Sabtu, 08 November 2014

#38.2 Transport in multicellular plants - Syllabus 2016 - 2018

7.1    Structure of transport tissues
7.2    Transport mechanisms


Flowering  plants do not have compact bodies like those of animals. Leaves and extensive root systems spread out to obtain  the light energy, water, mineral ions and carbon  dioxide that  plants gain from their environment to make organic molecules, such as sugars and amino  acids. Transport systems in plants move  substances from where they  are absorbed or produced to where they  are stored or used. Plants do not have systems for transporting oxygen  and carbon  dioxide; instead these gases diffuse  through air spaces within stems, roots  and leaves.

Learning Outcomes

Candidates should  be able to:

7.1    Structure of transport tissues

Plants  have two transport tissues: xylem and phloem.

a)   draw and label from prepared slides  plan diagrams of transverse sections of stems, roots  and leaves of herbaceous dicotyledonous plants  using an eyepiece graticule  to show tissues in correct proportions (see  1.1c)

b)   draw and label from prepared slides  the cells in the different tissues in roots,  stems and leaves of herbaceous dicotyledonous plants  using transverse and longitudinal sections

c)   draw and label from prepared slides  the structure of xylem vessel elements, phloem sieve  tube  elements and companion cells and be able to recognise these using the light microscope

d)   relate  the structure of xylem vessel elements, phloem sieve tube  elements and companion cells to their functions

7.2    Transport mechanisms

Movement of xylem sap and phloem sap is by mass flow. Movement in the xylem is passive as it is driven by evaporation from the leaves; plants  use  energy to move substances in the phloem.
Xylem sap moves in one direction  from the roots  to the rest  of the plant. The phloem sap in a phloem sieve  tube moves in one direction  from the location  where it is made to the location  where it is used or stored. At any one time phloem sap can be moving in different directions in different sieve  tubes.

a)   explain the movement of water between plant cells, and between them and their environment, in terms of water potential (see  4.2. No calculations involving water potential will be set)

b)   explain how hydrogen bonding  of water molecules is involved with movement in the xylem by cohesion-tension in transpiration pull and adhesion to cellulose cell walls

c)   describe the pathways and explain the mechanisms by which water and mineral ions are transported from soil to xylem and from roots to leaves (include reference to the symplastic pathway and apoplastic pathway and Casparian strip)

d)   define  the term  transpiration and explain that  it is an inevitable consequence of gas exchange in plants

e)   investigate experimentally and explain the factors that  affect transpiration rate  using simple  potometers, leaf impressions, epidermal peels, and grids for determining surface area

f)    make  annotated drawings, using prepared slides  of cross-sections, to show how leaves of xerophytic  plants are adapted to reduce water loss by transpiration

g)   state that  assimilates, such  as sucrose and amino  acids,  move between sources (e.g. leaves and storage organs)  and sinks (e.g. buds,  flowers, fruits, roots  and storage organs)  in phloem sieve  tubes

h)   explain how sucrose is loaded  into phloem sieve  tubes by companion cells using proton  pumping and the co-transporter mechanism in their cell surface membranes

i) explain mass flow in phloem sap down  a hydrostatic pressure gradient from source to sink