OCR - A-Level - Biology - Transport in Plants

Transport in Plants

Xylem

Phloem

Types of Xylem cell

Xylem vessel consist of dead hollow cells with lignified walls, making them waterproof

Linked end to end to create a hollow tube, with sidewalls that allow lateral movement of water

Parenchyma

Living cells with thin walls

Can store water, turgidity for support

Fibres

Lignified walls to provide strength

Water enters through root hair cells. Moves into xylem in the centre. All due to osmosis (cell sap has (in)organic molecules dissolved so low Ψ.

Moves through symplast or apoplast pathway. Through cell cytoplasm or between cell walls respectively

Continuous movement due to next cell having lower Ψ than current.

Water evaporates in leaf, then leaves through stomata (transpiration).

When water leaves, water from the top of the xylem is pulled into the leaf down water concentration gradient

Results in pressure at top being lower than pressure at bottom, pushing water up xylem

Surface tension, adhesion, etc. keeps water moving up xylem

All due to osmosis

Types of Phloem cell

Sieve tube elements - living tubular cells connected end to end. Ends have perforations (sieve plates)

Companion cells are next to sieve tube elements (one each)

Contains the organelles that sieve tube elements lack

Controls movement of solutes and provides ATP

Connected via plasmodesmata (strands of cytoplasm)

Parenchyma and Fibres

See Types of Xylem cell

Translocation

Movement of organic substances around the plant

Requires energy

(active process)

Sucrose loaded (actively) into phloem at source (photosynthesising leaf)

Hydrogen ions pumped out of the companion cell (ATP), creating a concentration gradient

H+ ions diffuse back in through cotransporter proteins that allow H+ in if with sucrose molecules

Cotransport

(secondary active transport)

Sucrose diffuses down the concentration gradient

Water moves in by osmosis because Ψ decreases

Sucrose unloaded into a sink, usually by diffusion before being converted to something else to maintain the concentration gradient

Mass flow of substances in phloem