Plant Biology Exam 2 Study Guide Lecture 11 Special features All plants have plastids they can be modified for several specialized functions about 100 genes o Chloroplasts photosynthesis DNA is about 150 kn and encodes Composed of outer and inner membranes granum lumen stroma and thylakoids Thylakoid membranes are where the photosynthetic light reactions take place Evolution of the chloroplast Loss of their own cell wall After the initial endosymbiosis The majority of genes transferred to the nucleus Development is under the control of the nucleus Various forms not every cell has a chloroplast but they all have some forms Development proplastid vesikel thylakoid granum thylakoidstapel o Amyloplasts starch storage Function in gravitropic response turning or growth movement by a plant in response to gravity o Chromoplasts carotenoid pigment storage Proplastid early stage of plastids little thylakoid present in meristems shoot and root Etioplast plastid in the dark The cell wall provides structural support o Composed of cellulose rods microfibrils cellulose is a very long polymer of glucose 1 4 linkages chains over 10 000 glucoses long H bonding between cellulose chains allows them to form very stable crystalline rods cellulose microfibrils cellulose synthase enzyme complex responsible for cellulose synthesis located in the plasmomembrane UDP glucose is fed from inside the side Cellulose microfibrils are deposited to the outside o Diverse group of shorter chain polysaccharides hold cellulose chains together o Proteins receptors ion channels o Two types Primary cell wall found in plant cells that are growing enlarging Rods of cellulose microfibrils provides strength embedded in a matrix of other shorter chain polysaccharides hemicelluloses and pectins Can be stretched and enlarged to accommodate cell growth Secondary cell wall laid down by plant cells when cell growth has ceased Composed of approximately equal parts cellulose microfibrils and lignin lignin is composed of phenolic compounds that are polymerized into a matrix that is a stiff rigid glue o lignin in the cell wall adds compressive strength and rigidity o lignin is the second only to cellulose as the most abundant compound on Earth Typically 50 cellulose and 50 lignin Cell wall thickening and cell expansion stops Once secondary wall formed the cell can no longer grow Found in collenchyma cells tracheids vessel elements and fibers sclerids Cotton fibers are secondary cell wall 90 cellulose Wood is secondary xylem cellulose lignin Primary cell wall reviewed Secondary cell wall reviewed o Cellulose semicellulose and pectin o Deposited early when cells are still expanding o Usually thin o Permit cell growth o No lignin o Cellulose semicellulose and pectin o Deposited later when cells no longer expand o Usually thick o Limit cell growth o Have lignin Hemp jute and flax fibers are from stems can be woven into cloth rope and Plant cells have a large central vacuole containing a concentrated solution of mats potassium salts o Primary function of central vacuole is to generate turgor pressure a positive internal hydrostatic pressure which keeps plant cells rigid and provides the driving force for plant cell enlargement o Vacuole takes a large volume of a cell o Most of the content in vacuoles is water o Vacuole contains salt and is membrane bound high osmoticum chemical agent employed to maintain the osmotic potential of a nutrient medium equivalent to that of the cultured cells o Vacuole and plant cell growth osmotically driven water uptake turgor pressure cell growth Plasmodesmata cytoplasmic connections between neighboring plant cells o Permit the movement of small molecules between plant cells o Molecules of molecular weight 800 can diffuse through o Proteins and RNAs can also be actively transported through o Contained within the primary pit field o The pits in the side walls of vessels and tracheids are the remnants of plasmodesmata plasmodesmata o Formation animal and plant cell division In animals fission Lecture 12 Photosynthesis Photosynthesis occurs in two separate reactions the light reaction and the Calvin cycle 3CO2 6H2O and light C3H6O3 3O2 3H2O o Photosynthesis is the only route by which virtually all energy enters the biosphere o Not all photosynthetic plants produce oxygen o Light reaction takes plant on the thylakoid membranes and light captured by chlorophyll leads to electron transport and finally to the formation of ATP from ADP and the reduction of NADP to NADPH Occurs in the grana thylakoid membranes Light energy is converted into chemical energy ATP H2O is split into O2 and H o The Calvin cycle dark reaction takes place in the stroma of the chloroplast and fixes CO2 into carbohydrates using ATP and NADPH CO2 is converted into sugar using the chemical energy produced in the light reaction occurs in the stroma 6 turns of the Calvin cycle are needed to make one glucose excess glucose can be stored as starch the dark reaction carbon fixation Rubisco enzyme that carries out CO2 fixation Converts a 5 carbon sugar RuBP into two 3 carbon sugars PGA Has dual activity carboxylase and oxygenase The oxygenase activity is unwanted When CO2 is low Rubisco binds O2 and breaks down RuBP to glycolyate and eventually to CO2 PHOTORESPIRATION RuBP CO2 H2O with Rubisco 2PGA Photorespiration reduces photosynthetic efficiency by 30 40 o Occurs in peroxisome and mitochondria C3 plants when temp is high C3 plants suffer from photorespiration o Fix CO2 into PGA a 3 carbon compound C4 plants nature s solution to the photorespiration problem o Sugar cane maize and tropical plants o Fix CO2 into a 4 carbon compound malate o More efficient than C3 plants at high temps and moderately dry conditions o Spatial separation of CO2 fixation and Calvin cycle in C4 plants Mesophyll cells site of CO2 fixation CO2 malate Bundle sheath cells site of Calvin cycle The chloroplasts in mesophyll and bundle sheath cells are connected o Make malate in regular mesophyll cells and do the Calvin cycle in Bundle sheath cells which surround the vascular bundles o C4 photosynthesis Stage 1 initial fixation of CO2 to form 4 carbon acids Stage 2 release of CO2 to Calvin cycle CAM photosynthesis Crassulacean acid metabolism a C4 modification o Temporal separation of CO2 fixation and the Calvin cycle o Use malate to fix CO2 stored in the vacuole as malic acid malic acid is also called Crassulacean Acid o CAM plants i e pineapple and cacti Save H2O Grow in dry
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