xylem and phloem

Question 1

where are the xylem and phloem located in leaves

Answer 1

• in the vascular bundle

Question 2

what is translocation

Answer 2

• the products of photosynthesis are transported away from the source to areas of the plant where they are used for growth or storage (sink)
• this is called the mass flow theory
• thought to be an active process

Question 3

explain the translocation of sugars

Answer 3

• hydrogen ions are actively transported out of the cell
• hydrogen ions return with sucrose down the concentration gradient
• sucrose diffuses into the sieve tube elements in the phloem
• water potential inside the sieve tube decreases and water moves in by osmosis
• hydrostatic pressure in the sieve tube at the source increases, the sugary fluid moves down the sieve tube from higher hydrostatic pressure to lower hydrostatic pressure
• sucrose molecules move from the sieve tube to the surrounding cells by facilitated diffusion
• sucrose enters root cells to be used in respiration
  water moves out of the sieve tubes by osmosis
• hydrostatic pressure at the sink falls

Question 4

describe a translocation experiment

Answer 4

• the ringing experiment
• a ring of bark is scraped away
• this also removes the phloem
• sugar will need to be transported down the stem but is stopped at the ring
• a bulge forms above the ring which suggests that sugars are transported down the phloem

Question 5

what is the xylem for

Answer 5

• it transports water and dissolved minerals from roots, up through the plant and eventually out through the stomata in the leaf
• only has one way transport

Question 6

what is the phloem for

Answer 6

• transports the dissolved products of photosynthesis in various directions around the plant

Question 7

how many types of tube make up the xylem and explain what they are

Answer 7

• 2
• tracheids = slightly narrower, made from cells which have perforated end walls, water flow is more obstructed, cells provide more support
• vessels = only found in flowering plants, form main conducting tubes, made from wide cells with reduced or absent walls
• both types of tubes are dead cells and have no cytoplasm which makes it easier to move water through them

Question 8

describe the structure of the xylem

Answer 8

• the cells are dead because of the lignin deposits
• this makes them impermeable
• lignin is deposited as rings or spirals which thickens the cell walls, they are there for support but the plant can still be flexible
• the lignin also allows the adhesion of water molecules
• there are pits where no lignin is deposited which allows sideways movement of water molecules

Question 9

describe transport in the xylem

Answer 9

• the column of water inside the xylem must be intact
• tension pulls the water up
• bubbles would break the cohesion between water molecules and prevent their movement
• the pits allow an intact column and allows it to bypass bubbles

Question 10

what are the two components of a phloem tissue

Answer 10

• sieve tubes
• companion cells

Question 11

describe the sieve tube structure

Answer 11

• elements are alive
• they have no nucleus
• they contain strands of cytoplasm
• end walls are present but they form structures called sieve plates which strands of cytoplasm pass through
• they cannot keep themselves alive

Question 12

describe companion cells structure

Answer 12

• the respire and excrete on the sieve tube elements behalf
• the cytoplasm is joined through plasmodesmata to the cytoplasm of sieve tube elements
• found next to each sieve tube element
• life support unit
• very dense cytoplasm
• contain many mitochondria
• very metabolically active