transport in plants Flashcards
Give 3 reasons why
multicellular plants need
transport systems?
Metabolic demands • O2 and glucose need to be transported around the plant, and the waste products of cell metabolism removed • Hormones need to be transported • Mineral ions absorbed by the roots need to be transported to all cells to make proteins and enzymes required for cell structure Size • Plants continue to grow throughout their lives • Perennial plants are large • An effective transport system is needed to move substances both up and down from the tip of the roots to the very topmost leaves and stems SA:V • Relatively small SA:V ratio so they can’t rely on diffusion alone to supply their cells with everything they need
What are dicotyledonous
plants (dicots)?
Plants that produce seeds containing two cotyledons, which act as food stores for the developing embryo and form the first leaves when the seed germinates
Describe the two types of
dicots
Herbaceous dicots • Soft tissues, relatively short life cycle Woody (arborescent) dicots • Hard lignified tissues, long life cycle
What is are vascular
bundles?
The vascular system of herbaceous
dicots, made up of xylem and
phloem tissue
Vascular bundles in the stem
Around the edge to give strength
and support
Vascular bundles in the roots
In the middle to help the plant
withstand the tugging strains that
result as the stems and leaves are
blown in the wind
Vascular bundles in leaves
• The midrib of a dicot leaf is the main vein carrying the vascular tissue through the organ • Helps to support the structure of the leaf • Many small, branching veins spread through the leaf, functioning in both transport and support
Describe the structure of the
xylem
• Xylem is a largely non-living tissue • Made up of several types of cells, most of which are dead • Xylem vessels are the main structures: long, hollow structures made by several columns of cells fusing together end to end • They have no cytoplasm or organelles present • Thick-walled xylem parenchyma packs around the xylem vessels storing food and containing tannin deposits (bitter tasting chemical that protects plant tissues from herbivore attacks) • Xylem fibres are long cells with lignified secondary walls that provide extra mechanical strength (and waterproofing) but do not transport water
How can lignin be laid down
in the walls of xylem
vessels?
• Can form rings, spirals or relatively solid tubes with lots of small unlignified areas called bordered pits • This is where water leaves the xylem and moves into other cells of the plant
What is the function of the
xylem?
• The transport of water and mineral ions • Support • The flow of materials in the xylem is up from the roots to the shoots and leaves
What is the phloem?
A living tissue that transports food in
the form of organic solutes around
the plant from the leaves where they
are made by photosynthesis
Describe the structure of the
phloem
Main transporting vessels are the sieve tube elements • Sieve tubes are made up of many cells joined end to end to form a long, hollow structure • Phloem tubes are not lignified • In the areas between the cells, walls become perforated to form sieve plates, which let the phloem contents flow through • Tonoplast, nucleus and some of the other organelles break down • Mature phloem cells have no nucleus • Companion cells are linked to the sieve tube elements by many plasmodesmata. They are active cells with a nucleus and all their organelles • Phloem tissue also contains supporting tissues including fibres and sclereids (cells with extremely thick cell walls)
How is water important for
the structure of plants?
Turgor pressure (or hydrostatic pressure) as a result of osmosis in plant cells provides a hydrostatic skeleton to support the stems and leaves • Turgor pressure in leaf cells is 100 times greater than human systolic blood pressure • Turgor also drives expansion - it is the force that enables plant roots to force their way through tarmac and concrete
How is water important for
the metabolism of plants?
• The loss of water by evaporation helps to keep plants cool • Mineral ions and the products of photosynthesis are transported in aqueous solutions • Water is a raw material for photosynthesis
What are root hair cells?
Cells found just behind the growing tip of a plant root that have long hair-like extensions that greatly increase the surface area available for the absorption of water and mineral ions from the soil
What is a root hair?
A long, thin extension from a root
hair cell, a specialised epidermal cell
found near the growing root tip
How are root hairs adapted
as exchange surfaces?
• Microscopic size means they can penetrate easily between soil particles • Each microscopic hair has a large SA:V ratio, and there are thousands on each growing tip • Each hair has a thin surface layer (just cell wall and cell-surface membrane) through which diffusion and osmosis can take place quickly • Concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil water and the cell
Why does water move into
root hair cells by osmosis?
• Soil water has a very low concentration of dissolved minerals so it has a very high water potential • Cytoplasm and vacuolar sap of the the root hair cell (and the other root cells) contain many different solvents including sugars, mineral ions, and amino acids so the water potential in the cell is lower • As a result water moves into the root hair cells by osmosis
What are the 2 pathways
that water can move across
the root to the xylem
through?
Symplast pathway - Movement of water through the symplast (continuous cytoplasm of living plant cells that is connected through plasmodesmata) Apoplast pathway - Movement of water through the apoplast (the cell walls and the intercellular spaces)
Describe the symplast
pathway
1. Water moves through the symplast by osmosis 2. Root hair cell has a higher water potential than the next cell along 3. This is the result of the water diffusing in from the soil which makes the cytoplasm more dilute 4. So water moves from the root hair cell into the next door cell by osmosis 5. This process continues from cell to cell across the root until the xylem is reached 6. As water leaves the root hair cell by osmosis, the water potential (Ψ) of the cytoplasm falls again, maintaining a steep Ψ gradient to ensure that as much water as possible continues to move into the cell from the soil
Describe the apoplast
pathway
1. Water fills the spaces between the loose, open network of fibres in the cellulose cell wall 2. 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 3. The pull from water moving into the xylem and up the plant along with the cohesive forces between the water molecules creates a tension that means there is a continuous flow of water through the open structure of the cellulose wall, which offers little or no resistance
How far does water move
across the root?
Water moves across the root in the apoplast and symplast until it reaches the endodermis (layer of cells surrounding the vascular tissue of the roots)
What is the Casparian strip?
A band of waxy material called
Suberin that runs around each of the
endodermal cells, forming a
waterproof layer
What happens when water
in the apoplast pathway
meets the casparian strip?
• The water can go no further and is forced into the cytoplasm of the cell, joining the water in the symplast pathway • To get to the cytoplasm, water must pass through the selectively permeable cell surface membranes, stopping any potentially-toxic solutes in the soil water from reaching living tissues, as the membranes would have no carrier proteins to admit them
How do the the endodermal
cells move mineral ions into
the xylem?
Active transport, as the solute
concentration in the cytoplasm of
the endodermal cells is relatively
dilute compared to cells in the xylem
What increases the rate of
water moving into the xylem
by osmosis?
• Endodermal cells move mineral ions into the xylem by active transport • As a result, the water potential of the xylem cells is much lower than that of the endodermal cells • This increases the rate of water moving into the xylem by osmosis down a water potential gradient from the endodermis through the symplast pathway
What happens as a result of the build up of sucrose in
the companion cell and sieve element?
• Water also moves in by osmosis • Leads to a build-up of turgor pressure due to the rigid cell walls • The water carrying the assimilates moves into the tubes of the sieve elements, reducing the pressure in the companion cells, and moves up or down the plant by mass flow to areas of lower pressure (the sinks) Phloem unloading • Diffusion of the sucrose from the phloem into the surrounding cells • Sucrose rapidly moves on into other cells by diffusion or is converted into another substance, so that a concentration gradient of sucrose is maintained between contents of the phloem and the surrounding cells
