Topic 4.4 Transport in Plants Flashcards
Xylem (GCSE)
Xylem transports water and minerals from the root to the leaves (dead cells).
Phloem (GCSE)
Phloem transports sucrose and amino acids up and down the plant (source to sink).
Cambium
-Unspecialised cells
-Divide by mitosis
-Differentiate into specialised cells
-Lead to formation of xylem and phloem
Structure and functions of xylem
-Movement of water
-Support
-Contain pits, allowing water to move sideways between vessels
-Thickened with a tough substance, providing structural support
Development of xylem: Start
-It starts off as living tissue
-The first xylem to for is protaoxylem
-Capable of stretching and growing as walls aren’t fully lignified
Development of xylem: Strengthening
-Stem strengthens, cellulose fibrils are laid more or less vertically
-Stem can withstand the weight of the plant
-As the stem ages, cells become impermeable and die
-Tissue becomes stronger and more supportive
-Lignin amounts increase (metaxylem - lignified tissue)
Development of xylem: Ageing
-As the stem ages, cells become impermeable and die
-Tissue becomes stronger and more supportive
-Lignin amounts increase (metaxylem - lignified tissue)
-End walls between cells break down so xylem forms hollow tubes running from the roots to the tip of the stems and leaves
Evidence for xylem: Eosin dye
-A cut end of a shoot is placed in eosin dye
-Eosin dye is a transported with water in the xylem
-When sections of tissue are examined under a light microscope, eosin is clearly seen in the xylem only
Evidence for xylem: Ringing experiments
-Ringing experiments involve killing a ring with a steam jet.
-This destroys living phloem cells but not the xylem cells.
-Eosin dye placed in the water shows that the upward movement of water through the plant is unaffected.
Evidence for xylem: autoradiography
-If the plant is provided with water containing radioactive isotopes, these can be traced by autoradiography as they move through the plant
-The movement of minerals in the xylem can be followed in the same way
Structure of the phloem
-Mature phloem is living tissue
-Transports food in the form of organic solutes and around the plant from the leaves where they are made by photosynthesis to where they are needed
-Can be transported both up and down the stems
-Sieve tube elements transports sugars around the plant
-Companion cells deigned for active transport of sugars into tubes
-Cytoplasms linked by plasmodesmata, allowing flow of substances between cells.
Transpiration
Movement of water and minerals up the xylem
Roots
-Water enters root hair cells by osmosis from the soil
-Very large SA:Vol ratio
-Water moves from root hair cell through more root cells to the xylem by: symplast pathway, apoplast pathway
Symplast pathway
-Water moves by diffusion down the concentration gradient
-From root hair cells to the xylem
-Through the interconnected cytoplasm (symplast) of the cells in the root system
-It moves through gaps in the cellulose cell walls that allow strands of cytoplasms to pass through them
-The contents of the two cells are in contact
Apoplast pathway
-Water is pulled across adjacent cell walls (apoplast) from the root hair cell to the xylem
-Up to half of the volume of cell wall can be filled with water
-Water entering the root hair from the soil has mineral ions dissolved in it and they are drawn up the apoplast pathway too
-The water moves across the cells of the roots in the cell walls until it reaches the epidermis which contains a waterproof layer called the casparian strip
Mass transport in xylem
- When he humidity of the atmosphere is less than that of the air spaces next to the stomata, the water vapour molecules diffuse out of the air spaces into the surrounding air.
- Water lost is replaced by water evaporating from the cell walls of the surrounding mesophyll cells.
- This is replaced by water reaching the mesophyll cells from the xylem.
- A column of water is pulled up the xylem because water molecules form hydrogen bonds between one another and hence tend to stick together.
Translocation in plants
-The substances transported in the phloem are called assimilates.
-Sucrose from sources is loaded into the sieve tube elements and transported to the sinks.
(Scientists are still not entirely clear about the process of translocation; research continues).
Describe how the structure of the xylem vessels allows them to transport water
-Hallow/no cytoplasm
-Vessels have no end walls/are open at ends
-Vessels have pits/are strong so don’t collapse
-Lignin makes the walls waterproof
How does lignin add strength to xylem tissue?
-Lignin holds the microfibrils
-Lignin keeps microfibrils parallel
The cohesion-tension theory
Water molecules form hydrogen bonds with each other, causing them to ‘stick’ together (cohesion). The surfaces tension of water also creates this sticking effect. Therefore as water os lost through transpiration, more can be drawn up the stem from the roots.
How does root pressure affect water movement?
High mineral content gives the root a low water potential, meaning there is strong osmotic flow into the roots. This creates a weak push effect, moving water from the roots into the stem.
How does temperature affect rate of transpiration?
A higher temperature increases evaporation, increases amount of water vapour before saturation (increases concentration gradient) therefore increasing rate of transpiration.
(Eventualy there will be a limiting factor).
How does light affect rate of transpiration?
A higher light intensity increases rate of photosynthesis, causing more stomata to open for gas exchange, therefore increasing rate of transpiration.
How does humidity affect rate of transpiration?
High humidity means the water content of the air next to the leaf is high. This reduces the concentration gradient, therefore decreasing rate of transpiration.
How does air movement affect rate of transpiration?
Large amounts of air movement blow moistures air away from the leaves, creating a steep concentration gradient. Therefore increases rate of transpiration.
Mass-flow hypothesis of translocation
-Sugar loaded into sieve tubes via active transport
-Lowers water potential, causing water to move in from the xylem
-Hydrostatic pressure causes sugars to move
Evidence for mass-flow hypothesis
-Sap is released when a stem is cut, therefore there must be pressure in the phloem
-There is a higher sucrose concentration in the leaves than the roots
phloem loading
1.sympast pathway - sucrose moves by diffusion down a conc gradient into companion cells through cytoplasm and into seive cells then water moves by osmosis into seive tube causing positive hydro static pressure - passive
2.apoplast pathway - moves by diffusion down a conc gradient through cell wall to companion cell across membranes by active transport producing a high conc in the cytoplasm of companion cells , passes into seive tubes through plasmodesmata. water then moves due to os producing positive hydrostatic presssure
parenchyma
thin walls of cellulose
collenchyma
cell walls with thickened areas of additional cellulose
sclerencyma
have lignified cell walls
2 types - sclereids and fiber (long and thin) cells
support
guttation
when transpiration rates are low and drops of water are forced out of the leaves
sources
site of sugar production
sinks
where sugar is used or stored
aerenchyma
when parenchyma is modified to create tissues with air spaces for buoyancy or aeration of tissues,
How are sugars stored after being transported around the plant?
- Glucose is converted to sucrose for transporting around the plant in the phloem
- Then converted back to glucose or starch for storage
The differences between the structures of xylem and phloem
- Xylem cell walls contain cellulose and lignin; phloem walls only contain cellulose.
- Xylem is hallow/no end walls; phloem ha sieve plates.
- Xylem has pits; phloem doesn’t.
- Xylem is dead; phloem has companion cells.
