Plant Cell Structure and Function PDF

Plant Adaptation and Membrane Lipids One major strategy by which plants adapt to temperature change is to decrease the degree of unsaturation of membrane lipids under high temperature and increase it under low temperature.  Triglycerides Triglycerides ––Composed of 1 gly...

Plant Adaptation and Membrane Lipids One major strategy by which plants adapt to temperature change is to decrease the degree of unsaturation of membrane lipids under high temperature and increase it under low temperature.  Triglycerides Triglycerides ––Composed of 1 glycerol and 3 fatty Composed of 1 glycerol and 3 fatty acids acids Fatty acids Fatty acids ––Need not be identical Need not be identical ––Chain length varies Chain length varies ––Saturated – no double bonds between Saturated – no double bonds between carbon carbonatoms atoms (Higher melting point) Higher melting point, animal origin – Unsaturated – 1 or more double bonds – Unsaturated – 1 or more double bonds (Lower melting point) Low melting point, plant origin The lipid composition of plasma Membranes is highly variable: Phospholipids Glycolipids Sterols Barley root cell membranes contain twice as many sterol molecules as phopholipids - this ratio is reversed in leaves. Plasmadesmota Plant cells are connected By plasmadesmata Important in cell-cell communication Lined by the cell membrane And contain a strand of Endoplasmic reticulum.  Plasmodesmata Plasmodesmata – Connects plasma membranes of adjacent plant cells – Extends through cell wall – Allows materials to move from cytoplasm of one cell to cytoplasm of next cell Plasmodesmata Endoplasmic reticulum & proteins thought to control the flow of materials through the channel  Symplast – name for continuous cytoplasm in set of cells connected by plasmodesmata Plant cells are connected- despite presence of cell walls that provide structural support.  Apoplast Space outside cell – Next to plasma membrane within fibrils of cell wall – Area of considerable metabolic activity – Important space in plant but questionable as to whether it is part of the plant’s cells Plasma membrane & cell wall interface Symplast and Apoplast Symplast vs. apoplast  The Cell Wall Provides Structural Support and Protection The shape of a plant cell is dictated by its cell wall. When living plant cells are treated with cell-wall degrading enzymes the resulting membrane- bound protoplast is invariably spherical  Cell Wall Structure Primary cell wall – Cell wall that forms while cell is growing – Microfibrils of cellulose (unbranched chain of the sugar glucose. Secondary cell wall – Additional cell wall layer deposited between primary cell wall and plasma membrane – Generally contains cellulose microfibrils and water-impermeable lignin – Provides strength to wood  Plant Cell Wall Components The cellulose micofibrils form the scaffold of all cell walls and are tethered together by cross-linking glycans. This framework is embedded in gel of pectic substances. Specialized types of cell walls – cutin covering cell wall or suberin imbedded in cell wall – Waxy substances impermeable to water – Cutinized cell walls Found on surfaces of leaves and other organs exposed to air Retard evaporation from cells Barrier to potential pathogens  The Plasma Membrane of a turgid plant cell is pressed tightly against the cell wall Cell wall: rigidity and strength Osmotic forces balanced by pressure exerted by cell wall – Creates turgor pressure – Causes cells to become stiff and incompressible – Able to support large plant organs – Loss of turgor pressure – plant wilts! Cell Wall: Osmotic forces balanced by pressure exerted by cell wall Osmotic forces balanced by pressure exerted by cell wall – Creates turgor pressure – Causes cells to become stiff and incompressible – Able to support large plant organs – Loss of turgor pressure – plant wilts Cell wall: rigidity and strength Elongating cell is stretched by Turgor  Cell Wall Expansion Wall loosening and incorporation of new wall materials Biosynthesis of the wall requires a coordination of activities  There are primary and secondary cell walls A major component of secondary cell walls is lignin.  In plants, plastids divide by fission and differentiate Seeds, embryonic, Dark grown meristems and photosynthetic reproductive tissues tissue Leaf Flower, fruit Storage of starch, oils and proteins  Parts of Chloroplasts Component Description Inner membranes of chloroplast, contain Thylakoids proteins that bind with chlorophyll Thick enzyme solution surrounding Stroma thylakoids Green pigment that gives plant tissue its Chlorophyll green color Storage form of carbohydrates produced Starch grains during photosynthesis  There are different types of Plastids Prefix Meaning Function Photosynthesis, convert light energy into chemical “yellow- energy, store Chloroplast “chloro –” green” carbohydrates as starch grains Store carbohydrates in Leukoplast “leuko –” “white” form of starch Leukoplasts that contain Amyloplast “amylo –” “starch” large granules of starch Stores carotenes and xanthophylls, give orange- Chromoplast “chromo –” “color” to-red color to certain plant tissues  Proplastids Proplastids – Small plastids always found in dividing plant cells – Have short internal membranes and crystalline associations of membranous materials called prolamellar bodies – As cell matures, plastids develop Prolamellar bodies reorganized Combined with new lipids and proteins to form more extensive internal membranes  Chromoplasts chromo – “color” Found in some colored plant tissues – tomato fruits, carrot roots – High concentrations of specialized lipids – carotenes and xanthophylls – Give plant tissues orange-to-red color  Amyloplasts amylo – “starch” Leukoplast that contains large starch granules  Leukoplasts leuko – “white” Found in roots and some nongreen tissues in stems No thylakoids Store carbohydrates in form of starch Microscopically appear as white, refractile, shiny particles  Chloroplasts Chloroplasts – Thylakoids Inner membranes Have proteins that bind to chlorophyll – Chlorophyll Green compound that gives green plant tissue its color – Stroma Thick solution of enzymes surrounding thylakoids  Chloroplasts Chloroplasts – Function Convert light energy into chemical energy (photosynthesis) Accomplished by proteins in thylakoids and stromal enzymes Can store products of photosynthesis (carbohydrates) in form of starch grains  Origin of Plastids Plastids are the product of an ancient symbiosis between a eukaryote and a cyanobacterium. All plastids trace back to a single endosymbiotic event : – Primary endosymbiosis There have also been – Secondary endosymbioses  Eukaryotic photosynthesis is derived from endosymbiosis A single primary Ancestral endosymbiotic event* about cyanobacterium 1.5 billion years ago gave rise to chloroplasts in chlorophytes (green algae and plants), red algae, and glaucophytes *A second more recent event that gave rise to Paulinella Rhodophytes Chlorophytes Glaucophytes chromatophora is described later Green plants This lesson focuses on Brown algae, diatoms, Secondary, tertiary photosynthesis as it dinoflagellates, endosymbiosis occurs in plants, green euglenoids…. algae and cyanobacteria Cyanobacterial cell The cytoplasm contains thylakoid membranes and aggregates of rubisco enzyme. A scheme for the origin and evolution of all plastids by primary and secondary endosymbiosis  Mitochondria Matrix – Viscous solution of enzymes within cristae Function – source of most ATP in any cell that is not actively photosynthesizing – Site of oxidative respiration – Release of ATP from organic molecules – ATP used to power chemical reactions in cell  Vacuole Functions – May accumulate ions which increase turgor pressure inside cell – Can store nutrients such as sucrose – Can store other nutritious chemicals – May accumulate compounds that are toxic to herbivores – May serve as a dump for wastes that cell cannot keep and cannot excrete  Vacuoles Large compartment surrounded by single membrane Takes up large portion of cell volume Tonoplast – Membrane surrounding vacuole – Has embedded protein pumps and channels that control flow of ions and molecules into and out of vacuole  Plant Vacuoles are Multifunctional 1. Turgor pressure: growth 2. Storage: ions, sugars, polysaccharides, pigments. Can be retrieved from vacuoles when needed. Flavors of fruits and vegetables. 3. Digestion: acid hydrolases: proteases, nucleases, glycosidases, lipases 4. Ph and ionic homeostasis: reservoirs of protons. 5. Defense against microbial pathogens and herbivores. Toxic compounds 6. Pigments 7. Compounds Toxic to plants.  Vesicles Small, round bodies surrounded by single membrane – Peroxisomes and glyoxysomes Compartments for enzymatic reactions that need to be separated from cytoplasm – Lysosomes Contain enzymes that break down proteins, carbohydrates, and nucleic acids May function in removing wastes within living cell Can release enzymes that dissolve the entire cell  Peroxisomes Nearly 50 enzymes have been localized to plant and animal Peroxisomes. Catalase which is always present. This enzyme breaks down hydrogen peroxide. Glycolate pathway Endoplasmic Reticulum: makes proteins & lipids for export ER Branched, tubular structure Often found near edge of cell Function: – Site where proteins are synthesized and packaged for transport to other locations in the cell – Proteins injected through membrane into lumen Endoplasmic Reticulum: makes proteins & lipids for export Types of ER – Rough ER – ribosomes attached to surface (=proteins) – Smooth ER – no ribosomes (= lipids) Carbohydrates transported with proteins – Often attached to proteins in the ER – Helps protect carbohydrates from breakdown by destructive enzymes Endoplasmic Reticulum (rough): makes proteins for export Packaging of proteins by ER – Considered to be packaged when separated from cytoplasm by membrane – Vesicle of membrane-containing proteins may bud off from ER – Vesicle carries proteins to other locations in cell