4.7 Transportation in plants

Question 1

Describe the structure of the xylem and how is it related to its structure

Answer 1

• Lined with polysaccharide lignin which is a water proof coat, this provides structure and doesnt allow any water too leak out of the xylem.
• No cytoplasm or cell organelles as this would slow the flow of water down up the xylem
• Contains unlignified bordered pits which allow water to leave and go to surrounding cells.
• Narrow enough so water travels upwards in an unbroken column

Question 1

Describe how xylem cells are formed

Answer 1

• Cellulose microfibrils are laid on the sides of the cell wall.
• As the stem ages and the cells stop growing more lignin is laid down in the cell walls
• Tissues become stronger due to lignin and this causes cell organelles start to die
• Ends of cell walls break down therefore a hollow tube forms down the xylem

Question 2

How does water move out to surrounding cells?

Answer 2

Water moves out via unlignified areas or through pits

Question 3

What is the process which slowly kills the living cells in the xylem

Answer 3

Lignification

Question 4

Describe the structure of the phloem

Answer 4

• Tubes are made up of living cells
• Made up of sieve tube cells which allows the transport of sugars around the plant
• Have little to no organelles to maximise flow of sap
• Companion cells designed for active transport of sugars into the tubes
• Plasmodesmata linked to the cytoplasm allowing flow of substances between cells

Question 5

What is the role of the phloem?

Answer 5

• To transport nutrients to storage organs and growing parts of the plant

Question 6

Where is water absorbed in plant cells?

Answer 6

• Root hair cells VIA osmosis

Question 7

How are root hair cells adapted to maximise the rate of osmosis

Answer 7

• Densely packed root hairs massively increases SA:V ratio of the root.
• Surface of root hair only contains cell membrane and cell wall , making it thin and giving it short diffusion distance
• Uses active transport to take in Mg ions and other sugars to reduce the water potential of the cell.

Question 8

Describe the symplast pathway

Answer 8

• Water moves from cell to cell via osmosis down a concentration gradient through the plasmodesmata

Question 9

Why is the symplast pathway relatively slower than apoplastic route

Answer 9

• Pathway for water is obstructed via cell organelles making it slower

Question 9

When is water forced to travel via symplast pathway only ?

Answer 9

• When it reaches casparian strip, water can no longer travel via the cell walls.
• Therefore water then travels via symplast pathway through cytoplasm and plasmodesmata

Question 10

What are the surface of leaves coated with to prevent water loss?

Answer 10

Waxy cuticle

Question 10

Describe the apoplastic route

Answer 10

• Water is pulled by the attraction of molecules from cell to cell via adjacent cell walls
• Cellulose cell walls have water filled pores therefore little resistance to pull of water
• Water travels via this route until it reaches the endodermis where casparian strip is found

Question 10

How do cells in the endodermis pump mineral ions into the xylem ?

Answer 10

• Active transport

Question 11

What is meant by cohesion ?

Answer 11

When the water molecules form hydrogen bonds between them due to the polar nature of water

Question 12

What is meant by adhesion

Answer 12

• Forces of attraction made by the polar water molecules in the xylem and the polar groups in lignin , therefore the water molecules can ‘stick’ to the xylem walls.

Question 13

Describe the cohesion tension theory

Answer 13

• Water molecules stick together and form hydrogen bonds , water molecules also bind to the xylem cell walls due to lignin having polar groups. This pulls water up as transpiration creates a hydrostatic pressure which is aided by the cohesion and adhesion of water molecules.

Question 14

What are the factors affecting the rate of transpiration ?

Answer 14

• Light intensity
• Wind
• Humidity
• Heat

Question 15

How does light intensity affect the rate of transpiration ?

Answer 15

• As light intensity increases, the rate of transpiration increases as stomata’s open in the light
• However, rate of transpiration will level off due to all stomata being open

Question 16

How does humidity affect the rate of transpiration?

Answer 16

• Higher humidity decreases the concentration gradient due to a higher amount of water vapour outside of the stomata
• As humidity increases, rate of transpiration increases

Question 17

How does temperature affect the rate of transpiration?

Answer 17

• As temp increases, there is a higher kinetic energy of water molecules, therefore a higher rate of transpiration
• As temp increases, the concentration gradient increases due to lower humidity out side of stomata
• Plataeus when something becomes limiting

Question 18

How does wind affect the rate of transpiration

Answer 18

• As wind increases, rate of transp. increases.
• This is because wind blows away still water vapour on the leaf, therefore increasing the conc gradient and increasing the rate of transpiration.