building tall structures
all plants use three basic priciples to build tall structures:
1. they produce strong cell walls out of cellulose, a polymer made from sugar molecules.
2. they build columns and tubes from specialised cells.
3. they stiffen these special cells with another polymer called lignin.
trees add a rng of this stiffened (lignified) tissue each year , and buiding up wood in this way allows trees to grow taller.
how are plant cells different from animal cells?
there are two main differences:
1. the plant cells has a rigid cell wall
2. the plant cell contains chloroplasts
these chloroplasts are the site of photosynthesis where energy from the sun is used to make storage molecules.
starch is found in storage vacuoles in the cytoplasm called amyloplasts.
another difference between plant and animal cells is that with plant cells there is often a large central vacuole surrounded by a vacuolar membrane.
how cellulose strenthens a plants cell walls.
A plants strength comes from the thin cellulose walls of plant cells and the 'glue' that hold them together.
cellulose is a polysaccharide which is a polymer of glucose. however the glucose in cellulose is slighly different with the -OH groups on the first carbon atoms being on opposite sides.
a condensation reaction betwen the -OH group on the 1st and 4th carbon of the adjacent glucose links the two glucose molecules.
in the cellulose there are only 1,4 glycosidic links making cellulose a long unbranched molecule.
microfibrils and bundles of cellulose molecules
typically a cellulose chain contains between 1000 and 10,000 glucose units which remains in a straight chain.
hydrogen bonds form between neighbouring cellulose chains forming bundles called microfibrils.
the large number of hydrogen bonds in the microfibril produces a strong structure.
these microfibrils are bundles of around 60-70 cellulose molecules arranged around the cell and stuck togther with a polysaccharide glue.
this glue is composed of short , branched polysaccharides known as hemicelluloses and pectins.
crossing the cell wall.
cell walls do not separate plant cell completely. narrow fluid-filled channels called plasmodesmata, cross the cell walls.
cell walls are also fully permeable to water and solutes.
at some areas the cell wall is thin becuase only the first layer of cellulose is deposited. the result is a pit in the cell wall.
plasmodesmata are often located in these pits , aiding the movement of substances between the cells.
tubes for transport and strength.
the build a tall plant like a tree some of the cells within the stem must be stiffened to provide mechanical support.
also some of the cells must allow water and minerals to pass from the roots and then leaves.
there are two types of cells which allow this to happen:
- xylem vessels - which form tubes for transport and their stiffened cell walls help support the plant.
- sclerenchyma fibres - columns of these cells with their stiffened cell walls also provide support