Features of Plant Cells

Question 1

Plant Cell Wall

Answer 1

shape, controls growth, structural barrier, food, fiber, energy

Question 2

Function of Cell Wall

Answer 2

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

Question 3

Composition of Cell wall

Answer 3

Primary wall and secondary wall

Question 4

Primary wall

Answer 4

• 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

Question 5

Secondary wall

Answer 5

• differentiated cells
• in only specialized cells
• lignin

Question 6

Primary and Secondary wall

Answer 6

• 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

Question 7

Primary wall composition

Answer 7

"fibers embedded in a matrix"
"steel rods in concrete"
90% polysaccharide 
(hemicellulose, pectin - matrix/ cellulose fibers)
10% protein

Question 8

Cellulose

Answer 8

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)

Question 9

Microfibrils

Answer 9

determine direction of growth:
- random: multidirectional (storage tissue)
- oriented: growth perpendicular to microfibrils, elongating cells (roots/stems)
outside of cell

Question 10

How is orientation achieved?

Answer 10

microtubules orient microfibrils

separated by cell membrane

Question 11

Microtubules

Answer 11

composed of tubulin, inside the cell

Question 12

Hemicellulose and Pectin

Answer 12

more complex than glucose

golgi -> vesicles -> cell membrane

Question 13

Hemicellulose

Answer 13

heterogenous, branched polysaccharide
beta 1 -> 4 backbone
monosaccharide side chains
xyloglucan - major hemicellulose
H-bonded to each other and microfibril

Question 14

Pectin

Answer 14

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

Question 15

Why is Ca+2 levels low in actively growing cells?

Answer 15

Ca2+ plays a key role in cross-linking acidic pectin residues. Low [Ca2+] levels increases the permeability of the plasma membrane.

Question 16

Hemicellulose and Pectin Synthesis

Answer 16

little is known
defining structure
complexity of polymers suggests a complex enzymatic system (delicate coordination)

Question 17

Proteins/Enzymes in the Cell Wall

Answer 17

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

Question 18

transglycosylase

Answer 18

specific for xylose (hemicellulose)

Question 19

expansins

Answer 19

break H-bonds between cellulose and hemicellulose

Question 20

Secondary Cell Wall

Answer 20

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

Question 21

Lignin

Answer 21

insoluble in water/ water-resistant
polymer of oxidized, aromatic alcohols
exact structure not known
fibers and wood (fuels)

Question 22

Functions of Secondary Cell wall

Answer 22

strengthens cell and plant
impermeable to water
component of the xylem (water transport)
wood = secondary cell wall
lignin strengthens and waterproofs cell wall

Question 23

Plasmodesmata

Answer 23

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

Question 24

Desmotubule

Answer 24

narrow cylindrical tube that runs through cytoplasmic channel
continuous with ER
16-20 nm diameter

Question 25

Cytoplasmic sleeve

Answer 25

main pathway for transport fluid-filled space continuous with cytosol

Question 26

Function of Plasmodesmata

Answer 26

1) cell-cell communication - prevalent in secretory tissues 2) transport (two transport systems) - desmotubule and cytoplasmic sleeve

Question 27

What drives transport through the plasmodesmata?

Answer 27

symplast and apoplast pathway systems tranport ions and molecules such as water

Question 28

Symplast

Answer 28

transport within cytoplasm, which are interconnected by plasmodesmata

Question 29

Apoplast

Answer 29

transport along cell wall and intercellular spaces | helps to transport water and ions from the soil through root to xylem elements.

Question 30

Vacuole

Answer 30

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

Question 31

Tonoplast

Answer 31

tonoplast thicker than cell membrane tonoplast has permeability characteristics

Question 32

Functions of Vacuole

Answer 32

Degradation, Storage, and Space-Filling

Question 33

Degradation

Answer 33

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)

Question 34

Storage

Answer 34

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

Question 35

Space Filling

Answer 35

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.

Question 36

Plastids

Answer 36

``` 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 ```

Question 37

Types of Plastids

Answer 37

1) Proplastid - precursor 2) Leucoplast - nonpigmented storage 3) Chromoplast - nonphotosynthetic pigments (fruit/flower) 4) Chloroplast - most important

Question 38

Chloroplast

Answer 38

``` 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 ```