paper 1: Mass Transport in Plants

Question 1

what type of cells are xylem

Answer 1

undifferentiated

Question 2

what is meant by a differentiated cell

Answer 2

they have specialised adaptions to help them carry out their functions

Question 3

what adaptions do xylem cells have to carry out their function

Answer 3

• long, dead, cells which form hollow tubes - so water potential is not effected and water stays in the xylem
• end walls break down - water can form a continuous column
• cell walls are strengthened by lignin - makes xylem waterproof and helps withstand tension
• no cytoplasm - so they don’t obstruct flow of water
• many narrow xylem vessels - increases surface rea for adhesion

Question 4

define transpiration

Answer 4

the evaporation of water from the leaf through the stomata

Question 5

describe and explain how water moves through xylem vessels

Answer 5

• water evaporates from the leaf through stoma
• leads to a lower water potential in mesophyll cells then in the xylem creating a water potential gradient
• water moves from xylem into mesophyll cells by osmosis. creates tension and pulls water up the xylem
• water molecules form a continuous column through cohesion as they are held together by H bonds
• H bonding causes adhesion of water molecules to walls of xylem to cause xylem to be pulled inwards

Question 6

explain why transpiration is important

Answer 6

• provides leaves with water for photosynthesis
• provides water to make cells turgid
• provides water for hydrolysis reactions

Question 7

list the factors effecting rate of transpiration

Answer 7

• air movement
• temperature
• humidity
• light intensity

Question 8

what is the function of the phloem

Answer 8

transport organic substances in plants

Question 9

what are the features of phloem and their functions

Answer 9

• end walls perforated with holes - allows continuous flow if substances through sieve tube
• sieve tubes have no nucleus and little cytoplasm- allows unobstructed flow of solutes
• companion cells contain many organelles - carry out functions for sieve tubes

Question 10

what is translocation

Answer 10

movement of organic substances or assimilates to where they are needed in the plant

Question 11

what is a source

Answer 11

area of plant which produces assimilates

Question 12

what is a sink

Answer 12

where assimilates are sued or stored `

Question 13

decsribe the process of the mass flow hypothesis

Answer 13

• at source, active transport is used to load the sucrose made from photosynthesis from companion cells into the sieve tubes in the phloem
• this lowers water potential in phloem so water moves form xylem to sieve tubes by osmosis
• creates a higher hydrostatic pressure at the source and of the phloem so there is mass flow of solutes towards the sink
• qt sink, solutes are removed from phloem down conc. grad. where they are used up or stored as another molecules at the sink
• as solutes leave phloem, the water potential inside sieve tubes increases so water moves out of sieve tubes by osmosis
• therefore a pressure gradient is maintained from source to sink