PLANT BIOLOGY

"Skill: Drawing of half-views of animal-pollinated flowers"

Drawing of Lamium album, which have large petals, nectaries and position of anther and stigma shows that it's adapted for insect pollination (usually bees): . . . . . .

XYLEM DRAWING: "Drawing the structure of primary xylem vessels in sections of stems based on microscope images."

*Recognize xylem in pg 115 DRAWING: . . . . . . . •Mature xylem vessels have no plasma membranes so water can move in/out freely •Thick cell wall containing lignin •Lumen filled with sap as the cytoplasm and nuclei of original cells break down. End walls (in the middle of lumen) also break down to form a continuous tube •Helical/ring-shaped thickenings of cell wall impregnated with lignin, which makes the hard to resist inward pressure. •Pores in outer cellulose cell walls conduct water out of xylem vessels and into cell walls of adjacent leaf cells.

SUMMARY OF COMPULSORY PRACTICALS NEED TO BE ABLE TO ANSWER QUESTIONS ABOUT

1) Light microscope to investigate structure of cells, calculating magnifications (pg 4) 2) Estimating osmolarity of tissue by bathing in hyper/hypotonic solutions (pg 11) 3) Investigating a factor affecting enzyme activity (pg 27) 4) Separating photosynthetic pigments by chromatography (pg 36) 5) Setting up sealed mesocosms to try establish sustainability (pg 55) 6) Monitoring ventilation rates in humans at rest, after mild, vigorous exercise (pg 78) 7) Measurement of transpiration rates using photometers (pg 111-112)

"Application: Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots and capillary tubing."

Models can show theories: 1) Water has adhesive properties: glass capillary tube sticking out of water and mercury: water adheres to glass so it rises up tube, mercury doesn't 2) Water is drawn through capillaries in cell walls: Strip of blotting/filter/chromatography paper made of cellulose cell walls put inside test tube with water in the bottom. Water rises up against gravity in pores in paper. 3) Evaporation of water can cause tension: Porous pot: water rises up tube into porous pot which has many narrow pores (capillaries) running through, and water adheres to it (similar to leaf cell). Water evaporates from the surface of the pot, so more water is drawn in to replace lost water.

"Skill: Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide."

Physiologists have developed method using aphids of obtaining samples of phloem sap from single sieve tubes. Aphids have long piercing mouthparts called stylets which they insert into stems/leaves and push inward through plant tissue to sieve tube. High pressure in sieve tube pushed phloem sap through stylet and into the gut of the aphid. To sample the phloem sap the aphid is cut off from its stylet when it started to feed, and sap continues to emerge through tube. Radioactively labelled CO2 (14CO2) is supplied to the leaf of a photosynthetic plant so radioactive sucrose loaded into phloem. Time it takes for radioactive sucrose to emerge from stylets and different distances from the leaf can give a measure of the rate of movement in the phloem sap.

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

Plants absorb water and mineral ions from soil using their roots: •Plants absorb potassium, phosphate, nitrate (& other mineral ions) by active transport, as concentration sully lower in soil than inside root cells. As a result water from soil absorbed in by osmosis. ADAPTATIONS OF ROOTS: 1) Branching of roots to increase SA 2) Growth of root hair (extensions) from epidermis cells (cells that cover roots) 3) Root hair cells have mitochondria and protein pumps in plasma membrane • Most roots only absorb mineral ions if they have oxygen to make ATP for active transport by aerobic respiration.

"Skill: Design of experiments to test hypotheses about factors affecting germination."

Seeds won't germinate unless external conditions suitable. 1) water to rehydrate seed 2) oxygen for cell respiration 3) warmth for enzyme activity Basic design of experiment to test one of these requires at least 2 treatments: 1) control treatment giving seed all factors needed 2) treatment deprived of one factor. •If seeds given control germinate but those denied don't, factor must be needed for germination. (e.g. for oxygen: sealed tubes with seeds in moist cotton wool with water, at 20˚C. One has air with oxygen, the other has alkaline solution that absorbs all oxygen)

ABIOTIC FACTORS AFFECTING TRANSPIRATION RATES

TEMP: Heat needed for evaporation of water from surface of mesophyll cells, so as temp increases so does transpiration rate. High temps also increase rate of diffusion through air spaces because of KE, and lower relative humidity of air outside of the leaf. In very high temps the stomata may close. GRAPH: (linear up) . . . . HUMIDITY: Humidity: water concentration in air, always high humidity within leaf. Water diffuses out of leaf when concentration gradient, so as atmospheric humidity decreases concentration gradient gets steeper and transpiration faster GRAPH (linear down): . . . . WIND SPEED: In still air humidity builds up so less transpiration. Moderate wind velocities move the water away to increase concentration gradient, though at very velocities stomata close. GRAPH: . . . .

"Adaptations of plants in deserts and in saline soils for water conservation."

XEROPHYTES: plants adapted to grow in dry habitats, e.g. giant cactus. 1) Vertical stems absorb sunlight early/late in day but not at noon when it's most intense 2) very thick waxy cuticle covering the stem 3) spines instead of leaves to reduce SA for transpiration 4) CAM physiology: opens stomata during cool night instead of hot day Plants that grow in sand dunes also have xerophytic adaptations, such as marram grass. Cross section of leaf (pg 113): 1) thick waxy cuticle covering leaf 2) hairs on underside of leaf 3) smaller airspace in mesophyll 4) few stomata, that are sunk in pits 5) cells that can change shape so leaf rolls up with lower epidermis/stomata on the inside. . HALOPHYTES: saline conditions have such high concentration of Na+ and Cl-that most plants can't grow. Saline soils found in coastal habitats & arid areas where water moves up in soil and evaporates, leaving dissolved ions on the surface. Adaptations: 1) To prevent water moving out by osmosis solute concentration much be high, though can't just raise Na+ inside cell because high concentration can impact cells e.g. protein synthesis. High concentrations of more harmless solutes like sugars of K+. 2) Na+ and Cl- concentrations above those of saline soil can be maintained in vacuoles as metabolic activities don't occur there 3) Getting rid of excess Na+: active transport back into soil, excretion from special glands in leaf, accumulating it in certain leaves then shedding them. •Also have adaptations for water conservation similar to xerophytes. •Some have water storage tissues so are succulents

"Application: Micropropagation of plants using tissue from the shoot apex, nutrient agar gels and growth hormones." "Application: Use of micropropagation for rapid bulking up of new varieties, production of virus-free strains of existing varieties and propagation of orchids and other rare species."

•Desirable varieties propagated asexually so all plants produced have them, in a new technique called micropropagation (name because propagation can be done with very small pieces of tissue taken from shoot apex) 1) small tissue removed (usually from shoot tip). Aseptic technique: all apparatus and tissue sterilized. 2) tissue placed on top of sterile nutrient agar gel (in test tube) with high auxin concentration to stimulate cell growth/division. 3) Callus (amorphous lump of tissue) grows, which can be cut up and made to grow more with same gel. 4) Eventually callus transferred to nutrient agar gel with less auxin but high cytokinin which stimulates plant lets with roots and shoots to develop. Gibberellin sometimes added to increase shoot growth and prevent dormancy. 5) Plantlets separated and transferred to soil where they grow strongly. Advantages: 1) new varieties can be bulked p rapidly 2) Virus-free strains of existing varieties can be produced as cells in shoot apex normally don't contain viruses that reduce plant growth (even if other cells in the original plant did) 3) large numbers of rare plants like orchids can be produced, reducing cost and making it unnecessary to take them out of their wild habitat.

STRUCTURE OF PHLOEM SIEVE TUBES "Structure-function relationships of phloem sieve tubes."

•Develop from columns of cells that break down nuclei and almost all of cytoplasmic organelles but remain alive (unlike xylem which is dead). Large pores develop in the cross walls between the cells, creating sieve plates that allow the sap to flow. •Cell walls of tube resists high pressure inside the sieve tube, sieve plates are cross-walls that strengthen the sieve tube with pores that allow sap to pass through in either direction •Plasmodesmata - narrow cytoplasmic connections with adjacent companion cells •Lumen of sieve tube has no organelles that would obstruct the flow of sap •Cell membrane on inside of wall holds sap inside the sieve tubes and has pumps to load/unload sucrose •Protein fibres (function uncertain) . . . . .

"Skill: Drawing internal structure of seeds."

•Flowering plants propagated (breed specimens) by sowing (planting) seeds. •Seeds contain an embryo plant and food reserves for embryo for germination. •Testa - seed coat. Cotyledons: 2 large modified embryo leaves for food reserves. •Beans (large seeds) diagram: External structure (still with testa): . . . . . Internal structure (with one cotyledon removed): . . . . .

"Success in plant reproduction depends on pollination, fertilization and seed dispersal." Guidance: Students should understand the differences between pollination, fertilization and seed dispersal but are not required to know the details of each process.

•Flowers are structures used by flowing plants for sexual reproduction. •Female gametes contained in ovules in ovaries, male gametes in pollen grains are produced by anthers. Pollination: transfer of pollen from an anther to a sigma, as pollen grains can't move without help from external agents. These germinate on the stigma and a pollen tube containing the male gamete grows down the style to the ovary, delivering the male gamete to the ovule. Pollination usually occurs by wind or animal. •Zygote formed by fusion of gametes in ovule = fertilization. •Fertilized ovules develop into seeds, ovaries into fruits which is used for seed dispersal, which is spreading seeds away from parent plant to sites where they can germinate and grow without competing.

PHLOEM TRANSPORT "Plants transport organic compounds from sources to sinks." "Incompressibility of water allows transport along hydrostatic pressure gradients." "Active transport is used to load organic compounds into phloem sieve tubes at the source." "High concentrations of solutes in the phloem at the source lead to water uptake by osmosis." "Raised hydrostatic pressure causes the contents of the phloem to flow towards sinks."

•Function of the phloem: transport organic compounds (sugars, AAs) from one part of the plant to another: •OCs loaded into phloem sieve tubes by active transport in sources (where photosynthesis occurs e.g. stems and leaves, and storage organs where stores are mobilized) and are unloaded at sinks (roots, storage organs like potato tubers, growing fruits including developing seeds). •The incompressibility of water allows transport along hydrostatic pressure gradients: •Hydrostatic pressure is pressure in a liquid •High concentration of solutes such as sugars in the phloem sieve tubes at the source leads to water uptake by osmosis and high hydrostatic pressure Low solute concentrate of phloem sieve tubes at sink lead to exit of water by osmosis and low hydrostatic pressure •There is therefore a pressure gradient that makes sap inside phloem sieve tubes flow from sources to sink. •(companion cells, which have nucleus, in between sources/sinks and phloem sieve tubes, phloem sieve tubes are elongated cells) •Main sugar carried by phloem sieve tubes is sucrose. Transported by active transport from sources but not by pumping molecules directly: 1) protons pumped out of phloem cells by active transport to create protein gradient 2) co-transporter proteins in the membrane of phloem cells use this gradient to move a sucrose molecule into the cell together with proton/simultaneously allowing protons out down the concentration gradient. •Some sucrose loaded directly into phloem sieve tubes by this process, and to speed it up adjacent companion cells also absorb sucrose by co-transport and then pass it to sieve tubes via narrow cytoplasmic connections called plasmodesmata.

MERISTEMS "Undifferentiated cells in the meristems of plants allow indeterminate growth."

•Meristems are regions where small undifferentiated cells continue to grow and divide. •At flowering plants meristems are at the tip of roots and stems, so they're apical meristems (as they're at apex of root and stem) •Growth in apical meristems allow roots/stems to elongate, and the shoot apical meristem also produces new leaves and flowers. •In animal embryos a fixed number of parts develop = determinant growth. •Apical meristems can continue to increase the length of stem and root throughout the life of a plant and can produce any number of extra branches of the stem or root = indeterminate growth. Can also produce any number of extra leaves or flowers.

"Most flowering plants use mutualistic relationships with pollinators in sexual reproduction."

•More than 85% of 250,000 flowering plant species depend on insects or other animal pollinators for reproduction. •Mutualistic: the plant benefits by flowers being pollinated and the pollinator benefits by obtaining nectar for energy and pollen for protein. •There is a trend for plant species to develop mutualistic relationships for pollination with one specific species of insect. E.g. vanilla orchid pollinated by species of Melipona bee. Advantage is that insect transfers pollen between flowers of the same species and not to other species (example of why we must protect entire ecosystems and not individual species and they all depend on each other).

AUXIN: "Plant hormones control growth in the shoot apex." "Plant shoots respond to the environment by tropisms." "Auxin efflux pumps can set up concentration gradients of auxin in plant tissue." "Auxin influences cell growth rates by changing the pattern of gene expression."

•Plant hormones are used to control growth at the shoot tip. The main hormone is auxin which acts as a growth promoter, and the process it controls is phototropism. •Tropism is directional growth in response to stimuli, and shoots are positively phototrophic meaning they grow toward the brightest light source. •Shoot tips can detect the source of the brightest light and also produce auxin. 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 toward the light: 1) Light detected using several types of pigment but 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, which is carried out by auxin pumps in the plasma membrane. They are efflux pumps as they move auxin from the cytoplasm out into the cell wall. Auxin molecules in the cytoplasm carry a negative change and it's these that are moved by the efflux pumps. 2) In the cell wall a proton binds to auxin and it can then diffuse into a cell, once it it loses the protein and is trapped in the cytoplasm util an efflux pump ejects it. 3) Auxin efflux pumps are moved in response to differences in light intensity so they set up a concentration gradience of auxin from lower on the lighter side to higher on the shadier side. 4) Plant cells contain and auxin receptor, and when auxin binds to it the transcription of specific genes is promoted. The expression of these genes causes secretion of hydrogen ions into the cell walls, which loosens the connections between cellulose fibers to allow cell to expand.

Nature of science (?): "improvements in analytical techniques allowing the detection of trace amounts of substances has led to advances in the understanding of plant hormones and their effect on gene expression"

•Plant hormones have low concentration in plant tissue, but even their traces can have effects on plant physiology because they usually act as regulators of gene transcription. •Difficult to detect because concentration at which plant hormones are active can be as low as picograms (1/mil gram) of hormone per gram of plant tissue. Another problem is that the 5 groups of plant hormones are chemically diverse, so different extraction methods needed. Techniques: 1) ELISA (enzymes-linked immunosorbent assays) 2) gas chromatography - mass spectrophotometry 3) liquid chromatography - mass spectrophotometry •Very low concentrations of hormones now detectible and previously unknown hormones have been discovered. Recently developed techniques employed in research. •Changes in the pattern of gene expression due to a hormone can be detected using microarrays. Proteins have been discovered to which specific hormones bind, which activates the protein and allows it to bind to promotors of pacific genes and cause their transcription. •For example, 5 genes have been show to be expressed on the shadier side of a shoot tip where the auxin concentration is higher.

DAY LENGTH AND FLOWERING "Flowering involves a change in gene expression in the shoot apex." "The switch to flowering is a response to the length of light and dark periods in many plants." "Application: Methods used to induce short-day plants to flower out of season" "Flowering in so-called short-day plants such as chrysanthemums, is stimulated by long nights rather than short days."

•The shoot apex produced stem/leaves until it receives a stimulus that makes it change to producing flowers, involving a change in gene expression in cells of the shoot apex. •Usually stimulus is change in length of light/dark periods. Some plants only flower at the time of year when days are short (short-day plants), and some when days are long (long-day plants). •Experiments show it's not the length of the day but the length of the night that is significant, for example chrysanthemums are short-day plants and only flower when they receive long continuous periods of darkness (14.5hours+), so naturally flower in autumn. •Growers can produce pots of flowing out of normal season by keeping them in greenhouses with blinds, to extend night artificially.

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

•The shoot of the plant is the stem and leaves, and at the tip there is a meristem called the shoot apical meristem. •Cells in this meristem carry out mitosis and cell division repeatedly to generate cells needed for extension of the stem and development of leaves. 1) some cells always remain in meristem and continue to go through cell cycle to produce more, this production of new cells causes other cells to be displaced to the edge of the meristem. Cells at the edge stop dividing and undergo rapid growth and differentiation to become either stem or leaf tissue. 2) Leaves are initiated as small bumps at the side of the apical dome, these bumps are called leaf primordial and though continued cell division and rapid growth they develop into mature leaves. DIAGRAM: . . . . . .

"Transpiration is the inevitable consequence of gas exchange in the leaf."

•Transpiration is the loss of water vapor from the stems and leafs of plants, the inevitable consequence of gas exchange in the leaf. •Leaves must absorb CO2 for use in photosynthesis, and excrete oxygen as a waste product. This gas exchange requires large moist surface provided by the mesophyll (tissue in leaf). •Top layer: palisade mesophyll •Lower part/middle: spongy mesophyll, with network of air spaces that increase SA of moist cell walls exposed to air •Lower layer: lower epidermis. •Unless air spaces fully saturated, water evaporates from moist cell walls of spongy mesophyll cells. Air spaces thus have high relative humidity so water vapor diffuses to air outside leaf. •Epidermis of most plant cells secrete wax to form a waxy cuticle - a waterproof coat to the leaf, which prevents excessive transpiration but also blocks gas exchange. •Pores (stomata) therefore needed for CO2 to enter the leaf and O2 leaves, controlled by guard cells. If open for gas exchange they allow water vapor to escape which is transpiration.

VASCULAR TISSUE DIAGRAMS/IDENTIFICATIONS "Skill: Identification of xylem and phloem in microscope images of stem and root."

•Vascular tissue; contains vessels used for transporting materials, main types are xylem and phloem which occur in stems. DIAGRAM OF VASCULAR TISSUE IN STEM OF YOUNG DICOTYLEDONOUS PLANT: . . . . . •Xylem can be identified from slices by presence of many large open xylem vessels (circles) and phloem tissue can be identified by areas of much smaller cells near the xylem. •In some stems, between them and the epidermis are tough lignified cells that provide support but aren't used for transport. •As stems grow thicker they develop more xylem and phloem. DIAGRAM OF XYLEM AND PHLOEM IN ROOTS (different position than stem): . . . . . ULTRASTRUCTURE of xylem/phloem in stem, roots and vertical section: pg 114

TRANSPIRATION EXPERIMENTS "Skill: Measurement of transpiration rates using potometers. (Practical 7)" "Skill: Design of an experiment to test hypotheses about the effect of temperature or humidity on transpiration rates."

•Water uptake usually measured for transpiration rate using potometer: As the plant transpires it draws water out of the capillary tube to replace the losses, and because capillary tube is narrow losses of water cause measurable movement of air bubbles. (repeated measurements of distance of bubble moved in one minute needed for reliable results) TEMP: heat lamp to vary temp, use infrared thermometer to measure leaf temperature. HUMIDITY: Transparent plastic bag enclosing leafy shoot with a mist sprayer to raise humidity. Desiccant bag containing silica gel to lower humidity. Use electronic hygrometer to measure relative humidity. WIND: Electric fan with varying velocities by either changing distance or rate of rotation. Use anemometer to measure speed of air moving across plant leaves.

XYLEM STRUCTURE: "Plants transport water from the roots to the leaves to replace losses from transpiration." "The cohesive property of water and the structure of the xylem vessels allow transport under tension." "The adhesive property of water and evaporation generate tension forces in leaf cell walls."

•Xylem are long tubular structures with strong side walls and very few cross walls (cell walls, bc they break down to form one continuous tube) that provides support and transports water. In flowering plants the xylem vessels are the main transport routes of water. •The main movement is from roots to leaves to replace water loss from transpiration, and this flow of water is called the transpiration stream: pulling forces (tension) cause water to move up, generated by the leaves by transpiration. •The stream is due to the adhesive property of water, as water adheres strongly to cellulose in plant cell walls. When water evaporates from leaf, more water is drawn through narrow pores (cellulose lined) in leaf cell walls from the nearest xylem vessels to replace it, generating the tension. •Tension transmitted between water molecules because of cohesive property, so tension transmitted all the way down columns of water till the roots. •At maximum transpiration the pressure inside the vessels can be very low so the side-walls must be strong to prevent inward collapse, achieved by depositing more cellulose in the walls and impregnating this thickening of the wall with lignin, making them harder and woody. •The first xylem formed by a shoot or root tip is the primary xylem, and the walls of this primary xylem are thickened in a helical/ring-shaped pattern which allows the vessel to elongate as the root or shoot grows in length. (water and minerals move in one direction)