9 - transport in plants

Question 1

What are reasons for the need of plant transport systems?

Answer 1

METABOLIC DEMAND - Green parts of plant photosynthesis and make their own glucose, however internal and underground plants don’t and need glucose transported to them - Mineral ions must be transported from roots to rest of plants to make proteins for enzymes and cell structure - Hormones must be transported - Waste cell metabolism products must be removed SIZE - Large perennial plants need effective transport systems to move substances up and down plant (roots to leaves) SA:V RATIO - Leaves have large SA:V, but stems trunks/roots make plants overall ratio small.

Question 2

What are herbaceous dicots?

Answer 2

Soft tissue plants with short life span (leaves and stems that die down at the end of growing season)

Question 3

Define vascular system.

Answer 3

Series of transport vessels that run through leaves, roots and stem of a dicot plant.

Question 4

Define vascular bundle.

Answer 4

The vascular system of herbaceous dicots comprising of xylem and phloems.

Question 5

label the structure of the stem

Answer 5

Parenchyma - In the middle, packing and supporting tissue Phloem vessel on outside - for translocation
Xylem vessel on inside - for transpiration
Vascular cambium - new xylem and phloem cells divide from it.
In a stem the vascular bundles around the edge provide structural support.

Question 6

label the structure of a root (dicot)

Answer 6

Root hair on the outside protruding
From root hair
Exodermis, epidermis, cortex, endodermis Phloem as 4 dots Xylem as a cross

Question 7

label the structure of a leaf (dicot)

Answer 7

Midrib of the leaf gives structural support as it is the main vein carrying the vascular tissue through the organ

Xylem and phloem

Palisade mesophyll - main photosynthetic tissue

Question 8

What is the function and structure of a xylem?

Answer 8

End walls removed forming a long tube
No cytoplasm or organelles - non-living tissue
Cell walls lignified (impregnated with lignin)
Makes wall waterproof and prevents xylem collapsing
Spiral, annular (ring-like) and reticulate (mesh-like) thickening to strengthen walls and prevent collapsing
Bordered pits to allow lateral water flow between xylem vessels
E.g. if the xylem has an obstruction, water can still flow but in another xylem vessel

Question 9

What is the function of a phloem vessel and what are its components?

Answer 9

Transports assimilates actively by mass flow - involves 2 cells
Sieve Tube Elements (STE) - long sieve tubes transporting assimilates
Companion Cells - supports metabolic function of STE, also involved in actively loading the phloem

Question 10

What are the adaptations of the sieve tube element?

Answer 10

Few organelles and no nucleus - long hollow tubular structure
Has end walls with sieve plates (perforated walls)
Thin layer of cytoplasm
Not lignified as the cell is not under tension

Question 11

What are the adaptations of companion cells?

Answer 11

ste = sieve tube element

Closely linked to STE
Connected to STE by plasmodesmata
microscopic channels of cytoplasm through cellulose cell walls linking cytoplasm to adjacent cells
Dense cytoplasm with many mitochondria and a large nucleus
Mitochondria gives energy to move sucrose into STE

Question 12

What are the roles of water in plants?

Answer 12

Turgor pressure (hydrostatic) as a result of osmosis in plant cells, gives a hydrostatic skeleton - supports stem and leaves.
Loss of water by transpiration keeps plants cool
Water is a transport medium for mineral ions and assimilates
Water is a raw material for photosynthesis.

Question 13

What is the role and what are the adaptations of a root hair cell?

Answer 13

Exchange surface where water is taken into plant
Long thin extension from root hair cell is root hair
Adaptations

Small size - can penetrate soil particles
Each hair has a large SA:V ratio
Each hair has a thin surface layer to reduce diffusion and osmosis distance to quicken the process
Solute concentration in root hair cell cytoplasm maintains water potential gradient between soil and cell.

Question 14

What is the symplast pathway?

Answer 14

Symplast - continous cytoplasm of living plant cells that are connected through plasmodesmata
Water moves by osmosis down the water potential gradient from cell to cell, starting at the root hair cell until the xylem

Question 15

What is the apoplast pathway?

Answer 15

Apoplast - cell wall and extracellular spaces of the cell

Carries water through cell wall and between cells
Water does not enter the cytoplasm or pass the plasma membranes

Question 16

Describe the movement of water into the xylem.

Answer 16

After water move by symplast and apoplast it reaches the endodermis (layer of cells surrounding xylem and phloem
Casparian Strip
Band of waxy material (suberin) that surrounds endodermal cells forming a water proof layer.
It stops water from entering via the apoplast pathway, it is forced into the symplast pathway
For water to enter the symplast pathway from apoplast, it must pass the selectively permeable membrane which stops toxic solutes from entering from the soil as there are no carrier proteins to admit them

Question 17

After entering the xylem does the water stay in the symplast pathway?

Answer 17

No it re-uses the apoplast pathway

Question 18

What is root pressure?

Answer 18

Root endodermis uses metabolic energy to pump mineral ions into root medulla
Reducing water potential in medulla and xylem to lower than the cortex, so water moves across the endodermis into the xylem
Water cannot move back to the cortex as it is blocked by the Casparian Strip, creating pressure in the cortex pushing water up the xylem

Question 19

Define transpiration.

Answer 19

Loss of water vapour from the upper segments of the plant (mainly leaves)
Occurs through evaporation from leaf surface and through stomata

Question 20

What are the components of the transpiration stream?

Answer 20

Transpiration stream: movement of water from roots to leaves

Capillary action
Transpiration pull

Question 21

What are the stages of transpiration?

Answer 21

Water moves by osmosis from the xylem to mesophyll cell in the leaf
Water evaporates from the surfaces of the spongy mesophyll into air spaces
Water vapour diffuses out of the leaf from the air spaces through the stomata down the conc. gradient
The stomata remain open during the day to allow for gas exchange enabling photosynthesis
Since the stomata are open, water vapour is lost so transpiration is a consequence of gaseous exchange

Question 22

What is capillary action?

Answer 22

Water rising through narrow tubing against gravity

Needs cohesion + adhesion

Adhesion: Formation of H-bonds between water molecules and carbs in xylem walls

Cohesion: Formation of H-bonds between water molecules

Question 23

What is the transpiration pull?

Answer 23

Water moving out of xylem creating a pull to replace loss of water vapour from the leaves
Since water is cohesive it moves as on body under tension, which pulls the water up the stem
known as cohesion-tension theory

Question 24

Show evidence for cohesion tension theory.

Answer 24

Change in tree diameter - rate of transpiration is highest during the day, xylem vessel tension is highest - tree diameter shrinks

At night transpiration rate is low and the tension on xylem is low, increasing tree diameter

Xylem vessel damage - air is drawn into xylem, plant can’t transpire, continuous water stream is broken

Question 25

What happens when stomata are flaccid?

Answer 25

Turgor pressure is low (guard cells are flaccid)

Pore closes

Question 26

What happens when stomata are turgid?

Answer 26

When conditions are favourable guard cells actively pump solutes in increasing turgor
Cellulose hoop stop cells from swelling widthways and they extend lengthways
Inner cell wall less flexible than outer making a bean shape
If water becomes scare - hormonal signals from roots trigger guard cell water loss closing the aperture.

Question 27

What are factors affecting transpiration?

Answer 27

Light intensity - light needed for photosynthesis - stoma open, at night, no photosynthesis, stoma close
Relative humidity - amount of water vapour in air
High humidity = low rate of transpiration due to decreased water vapour gradient vice versa
Temperature - increases evaporation so there will be a higher water potential in leaf
Temp rise - also increases amount of water external air can hold before saturation, decreasing humidity and increasing water vapour gradient
Air movement - if there is more wind this blows water vapour (trapped in the hairs at the surface of the leaf) increasing the water vapour gradient
Soil-water availability - if there is not enough water in soil, plant will be under water stress reducing rate of transpiration.

Question 28

Define translocation.

Answer 28

Movement of organic solutes in phloem from sources to sinks.

• Active process (requires energy), substances transported up and down plants

Question 29

Define assimilates.

Answer 29

Products of photosynthesis that are transported around plant
- Glucose is made during photosynthesis but the main assimilate is sucrose
Cell sap sucrose level - 0.5%
Phloem sap sucrose level: 20-30%

Question 30

What are sources of assimilates?

Answer 30

Green leaves and green stem
Storage organs (in tubers and tap roots) - they unload stores at beginning of growth period
Food stores in seeds when they germinate

Question 31

What are sinks of a plant?

Answer 31

Roots that are growing and/or actively absorbing mineral ions
Actively dividing meristems
Any part of plants that are laying down food stores such as developing seeds, fruit stores in organs

Question 32

What are the 2 ways for phloem loading?

Answer 32

Passive route

Active route (uses apoplast route): Sucrose travels from source, through cell walls and inter-cell spaces to companion cells & sieve-tube-elements
- By diffusion down conc. gradient - gradient maintained by removal of sucrose from phloem vessels.

Question 33

What is a co-transporter?

Answer 33

Intrinsic protein that transports 2 substances simultaneously across a biological membrane

Question 34

Describe the process of active phloem loading.

Answer 34

H+ ions actively pumped (uses ATP, ATP + H2O > ADP + Pi + energy) out of companion cell into surrounding cell (creating conc. gradient)
Co-transporter transports H+ ions and sucrose molecules into the companion cells down the concentration gradient into the sieve-tube-elements.
(Sucrose moves through plasmodesmata into STE)
Since solute conc. increases, water potential decreases, so water moves into the STE by osmosis increasing turgor pressure
- Turgor pressure moves water carrying assimilates up and down plant.

Question 35

What are the features of companion cells?

Answer 35

Many infoldings in cell membrane to increase SA for active transport of sucrose
Contains many mitochondria to provide ATP for transport pump

Question 36

What is the effect of an increase in turgor pressure?

Answer 36

Creates pressure difference between phloem and sink, allowing rapid transportation of solutes and water up and down plant.

Question 37

Define mass flow

Answer 37

Assimilates flowing from source to sink down pressure gradient.

Question 38

Describe the process of phloem unloading.

Answer 38

Sucrose unloaded to sink via diffusion down conc. gradient

Sucrose moves to other cells to maintain the conc. gradient between cells and phloem
Increases water potential in STE, H2O then flows into sink, or into xylem and subsequently the transpiration stream.

Question 39

Give evidence for translocation.

Answer 39

Microscopy allows us to see adaptations of companion cells
If mitochondria in companion cells are poisoned, translocation is stopped
Flow of sugars in phloem is 10,000x faster than diffusion, suggesting it is an active process
Aphids feed off of plant tissue, they insert their stylet (mouth), if aphids are anaesthetised and removed, sap continues to move out of phloem.

Question 40

Define xerophyte and give examples.

Answer 40

Plants with adaptations that enable them to survive in dry habitats with short environmental water supply. Cacti, Marram Grass

Question 41

With regards to leaves, what adaptations do xerophytes have?

Answer 41

Reduced Leaves - Reduces SA:V ratio, minimises transpiration water loss. E.g. thin needles in conifers

Hairy Leaves - e.g. spine of cacti, creates microclimate of humid air, reducing water vapour potential gradient. -

Curled Leaves - Confines stomata to microclimate of still humid air. e.g. Marram grass

Leaf Loss - when water isn’t available, some plants shed leaves to prevent water loss

Question 42

With regards to stomata, what adaptations do xerophytes have?

Answer 42

Sunken Stomata - stomata located in pits, reduce air movement, produces microclimate of humid air

reduces water vapour potential gradient - e.g. in cacti, marram grass,
Reduced Stomata No - reduces water loss by transpiration, but also reduces gas exchange capabilities.

Question 43

What root adaptations do xerophytes have?

Answer 43

Long tap roots, grow deep into soil to access water
Widespread shallow roots
large surface area to absorb any water e.g. cacti roots grow 12-18m
Marram grass roots - grow vertically down, have a mat of horizontal rhizomes (modified stems) - roots develop from them

Question 44

What other adaptations do xerophytes have?

Answer 44

Thick Waxy Cuticle - 10% of water loss in plants attributed to cuticle by transpiration, thick cuticle prevents and minimises this

Succulents - store water in specialised parenchyma tissue in stems and roots

they are swollen (thick flesh), store water for times of drought (e.g. Aloe plants)
Some plants lose leaves and become dormant
Some die and let new seeds germinate
Some survive as storage organs (bulbs, tubers)

Question 45

Define hydrophyte and give an example.

Answer 45

Plants adapted to survive in wet habitats, submerged or on surface of water E.g. water lillies

Question 46

What issues do hydrophytes face even though they have an abundant supply of water?

Answer 46

Water-logging air spaces in plants, air spaces must have air

Question 47

Water-logging air spaces in plants, air spaces must have air

Answer 47

Reduced Plant Structure - water provides buoyancy for leaves and flowers

Wide Flat Leaves - e.g. water lilly, spread across surface to capture light small roots - water diffuses directly so no need for sole water uptake via a root

Thin/No Waxy Cuticle - don’t need to conserve water as water lost by transpiration is always replaced

Air Sacs - enables leaves to float

Large Stem/Roots - for underwater plants, maximises area for photosynthesis and oxygen diffusion

Question 48

What are the stomatal adaptations of a hydrophyte?

Answer 48

Always Open Stomata

on upper surfaces, maximised gas exchange, no risk of loss of turgor as water is always there
guard cells inactive
on water surface plants stomata must be on upper surface for gas exchange