plant cell walls

purpose of the cell wall

• maintaining/ determining cell shape

• support and mechanical strength

• prevents the cell membrane from bursting in a hypotonic medium (resists water pressure)

• controls the rate and direction of cell growth and regulates cell volume

• physical barrier to pathogens & water in suberized (root) cells

• carbohydrate storage

• signaling

• economic products

• biofuels

carbohydrate storage in the cell wall

components of the cell wall can be reused in other metabolic process (especially seeds

signaling in the cell wall

• fragments of the wall, oligasaccharins, act as hormones

  • stimulate ethylene synthesis

  • induce phytoalexin (defense chemicals produced in response to a fungal/ bacterial infection)

  • induce chitinase

  • increase cytoplasmic calcium levels

  • cause an “oxidative burst”

biofuels from the cell wall

• via fermentation

• cellulosic ethanol

  • produced from wood, grasses, or the inedible parts of plants

• produced from lignocellulose

  • structural material that comprises much of the mass of plants

  • comprised of cellulose, hemicellulose, and lignin

what is the main ingredient in cell walls?

polysaccharides

• carbohydrates

• lipids (fats)

• proteins

components of the cell wall

• cellulose

• cross-linking glycans

• pectic polysaccharides

• protein

• lignin

• suberin, wax, cutin

• water

cellulose in the cell wall

1→4 beta linkage (alternating)

cross linking glycans in the cell wall

it’s short length & branches allows it to link microfibrils

pectic polysaccharides in the cell wall

• thickening agent in jams & jellies

• pectin 2 is a polysaccharide

  • monomer is glucose (alpha 1→ 4 linkage)

lignin in the cell wall

• only found in the secondary cell well

• comprised of a hydroxyl group & benzene ring

primary meristem

refers to the three fundamental tissue types (protoderm, ground meristem, procambium) that the apical meristem produces, which then from the plant’s primary tissues (epidermis, ground tissue, vascular tissue) and contribute to growth in length

apical meristem

the specific location (tips of roots & shoots) that are the source of primary growth

phragmoplast

the dynamic cellular machinery (microtubules, actin, vesicles) that builds the cell plate

• the construction crew & the tools

• cell plate is the actual wall being built between the two new daughter cells

where is cellulose made?

• cell surface

• catalyzed by cellulose synthase

  • made in ER by ribosomes

  • ER → vesicles → golgi → cell membrane

  • two catalytic sites that transfer two glucoses at a time

cellulose synthase

• large protein complex embedded in the plasma membrane of plants

• synthesizes cellulose

  • does so by linking glucose units from UDP-glucose into long chains

  • forms microfibrils for structural support

UDP- glucose

a vital, activated sugar molecule used by all living cells as a building block and donor for synthesizing complex carbohydrates like glycogen (energy storage), sucrose (plant sugar), and lipopolysaccharides (bacterial cell walls)

acid growth hypothesis

explains how the plant hormone auxin promotes cell expansion by acidifying the cell wall (apoplast), causing it to loosen and stretchy under turgor pressure

extansins

patch holes in the cell wall

expansins

appear to be the primary wall-loosening enzymes