BIOLOGY 2 AP Ch 33 Lecture Notes

Plants and other photosynthetic autotrophs play a critical role in the energy flow and chemical cycling of ecosystems by transforming:

· Light energy into chemical bond energy.

· Inorganic compounds into organic compounds.

To accomplish these tasks, plants require:

· Sunlight as the energy source for photosynthesis.

· Inorganic, raw material, such as:

o Carbon dioxide

o Water

o Variety of inorganic, mineral ions in the soil

To acquire the resources of light and inorganic nutrients, plants have highly ramified root and shoot systems with large surface areas for exchange with their environment – soil and air.

I. Plants require at least seventeen essential nutrients

A. The Chemical Composition of Plants

Early ideas about plant nutrition:

· In the fourt5h century B.C., Aristotle thought that soil was the substance for plant growth and that leaves only provided shade for fruit.

· In the 1600’s, Jean-Baptiste van Helmont concluded plants grow mainly from the water it absorbs. To discover if plants grew by absorbing soil, he:

o Planted a seed in a measurement amount of soil;

o Weighed the grown plant;

o Measured how much soil was left. The soil did not decrease proportionately.

· In the 1700’s, Stephen Hales postulated plants were nourished mostly by air.

Plants grow mainly by accumulating water in their cells’ central vacuoles.

· Water is a nutrient since it supplies most of the hydrogen and some oxygen incorporated into organic compounds by photosynthesis.

· 80-85% of an herbaceous plant is water.

· More than 90%of the water absorbed is lost by transpiration.

· Retained water functions as a solvent, allows cell elongation, and keeps cells turgid.

By weight, the bulk of a plants organic material is from assimilated CO2. The composition of the dry weight of plants is:

· 95% organic substances, mostly carbohydrates.

· 5% minerals, to some extent determined by soil composition.

B. Essential Nutrients

An essential nutrient is one that is required for a plant to grow from a seed and completes its life cycle.

· Determined by hydroponic culture, in which roots are bathed in an aerated aqueous solution of known mineral content. If a mineral is omitted and plant growth is abnormal, it is essential.

Macronutrients = Elements required by plants in large amounts

Micronutrients = Elements required by plants in small amounts.

· These function primarily as cofactors of enzymatic reactions.

· Optimal concentrations vary for different plant species.

II. The symptoms of a mineral deficiency depend on the function and mobility of the element.

Symptoms of mineral deficiencies depend on:

1. The role of the nutrient in the plant.

2. Its mobility within the plant.

· Deficiencies of nutrients mobile in the plant appear in older organs first since some are preferentially shunted to growing parts.

· Deficiencies of immobile nutrients affect young parts of plant first because older tissues may have adequate reserves.

Deficiencies of N, K, and P are the most common.

· Shortage of micronutrients is less common and often localized.

· Overdoses of some micronutrients can be toxic.

III. Soil characteristics are key environmental factors in terrestrial ecosystems.

Plants growing in an area are adapted to the texture and chemical composition of the soil.

A. Texture and Composition of Soil

Soil is produced by the weathering of solid rock. Living organisms may accelerate that process once they become established.

Horizons = Distinct soil layers.

Topsoil = Mixture of decomposed rock of carrying textures, living organisms, and humus.

· Coarse sand has diameter of 0.2 -2 mm.

· Sand is 20-200 mm.

· Clay is less than 2 mm.

Loams = Mixture of sand, silt and clay; are the most fertile soils.

· Fine particles retain water and minerals

· Coarse particles provide air spaces with oxygen for cellular respiration.

Soil contains many bacteria, fungi, algae, protista, insects, earthworms, nematodes and plant roots. These affect the soil composition. For example, earthworms aerate soil

Humus = Decomposing organic material.

· Prevents clay from packing together.

· Builds a crumbly soil that retains ater but is will porous for good root aeration.

· Acts as a reservoir of mineral nutrients.

Soil composition determines which plants may grow in it, and plant growth, in turn, affects soil characteristics.

B. The Availability of Soil Water and Mineral.

Some water is bound so tightly to hydrophilic soil that it cannot be extracted by plants.

Water bound less tightly is generally available to the paltn as a soil solution containing minerals. This solution is absorbed into the root hairs and passes via the apoplast to the endodermis.

Positively charged minerals (K+, Ca+, Mg+) adhere by electrical attraction to negatively charges clay particles.

· Clay provides much surface area for binding.

· Prevents leaching of mineral nutrients

Cation exchange = H ions in soil displace positively charges mineral ions from clay, making them available to plants.

· Stimulated by roots which release acids to add H+ to the soil solution.

Negatively charges minerals (NO3- , H2PO4-,SO4-) are not tightly bound to soil particles.

· Tend to leach away more quickly.

Water and minerals are depleted from soil surrounding roots, but roots grow to bring the plant into contact with a new supply of nutrients.

IV. Soil conservation is one step toward sustainable agriculture

Good soil management is necessary to maintain soil fertility which may have taken centuries to develop through decomposition and accumulation of organic matter.

· Agriculture is unnatural and depletes the mineral content of the soil, making soil less fertile. Crops also use more water than natural vegetation.

· Three important aspects of soil management are:

1. Fertilizers

2. Irrigation

3. Erosion prevention

1. Fertilizers

Fertilizers may be mined, chemically produced or organic.

· Usually enriched in N, P,K

· Marked with 3 numbers corresponding to % nitrogen (as ammonium or nitrate), % phosphorus (as phosphoric acid), and % potassium (as potash).

Organic fertilizers are manure, fishmeal, and compost.

· Release minerals more gradually than chemical fertilizers, thus, soil retains minerals longer. Excess minerals from chemical fertilizers may be leached from soil and may pollute streams and lakes.

To properly use fertilizers, one must consider the soil pH.

· Acidity affects cation exchange and the chemical form of the mineral.

· A change in soil pH may make on essential element more available while causing another to adhere so tightly to soil particles that it is unavailable.

2. Irrigation

Availability of water limits plant growth in many environments.

Problems of irrigation arid land:

· Huge drain of water resources

· Can gradually make soil salty and infertile.

Solutions to these problems:

· use of drip irrigation. Perforated pipes slowly drip water close to plant roots, which reduce water evaporation and drainage.

· Development of plant varieties that require less water or can tolerate more salinity .

3. Erosion

Wind and water erode away much of the topsoil each year. Measures to prevent these losses include:

· Rows of trees to divide fields act as windbreaks.

· Terracing hillsides help prevent water erosion.

· Planting alfalfa and wheat provide good ground cover and protection.

Proper management makes soil a renewable resource which will remain fertile for many generations.

V. The metabolism of soil bacteria makes nitrogen available to plants.

Plants require nitrogen to produce protein, nucleic acids and other organic molecules.

· Plants can not use nitrogen in gaseous form (n2)

· To be assimilated by plants, nitrogen must be in the form of ammonium (NH4+) or nitrate.

There is a complex cycling of nitrogen in ecosystems.

· Over the sort term, the main source of nitrogenous minerals is the decomposition of humus microbes.

- For example, ammonifying bacteria

- Nitrogen in organic compounds is repackaged into inorganic compounds that can be absorbed as minerals by roots.

· Nitrogen is lost fro this local cycle when soil denitrifying bacteria convert.

· Nitrogen fixing bacteria restock nitrogenous minerals in the soil by converting N2 to NH3 (ammonia), a metabolic process called nitrogen fixation.

A. Nitrogen Fixation

Nitrogen fixation = The processs of converting atmospheric nitrogen (gaseous state) to nitrogenous compounds that can be directly used by plants (nitate or ammonia).

N2 + 8e- + 16ATP -à Nitrogenaseà 2NH3 + H2 + 16 ADP = 1 P

· Some soil bacteria possess nitrogenase

· Very energy consuming process, costing the bacteria at least 12 ATPs for eat ammonia molecule synthesized.

· In the soi, ammonia is converted to the ammonium ion which plants can absorb:

NH3 + H+ à NH 4+

· Plants acquire most of their nitrogen in the form of nitrate (NO3-), which is produced in soil by nitrifying bacteria that oxidize ammonium.

· Nitrogen absorbed in the plant is incorporated into organic compounds.

· Most plant species export nitrogen from the roots to the shoot (through xylem) in the form of organic compounds and amino acids.

B. Symbiotic Nitrogen Fixation

Legumes (e.g. Peas, beans, soybeans, peanuts, alfalfa, clover) have a built-in source of fixed nitrogen because they possess root nodules.

Nodules = Root swellings composed of plant cells that contain nitrogen-fixing bacteria of the genus Rhizobium. Each species of legumes is associated with a particular species of Rhizobium.

Nodules form as follows:

· Roots secrete chemicals that attract nearby bacteria

· Attracted bacteria emit chemicals which stimulate root hairs to elongate and curl to prepare for bacterial infection.

· Bacteria enter the root through an “infection thread” that carries them to the root cortex.

· Bacteria become enclosed in vesicles and assume a form called bacteroids.

· Bacteroids produce a chemical that induces the host’s cells to divide and form a nodule.

· Nodules continue to grow and a connection with the xylem and phloem develops.

This association is mulatistic; the bacteria supplies fixed nitrogen, and the plant supplies carbohydrates and other organic compounds.

Leghemoglobin = An iron-contianing protein that binds oxygen.

Most of the ammonium produced is used by the nodules to make amino acids for export to the shoots and leaves.

The basis for crop rotaion is that, under favorable conditions, root nodules fix more nitrogen than the legumes uses. The excess is secreted as ammonium into the soil.

· One year a nonlegume crop is planted, and the next year a legume is planted to restore the fixed nitrogen content of the soil.

· Legumes may be plowed under to further increase the fixed nitrogen content of the soil.

Some non-legumes host nitrogen-fixing symbionts.

· Alders and tropical grasses may host nitrogen-fixing actinomycetes.

· Rice farms cultre a fern (Azolla), containing symbiotic nitrogen-fixing cyanobacteria, with the rice.

VI. Improving the protein yield of crops is a major goal of agricultural research

A majority of the world’s population depends mainly on plants for protein. Some food plants have low protein content while others may be deficient in certain amino acids.

Ways in which to increase the protein content of plants are:

1. Plants breeding to create new varieties enriched in protein.

· Unfortunately these ‘super’ varieties require large quantities of nitrogen in the form of commercial fertilizer too expensive for many countries to afford.

2. Improving the productivity of symbiotic nitrogen fixation.

· Mutant strains of Rhizobium have been isolated that continue to produce nitrogenase even after fixed nitrogen accumulates, thus releasing the excess into the soil.

· Rhizobium varieties may be selecte that fix N2 at a lower cost in photosynthestic energy, yielding a higher total food content in the plant.

3. Genetic engineering.

· May create varieties of Rhizobium that can infect nonlegumes.

· May transfer the genes required for nitrogen-fixation directly into plant genomes, using bacterial plasmids as vectors.

VII. Predation and symbiosis are evolutionary adaptions that enhance plant nutirion.

Some plant adaptions enhance nutrition through interactions with other organisms.

A. Parasitic Plants

Some parasitic plants:

· Are photosynthestic, and only supplement nutrition by using haustoria (not homologous to those of parasitic fungi) to obtain xylem sap from its host plant.

· Have lost photosynthesis entirely, drawing all nutrients from the host plant.

Epiphytes are plants that:

· Grow on the surface of other plants, anchored by roots, but are not parasitic.

· Nourish themselves from the water and minerals absorbed from rain.

· Examples include Spanish moss and staghorn ferns.

B. Carnivorous Plants

Carnivorous plants:

· Live in habitats with poor (usually nitrogen deficient) soil conditions.

· Are photosynthetic, but obtain some nitrogen and minerals by killing and digesting insects.

· Most insects traps evolved by modifications of leaves and usually are equipped with glands that secrete digestive juices.

C. Mycorrhzae

Mycorrhizea = Symbiotic associations (mutualistic) between plant roots and fungi. The fungus either forms a sheath around the root or penetrates root tissue.

· Helps the plant absorb water.

· Absorbs minerals and may secrete acid that increases mineral solubility and converts minerals to forms easily used by the plant.

· May help protect the plant against certain soil pathogens.

· The plant nourishes the fungus with photosynthetic products.

Almost all plants are capable of forming mycorrihzae if exposed to the proper species of fungi.

Mycorrihzae may have permitted early plants to colonize land.

· Fossils indicate the earliest land plants possessed mycorrhizea.

· This mutualistic association may have allowed the early plants to obtain enough nutrients to survive colonization.