Transport in Plants Flashcards
Why do all multicellular plants need a transport system?
Every cell in a multicellular plant needs a regular supply of water and nutrients. In a large multicellular plant, the epithelial cells, which are close to the supply, could gain all they need by simple diffusion. But there are many cells inside the plant that are further from the supply. These cells would not receive enough water or nutrients to survive.
-The problem in plants is that the roots can obtain water fairly easily, but they cannot absorb sugars from the soil. The leaves can produce sugars, but cannot obtain water from the air.
Vascular Tissues: What are vascular tissues?
It is the transport system in plants. Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally.
Vascular Tissues: What are two types of vascular tissues?
- Xylem
- Phloem
Vascular Tissues: What are vascular bundles?
The vascular tissue is distributed throughout the plant but the xylem and phloem are found together in vascular bundles. These bundles often also contain other types of tissue that give the bundle some strength and help support the plant.
Vascular Tissues: What is the arrangement of the vascular bundle in a young root?
The vascular bundle is found at the centre of a young root, which is good as they are well protected and hard to damage. There is a large central core of xylem, often in the shape of an X. The phloem is found in between the arms of the X-shaped xylem. This arrangement provides strength to withstand the pulling forces to which roots are exposed.
-Around the vascular bundle there is a special sheath of cells called the endodermis, which has a key role in getting water into the xylem vessels. Just inside the endodermis is a layer of meristem cells (that remain able to divide) called the pericycle.
Vascular Tissues: What is the arrangement of the vascular bundle in the stem?
The vascular bundles are found near the outer edges of the stem.
- In non-woody plants the bundles are separate and discrete.
- In woody plants the bundles are separate in young stems but continuous in older stems. This means there is a complete ring of vascular tissue just under the bark of a tree. This arrangement provides strength and flexibility to withstand the bending forces to which stems and branches are exposed.
Vascular Tissues: How is the ring of vascular tissue arranged in the stem?
The vascular tissue is arranged in a ring shape around the periphery. The xylem is found towards the inside of each vascular bundle, this supports the plant when full of water. The phloem is found towards the outside of the bundle. In between the xylem and phloem is a layer of cambium. This is a layer of meristem cells that divide to produce new xylem and phloem.
Vascular Tissues: Why are the vascular tissues so important for the plant?
Without the vascular tissues, the plant will not be able to survive as it will not be possible for them to transport water or sugars.
-If a ring of bark is stripped from a tree, it is likely it will damage the phloem, on the outside of the ring of vascular bundles. This means the plant will not longer be able to transport sugars and so will die.
Vascular Tissues: What is the arrangement of vascular bundle in the leaf?
The vascular bundles form the midrib and veins of a leaf. These are two main groups of flowering plants, dicotyledons and monocotyledons. These two groups have different patterns of veins. A dicoltyledon leaf has a branching network of veins that can get smaller as they spread away from the midrib. Within each vein, the xylem can be seen on top of the phloem.
What is the difference between a root and a stem?
Root: -Vascular bundle in central X position -Stomata absent -Endoderm present Stem: -Vascular bundle arranged around periphery. -Stomata present -Endoderm absent
What is an epidermis?
The outermost layer of a root. In primary root it has root hairs (increases surface area to reabsorb more water and mineral ions, e.g. Mg, and K).
What is a cortex?
In a root, it is made up of parenchyma cells and can store starch.
What is an endodermis?
Single layer of cells with a Casparian strip (made of suberin) in cell wall. It is impermeable to water and mineral ions and so when water is going up a root, suberin will direct it to certain channels so it won’t just go anywhere.
Xylem: What is the xylem?
Xylem is used to transport water and minerals from the root up to the leaves and other part of the plant. Xylem tissue consists of tubes to carry the water and dissolved minerals, fibres help support the plant and living parenchyma cells.
Xylem: What are the cell types in the Xylem?
- Tracheids
- Xylem vessel elements
- Parenchyma/Collenchyma
- Fibres (e.g. collagen)
Xylem: What are tracheids?
- Elongated cells with tapered ends
- Have numerous pits through which water can pass freely.
Xylem: What are xylem vessel elements?
- Shorter and fatter than tracheids
- Long cell walls that have been impregnated with lignin.
- As the xylem develops, the lignin waterproofs the walls of the cells. As a result, the cells die and their end walls and contents decay. This leaves a long column of dead cells with no contents - a tube with no end walls.
- The lignin strengthens the vessel walls and prevents the vessel from collapsing. This keeps the vessel open even at time when water may be in short supply.
Xylem: What patterns in the cell wall of xylem vessel elements does lignin form?
These may be spiral, annular (rings) or reticulate (a network of broken rings). This prevents the vessel from being too rigid and allows flexibility if the stem or branch.
Xylem: Why may some parts of the xylem vessel elements be pitted?
In some places, lignification is not complete. It leaves pores in the wall of the vessel, which are called pits or bordered pits. These allow water to leave on vessel and pass into another adjacent vessel, or pass into the living parts of the plant.
Xylem: How is the xylem adapted to its function?
Xylem tissue can carry water and minerals from roots to the very top of the plant because:
- It is made from dead cells aligned end-to-end to form a continuous column.
- The tubes are narrow so the water column does not break easily and capillary action can be effective.
- Pits in the lignified walls allow water to move sideways from one vessel to another.
- Lignin deposited in the walls in spiral, annular or reticulate patterns allows xylem to stretch as the plant grows and enables the stem or branch to bend.
Xylem: Why is the flow of water not impeded?
- There are no end walls
- There are no cell contents
- There is no nucleus or cytoplasm
- Lignin thickening prevents the walls from collapsing.
Phloem: What is the phloem?
The function of the phloem is to transport sugars from one part of the plant to another. This could be up or down the stem.
Phloem: What are the cell types in the Phloem?
- Sieve tube elements
- Companion cells
- Parenchyma/Collenchyma
- Fibres (e.g. collagen)
Phloem: What are sieve tube elements?
The sieve tube elements are not true cells as they contain very little cytoplasm and no nucleus. They are lined up end-to-end to form a tube, in which the plant transports sugars (usually sucrose). The sucrose is dissolved in water to form sap. Unlike xylem vessels, this tube contains cross-walls at intervals. These cross-walls are perforated by many pores to allow the sap to flow. Hence the cross walls are called sieve plates and the tubes are called sieve tubes. The sieve tubes have very thin walls and are usually five- or six-sided.
- Living cytoplasm and nucleus disappears as cells differentiates vacuole and organelles, so membranes disappear.
- Inside is hollow so glucose can be transported.
Phloem: What are companion cells?
In between the sieve tubes are small cells, each with a large nucleus and a dense cytoplasm. They have numerous mitochondria to produce the ATP needed for active processes. The companion cells carry out the metabolic processes needed by the sieve tube elements. This includes using ATP as a source of energy to load sucrose into the sieve tubes. The cytoplasm of the companion cells and the sieve tub elements are linked through many plasmodesmata. These are gaps in the cell walls, allowing communication and flow of minerals between the cells.
What are plasmodesmata?
Plasmodesmata are gaps in the cell walls, allowing communication and flow of minerals between the cells. They contain a thin strand of cytoplasm. This allows it to link the contents of adjacent cells.
Water: What is water potential
A measure of the tendency of water molecules to diffuse from one place to another.
- Water always moves from a region of higher water potential to a region of lower water potential. The water potential of pure water is zero.
- In a plant cell, the cytoplasm contains salts and sugars (solutes) that will reduce the water potential. This is because there are fewer ‘free’ water molecules available than in pure water. As a result, the water potential in plant cells is always negative.
Water: What happens if you place a plant cell in pure water
If you place a plant cell in pure water, it will take up water molecules by osmosis. This is because the water potential in the cell is lower (more negative) that the water potential of the water. But the cell will not continue to absorb water until it bursts. This is because the cell has a strong cellulose wall. Once the cell is full of water it is described as being turgid. The water inside the cell starts to exert pressure on the cell wall, called the pressure potential. As the pressure potential builds up it reduces the influx of water.
Water: What happens if you place a plant cell a concentrated salt solution
If a plant cell is placed in a solution with a very low water potential (a concentrated salt solution), it will lose water by osmosis. This is because the water potential of the cell s higher than the water potential of the solution. So water diffuses down its water potential gradient out of the cell. The cell loses its turgidity. If water loss continues the cytoplasm and vacuole shrink. Eventually the cytoplasm no longer pushes against the cell wall. This is called incipient plasmolysis. If water continues to leave the cell, the plasma membrane will lose contact with the wall, a condition known as plasmolysis.
Water: How does water move between cells
When plant cells are touching each other, water molecules can pass from one cell to another. The water molecules will move from the cell with the higher water potential (less negative) to the cell with a lower water potential (more negative).
Water: What are the routes water can take between cells
- Apoplast Pathway
- Symplast Pathway
- Vacuolar Pathway
Water: What is the apoplast pathway
The cellulose cell walls have many water-filled spaces (intercellular spaces) between the cellulose molecules. Water can move through these spaces and between the cells. In this pathway, the water does not pass through any plasma membranes. This means that dissolved mineral ions and salts can be carried with the water.
- The Casparian strip blocks this pathway, forcing the water to pass into the cytoplasm through cell membranes.
- 90% of water travels through this pathway.
Water: What is the symplast pathway
Water enters the cell cytoplasm through the plasma membrane. It can then pass through the plasmodesmata from one cell to the next. The plasmodesmata are gaps in the cell wall that contain a thin strand of cytoplasm, therefore the cytoplasm of adjacent cells is linked. Once inside the cytoplasm, water an move through the continuous cytoplasm from cell to cell.
-9% of water travels through this pathway.
Water: What is the vacuolar pathway?
This is similar to the symplast pathway, but the water is not confined to the cytoplasm of the cells. It is able to enter and pass through the vacuoles as well.
-A small amount of water travels through this pathway.
Water: How is water taken up from the soil?
Plant roots are surrounded by soil particles. The outermost layer of cells (the epidermis) contains root hair cells that increase the surface area of the root. These cells absorb minerals from the soil by active transport using ATP for energy. The minerals reduce the water potential of the cell cytoplasm. This makes the water potential in the cell lower than that in the soil. Water is taken up across the plasma membrane by osmosis as the molecules move down the water potential gradient.
Water: What minerals are taken up from the soil with the water?
- Nitrates (to make proteins)
- Magnesium (to make chlorophyll)
Water: What is the movement of water across the root driven by?
-The movement of water across the root is driven by an active process that occurs at the endodermis. The endodermis is a layer of cells surrounding the xylem. It is also known as the starch sheath as it contains granules of starch - a sign that energy is being used. The endodermis consists of special cells that have a waterproof strip in some of their walls, called the Casparian strip. This blocks the apoplast pathway, forcing water into the symplast pathway.
Water: How is water moved across the root?
- The endodermis cells move minerals by active transport from the cortex into the xylem. This decreases the water potential in the xylem. As a result, water moves from the cortex through the endodermal cells to the xylem by osmosis.
- This reduces the water potential in the cells just outside the endodermis. This, combined with water entering the root hair cells, creates a water potential gradient across the whole cortex. Therefore water is moved along the symplast pathway from the root hair cells, across the cortex and into the xylem.
- At the same time, water can move through the apoplast pathway across the cortex. This water moves into the cells to join the symplast pathways just before passing through the endodoermis.
