Xylem and Phloem

Things we know: Plants need water, minerals, sugars, but they also need to get rid of waste Multicellular = small SA:V Diffusion is too slow TF they need a transport system!

The vascular bundle Vascular tissue is distributed throughout the plant Vascular tissue is made up of xylem and phloem tissue As well as the transport of substances the vascular bundle has structural support purposes

Transpiration System The movement of water molecules and dissolved minerals ions Xylem vessels Passive process Translocation System The movement of sugars (Sucrose) & amino acids Phloem vessel – sieve & companion cells Active process

Xylem The X on the root diagram, always on the inside - strongest Transports water and mineral ions Phloem On the outside Transports dissolved substances e.g. sugar Hint: ‘f’ sound – phloem = food

Where? All parts of the plant! In the root they are at the centre to provide support as it pushes through the soil In other parts of the plant (e.g. stem) they are found around the edges – scaffolding In the leaf we see them as veins – still supporting!

Xylem Continuous tubes made of cells called vessel elements. No end walls on cells – water and mineral ions can flow (1D!) from the root DEAD cells = no cytoplasm Lignin spirals INSIDE the walls – stops tube collapsing inwards Older cells have more lignin Water and ions move in through pits where there is no lignin (like pores) PASSIVE TRANSPORT

Root pressure – passive transport Water in xylem flows from root to tip Osmotic pressure causes water to rise up the plant at night Mineral ions build up in the roots, leading to a decrease in water potential This leads to water entering the root from the soil by osmosis The pressure of this water pushing on the rigid xylem cells causes the water to rise up the stem Fun fact: root pressure can raise water up to 7m!

Phloem Also in tubes Not used for support – no lignin Solutes flow in both directions Two important parts for solute transport: sieve tube elements and companion cells ACTIVE TRANSPORT

Phloem tissues used for transport Sieve tube elements Living cells that make up the tubes Have holes on the end walls for solutes to pass through No nucleus, very little cytoplasm, and few organelles – cannot live on their own! So they have… Companion cells One for every sieve tube element Carry out the functions for both cells to provide energy for active transport of solutes

Pressure Flow (Hypothesis) – active transport Sugars travel from areas of high water potential, called sources (leaves), to regions of low water potential, called sinks (growing tissues). Sugar -> sieve tube elements - active transport Water -> sieve elements - osmosis Water creates turgor pressure in the sieve elements, which forces the sugars and fluids down the phloem tubes toward the sinks At the sinks, the sugars are actively removed from the phloem and water follows osmotically High water potential and low turgor pressure are restored, pressure flow can continue!

Exam Question (b) hint: think about the structure of the xylem cells

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