrane domains.
- The Na+ gradient driving the glucose symport is maintained by the Na+-K+ ATPase in the basal and lateral plasma membrane domains, which keeps the internal concentration of Na+ low.
- Adjacent cells are connected by impermeable junctions (called tight junctions). The junctions have a dual function in the transport process illustrated: 1. they prevent solutes from crossing the epithelium between cells, allowing a concentration gradient of glucose to be maintained across the cell sheet 2. they also serve as diffusion barriers within the plasma membrane, which help confine the various carrier proteins to their respective membrane domains
35
Q
Double membrane of an E. coli bacterium.
A
- The inner membrane is the cell’s plasma membrane.
- Between the inner and outer lipid bilayer membranes there is a highly porous, rigid peptidoglycan composed of protein and polysaccharide that constitutes the bacterial cell wall; it is attached to lipoprotein molecules in the outer membrane and fills the periplasmic space(only a little of the peptido-glycan is shown).This space also contains a variety of soluble protein molecules.
- Polysaccharide chains of the special lipopolysaccharide molecules that form the external monolayer of the outer membrane; for clarity, only a few of these chains are shown.
- Bacteria with double membranes are called gram negative because they do not retain the dark blue dye used in the gram staining procedure.
- Bacteria with single membranes (but thicker cell walls), such as staphylo-cocci and streptococci, retain the blue dye and therefore are called gram positive; their single membrane is analogous to the inner (plasma) membrane of gramnegative bacteria
36
Q
The auxiliary transport system
A
- Porins: The solute diffuses through channel-forming proteins in the outer membrane and binds to a periplasmic substrate-binding protein.
- As a result, the substrate binding protein undergoes a conformational change that enables it to bind to a transport ATPase in the plasma membrane, which then picks up the solute and actively transfers it across the bilayer in a reaction driven by ATP hydrolysis.
- The peptidoglycan is omitted for simplicity; its porous structure allows the substrate-binding proteins and water-soluble solutes to move through it by simple diffusion
37
Q
The transporter and 4 domains
A
- The transporter consists of four domains:
- two highly hydrophobic domains, each with six putative membrane-spanning segments that somehow form the translocation pathway, and two ATP-binding catalytic domains (or cassettes).
- In some cases the two halves of the transporter are formed by a single polypeptide (as shown), whereas in other cases they are formed by two separate polypeptides.
- Sugar transporters: passive glucose transporters in mammalian cells some H+-driven sugar transporters in bacteria
- Cation-transporting ATPases Na+-K+ ATPases; Ca2+ ATPases
- ABC transporters multidrug resistance (MDR) ATPase in mammalian cells; periplasmic substrate-binding-protein-dependent ATPases in bacteria; chloroquine-resistance ATPase in P. falciparum; mating pheromone exporter in yeast; peptide pump in vertebrate ER membrane; cystic fibrosis transmembrane regulator (CFTR) protein
38
Q
3 Antiporters and 1 symporters
A
- Anion (Cl–HCO3 -) antiporters band 3 in red blood cells; anion exchangers in other cells
- Cation antiporters Na+-H+exchanger
- Cation/anion antiporters Na-dependent Cl–HCO3 - exchanger Na+-driven symporters Na+-glucose symporter in intestinal cells; Na+-proline symporter in bacteria;
- Na+-HCO3 - symporter in glial cells
39
Q
Ion Channels
A
- Channel proteins form aqueous pores across the lipid bilayer and allow inorganic ions of appropriate size and charge to cross the membrane down their electrochemical gradients at rates that are about 1000 times greater than those achieved by any known carrier.
- These ion channels are “gated” and usually open transiently in response to a specific perturbation in the membrane, such as a change in membrane potential (voltage-gated channels) or the binding of a neurotransmitter (transmitter-gated channels)
- K+-selective leak channels play an important part in determining the resting membrane potential across the plasma membrane in most animal cells.
- Voltage-gated cation channels are responsible for the generation of self-amplifying action potentials in electrically excitable cells such as neurons and skeletal muscle cells.
- Transmitter-gated ion channels convert chemical signals to electrical signals at chemical synapses: excitatory neurotransmitters, such as acetylcholine and glutamate, open transmitter-gated cation channels and thereby depolarize the postsynaptic membrane toward the threshold potential for firing an action potential; inhibitory neurotransmitters, such as GABA and glycine, open transmitter-gated Cl- channels and thereby suppress firing by keeping the postsynaptic membrane polarized.
- Gap junctions
- A subclass of glutamate-gated ion channels, called NMDA-receptor channels, are highly permeable to Ca2+, which can trigger the long-term changes in synapses that are thought to be involved for some forms of learning and memory
- Ion channels work together in complex ways to control the behavior of electrically excitable cells. A typical neuron, for example, receives thousands of excitatory and inhibitory inputs, which combine by spatial and temporal summation to produce a grand postsynaptic potential (PSP) in the cell body. The magnitude of the grand PSP is translated into the rate of firing of action potentials by a mixture of cation channels in the membrane of the axon hillock.
40
Q
Ion channels opening and closing
A
- Ion channel, which fluctuates between closed and open conformations.
- A transmembrane protein complex: forms a hydrophilic pore across the lipid bilayer only when the gate is open.
- Polar amino acid side chains are thought to line the wall of the pore, while hydrophobic side chains interact with the lipid bilayer. The pore narrows to atomic dimensions in one region (the “ion-selective filter”), where the ion selectivity of the channel is largely determined.
- Gated ion channels: Voltage gated, Ligand-gated extracellular ligand, Ligand-gated intracellular ligand, mechanically gated
41
Q
The flow ions
A
- A small flow of ions carries sufficient charge to cause a large change in the membrane potential.
- The ions that give rise to the membrane potential lie in a thin surface layer close to the membrane, held there by their electrical attraction to their oppositely charged counterparts (counterions) on the other side of the membrane.
- 1 microcoulumb of charge:(6 x 1012 univalent ions)
42
Q
Vertebrate Neuron
A
- Vertebrate neuron: The single axon conducts signals away from the cell body, while the multiple dendrites receive signals from the axons of other neurons. The nerve terminals end on the dendrites or cell body of other neurons or on other cell types, such as muscle or gland cells.
43
Q
The encoding of the grand PSP in the form of the frequency of firing of action potentials by an axon.
A
Firing frequency of an axon increases with an increase in the grand PSP
44
Q
The principal temporal summation
A
Each presynaptic action potential arriving at a synapse produces a small postsynaptic potential, or PSP (black lines). When successive action potentials arrive at the same synapse, each PSP produced adds to the tail of the preceding one to produce a larger combined PSP (green lines). The greater the frequency of incoming action potentials, the greater the size of the combined PSP
