3.2b plant nutrition (translocation)

define the term translocation

the transport of manufactured food substances (eg sucrose and amino acids) in plants (phloem tissue)

describe the translocation study for “ringing” experiment

• remove a ring of bark with the phloem of a woody plant

• stem above the cut region swells as the sucrose from the leaves accumulate above the cut region

describe the translocation study for using aphids

• aphids are anaesthetised with CO₂ while feeding on plant juices

• the body of the aphid is removed, leaving behind the stylet

• analysis of the liquid in the stylet shows sucrose and amino acids

describe the translocation study for using isotopes

• leaf is supplied radioactive isotope, Carbon-14 (¹⁴C)

• during photosynthesis, leaf uses ¹⁴C to synthesise sucrose

• when exposed onto X-ray photographic film, reactive substances are present in phloem

describe the distribution of the chloroplasts for photosynthesis and its significance

• the closely packed, long and cylindrical palisade mesophyll cells contain the most number of chloroplasts

  • function (significance): more light can be absorbed near the upper leaf surface

• the loosely packed and irregular-shaped spongy mesophyll cells contain chloroplasts

• the guard cell contains chloroplasts

• upper epidermis and lower epidermis cells are closely packed and form one layer respectively; they do not contain chloroplasts

list the adaptations of a leaf for photosynthesis

1. waxy cuticle on upper and lower epidermis

2. stomata present in the epidermal layers

3. chloroplasts containing chlorophyll in all mesophyll cells

4. more chloroplasts in upper palisade tissue

5. interconnecting system of air spaces in the spongy mesophyll 

6. veins containing xylem and phloem situated close to mesophyll cells

describe the function of this adaptation: waxy cuticle on upper and lower epidermis

• reduces water loss through evaporation from the leaf

• it is transparent for light to enter the leaf

describe the function of this adaptation: stomata present in the epidermal layers

stomata open in the presence of light, allowing CO₂ to diffuse in and O₂ to diffuse out of the leaf

describe the function of this adaptation: chloroplasts containing chlorophyll in all mesophyll cells

chlorophyll absorbs energy from light and transfers it to chemical stores of energy in glucose molecules (carbohydrates)

describe the function of this adaptation: more chloroplasts in upper palisade tissue

more light can be absorbed near the upper leaf surface

describe the function of this adaptation: interconnecting system of air spaces in the spongy mesophyll 

the air spaces allow rapid diffusion of CO₂ and O₂ into and out of mesophyll cells

describe the function of this adaptation: veins containing xylem and phloem situated close to mesophyll cells

• xylem transports water and mineral salts to mesophyll cells

• phloem transports sucrose away from the leaf

describe how CO₂ reaches mesophyll cells in a leaf

1. CO₂ diffuses into the leaf through the stomata

2. it dissolves in the water around the cells

3. it then diffuses into the cells

describe how leaf cells receive water and mineral salts

1. water and mineral salts are transported through the xylem vessels from the roots

2. they move from cell to cell through the mesophyll

state the functions of a xylem vessel

• conduct water and dissolved mineral salts from the roots to the stems and leaves

• provide mechanical support for the plant

describe the adaptations of a xylem vessel

• long hollow tube made up of dead cells without cross walls (reduces resistance to water flowing through the xylem)

• there is lignin on the inner walls (to strengthen)

• end walls broken down completely to allow unobstructed flow of water & mineral salts

state the direction of flow in the xylem vessel and process by which substances are moved

• unidirectional — upwards

• transpiration

state the function of phloem

transport food from the leaves to other parts of the plant

describe the adaptations of the phloem

• a column of sieve tube cells (/elements) forms a long sieve tube

• the sieve plates, which are ‘cross-walls’ between the cells have many minute pores

• the companion cell of each sieve tube cell provides nutrients and helps it to transport food

• end walls broken down partially

• lignin: absent

state the direction of flow in the phloem and process by which substances are moved

• bidirectional — up and downwards

• translocation