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Why do plants need a transport system (3 points)?
1. Metabolic demands - every cell needs oxygen and glucose for respiration, and mineral ions for cell structure 2. Size - very varied sizes 3. SA:V - diffusion works well in the leaves, but not enough to supply the whole plant
Why does simple diffusion not work for plants?
The large transport distance means that diffusion wouldn't be fast enough to meet the metabolic requirements of cells
Give 3 ways plants adapt to increase their SA:V ratio
Roots have root hair cells, leaves are flat and thin and plats have a branching body shape
What are the two key types of transport tissue?
Xylem and phloem
What other types of transport tissue may be seen in vascular bundles (depending on their location)?
Parenchyma, collenchyma, sclerenchyma and cambium
What is the role of parenchyma?
Packing tissue that fills spaces between other tissues. In roots, these cells may store starch.
What is the role of collenchyma?
They strengthen the vascular bundles and outer parts of stems, but allow flexibility. They have thick cellulose walls
What is the role of sclerenchyma?
They support tissue made of dead cells with heavily thickened cell walls containing lignin. These cells strengthen stems and leaf midribs, but are not always present (found in woody plants).
What is the role of the cambium?
Meristem tissue. It divides by mitosis to produce xylem and phloem
What does vascular tissue look like in the root?
There is a central core of xylem in an X shape, and the phloem is found between the xylem. The vascular bundle is surrounded by endodermis, with a layer of meristem tissue inside called the pericycle
How does the shape of the vascular tissue in the leaf help to support the plant?
It provides strength in the core to resist forces of pulling or uprooting
What does vascular tissue look like in the stem?
The vascular bundles are in a ring towards the edge of the stem, with the xylem inside of the phloem and cambium tissue in between.
How does the shape of the vascular tissue in the stem help to support the plant?
It provides flexibility for blowing the the wind, but strength to maintain structure
What does vascular tissue look like in the leaf?
Vascular bundles form the midrib and veins in the leaf, with the xylem towards the upper surface
What is the midrib?
The central vein in a leaf
How do the veins change in dicotyledonous plants compared to monocotyledonous plants?
In dicots the veins branch and are smaller towards the edge of the leaf. In monocots the veins are parallel.
When asked for a low power drawing (plan diagram), what should you do?
Don't draw individual cells, draw all tissues completely enclosed by lines and draw a correct interpretation of the distribution of tissues
When asked for a high power drawing, what should you do?
Draw only a few representative cells and draw the cell wall of all plant cells
What is the structure of the xylem?
It consists of lignified xylem vessels and pits, joined end to end to form tubes. There The cells are dead and there are no organelles.
What four cells make up the xylem?
Tracheids, vessel elements, xylem parenchyma and sclerenchyma cells
What are tracheids?
Long narrow tapered cells with no perforations in the cell walls. They are lignified.
How does the pattern of lignin change in a plant?
As the plant gets older, the patterns of lignin change - they can be spiral, annular (rings) or reticulate (broken rings)
What is the function of lignin in xylem vessels?
To prevent collapse
What is the function of pits in xylem vessels?
They are non-lignified and allow a small amount of lateral movement of water between xylem vessels
What is the structure of the phloem?
It consists of sieve tube elements and companion cells. The sieve tube elements contain cross walls called sieve plates, and are perforated by pores. The companion cells are linked to the sieve tube elements through plasmodesmata
Do sieve tube elements contain a nucleus, vacuole or ribosomes?
No, to maximise the the space for translocation
Do companion cells elements contain a nucleus and other organelles?
Yes, to provide metabolic support to sieve tube elements, and help with the loading and unloading of assimilates
When is a cells plasmolysed?
In a hypertonic solution - the water potential is lower outside the cell
When is a cell turgid?
In a hypotonic solution - the water potential is higher outside the cell
Why does water not continue to move in when a cell is turgid?
Pressure on the cell wall creates a pressure potential which prevents further entry of water
Name the 3 main pathways for water through a plant
Apoplast pathway, symplast pathway and vacuolar pathway
How does the apoplast pathway work?
Water moves by mass flow between cells through spaces in the cell walls. It is the path of least resistance
How does the symplast pathway work?
Water passes through the plasma membrane and into the cytoplasm, then uses the plasmodesmata to move between cells
How does the vacuolar pathway work?
It is similar to the symplast pathway but also involves entering the vacuole
What is transpiration?
The loss of water vapour from the upper plants of a plant
Where is most water lost during transpiration?
Through the stomata
Why does transpiration happen?
It transports mineral ions from the root to leaf, maintains cell turgidity throughout the plant and supplies the water needed for photosynthesis, growth and cell elongation
Name 5 factors affecting the rate of transpiration
Light intensity, temperature, humidity, wind and water availability
How does light intensity affect the rate of photosynthesis?
Stomata close in the dark, but once the stomata open any increase in light intensity has no effect on the rate of transpiration
How does temperature affect the rate of photosynthesis?
Increase in temp = increase in KE of molecules, therefore increase the rate of transpiration since diffusion occurs at a faster rate. If the temp is too hot the stomata close to prevent excess water, reducing the rate of transpiration
How does humidity affect the rate of transpiration?
High humidity reduces the concentration gradient between the leaf and the surrounding air, reducing transpiration rates. At a certain level of humidity equilibrium is reached so there is no net movement
How does wind affect transpiration rates?
Air currents can sweep away molecules from the leaf's surface, maintaining the concentration gradient and increasing the rate of transpiration
How can we measure transpiration?
A potometer. You can measure the movement of an air bubble
Explain how transpiration works
Water enters via the root hair cells, and moves through the root cortex to the endodermis by the symplast and vacuolar pathway (and a bit of the apoplast pathway). Movement up the stem is uni-directional and is driven by three processes: root pressure, transpirational pull and capillary action. Water then leaves the leaf mostly through the stomata, as it evaporates from cells, lowering the water potential and causing water to move directly into the cells by osmosis.
Why is water blocked by moving via the apoplast pathway in transpiration?
A layer of suberin in the casparian strip surrounding the endodermis blocks the water moving back into the cortex
What is the name given to a plant that is adapted to live with little water?
Xerophyte
Give 3 ways plants may reduce the rate of transpiration
Stomata in pits - moist air is trapped close to the air of water loss reducing the diffusion gradient
Thick waxy cuticle reduces water loss
Rolled leaves trap transpiration
Name an example of a xerophyte and list 3 adaptations
Marram grass - rolled leaves, thick waxy cuticle and stomata in pits covered by hairs
What is a hydrophyte and give an example
Hydrophytes are adapted to live in water. Their leaves contain air spaces for buoyancy and they have stomata on the upper surface. An example is the water lily
What is a hydathode?
A type of pore in the epidermis that releases water and sap.
What is translocation?
The process of moving assimilates from source to sink
What are assimilates?
Substances that will become incorporated into biological tissue
Give an example of a source and a sink for translocation
The source could be green leaves where photosynthesis has produced glucose. The sink could be growing roots
Why are carbohydrates transported as sucrose in plants?
It allows for efficient energy transfer and increased energy storage as sucrose is a disaccharide. It is also less reactive than glucose so no intermediate reactions can occur when it is being transported
Explain how sucrose is loaded
Through the apoplastic pathway, modified companion cells pump hydrogen ions via a proton pump into their cell walls. This is an active process so requires ATP. The hydrogen ion gradient between the surrounding cells and the companion cell causes the hydrogen ions to move back into the companion cells down the concentration gradient through a co-transporter protein, along with a sucrose molecule against the concentration gradient. The sucrose molecules then move into the sieve tubes via the plasmodesmata
Explain the mass flow hypothesis
The movement of sucrose into the sieve tube element reduces the water potential, causing water to move into the phloem by osmosis. This increases the hydrostatic pressure. There is a pressure gradient between the high hydrostatic pressure near the source cell and lower hydrostatic pressure near the sink cells. The solutes then move down the pressure gradient towards the sink. The solutes are unloaded at the sink, increasing the water potential and causing water to move out of the phloem.
How is sucrose unloaded?
The unloading of assimilates occurs at the sinks. The sucrose is actively transported out of the companion cell and then moves out of the phloem tissue via apoplastic pathways. To maintain a concentration gradient in the sink tissues, sucrose is converted into other molecules using enzymes