MCDB Plant Physiology PDF

Lecture 8 What is a photoreceptor? ​ Receptors that respond to light. 2 groups: Blue-light and Red-light. ​ Blue: Phototropins, Zeaxanthin, and Cryptochromes. ​ Red and far-red: Phytochromes ​ Phytochromes act as a molecular light switch switching between an inactive Pr that ab...

Lecture 8 What is a photoreceptor? ​ Receptors that respond to light. 2 groups: Blue-light and Red-light. ​ Blue: Phototropins, Zeaxanthin, and Cryptochromes. ​ Red and far-red: Phytochromes ​ Phytochromes act as a molecular light switch switching between an inactive Pr that absorbs red light and an active Pfr that absorbs far-red light. When the plant is absorbing red light from the sun Pr changes to Pfr that leads to a cellular response such as seed germination. (basically there has to be enough red light to promote response, germination). When there isn’t enough red light or it is dark Pfr switches back to Pr. Plants with a lot of sunlight produce more germination, more Pr → Pfr, more branches leaves and are shorter. Those in shades have to switch back from Pfr → Pr more so they are taller with less branches as there isn’t a lot of germination, but more vertical growth. Plants respond to mechanical stimuli including gravity and touch through action potentials that are driven by electrochemical gradients in parenchyma cells (large central vacoule, thin and flexible) in plant cells (neurons). What are action potentials? ​ A rapid change in the electrical potential across the cell membrane. Membrane potential: difference in electrical charge/ voltage between the inside and outside the plasma membrane. In order to get an action potential, the neuron must be stimulated so that the difference in charge inside and outside the its plasma membrane is above -55 mV. Once it reaches the threshold, the voltage-gated sodium channels open which allows the sodium to diffuse into cell which depolarizes the membrane (bc inside the cell membrane is negative). Electrochemical gradient: different amount of different ions in/outside the membrane. The inside is usually more neg.  When a leaf is touched an action potential is driven by the electrochemical gradient where Ca2+ moves into the cell and Cl- and K+ moves out. In order for calcium to go in, it must go through ion bchannels. Water is then pulled out by ions through aquaporins because it moves from a low to high ion concentration, osmosis. Statoliths are plant’s response to gravity. In roots, auxin inhibits cell elongation, as they display positive gravitropism. Lecture 7 1.​ Signal Reception: there is a receptor in the plasma membrane that detects or receives signal or stimuli such as hormones, (physical and chemical signs). 2.​ Signal transduction: Signal goes to signal transductors that takes signal through the cell. A way to relay signal to the rest of the cell (proteins and secondary messengers) 3.​ Cellular response: The cell responds to signal (effectors) Creating photosynthetic tissue. 1.​ Light detected by phytochrome receptor 2.​ Cyclic GMP amplifies signal and relays signal to protein kinase where Ca2+ channels open and increased Ca2+ activates protein kinase 2  3.​ Gene expression changes for proteins in greening response Auxin (IAA) is a hormone , a chemical signaling molecule that travels through the plant and changes the functioning of the target cell. -​ Found in shoot apical meristems and yoing leaves. Root depends on the shoot for its auxin. Developing seeds and fruits contain high level of auxin. -​ Promotes stem elongation, differentiation of meristems into vascular tissue, leaf development and phototropism, or how plants grow toward light. -​ Directional light causes auxin to move to shaded side of shoot tips where it promotes greater cell elongation on shady side. Shoots grow toward light, positive phototropism but also roots bend downward, positive gravitropism.. Cell wall is more acidic pH=5 than the cytoplasm, pH=7. Auxin is uncharged in the cell wall and hence crosses the plasma membrane easily through passive discussion But when auxin reaches the cytoplasm it becomes negatively charged and thus needs transport proteins to exit the cell. There can be transport proteins on the bottom or on the side hence there is unidirectional transport from shoots into roots called polar transport. Where auxin goes depends where these transport proteins are. In regulating cell elongation auxin allows for the loosening of the cell wall because it promotes H+ secretion that lowers the pH of the cell wall.(more H+, more acidic). This low pH activates expansins that disrupt hydrogen bonds between polysaccharides and because cellulose microfibrils can slide past each other in loosened cell walls, cells can expand if there's a lot of turgor pressure. Lecture 6 Photosynthesis occurs on the leaves of plants. There is tissue in the inside of leaves called mesophyll that contain cells with chloroplasts. These chloroplasts have an inner membrane where there is also a third membrane called the thylakoid membrane that contains pigment molecules (chlorophyll). Stacks of thylakoids=granum. The region between the thylakoids and the intermembrane space is a fluid rilled region called the stroma. The Chloroplasts also have an outer membrane. (carbon dioxide and oxygen eneter & leave via stomata). It converts light energy into chemical energy through Light Reactions : Photosystem II and I, and through the Calvin cycle. PS are light dependent while the Calvin cycle is independent. -​ Light reactions occur in Thylakoid membranes, turn solar energy into chemical energy: ATP & NADPH -​ Calvin Cycle happens in the Stroma, turns CO2 to glucose with the help of ATP & NADPH Light provides photons that hit an electron at ground state and excite it to the excited state and makes it a high-energy electron. -​ Light-harvesting complexes is what captures the photons and the breaking of water is what produces O2. When the electrons from the breaking of water hit P680 (special pair of chlorophyll a molecules) they go from ground to to an excited state, jumping to the primary electron acceptor, this jumping is called the reaction-center complex. Then they  undergo the electron transport chain, going from Pq to cytochrome complex to Pc where ATP is generated. -​ Electrons exit PSII enter the ETC and through this H+ is pumped into thylakoid. From water H+ is also broken off and chemiosmosis, or the energy stored in the H+ chemical gradient is then used to synthesize ATP from ADP + Pi (running down an ATP synthase. -​ Same process for Chloroplast and Mitochondria but in the chloroplast it happens in thylakoid membrane, space and the stroma as opposed to inner membrane, inter-membrance space and matrix. -​ When electrons exit PSII they enter the ETC that pump H+ into thylakoid. In PSI light hits pigment molecules and provides an electron alongside another electron PSII provides to P700 up to primary electron acceptor, through ETC again. Electrons now run from Fd to the NADP+ reductase where NADP+ and H+ and 2e- is reduced to NADPH. PSI can produce solely ATP via cyclic photophosphorylation Both pump H+ from matrix or Stroma into the intermembrane space or thylakoid space by the ETC. Low to High solute conce. We have a diffusion back down through the ATP synthase that produces ATP from ADP +Pi.. In the Mitochondria, NAD+ is reduced to NADH and in the chloroplast NADP+ is reduced to NADPH. The Calvin cycle uses the chemical energy of ATP and NADH to reduce CO2 into sugar because first a molecule of CO2 enters alongside rubicose and it undergoes carbon fixation. 3 CO2 are needed to produce enough for G3P a 3 Carbon sugar. Then 6 ATP is oxidized from ATP to ADP Then 6 NADPH is used and releases NADP+. Then 6Pi is released. NADPH donates electrons to, or reduces, a three-carbon intermediate to make G3P. Then we go to Reduction where we then produce 6 molecules of G3P. Most of it stays in order to Regenerate RuBP. In this regernatrion, 3 ATP is oxidized to 3ADP + 2Pi. We go from 3 5-carbon chain with 2 P to 6 3-carbon chain with 1 P as we introduce 3 CO2. Rubisco acts as an enzyme that catalyzes the first step of carbon fixation, the reaction of CO2 and RuBP (ribulose, a five-carbonsigar). Where does Mass Energy Energy Process Mass input it occur? output input output Photosystem Excited Thylakoid Water Oxygen and Light energy II (light electrons membrane ADP + Pi ATP reactions) Photosystem Thylakoid NADP+ Light energy I (light NADPH H+ gradient membrane reactions) Chemical stroma CO2, RuBP, G3P, ADP, Chemical bonds Calvin cycle ATP and NADP+, and bonds of between NADPH Pi RuBP the second and third phosphate groups in ATP PLANTTTTSSSS AND UPTAKE OF SUGAR The cell wall provides support, maintinas cell shape and direction of cell growth, prevents excessive uptake of water. Primary and secondary wall. Secondary is more inner. Think of it from the outside. Cellulose is an unbranched polysaccharide carbohydrate that is used to support and provide strength in the cell wall. The cell wall is made of cellulose microfibrils that are made of many OH groups that are linked together by glycosidic linkage. Because of these large amounts of OH polar groups the primary cell wall is hydrophilic. The H is free to bond with other hydroxyl groups due to the electronegativity of oxygen. The cellulose microfirbils allow for the cells to grow by water intake through the vacoule and supports the cell wall as it expands due to this water intake. The cell elongates perpendicularly to encircling the cell wall cellulose microfibrils. Secondary cell wall: -​ 1o= hemicellulose (structural support= rigidity), more pectin (gelatinous matrix, fluidity), cross-linking glycan, cellulose microfibris More rigidity hemicellulose in 2o think wood. Primary wall more flexible 2o same thing as primary, more hemicellulose plus LIGANDS, it is also hydrophobic due to waxy substances ​ Ligands are the woody parts Primary wall set first where water can enter, then secondary where water is no longer able to enter and can’t fill the vacoule. The middle lamella connects neighboring plant cells as it is a thin layer rich in strong polysachharides, pectin (hydrophillic) that glues them together between the cells’ primary walls. The plasmodesmata are pores and channels that allow for the flow of ions and molecules by the cytosol or different cells. Little holes in the midst of them. Ground: Storage, photosynthesis and support. Internal to dermal tissue Dermal: Outer protective layer of epidermis Vascular: Transport water and minerals through the plant and provides mechanical support ​ The vascular tissue is made up of two specialized tissues called the phloem and xylem.  Phloem: Transports sugars, products of photsytnhesis from where made leaves to where needed or stored (roots or sites of growth) ​ Xylem: Water and mineral uptake from roots into shoots ​ Meristems: localized regions of undifferentiated cells. Primary growth: Vertical growth with the help of shoot apical meristems. Apical meristems: undifferentiated cells whom when they undergo division one daughter remains meristem and some go on to becoming differentiated primary meristems → protoderm, ground meristem, procambium these then go to become ​ epidermis, ground tissue, primary phloem/xylem Secondary growth: Horizontal growth lateral meristems. Increases diameter of stems and roots in woody plants ​ Lateral meristems: Vascular and Cork Cambium Secondary xylem (wood) is added to the inside of the vascular cambium cell and secondary phloem is added to the outside. ​ Cork cambium produce cork tissue Vascular inside of cork cambium Lecture 3 Transmembrane transport: molecules move through plasma membrane and cell walls Symplastic transport: transport through the cytosols and plasmodesmata Apoplastic transport: transport through the cell walls Turgid: Lots of water coming in, there is alot of pressure against the cell wall, it is rigid. The plasma membrane presses tightly against the cell wall Flaccid: Water comes in and out, the plasma membrane does not press tightly against the cell wall Plasmolyzed: Lots of water leaving the cell, it folds. So much water has been lost by osmosis that the plasma membrane contorts away from the wall Water Potential= Water solute + Pressure potential ​ ​ (always negative) (neg or pos) Water moves from a region of HIGH to LOW water potential Osmosis: water moves from low to high concentration. Passive transport simple diffusion Water potential : potential energy of water or water’s capacity to perform work when it works Root endodermis well the roots actually allow for a larger absorption of water. Root hairs covered in epidermal cells help increase surface area that allows for absorption of water and minerals. The movement through the epidermis (cortex) may be apoplastic or symplastic. In order for water to enter the vascular tissues (whether the movement started as apo or sym), it needs symplastic movement across the endodermis (casparian strips) Passage is blocked into xylem when water and minerals reach the root endodermis via apoplast by Casparian strips which are like waxy that create an opportunity for selective uptake. Passive transport. Via Symplastic requires energy, Proton pump. Bulk flow: movement of liquid in response to a pressure gradient Cohesion-tension theory: movement of xylem sap is driven by a water potential difference between leaf end and root end of xylem, more neg at the top, moves high to low so bottom to top. NEG PRESSURE NEGATIVE PRESSURE POTENTIAL : water and minerals pulled up by roots : XYLEM POSITIVE PRESSURE: sugars are pushed from where produced (source) to where needed (sinks) roots. Transportation: water vapors evaporates from the leaves Cohesion: water attracted to itself , oxygen is very electroneg Adhesion: watter attracted to something other than itself, water attracted to the sides of the xylems, as water moves upward it moves more water upward because of cohesion (monkey barrels) Stoma: where water evaporates Stoma: made of two guard cells that open and close REGULATE. Dont want too much water to evaporate close but need it to open to allow for CO2 to diffuse out There are two types of xylem cells (dead): Tracheids and Vessel elements Phloem conducting cells (alive): Sieve-Tube elements and Companion cells (dead) Secondary cell walls maintain structures and prevent collapse under high tension of the neg pressure of water transport Lecture 4 Stomata opens and closes with the help of guard cells to regulate transpirational water loss. When they open they are considered turgid and flaccid when they close. Needs to open to allow for CO2 to come in and close to avoid the excess the loss of excess water Stomata can open when there is an increase in K+ inside guard cells and water goes in. OPENS: Lots of K+, low CO2, a lot of sunlight (needed for photosynthesis), CLOSES: When water is leaving, an increase in ABA drought stress hormone, decrease in stomata S Translocation: phloem sap moves from sugar prod to consumption Symplastic: mesophyll cell, companion, sieve tube elements via plasmodesmata Apoplastic: mesophyll cell, apoplast, companion or sieve tube elements via active transport