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#plant cells

26 public questions tagged with this topic.

Basics of Cell Biology

Which of the following best describes a plastid?

Plastids comprise diverse double-membrane organelles in plant and algal lineages originating from primary endosymbiosis of cyanobacterium engulfed by eukaryote over billion years ago, retaining circular plastid genome encoding photosystem components and ribosomal RNAs and ability to divide by binary fission via FtsZ ring. Differentiation depends on tissue and environmental cues: chloroplasts in mesophyll contain thylakoid stacks grana where photosystem II, cytochrome b6f, photosystem I perform light reactions generating proton gradient for ATP synthase and NADPH for Calvin cycle fixing CO2 via Rubisco, chlorophyll a and b absorbing 680 and 700 nm light; chromoplasts develop from chloroplasts during fruit ripening accumulating carotenoids lycopene in tomato and beta-carotene in pepper conferring orange-red coloration and antioxidant nutritional value attracting frugivores; amyloplasts in roots and seeds store amylose and amylopectin starch granules and function as statoliths sedimenting for gravity perception; etioplasts in dark-grown seedlings hold prolamellar body. Unlike mitochondria focused on oxidative phosphorylation, plastids specialize in pigment storage and photosynthesis.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 14: Plastids, Chloroplasts, and Photosynthesis.

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Membrane Transportation

Which ATPase is crucial for acidifying vacuoles in plant cells?

Plant central vacuole occupies major cellular volume functioning in turgor generation, storage of ions, sugars, pigments, anthocyanins and toxic metabolites, pH regulation and degradation analogous to animal lysosomes. Luminal pH typically 5 to 5.5 maintained acidic relative to cytosol 7.5 to support proton coupled antiport of calcium via CAX, sodium via NHX, sugars and sequestration of xenobiotics. Acidification achieved by two proton pumps coexisting in tonoplast: V-type proton ATPase and proton pyrophosphatase using pyrophosphate. Dominant contributor is V-ATPase, multisubunit rotary machine similar to lysosomal pump, comprising V1 domain A3B3 hexamer hydrolyzing ATP and Vo domain containing c-ring proteolipids and subunit a forming proton path, consuming cytosolic ATP to move protons inward without phosphoenzyme intermediate, sensitive to bafilomycin and concanamycin. Pyrophosphatase provides backup using PPi from metabolism. F-type ATP synthase located exclusively in mitochondria chloroplasts synthesizes ATP driven by proton motive force, does not reside in vacuole. Na+/K+ ATPases absent in higher plants; calcium ATPases distinct. Thus plant vacuolar acidification crucially depends on V-ATPase creating proton motive force.

Ref: Sze et al., Trends Plant Sci, Plant Vacuolar V-ATPase Acidification of Vacuoles and Tonoplast.

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Membrane Transportation

Which type of solution would cause plasmolysis in plant cells?

Plant cell physiology depends heavily on water status because central vacuole can occupy 90 percent volume generating hydrostatic turgor pressure essential for cell expansion and mechanical support. Turgor results from water entry driven by intracellular solute accumulation, pressing protoplast against rigid cellulose cell wall. Membrane surrounding protoplast semipermeable allows water but restricts many solutes. If external soil solution or surrounding medium becomes hypertonic containing higher total effective concentration of non-penetrating solutes such as sodium chloride, mannitol or sucrose compared with vacuolar sap, extracellular water potential drops. Net water efflux occurs via aquaporins PIP and TIP, reducing vacuolar volume, decreasing pressure and causing plasma membrane to detach from cell wall. Cytological consequence visible under microscope as plasmolysis with protoplast rounding and chloroplasts clustering, wilting at whole plant level. Reversibility possible if protoplast returns to hypotonic solution restoring influx. Isotonic maintains volume, hypotonic increases turgor. Therefore hypertonic external conditions trigger plasmolysis in plant cells.

Ref: Taiz & Zeiger, Plant Physiology, Chapter 3: Plasmolysis and Hypertonic Effects in Plant Cells.

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Cell Organelles

Which plastid stores lipids?

Plastid diversity reflects differentiation of proplastids in meristems according to tissue identity and developmental signals regulated by light, hormones and transcription factors. Elaioplasts are specialized non-photosynthetic leucoplasts lacking chlorophyll that function in lipid storage and metabolism crucial for reproduction. They are abundant in oil seeds like Brassica, citrus exocarp oil glands, and anther tapetum where lipid reserves for pollen coat tryphine formation and pollinator attraction are needed. Ultrastructurally they contain numerous electron-dense plastoglobuli and oil droplets bounded by single inner envelope, with few internal thylakoids and extensive physical connections to endoplasmic reticulum for fatty acid trafficking via touching domains. They possess complete enzymes for fatty acid synthesis, including acetyl-CoA carboxylase, fatty acid synthase complex, and stearoyl-ACP desaturase, converting imported photosynthate like sucrose-derived pyruvate into triacylglycerols and sterol esters stored as osmiophilic globules. In contrast amyloplasts accumulate starch via ADP-glucose pyrophosphorylase, proteinoplasts store proteins in crystalline protein bodies, and chromoplasts accumulate carotenoids for coloration, illustrating functional specialization supporting plant physiology.

Ref: Wise & Hoober, The Structure and Function of Plastids, Chapter 9: Classification of Leucoplasts and Elaioplasts.

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Cell Organelles

In plants, the Golgi apparatus plays a role in:

In animal cells Golgi modifies proteins, but plant cells possess additional requirement to produce noncellulosic cell wall polysaccharides that constitute matrix embedding cellulose microfibrils. Golgi stacks, often four hundred per cell moving along actin tracks, synthesize pectins homogalacturonan methylesterified in cis medial then de-esterified and calcium crosslinked in trans and at wall, rhamnogalacturonan I II highly branched, and hemicelluloses xyloglucan backbone synthesized by CSLC glucan synthases, xylan, arabinan, galactan sidechains added by glycosyltransferases GT47, GT8 families. Nucleotide sugars UDP-galacturonic acid imported by transporters from cytosol where converted from glucose. Trans Golgi network vesicles fuse with phragmoplast cell plate during cytokinesis depositing wall material, also delivering cellulose synthase complexes to plasma membrane via secretory vesicles. ATP production confined to mitochondria chloroplasts, protein degradation vacuole, DNA replication nucleus. Mutants deficient in Golgi enzymes show dwarfism, altered extensibility, confirming central participation in wall biogenesis supporting turgor pressure and growth. Integration with cell cycle kinases, calcium signaling and mechanical cues ensures coordinated remodeling during growth, migration and differentiation.

Ref: Richmond Annu Rev Plant Biol; plant Golgi pectin hemicellulose synthases cell plate wall.

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