B9 - Transport in plants

Question 1

Why do plants need transport systems?

Answer 1

• 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

Question 2

What are dicotyledonous plants?

Answer 2

• they are plants that produce seeds containing 2 cotyledons (organs that act as food stores and form first leaves)

Question 3

What are the two types of dicots?

Answer 3

• herbaceous
  
  • soft tissues, short life cycle
• woody/arborescent
  
  • hard/lignified tissues, long life cycle

Question 4

What is the difference between the vascular system and the vascular bundle?

Answer 4

• 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

Question 5

Where are vascular bundles found in different areas of the plant?

Answer 5

• 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

Question 6

What is the structure of the xylem?

Answer 6

• 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

Question 7

What are the functions of the xylem?

Answer 7

• 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

Question 8

What is the structure of the phloem?

Answer 8

• 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

Question 9

What are the functions of the phloem?

Answer 9

• 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

Question 10

What is plasmodesmata?

Answer 10

• gaps in the cell wall containing cytoplasm that connects two cells

Question 11

What is the role of water in plants?

Answer 11

• 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

Question 12

What are the adaptations of root hairs?

Answer 12

• 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

Question 13

What is the symplast pathway?

Answer 13

• water enters the cytoplasm through the
  plasma membrane and moves from one cell to the next through plasmodesmata

Question 14

What is the apoplast pathway?

Answer 14

• 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 **

Question 15

How does the water reach the xylem?

Answer 15

• 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)

Question 16

What is water potential?

Answer 16

• the measure of the tendency of water molecules to move from one place to another
• high to low water potential
• pure water = 0 (highest)

Question 17

What happens during water uptake/loss?

Answer 17

• 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

Question 18

What is transpiration?

Answer 18

• 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

Question 19

What is the stomata?

Answer 19

• microscopic pores in the leaf
• opened and closed by guard cells (pair of epidermal cells that control the opening/closing of the stomata)

Question 20

What happens during transpiration?

Answer 20

• 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)

Question 21

What is the transpiration stream?

Answer 21

• the movement of water from the soil, through the plant, to the air surrounding the leaves

Question 22

What is the transpiration pull?

Answer 22

• 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)

Question 23

What are the pros/cons of transpiration?

Answer 23

• 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

Question 24

How do guard cells control the opening/closing of the stomata?

Answer 24

• 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

Question 25

What are the factors affecting transpiration?

Answer 25

• 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

Question 26

What is translocation?

Answer 26

• 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)

Question 27

What are the main sources of assimilates?

Answer 27

• green leaves/stems
• storage organs (tubers, tap roots)
• food stores in seeds

Question 28

What are the main sinks in a plant?

Answer 28

• roots that are growing/actively absorbing mineral ions
• actively dividing meristems
• parts of plant laying down food stores (developing seeds, fruits, storage organs)

Question 29

What is phloem loading?

Answer 29

• 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)

Question 30

What is phloem unloading?

Answer 30

• 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

Question 31

What are xerophytes?

Answer 31

• plants that are adapted to living in dry (or very icy/frozen) conditions
• e.g. marram grass, cacti

Question 32

What are the adaptations of xerophytes?

Answer 32

• 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)

Question 33

What are hydrophytes?

Answer 33

• plants that are adapted to living in water (submerged/on surface/edges of bodies of water)
• e.g. water lilies

Question 34

What are the adaptations of hydrophytes?

Answer 34

• very thin/no waxy cuticle:
  
  • they do not need to conserve water