9.2 Transport in Angiospermophytes

9.2.1  Outline how the root system provides a large surface area for mineral ion and water uptake by means of branching and root hairs

  • Plants take up water and essential minerals via their roots and thus need a maximal surface area in order to optimise this uptake
  • The monocotyledon root has a fibrous, highly branching structure which increases surface area for maximal absorption
  • The dicotyledon root has a main tap root which can penetrate deeply into the soil to access deeper reservoirs of water and minerals, as well as lateral branches to maximise surface area
  • The root epidermis may have extensions called root hairs which further increase surface area for mineral and water absorption
  • These root hairs have carrier proteins and ion pumps in their plasma membrance, and many mitochondria within the cytoplasm, to aid active transport

Transport in the Root System

9.2.2  List the ways in which mineral ions in the soil move into the root

Minerals move into the root system via the following pathways:

  • Diffusion:  Movement of minerals along a concentration gradient
  • Mass Flow:  Uptake of mineral ions by means of a hydrostatic pressure gradient
    • Water being taken into roots via osmosis creates a negative hydrostatic pressure in the soil
    • Minerals form hydrogen bonds with water molecules and are dragged to the root, concentrating them for absorption
  • Fungal Hyphae:  Absorb minerals from the soil and exchange with sugars from the plant (mutualism)

9.2.3  Explain the process of mineral ion absorption from the soil into roots by active transport

  • Minerals that need to be taken up from the soil include K+, Na+, Ca2+, NH4+, PO43- and NO3-
  • Fertile soil invariably contains negatively charged clay particles to which positively charged minerals may attach
  • Root cells contain proton pumps that actively pump H+ ions into the surrounding soil, which displaces the positively charged minerals allowing for their absorption (the negatively charged minerals may bind to the H+ ions and be reabsorbed with the proton)

Mineral Ion Absorption

  • This mode of absorption is called indirect active transport - it uses energy (and proton pumps) to establish an electrochemical gradient by which mineral ions may be absorbed via diffusion
  • Alternatively, the root cells may absorb mineral ions via direct active transport - using protein pumps to actively translocate ions against their concentration gradient

9.2.4  State that terrestrial plants support themselves by means of thickened cellulose, cell turgor and lignified xylem

Three ways by which terrestrial plants may support themselves are:

  • Thickened cellulose:  Thickening of the cell wall provides extra structural support
  • Cell turgor:  Increased hydrostatic pressure within the cell exerts pressure on the cell wall, making cells turgid
  • Lignified xylem:  Xylem vessels run the length of the stem and branches, lignification of these vessels provides extra support

9.2.5  Define transpiration

Transpiration is the loss of water vapour from the leaves and stems of plants

9.2.6  Explain how water is carried by the transpiration stream, including the structure of xylem vessels, transpiration pull, cohesion, adhesion and evaporation

  • Some of the light energy absorbed by leaves changes into heat, converting water in the spongy mesophyll into vapour
  • This vapour diffuses out of the stomata and is evaporated, creating a negative pressure gradient in the leaf 
  • New water is drawn from the xylem (mass flow), which is replaced by water from the roots (enters from soil via osmosis)
  • The flow of water through the xylem from the roots to the leaf is called the transpiration stream
  • Water rises through xylem vessels because of two qualities:
    • Cohesion:  Water molecules are weakly attracted to each other via hydrogen bonds
    • Adhesion:  Water molecules form hydrogen bonds with the xylem cell wall
  • These properties create a suction effect (or transpiration pull) in the xylem
  • The xylem has a specialised structure to facilitate transpiration:
    • The inner lining is composed of dead cells that have fused to create a continuous tube
    • These cells lack a cell membrane, allowing water to enter the xylem freely
    • The outer layer is perforated (contains pores), allowing water to move out of the xylem into the leaves
    • The outer cell wall contains annular lignin rings which strengthens the xylem against the tension created by the transpiration stream

9.2.7  State that guard cells can regulate transpiration by opening and closing stomata

  • The transpiration pull is generated by the negative hydrostatic pressure created by the evaporation of water vapor from the leaf
  • Guard cells line stomata and regulate transpiration by controlling how much water vapor can exit the leaf
  • When stomata are open the rate of transpiration will be higher than when they are closed

9.2.8  State that the plant hormone abscisic acid causes the closing of the stomata

  • When a plant begins to wilt from water stress, dehydrated mesophyll cells release the plant hormone abscisic acid (ABA)
  • Abscisic acid triggers the efflux of potassium from guard cells, decreasing the water pressure within these cells and making them flaccid
  • This causes the stomatal pore to close

Opening and Closing of Stomata by Abscisic Acid

9.2.9  Explain how the abiotic factors light, temperature, wind and humidity, affect the rate of transpiration in a typical terrestrial plant


  • Increasing the intensity of light increases the rate of transpiration
  • Light stimulates the opening of stomata (gas exchange required for photosynthesis to occur) 
  • Some of the light energy absorbed by leaves is converted into heat, which increases the rate of water evaporation


  • Increasing the temperature increases the rate of transpiration
  • Higher temperatures cause an increase in water vaporisation in the spongy mesophyll and an increase in evaporation from the surface of the leaf
  • This leads to an increase in the diffusion of water vapour out of the leaf (via the stomata) which increases the rate of transpiration


  • Greater air flow around the surface of the leaf increases the rate of transpiration
  •  Wind removes water vapour (lower concentration of vapour on leaf surface), increasing the rate of diffusion from within the spongy mesophyll


  • Increasing the humidity decreases the rate of transpiration
  • Humidity is water vapour in the air, thus a high humidity means there is a high concentration of water vapour in the air
  • This reduces the rate of diffusion of water vapour from inside the leaf (concentration gradient is smaller resulting in less net flow)

9.2.10  Outline four adaptations of xerophytes that help to reduce transpiration

Xerophytes are plants that can tolerate dry conditions (such as deserts and high altitudes) due to a number of specialised adaptations:

  • Reduced leaves:  Reducing the surface area of the leaf will reduce the area for water loss and thus reduce transpiration
  • Rolled leaves:  Rolling up leaves (lower epidermis inside) reduces exposure of stomata to air and thus reduces transpiration
  • Thick waxy cuticle:  A thickened cuticle prevents water loss from the surface of the leaf and thus reduces transpiration
  • Stomata in pits:  Having stomata in pits, surrounded by hairs, concentrates water vapour near the stomata, reducing the rate of transpiration
  • Low growth:  Plants located near the ground are less exposed to wind and may be shaded, reducing the rate of transpiration
  • C4 / CAM physiology:  Plants with C4 or CAM physiology require less amounts of CO2, meaning stomata can stay closed for longer 

9.2.11  Outline the role of the phloem in active translocation of sugars (sucrose) and amino acids from source (photosynthetic tissue and storage organs) to sink (fruits, seeds, roots)

  • Organic molecules (sugars, amino acids) move from their source (photosynthetic tissue or storage organs) into a tube system called the phloem
  • Sugars are transported as sucrose (because it is soluble but metabolically inert) in the fluid of the phloem (called the sap)
  • They are actively loaded into the phloem by companion cells, creating a high concentration which draws water from the xylem via osmosis
  • The sap volume and pressure consequently increase to create mass flow which drives the sap along the phloem
  • The organic molecules are actively unloaded by companion cells and stored in the sink (fruits, seeds, roots)
  • Sucrose is stored as starch (insoluble), while the water in the phloem is released (now that solute concentration is low) and returned to the xylem

Active Translocation in the Phloem