Transport in Plants

Question 1

Why is a transport system needed in multicellular plants?

Answer 1

1. Size can vary but need to be able to move substances both up and down, from the tip of the roots to the topmost leaves and stems
2. High metabolic rate
3. Surface area to volume ratio - larger plants have a smaller SA:V ratio so need to have specialised exchange surfaces and a transport system otherwise the diffusion distance is too long and rate of diffusion is too slow to meet metabolic requirements

Question 2

What direction does the xylem transport materials?

Answer 2

Water and soluble mineral ions travel UPWARDS in the XYLEM tissue

Question 3

What direction does the phloem transport materials?

Answer 3

Assimilates (i.e. sugars) travel UP/DOWN in the PHOLEM tissue

Question 4

What is the vascular (transport) tissue in plants?

Answer 4

Xylem and phloem

Question 5

Where are the xylem and phloem found?

Answer 5

Found together in vascular bundles which contain other types of tissue (collenchyma/sclerenchyma – give the bundle strength and help to support the plant)

Question 6

Describe the distribution of vascular tissue in a young root.

Answer 6

o Vascular bundle found at the centre of the young root
o Central core of xylem (often, X-shape)
o Phloem is between the arms of the X-shaped xylem tissue
o This arrangement provides strength to withstand the pulling forces (roots)
o Around the vascular bundle is the endodermis (special sheath of cells) – has a key role in getting water to the xylem vessel
o Inside the endodermis is a layer of meristem cells (cells that remain able to divide – stem cells for plants) – this is called the pericycle

Question 7

Describe the distribution of vascular tissue in the stem

Answer 7

o Vascular bundles are found near the outer edge of the stem
o Bundles are separate
o This arrangement provides strength and flexibility to withstand the bending forces that the stems and branches are exposed to
o Xylem found towards the inside of each vascular bundle / phloem towards the outside
o Between the xylem and phloem is a layer of cambium – layer of meristem cells (to produce more xylem and phloem)

Question 8

Describe the distribution of vascular tissue in the leaf

Answer 8

o Vascular bundles form at the midrib and veins of the leaf

o Within each vein, the xylem is located on the top of the phloem

Question 9

What is the structure and function of xylem tissue?

Answer 9

o Vessels – carry the water and dissolved mineral ions
o Fibres – to help support the plant
o Living parenchyma cells – act as packing tissue to separate and support the vessels

Question 10

What is the role of the xylem?

Answer 10

The xylem is tissue used to transport water and mineral ions from the roots up to the leaves and other parts of the plant

Question 11

What are the adaptations of xylem?

Answer 11

o Made of dead cells (lignin kills the cells) aligned end to end to form a continuous column
o Tubes are narrow so that the water column does not break easily and the capillary action can be effective
o Bordered pits (when lignification is incomplete) allows water to move sideways from one vessel to another/leave the xylem
o Lignin deposited in cell walls which strengthens the vessel walls and prevents it from collapsing
o Flow of water is not impeded: as no cross-walls/cell contents/nucleus/cytoplasm

Question 12

What formation is the lignin in and how does this aid xylem function?

Answer 12

The lignin thickening forms patterns in the cell wall – may be spiral/annular (rings) – prevents the vessel from being too rigid and allows some flexibility in the stem/branch

Question 13

What is the role of the phloem?

Answer 13

Phloem is a tissue used to transport assimilates (mainly sucrose/amino acids) around the plant

Question 14

What is the structure and function of phloem tissue?

Answer 14

o Phloem consists of sieve tubes (made of sieve tube elements) and companion cells
o Elongated sieve tube elements are lined up end to end forming sieve tubes
o Contain no nucleus, very little cytoplasm – leaving space for mass flow of sap to occur
o The ends of the sieve tube elements are perforated cross-walls called sieve plates, allowing movement of the sap from one sieve tube element to the next
o The sieve tubes have very thin walls and when seen in transverse section, are usually 5/6 sided
o Between the sieve tubes are small companion cells, each with a large nucleus and dense cytoplasm
o Have numerous mitochondria – produce ATP needed for active processes
o Carry out the metabolic processes needed to load assimilates into the sieve tubes

Question 15

Apoplast pathway

Answer 15

Water, with dissolved mineral ions and salts, move through spaces in the cell walls and between cells. The water does not pass the plasma membrane so the water moves by mass flow, not osmosis.

Question 16

Symplast pathway

Answer 16

Water enters the cytoplasm (only) via the plasma membrane. It passes the plasmodesmata to move between cells.

Question 17

Vacuolar pathway

Answer 17

The water can enter and move through the vacuole, as well as the cytoplasm

Question 18

Transpiration

Answer 18

It is loss of water vapour from aerial parts of the plant

Question 19

What is a consequence of stomatal opening?

Answer 19

Water vapour (transpiration) is mainly evaporated from the stomata – open for gaseous exchange for photosynthesis (so transpiration usually occurs in the day)

Question 20

How does transpiration occur?

Answer 20

 Water enters leaf via the xylem, moving into the cells of the spongy mesophyll by osmosis
 Water evaporates from the cell walls of the spongy mesophyll
 Water moves out via diffusion due to open stomata – relies on difference in concentration gradient of water molecules outside the region (water vapour potential gradient)

Question 21

Why is transpiration important?

Answer 21

• Transports useful minerals up the plant
• Maintains cell turgidity
• Supplies water for growth/cell elongation/photosynthesis
• Supplies water, which evaporates on a hot day, to keep the plant cool

Question 22

How is transpiration rate measured?

Answer 22

• Use a potometer – an estimate is given because it actually measures the rate of water uptake via a shoot
• Water vapour lost by the leaves is replaced by water from the capillary tube
• The movement of the meniscus at the end of the water column is measured by a ruler
• To find the volume – calculate the volume of a cylinder (V=(pi)r2l), where ‘r’ is the radius of the capillary tube and ‘l’ is the length of the capillary tube
• Then work out the rate (rate=volume/time)
• Measures water uptake (NOT LOSS) so assumes all that is take up is also lost – can be unreliable to an extent as some may be used in photosynthesis

Question 23

What should be checked before using a potometer?

Answer 23

o Cut the healthy shoot under water (stop air entering xylem)
o Cut shoot at a slant (increase surface area)
o Check apparatus is full of water (no air bubbles)
o Insert short into apparatus under water
o Remove potometer from water and ensure the joints around the shoot are airtight
o Dry leaves
o Allow time for the shoot to acclimatise
o Shut screw clip
o Keep ruler fixed and record the position of the air bubbles on scale
o Use a healthy shoot

Question 24

What factors affect transpiration rate?

Answer 24

Light intensity, temperature, relative humidity, air movement (wind), water availability, number of leaves

Question 25

How does light intensity affect transpiration rate?

Answer 25

o Higher light intensity, results in a higher transpiration rate
o In light, the stomata open to allow gaseous exchange for photosynthesis

Question 26

How does temperature affect transpiration rate?

Answer 26

Higher temperature means a higher transpiration rate because:

• Increase rate of evaporation from cell surfaces so water vapour potential rises in the leaf
• Increase in rate of diffusion through stomata due to water molecules having more kinetic energy
• Decrease the relative water vapour potential in the air, faster diffusion

Question 27

How does air movement (wind) affect transpiration rate?

Answer 27

Air moving outside the leaf carries water vapour away, which just diffused out of the leaf – maintaining a high water vapour potential gradient

Question 28

How does water availability affect transpiration rate?

Answer 28

o Little water in soil means the plant cannot replace lost water
o If the water remains insufficient, the stomata will close and the leaves will wilt

Question 29

How does number of leaves affect transpiration rate?

Answer 29

The more leaves a plant has, more surface area, the higher the transpiration rate

Question 30

Describe the dissection of plant material

Answer 30

1. Cut a very thin section longitudinally or transversely of the plant stem using a scalpel.
2. Place it on a slide and place a few drops of TCO (specific to lignin) and wait for a minute (xylem goes blue)
3. Cover with a coverslip (tweezers) and observe under a light microscope.

Question 31

Transpiration stream

Answer 31

• The movement of water from the soil, through the plant, to the air surrounding the leaves
• Driven by water potential gradient between the soil and the air in the leaf air spaces

Question 32

How does the root uptake water?

Answer 32

• Root hair cells – increase the surface area of the root and absorb mineral ions and water from the soil
• Water moves across the root and down the water potential gradient, moving into the cell by osmosis
• Water potential is most negative in the xylem, due to active uptake of ions and minerals so water
• Transpiration pull (cohesion-tension in the xylem)
• The passage is via the cells walls and cytoplasm(s)
• Casparian strip blocks the apoplast pathway so water crosses the membrane and enters the symplast pathway
• Root pressure only pushes the water a few metres up the stem though

Question 33

How does water move up the stem to the leaf?

Answer 33

• Water travels up the stem and into the leaf in the xylem vessels, down a water potential gradient, which is most negative at the leaf
  o Loss of water from evaporation on the leaves must be replaced by water in the xylem
• Transpiration sets up a gradient, placing water in the xylem under tension
• Movement of water through the xylem is by mass flow
• Water molecules are attracted to each other, due to hydrogen bonding, so there are forces of cohesion, which are strong enough to hold the molecules in a continuous water column
  o Whole column can be pulled up as a single chain
• Water molecules attracted to the sides of the narrow xylem vessel – adhesion (xylem is very narrow so it can pull water up the sides)
• Mass flow
• But if it is broken in one xylem vessel, the water column can still be maintained through another vessel via bordered pits

Question 34

What is mass flow?

Answer 34

A flow of water and mineral ions in the same direction

Question 35

How does water leave the leaf?

Answer 35

• Most water that leaves the leaf exists as vapour, leaving through the stomata or the waxy cuticle
• Water evaporates from the cells lining the cavity, situated immediately above the guard cells (the sub-stomatal air space), which lowers the water potential in these cells, causing water to enter them by osmosis from neighbouring cells
• Water is drawn from the xylem into the leaf via osmosis
• Water may also reach these cells via the apoplast pathway from the xylem

Question 36

Xerophyte

Answer 36

A plant adapted to living in arid conditions – low water availability in the environment

Question 37

Adaptations of marram grass (found on sand dunes and in windy conditions)

Answer 37

 Leaf is rolled longitudinally so air is trapped inside, making the air humid, reducing water loss from the leaf (which rolls more tightly in very dry conditions)
 Thick waxy cuticle on the upper epidermis of the rolled leaf to prevent evaporation
 Stomata are on the lower epidermis of the rolled leaf so are protected by the enclosed air space
 Stomata are in pits in the lower epidermis, which is folded and covered in hairs – reduced air movement and therefore, loss of water vapour
 Spongy mesophyll is very dense (few air spaces), less surface area for evaporation of water

Question 38

Hydrophytes

Answer 38

Plants living in water (i.e. water lilies) have high water availability in the environment

Question 39

Adaptations of a water lily

Answer 39

 Lots of large air spaces in the leaf – keeps leaves afloat so they are in the air and can absorb sunlight
 Stomata are on the upper epidermis – exposed to the air allowing gaseous exchange
 Leaf stem has many large air spaces – helps with buoyancy and allows oxygen to diffuse quickly into the roots for aerobic respiration
 Flat leaves – acts as a floating portion of the plant

Question 40

Describe the mechanism of translocation (active loading)

Answer 40

 Sieve plates perforated for ease of flow
 Companion cells have many mitochondria
 Sucrose is actively loaded into the companion cells by an active process
 ATP is used to actively transport H+ ions out of the companion cells (proton pump), increasing the concentration outside the cells – creates a concentration gradient
 The H+ ions diffuse back into the companion cell via cotransporter proteins, which only allow the movement of H+ ions into the cell, if accompanied by a sucrose molecule – this is cotransport (sucrose moved against concentration gradient)
 As the concentration of sucrose in the companion cell increases, it can diffuse through the plasmodesmata into the sieve tube

Question 41

Where does active loading occur?

Answer 41

Occurs in the phloem and is the movement of assimilates throughout the plant

Question 42

How does sucrose move around the plant?

Answer 42

• Sucrose moves along the phloem via mass flow
• A solution of sucrose, amino acids and other assimilates (sap) flows along the phloem, going up/down, where required – from source to sink
• The flow is caused by a difference in hydrostatic pressure between the two ends of the tube, producing a pressure gradient
• Water enters the tube at the source (increases pressure) and leaves at the sink (reducing)

Question 43

Source

Answer 43

• Source (leaves) when root converts starch into sugars
• High hydrostatic pressure, loading
• Part of the plant that loads materials into the transport system

Question 44

Sink

Answer 44

• Sink (roots, meristems) when root either stores starch or uses carbohydrates for respiration/growth
• Low hydrostatic pressure, unloading
• Part of the plant where those materials are removed from transport systems

Question 45

Plasmodesmata

Answer 45

gaps in the cell wall containing cytoplasm that connects two cells

Question 46

Adhesion

Answer 46

the attraction between water molecules and the walls of the xylem vessel

Question 47

Cohesion

Answer 47

The attraction between water molecules caused by hydrogen bonds

Question 48

Dicotyledonous plants (dicots)

Answer 48

plants with two seed leaves and a branching pattern of veins in the leaf (without a woody stem)