Transport in Plants

• epidermis


  covers the outer surfaces of the plant for protection
• palisade mesophyll

  the main site of photosynthesis in the leaf
• spongy mesophyll

  air spaces between the cells allow gases to diffuse through the leaf
• xylem

  transports water and minerals through the plant from the roots to leaves. Also supports the plant
• phloem


  transports dissolved food materials through the plant
• meristem

  found mainly at the tips of the roots and shoots, where it can produce new cells for growth
• plant tissues are gathered together to form organs
• the leaf is a plant organ
• many chloroplasts are close to the top of the leaf to trap maximum sunlight
• lots of air spaces to allow gases to diffuse
• stomata allow the diffusion of gases in and out of the leaf and can be opened and closed by guard cells
• water enters the plant from the soil, through the root hair cells, by osmosis
• root hair, xylem and phloem cells are specialised to transport water, minerals and sugars around the plant. This water contains dissolved minerals
• the water and minerals are transported up the xylem vessels, from the roots to the stems and leaves
• at the leaves, most of the water will evaporate and diffuse out of the stomata
• the loss of water from the leaves is called transpiration
• there are many factors that can affect the rate of transpiration:

  • an increase in temperature will increase the rate, as more energy is transferred to the water to allow it to evaporate
  • faster air flow will increase the rate, as it will cause stomata to open
  • an increase in humidity will decrease the rate as the air contains more water vapour, so the concentration gradient for diffusion is lower
• in the leaf, the role of guard cells is to open and close stomata
• at night the stomata are closed. This is because carbon dioxide is not needed for photosynthesis during the night, so closing the stomata reduces water loss
• when water is plentiful, the guard cells take up water and bend. This causes the stomata to open, so gases for photosynthesis are free to move in and out of the stomata along with water from transpiration
• when water is scarce, losing water makes the stomata change shape and close. This stops the plant from losing more water through transpiration
• Measuring water uptake by a leafy shoot using a potometer

  A) resevoir

  B) cut shoot

  C) beaker of water

  D) capillary tube

  E) volume scale

  F) air bubble

  
• the rate of transpiration from a cut shoot can be estimated by measuring the rate at which the shoot takes up water. This is only an estimate because not all of the water taken up by a shoot is lost - a very small percentage is used in the leaf
• phloem tissue transports dissolved sugars from the leaves to the rest of the plant. This is called translocation
• in most plants, the stomata are mainly found on the bottom of the leaf. This means that the sun does not shine directly on them, reducing water loss.
• plants cannot stop transpiration completely. This is because carbon dioxide is needed for photosynthesis, so water will always escape
• Leaf

  A) wax cuticle

  B) upper epidermis

  C) palisade mesophyll

  D) spongy mesophyll

  E) lower epidermis

  F) guard cells