Chapter 9 Transport In Plants Flashcards
What are the 3 reasons why multicellular plants need transport systems?
(1) METABOLIC DEMANDS
- many parts of the plant don’t photosynthesise
- need oxygen and glucose transported to them and the waste products of cell metabolism removed
- hormones need to be transported
- mineral ions absorbed by the roots need to be transported (to make proteins)
(2) SIZE
- plants can grow tho become very large
- this means plants need very effective transport systems to move substances both up and down from the tip of the roots to the topmost leaves and stems
(3) Surface Area : Volume (SA:V ratio)
- leaves are adapted to have very high SA:V (for gas exchange)
- however they have an overall relatively small SA:V
- this means they cannot rely on diffusion alone to supply their cells
What are dicotyledonous plants (dicots)?
They are plants which make seeds that contain 2 COTYLEDONS (organs that act as food for the developing embryo plant and form the first leaves when the seed germinates).
They have a series of transport vessels running through the stem, roots, and leaves = vascular system.
Describe the 2 types of dicots?
(1) HERBACEOUS = soft tissues, relatively short life cycle, and leaves/stems die down at the end of the growing season to soil level
(2) woody (ARBORESCENT) = hard, lignified tissues, long life cycle
What are the 2 main types of transport vessels in herbaceous dicots?
XYLEM and PHLOEM (arranged together in vascular bundles - in the leaves, stems, and roots).
What are the 2 main functions of the xylem?
- Non-living tissue
- transports WATER and MINERAL IONS
- support
- flows from roots towards the shoots and leaves
- made of several types of cells, most of which are dead
- xylem vessels are main structures - long, hollow, and made up of columns of cells fusing together (end-to-end)
What are the other tissues associated with the xylem?
Thick walled xylem PARENCHYMA
- packs around the xylem vessels (storing food and containing TANNIN deposits)
- TANNIN = a bitter astringent, tasting chemical that protest plant tissues from attack by herbivores
Xylem fibres = long cells with LIGNIFIED secondary walls that can provide extra mechanical strength (do not transport water)
Lignin can be laid down in the walls of the xylem vessels in several different ways. it can form rings, spirals or relatively solid tubes with lots of small unlignified areas called bordered pits.
- where water leaves the xylem and moves into other cells of the plant
What is the structure and functions of the phloem?
Phloem is a living tissue that transports food in the form of organic solutes around the plant from the leaves where they are made by photosynthesis.
Supplies the cell with the sugars and amino acids needed for cellular respiration.
Can go UP and DOWN the plant.
Main transporting vessels of the phloem = sieve tube elements.
- made up of many cars joined end to end to form a long hollow structure.
NOT LIGNIFIED
In areas between the cells, the walls become perforated forming sieve plates.
- as the large pores appear in these cell walls, the tonoplast (vacuole membrane), the nucleus break down
- phloem = tube filled with phloem sap AND mature phloem cells have no nucleus.
Closely linked to the sieve tube elements = companion cells (linked by plasmodesmata)
PLASMODESMATA = microscopic channels through the cellulose cell walls linking the cytoplasm of adjacent cells
- they maintain their nucleus and all their organelles
Companion cells are very active and it is thought that they function as a ‘life support system’ for the sieve tube cells, which have lost most of their normal cell functions.
What is TURGOR pressure?
Turgor pressure (hydrostatic pressure) as a result of osmosis in plant cells provides a hydrostatic skeleton to support the stems and leaves.
Also drives cell EXPANSION - force that enables plant roots to force their way through tarmac and concrete.
What are root hair cells and HOW are they adapted?
Root hair cells = exchange surface in plants where water is taken into the body of the plant from the soil.
- a root hair is a long, thin extension from a root hair cell (a specialised epidermal cell found near the growing root tip)
(1) MICROSCOPIC SIZE
(2) LARGE SA:V ratio
(3) thin surface layer (faster diffusion/osmosis rate)
(4) concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil water and cell
- soli water has a very low concentration of dissolved minerals = HIGH water potential
- cytoplasm/vacuoles sap contain many different solvents (sugars, mineral ions, amino acids = LOW water potential)
As a result water moves into the root hair cell via OSMOSIS.
What is the movement of water across the root?
Continues to move across the root to the XYLEM.
(1) SYMPLAST pathway
(2) APOPLAST pathway
SYMPLAST pathway??
Water moves through the symplast.
SYMPLAST = continuous cytoplasm of the living plant cells that is connected through the plasmodesmata (via osmosis).
The root hair cell has a higher water potential than the next cell along, so water moves via osmosis. Process continued till XYLEM is reached.
As water leaves the root hair cell, the water potential of the cytoplasm falls again - maintaining a STEEP concentration gradient to ensure that as much water as possible continues to move into the cell from the soil.
APOPLAST pathway??
APOPLAST = cell walls and intercellular spaces
Water fills the spaces between the loose, open networks of fibres in the cellulose cell wall.
As water molecules move into the xylem, more water molecules are ‘pulled through the apoplast behind them due to the COHESIVE forces between the water molecules.
- the pull from water moving into the xylem and up the plant along with the cohesive forces = TENSION = continuous flow of water through the open structure of the cellulose wall (offers no resistance).
Outline the movement of water INTO the XYLEM??
Water moves across the root in the apoplast/symplast pathways until it reaches the ENDODERMIS - the layer of cells surrounding the VASCULAR TISSUE of the roots.
Endodermis is noticeable in the roots because of the effect of the CASPARIAN STRIP = a band of waxy material (SUBERIN) that runs around each of the endodermal cells.
- waterproof layer
- selectively-permeable (rids of any toxic solutes in the soil water) AS the membranes will have NO CARRIER PROTEINS to admit them
Water in the apoplast pathway is forced into the cytoplasm of the cell, joining the water in the symplast pathway.
The solute concentration in the cytoplasm of the endodermal cells is much LOWER compared to the solute concentration of cells in the xylem.
- INCREASES rate of water moving into the xylem by osmosis
What happens to the movement of water once it reaches teh xylem>
Once inside the vascular bundle, water RETURNS to the APOPLAST pathway to enter the xylem itself and move up the plant.
The active pumping of minerals into the xylem to produce movement of water by osmosis = ROOT PRESSURE and it is independent of any effects of transpiration
- root pressure gives water a push up the xylem (but it isn’t a major factor in the movement of water up from the roots to the leaves)
EVIDENCE for the role of ACTIVE TRANSPORT in root pressure?
- cyanide affects mitochondria and production of ATP - if it is applied to root hair cells, there is no energy supply, root pressure DISAPPEARS
- root pressure increases with a rise in temperature and falls with a fall in temperature (suggesting chemical reactions are involved)
- if levels of oxygen or respiratory substrates fall, root pressure falls
- XYLEM SAP may exude from the cut ends of stems at certain times
