chapter 9: transport in plants Flashcards
1
Q
What are the two functions of xylem?
A
- conducting water and dissolved mineral salts from the roots to the stems and leaves
- providing mechanical support for the plant
2
Q
What are xylem vessels and what are they strengthened by?
A
- a xylem vessel is a long hollow tube stretching from the root to the leaf
- the structure is made of many dead cells
-the inner walls of xylem is thickened by deposits of a substance called
> LIGNIN - lignin may be deposited in form of rings, spirals or the whole vessel is lignified except in regions called pits
3
Q
How is a xylem vessel adapted for its functions?
A
- they xylem has an empty lumen without protoplasm
> reduces resistance to water flowing through the xylem - it’s walls are thickened with lignin
> lignin is a hard an rigid substance which prevents collapse of the vessel
> all the xylem vessels together provide mechanical support to the plant
4
Q
How is the phloem adapted for its function?
A
- companion cells have many mitochondria
> provide the energy needed for the companion cells to load sugars from the mesophyll cells into the sieve tubes by active transport - the hole in the sieve plates allow rapid flow of manufactured food substances through the sieve tubes
5
Q
What is phloem and what does it consist of?
A
- phloem conducts manufactured food ( sucrose and amino acids)
> from the green parts of the plant (esp leaf) to other parts of the plant - phloem consists mainly of sieve tubes and companion cells
- SIEVE TUBES: consists of columns of elongated, thin-walled living cells called
> sieve tube cells or sieve tube elements
> the ‘end walls’ separating the cells have many minute pores and are called SIEVE PLATES - a mature sieve tube cell has only a thin layer or cytoplasm inside the cell
> cytoplasm is connected to the cells above and below through sieve plates - each sieve tube has lost its central vacuole, nucleus and most organelles
- each sieve tube cell also has a companion cell beside it > carries out the metabolic processes needed to keep the sieve tube cell alive
- companion cell is narrow, thin-walled cell with many mitochondria, cytoplasm and nucleus
- companion cells provide nutrients and help the sieve tube cells to transport manufactured food
6
Q
How are vascular tissues organised in stems?
A
- In a dicotyledonous stem, the xylem and phloem are grouped together to form vascular bundles
- The vascular bundles are arranged in a ring around a central region called a pith
- the phloem lies outside the xylem with a tissue called the cambium between them
> cambium cells can divide and differentiate to form new xylem and phloem tissues giving rise to the thickening of the stem - the region between the pith and the epidermis
> CORTEX
> both the cortex and the pith serve to store up food substances such as starch - stem covered by a layer of cells called epidermis
> epidermal cells are protected by a waxy waterproof cuticle that greatly reduces evaporation of the water from the stem
7
Q
how are the vascular tissues organised in roots
A
- in a dicotyledonous root, the xylem and phloem are not bundled > they alternate each other
- the cortex of the root is also a storage tissue
- the epidermis of the root is the outermost layer of cells
> bears root hairs and is also called the piliferous layer - each root hair is a tubular outgrowth of an epidermal cell
> outgrowth increases the surface area to volume ratio ‘
> absorption of water and mineral salts is increased
8
Q
what is the meaning of translocation?
A
- transport of manufactured food substances such as sugars and amino acids in plants is known as translocation
9
Q
how are aphids used in translocation studies?
A
- insects like aphids feed on plant juices
- the long mouthpart of each aphid penetrates the leaf or the stem
- the aphid can be anaesthetised with carbon dioxide while it is feeding
> body cut off only leaving the feeding stylet in the plant tissues - a liquid will exude out from the cut end of the proboscis
> liquid contains sucrose and amino acids - when the stem is sectioned and examined under microscope
>can see that the feeding stylet of the aphid is inserted into the phloem sieve tube
> shows the translocation of sugars and amino acids occurs in the phloem
10
Q
how are isotopes used in translocation studies?
A
- carbon-14 is a radioactive carbon isotope
> its presence can be detected by an X-ray photographic film - a leaf is provided with carbon dioxide ( containing the radioactive carbon)
- when photosynthesis takes place, the sugars formed will contain radioactive carbon
- the stem is then cut and a section of it is exposed onto an X-ray photographic film
- it is found that radioactive substances are present in the phloem
> since radioactive substances cause the X-ray film to darken
11
Q
how does water enter a plant?
A
- each root hair > a fine tubular outgrowth of an epidermal cell
> grows between the soil particles, coming into close contact with the soil solution surrounding them - the thin film of liquid surrounding each soil particle is >a dilute solution of mineral salts
- the sap in the root hair cell is relatively concentrated solution of sugars and various salts
> sap has lower W.P than soil solution
- two solutions are separated by a partially permeable cell surface membrane of the root hair cell
> water enters root hair by osmosis - entry of water dilutes the sap
- sap of root hair cell A have a higher water potential than the next cell
> water passes by osmosis from the root hair cell into the inner cell - similarly, water passes from B to cell C of the cortex
> process continues until the water enters the xylem vessels and moves up the plant
12
Q
how do root hair cells absorb ions or mineral salts?
A
- by active transport, when the concentration of ions in the soil solution is lower than the root hair cell sap
> root hairs have to absorb the ions against a concentration gradient by active transport
> energy for this process comes from cellular respiration in the root hair cell - by diffusion when concentration of certain ions in the soil solution is higher than the root hair cell
13
Q
how has the root hair cell adapted to its function of absorption
A
- long and narrow: increases the SA / V which increases the rate of absorption of water and mineral salts by the root hair cell
- cell surface membrane prevents the cell sap from leaking out
>contains sugars, amino acids and salts
> has lower WP than soil solution so water can enter the root hair by osmosis - root hair cell contains many mitochondria
> aerobic respiration in the mitochondria releases energy for active transport for the active transport of ions into the cell
14
Q
what is root pressure?
A
- the living cells around the xylem vessels in the root use active transport to pump ions into the vessels
> lowers water potential in the xylem vessels
> water passes from the living cells into the xylem vessels by osmosis and flows upwards
> called root pressure
*root pressure alone is not sufficient to bring water up to the leaves in tall trees
15
Q
what is capillary action?
A
- water tends to move up inside very narrow tubes ( capillary tubes)
>due to the interaction between water molecules and the surfaces of the tubes
> called capillary action - interaction water molecules is known as cohesion
- the forces of attraction between unlike molecules such as water molecules and lumen of tubes known
> known as adhesion - since xylem vessels in the plant are very narrow tubes, capillary action helps moving water up the vessels
- capillary action plays a part in the upward movement of water in SMALL plants
