Biology - An Illustrated Guide to Science (794127), страница 5
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The movements of carbon dioxideand oxygen are good examples ofpassive transport in living cells.● Active transport can occur either upor down a concentration gradient,so active transport can movematerials from areas of lowconcentration to areas ofhigh concentration. Activetransport requires anenergy input by a cell.Absorption of vitaminsby the gut in mammalsis a good example.●Active transportEnergy released fromadenosinetriphosphate (ATP)—the main chemicalenergy carrier in allorganisms—pumpsmaterials across themembrane.● Carrier proteins in the cellmembrane may change shapeto take in particles (calledpassenger molecules) on oneside, twist configuration and thenrelease the particle on the other side.● Low oxygen concentrations or lowtemperatures will slow down activetransport by reducing the energyavailable for this reaction.● Some active transport mechanisms areused to create electrical imbalancesbetween the inside and outside ofcells.© Diagram Visual Information Ltd.●A passenger molecule movestoward a carrier protein.passenger moleculecarrier proteinplasma membraneinside cellThe passenger moleculeis released into thecytoplasm.ADPATPThe passenger moleculebinds to the carrierprotein.Energy release from ATP(adenosine triphosphate)causes a conformationalchange in carrier protein.27Plasma membrane:endocytosisLarge particletaken up byphagocytosis.Key wordscytoplasmendocytosisenzymeexocytosislysosomeplasmamembraneExocytosis ofwaste product.Particle isenclosedin vacuole.Vacuole withwaste productsmoves towardcell membrane.cytoplasmDigestion occursin vacuole andproducts areabsorbed.lysosomeLysosome fuseswith vacuole.Pinocytosisplasmamembranesmall particlesadsorption tocell surfacecytoplasmphagocytosisvacuoleTaking materials into thecellEndocytosis moves materials into thecell.
The Greek word endo means“inside.”● Endocytosis is further broken downinto two forms: phagocytosis, whichmoves relatively large particles into thecell, and pinocytosis, which movessmaller particles (often in groups) intothe cell.●PhagocytosisIn microorganisms, food particles areoften absorbed by phagocytosis.● The cytoplasm of the cell flowsaround small microorganisms andencloses them in a vacuole. Lysosomes,which contain digestive enzymes, nextfuse with the cell. Enzymes then breakdown the particles into simplerchemicals, which can then beabsorbed into the cell.● Indigestible materials in a phagocytoticvacuole are often released back to theoutside of the cell through a processcalled exocytosis (see page 28).●Pinocytosis●Pinocytosis is a slightly simplerprocedure than phagocytosis becausethe contents of the pinocytotic vacuolegenerally need less processing beforethey can be absorbed into the cell.pinocytoticvacuole formedVacuole breaks down,releasing particlesinto cytoplasm.© Diagram Visual Information Ltd.PhagocytosisUNITY28UNITYKey wordsactive processexocytosisGolgi bodymytochondrionplasmamembraneroughendoplasmicreticulumPlasma membrane:exocytosisPancreatic ductTransverse sectionMoving materials out ofthe cellmoves materials out of thecell.
The Greek word exo means“outside.” These materials may besecretory, excretory, or may be theundigested remains of materials infood vacuoles.● Exocytosis is an active process—itrequires energy input from the cell.● Exocytosis is common in cells thatproduce secretions, such as the acinarcells of the pancreas, which furnishpancreatic juice.fine branch ofpancreatic ductacinar cell● ExocytosisManufacture of chemicalsIn the example at right, the roughendoplasmic reticulum deep in thecell uses energy produced by aerobicrespiration in the mitochondrion tosynthesize and transport proteins.● The proteins are collected in the Golgibody and then packaged in smallvacuoles made of plasma membrane.● Vacuoles are “pinched off ” the Golgibody and move toward the outside ofthe cell.●Release of materials●When the vacuoles reach the outer cellmembrane, the membrane formingthe vacuole merges with the plasmamembrane.
The vacuole then releasesits contents (such as the inactiveenzyme zymogen) to the outsideworld.Pancreatic duct (acinar) cell illustrating secretory exocytosisinactive enzyme (zymogen)exocytosismature secretory vesicleGolgi bodyproteinsmove throughGolgi bodyvesicle fromrough endoplasmicreticulumrough endoplasmicreticulum producesand transportsproteinsnucleus© Diagram Visual Information Ltd.energy usedin proteinsynthesisplasmamembranemitochondrion29LysosomesUNITYKey wordsLysosomes and phagocytosissecondary lysosomemerged with phagocystic vacuolecytoplasmenzymeexocytosisGolgi bodylysosomedigestionphagocytosisresidual bodyphagocytosisWaste disposal systems●exocytosisphagocyticvacuoleLysosomes are vacuoles that contain apowerful collection of enzymes thatcan break down a range of compoundsinto simpler molecules that can beabsorbed through a cell membrane.Lysosomes andphagocytosisroughendoplasmicreticulumWhen a relatively large particle isengulfed by phagocytosis, it cannot beabsorbed into the cell until it has beenbroken down.● Lysosomes, produced by the Golgibody, merge with the phagocytoticvacuole so that the enzymes arereleased into the vacuole and can startto act on the engulfed particle.● Once the enzymes have broken downthe particle, the products can beabsorbed.
Any indigestiblecomponents are released to theoutside world through exocytosiswhen the phagocytotic vacuole mergeswith the plasma membrane of the cell.●Golgi bodyprimary Iysosomeproduced byGolgi bodyLysosomes and autophagyresidualbodyplasma membraneLysosomes and autophagyLysosomes destroy worn out ordamaged cell components through aprocess called autophagy.● The cell component is surrounded bya membrane, and lysosomes thenmerge with this vacuole.
The enzymesbreak down the damaged cellstructure, and the importantcomponents can be reabsorbed intothe cytoplasm through the membrane.digestionprimary Iysosomeproduced byGolgi bodyroughendoplasmicreticulumGolgi bodyworn outmitochondrionin vacuolesecondarylysosome© Diagram Visual Information Ltd.●30UNITYKey wordschloroplastgranumlight-dependentreactionlight-independentreactionorganellephotosynthesisthylakoidSummary ofphotosynthesisPlant cell213Reaction pathwaysPhotosynthesis is a biochemicalprocess (see top diagram) by whichplants harness the energy from light(1) to take carbon dioxide (2) andwater (3) and produce glucose (4) andoxygen (5).● Photosynthesis is a complex series ofreactions that fall into two groups: thelight-dependent reaction and the lightindependent reaction.● Both of these reactions occur inorganelles called chloroplasts.
Withinthe chloroplasts are disk-shapedmembrane structures calledthylakoids, which contain thechlorophyll needed forphotosynthesis. Chloroplasts are madeup of stacks of these disks calledgrana (see middle diagram).●412345Sunlight energyCarbon dioxideWaterGlucoseOxygen5chloroplastSimple equationfor photosynthesis6CO2 + 6H2Osunlight energyC6H12O6 + 6O2chlorophyllLight-dependent reactionThe light-dependent reaction (LDR),also called photolysis, captures energyin light (bottom diagram 1) andconverts it into chemical energy in theform of adenosine triphosphate (ATP)and nicotinamide adeninedinucleotide phosphate (NADP).
Theenergy is then available for the rest ofthe photosynthetic reaction.● The LDR produces oxygen by splittingwater molecules (bottom 2, 3).●ChloroplastgranumthylakoidLight-independent reactionThe light-independent reaction (LIR),sometimes called carbon fixation,occurs in the light and the dark—provided the LDR has providedenough energy and raw materials todrive it.● Energy captured by the LDR is used toreduce carbon dioxide in a complexseries of reactions to produce glucose(bottom 4, 5).Light-dependentstage in grana1Light-independentstage in stroma© Diagram Visual Information Ltd.●NADPH242ATP12345Sunlight energyWaterOxygenCarbon dioxideGlucose3531Chloroplast: structureUNITYKey wordsPlant cellchloroplastcytoplasmgranumlight-dependentreactionlight-independentreactionstromathylakoidSize and distributionChloroplasts are found in allphotosynthetic plants and are usuallylarge enough to be seen with the lightmicroscope as green disks embeddedin the cytoplasm.● Chloroplasts are not present in cellsthat receive no light, e.g., cells of theroot or deep inside plant bodies.● Chloroplasts are particularly rich incells in leaves and green stems.● Chloroplasts are the site of starchproduction in photosynthesis and ofstarch storage.● Chloroplasts are bounded by a doublemembrane—one derived from theenclosing cell and one from thechloroplast itself.●chloroplastchloroplast membraneouter membraneinner membraneChloroplastGranaEmbedded in thestroma is a complexnetwork of stackedsacs called grana.● The grana consist ofinterconnectedthylakoids.
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