Plant Cell: Overview Fiona Doris, University College, Dublin, Ireland Martin Steer, University College, Dublin, Ireland Although plant cells share the same eukaryotic ancestor cells as animal cells, and hence bear strong resemblance to one another, there are many differences between the two. These differences reflect the distinct structural, developmental and environmental pressures which plant cells experience and include the presence of cell walls, differences in membrane structure and changes to the endomembrane system and structures associated with cell division. Introduction Plantsevolvedfromthesameeukaryoticancestorcellsas animals,soitisnotsurprisingthatthecellandmolecular biology of plants bears a strong resemblance to that of animals. However, there are differences. Some may just have evolved with the passage of time, while others are directed to optimizing the efficiency of a rather different modeofnutritionandlifeform.Amajordifferenceisthat plants exhibit indeterminate growth, generated by shoot androotmeristems.Also,cellsfromalmostanypartofthe plantbodycanbeinducedtoundergodivisionstoformnew meristems, which can give rise to new plants. This ‘totipotency’,theabilitytosummonuptheentirenuclear genome programme of a differentiated cell, is in marked contrasttotheterminaldifferentiationofanimalbodycells. The evolution of a stable, more or less permanent, cell wallmarkedasignificantstepinthedevelopmentofplants. Cell walls may have arisen as the result of the ready availabilityofcarbohydratemonomers,resultingfromthe photosynthetic activity of chloroplasts. The cell wall imposed a physical barrier on the access of everything exceptsolutesandthesmallestparticles( 5 10000Da)to theouterfaceoftheplasmamembrane.Theevolutionof multicellular forms means that cells of these individuals live in boxes of carbohydrate that are protective in providinganessentialexternalwaterphase,butrestrictive inwhattheyallowtopassfromcelltocell. Anadditionaladvantageofthecellwallisthatitreleases the organism from a requirement for active water regulationtocopewiththeosmoticuptakeofwater.The osmotic potential gradient drawing water into the cell is matchedbyaninternalhydrostaticpressuregeneratedby theresistanceofthecellwalltoexpansion.Thelargecell vacuoleactsasasinkforsolutesandservestospreadthe cytoplasm in a thin layer against the plasma membrane. Thisovercomesthemajorlimitationoncellsize,theneed tomaintainaconstantsurfacearea-to-volumeratio. Ageneralizedcellisshownin Figure 1.Thefunctionsof themaincomponentsandthedifferencesbetweenthemin animalsandplantsarediscussedbelow. Plasma Membrane Aswithanimalcells,researchonplantplasmamembranes has identified specific features associated with ion trans- port, glycoprotein insertion and cytoskeletal attachment. Aspectsoftheplasmamembranethatareuniquetoplants include cell wall synthesis, the cell-to-cell transport channels,plasmodesmataandlipidcomposition. Evidence for the crosslinking of the plasma membrane to the cell wall comes from observations during plasmo- lysis. First, the solute concentration required to cause plasmolysis is higher than predicted on the basis of the internal cell solute concentration, suggesting that some additional resistance must be overcome. Second, micro- scope observations reveal the presence of connecting strands (Hechtian strands) between the plasma mem- brane and the detached cell wall of partially plasmolysed cells. Onthecytoplasmicside,microtubulesarefoundcross- linked to the plasma membrane. Antimicrotubule agents causechangesintheplasmamembrane,disruptingcellwall formation.Antiactindrugsdodisruptsomekeyprocesses such as cell extension and tip growth, but the molecular interactionshaveyettobecharacterized. Cell Walls Plantcellwallsservetwofunctions:theyprovidemechan- ical support and maintain an aqueous phase at the extracellularleafletoftheplasmamembrane. Mechanical support can be exerted through turgor pressureoftheprotoplastpushingoutonthecellwall,like a balloon skin. In nonwoody stems and leaves the outer wallsoftheepidermalcellsarespeciallythickenedtoresist thestretchingforcesofthetissuecellswithin,holdingthe structuresemirigid.Wiltingisduetolossofturgorpressure andcollapseoftheinflationpressure. Article Contents Introductory article . Introduction . Plasma Membrane . Cell Walls . Endomembrane System . Nucleus and Cell Division . Vacuoles . Microbodies . Plastids . Mitochondria . Cytoskeleton 1 ENCYCLOPEDIA OF LIFE SCIENCES © 2001, John Wiley & Sons, Ltd. www.els.net