Plant Transport

• Why do multicellular plants need transport systems?

  • Metabolic demand - The cells of the green part of the plant make their own glucose and oxygen by photosynthesis, but internal and underground parts are unable to photosynthesise, and need products transported to them.
  • Size - large plants (perennial) require transport systems to transport substances from the roots to the leaves.
  • Surface area to volume ratio - leaves are adapted to have large SA:V for gas exchange, however the stems, trunks, and roots have small SA:V, and therefore cannot rely of diffusion alone.
• What are the substances that require transport?

  Oxygen, glucose, hormones, mineral ions, and waste products for removal.
• What is the role of mineral ions absorbed at the roots?

  Mineral ions are transported to all cells to make the proteins required for enzymes and the structure of the cell.
• What are dicotyledonous plants (dicots)?

  These are plants that make seeds that contain two cotyledons, organs that act as food stores for the developing embryo plant, and form the first leaves when the seed germinates.
• What are the two type of dicots?

  Herbaceous dicots and arborescent (woody) dicots.
• What are herbaceous dicots?

  These are dicots with soft tissues and a short life cycle, as their leaves and stems die at the end of growth season.
• What are arborescent dicots?

  These are dicots with hard, lignified tissues and a long life cycle, as long as hundreds of years.
• What is the vascular system of dicot plants?

  A series of transport vessels running through the stem, roots, and leaves, and consisting of the xylem and phloem.
• How are these transport tissues arranged?

  In vascular bundles in the leaves, stems, and roots.
• Where are these vascular bundles located in the leaves?

  They are located in the centre of the leaf in the form of the midrib (primary vein), that has veins branching from it and through the leaf for transport and support.
• Where are these vascular bundles in the stem?

  They are located near the edge of the stem, with the phloem on the outside and xylem on the inside, for strength and support required to resist compression from wind and weight.
• Where are these vascular bundles in the roots?

  They are located in the centre of the root to help the plant withstand the tugging strains resulting from the stems and leaves blowing in the wind.
• What is the structure of xylem?

  Long, hollow structures formed by several columns of dead cells fusing together end to end, due to their lack of end cell walls; The cells lack organelles and cytoplasm to create space for transport, and walls are lignified.
• What is the function of a xylem?

  Support and transport of water and mineral ions, unidirectionally from the roots to the shoots and leaves.
• Why are the walls of xylem lignified?

  Acts as a secondary wall that provides mechanical strength, with the lignin forming rings, spirals, or tubes.
• What are bordered pits?

  Small unlignified areas in the lignin cell wall lining, that allow water to leave the xylem and move into the required cells.
• What is a tracheid?

  A long, tapered (gradually narrower), lignified, conductive cell in the xylem that aid transport.
• What is a parenchyma pack?

  Comprised of parenchyma cells, they are responsible for storing food, and containing tannin deposits (a bitter chemical that protects plants from attack).
• What is the structure of phloem?

  Living tissue consisting of non lignified sieve tube elements, that are made up of many cells joined end to end to form a long, hollow structure; the walls of the areas between cells are perforated to form sieve plates that allow contents to flow.
• What occurs when large pores appear in the perforated cell walls?


  The tonoplast (vacuole membrane), nucleus, and some organelles break down, causing the phloem tubes to fill with phloem sap, and the mature phloem cells to have no nucleus.
• What are companion cells?

  These are very active cells that are linked to the sieve tube elements by many plasmodesmata, which are microscopic channels through the cellulose cell walls linking the cytoplasm of adjacent cells, and act as the life support system for sieve tube cells that have lost their normal cell function, due to a lack of nucleus.
• What are the supporting tissues of the phloem?

  Fibres and sclereids.
• What are sclereids?

  A reduced form of sclerenchyma cells with thick, lignified cells walls that form small bundles of durable layers of tissue in the phloem.
• Why is water important in plants?

  - Turgor pressure (hydrostatic) as a result of osmosis provides a hydrostatic skeleton to support stems and leaves.

  - Turgor drives cell expansion, and is the force that enables roots to force their way through tough ground.

  - Water loss by evaporation helps keep plants cool.

  - Mineral ions and the products of photosynthesis are transported in aqueous solutions.

  - Water is a raw material of photosynthesis.
• How is water obtained by the plant?

  Root hair cells, specialised epidermal cell found near the growing root tips.
• How are root hair cells well adapted?

  - Their small size allows them to penetrate easily between soil particles.

  - Each hair has a large SA:V ratio and there are thousands on each growing root tip.

  - Each hair has a thin surface layer, allowing for fast diffusion and osmosis.

  - The concentration of solutes in the root hair cell cytoplasm maintains a water potential gradient between soil and cell.
• How does a high solute concentration in the cell allow for osmosis?

  The soil has a low solute concentration and high water potential, while the cytoplasm and vacuolar sap of root hair cells contains a high solute concentration of sugars, mineral ions, and amino acids, resulting in a low water potential, and allowing water to pass into the root hair cells.
• What are the two pathways that water takes in pants?

  The symplast and apoplast pathway.
• What is the symplast pathway?

  This is when water moves via osmosis through the symplast, which is the cytoplasm of the plant cells that is connected through the plasmodesmata. Each root hair cell has a higher water potential than the next, allowing for continuous movement down a water potential gradient.
• What is the apoplast pathway?

  This is when water moves via osmosis through the apoplast, which are cell walls and intercellular spaces, through water filling the spaces between the open network of fibres in the cellulose cell walls. As water molecules move into the xylem, more water molecules are pulled through the apoplast behind them due to waters cohesive nature, and this tension allows for a continuous flow of water with little resistance.
• What happens to the water after moving across the root in these pathways?

  It reaches the endodermis, a layer of cells surrounding the xylem and phloem (vascular tissue) of the roots, which contains a Casparian strip.
• What is a Casparian strip?

  A band of waxy material called suberin that runs around each of the endodermal cells to form a waterproof layer.
• What happens to the water when it reaches the endodermis?

  Water in the apoplast pathway is forced into the cytoplasm of the endodermal cells, joining the water in the symplast pathway. The water potential of the vascular bundle is lower, causing a steep water potential gradient to form, allowing water to move in by osmosis.
• What happens to the water once inside the vascular bundle?

  Water returns to the apoplast pathway to enter the xylem and move up the plant.
• What else do endodermal cells move into the xylem?

  Mineral ions by active transport, further increasing the water potential gradient, and results in root pressure that gives water a push up the xylem.
• What is guttation?

  This is when xylem sap is forced out of special pores at the ends of leaves in certain conditions, and acts as evidence for root pressure.
• What is transpiration?

  During photosynthesis carbon dioxide moves into and oxygen moves out of leaves by diffusion through the stomata, causing water vapour to be lost also.
• Why must some stomata be open at all times?

  In the day the plant requires carbon dioxide, and in the night when photosynthesis cannot occur and oxygen is not produce, the plant requires oxygen for cellular respiration.
• What is the transpiration stream?

  Water enters the root via osmosis, where it is then transported up the plant and to the leaves via the xylem. It then evaporates from the cellulose cell walls of mesophyll cells in the leaves into the air spaces, where it then moves out of the stomata as vapour, which lowers the water potential of the cells and draws water from adjacent cells.
• What is adhesion?

  This is when water molecules form hydrogen bonds with the carbohydrates in the walls of the narrow xylem vessels.