Transport in Plants

Question 1

Function of the xylem

Answer 1

Transport of water and mineral ions from roots to other parts of the plant
Provide mechanical support to the plant

Question 2

Structure of the xylem

Answer 2

Hollow and dead (no cell organelles): space available in the xylem for transport of water and mineral ions

Continuous !from roots to stem to leaves!: allows water absorbed at roots —> leaves

Lignified cell walls: strong and waterproof, provides mechanical support

Question 3

Function of the phloem

Answer 3

The phloem transports manufactured food (dissolved sucrose and amino acids) from leaves to other parts of the plant (translocation)

Question 4

Structure of phloem

Answer 4

Sieve plates found in sieve tube elements, facilitates the flow of substances from cell to cell along the sieve tube element

Companion cell is connected to sieve tube element and keeps it alive, to provide the energy required for translocation (transport of manufactured food)

Sieve tube element is hallow (lacks cell organelles), space available for substances to be transported in the plant

Question 5

Describe what the phloem is made out of

Answer 5

Sieve tube elements separated by sieve plates. Each element has a companion cell

Question 6

Similarities between xylem and phloem tissues

Answer 6

Both are tissues that are involved in the transport of substances
Contents in both tissues are dissolved in solution

Question 7

Differences between xylem and phloem tissues

Answer 7

Substances transported: xylem (water and mineral ions), manufactured food (dissolved amino acids and sucrose)

Direction of transport: xylem (from the roots to leaves, upwards), phloem (from the source to sink, upwards and downwards)

Living or non-living tissues: xylem (non-living), phloem (living, sieve tube element and companion cell, not sieve plate)

Walls of the vessel: xylem (lignified, provides mechanical support), phloem (no lignin no support)

Question 8

What are vascular bundles?

Answer 8

Structures where xylem and phloem tissues are found together

Question 9

What is the cambium?

Answer 9

The cambium of vascular bundles is a group of actively dividing cells that will differentiate eventually into phloem or xylem tissues
It is found in roots, stems and leaf vascular bundles

Question 10

What are root hair cells?

Answer 10

They are specialised epidermal cells found in roots that function to absorb water and mineral ions

Question 11

Structure of a root hair cell

Answer 11

Concentrated cell sap: increases steepness of water potential gradient between cell and soil

Elongated extension (root hair): increases the SA:V ratio, allowing efficient absorption fo water and mineral ions from the soil

Large vacuole: allows more water and mineral salts to enter the cell

Many mitochondria: energy released during aerobic respiration is used for active transport of mineral salts into the cell

Absence of chloroplasts: unnecessary bc underground

Question 12

Describe the uptake of water and mineral ions into the root hair cell

Answer 12

Water is taken into the root hair cell via osmosis
Water molecules move from a region of less negative water potential in the soil to an area of more negative water potential in the cell sap, across a partially permeable membrane (the cell membrane)

Mineral ions are taken into the root hair cell via active transport
Mineral ions move from an area of higher concentration in the soil to an area of lower concentration in the cell sap against a concentration gradient, requiring energy

Question 13

What are the routes water molecules absorbed at the root hair cells can take to the xylem in the centre of the roots?

Answer 13

Apoplast pathway
Water and mineral ions move along the continuum of the interconnected porous cell walls (without entering the cells)

Symplast pathway
Water and mineral ions move from cell to cell along the continuum of the living cytoplasm where cells are connected by cytoplasmic bridges called plasmodesmata

Vacuolar pathway (transmembrane)
Water and mineral ions move in and out of cells passing across cell membranes and cell walls

Question 14

What is the endodermis and casparian strip, and what is its function?

Answer 14

The endodermis is a layer of cells located just outside the vascular bundle, and it contains a waterproof substance known as the casparian strip

The casparian strip forces water and mineral ions to pass through the cell membrane and enter the xylem to be transported to the rest of the plant, preventing the leaking back of mineral ions into the cells in the roots and the soil

Water and mineral ions that enter the xylem will be transported up the stem towards the leaves, by a force known as transpiration pull

Question 15

How is xylem sap transported up the plant?

Answer 15

Transpiration pull: main suction force that is generated by transpiration

Capillary action: due to the adhesive and cohesive nature of water

Root pressure: pushing force caused by water entering the xylem at the roots

Question 16

Define transpiration

Answer 16

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

Question 17

Describe the process of transpiration

Answer 17

Stomata are open during the daytime, to allow CO2 to enter for photosynthesis
Water molecules move out of the xylem vessel to the surrounding mesophyll cells
Water molecules evaporate from the surface of the mesophyll cells forming water vapour in the intercellular air spaces of the spongy mesophyll
Diffusion: water molecules moved from the intercellular air spaces (area of higher concentration) to the surrounding atmosphere (area of lower concentration), due to the concentration gradient

Question 18

What are the functions of transpiration?

Answer 18

To create a suction force called transpiration pull responsible for transporting water and mineral ions up the plant in the xylem
Cooling down the plant, evaporation of water from the stomata of the plant removes the latent heat of vaporisation

Question 19

Summary of the route of water up the plant

Answer 19

Water molecules enter the plant at the roots, absorbed by root hair cells via osmosis
Water molecules move from the root hair cells to the xylem via the apoplast, symplast and vacuolar pathways
Water molecules are transported up the xylem by transpiration pull, suction force and adhesion and cohesion
Water molecules move out of the xylem to the surrounding mesophyll cells in the leaves and is lost in the form of water vapour to the surroundings via transpiration

Question 20

How does light intensity affect transpiration rate?

Answer 20

As light intensity increases, transpiration rate increases

Since light is a condition required for photosynthesis, stomata open larger under high light intensities to provide more CO2 for more photosynthesis, so increased gaseous exchange occurs and there is more evaporation of water through the stomata of the leaves

Question 21

How does air movement affect transpiration rate?

Answer 21

As air movement increases, transpiration rate increases

More wind sweeps water vapour away, resulting in a lowered concentration of water vapour on the leaf surface, so the concentration gradient between the intercellular air spaces and surroundings remain high, speeding up diffusion of water molecules

Question 22

How does humidity affect transpiration rate?

Answer 22

As humidity increases, transpiration rate decreases

Since there is more humidity and a higher concentration of water vapour in the atmosphere, the concentration gradient between the surroundings and the intercellular air spaces of the leaf decreases, so the rate of diffusion of water vapour decreases

Question 23

How does temperature affect transpiration rate?

Answer 23

As temperature increases, transpiration rate increases

Higher temperatures result in a higher rate of evaporation of water from the surface of the mesophyll cells to the intercellular air spaces in the spongy mesophyll layer, resulting in a steeper concentration gradient between intercellular air spaces and the surroundings, so the rate of diffusion is faster

Question 24

Why do plants wilt and under what conditions?

Answer 24

Wilting occurs when the amount of water lost via transpiration is higher than the rate of water absorption at the roots

High transpiration: high light intensity, more air movement, lower air humidity, higher temperatures
Low water absorption rate: more negative water potential of soil caused by infrequency watering, use of excessive fertiliser, or soil with high salinities

Question 25

What are plant cells and stomata in wilted plants like?

Answer 25

Plant cells: plasmolysed

Stomata: may close to decrease transpiration rate as a protective function, decreasing photosynthesis rate

Question 26

Adaptations to prevent wilting

Answer 26

1. Thick cuticle: barrier to evaporation and shiny surface reflects heat
2. Sunken stomata: harder for wind to blow water away
3. Rolled leaf: trap water vapour, smaller SA of leaf exposed to atmosphere
4. Trichomes/hairs on leaf surface: trap water vapour and reduce water loss
5. Small, needle-like leaves: less SA, smaller evaporating surface
6. Thick, fleshy stems to store water
7. Fewer stomata
8. Stomata opening in cooler evening

Question 27

Function, components, and units of a potometer

Answer 27

Function: to measure the rate of water absorption/uptake of a plant

Components: water reservoir (supplies water for plant), air bubble (indicates how fast water is being moved), scale (quantifier), oil (prevents evaporation)

Units: mm/s, cm/s (speed of air bubble’s movement)
mm^3/s, cm^3/s (rate of volume of uptake of water by plant)

Question 28

Define translocation

Answer 28

Transport of manufactured food substances such as sucrose and amino acids in the phloem tissue of plants

Translocated from the source (leaf the producer, or root the storage) to the sink (plant organ that consumes or stores sugar)

Upwards or downwards

Question 29

Describe translocation

Answer 29

At the source, sucrose is actively transported into the sieve tube element using energy from the companion cell

The high concentration of sucrose in the sieve tube element at the source is a region of more negative water potential, so water molecules from the surrounding xylem will enter the sieve tube element via osmosis
The entry of water at the sieve tube element at the source creates an area of high hydrostatic pressure, and forces manufactured food in the sieve tube element to flow along the phloem
At the sink, sucrose is unloaded and removed from the sieve tube element, resulting in a region of less negative water potential. Water molecules in the sieve tube element will leave via osmosis back into the xylem.

Question 30

Explain tissue ringing experiments

Answer 30

Part of the stem is removed, only the phloem not the xylem
Leaves are unaffected, but stem area above the ring became swollen and the stem area below the ring had reduced growth, since sucrose cannot be transported from the leaves to the other parts of the plant and accumulates above the ring

Question 31

Describe and explain the aphid studies

Answer 31

Aphids feed on plant sap from the sieve tube element using their stylets
CO2 gas is used to knock out the aphids while they feed, and they are decapitated so the stylets are still embedded in the phloem and the plant sap will leak out the stylets
Based on the rate of flow of plant sap out of the stylets
Do food tests on the plant sap to determine its composition

Question 32

Explain radioactivity experiments

Answer 32

Using carbon isotopes and radioactive 14CO2 to supply to leaves, so 14C glucose synthesised and transported in the phloem, so X-ray film identifies it as shaded/grey regions, proving products of photosynthesis are transported via the phloem

Question 33

What are root pressure and capillary action?

Answer 33

Root pressure is upward push of xylem sap driven by the positive pressure as a result of active transport of ions

Capillary action is the upward movement of water as a consequence of its adhesive and cohesive nature