Features of Plant Cells Flashcards
Plant Cell Wall
shape, controls growth, structural barrier, food, fiber, energy
Function of Cell Wall
rigidity and form to the plant
barrier: permeable only to small molecules (water, ions, etc.)
exist with small molecules from the environment
large molecules are synthesized
Composition of Cell wall
Primary wall and secondary wall
Primary wall
- young, undifferentiated cells
- in nearly all cells
- resists the pressure that builds up when water accumulate by osmosis
- composed of cellulose, hemicellulose, pectin, and protein
Secondary wall
- differentiated cells
- in only specialized cells
- lignin
Primary and Secondary wall
- both external to cell membrane
- extremely complex
- little known about regulation of synthesis
- provide support and structure to the plant but they also limit what can enter cells
Primary wall composition
"fibers embedded in a matrix" "steel rods in concrete" 90% polysaccharide (hemicellulose, pectin - matrix/ cellulose fibers) 10% protein
Cellulose
least complex long, unbranched polymer of glucose beta 1 -> 4 linkage not known what controls length 2 cellulose chains H-bonded forms dimer synthesized at cell surface: UDP-glucose precursor (2 binding sites)
Microfibrils
determine direction of growth:
- random: multidirectional (storage tissue)
- oriented: growth perpendicular to microfibrils, elongating cells (roots/stems)
outside of cell
How is orientation achieved?
microtubules orient microfibrils
separated by cell membrane
Microtubules
composed of tubulin, inside the cell
Hemicellulose and Pectin
more complex than glucose
golgi -> vesicles -> cell membrane
Hemicellulose
heterogenous, branched polysaccharide beta 1 -> 4 backbone monosaccharide side chains xyloglucan - major hemicellulose H-bonded to each other and microfibril
Pectin
heterogenous, branched polysaccharide many galacturonic residues Ca+2 binds to negative charges covalently attached to hemicellulose l.is abundant in the middle lamella middle lamella is a pectin layer which cements the cell walls of two adjoining plant cells together, outside the cell membrane
Why is Ca+2 levels low in actively growing cells?
Ca2+ plays a key role in cross-linking acidic pectin residues. Low [Ca2+] levels increases the permeability of the plasma membrane.
Hemicellulose and Pectin Synthesis
little is known
defining structure
complexity of polymers suggests a complex enzymatic system (delicate coordination)
Proteins/Enzymes in the Cell Wall
Visualize with antibodies
Copurification with cell wall
Enzyme activity lost during protoplast isolation
Substrates can be modified that don’t penetrate cell wall
enzymes include transglycosylases and expansins
transglycosylase
specific for xylose (hemicellulose)
expansins
break H-bonds between cellulose and hemicellulose
Secondary Cell Wall
not present in all cells
outside cell membrane, inside primary wall
(PM -2 CW - 1 CW)
thicker than primary wall
can kill protoplast
50% cellulose, 25% hemicellulose, 25% lignin
Lignin
insoluble in water/ water-resistant
polymer of oxidized, aromatic alcohols
exact structure not known
fibers and wood (fuels)
Functions of Secondary Cell wall
strengthens cell and plant impermeable to water component of the xylem (water transport) wood = secondary cell wall lignin strengthens and waterproofs cell wall
Plasmodesmata
a system of small, narrow cytoplasmic channels (about 40-50 nm diameter)
700-1000 daltons MW can pass through
allows communication and transport between cells
includes desmotubule and cytoplasmic sleeve
formed during cytokinesis
Desmotubule
narrow cylindrical tube that runs through cytoplasmic channel
continuous with ER
16-20 nm diameter
Cytoplasmic sleeve
main pathway for transport
fluid-filled space
continuous with cytosol
Function of Plasmodesmata
1) cell-cell communication
- prevalent in secretory tissues
2) transport (two transport systems)
- desmotubule and cytoplasmic sleeve
What drives transport through the plasmodesmata?
symplast and apoplast pathway systems tranport ions and molecules such as water
Symplast
transport within cytoplasm, which are interconnected by plasmodesmata
Apoplast
transport along cell wall and intercellular spaces
helps to transport water and ions from the soil through root to xylem elements.
Vacuole
large, central compartment surrounded by a differentially permeable outer membrane called tonoplast
immature cells have many small vacuoles
mature cells have one large vacuole
vacuole is in osmotic equilibrium
vacuole composition is different from
cytoplasm
Tonoplast
tonoplast thicker than cell membrane tonoplast has permeability characteristics
Functions of Vacuole
Degradation, Storage, and Space-Filling
Degradation
comparable to lysosome
contains hydrolytic activities
enzymes may be exocytosed (out of cell)
organelles endocytosed (into cell) - active ingestion, entrapment
recovery of nutrients at senescence (aging)
Storage
vacuole content different from cytoplasm
lower pH (3.5-5.5) than cytoplasm (7)
molecules accumulate in vacuole by membrane bound transporters
vacuole helps maintain cell homeostasis
regulate enzyme activity - energy required (ATP)
to accumulate ions, amino acids, organic acids, glucose/sucrose, pigments (anthocyanins) via transport reactions (H+-ATPase, proton cotransport via symporter) proton gradient across tonoplast transport ions and molecules into the vacuole
Space Filling
costly for cell to be filled with cytoplasm
high solute concentration in vacuole
water enters vacuole by osmosis
water accumulation in vacuole creates pressure (turgidity)
despite resistance of cell wall, turgidity results in cell expansion and plant growth.
Plastids
virtually in every cell (except sperm) double outer membrane, large cytoplasmic organelles has an internal membrane system (thylakoids) - grana - stroma lamellea Has a stroma - multiple copies of DNA interconvertible capable of division differentiate into each other
Types of Plastids
1) Proplastid - precursor
2) Leucoplast - nonpigmented storage
3) Chromoplast - nonphotosynthetic pigments (fruit/flower)
4) Chloroplast - most important
Chloroplast
plastid that contains chlorophyll and in which photosynthesis takes place has an outer and inner membrane stroma - space thylakoids - flattened sacs grana - stacks of thylakoids stroma lamellae - connects grana
