Biology 3.1.3- Transport in plants

• Why do multicellular plants need transport systems? (4)

  Larger SA:Vol

  Diffusion is too slow

  Cells are differentiated and need to exchange substances

  They have metabolic requirements
• Structure of xylem vessels

  Long arranged end to end

  No end walls and no cell contents (for water flow)

  Strengthened with lignin

  Bordered pits allow lateral flow of water

  
• Structure of Phloem (sieve tube element)

  Many elongated cells, joined vertically

  End walls known as sieve plates with sieve pores

  Cell surface membrane and a thin cell wall made of cellulose

  Thin layer of cytoplasm

  
• Structure of phloem (companion cells)

  attached to seive tube elemnts by plasmodesmata

  Living cells with dense cytoplasm, containing cell components

  Lots of mitochondria to make ATP
• Function of Phloem

  Transport organic solutes, made from photosynthesis, (e.g. Sucrose) up and down a plant
• Function of Xylem

  Transports water and minerals

  Mechanical strength
• Function of spongy mesophyll

  Large air space for circulation of carbon dioxide for photosynthesis
• Function of lower epidermis

  Contains pores (stomata) for gas exchange
• Function of palisade mesophyll

  Parenchyma cells containing chloroplasts for photosynthesis
• What is transpiration

  The process where by water vapour evaporates and is lost from the Arial parts of the plant (leaf)

  Water vapour evaporates from the spongy mesophyll cells and then diffuse out of the stomata
• Describe the two main pathways water takes to get from the root to the xylem (3rd less common path)

  Apoplastic pathway- Through cell wall and intercellular space

  Symplastic pathway- Through cytoplasm and plasmodesmata

  (Vacuolar pathway- Jumps between vacuoles, rarely)
• How does leaf structure prevent water loss (5)

  - Stomata on lower side

  - Guard cells cause stomata to close

  - Fewer stomata

  - Waxy cuticle

  - Spongy mesophyll cells are close together
• How have leaves adapted to prevent water loss (5)

  - Leaves fall off

  - Leaves could have trichomes (hairs) to trap humid air

  - Leaves wilt

  - Lower surface area of leaf

  - Rolled leaf
• How have xerophytes adapted to low water levels (6)

  Succulent- store fluid

  Cells can withstand low water potential- very concentrated

  CAM photosynthesis- requires less water

  Spines- very few leaves

  Low SA:Vol

  Tap root and dew root
• How have hydrophyts adapted

  Stomata on top

  Little cuticle

  Air sacs- specialised cells, aerenchyma

  Very little lignin

  Root prevents floating away
• Describe the formation of sucrose and how it gets to the phloem

  1. Photosynthesis in palisade mesophyll cells forms trios sugars

  2. Some sugars convert into hexose sugars (glucose fructose)

  3. Sucrose forms through condensation of glucose and fructose

  4. Sucrose moves to phloem by apoplastic or symplastic pathway

  
• Describe the process of active loading

  1. Proton pumps, in companion cells, use ATP to pump H+ (protons) out of the cytoplasm

  2. H+ concentration outside the cell builds up, creating high H+ concentration

  3. H+ move back in down the concentration gradient through co-transporter protein also bringing sucrose against its concentration gradient (no ATP needed)

  4. Sucrose moves from companion cell to sieve tube via diffusion, through plasmodesmata

  
• What is translocation

  An energy-requiring process transporting assimilates in the phloem, between sources and sinks
• What are assimilates

  Products of photosynthesis that are transported
• What are the main sources of assimilates in a plant

  Green leaves and green stem

  Storage organs that are unloading their stores at the beginning of a growth period, e.g. tubers and tap roots

  Food stores in germinating seeds
• What are the main sinks in a plant

  Roots that are growing and/or actively absorbing mineral ions

  Actively dividing meristems

  Parts of the plant that are laying down food stores, such as developing seeds, fruits or storage organs
• What is the evidence of transpiration

  Adaptations of companion cells, seen by microscopy

  If mitochondria in the companion cells are poisoned, translocation stops

  Flow of sugars in the phloem is about 10,000 time faster than it would be by diffusion alone

  Aphid studies show there is a positive pressure in the phloem that forces the sap through the stylet
• Describe the process of mass flow

  Mass flow is due to the active loading of sucrose in the leaves causing the influx of water by osmosis

  This creates a hydrostatic pressure gradient

  Sucrose flows in solution from a high hydrostatic pressure (source) to a lower hydrostatic pressure (sinks)

  Sucrose and water are removed in the sinks