3.3 - Transport in Plants

Question 1

Explain metabolic demands as a reason why multicellular plants need transport systems.

Answer 1

• internal and underground parts of a plant do not photosynthesise so require oxygen and glucose to be transported to them.
• waste products of cell metabolism to be removed.
• hormones produced in one part need go be transported to the areas where they have an effect.
• mineral ions absorbed by the roots need to be transported to all cells to make proteins required for enzymes and structure.

Question 2

Explain size as a reason why multicellular plants need transport systems.

Answer 2

• many perennial plants are very large. For example trees.
  This means they need effective transport systems to mive substances up/down from the root tips to the top most leaves and stems.

Question 3

Explain SA:V as a reason why multicellular plants need transport systems.

Answer 3

• although leaves are adapted to have a relatively large sa:v ratio, once the roots and stems are taken into account plants have a small sa:v ratio overall.
• therefore they cannot rely on diffusion alone to supply their cells with everything they need.

Question 4

What are dicotyledonous plants and the 2 types?

Answer 4

Dicots are plants whose seeds are made up of two cotyledons.

• herbaceous dicots: leafy, soft tissued plants with a short life cycle.
• arborescent dicots: woody, hard tissued plants with a long life cycle.

Question 5

What are cotyledons?

Answer 5

organs that act as food stores for the developing embryo plant and form the first leaves when the seed germinates.

Question 6

Explain what the vascular system and bundles are in herbaceous dicots.

Answer 6

• vascular system= the series of transport vessels running through the stem, roots and leaves in dicotyledonous plants.
• in herbaceous dicots the vascular system is made up of the xylem and phloem transport vessels.
• these transport tissues are arranged in vascular bundles in the leaves, stems and roots.

Question 7

What is the xylem?

Answer 7

• a large non-living tissue with two main functions:
  1. Transport of water and mineral ions.
  2. Support.
• the flow of materials is from the roots to the shoots and leaves in one direction.
• made up of several cell types, most are dead when they are functioning. The cells dont contain cytoplasm or nuclei.

Question 8

What makes up the main structure of the xylem tissue.

Answer 8

• xylem vessels are long hollow structures made by several columns of cells fusing end to end. (are the main structures)
• one way tube so that adhesion and cohesion is not interrupted.
• thick walled xylem parenchyma packs around the vessels, storing food and containing tannin deposits.
  Tannin= bitter chemical that protects plants from herbivore attacks.

Question 9

What are xylem fibres and the role of lignin?

Answer 9

They are long cells with lignified secondary walls that provide extra mechanical strength but do not transport water.

• lignin can be laid down in the walks of the vessels by forming either rings, spirals or solid tubes with lots of small unlignified areas called bordered pits.
• the bordered pits is where the water leaves the xylem and moves into other cells.
• the main role of lignin is to reinforce the vessels so they don’t collapse under the transpiration pull.
• amount of lignin increases as cell gets older.

Question 10

What is the locations of xylem and phloem in roots?

Answer 10

In the roots, the vascular bundle is in centre.

• there is a central core of xylem in X shape. Phloem located between the arms of the X.
• this provides strength to withstand the pulling forces the roots are exposed to.
• around vascular bundle there is layer of cells called the endodermis which contain a layer of meristem cells called the pericycle.

Question 11

Locations of xylem and phloem in stems?

Answer 11

• vascular bundles are found near outer edge of the stems.
• xylem is found towards the inside of the bundle and phloem towards the outside.
• have a layer of cambium in between. These are meristems that divide to form new xylem and phloem.

Question 12

Locations of xylem and phloem in leafs?

Answer 12

Vascular bundles form the midrib and veins of a leaf.

- within each vein, xylem is located on top of the phloem.

Question 13

What is the Phloem?

Answer 13

Living tissue that transports food in the form of organic solutes around the plant from the leaves.

• supplies cells with the sugars and amino acids needed for cellular respiration and for the synthesis of needed molecules.
• flow of material can go either direction.
• phloem tubes are not lignified.

Question 14

What are sieve tube elements?

Answer 14

• sieve tube elements are the main transporting vessels of the phloem.
  They are made up of many elongated cells joined end to end to form long hollow structures.
• they have no nucleus or tonoplast and very little cytoplasm which leaves space for the phloem sap to flow.

Question 15

What are sieve plates and their role?

Answer 15

The ends of sieve tube elements have perforated walls called sieve plates which let the phloem contents flow through from one sieve tube element to the next.

• they support the lumen, keeping it open.
• they become blocked if the sieve tube element becomes injured to prevent the sap escaping.

Question 16

What are plasmodesmata?

Answer 16

Microscopic channels through the cellulose cell walls linking the cytoplasm of adjacent cells.
(Phloem)

Question 17

Companion cells?

Answer 17

Closely linked to the sieve tube elements by many plasmodesmata.

• they have a nucleus and all other organelles.
• they are very active and function as a life support system fir the sieve tube cells.
• contain lots of mitochondria needed to help load assimilates into the sieve tubes.

Question 18

Why is water/transpiration vital for plants?

Answer 18

• raw material for photosynthesis.
• loss off water by evaporation acts as cooling mechanism.
• mineral ions and photosynthesis products transported in aqueous solutions.
• turgor pressure as a result of osmosis provides a hydrostatic skeleton to support stems and leaves.
• turgor drives cell expansion.

Question 19

What are root hair cells?

Answer 19

The exchange surface in plants where water is taken into the body of the plant from the soil.
- it is a specialised epidermal cell found near the growing tip with a long thin extension called the root hair.

Question 20

How are root hairs adapted as exchange surfaces?

Answer 20

• microscopic size means they can penetrate easily between soil particles.
• each hair as a large SA:V and there are thousands on each root tip.
• thin surface layer- short diffusion/osmosis distances.
• conc of solutes in cytoplasm of cells maintains a water potential gradient between the soil water and cell.

Question 21

Explain the symplast pathway.

Answer 21

Explain the symplast pathway.

Question 22

What is the symplast and apoplast?

Answer 22

Symplast - the continuous cytoplasm of the living plant cells that is connected through the plasmodesmata.

Apoplast- the cell walls and intercellular spaces.

Question 23

Explain the apoplast pathway.

Answer 23

• water moves through the apoplast.
• water fills between loose, open network of fibres in cellulose cell wall.
• as water molecules move into xylem, more molecules behind are pulled through apoplast due to cohesive forces between the water molecules.
• the pull of water through plant to xylem along with cohesive forces creates a tension meaning there is continuous flow of water through the open structure of cellulose wall.

Question 24

Explain the movement of water into the xylem.

Answer 24

1. Water moves across the root in apoplast and symplast pathways until it reaches endodermis.
2. Water in apoplast pathway cant go further due to casparian strip so is forced into cytoplasm, joining symplast pathway.
3. Solute conc in endodermal cells is dilute compared to cells in the xylem.
4. Endodermal cells move mineral ions into xylem by active transport.
5. As a result, endodermal cells have higher water potential than xylem which increases rate of water moving into xylem down water potential gradient.
6. Once inside vascular bundle, water returns to apoplast pathway to enter xylem itself.
7. Root pressure gives water a push up the xylem

Question 25

Explain root pressure.

Answer 25

Root pressure is created by the active pumping of minerals into the xylem to produce movement of water by osmosis.
- it gives water a push up the xylem but isn’t the major factor in movement of water from roots to leaves.

Question 26

What is the role of active transport in root pressure?

Answer 26

• root pressure increases with rise in temp and decreases with decrease in temp. Shows chemical reactions are involved.
• if levels of oxygen or respiratory substrates fall, root pressure falls.
• cyanide (poison) affects mitochondria so prevents ATP production. If applied to root cells, root pressure disappears.

Question 27

What is transpiration?

Answer 27

The loss of water vapour from the leaves and stems of plants.
- is an inevitable consequence of gaseous exchange.

Question 28

What is the casparian strip and it’s importance?

Answer 28

A band of waxy material called suberin that runs around each of the endodermal cells forming a waterproof layer.

• prevents harmful substances entering xylem, as they would now need to travel via the symplast route.
• prevents leakage of water from xylem vessel and aids the development of root pressure.

Question 29

Explain the features of leaves that affect transpiration.

Answer 29

1. Large SA covered with waxy cuticle which makes them waterproof. Prevents leaf losing water by evaporation from the surface.
2. Stomatal frequency, size and position. They can open and close to control the amount of water lost by a plant. Some stomata needs to always be open as there is always exchange happening.
3. Thickness of epidermis and cuticle.

Question 30

What is the transpiration stream?

Answer 30

The movement of water through a plant from the roots until it is lost by evaporation from the leaves. It is a passive process (xylem vessels =dead)

• water moves by osmosis across membranes and by diffusion in the apoplast pathway from the xylem to leaves.
• water molecules evaporate from surface of mesophyll cells into air spaces in leaf before moving out the stomata by diffusion down a conc gradient.
• this loss of water lowers water potential of the mesophyll cell=water moves in from an adjacent cell along the symplast+apoplast pathway.
• this is repeated across the leaf to the xylem.

Question 31

Explain the cohesion-tension theory.

Answer 31

The movement of water from the soil in a continuous stream up the xylem and across the leaf.

• water molecules form hydrogen bonds with carbohydrates in walls of narrow xylem vessels = adhesion.
• water molecules form hydrogen bonds with each other so stick together = cohesion.
• combined effect of adhesion+cohesion results in water exhibiting capillary action. Meaning it can rise up narrow tube against force of gravity. Called transpiration pull.
• transpiration pull results in tension in xylem, helping move water across roots from soil.

Question 32

What is the evidence for cohesion-tension theory?

Answer 32

1. Changes in tree diameter:
   - during day= transpiration at highest= tension is high= so diameter shrinks.
   - during night=transpiration at lowest=tension is low= increased diameter.
2. When xylem vessel is broken, air is drawn into xylem rather than water out. Continuous stream of water molecules held together by cohesive forces= broken so water can’t move up.

Question 33

How can transpiration be measured and key points when setting it up?

Answer 33

Using a potometer.

• all joints sealed with waterproof jelly to ensure all water loss is result of transpiration.
• fresh shoot is cut under water to avoid air bubbles in stem. No water should get onto leaves.

Question 34

Describe the procedure when using a potometer

Answer 34

-

Question 35

How do light, humidity and temp affect the rate of transpiration?

Answer 35

1. Light- needed for photosynthesis. Stomata open in light and most close in dark. Increasing light intensity= increase no of open stomata= increase transpiration.
2. Humidity. High relative humidity= reduces water vapour potential gradient= decrease transpiration. Very dry air has opposite effect.
3. Temperature. Increase in temp= increase kinetic energy of water molecules= increase evaporation.
   Increase in temp= increases conc of water vapour the external air can hold = decreases relative humidity and water potential= increases transpiration.

Question 36

How does air movement and soil-water availability affect transpiration?

Answer 36

1. Air movement. Each leaf has a layer of still air trapped around it. Has features like hairs on surface to reduce air movement.
   Water vapour that diffuses out accumulates here= water potential around stomata increases= diffusion gradient reduced. Therefore increased movement= increased diffusion gradient = increase in transpiration.
2. Water-soil availability. Dry soil=water stress = reduced transpiration rate.

Question 37

what factors affect transpiration?

Answer 37

• light intensity.
• humidity.
• temperature.
• air movement.
• soil water availability

Question 38

What are xerophytes?

Answer 38

Plants that are adapted to survive in dry habitats where water availability is very low
- cacti, marram grass, conifers etc.

Question 39

What adaptations of the stomata do xerophytes have?

Answer 39

1. Sunken stomata- located in pits which reduces air movement. Creates microclimate of still, humid air so reduces vapour potential gradient and therefore transpiration.
2. Reduced numbers of stomata- reduces water loss but also their gas exchange ability.

Question 40

Adaptations of the leaves in xerophytes?

Answer 40

1. Reduced leaves- often reduced to thin needle like structures which have a greatly reduced SA:V. Minimising transpiration.
2. Hairy leaves- create microclimate of still humid air, reducing water vapour potential gradient, reducing transpiration.
3. Curled leaves- confines all stomata in still humid air, reducing diffusion of water vapour from stomata.
4. Leaf loss- some plants lose their leaves when water isn’t available. Trunk and branches turn green and photosynthesise with minimal water loss.

Question 41

Other adaptations in xerophytes?

Answer 41

1. Thick waxy cuticle- most plants lose 10% of water loss by transpiration through cuticle. This minimises it.
2. Succulents- succulent plants store water in specialised parenchyma tissue in stem/roots. Water is used in times of drought.
3. Root adaptations:
   - can have long tap roots to reach deep into ground, to access water below.
   - can have shallow widespread roots with large SA to absorb max before rainfall evaporates.
4. Some plants become dormant or die and leave seeds behind to germinate.

Question 42

Adaptations of hydrophytes?

Answer 42

1. Thin/ no waxy cuticle- don’t need to conserve water.
2. Many always open stomata in upper surface- maximises gas exchange.
3. Reduced structure- water supports leaves/flowers.
4. Wide, flat leaves- spread across water surface to capture max light for photosynthesis.
5. Small roots- water can diffuse directly into stem/leaf tissue.
6. Large SA of stems/roots under water- max area for photosynthesis and oxygen to diffuse into submerged plants.
7. Air sacs- enable leaves/flowers to float on water surface.

Question 43

What is aerenchyma?

Answer 43

• specialised spongy tissue that forms in leaves/stem/roots of hydrophytes.
• has many large air spaces (filled with O2), formed by apoptosis in normal parenchyma.
  Functions in plants:
• making leaves and stems more buoyant.
• forming a low-resistance internal pathway for movement of substances like oxygen to tissues below the water. (Helps cope with anoxic, low oxygen, conditions in mud)

Question 44

What are pneumatophores?

Answer 44

Special aerial roots that grow upwards into the air.
They have many lenticels (raised pores), which allow the entry of air into the woody tissue.
- grow in situations where there is plentiful water supply and the roots can become waterlogged.

Question 45

In what form are assimilates transported in plants?

Answer 45

• the leaves produce large amounts of glucose, which is needed by cells for respiration.
• the glucose is converted to sucrose for transport.
• once it reaches the cell it is converted back to glucose or starch for storage.

Question 46

What is translocation?

Answer 46

The process by which plants transport organic compounds in the phloem from sources to sinks (tissues that need them).

• active process that requires ATP.
• although glucose is made in photosynthesis, sucrose is the main assimilate transported.
• can be transported up and down

Question 47

What are the main sources of assimilates in a plant?

Answer 47

• green leaves and stems.
• food stores in seeds when they germinate.
• storage organs like tap roots that are unloading their stores at the beginning of a growth period.

Question 48

What are the main sinks in plants?

Answer 48

• roots that are growing and/or actively absorbing mineral ions.
• actively dividing meristems.
• any parts that are laying down food stores such as developing seeds, fruits or storage organs.

Question 49

Why is sucrose the main assimilate transported in translocation?

Answer 49

It is not used in metabolism as readily as glucose, so it less likely to be used up in the transportation process.

Question 50

Explain the Phloem loading in translocation.

Answer 50

• sucrose travels down the apoplast route by diffusion down a concentration gradient.
• in companion cells= sucrose moved into cytoplasm in an active process.
• H+ ions actively pumped out of the companion cell using ATP and return to it down a conc gradient via co-transport protein.
• sucrose is co-transported which increases sucrose conc in companion cells and sieve tube elements.

Question 51

What is the evidence for translocation?

Answer 51

• advances in microscopy allow the adaptations of companion cells for active transport to be seen.
• in mitochondria of companion cells are poisoned, translocation stops.
• flow of sugars in phloem is alot faster than it would be with diffusion alone, suggesting an active process is driving the mass flow.
• aphids have shown there is a +ve pressure in phloem that forces the aap out through the stylet. Therefore pressure is lower closer to the sink and conc of sucrose higher near the source.