Chapter 9 Transport In Plants Flashcards
Why do plants need transport systems?
Small surface area to volume ratio
High metabolic rate- require water, mineral ions and sugars and also need to get rid of waste substances
Exchanging substances by diffusion alone would be too slow
What is the function of the xylem?
Transports water and mineral ions from the roots to the leaves
What is the function of the phloem?
Transports sugars and assimilates in solution both up and down the plant in a bio directional pathway
What is the vascular system of a plant?
A series of transport vessels consisting of the xylem and the phloem running through the stem, roots and leaves.
How is the phloem and xylem situated in a root cross section?
The xylem is at the centre surrounded by the phloem.
Identify the xylem with a cross shape with the phloem surrounding it.
Why are the vascular bundles situated at the centre of the root cross section?
Help withstand the tugging strains that result as stems and leaves are blown in the wind.
Also, it provides support for the root as it pushes through the soil.
How is the phloem and xylem situated in a stem cross section?
In the stems, the xylem and phloem are around the outside of the stem.
Identify the phloem facing the outside of the oval and the xylem facing the inside of the oval
Why are the vascular bundles situated around the outside of the stem?
Provide structural support and strength to reduce bending.
How are the xylem and phloem situated in a leaf cross section?
Midrib vein carries the vascular tissue, with many small branching veins spread throughout the leaf.
Identify the Phloem at the bottom of the vein and xylem at the top of the vein.
Why are the vascular bundles situated in the veins of the leaf?
The xylem and phloem make up a network of veins that support the structure of the thin leaves
Explain the structure of a xylem vessel.
-The xylem vessels form long, tube-like structures formed from the cells (vessel elements) joined end to end
-There is no end walls so the tube is uninterrupted allowing a continuous column of water to flow through
-The walls are thickened with lignin which helps to strengthen the structure and provide support preventing the vessel from collapsing when water is in short supply.
-The cells are dead so contain no cytoplasm
-Where lignification is incomplete there are gaps known as bordered pits which are aligned to an adjacent vessel so water or mineral ions can move between vessels
-The xylem is also narrow allowing water to be pulled up by capillary action
Explain the structure of the phloem vessel.
The Phloem vessel is composed of phloem fibres, phloem parenthyma, sieve tube elements and companion cells.
Sieve tube elements:
-The sieve tube elements are living cells which form the tube for transporting the solutes through the plant.
-The sieve tube elements are joined end to end to form sieve tubes
-At the end walls there are many holes to allow solutes to pass through
-Contain no nucleus, thin layer of cytoplasm and few organelles
-The cytoplasm of adjacent cells is connected through the holes in the sieve plates
Companion cells
-Situated between sieve tubes
-Contain a large nucleus and dense cytoplasm with an abundance of mitochondria
-Carries out the living functions for both themselves and sieve cells
For example, providing the ATP required for active processes like loading assimilates into the phloem.
Explain the process of Transpiration (Part 1)
1- Water enters through the root hair cells via osmosis meaning it travels down a water potential gradient
(From a less negative or higher water potential to a more negative solution or a lower water potential)
-It then passes through the root cortex, including the endodermis, to reach the xylem
-The water potential gradient is maintained from the roots to the leaves as soil has a high water potential while the leaves have a low water potential as it is constantly evaporated.
Explain the process of transpiration (Part 2)
2- Water then travels from the roots via the root cortex into the xylem by the symplast pathway
-This is where water travels through the cytoplasm of cells by osmosis.
-This is because the cytoplasm of neighbouring cells connect through plasmodesmata.
-Water can also travel from the roots via the root cortex by the apoplast pathway
-This is where water travels through the cell walls and intracellular spaces of neighbouring cells
-The water can carry solutes and move from areas of high hydrostatic pressure to areas of low hydrostatic pressure (mass flow)
-However, when the water reaches the endodermis cells in the root, its path is blocked by a waxy strip known as the Casparian strip.
-The water then has to take the symplast pathway, through the cell membranes which is useful as it is partially permeable to control what substances in the water can pass through.
-This stops any toxins from entering the plant
-Following this water enters the xylem.
Explain the process of transpiration (Part 3)
3- At the leaves the water moves out of the xylem and into the cells by the apoplast pathway.
- Water evaporates from the cell walls into the spaces between the cells
- When the water stomata open the water diffuses out of the leaf down a water potential gradient into the surrounding air.
