B9 - Transport in plants Flashcards
1
Q
Why do plants need transport systems?
A
- metabolic demands:
- only the cells of the green parts of the plant are able to photosynthesise (produce glucose/oxygen)
- internal/underground parts need glucose/oxygen transported to them and the waste products removed
- the hormones produced also need to be transported
- mineral ions need to be transported to cells that make proteins (enzymes/cell structure)
- size:
- many plants grow throughout their lives and are very large
- due to this, they need effective transport systems to move substances up and down from the tip of the roots to the topmost leaves/stems
- surface area : volume ratio:
- leaves have a large SA:V ratio for the exchange of gases
- multicellular plants have a relatively small SA:V ratio because of the extra parts (stems, trunks, roots) that are taken into account
- diffusion alone will not be able to supply their cells with what they need
2
Q
What are dicotyledonous plants?
A
- they are plants that produce seeds containing 2 cotyledons (organs that act as food stores and form first leaves)
3
Q
What are the two types of dicots?
A
- herbaceous
- soft tissues, short life cycle
- woody/arborescent
- hard/lignified tissues, long life cycle
4
Q
What is the difference between the vascular system and the vascular bundle?
A
- vascular system:
- a system of transport vessels in animals/plants
- this includes the xylem and phloem
- vascular bundle:
- the transport tissues arranged together in small bundles in stems, leaves, and roots
- found in herbaceous dicots
5
Q
Where are vascular bundles found in different areas of the plant?
A
- stem:
- around the edge to provide strength and support
- root:
- in the middle (xylem = ‘x’ shape, phloem around it)
- helps withstand tugging strains
- leaf:
- midrib of leaf (main vein) which helps to support structure
- branching veins function in transport/support
6
Q
What is the structure of the xylem?
A
- a non-living tissue
- xylem vessels (long, hollow structures)
- fibres (provides extra mechanical strength)
- living parenchyma cells (support/separate vessels)
- lignin can form bordered pits where water leaves the xylem
7
Q
What are the functions of the xylem?
A
- transports water and dissolved minerals from the roots to the very top
- dead cells form a continuous column
- vessels are narrow where capillary action can effectively take place
- xylem parenchyma = stores food, contains tannin (protects plant tissues from herbivores)
- bordered pits allow the water to move sideways
8
Q
What is the structure of the phloem?
A
- a living tissue
-
sieve tube elements (transporting vessels)
- contain no nucleus, very little cytoplasm
- lined up end-to-end to form sieve tubes
- walls become perforated to form sieve plates (allowing movement of sap)
- companion cells
- large nucleus, dense cytoplasm, numerous mitochondria
9
Q
What are the functions of the phloem?
A
- used to transport assimilates (sucrose and amino acids) around the plant
- they provide cells with the materials needed for cellular respiration and the synthesis of other molecules
- flows up and down the plant
10
Q
What is plasmodesmata?
A
- gaps in the cell wall containing cytoplasm that connects two cells
11
Q
What is the role of water in plants?
A
- turgor pressure:
- provides a hydrostatic skeleton to support stems/leaves
- cell expansion:
- allows for roots to go through tarmac/concrete
- loss of water by evaporation keeps plants cool
- mineral ions/products of photosynthesis are transported in aqueous solutions
- water is needed for photosynthesis
12
Q
What are the adaptations of root hairs?
A
- microscopic size:
- can easily penetrate between soil particles
- large SA:V ratio
- thin surface layer:
- short diffusion/osmosis distance
- solute conc. in root hair cell cytoplasm maintains water potential gradient:
- soil water = high water potential
- root hair cell = lower water potential
- water moves into cell by osmosis
13
Q
What is the symplast pathway?
A
- water enters the cytoplasm through the
plasma membrane and moves from one cell to the next through plasmodesmata
14
Q
What is the apoplast pathway?
A
- water moves through the water filled
spaces between cellulose molecules in the cell walls - when it reaches the endodermis (Casparian strip - waxy material) it enters the symplast pathway
- ** water passes through the selectively permeable cell surface membranes (filters any toxic solutes) and joins symplast pathway **
15
Q
How does the water reach the xylem?
A
- moves mineral ions into the xylem by active transport:
- water potential of endodermal cells is much lower than water potential of xylem cells
- increases rate of osmosis (down w.p. gradient)
- once inside vascular bundle, water returns to apoplast pathway to enter xylem
- movement of water in xylem causes root pressure which pushes water up (the xylem)
16
Q
What is water potential?
A
- the measure of the tendency of water molecules to move from one place to another
- high to low water potential
- pure water = 0 (highest)
17
Q
What happens during water uptake/loss?
A
- uptake:
- plant cell placed in solution with very high w.p.
- water moves in by osmosis
- causes cell to become turgid, water exerts pressure on cell wall (pressure potential)
- loss:
- plant cell placed in solution with very low w.p.
- water moves down w.p. gradient and out of the cell
- causes plasma membrane to lose contact with cell wall (plasmolysis)
- tissue becomes flaccid
18
Q
What is transpiration?
A
- the loss of water vapour from the leaves/stems of the plant
- ** mostly through stomata which opens during gaseous exchange for photosynthesis **
- so majority of water is lost during the day
19
Q
What is the stomata?
A
- microscopic pores in the leaf
- opened and closed by guard cells (pair of epidermal cells that control the opening/closing of the stomata)
20
Q
What happens during transpiration?
A
- water enters leaves through xylem and moves into the spongy mesophyll cells (by osmosis)
- water is then lost from those cells by evaporation (lowers w.p. of cell so water moves in from adjacent cell)
- repeated across leaf to xylem (osmosis)
- the water vapour moves out of the leaf by diffusion (lower w.p. outside)
21
Q
What is the transpiration stream?
A
- the movement of water from the soil, through the plant, to the air surrounding the leaves
22
Q
What is the transpiration pull?
A
- adhesion = water molecules form hydrogen bonds with the carbohydrates in the walls of xylem vessels
- cohesion = water molecules form hydrogen bond with each other
- ** combined effects exhibit capillary action **
- this pull causes tension in the xylem where the water from the soil moves in a continuous stream up the xylem (cohesion-tension theory)
23
Q
What are the pros/cons of transpiration?
A
- pros:
- transports mineral ions up the plant
- maintains the cell’s turgidity (as water is replaced)
- supplies water for growth, cell elongation, and photosynthesis
- keeps plant cool
- cons:
- can cause cell to lose turgor pressure (high rates of transpiration)- cells may become plasmolysed
24
Q
How do guard cells control the opening/closing of the stomata?
A
- low turgor = asymmetric configuration closes the pore
- high turgor = cells pump in solutes, cellulose hoops cause them to extend lengthways (bean shaped), opens the pore
25
What are the factors affecting transpiration?
- light intensity:
- higher = more open stomata, more water vapour diffuses out
- increases evaporation from surfaces of the leaf
- relative humidity:
- very high = reduces w.p. gradient between inside/outside
- increases diffusion outside the cell
- temperature:
- increases kinetic energy of water molecules (rate of evaporation from spongy mesophyll cells into air spaces)
- decreases relative humidity outside and lowers water potential
- air movement:
- carries water vapour around leaf away which maintains the water vapour gradient
- increases rate of transpiration
- water availability:
- low water levels in soil will reduce rate of transpiration as plant will be under *water stress*
26
What is translocation?
- the movement of assimilates (products of photosynthesis) throughout the plant
- an active process
- main assimilate transported is sucrose (converted from glucose as it is less likely to be *metabolised*)
27
What are the main sources of assimilates?
- green leaves/stems
- storage organs (tubers, tap roots)
- food stores in seeds
28
What are the main sinks in a plant?
- roots that are growing/actively absorbing mineral ions
- actively dividing meristems
- parts of plant laying down food stores (developing seeds, fruits, storage organs)
29
What is phloem loading?
- the sucrose follows the apoplast route from the source and into the companion cells/sieve tube elements
- it is moved into the companion cells by active transport
- the H+ ions are *actively* pumped out of the c.c. through the proton pumps (ATP --> ADP + Pi)
- this increases the H+ conc. outside which causes the ions to diffuse back in
- the H+ ions binds with a co-transporter as well as the sucrose
- this moves the sucrose into the c.c.
- the many plasmodesmata linking the c.c. and s.t.e. causes the sucrose to move into the s.t.e.
- this decreases the w.p. of the s.t.e. so water moves in
- the build up of sucrose and water generates turgor pressure which causes mass flow (allows for movement of assimilates up and down plant)
30
What is phloem unloading?
- sucrose is unloaded whenever it is required by cells
- the sucrose rapidly moves on into other cells by diffusion from the phloem (maintains conc. gradient)
- it may be converted into another substance (glucose for respiration, starch for storage)
- loss of solutes from the phloem leads to higher w.p. so water moves out
- water carrying solutes may join transpiration stream in xylem
31
What are xerophytes?
- plants that are adapted to living in dry (or very icy/frozen) conditions
- e.g. marram grass, cacti
32
What are the adaptations of xerophytes?
- thick waxy cuticle:
- reduces evaporation (common in evergreen plants)
- sunken stomata:
- located in pits which reduces air movement
- this reduces the water vapour gradient (reduces transpiration)
- reduced no. of stomata:
- reduces water lost by transpiration (but also reduces gas exchange capabilities)
- reduced leaves:
- reduced leaf area reduces water loss
- (leaves of conifers are very narrow which greatly reduces SA:V ratio)
- hairy leaves:
- creates microclimate of still, humid air which reduces the water vapour potential gradient
- curled leaves :
- confines stomata within microenvironment of humid air
- this reduces the diffusion of water vapour from stomata
- succulents:
- water is stored when in plentiful supply (used in times of drought)
- stored in specialised parenchyma tissue (stems/roots)
- leaf loss:
- prevents water loss from leaves altogether
- e.g. desert tree loses all leaves and trunk/branches turn green for photosynthesis
- root adaptations:
- long tap roots = penetrate several metres (can access water way below surface)
- widespread/large SA = able to absorb water before rain shower evaporation
- avoiding the problems:
- plants may lose leaves and become dormant/die, they leave seeds behind to germinate and grow
- some survive as storage organs (bulbs, corms, tubers)
33
What are hydrophytes?
- plants that are adapted to living in water (submerged/on surface/edges of bodies of water)
- e.g. water lilies
34
What are the adaptations of hydrophytes?
- very thin/no waxy cuticle:
- they do not need to conserve water
