Need for Transport Systems
- There are three main reasons why multicellular plants need transport systems in plants; metabolic demands, size and surface area: volume ratio
- Metabolic Demands: There are many internal and underground parts of plants that don’t photosynthesise so cannot make their own oxygen or glucose so oxygen and glucose need to be transported to these parts
- Hormones also need to be transported from one place to another and mineral ions need transporting to every cell.
- Size: Plants are too big to have their demands supplied by diffusion alone. So, they need an effective transport system to move substances to every part of the plant.
- Surface area to volume ratio: The surface area: volume ratio is relatively small. they can’t rely on diffusion alone to supply their cells with substances they need.
Structure and Functions of Xylem
- The xylem is a large, non-living tissue. It’s a hollow structure made by several columns of cells fusing together.
- In the transport system of plants, xylems transport water and mineral ions
- Xylems have lignified secondary walls for extra mechanical strength.
- In herbaceous dicots, the xylem is thick walled as it has parenchyma packed around the xylem vessels. It stores food and contains tannin deposits
Structure and Function of Phloem
- The phloem is a living tissue and transports food in the form of organic solutes
- In the transport system of plants, pholem supplies the cells with sugar and amino acids needed for cellular respiration
- The main vessels of the phloem are the sieve tube elements, which are cells joined to make a hollow structure
- The walls of the sieve tube elements are perforated to form sieve plates. Companion cells form with sieve tube elements.
Movement of water across root
- Symplast Pathway: The symplast pathway is the movement of water and solutes through the cytoplasm of living cells via the plasmodesmata by diffusion
- Apoplast pathway: The apoplast pathway is the movement of substances through the cell walls and cell spaces by diffusion and into the cytoplasm by active transport.
- Transpiration is the loss of water vapour from leaves and stems in plants.
- Transpiration stream is how water moves by osmosis, across membranes and by diffusion in the apoplast pathway from the xylem, through the cells of the leaf.
- It then evaporates through the permeable cellulose cell walls of mesophyll cells into the air spaces.
- Water vapour then moves into external air through the stomata along a diffusion gradient.
- The loss of water through the mesophyll cells lowers the water potential, so water moves in by osmosis in both apoplastic and symplastic pathways.
- This is repeated across the leaf to the xylem, as water moves out of the xylem by osmosis into the cells of the leaf.
This is the end of the topic
Drafted by Harris (Biology)