Transport in plants

• Why do plants need transport systems?

  • They are multicellular with a low SA:V
  • Diffusion is too slow to meet their metabolic needs
  • Substances must be moved along long distances
• What does the xylem do?

  • Transports minerals and ions around plants.
  • It provides structural support
  • Mostly made up of xylem vessels
• What are adaptations of xylem tissue?

  • They are elongated with hollow tubes without end walls
  • They lack organelles
  • Their walls are thickened with lignin for support
  • They have non-lignified pits that allow movement of water and ions in and out of vessels
• What does the phloem do?

  • Transports sugars and amino acids (assimilates) around plants
  • Its mostly made up of sieve tube elements and companion cells
• What are adaptations of sieve tube elements?

  • They are connected end-to-end to form sieve tubes
  • They have sieve plates with pores at their ends to allow flow of sugars and amino acids
  • They lack nuclei and most organelles
  • They have only a thin layer of cytoplasm
• What are adaptations of companion cells?

  • They are connected to sieve tube elements through plasmodesmata
  • The cytoplasm contains a large nucleus and many mitochondria to release energy for active transport of substances through the sieve tube elements and many ribosomes for protein synthesis
• What are the phloem and xylem tissues arranged into?

  • Theyre arranged into vascular bundles
  • Which are the arranged to stem, roots and leaves

  A) Phloem

  B) xylem

  C) xylem

  D) phloem

  E) ambium
• What is ligin?

  • A woody substance that provides stregnth and rigidity
  • Allows the vessels to withstand negative pressure and tension generated during the upward movement of water
  • Ensuring they dont collapse
• How does water move through plants?

  • Water enters a plants root hair cell via osmosis
  • It moves through a cells cytoplasm or cell walls towards the xylem
  • The xylem transports water from the roots up to the leaves
  • Water is used for photosynthesis
  • Some water evaporates from the leaf cells by transpiration and diffuses out of the plant
• What happens in the apoplast pathway?

  • Water moves through spaces in cell walls and between cells
  • This occurs due to the cohesive and adhesive properties of water
• What happens in the symplast pathway?

  • Water moves from cell to cell through the cytoplasm and plasmodesmata
  • This occurs due to water potential gradients
• Whats the casparian strip?

  • Way how apoplast pathway is blocked at root endodermis by the casparian strip
  • Band of waterproof substance called suberin that surrounds the endodermis cells
  • It forces water out of the apoplast pathway into the symplast pathway
• How does water leave the plant?

  • Water then evaporates from cell walls in leaf into air spaces so it can exit the plant walls through its stomata
• What is transpiration?

  Transpiration is the evaporation of water aerial parts of plants, especially leaves
• Why does transpiration occur?

  • Water evaporates from moist surfaces of the mesophyll cells
  • Stomata open so they can absorb carbon dioxide for photosynthesis
  • This provides a pathway for water vapour loss through the open stomata
  • Water vapour moves down a water potential gradient from the air spaces in the leaf into the atomsphere
• What are the 4 main factors affecting transpiration?

  • Light intensity
  • Temperature
  • Humidity
  • Wind speed
• How does light intesnity effect transpiration?

  • At high intensities stomata opens for maximum CO2 absorption for photosynthesis
  • Increasing the transpiration rate
• How does temperature effect transpiration?

  • At high temp evaporation of water molecules is faster due to higher kinetic energy
  • Increasing the transpiration rate
• What does humidity effect transpiration?

  • Low humidity increases the water vapour gradient between the leaf and atmosphere
  • Increasing the transpiration rate
• How does wind speed effect transpiration?

  • High wind speed increases the water vapour gradient between the leaf and atmosphere
  • Increasing the transpiration rate
• What are the stages of using a potometer?

  1. Cut the shoot underwater at slant to increase the surface area for water uptake.
  2. Assemble the potometer with the shoot submerged in water.
  3. Keep the capillary tube end of the potometer submerged throughout the experiment.
  4. Check that the apparatus is airtight.
  5. Dry the leaves, and give the shoot time to acclimatise.
  6. Shut the tap, form an air bubble and record its position.
  7. Measure the distance the air bubble moves and the time taken.
  8. Change one variable at a time and keep everything else constant.
• What are the adaptations of leaf structures for gas exchange?

  • Air spaces - provide a network for gases to quickly diffuse in and out of the leaf and acssess photosynthesizing cells
  • Mesophyll cells - These are dispersed throughout the leaf, providing a large SA across which gases can diffuse
  • Stomata - These open when conditions are suitable for photosynthesis allowing inward diffusion of CO2 and outward diffusion of oxygen and close to minimise water loss
• How can plants limit water loss?

  • They have a waxy cuticle on their leaves
  • They have guard cells that can close the stomata when needed
• What are Xerophytes?

  Plants adapted to live in dry environments with limited water availability
• Adaptations of xerophytes to reduce water loss?

  • Thick waxy cuticle - reduces water loss through evaporation
  • Rolling or folding of leaves
  • Hairs on leaves - traps moist air against leave surface to reduces diffusion of water vapour
  • Sunken stomata in pits - reduces air flow and evaporation of water
  • Water storage organs - conserve water for when it is low in supply
• Whats the mass flow hypothesis?

  • Proposes that translocation occurs due to pressure gradients
• Whats the stages of mass flow hypothesis?

  1. At the source, solutes like sucrose are actively loaded into sieve tube elements from companion cells.
  2. This decreases the water potential in sieve tube elements.
  3. Water enters the sieve tube elements from the xylem and companion cells by osmosis.
  4. This increases hydrostatic pressure in the sieve tube elements at the source.
  5. At the sink, solutes are actively removed from the sieve tube elements.
  6. This increases the water potential in sieve tube elements at the sink.
  7. Water leaves the phloem by osmosis, decreasing the hydrostatic pressure at the sink.
  8. This creates a pressure gradient, pushing solutes from the source to areas of lower pressure at the sink.
• What are the stages of active loading?

  • H+ ions are actively transported out of companion cells into surrounding source cells
  • H+ is co transported along its concentration gradient back into companion cells with sucrose
  • Sucrose can then diffuse along its concentration gradient through plasmodesmata from companion cells to sieve tubes elements
• What is translocation?

  Mass of assimilates from one part of a plant, the source, to another part of the same plant, the sink
• What are assmiliates?

  • Substances that have been manufactured or modified in the plant such as sucrose or amino acids, that are transported in the phloem sieve tube during translocation
• What are features of translocation?

  • It requires energy
  • It transports from sources to sink
  • Water provides the medium in which these substances dissolve for transport in the phloem
  • It maintains the concentration gradient using enzymes
• What are the main sources of assimilates in plants?

  • green leaves and green stems
  • storage organs such as tubers and tap roots that are unloading their at the beginning of growth period
  • food stores in seeds when they germinates
• What do the main sinks in plants include?

  • roots that are growing and/ or actively absorbing mineral ions
  • meristems that are actively dividing
  • any parts of the plant that are laying down food stores, such as developing seeds, fruit or storage organs