Transport and transpiration in plants

• Although protoxylem is typically formed in young growing regions and metaxylem in more mature parts of the plant, the two types of tissue often occur together. New xylem cells are produced in the meristematic region (cambium) between the xylem and phloem. Consequently, in root the protoxylem is pushed to the outer edge of the stele as the metaxylem forms behind it. The protoxylem cells are typically smaller and have less thickened walls than the metaxylem.
• The cross-sections of plant roots and stems covered in this section describe the pattern in the herbaceous (non-woody) plants. The arrangement is different in woody plants (trees and shrubs). Wood is formed of xylem tissue, therefore woody plants in cross-section are almost all xylem, with very small amounts of phloem and other tissues.
• Transpiration can be defined as the evaporation of water from the mesophyll surface and the consequent diffusion of water vapour through the stomata and into the atmosphere.
• Changing diameters in tree trunks is more obvious than in herbaceous plants due to the fact that the a tree trunk is almost all xylem and therefore much of it is involved in water uptake
• Lignin in the xylem vessels gives it two important properties; it’s waterproof, avoiding the loss of water and it’s very strong, allowing the vessels to withstand the pressures exerted on them by the transpiration pull.
• The missing end walls and absence of cell contents in xylem vessels make them highly adapted for water transport by the cohesion-tension method.
• In most leaves, the cuticle on the upper surface of the leaf is thicker than the cuticle on the lower.
• While most water vapour diffuses out of the stomata, some water is lost by evaporation through the waxy cuticule - cuticular transpiration
• If a plant is dehydrated, the stomata will automatically close. This is a defensive mechanism to conserve water in times of significant water stress
• What needs to move in plants by a number of processes?

  In multicellular plants, water and other minerals need to move from cell to cell by a number of processes.
• What type of tissues do plants have + what is their function?

  Plants have specialised tissues and cells that are adapted for the mass flow of water, sucrose and mineral ions.
• Why do plants have a lower metabolic rate than animals - what does this mean in terms of transport systems?

  - Plants are relatively inactive, so they have a lower metabolic rate than most animals.

  
  

  - This means that they do not require a complex circulatory system.

  
  

  - However, in many flowering plants the distances over which substances have to move can be very large and so a transport system is necessary.
• What are the two transport systems within a plants vascular systems?

  There are 2 transport systems within a plant's vascular tissues - these are phloem and xylem.

  
• What do the phloem + xylem form?

  They form a central stele (cylinder) in the root, and peripheral bundles in the stem.
• What is the root specialised for?

  The root is specialised for the transport of water and ions (xylem) and organic molecules e.g. sugars & amino acids (phloem).

  
• What are four specialisations of plant roots?

  - The outer layer of the root is composed of an epidermis

  - Vascular tissue (mainlyxylem& smaller amount ofphloem)centralised in astele(vascular cylinder).

  - There is a singleepidermallayer of cells immediately outside the stele.

  - Thecortexcontains undifferentiated cells (parenchyma) which have air spaces in between them, and the cells are often rich in starch grains (storage).- When turgid they become tightly packed and so provide support, in conditions of drought they become flaccid and cause the plant tissue to wilt
• What is the distribution of the xylem + phloem + the distribution of tissues in the plant stem - explain why.

  - Xylem and phloem have the same distribution as describes in the root, however, the distribution of tissues is different, with the vascular bundle being on the outside of the stem.

  
  

  - This provides greater support for the branches and the leaves.
• What is the location of the protoxylem in the plant stem?

  The protoxylem is closer thot the centre of the stem and the metaxylem is closer to the edge of the stem.
• What do stems branch into?

  Stems branch in to smaller stems until they form leaves.
• Where is the vascular bundle as it enters the leaf?

  The vascular bundle enters the leaf as the midrib, which branches into smaller veins that are distributed throughout the leaf.
• Where are leaf veins typically found?

  Leaf veins are typically found in the spongy mesophyll layer.
• What were the type of plant that the cross sections described + how does this differ from woody plants?

  - The cross sections described above are from herbaceous plants (non woody).

  
  

  - The arrangement in woody plants is different as in cross section, they are nearly all xylem, with much smaller amount of phloem and other tissues.
• What is the xylem described as?

  - Xylem is described as a mixed tissue as it contains different types of cells:

  => Xylem vessels

  => Fibres

  => Parenchyma cells

  => Tracheids
• What are the xylem vessels + What is their function?

  - Xylem vessels are chains ofdead cellswith no cell contents, no cytoplasm, mitochondria, nucleus or other cell organelles.

  - These are the cells that transport the water.
• How are the cells arranged?

  The cells are arranged on top of each other and forma continuous tubeonce the end walls of each cell break down.
• What is there no barrier to?

  There isno barrierto the movement of water and minerals.
• Describe the secondary + primary wall as well as a property of lignin.

  - In xylem vessels, a secondary wall, impregnated withLigninis deposited inside the primary cellulose cell walls providing strength whichprevents the vessel collapsingwhen pressure inside falls and makes the vessel waterproof.

  - As lignin isimpermeable to water, mature xylem vessels aredead.
• What do gaps in the lignin allow?

  Gaps in the lignin –pits– allow the lateral movement of water to side branches.
• How can lignin be deposited?

  The lignin can be deposited in a variety of patterns all givingstructural supportandwaterproofingthe tube.
• How is lignin deposited in the growing regions?

  In the growing regions behind root and shoot tips, the formation of primary xylem (protoxylem) lignin is deposited inrings(annular) andspirals(helical) so that flexibility and further elongation of the stem are possible.

  
• What is the lignin like in the mature parts of the stem + why?

  In mature parts of the stem(metaxylem), lignin consisting ofscalariform,reticulateandpittedvessels forms almost a complete layer and is therefore more rigid, so prevents further growth.
• How are the two types of vessels thickened?

  - Reticulated vessels are thickened by interconnecting bars of lignin

  
  

  - Pitted vessels are uniformly thickened except at pores, seen as pits that allow rapid movement of water and ions out of vessels to surrounding cells.
• Do the protoxylem + metaxylem occur together or apart?

  Although protoxylem is typically formed in young growing regions, and metaxylem in more mature parts of the plant, the two tissues often occur together.
• Where are new xylem cells produced + what does this mean for the protoxylem in the roots?

  - New xylem cells are produced in meristematic region called thecambiumbetween the xylem and the phloem.

  - Therefore in roots, the protoxylem is pushed to the outer edge of the stele as the metaxylem forms behind it.
• What is the difference between the protoxylem cells + the metaxylem cells?

  The protoxylem cells are typically smaller and have less thickened walls that the metaxylem.
• How are xylem vessels well adapted for their function of transporting water up the stem? (5)

  - Continuous– as the vessels form, the end walls break down so an unbroken column of water is possible due to cohesion

  - Narrow– capillarity increases adhesion

  - Lignified– lignin provides strength and makes xylem waterproof, so water does not seep out. It prevents cells from collapsing when under tension.

  - Pits– holes in lignin for water to pass laterally

  - Dead– flow is not restricted as the lumen is empty, so movement requires less pressure.

  
• What are the four cell types that the phloem consists of?

  - Sieve tube elements

  - Thin cellulose cell wall

  - Microtubules

  - Companion cells

  
• Explain the function of sieve tube elements in the phloem. (4)


  -Sieve tube elementsare the cells primarily concerned with transport of the sucrose.

  - They are living cells with cell contents, but when they are fully grown they have no nuclei, very few organelles, and the cytoplasm is displaced to the cell walls.

  - The elements lie end to end forming a continuous row – asieve tube.

  - They do have end walls, but these are perforated withsieve poresto formsieve plates.
• Explain the function of cellulose wall in the phloem.

  The thincellulose cell wallat the ends of the sieve tube elements are perforated to form sieve plates making movement between the sieve tube elements easier.

  
• Explain the function of microtubules in the phloem.

  Microtubulesextend between the sieve elements and the sieve pores, these are involved in the translocation of solutes.