Transport in plants

Transport in PlantsTransport in Plants

Looking at the role of xylem, phloem, root hairs and stomata and the effect of

transpiration has on these

The transport systemThe transport system

• The transport system is far less elaborate than in mammals due to:– plants are less active and therefore ‘supplies’

don’t run out so quickly– because of the branching system, gases for

respiration and photosynthesis Can be obtained from diffusion in the air

– two separate systems; xylem and phloem

XylemXylem

• Xylem carries the water and minerals

• It is made of many hollow dead cells joined end to end of which the end cell wall has disappeared to form a long tube

• Xylem vessels run from root to every leaf

• Xylem vessels contain no cytoplasm or nuclei

• Their walls are made of cellulose and lignin

• Lignin is very strong and so xylem help keep the plant upright

Lignified cell wall of XylemLignified cell wall of Xylem

Close up of Hibiscus rosa xylemClose up of Hibiscus rosa xylem

Vascular bundlesVascular bundles

• Xylem and phloem tubes are normally found close together, when they are this is called a vascular bundle

• In a root vascular tissue is found at the centre

• In a shoot they are found near the outside edge to help support the plant

Transverse section of a stemTransverse section of a stem

The transport of waterThe transport of water

• Plants take in water from the soil through the root hairs and is carried in the xylem throughout the plant

• Water gets into root hair by osmosis.

• The cytoplasm and cell sap inside it are quite concentrated solutions and the water in soil is normally quite dilute

• Water therefore diffuses down its concentration gradient through a partially permeable membrane

A root tip showing root hairsA root tip showing root hairs

OSMOSISOSMOSIS

Water is a polar molecule.  When water molecules approach they form a Hydrogen Bond.  The negatively charged oxygen atom of one water molecule forms a hydrogen bond with the positively charged hydrogen atom in another water molecule. Water enters the xylem in the roots by Osmosis.  Once in the xylem the water molecules hydrogen bond forming a continuous string of water molecules up to the leaf. Water is constantly lost by Transpiration in the leaf.  When one water molecule is lost another is pulled along.  Transpiration pull is the main cause of water movement.

TRANSPIRATION PULLTRANSPIRATION PULL

The Transpiration StreamThe Transpiration Stream

Water enters the xylemWater enters the xylem

• The root hairs are on the edge of the root and the xylem is in the centre

• It travels by osmosis through the cortex from cell to cell or in the spaces between the cells by the following process

– water passes across the root, from cell to cell, by osmosis, it also seeps between the cells

– water enters root hairs by osmosis

– water is drawn up the xylem vessel because water is continually being removed by transpiration

TRANSPIRATION PULLTRANSPIRATION PULL

TranspirationTranspiration

• The evaporation of water from the plant

• Most of which takes place from the leaves through the stomata

• Guard cells around the stomata control the rate of transpiration by opening and closing

TranspirationTranspiration

• When water is lost through transpiration water from the xylem vessel in the leaf will travel to the cells to replace it

• Water is constantly being taken from the top of the xylem vessel to supply the cells in the leaves

• This reduces the pressure at the top of the xylem so water flows up

• This process is known as the transpiration stream, or transpiration ‘pull’

Definition of transpirationDefinition of transpiration

Evaporation of water at the surfaces of the mesophyll cells followed by loss of water vapour from plant leaves, through the stomata

TRANSPIRATION TRANSPIRATION

1. Water leaves the xylem vessels. Some enters the mesophyll cells.

2. Water evaporates into the air spaces and forms water vapour.

3. Water vapour diffuses out of the open stomata into the air.

4. Air movements take the water vapour away.

Conditions that affect Conditions that affect transpiration ratetranspiration rate

• Temperature

• Humidity

• Wind speed

• Light intensity

• Water supply

THE TRANSPORT OF FOODTHE TRANSPORT OF FOOD

Phloem Translocates organic Phloem Translocates organic foodfood

• TRANSLOCATION is the movement of sucrose and amino acids in phloem, from regions of production to regions of storage, or to regions of utilization in respiration or growth.

Source and SinkSource and Sink

• The part of the plant from which sucrose and amino acids are being translocated is called a SOURCE.

• The part of the plant to which they being translocated is called a SINK.

• Leaves are generally the major sources of translocated material.

• Sinks include the roots, flowers and fruits.

TRANSLOCATIONTRANSLOCATION

1. Sugar is made in the leaves by photosynthesis.

2. Sugar moves into the phloem tubes.

3a. The phloem tubes carry sugar to growing buds, which need it for energy.

3b. The phloem tubes carry sugar to the roots, where it may be used for energy or changed into starch and stored.

Transport of Food Transport of Food in plantsin plantsTransport the ‘food’ (sucrose and amino acids)

They are also made of many cells joined end to end, however their end wall is not completely broken down; instead they form sieve plates. The cells are called Sieve tube elements.

The cells contain cytoplasm but no nucleus and they do not have lignin in their cell walls.

Each sieve cell has a companion cell next to it which does contain a nucleus and many other organelles and supply sieve tube elements with some of their requirements.

Movement of water and foodMovement of water and food