AS SNAB Topic 4-Plants

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Cell wall-


Cell wall- gives uniformed shape + support- largely composed of insoluble cellulose.

Cellulose like complex carbs. E.g. starch- made up long chains of glucose joined by glycosidic bonds.

Made of glucose isomers α and β

Glucose alpha/beta molecules = 1,4-glycosidic bonds- join in condensation reaction, break in hydrolysis

Between molecules glucose, hydrogen bonds form due to OH sticking out from beta glucose.

Cellulose thus = strong.

Cellulose straight chains, unlike starch that compacts

Cellulose fibres form microfibres-in layers, held by matrix of hemicelluloses + short chain carbs.

Microfibres spiral- spiral around ‘box of cell’ Closer/more vertical spirals= stronger wall is.

Cells are turgid, when wilt due to reduced water = flaccid.

Typically- cell wall permeable to anything dissolved in water- does not act as barrier.

But cell can become laden with suberin or with lignin, to produce wood, these block the cell.

Middle lamella- first layer when new cell is formed from 2 cells dividing.

Largely made of Pectin- a polysaccharide acts as holding agent and holds neighbouring cell walls together.

Pectin has many neg. charged carboxyl (----COO)- groups, these combine with +calcium ions to =

Calcium pectate- binds cellulose that forms either side.

Cellulose builds up either side of Middle Lamella,- initially flexible, (primary cell walls), with ages, secondary thickening occurs- with second wall with matrix of microfibrils = more rigid.

Hemicelluloses harden it further.

Long cells with cellulose cell walls that are heavily lignified (lignin makes wood) = plant fibres

Cells have intracellular exchange through cytoplasmic bridges between cells- PLASMODESMATA

Are produced as cells divide- cells don’t completely separate- threads cytoplasm remains. Threads pass through gaps in new cell wall and allow transfer of signalling substances. The interconnected cytoplasm of the cells = SYMPLAST.

Stem= support= holding leaves in best position for photosynthesis

= hold flowers for pollinating

=flexible to not be damaged by rain/wind

=movement of materials

Stems are organs that contain different tissues

Cross section of a stem: axis of a plant.
Epidermis: outer layer of the stem.
Phloem vessels: tubes that carry sap.
Cambium: new parts of the stem.
Cork: protective covering of the stem.
Pith: central part of the stem.
Xylem vessels: woody part of the stem.

Epidermis- secretes cutin- waxy substance, helps prevent water loss/pathogen entrance . epidermis doesn’t support- protects cells beneath it.

Epidermal cells may form hairs- as single cell extension or modified epidermal cells= insulating, trapping moist air to reduce transpiration (water loss). Some of hairs are gripped to help climbing- some protective like nettle.

Most common type of plant cells= ‘packing tissue’= PARENCHYMA cells. They are unspecilized cells but can be modified to make suitable for storage and photosynthesis. E.g. Outer later of PARNCHYMA cells in stem may contain some chloroplasts.

COLLENCHYMA cells- thick primary walls- thickest at corners= hexagonal shape. This gives tissue strength. Found outside the stem- inside epidermis. Remain living so they stretch as plant grows. Collenchyma makes up the strings in


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