9. 1 Transport in the xylem of plants

>> Transpiration is the inevitable consequence of gas exchange in the leaf

Plant leaves are where the process of photosynthesis occurs. It involves the assimilation of carbohydrates(glucose-C6H12O6) using light energy. Carbon dioxide is used as a raw material and oxygen is a waste product. Gas exchange is essential for photosynthesis to occur. Absorption of CO2 is essential for photosynthesis, as the waxy cuticle has low permeability, it needs pores on its surface known as stomata. A common problem among plants and other organisms is having gas exchange without water loss. This loss of water vapor from the leaves and the stem of the plant is known as transpiration. Water loss is minimized using guard cells. Theyre found in pairs on either side of the stoma. The guard cells control the opening of the stoma and can adjust it. A group of plants without any stomata are known as liverworts.

>>> Plants transport water from the roots to the leaves to replace losses from transpiration

The water leaving through stomata by transpiration is replaced by the water from the xylem. The water in the xylem climbs the stem through the pull of transpiration combined with the forces of adhesion and cohesion. Water moves from the soil into the roots by osmosis due to the active transport of minerals into the root. Once the water is in the root it travels to the xylem through cell walls(apoplast pathway) and through the cytoplasm(symplast pathway)

>> The cohesive property of water and the structure of the xylem cells allow transport under tension

The plants are able to transport water very efficiently due to the structure of the xylem vessel. Water molecules are cohesive, they are closely stuck together. Theyre not broken down by negative pressure or suctions. This is caused by the hydrogen bonding.

Properties/Structure of the xylem vessel

The xylem walls are thickened. The reason they can withstand low pressure without collapsing is that the thick walls are saturated with a polymer known lignin. Xylem vessels are formed from files of cells, lined end to end. Sometimes in flowering plants the cell wall material in areas between adjoining cells is largely removed. The cells contents and plasma membrane breakdown. Mature xylem cells are non-living, so the flow of water along is a passive process. The atmospheric pressure is higher than the pressure inside the xylem cell, but the tube doesnt collapse due to the rigid structure.

>> The adhesive property of water and evaporation generate tension forces in leaf cell wall

Pulling forces(tension) causes the water to move up to the leaves. These forces are generated by leaves and are due to the adhesive property of water. Water strongly adheres to the cellulose in plant cells When water evaporates from mesophyll cell walls in the leaf, more water is drawn through narrow cellulose-lined pores in leaf cell walls from the nearest xylem vessels to replace it, generating the tension.

>> Active uptake of mineral ions in the roots causes absorption of water by osmosis

>> Adaptation of plants in deserts and in saline soils for water conservation Desert - Succulence - Succulent plants are water hoarders. They store water in stems, root or fleshy leaves; special structures good at moisture retention.- Toleration- Evasion Saline- Salt excretion - These plants have leaves with glands that excrete salt.- Leave dropping - The plants store the salt in some leaves and if the load becomes too high, they drop the leaves.- Stomata control - The plants can restrict the stomata opening allowing them to conserve fresh water. - Turned leaves - The plants usually turn the leaf to reduce the surface area exposed to the hot sun and to capture evaporating water and increase humidity.

9. 2 Transport in the phloem of plants

>> Plants transport organic compounds from sources to sinks. The transport of organic solutes in a plant is called translocation Examples of Sources- Mature green leaves- Green stems- Storage tissues in germinating seeds- Tap roots or tubers at the start of the growth season. Examples of Sinks- Growing roots- Developing fruits- Developing seeds- Growing leaves- Developing tap roots or tubers

>> Active transport is used to load organic compounds into phloem sieve tubes at the source

Sieve Plate: These are the remnants of cell walls that separated the cells. The perforated walls in combination with the reduced cytoplasm means that resistance to the flow of phloem sap will be lower. Companion cells: These cells perform many of the genetic and metabolic functions of the sieve tube cells and maintain the viability of the sieve tube cell. Sieve Tube Elements: Sieve tubes are formed by living cells known as a sieve tube element. They form rigid walls which allow for the establishment of pressure necessary to achieve the flow of phloem in the sieve tube cell. Cytoplasm: This viscous liquid is used for movement in the cell.

>> Incompressibility of water allows transport by hydrostatic pressure gradients The build up of sucrose and other carbohydrates draws water into the companion cell through osmosis. the rigid cell walls combined with the incompressibility of water results in a buildup of pressure. Water will flow from this area of high pressure to an area of low pressure. >> Experiments using aphid stylets Aphids penetrate plant tissues to reach the phloem using mouth parts called stylets. If the aphid is anaesthetized and the stylet severed, phloem will continue to flow out of the stylet and both the rate of the flow and the composition of the sap can be analysed. The closer the stylet is to the sinks, the slower the rate at which the phloem sap will come out.

>> Radioisotopes are important tools in studying translocation Carbon 14 is an isotope of carbon that is radioactive. Radioactively-labelled carbon within carbon-dioxide can be fixed by plants during photosynthesis. It will release radiation that can be detected using either film or radiation detectors. As the carbon is metabolized, it will be found in different molecules within the plant. The formation and movement of radioactive molecules can be traced.

>> Identification of xylem and phloem in microscope images of stem and root.

9.3 Growth in plants

>> Undifferentiated cells in meristems of plants allow indeterminate growth. Most have a determinate growth which stops when it reaches a certain size. Cells are totipotent (can differentiate into all cells) - important for tissue cultures. All growth is confined to meristems (composed of undifferentiated cells that are undergoing active cell division.) Flowering plants have meristems at the tip of the root and the tip of the stem. They are apical meristems as they are at the apex of the root and the stem. Growth in apical meristems allows roots and stems to elongate. The shoot apical meristem also produces new leaves and flowers. In animal embryos, a fixed number of parts develop. This is called determinate growth. The growth of plants by contrast is indeterminate, because apical meristems can continue to increase the lengths of stem and root throughout the life of a plant and can produce any number of extra branches of the stem or root. They can also produce any number of extra leaves or flowers.

>> Mitosis and cell division in the shoot apex provide cells needed for extension of the stem and development of the leaves .

The leaves of a plant are attached to the stem. The shoot of the plant is the stem together with the leaves. At the tip of the shoot, there is a meristem called the shoot apical meristem. The cells in this meristem carry out mitosis and cell division repeatedly, to generate the cells needed for extension of the stem and development of leaves. Some of the cells always remain in the meristem and continue to go through the cell cycle producing more cells This production of new cells causes other cells to be displaced to the edge of the meristem

Cells at the edge stop dividing and under grow rapid growth at differentiation to become either stem or leaf tissue Leaves are initiated as small bumps at the side of the apical dome. These bumps are called leaf primordia and through continued cell division and rapid growth they develop into mature leaves.

>> Plant hormones control growth in the shoot apexPlant hormones are used to control growth at the shoot tip. The main hormone is auzin, which acts as a growth promoter. One of the processes that auxin controls is phototropism.

Tropisms are directional growth responses to directional stimuli. Shoots are positively phototropic - they grow towards the brightest source of light.

Shoot tips can detect the source of the brightest light and also produce auxin. According to long-standing theory, auxin is redistributed in the shoot tip from the lighter side to the shadier side. It then promotes more growth on the shadier side, causing the shoot to bend towards the light.

>> Plant shoots respond to the environment by tropisms

Light is detected using several types of pigment, but the most important are a group of proteins called phototropins. When these detect differences in the intensity of blue light in the shoot tip they trigger off movements of auxin by active transport. This is carried out by auxin pumps in the plasma membrane.

>> Auxin efflux pumps can set up concentration gradients of auxin in plant tissue. The auxin pumps in the plasma membranes are efflux pumps as they move auxin from the cytoplasm out into the cell wall. Auxin molecules in the cytoplasm carry a negative charge and it is these that are moved by the efflux pumps.

In the cell wall and proton binds to the auxin and it can then diffuse into a cell through the plasma membrane. Once in a cell the auxin loses its proton again and is trapped in the cytoplasm until an efflux pump ejects it.

Auxin efflux pumps are moved in response to the differences in light intensity so they set up a concentration gradient of auxin from lower on the lighter side to higher on the shadier side.

>> Auxin influences cell growth rates by changing the pattern of gene expression. Plant cells contain an auxin receptor. When auxin binds to it, transcription of specific genes is promoted. The expression of these genes causes secretion of hydrogen ions into cell walls. This loosens connections between cellulose fibres allowing cell expansion.