Plant tissues

The stem

Complex parts

Vascular bundle, which is formed of = Xylem, Phloem and sclerenchyma fibres

Xylem

Transports mineral ions and water upwards through the stem.

It also provides support

It is a long, hollow tube made up of dead cells

Red cells = wide hollow with a large lumen and thick ish walls

Secondary thickening

lignin forms during a process called secondary thickening

When XYLEM and SCLERENCHYMA CELLS which have finished growing produce a secondary cell wall between the normal cell wall and the cell membrane.

This second cell wall is thicker than the first cell wall and contains a higher proportion of lignin

Xylem structure

Hollow tube, dead cells = TRANSPIRATION: transports water and mineral ions up the stem

No walls between cells = capillary action, uses cohesive and adhesive forces to draw water up the stem

Lignin in xylem walls (spirals or rings) = supports stem, gives tensile strength and water proofing

Pits = so water and minerals can move out of the xylem

Phloem

Blue cells = hollow lumen and thin walls. Between xylem and fibres

Phloem structure

Sieve tube elements = translocation of solutes

Living cells = need to transport substances up and down the plant ( doesn’t rely on capillary action like xylem )

No nucleus, thin cytoplasm, few organelles, shared cytoplasm = to allow space for transport of sucrose, amino acids

Companion cell = to conduct essential processes e.g. respiration

Sieve tube

Sieve plates with sieve pores = allows for the continuous movement of the organic compounds

Cellulose cell wall = strengthens the wall to withstand the hydrostatic pressure that move the assimilates

No nucleus, vacuole or ribosomes in mature cells(some ER and mitochondria are present) = maximises the space for translocation of the assimilates

Thin cytoplasm = reduces friction to facilitate the movement of the assimilates

Companion cells

Nucleus and other organelles present e.g. RER = provides metabolic support to sieve tube elements and helps with the loading and unloading of the assimilates

Transport proteins in plasma membrane = moves assimilates into and out the sieve tube elements

Large numbers of mitochondria = to provide ATP for the active transport of assimilates into or out of the companion cells

Plasmodesmata (channels in the cell wall) = the link to sieve tube elements which allows organic compounds to move from the companion cells into the sieve tube elements

Sclerenchyma structure

Long fibres run length of stem = support the stem

Cell walls impregnated with lignin and lots of cellulose (thick walls) = very strong

No pits = don’t transport anything

Hollow lumen and end walls = strong and supportive

Dead cells = don’t transport anything. Allows lignification

Explain how the structural features of xylem help carry outs its function

Transport mineral ions and water upwards through the stem

Hollow tubes made up of dead cells

Lignin in xylem walls (rings) supports the stem gives tensile strength and water proofing

Pits = strong so they do not collapse

Describe the structure of a cellulose cell wall

Cellulose molecules held together by H bonds as a microfibril

Microfibrils in a parallel and forms a mesh layer

Unbranched beta glucose which are held together by glycosidic bond

Hemicellulose

Cellulose

Give two reasons why a sieve tube element does not require RER and ribosomes

No transcription and no mRNA, so ribosomes not required for translations

No proteins will be synthesised to be processed in endoplasmic reticulum