CHAPTER 8: TRANSPORT SYSTEM IN PLANTS

Transport of Water

Introduction

Plants need water to:

  • keeping cells turgid - leaves and petals in proper position
  • solvent for metabolic reaction
  • reactant in photosynthesis

Water taken up via root hair ➡ seeps across root towards centre ➡ enter xylem vessels ➡ transported to stem & leaves (other parts of plants)

Transpiration

Actions of water being evaporated out from plants cells via stomata.

Water diffusion: osmosis

Water ➡ root hairs ➡ centre (xylem vessels) ➡ stem (transported through vascular bundles) ➡ leaves ➡ water evaporated from cells (in air spaces) ➡ water vapour diffuses into the air through the stomata ➡ into environment

Produces tension: transpiration pull - enable water to be pulled upwards (against gravitational pull)

  • water cohesion: enable water to be pulled in a continuous stream - water molecules tend to stick to each other, preventing the column from breaking
  • air lock: when water in column breaks, air will fill in the gap - water stops flowing upwards

How does it works?

  1. Water loss at the top portion of the plants (leaves & shoots) - reduces water pressure at the top
  1. Water at the bottom parts (roots) having higher water pressure
  1. Water at the bottom parts (higher water potential) 'push' upwards to the top (lower water potential) - down water potential gradient
  1. Deliver water throughout the plants

Water potential drives transpiration:

Water potential in air < roots: water goes into the roots

Water potential in air (depends on the humidity) < leaves: water transpire out

Water potential in cell walls (of mesophyll cells) < xylem vessels: water being evaporated out

Water potential in upper part of vessel system (leaves) < lower part of vessel system (roots): movement of water upwards through xylem vessels

Water moves into the roots:

Via root hairs (constantly being produced due to short lifespan)

  • short lifespan (always get destroyed)
  • grow in between soil particles
  • increase surface area for water absorption - speeds up absorption
  • also takes up mineral salts (inorganic ions)

KEYPOINTS: Water moves into roots:

  • via osmosis into root hairs
  • soil has higher water potential than cytoplasm in root hair cells

Water moves across the roots:

Water moves from root hairs ➡ across cells in cortex ➡ into xylem system

Water seeps through the cells in cortex, without going into the cells

Water moves up xylem vessels:

Xylem vessels:

  • long hollow dead cells
  • placed end to end - form a long continuous tube
  • having pits (small holes) at the sides facilitate water movement into & out from xylem vessels
  • No cytoplasm in the cells
  • Water & mineral ions - free to pass

Effects of Environmental Conditions on Transpiration Rate

To fasten transpiration rate:

High temperature

  • increase rate of evaporation in leaves

Dry Air

  • increase rate of evaporation in leaves
  • lower water potential in the air than in the leaves

Wind Speed

  • take away humid air around the leaves
  • lower water potential in the air than in the leaves

Light

  • stomata opens up during photosynthesis - letting out CO2
  • water vapour will diffuse out from the leaves via the opened stomata
  • wihtout light, the diffusion of water vapour out from stomata is significantly reduced

Adaptations to different environment

In hot temperate regions (deserts) - cactus:

  • reduced area-to-volume ratio - less area for transpiration (almost spherical in shape & no broad leaves)
  • photosynthesis in stem instead of leaves
  • water reservoir in stem
  • extensive rooting system

In water-logged environment (ponds) - water lily:

  • leaves lies on top of water (to allow transpiration & to get plenty of sunlight)
  • stomata on upper leaves surface (instead of lower surface) - to obtain CO2 from air
  • air spaces in roots - absorbing oxygen

Translocation

Introduction

Action of photosynthesized products (e.g. glucose) being transported to the other parts of the plant

Using phloem tubes (next to xylem tubes)

Components involved in translocation:

  • photosynthesized products: glucose
  • complex sugar: sucrose (changed from glucose in leaves)
  • amino acids

What happens to sucrose after after arriving at site:

In roots:

  • sucrose ➡ glucose ➡ used in respiration
    OR
  • sucrose ➡ starch ➡ stored

In flowers:

  • sucrose ➡ sweet nectar (to attract pollinators)

In fruits:

  • sucrose ➡ sweet flesh of fruit (to attract pollinators)

In growing part of plants:

  • sucrose ➡ polysaccharide cellulose (cell walls of new cells)

Source and sinks

Source: site of production of glucose or sucrose

  • e.g.: leaves, tuber

Sinks: site of product being used

  • e.g.: fruits, growing shoots

Translocation of chemicals

Contact pesticides

  • touch pests to kill
  • easy to be missed - pests hide underside leaves or between crevices

Systemic pesticides

  • plants absorb the pesticide and distribute the pesticide evenly throughout the plant
  • pest feeds on the plant will ingest pesticides too