Chapter 9 - Transport in Plants

What is the need for transport systems in plants

• Metabolic demands - Oxygen and glucose needs transportation as not all cells photosynthesise, hormones need transporting to different areas, mineral ions need to be transported to all cells

• Size - Plants continue to grow throughout their lives and some can be giant so they need effective transport systems to move substances both up and down the plant

• SA:V ratio - Although some parts of the plant have relatively high SA:V ratio, when all the parts of the plant are taken into account, they have a relatively low SA:V ratio meaning they can’t rely on diffusion alone to supply their cells

What are dicotyledonous plants (dicots)

These plants make seeds that contain cotyledons, organs that act as food stores for developing embryo plant and form the first leaves when the seed germinates

What are two different types of dicots

Herbaceous dicots - Soft tissues, relatively short life cycle

Woody (arborescent) dicots - Hard lignified tissues, long life cycle

What are the transport systems in dicotyledonous plants

Series of transport vessels running through stem, roots and leaves known as the vascular system.

In herbaceous dicots this is made of two types of transport vessels, xylem and phloem

Transport tissues are arranged into vascular bundles which can be seen in the transections

What does the transaction of a stem of a young herbaceous plant look like

The vascular bundles are around the edge to give strength and support

What does the transaction of a root of a young herbaceous plant look like

The vascular bundles are in the middle to help the plant withstand the tugging strains that result as the stems and leaves are blown in the wind

What does the transaction of a dicot leaf look like

The midrib of a dicot leaf is the main vein carrying the vascular tissue through the organ, it also helps to support the structure of the leaf.

There are also many small branching veins through the leaf which have a function in transport and support.

Describe the structure of the xylem tissue

• Non-living tissue

• One way transport upwards from the roots to the shoots/leaves

• Long

• Hollow

• Made by several columns of cells fusing together end to end

Two other tissues associated with xylem:

• Thick-walled parenchyma - packs around xylem vessels, storing food and containing tannin (bitter tasting chemical to protect from herbivores) deposits

• Xylem fibres - Long cells with lignified secondary walls which provide mechanical strength but don’t transport water

How can lignin be laid down in the walls of xylem vessels

• Rings

• Spirals

• Solid tubes with lots of small unlignified areas called bordered pits. These are areas where water leaves the xylem and moves into other cells of the plant

Describe the function of the xylem

• Transport of water

• Support the plant

Describe the structure of the phloem

• Living tissue

• Two way transport up and down the plant

• Sieve tube elements (main transporting vessels)

• Sieve tubes made of many cells joined end to end

• Long

• Hollow

• Not lignified

• Perforated sieve plates in between cells to allow contents to flow through

• Mature phloem cells have no nucleus

• Companion cells linked to sieve tubes made of elements

• These cells are linked by plasmodesmata

• Have supporting tissues: fibres, sclereids (cells with thick cell walls)

What happens when large pores appear in cell walls

Tonoplast, nucleus, other organelles break down and phloem becomes tube filled with phloem sap

What are plasmodesmata

Microscopic channels through the cellulose cell walls linking the cytoplasm of adjacent cells

Describe the functions of the phloem

• Transports food in the form of organic solutes around the plant from the leaves where they’re made

• Supplies cells with sugars and amino acids needed for cellular respiration and for synthesis of all other useful molecules

• Companion cells maintain their nucleus and organelles so function as a life support system for sieve tubes made of cells

Describe how water plays a key role in plants

• Turgor pressure - Provides a hydrostatic skeleton to support stems and leaves of plant as a result of osmosis

• Turgor - Drives cell expansion enabling the plant roots to force their way through tarmac and concrete

• Water loss - Helps to keep plants cool

• Aqueous solutions - Helps to transport mineral ions and products of photosynthesis

• Photosynthesis - It is a raw material for photosynthesis

How are root hair cells adapted

• Microscopic size - Can easily penetrate soil particles

• Large SA:V ratio

• Thin surface layer

• Water potential gradient is always maintained

How is the water potential gradient between soil and cell maintained

• Concentration of solutes in cytoplasm of root hair cells is relatively high meaning low water potential in cell

• Concentration of solutes in soil water is very low so there is high water potential in the soil water

• As a result, water moves into root hair cells by osmosis

Describe the symplast pathway

• Water moves through the symplast, a continuous cytoplasm of living plant cells connected via plasmodesmata

• Root hair cell has higher water potential than the next cell along so as a result water moves from cell to cell by osmosis until the xylem is reached

• As water leaves the root hair cell, its water potential falls again so this maintains a steep water potential gradient

Describe the apoplast pathway

• Water moves through the apoplast, this is the cell walls and intracellular spaces

• Water fills spaces between open network of fibres in cell wall

• As water molecules move into the xylem, more water molecules are pulled through the apoplast behind them due to cohesive forces

• This pull creates tension meaning there is continuous flow of water this way, offering essentially no resistance

How does water move into the xylem

• Water will reach the endodermis, the layer of cells surrounding vascular tissue

• Endodermis is noticeable in the roots because of the Casparian strip

• Water in the apoplast water can’t go further so it is forced into the cytoplasm to join the symplast pathway

• This diversion to the cytoplasm means that water must pass through selectively permeable surface membranes, so any potentially-toxic solutes in soil water can not reach living tissues as their are no carrier proteins to admit them

• Water potential of xylem cells is lower than water potential of endodermis cells because endodermal cells move mineral ions into xylem by active transport

• This means water moves into xylem very easily

• When water is inside the vascular bundle it returns to the apoplast pathway to enter the xylem and move up

What is the Casparian strip

A band of waxy material called suberin which runs around each of the endodermal cells forming a waterproof layer

What is root pressure and how does it come about

• Root pressure gives water a push up the xylem

• The active pumping of mineral ions into the xylem produces movement of water by osmosis resulting in root pressure

What are some pieces of evidence for the role of active transport in root pressure

• Some poisons affect mitochondria and prevent production of ATP. E.g. if cyanide is applied to root cells, no energy supply, root pressure disappears

• Root pressure increases with temperature suggesting chemical reactions are involved

• If levels of oxygen or respiratory substrates fall, root pressure falls

• Xylem sap can exude from cut ends of stems. In the natural world, xylem sap is forced out of special pores at the ends of leaves in some conditions, e.g. at night when transpiration is low. This is guttation.

Describe transpiration and adaptations for this process

• Lead surface covered with a waxy cuticle which makes them waterproof, preventing them from losing water rapidly

• Carbon dioxide and oxygen move into and out of leaves down the concentration gradient via small pores called stomata

• Stomata are opened and closed by guard cells to control the amount of water loss

How does the transpiration stream work

• Water enters roots by osmosis and transported up the xylem to the leaves

• It moves across membranes and throughout cell walls and intracellular spaces

• The water vapour then moves out of the leaves via stomata

• As water is lost by evaporation, water potential of the cell is lowered so that more water moves into the cell from adjacent cells by osmosis

• Water exhibits capillary action moving it up the cell because of adhesion and cohesion

• Water is continuously pulled up the xylem to replace the water that was lost by evaporation, this is the transpiration pull

Describe some pieces of evidence for the cohesion-tension theory

• Changes in diameter of trees - When transpiration is at its peak, tension in xylem vessels is at its peak so tree shrinks in diameter. When transpiration is low at night, tension in xylem vessels is low so diameter of tree increases

• When xylem vessel is broken - When vessel is broken, air is pulled into the xylem so water can’t be pulled up in a continuous stream as the cohesive forces have been broken

How are stomata opened and closed

Stomata are opened and closed by guard cells, this opening and closing is a turgor-driven process.

• When turgor is low, asymmetric configuration of guard cell walls closes the pore

• When environmental conditions are favourable, guard cells pump in solutes by active transport, increasing their turgor

• Cellulose hoops prevent cells from swelling in width so they extend lengthways