OCR - A-Level - Biology - Transport in Plants

Flussdiagrammknoten

• 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