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PLANT TRANSPORT - Coggle Diagram
PLANT TRANSPORT
TRANSLOCATION
Translocation - the products of photosynthesis transported away from the source to areas of the plant where they are used for growth or storage - the sink (movement sucrose and amino acids around the plant via the phloem)
- This is the mass flow theory and was originally thought to be passive
- Movement of organic substances in the phloem is now thought to be an active process
Mass flow theory:
- Water moves into a area of high sugar concentration(source) via osmosis down a water potential gradient. This causes hydrostatic pressure
- Water and sugar solution move from an area of high hydrostatic pressure (source) to an area of low hydrostatic pressure (sink)
Evidence against mass flow:
- No function for sieve plates
- No explanation why companion cells are along the length of the phloem
- Doesn't explain why sucrose and amino acids can move at different rates and in different directions at the same time in the same tissue
- Doesn't why the phloem has a high rate of oxygen consumption/translocation is slower/stopped by potassium cyanide
- Campion cells contain many mitochondria. Mass flow doesn't suggest a role for the companion cells
New Theories:
- Translocation is active - loading of sucrose into the phloem. H+ ions pumped out, re enter by diffusion via co - transporter which allows entry of sucrose
- Cytoplasmic streaming could be responsible for bi - directional movement in individual sieve tubes
- Protein filaments pass through sieve pores and suggest that different solutes are transported along different filaments
Translocation of sugars:
- Hydrogen ions are actively transported out of the companion cells
- Hydrogen ions return companion cell with sucrose down the diffusion gradient (facilitated diffusion). This diffusion occurs through co - transporter proteins
- Sucrose diffuses into sieve tube elements through plasmodesmata
- Water potential inside the sieve tube decreases; water moves into the sieve tube elements via osmosis
- Hydrostatic pressure in the sieve tube at the source increases. Sugary fluid moves down sieve tube element from higher hydrostatic pressure to lower hydrostatic pressure (source to sink)
- Sucrose molecules moves sieve tube into the surrounding cells by facilitated diffusion or active transport. Sucrose enters root cells (sink) to be used in respiration or to be converted into starch for storage
- Water out of the sieve tube by osmosis. Hydrostatic pressure at the sink drops
Translocation experiments:
- Ringing experiment - A ring of bark is scraped away, also removing the phloem. After a while sugar is trying to be transported down the stem but it is stopped by the ring. A bulge of sugar forms above the ring. This suggests that sugar moves down the stem in the phloem
- Aphids - Aphids have a specialised mouthpart called a stylet, which they use to penetrate phloem tubes. If the aphids are anaesthetised with carbon dioxide and the stylet is cut off, so it remains in the phloem, pure phloem sap can be collected through the stylet for analysis
- Use of radioactive tracers:
- APHIDS - Radioactive labelled CO2 is placed into a bag surrounding an illuminated individual leaf. The CO2 is incorporated into sugars and transported in the phloem. Aphids feeding on the sugar in the phloem can be used to trace the movement of the sugar in the plant from source to sink by doing the aphid practical.
- AUTORADIOGRAPHY - Radioactive labelled CO2 is placed into a bag surrounding an illuminated individual leaf. The source and sink leaves are placed firmly on photographic film in the dark for 24hrs. When the film is developed, the presence of radioactivity shows up as 'fogging' of the negatives. Result: sugar transports both up and down the stem.
TRANSPIRATION
Uptake of water:
- Plants lose water during gas exchange which must be replaced by absorption from the soil through root hairs
- Soil is a dilute solution so has a high water potential
- In the vacuoles of root hair cells is a concentrated solution of sugars and salts so it has a low water potential
- Water enters the root hair cells by osmosis down a water potential gradient
- The cells of the endodermis have a waterproof layer called the casparian strip
- This is made of a waxy waterproof substance called suberin which blocks the apoplast pathway and water and dissolved mineral ions must enter the cytoplasm of the endodermal cells and follow the symplast pathway before entering the xylem vessels
Transpiration - Loss of water vapour from leaves giving rise to the transpiration stream
- Water is lost through the stomata and moves down the water potential gradient
- The continuous removal of water molecules from the top of xylem vessels results in a tension that pulls on the xylem column pulling water up from the roots
Factors affecting transpiration:
- Light intensity - this affects the degree of opening and closure of the stomata. The higher the light intensity, the more photosynthesis takes place, so more gas is needed so the stomata open
- Temperature - Higher temperatures increase the kinetic energy of the water molecules so they diffuse more quickly. Warm air also has more kinetic energy and holds more water
- Humidity - Dry air outside the leaf creates a steeper diffusion gradient between the internal air spaces and the environment, thus increasing transpiration rate. High humidity reduces the rate of transpiration.
- Air movement - this maintains a diffusion gradient, by blowing away humid air which accumulates around the stomata and replacing it with fresh air that is less saturated with water vapour
Measuring the rate of water absorption:
- Measured using a potometer
- Leafy shoot, cut at angle and insert under water, ensure airtight fit with no air bubbles in the xylem
- Air bubble introduced
- Time take for movement along the scale (mm/s)
- Reservoir tap opened to return bubble to start point
- Repeat measures taken and mean calculated (reliability)
- Rate of water absorption is not equal to rate of transpiration because not all the water absorbed is transpired, some is used for photosynthesis etc.
PLANT TRANSPORT
- Plants need to absorb water from the soil through their roots and transport it to the photosynthesising leaves
- They may also need to transport the products of photosynthesis to all calls for use in respiration
- Water and mineral salts are transported via the xylem
- Sugars and amino acids are transported via the phloem
ROOT HAIR CELLS
Visible features:
- Large surface area for water to enter by osmosis (thousands of root hair cells per root)
- Cellulose cell wall - freely permeable to water
Other features:
- Large numbers of mitochondria to provide ATP for active transport of mineral ions
- Large numbers of protein carriers embedded in membrane for active transport of mineral ions
(learn part of cross section of root)
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XYLEM & PHLOEM
Distribution: - Leaves - vascular bundles in midrib, network of veins gives lateral support to leaf - In the root the xylem and the phloem have a central arrangement to anchor the plant - In the stem the xylem and the phloem have a peripheral arrangement to resist bending
XYLEM
- Transports water and dissolved minerals from the roots, up through the plant and eventually out through the leaf stomata. One - way transport
Structure:
- Two different types of tube make up the xylem
- Tracheids - slightly narrower and are made from cells which have perforated end walls. Water flow is more obstructed then in vessels but these cells do provide more strength and support
- Vessels - only found in flowering plants. They form main conducting tubes. They are made from wide cells with reduced or absent end walls. This means they form a continuous tube
- Both contain no living material. They are dead cells - no cytoplasm, easier for water to flow up
- Their walls are strengthened with lignin, the major constituent of wood. These walls have thinner areas known as pits, which allow water to leak through
- Also have fibres and parenchyma
- Fibres - support role only, don't transport water
- Parenchyma - living packing tissue, keeps all xylem elements in place
- Cells are dead due to the deposition of lignin on the cellulose cell walls which makes them impermeable
- Lignin is deposited as rings/spirals and thickens the cell wall
- Lignin provides mechanical strength - prevents the collapse of the xylem, supports the plant and allows adhesion of water molecules
- Pits where no lignin is deposited allows sideways movements between vessels
Transport in the xylem:
- For tension to successfully pull water to the top of the plant one particular rule must be obeyed: the column of water in the xylem must be intact - any breaks in the column or bubbles would break the cohesion between the water molecules and hence prevent upwards movement.
- Bubbles occur frequently in the xylem vessels and any damage to the plant is likely to break the column of water
- However puts between adjacent vessels allow lateral movement of water, maintaining an intact column or allowing the column to bypass bubbles
PHLOEM
- Transports the dissolved products of photosynthesis in various direction around the plant
Structure:
- There are two main cell types in the phloem: sieve tubes and companion cells
- The individual sieve tube elements that make up the phloem are alive, although they have no nucleus, very few organelles and only strands of cytoplasm
- Unlike in xylem vessels, the end cell walls do not disappear, but instead form structures called sieve plates, through which strands of cytoplasm can pass.
- Because of their greatly reduced contents, sieve tube elements cannot keep themselves alive and have to be aided by a companion cell which respire, excrete etc. on the elements behalf. The cytoplasm of the companion cells and their sieve tube elements are joined through plasmodesmata
- Companion cells are found next to each sieve element
They are essentially the life support unit for the sieve element
- They have very dense cytoplasm containing many mitochondria and ribosomes and are very metabolically active
- There are also other cell types:
- Phloem fibres - support role, no transport
- Parenchyma - packing tissue, keeps all the phloem elements in place