Transport in plants Flashcards
Define translocation
energy-requiring
process transporting assimilates, especially sucrose,
in the phloem between sources and
sinks
Give examples of sources in a plant
- green photosynthesising leaves and stems
- storage organs eg. Tubers, tap roots
- food stores in seeds when they germinate
Give examples of sinks
- growing roots
- meristems that are actively dividing
- any part of plant that is laying down food stores eg. Developing seeds, fruit or storage organs.
Why are carbs transported in form of sucrose?
- it alls for efficient energy transfer and increased energy storage, because sucrose is disaccharide and therefore contains more energy
- it is less reactive then glucose as it is a non-reducing sugar and therefore no intermediate reactions take place as its being transported
What are the two main ways sucrose is loaded?
Symplast Route
Apoplast route
Describe the symplast route of sucrose loading
Sucrose from the source moves through the cytoplasm and on into the sieve tube by diffusion through the plasmodesmata
- this is a passive process
Describe the apoplast route of sucrose loading
Through the cell walls, this is an active process
1. companion cells have proton pumps, which use ATP to actively transport H+ ions out of cytoplasm into surrounding tissue
2. A H+ diffusion gradient is set up so they return to companion cell, down their concentration gradient via co-transport protein, sucrose is co-transported into companion cell
3. Sucrose builds up in the companion cells, causing it to diffuse across into the sieve the elements via plasmodesmata
Companion cell adaptions for sucrose loading
Many infolding on cell surface membrane
- give increase surface area
- more proton pumps and co-transporter pumps
Many mitochondria
- to provide ATP needed for active transport for the proton pumps
Scientific evidence to support principles of translocation
- advances in microscopy allow us to see the adaptations of the companion cells for active transport
- if the mitochondria is poisoned, translocation stops
- aphid studies show that there is a positive pressure in the phloem that forces the sap out through the stylet. The pressure and therefore flow rate in phloem is lower closer to the sink than it is to the source. The conc of sucrose in sap is higher near to the source than the sink
Describe Sucrose unloading
Occurs a the sinks (active cells)
- either sucrose rapidly moves into cells by diffusion
- or is converted into another molecule (glucose - respiration or starch - storage) in order to maintain the sucrose conc gradient between phloem and sinks
Why do multicellular plants need transport systems ?
Metabolic Demands = glucose created in photosynthesis has to be transported to all cells for respiration. Waste such as excess oxygen must be removed. Hormones made in one area need to be transported to areas where they are needed. Mineral ions absorbed by roots need to be transported to all cells for protein synthesis , creation of enzymes
SIZE= plants need effective transport systems to transport substance up and down from the tip of the roots to top most leaves and stems
Surface area to volume ratio = leaves are adapted to have large SA:V ratio but the overall plant does not, so the surface alone wouldn’t be enough to get water and surgery around the plant quickly enough. The plants cannot rely of diffusion alone t supply all the needs of all the different areas of the plant.
Describe the transport system in herbaceous dictots
Have a series of transport vessels running through the roots, stems and leaves. It is known as the vascular system, consists of :
1- phloem
2- xylem
These transport tissues are arranged together in vascular bundles
Describe the arrangement of the vascular bundle in the roots
Vascular bundles are in the middle of the root to help the plant withstand the tugging strains that result as the stems and leaves are blown in the wind
- Xylem = lies at the centre of root arranged in an X-shape
- Phloem = lies around the xylem
- Endodermis = cylindrical layer of cells that encloses the xylem and phloem
- Pericycle = found beneath the endodermis and consists of meristem cells
- Epidermis = single layer of cells on outside of root, root hair cells are slender extension of specialised epidermal cells
- Cortex = space between epidermis and endodermis, several layers of parenchyma cells (packing tissues that provides support, is permeable to water and dissolved solutes)
Describe arrangement of vascular bundle in stems
The vascular bundles are around the edge to give strength and support
- xylem and phloem are separated by a layer of cambium, meristematic tissue that can produce new xylem and phloem tissue
- epidermis = surrounds outside of stem
- cortex = region between epidermis and pith
- pith= area in the centre, consists of parenchyma cells
Describe arrangement of vascular bundle in leaves
The midrib of a dicot leaf is the main vein carrying the vascular tissues through the organ. It also helps to support the structure of the leaf, many small branching veins spread through the leaf functioning both in transport and support
The structure and function of XYLEM
Transport water and mineral ions!
They are long tubes of dead tissue with open end, therefore they can form a continuous column
- intially they are elongated living cells but as the cellulose walls are strengthened with lignin the cell contents die, cytoplasms disappears and the end wall of each cell wall breaks down
- xylem vessels are reinforced with lignin so they don’t collapse under the transpiration pull
- lignin can be laid down in walls of xylem forming rings, spirals or relatively solid tubes
- walls also contain pits, which enable water to move sideways between other vessels or cells
Two other tissues associated with xylem:
1- Xylem Parenchyma= thick walled cells that pack around the vessels for support, store food and contain tannin (bitter chemical that protects plant from herbivores)
2- Xylem fibres = long cells with lignified secondary walls that provide extra mechanical support but do not transport water
The structure and function of PHLOEM!
Involved in translocation!
Consists of:
- Sieve tube elements = living cells that are lined up end to end to form sieve tubes. NO lignin. Contain perforated cross walls called sieve plates, as the pores develop to oblast and organelles break down, nuclear disappears
- Companion cells= small living cells that linked to sieve tube elects by plasmodesmata. Large nucleus, dense cytoplasm, numerous mitochondria. Very metabolically active cells that carry out processes for both companion cells and sieve tubes. Produce ATP for loading sucrose into sieve tubes.
- Also contains fibres and sclereids (cells with extremely thick cell walls) both tissues provide support
Why is water important to plants
1- Turgor pressure (hydrostatic pressure) as result of osmosis in plant cells provide a hydrostatic skeleton to support the stems and leaves, this turgor pressure also drives cell expansion, force that enables plant roots to force through tarmac and concrete
2- loss of water by evaporation from the stomata in leaves keeps plants cool
3- mineral ions + products of photosynthesis is transported in aqueous solution
4- water is a reactant for photosynthesis
Root hair cell adaptions as exchange surface
- microscopic size means they can penetrate easily between soil particles
- each hair has a large SA:V ratio and there are thousands on each growing root tip
- each hair has a thin surface layer through which diffusion and osmosis can take place place quickly
- the conc of solutes in cytoplasm of root hair cells maintains a water potential gradient between the soil water and the cell, therefore water moves into root by osmosis
Describe how the water moves across the root into the xylem via the apoplast pathway
Water moves through the apoplast = cell walls and intracellular spaces
- water fills the spaces between the open network of fibres in the cellulose cell wall
- cohesive forces between molecules of water, form a continuous stream of water which moved towards the xylem
- when water reaches part of root called the endodermis, a layer of waxy Suberin called Casparian strip is impermeable to water, therefore in order for it to cross the endodermis the water leaves cell walls and enters the cytoplasm. (Joins symplast pathway)
- the water must pass through the selectively permeable cell membrane, this excludes and potentially toxic solutes in the soil from reaching the tissues
Describe how the water moves across the root into the xylem via the symplast pathway
Water moves through the symplast = the continuous cytoplasm of living plant cells that is connected through the plasmodesmata by osmosis
- as water diffuses in from the soil, the root hair cytoplasm is more dilute (higher water potential) than the next cell along, therefore water moves from high water potential to lower water potential and travels along the root cells till it reaches the xylem
