Plant organs and transport

Recall a leaf's level of organisation

Roots and shoots form one connected plant

In a flowering plant, the root system and shoot system cooperate; stems and leaves belong to the shoot system

Roots take up water and dissolved mineral ions from soil; leaves with chloroplasts use light to make sugars from carbon dioxide and water

The stem supports the shoot and contains transport tissues connecting organs; water and mineral ions enter through roots, while leaves make sugars

This topic follows a flowering plant; it does not claim that every organism classified as a plant has the same transport structures

Connect root cells with uptake

Root hair cells have extensions that increase contact surface with soil water; water and mineral ions enter through living root surfaces

Water then crosses root tissues before entering xylem; a root hair is not itself a pipe extending all the way to the leaf

Mineral ions are needed substances but are not sugars or a plant's main source of stored chemical energy; plants make sugars by photosynthesis or use stored supplies

Identify what roots take up

Distinguish xylem from phloem

Mature xylem vessel elements have lost their living contents; they can still serve as structural parts of the plant's transport tissue

Xylem and phloem are tissues within organs such as stems and roots, not two separate whole organisms

Link a hollow pathway to transport

Follow the water path

Follow this route: soil water enters root surfaces, crosses root tissues, enters root xylem, moves through stem xylem and reaches leaf xylem

Water moves into leaf cells; some is used in photosynthesis and much can evaporate into leaf air spaces and leave as water vapour through stomata

This water loss is called transpiration; evaporation from leaves helps draw a connected water column through xylem

The plant has no heart pumping blood; xylem transport is not animal circulation

Trace water from soil towards a leaf

Follow supply and demand for sugar

A mature leaf in sufficient light can make more sugar than it uses; phloem can carry sugars from this supplying region to growing roots, shoots, flowers, fruits or storage organs

A region supplying sugar is called a source; a region using or storing the incoming sugar is called a sink

A storage organ can become a source when it releases stored carbohydrate for a growing shoot; identify the source and sink given for that plant at that time

Phloem movement is therefore not always downwards; the route depends on where sugar is supplied and needed

Trace sugar to a growing root

A lit mature leaf makes more sugar than it uses

A growing root needs sugar; connected phloem transports sugar from the supplying leaf to the root

Which transport statement follows for this plant at this time?

The leaf is the stated sugar supplier and the growing root is the stated user; phloem connects them, so sugar travels from the leaf towards that root in this case

Sugar moves through phloem from the supplying leaf towards the growing root

Separate water and sugar routes

Use a storage organ as a sugar source

R means root, S stem, L a supplying mature leaf and F a growing fruit; compare water in xylem with sugar in phloem; switching the display selects a route, not a time step or a change in the real plant; the layout is symbolic rather than anatomical

The stem S lies on both displayed routes; what follows?

Keep a local change local

To predict an effect, identify the changed tissue, the substances it carries and the organs linked by the affected route

Blocking a xylem route directly restricts water and mineral-ion supply along that route; blocking a phloem route directly restricts sugar transport along that route

Other changes can follow later, but a local blockage does not establish that every transport path, leaf or organ has stopped working

Use an unchanged water route

A phloem path from a supplying leaf to a growing fruit is blocked

Water can still reach the fruit through its xylem

Which transport is directly affected?

The obstruction affects sugar supply along the named phloem path while the record retains xylem water transport; no whole-plant gas-exchange change is stated

Sugar supply along that phloem path to the fruit

Predict a direct xylem effect

Balance exchange and water supply

Stomata are small pores controlled by guard cells; open pores allow carbon dioxide exchange and water vapour loss

Closing stomata can reduce water loss through them, but also restricts carbon dioxide entry through the same openings

A plant's stored water decreases when water output exceeds input over an interval; roots being present does not guarantee that uptake keeps pace with loss

For a stated water balance, subtract output from input and say whether stored water rises or falls; do not add the two quantities to calculate the change

Explain an exchange trade-off

Compare water input with water output

Construct two labelled routes

Written and drawing task for discussion, not automatic marking: sketch a flowering plant with roots, a stem, a mature leaf in light and a growing fruit

Add one labelled route for water from soil to the leaf and another for sugar from the leaf to the fruit; name the transport tissue on each route

Then explain the direct effect of a blockage confined to the fruit's phloem while its xylem stays open; state one conclusion the evidence does not justify

Check substances, tissues and consequences

Model water route: soil water, root surfaces and tissues, root xylem, stem xylem, leaf xylem and leaf cells

Model sugar route: supplying mature leaf, phloem through the connecting tissues, growing fruit using or storing sugar

Blocking the stated phloem path directly reduces sugar supply along that path; it does not establish that water supply through the still-open xylem has stopped

The sketch is a model of routes, not a measurement of tissue size or an observation of actual transport speed

Trace a substance through cooperating plant organs

Connect uptake at roots, transport through stems, and photosynthesis and exchange in leaves

Distinguish xylem water and mineral-ion routes from phloem sugar routes; use the stated source and destination rather than assuming a fixed sugar direction

For a change, identify the affected route and its direct effect; keep unchanged routes and later possibilities separate