• Plants have transport systems to move food, water, and minerals around
• These systems use continuous tubes called xylem and phloem
• Xylem vessels carry water and minerals from the roots to the leaves
  • The xylem vessel is specialized to transport water and dissolved minerals from the root up to all the other parts of the plant
  • It also helps in supporting the stem and strengthening it
• Phloem tubes carry sugar and other organic nutrients made by the plant from the leaves to the rest of the plant
  • The function of the phloem tissue is to transport food nutrients, such as glucose and amino acids, from the leaves to all other cells of the plant, this is called translocation

The Position of Xylem and Phloem as Seen in Sections of Roots, Stems, and Leaves, Limited to Non-woody Dicotyledonous Plants

• In the roots, xylem and phloem are in the center to withstand stretching forces
  
• In the stems, they are arranged in bundles near the edge to resist compression and bending
  

Root Hairs and Water Uptake by Plants

Root Hairs

• Plants take in water from the soil, through their root hairs:
  • At the very tip is a root cap. This is a layer of cells which protects the root as it grows through the soil
  • The rest of the root is covered by a layer of cells called the epidermis
  • The root hairs are a little way up from the root tip. Each root hair is a long epidermal cell. Root hairs do not live for very long. As the root grows, they are replaced by new ones
• Functions of root hair cells:
  • Increase the external surface area of the root for absorption of water and mineral ions (the hair increases the surface area of the cell to make it more efficient in absorbing minerals)
  • Provide anchorage for the plant

Water Uptake

• The water tension developed in the vessels by a rapidly transpiring plant is thought to be sufficient to draw water through the root from the soil.
• The water enters the root hair cells and is then passed on to cells in the root cortex
• It enters the xylem vessels to be transported up the stem and into the leaves, arriving at the leaf mesophyll cells

Transpiration in Plants

• In the leaves, water molecules leave the xylem vessels and move from cell to cell
• They move through the spongy mesophyll layer by osmosis along a concentration gradient.
• Water then evaporates into spaces behind the stomata and diffuses through the stomata into the surrounding air
• Transpiration is the evaporation of water at the surfaces of the mesophyll cells, followed by loss of water vapor from plant leaves, through the stomata
• Water in the leaf cells forms a thin layer on their surface.
• The water evaporates into air spaces in the spongy mesophyll.
• This creates a high concentration of water molecules
• They diffuse out of the leaf into the surrounding air, through the stomata

Mechanism of Water Movement through a Plant

• Water molecules are attracted to each other (cohesion)
• A water vapor evaporating from a leaf creates a kind of suction; pressure of water at the top of the vessels is lower than that of the bottom
• Water moves up the stem in the xylem; more water is drawn into the leaf from the xylem
• This creates a transpiration stream, pulling water up from the root

Factors that Affect the Rate of Transpiration

• Transpiration is the evaporation of water from the leaves, so any change that increases or reduces evaporation will have the same effect on transpiration

• Light Intensity
  • In daylight, the stomata of the leaves are open and this allows the water vapor in the leaves to diffuse out into the atmosphere
  • At night, when the stomata closes, transpiration is greatly reduced
  • Transpiration speeds up when light intensity increases
• Humidity
  • If the air is very humid, i.e. contains a great deal of water vapor, it cannot accept more from the plants and so transpiration slows down
  • In dry air, the diffusion of water vapor from the leaf to the atmosphere will be rapid
• Air Movement
  • In still air, the region round a transpiring leaf will becomes saturated with water vapor, so that no more can escape from the leaf. In these conditions, transpiration would slow down.
  • In moving air, the water vapor will be swept away from the leaf as fast as it diffuses out. This will speed up transpiration
• Temperature
  • Warm air can hold more water vapor than cold air
  • Thus evaporation or transpiration will take place more rapidly in warm air

Translocation

• Translocation is the movement of organic food, such as sucrose and amino acids, in the phloem; from regions of production to regions of storage OR regions of utilization in respiration or growth
• Sucrose and amino acids are transported to every tissue of the plant, each cell uses it in a different way:
  • Root cells convert sucrose into glucose for respiration and store it
  • Growing cells make cellulose for cell walls from sucrose and use the amino acids to make proteins for growth
  • Fruits use the sucrose to make the attractive scent and tasty nectar to attract insect
• The areas of the plant where sucrose is made are called sources, and where they are delivered to and made use of is called sinks