Chapter Summaries 36.4 - 36.6

1. Chapter Summaries36.4 - 36.6 By: Rida and Shakir

2. INTRO Ever wonder why dew builds up on the leaves of plants early in the morning? Or how organic materials move from roots to leaves and vice-versa? How about if plasmodesmata are highly dynamic structures that change in permeability and number or not? All these questions will be answered by this amazing presentation! Plants are a very complex species. But like all complex species, they need various substances and functionality of internal systems to sustain life. The loss of compulsory substances such as water and the functionality of processes such as translocation are both very important when studying plant structure. Therefore, being students of biology, it is important for you to understand the information that will be presented to you through this presentation.

3. Chapter 36.4; Stomata help regulate the rate of transpiration • A plant’s tremendous requirement for water is negatively effected by the presence of stomata (which are required for the shoot system) as water escapes through these tiny holes. However, guard cells open and close the stomata to balance the plants requirement to conserve water and its requirement for photosynthesis. • The loss of water vapor from parts of plants is called transpiration. 95% of transpiration occurs through the stomata. Each stomata is planked by guard cells that change shape to widen or narrow the gap between each other. • Turgor pressure is the main pressure of water inside plant cells against the cell wall and is what gives the plant its shape. The change in turgor pressure results from the absorption and loss of potassium ions. • Guard cells have 2 shapes, turgid (stoma open) and flaccid (stoma closed). When guard cells take in water from neighbouring cells, they become turgid (turgor pressure increases) . When they lose water, they become flaccid (turgor pressure decreases).

4. Chapter 36.4; continued • The flow of potassium ions is coupled with the generation of a membrane potential by a proton pump. Membrane potential is the is the difference in voltage between the interior and exterior of a cell (Vinterior − Vexterior) • First, protons leave the guard cell through active transport (proton pump). This creates a membrane potential so that potassium ions can enter the guard cells. • Water potential is the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects such as surface tension. The addition of solutes to water lowers its potential (makes it more negative). If possible, water will move from an area of higher water potential to an area that has a lower water potential. • When potassium ions enter the guard cells, water potential decreases inside the cell and therefore water from outside the cell enters, thus turgor pressure increases and the stoma opens

6. Chapter 36.4; Continued • Stoma closing results from a loss of potassium ions from the guard cell to neighbouring cells which results in the osmotic loss of water. In general stoma are open during day and close during night. All plants regardless of if they are in light or dark environments will close during ‘night time’ and open during ‘day time’ due to the circadian rhythm. • There are 3 main factors that effect stoma opening/closing: Light, CO2 depletion, and internal ‘clock’ in guard cells • Light stimulates guard cells to accumulate potassium ions and turn turgid. • If CO2 concentrations decrease in the environment, the stoma will open to allow more CO2 to enter if water supply is sufficient.

7. Chapter 36.4; continued • Transpiration is greatest on sunny, warm, dry and windy conditions since this aids in evaporation • If transpiration cannot pull sufficient water towards the leaves, the shoot system will become withered since the leaves will lose turgor pressure. • Transpiration also cools the plant due to the cooling effects of evaporation. The leaves could be up to 10ºC cooler as compared to its surrounding air. This prevents the leaf from reaching temperatures that could denature it’s proteins.

8. Chapter 36.5: Sugars Are Transported From Leaves to Sites of Use and Storage • Mature leaves are the main source of sugars. However, during seasonal changes, storage organs become the sugar source. • phloem loading and unloading occurs depending on the active transport of sucrose • Loading and unloading of sugar creates a pressure difference that keeps sap moving through the sieve tubes

9. Chapter 36.6: The Symplast is Highly Dynamic • As a plant grows and develops, its transport needs also change. The symplast is living tissue and largely responsible for these changes. • Plasmodesmata are structures that can change in number and permeability in response to changing turgor pressure, calcium levels, and pH. • As a leaf matures, plasmodesmata are either closed or removed, which causes phloem unloading to cease • Plant viruses also affect plasmodesmata (viral movement proteins), causes plasmodesmata to dilate which allows viral RNA to pass between the cells. Viruses mimick the cell’s plasmodesmata regulators (each cell regulates plasmodesmata as part of a communication network)

11. Chapter 36.6 Continued… • High levels of cytoplasmic connectedness is found in the sytoplasmic domain. Molecules such as proteins and RNA coordinate development within the sytoplasmic domain • If this domain is altered/mutated, growth is largely impacted • Another feature of the symplast is long distance electrical signalling through the phloem. In this sense, the phloem has a nerve like function that provides electrical communication (e.g. change in hormone levels)

12. Chapter 36.6 Continued… • Phloem is also a “superhighway” that transports macromolecules (such as proteins and various forms of RNA) and viruses • This communcation allows all the functions of the plant to be integrated. Example: defense response to localized infection (activates defense genes) • Transport of materials and information is essential to plant survival

13. CONCLUSION • Guard cells cause the opening and closing of the stomata, where the majority of transpiration (loss of water) occurs. When potassium ions enter the guard cell, the stomata opens, it closes when there are less potassium ions. 3 main factors effect stoma opening/closing: Light, CO2 depletion, and internal ‘clock’ in guard cells. Transpiration is required so the shoot system does not wither • Mature leaves are the main source of sugars. However, during seasonal changes, storage organs become the sugar source. Loading and unloading of sugar creates a pressure difference that keeps sap moving through the sieve tubes • The symplast and plasmodesmata are dynamic structures that aid the transport of sugar between cells. Plasmodesmata can change in number and permeability. They are affected by turgor pressure, and pH levels, as well as viruses that use the plasmodesmata to travel between cells. The Phloem is a “superhighway” that transports molecules and information.

14. Bibliography Campbell, Neil, and Jane Reece. AP Edition Biology. 8th ed. 1995. Print. Malmberg, Julie. (2008). Retrieved from www.colorado.edu Basu, Surish. (n.d.). Of Waltzing proteins and whispering rna. Retrieved from www.engineering.cornell.edu NSW, . (n.d.). Retrieved from http://www.sydneyr.det.nsw.edu.au/ Nikon, Initials. (2000). Digital eclipse image gallery. Retrieved from www.microscopyu.com