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Grace Darling
Cells storing and manufacturing carbohydrates transport the carbohydrates through to the phloem tissue to be transported elsewhere. When the sugars exit these areas it causes the sieve tube to have a high concentration (near the production site) which leads to the movement of sugars down the phloem to areas of low concentration- these are the areas which require the sugars. This is done to achieve an equilibrium in concentration levels.
One of the main drivers behind the movement of water through the xylem: it must replace the water lost through transpiration
-Kinnear. J, Martin. M (2013). Nature of Biology Unit One and Unit Two. Milton, Qld: John Wiley & Sons Australua Ltd. p.166-174, 586, 587.
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The sieve tube element of the phloem tissue. Sugars pass through the sieve plate which connect each elongated sieve tube cell to one another
Organic substances are synthesised in leaves and then are transported from their origin through the phloem tissue to other areas where they are either stored or used. These areas could include: root hair cells or developing flowers.
So, after a carbohydrate is formed, the phloem transports the sucrose, the main carbohydrate which is actively transported in plants, to different locations around the plant. This is done in the phloem tissue, through the sieve tube elements. Companion cells readily provide this active transport with energy.
When sugar is in an area of high pressure, water enters the the sieve tube from the xylem, meaning that it enters the phloem against the concentration gradient. Consequently, there is a build up of turgor pressure which leads to a mass flow of water, and the soluble sugars, down the gradient from the leaves to the recipient plant cell
When the sucrose enters the cell it will be either: stored as starch or used in the metabolism after being further broken down into monosaccharides.
During the process where sugar enters the cell, water moves through osmosis from the phloem back into the xylem , hence decreasing turgor pressure in the sieve cells
The root hairs of the root system absorbs water from the soil through osmosis. These water molecules pass through the epidermis, cortex, and endodermis and then enter the xylem column. The water is now in the vascular tissue, meaning that it can be transported throughout the plant.
The xylem tissue has three different types of cells:
-xylem parenchyma cells
-xylem fiber cells
-xylem tracheary elements (TE)
The parenchyma (storage site in the xylem tissue of water, mineral nutrients and carbohydrates, and respond to wounding) and fiber cells provide structural support for the TE. There are two types of tracheary elements: vessel elements and tracheids. Both perform the same tasks yet there are some structural differences (refer to the next slide). Once the vessel element and tracheid have matured, they die, meaning that their role as water and soluble mineral transporters is passive. Movement happens due to root pressure and transpirational pull.
From the root system, water molecules continue their journey up to the leaves. This is only possible when water vapour moves out of the stomata (in the leaf), forcing the water in the xylem vessels to replace the lost water by sucking more up the whole column, hence moving all the molecules along.
Water molecules remain together because of their cohesive properties and the smaller the diameter of a xylem vessel the greater the tensile strength. As water is sucked up the xylem column the molecules remain stuck together and to the walls- preventing them from pulling away.
The pathway in which water flows around a plant of any sort depends on the xylem. There are two factors influencing the moment of water, and dissolved minerals, and they are: root pressure and the pull from the process of transpiration. As well as these factors there are additional ones influencing the rate of transpiration: light, temperature, humidity, wind, and soil water. Before we learn about transpiration, we must understand how the water gets to the leaves in the first place.
Due to the tapered cylindrical structure, holes are found in the cell walls to allow for a continual flow
Xylem is a specified tissue in plants which assists the transportation of water and dissolved substances. For normal growth, plants require macro-nutrients in substantially large amounts, while micro-nutrients (trace elements) are the the opposite; they are only required in small doses. This transportation network is made of tubes and transport cells which, ultimately, create vessels and work to carry substances from one end of the plant to another.
Plants are incredibly sophisticated and are
comprised of many different networks, particularly the translocation and transpiration system.
Translocation: transport of organic material, including sugars, through the phloem of a vascular plant
Transpiration: loss of water from the surfaces of a plant
These two systems allow the plant, whether it be a fern, grass, shrub, flower, or tree, to support all the different areas from root to leaf.
These two systems would be unable to function if it weren't for the xylem and phloem,
found in the vascular tissue.
Vascular tissue: plant tissue specialised for
transport of nutrients, water, and minerals,
and which provides a plant with
support.
The mesophyll cells, found just below the upper epidermis of the leaf, are moist and contain water vapour in the air spaces around each cell. Diffusion of water vapour occurs through the stomata when the sun shines. 'Water evaporates from the wall of the mesophyll cells to replace that lost from the air spaces.' The walls of the mesophyll cells are kept moist by the movement of water out of the cell, as water evaporates from the surface of each (mesophyll cell). This causes water molecules flowing past in the xylem vessel to move into the mesophyll cell to replace the lost water. Because the movement of water in the xylem vessels is so dependent on the evaporation of water through the stomata, the molecules in the xylem are very sensitive and under tension. When movement is underway it is transmitted along the whole water column moving every water molecule along.
The vessel elements are stacked together and look like hollow cylinders
Sieve plates contain sieve-like pores and appear when enzymes break up the ends of the tubes.
Sieve tube
members are elongated and equipped to
transport substances rapidly and over
long distances (absence of nuclei).
Phloem tissue differs in composition to the
xylem tissue and hence, performs a different role. When cells containing chloroplasts manufacture carbon dioxide and water into carbohydrates (sucrose: the form of carbohydrate for transport in plants, and glucose) and oxygen, the phloem transports this sucrose to various regions of the plant- particularly those which cannot photosynthesise. This is important as all areas of the living plant must receive carbohydrate nutrients to function. Some cells cannot produce their own carbohydrates, so they must take it from others- with the help of the phloem.
Along with transporting carbohydrates, the phloem also moves hormones and other
organic material created by the plant to
other areas.
These cells aid in the transportation of substances
Many of the structures within the phloem tissue are living, the opposite to that of the xylem tissue