(U2) T&EM - Transport In Plants And Transpiration

Question 1

State the features of roots in plants

Outside to inside

Answer 1

1. Root hair cells / tissue
2. Epidermis
3. Cortex
4. Stele:

• endodermis (with casparian strips)
• phloem
• xylem

Question 2

State the features of the stem in plants

Outside to inside

Answer 2

1. Epidermis
2. Cortex
3. Vascular bundle:

• phloem (outside)
• cambium (separating both)
• xylem (inside)

4. Pith

Question 3

What are the 2 types of protoxylem?

Describe the features of each (3)

Answer 3

1. Annular
2. Spiral

• no cell contents - little resistance to water flow
• thin cell walls - allows stretching during growth
• rings/spirals of lignified cell wall - prevent inward collapse under tension (also hydrophobic)

Question 4

What are the 2 types of metaxylem?

Describe the features of each (4)

Answer 4

1. Reticulated
2. Pitted

• no cell contents - little resistance to water flow
• large empty lumen - large volumes of water can flow
• thick, heavily lignified cell walls - prevent inward collapse under tension
• (only in pitted) pits - allow water to flow laterally between xylem vessels to surrounding cells

Question 5

Why are Xylem vessels described as being dead? (2)

Answer 5

• they do not contain mitochondria
• lignin is impermeable and increases with age - lack of water confirms they are dead

Question 6

Where is lignin situated in xylem vessels? (2)

Answer 6

• Impregnated in the secondary wall,
• formed inside the primary cellulose wall

Question 7

What is translocation? (3)

Why is it described as two-way flow?

Answer 7

• movement of organic solutes (ions or, often sucrose) through phloem vessels
• from source to sink (ie from chloroplasts in leaf to outer parts of the plant such as flowers)
• requiring ATP

2. The organic solute can be moved up to a bud up the plant or down to the roots at the base of the plant

Question 8

Why do sieve tube elements have sieve plates between each other?

Answer 8

Allows phloem sap to flow between the sieve tube elements, allowing diffusion of substances through the phloem

Question 9

What are the two types of cell in phloem tissue?

Answer 9

• phloem sieve tubes
• companion cells

Question 10

What is contained within a companion cell? (4)

Answer 10

• cytoplasm
• many mitochondria
• a nucleus

+ sucrose is stored here

Question 11

How can sucrose move into phloem sieve tubes?

Answer 11

diffusion via plasmodesmata between companion cells and sieve tubes

Question 12

How are:

• water
• mineral ions

Absorbed into root hair cells

Answer 12

• water: osmosis
• mineral ions: active transport / facilitated diffusion

Question 13

What is the apoplast pathway?

Answer 13

Where water moves along cellulose fibrils of cell walls

Question 14

What is the symplast pathway? (2)

Answer 14

• Where water moves through protoplasts
• and between cell cytoplasm via plasmodesmata

Question 15

Which pathway is most common for water transport in plants?

Why?

Answer 15

• apoplast pathway
• offers least resistance to water flow

Question 16

Where are Casparian strips?

What are they made of?

What do they cause?

Answer 16

• parts of the top and bottom of cells in the endodermis of roots in plants
• suberin: a hydrophobic substance that blocks the apoplast pathway
• water only travels via symplast pathway instead, into stele

Question 17

What is the purpose of the waxy cuticle?

Answer 17

Limits cuticular evaporation (during transpiration) due to its hydrophobic properties

Question 18

What internal factors affect rate of transpiration? (3)

Answer 18

• leaf surface area (more stomata exposed to the air)
• stomatal density (more stomata per unit area)
• cuticle thickness (thicker means reduced cuticular evaporation)

Question 19

What external factors affect rate of transpiration? (5)

Answer 19

• temperature
• air movements (which blow away diffusion shells of water outside stomata)
• humidity (high humidity decreases diffusion gradient)
• light (which opens stomata)
• soil water availability (stomata close to conserve water)

Question 20

Explain the cohesion-tension theory (4)

Answer 20

• water transpires, creating tension in the plant
• this creates a water potential gradient which causes water to be sucked up toward the leaves (transpiration stream)
• cohesive forces between water molecules also aid movement (by pulling molecules together, forming a water column)
• also adhesive forces between the water and xylem vessel walls prevent the water column falling under gravity

Question 21

Explain the root pressure hypothesis (4)

Is this linked to transpiration?

Answer 21

• ions are actively transported into the xylem
• a water potential gradient is formed
• water follows this gradient and diffuses via osmosis into the xylem
• this creates root pressure - pressure in the xylem that forces water upward

2. No

Question 22

State evidence for metabolic energy expenditure being involved in translocation (3)

Answer 22

• translocation cannot occur with metabolic poisons
• phloem companion cells have many mitochondria
• the rate of flow is faster than diffusion

Question 23

State evidence for two-way movement being involved in translocation

Answer 23

Radioactively labelled sucrose can be found in the roots and in shoot tips

Question 24

What are the general adaptations of xerophytic plants? (5 categories)

Answer 24

Decreased SA for transpiration:

• leaf curvature via hinge cells
• spines instead of leaves

Localised humidity via trapping water vapour: (reduced water potential gradient)

• hairs
• leaf curvature
• sunken stomata

Other transpiration limiters:

• thick waxy cuticle

Maximised water uptake:

• deep network of roots or
• extensive shallow network of roots

Question 25

What are the *general* **adaptations** of **hydrophytic** plants? (**4 categories**)

Answer 25

Diffusion enhancing features: - *stomata* on **upper epidermis** (prevents flooding) - air enchyma (aids maintaining a high water potential gradient in ICAS) Greater buoyancy (for **more photosynthesis**): - **air enchyma** (doesn’t break surface tension of water) Anchorage: - roots are deep Greater photosynthesis: - large air enchyma (*decreased shading*)