2.3 Plant (extra edition)

Question 1

Explain root structure

Answer 1

• root hairs that are thin, permeable extensions of epidermal cells
• cortex consists of parenchyma where starch may be stored
• endodermis is once cell thick and has impermeable casperian strips

Question 2

Explain uptake and transport in roots

Answer 2

• water and ions mainly by root hairs which provide large surface area
• ions absorbed by diffusion and active transport
• water uptake is by osmosis along a water potential gradient - soil>root hair
• water potential gradient maintained by water continually moving up the xylem and ions dissolved in the xylem sap

Question 3

Explain mass flow hypothesis

Answer 3

• translocation of phloem due to mass flow of solutions of organic substances along a hydrostatic pressure gradient
• sugars produced in mesophyll cells known as source
• sugars are actively transported into sieve tubes, lowering their water potential
• water enters by osmosis and creates high hydrostatic pressure in sieve tubes in leaf
• in roots, sugars leaving the phloem to be used in respiration and are sinks
• water potential of sieve tubes of sinks becomes higher so water leaves by osmosis and hydrostatic pressure falls
• photosynthetic products transported along pressure gradient

Question 4

What are the two main hypothesis for movement of water and minerals from roots to leaves in the xylem?

Answer 4

• root pressure
• cohesion-tension hypothesis

Question 5

Explain root pressure hypothesis

Answer 5

• positive hydrostatic pressure
• active transport of mineral ions by endodermal cells lowers the water potential of xylem
• water move into xylem by osmosis raising the hydrostatic pressure

Question 6

Explain cohesion tension hypothesis

Answer 6

• light energy cause transpiration fo water from leaves
• water evaporate from mesophyll cells and diffuses out of stomata
• water potential is lower commmpared to the inner mesophyll
• water moves into across the water potential gradient
• water potential gradient extends to the xylem creating tension in the xylem pulling water up
• water in xylem forms a continuous column from leaves to roots
• column is maintained by cohesive and adhesive forces of water due to hydrogen bonding
• upward movement of water from xylem in roots maintains a water potential gradient across root cortex cells for water uptake from soil

Question 7

What measurements need to be made in a potometer to determine rate of transpiration?

Answer 7

Distance traveled and time taken

Question 8

Explain how a sunken stomata is able to reduce transpiration

Answer 8

• Hugh humidity in air chamber as water vapour is trapped
• this reduces the water potential gradient between the inside of the leaf and the air chamber
• leading to a decreased rate of diffusion