Chapter 5 - Transport in Plants

Question 1

Pith and cortex (function)

Answer 1

Store food substances (e.g. starch)

The size of the pith and cortex will vary depending on the plant

Question 2

Epidermis (structure)

Answer 2

A layer of cells which cover the stem

Protected by waxy, waterproof cuticle → reduces evaporation of water

Question 3

Xylem (function)

Answer 3

Conducts water and dissolved mineral

salts from the roots to stem and leaves

Question 4

Xylem (structure)

Answer 4

• Long hollow tube stretching from the root to the leaf
• Made of many dead cells
• Long and empty lumen, without cross-walls or protoplasm
• Walls are thickened with lignin

Question 5

Sieve tubes (structure)

Answer 5

• Consist of sieve tube cells/elements which are columns of elongated, thin-walled living cells
• Degenerate protoplasm
• No nucleus, no vacuole and most organelles are also lost
• Thin cytoplasm which is connected to cells above and below through sieve plates

Question 6

Lignin (function)

Answer 6

Provides mechanical support for the plant

Prevents collapse of the plant

Question 7

Lignin (structure)

Answer 7

• Hard and rigid substance

- Can be annular, spiral or pitted

Question 8

Phloem (function)

Answer 8

Transports manufactured food (sucrose and amino acids) from the leaves to other parts of the plant

Question 9

Phloem (structure)

Answer 9

A column of sieve tubes and companion cells

Question 10

Companion cells (function)

Answer 10

• provide nutrients → keep sieve tube cells alive
• many mitochondria → provide energy for transporting sugars from the mesophyll cells into sieve tubes by active transport

Question 11

Sieve plates

Answer 11

• presence of pores → rapid flow of manufactured food substances through sieve tubes

Question 12

Cambium

Answer 12

• thickening of stem
• between the phloem and xylem
• cells divide and differentiate to form new xylem and phloem tissues

Question 13

Dissolved mineral salts (ions) enter root hair cells by diffusion when…

Answer 13

concentration of certain ions and mineral salts in the soil solution is higher than in the cell sap

Question 14

Dissolved mineral salts (ions) enter root hair cells by active transport when…

Answer 14

concentration of certain ions in the soil solution is lower than in the cell sap
(ions are pumped into the cells)

Question 15

During active transport, ions are ________ into cells

Answer 15

pumped

Question 16

After entering the root hair cells, dissolved mineral salts (ions) continue to move through the root cells by ________ until they reach the xylem cell

Answer 16

diffusion

Question 17

Water enters root hair cells from the soil by ________

Answer 17

osmosis

Question 18

Osmosis process: thin film of liquid around soil particles

• is a ________ solution of ________
• has ________ water potential

Answer 18

• dilute, mineral salts

- less negative

Question 19

Osmosis process: cell sap

• is a ________ solution of ________
• has ________ water potential

Answer 19

• concentrated, sugars and salts

- more negative

Question 20

Water transport between plant cells through cell wall = ________ pathway

Answer 20

apoplast

Question 21

Water transport between plant cells through cytoplasm = ________ pathway

Answer 21

symplast

Question 22

Water transport between plant cells through vacuole = ________ pathway

Answer 22

vacuolar

Question 23

Plasmodesma

Answer 23

Small channels that directly connect the cytoplasm of neighbouring plant cells to each other to facilitate transport of materials

Question 24

Entry of water _________ the cell sap in root hair cell.
Cell sap in root hair cell has a ________ negative water potential compared to that in inner root cells.
Water moves from root hair cell to the inner root cells via ________ until it reaches the vascular cylinder.

Answer 24

dilutes
less
osmosis

Question 25

Casparian strip (structure)

Answer 25

a belt of waxy material in the endodermal layer

Question 26

Casparian strip (function)

Answer 26

blocks the passage of water and ions via the apoplast pathway (regulates the flow of water)

Question 27

Describe root pressure

Answer 27

Active transport of ions into the vascular cylinder → Positive pressure → Inward movement of water into the region → Upward push of xylem sap

Question 28

Describe capillary action

Answer 28

Adhesive and cohesive nature of water → Upward movement of water

Question 29

Define "cohesion"

Answer 29

hydrogen bond between water molecules

Question 30

Define "adhesion"

Answer 30

hydrogen bond between water molecules and cell wall of xylem vessel in stems

Question 31

Define transpiration

Answer 31

The loss of water vapour from the aerial parts of the plant, through the stomata of the leaves

Question 32

Define transpiration pull

Answer 32

The negative pressure (tension) or 'suction force' generated by transpiration

Question 33

Water lost through transpiration are:

Answer 33

- excess water absorbed from the soil and not used by the plant - water produced by plant cells during respiration

Question 34

Movement of water inside a leaf

Answer 34

→ Water continuously moves out of the mesophyll cells to form a thin film of moisture over their surfaces → Water evaporates from this thin film of moisture and moves into the intercellular air spaces. Water vapour accumulates in the large air spaces near the stomata (sub-stomatal air spaces) → Water vapour then diffuses through the stomata to the drier air outside the leaf (=transpiration) → As water evaporates from the mesophyll cells, the water potential of the cell sap decreases. The mesophyll cells begin to absorb water by osmosis from the cells deeper inside the leaf. These cells, in turn, remove water from the vein (xylem vessels). → This results in a suction force which pulls the whole column of water up the xylem vessel

Question 35

What happens in the leaf during transpiration?

Answer 35

- Water diffuses from xylem to the neighbouring cells in the spongy mesophyll layer - Water dissolved into the thin layer of moisture surrounding the spongy mesophyll cells - Water vapour leaves via stomata

Question 36

What happens in the stem during transpiration?

Answer 36

- Loss of water vapour from leaves creates a 'suction force' = transpiration pull which draws water up the stem

Question 37

What happens in the root during transpiration?

Answer 37

- Water moves into root hair cells by osmosis | - Water moves from root hair cells to neighbouring cells in the root by osmosis until it reaches the xylem vessel

Question 38

What is a transpiration stream?

Answer 38

Upward movement of water molecules from roots to leaves

Question 39

Rate of transpiration increases when... [4]

Answer 39

1. humidity is low 2. there is wind/air movement 3. temperature is high 4. there is light

Question 40

Why does humidity affect transpiration?

Answer 40

When humidity is low, | air is dry and water vapour diffuses out from the leaves faster

Question 41

Why does wind affect transpiration?

Answer 41

- Wind blows away the water vapour that diffuses out from the leaves - Maintains the water vapour concentration gradient

Question 42

Why does temperature affect transpiration?

Answer 42

When temperature is high, | rate of evaporation increases

Question 43

Why does light affect transpiration?

Answer 43

When there is light, | stomata open and more water vapour can diffuse out

Question 44

Importance of transpiration pull [5]

Answer 44

1. Draws water and mineral salts from roots to stem and leaves 2. Removes latent heat of vapourisation → cools the plant 3. Water is used by leaves during photosynthesis 4. Keep cells turgid to spread out the leaves widely to trap sunlight 5. Replace water lost by cells

Question 45

How can the rate of transpiration be measured?

Answer 45

1. Using a spring balance 2. Using a potometer (for detailed set-ups, check slides or textbook)

Question 46

Spring balance: How to use the results to calculate rate of transpiration?

Answer 46

loss in mass / time taken | units: g/h

Question 47

Potometer: How to use the results to calculate rate of transpiration?

Answer 47

change in volume of water in column / time taken | units: cm^3/min

Question 48

In a normal, healthy plant, ...

Answer 48

- Leaves are __ and the cells are turgid | - Spread out widely

Question 49

In a wilted plant, ...

Answer 49

- Leaves are folded and cells are plasmolysed (lost of turgor pressure) - Folded up

Question 50

Wilting

Answer 50

→ Plant cells lose water vapour faster than the roots can absorb water (i.e rate of transpiration > rate of water absorption) → Turgor pressure in plant cells decreases → Cell membrane and cytoplasm shrinks away from cell wall → Plant cells become plasmolysed/flaccid → Stem becomes soft and limp and leaves are folded up

Question 51

Xerophytic plants: What are adaptations do they have? [3]

Answer 51

- The upper epidermis on the outside of the leaf has a thick cuticle and no stomata - Stomata are confined to furrows in the inner surface of the leaf, where they are sheltered from air currents - The inner epidermis has many stiff hairs

Question 52

Xerophytic plants: How do the adaptations help in the harsh environment?

Answer 52

Results in humid air becoming trapped inside the rolled leaf, reducing the rate of transpiration

Question 53

Xerophytic plants: Leaf unrolls when ________, but rolls up when transpiration is _______ Rolling is brought about by ________ in the leaf cells

Answer 53

water is available excessive osmotic changes

Question 54

What is translocation?

Answer 54

Transport of manufactured food and amino acids in plants

Question 55

Sugars are transported from ________ to ________ via phloem

Answer 55

sources | sinks

Question 56

What are sources?

Answer 56

Plant organs in which sugar is being produced by photosynthesis or the hydrolysis of starch

Question 57

What are sinks?

Answer 57

Organs which consume or store sugar | e.g. roots, buds, stems, fruits

Question 58

What is translocation?

Answer 58

The transport of manufactured food substances such as sucrose and amino acids in plants

Question 59

Mechanism of translocation

Answer 59

→ Active transport of sucrose into the sieve tube at the source reduces water potential inside the sieve-tube elements → Inward movement of water into the phloem by osmosis → Generates a positive pressure that forces the sap to flow along the tube → The pressure is relieved by the unloading of sugar and the consequent loss of water at the sink → Xylem recycles water from sink to source (in leaf-to-root translocation)

Question 60

What is the hypothesis behind the mechanism of translocation?

Answer 60

Bulk flow by positive pressure (pressure-flow hypothesis)

Question 61

"Ringing" experiment: to show phloem as the translocation tissue

Answer 61

1. Cut of a complete ring of the bark, including the phloem and the cambium, from the stem of a twig 2. Place the twig in water with the ring immersed 3. Set up another twig that has a cut ring above water 4. Set up a control with an unringed twig 5. Observe for swellings

Question 62

Swellings will be formed ________ the cut region due to accumulation of ________

Answer 62

above | food substances

Question 63

Using aphids: to show phloem as the translocation tissue

Answer 63

- Aphids feed on plant juices - Studies have shown that aphids insert their stylet into the phloem sieve tube to feed on the sugars → Shows that translocation of sugars and amino acids occurs in phloem

Question 64

Using isotopes and autoradiography: to show phloem as the translocation tissue

Answer 64

1. Provide a leaf with carbon dioxide containing radioactive carbon (14C) 2. During photosynthesis, glucose formed contains 14C 3. Expose a section of the cut stem onto an X-ray film → Only the phloem will show radioactivity