Chapter 36

Nutritional Adaptations of plants

1. 99% of living plants are autotrophs, synthesizing their own sugar through photosynthesis -95% of vascular plants take up nutrients from soil -80% of plants obtain soil nutrients from mycorrhizal fungi 2. However, a few plants don't follow these rules -Some plants parasitize other plants -Some plants appear to live on air -Some plants catch and digest insects

Nitrogen Fixation

1. Although Nitrogen gas makes up 80% of the atmosphere, plants and other eukaryotes cannot use nitrogen in this form 2. A few species of bacteria and archaea are able to absorb N2 from the atmosphere and convert it to ammonia (NH3), nitrites (NO2), and nitrates (NO3) in a process called nitrogen fixation -Plants normally absorb nitrogen in the form of ammonium (NH4+) or nitrate (NO3-) ions 3. Nitrogen fixation requires a series of specialized enzymes and cofactors, including a large multi-enzyme complex called nitrogenase 4. The process is extremely energy demanding -An expenditure of 8 high-energy electrons and 16 ATP molecules is required for nitrogenase to reduce one molecules of N2 to two molecules of NH3. 5. In some cases, nitrogen-fixing bacteria take up residence inside plant root cells 6. The infected root cells of legumes form distinctive nodules, where nitrogen-fixing rhizobia are found -The nodules are pink because they contain leghemoglobin, and oxygen-binding molecule similar to hemoglobin -Leghemoglobin protects nitrogenase, which is poisoned by oxygen, by maintaining low levels of free oxygen

Which Nutrients Are Essential?

1. An essential nutrient is an element or compound that is required for normal growth and reproduction 2. Most vascular plants have 17 essential elements -Three elements make up 96% of the dry mass of the plant: Carbon, Hydrogen, Oxygen

Carnivorous Plants

1. Carnivorous plants use modified leaves to trap insects and other animals -These plants kill their prey and absorb the prey's nutrients 2. Carnivorous plants make their own carbohydrates via photosynthesis, using carnivore to supplement the nitrogen available in the environment -Most are found in bogs or other habitats where nitrogen is scarce

How Nitrogen-Fixing Bacteria infect Root Hairs

1. Each legume species produces a different flavonoid that acts as a recognition signal, and each rhizobium species responds with one or more unique nod factors 2. When Nod factors bind to the root-hair surface, they set off a chain of events that leads to dramatic morphological changes in the host legume 3. The bacterial cells contact the root-hair surface. Rhizobia multiply and move through and opening of the root-hair plasma membrane called an infection thread. The infection thread invades the root cortex. The infection thread buds off, forming membrane-bound clusters filled with rhizobia within cortex cells. The infected cortex cells divide, forming root nodules.

How Mycorrhizal fungi transfer the nutrients

1. Ectomycorrhizal fungi (EMF) -Have hyphae that wrap around the epidermal cells of roots 2. Arbuscular mycorrhizal fungi (AMF) -Penetrate the walls of plant root cells

Macro & Micro nutrients

1. Essential nutrients can be obtained from three sources: -Water -Air -Soil 2. Macronutrients are elements in the soil that plants require in large quantities -These nutrients are major components of nucleic acids, proteins, and phospholipids 3. Nitrogen, phosphorus, and potassium often act as limiting nutrients -Their availability limits plant growth -N:P:K fertilizers! 4. Micronutrients are required in small quantities -They usually act as cofactors of specific enzymes -Examples include iron, zinc, boron, copper, and nickel 5. Micronutrients may be vital to plant health, even though they are required in tiny amount.

Nutrient uptake via Mycorrhyzal Fungi

1. Fungi and plant roots that live in association are called mycorrhizae -Over 80% of all vascular plant species have relationships with mycorrhizal fungi 2. The symbiotic relationship between plant and fungi is mutualistic -Fungal symbionts obtain sugars from the plant, while plant symbionts receive soil nutrients such as nitrogen from the mycorrhizal fungi 3. Fungi are particularly efficient at acquiring the nutrients required by plants, for two reasons: -Networks of filamentous hyphae increase the surface area available for absorbing nutrients by up to 700% -Fungi can acquire nutrients from macromolecules in soil that are unavailable to non-mycorrhizal plants

Hydroponic Growth

1. Hydroponic growth can be used to study the effects of nutritional deficiencies -Hydroponic growth takes place in liquid cultures, without soil, so the availability of nutrients can be precisely controlled 2. Different deficiencies have distinctive symptoms -Example: Tomatoes lacking the element copper will grow poorly, with stunted shoots, light foliage, and curled leaves -Copper is used as a cofactor of respiratory enzymes

Using a Proton Gradient to import Cations

1. In the membranes of root-hair cells, the electrochemical gradient established by proton pumps favors the entry of cations via ion channels -The gradient may be strong enough to overcome the concentration gradient, which opposes the entry of the cations 2. Anions enter root hairs through membrane transport proteins called cotransporters, which transport two solutes at once. -Enough energy is released when protons enter the cell along their electrochemical gradient that anions can be cotransported against their electrochemical gradients 3. Thus the electrochemical gradient set up by proton pumps makes it possible for plant roots to absorb key cations and anions via ion channels and symporters.

Mechanisms of Ion Exclusion

1. Many metal ions found in soil are poisonous to plants -Some natural or contaminated soils contain enough cadmium, lead, zinc, nickel, or other metals to poison plant enzymes 2. Plants my exclude detrimental ions by two methods: -In passive exclusion, root cells lack transporters to bring in toxic ions -In active exclusion, plants have mechanisms from coping with toxins that enter their cells

When Key Nutrients are in Short Supply

1. Mobile nutrients such as Nitrogen, Potassium, and Phosphorous are readily transferred from older leaves to newer leaves when in short supply. -Scarcity is reflected in the deterioration of older leaves 2. Immobile nutrients such as Calcium, Iron, and Copper remain tied up in older leaves -Scarcity is reflected in deficiency symptoms in newer leaves

Passive Exclusion

1. Passive exclusion of ions occurs in root hairs and by the Casparian strip 2.Some ions cannot enter the root system, because root-hair cells lack the membrane protein required for that ion to enter the cell -Ions that enter the root and move through the root cortex via the apoplectic pathway maybe blocked from entering xylem by the Casparian strip -Corn and rice are sensitive to salt buildup and may have fewer sodium channels

Modified Leaves of Carnivorous plants

1. Pitcher plant leaves form tubes and produce an odor to attract insects -Once inside the leaf, the insect has a difficult time climbing out and eventually drowns in the water at the bottom of the leaf 2. Sundew have modified leaves containing sticky hairs that trap insects 3. Venus flytraps capture prey by snapping shut when sensory hairs are triggered by the presence of an insect

Active Exclusion by Metallothioneins and Phytochelatins

1. Plants also have mechanisms for coping with toxins after they enter cells 2. Small proteins called metallothioneins and short peptides called phytochelatins bind to metal ions and prevent them from acting as a poison -These protective macromolecules are also found in bacteria, fungi, and animals

Importance of Soil Conservation

1. Soil erosion occurs when soil is carried away from a site by wind or water 2. Sustainable agriculture describes farming techniques for preventing soil erosion -Planing rows of trees as windbreaks -Minimizing the amount of plowing and tilling needed to control weeds -Planting crops in strips that follow the contour of hillsides 3. Natural soil erosion -The action of rivers -Soil formation vs soil erosion 4. Reasons for the removal of plant cover: -Deforestation -Suburbanization -Farming 5. Soils conservation is critically important for food supply

The role of soil pH

1. Soil pH can also influence the availability of essential elements -Acidic soils are found in regions with comfier forests, where the decomposition of organic matter produces carbonic, phosphoric, or nitric acid -Alkaline soils are found in areas rich in limestone (CaCO3) 2. Cation exchange occurs when protons or other soluble cations bind to negative charges on soil particles, causing the release of bound cations such as Mg2+ and Ca2+ -Nutrients released by cation exchange are available for uptake by plant roots 3. Plants influence cation exchange as roots release CO2, as a by-product of cellular respiration -CO2 reacts with H2O to form carbonic acid, which releases protons

Active Exclusion by Antiporters

A second mechanism for actively neutralizing specific toxins involves transport proteins located in the tonoplast-the membrane surrounding the large central vacuole -Proteins in the tonoplast allow plant cells to remove toxic substances from the cytosol and store them in the vacuole, where they cannot poison enzymes

Cost and Benefits of Carnivory

Carnivorous plants produce fewer insect trapping leaves and more leaves that function primarily in photosynthesis when provided with nitrogen-based fertilizers -Carnivory is a trait that shows phenotypic plasticity -Plants increase investment in prey-capture structures when nitrogen is rare and decrease investment in prey-capture structures when nitrogen is readily available.

Nutrient Uptake

Most nutrient uptake occurs just above the growing root tip, in the region called the zone of maturation -Epidermal cells in this region have root hairs that increase the surface area available for nutrient and water absorption -Root hairs create a "zone of nutrient depletion" in the soil surrounding them -Continuous growth of the root tip is vital to a plant's health

Parasitic plants

Parasite live on or in a host obtaining water o nutrients from the host and reducing the host's fitness -Some parasitic plants are heterotrophs, producing structures called haustoria that can penetrate the host vascular system to obtain water and nutrients. -Most parasitic plants are photosynthetic and use haustoria to extract water and ions from the xylem of the host plant

Mechanisms of Nutrient Uptake

While nutrients pass through plant cell walls freely, plasma membranes are selectively permeable -Root hairs contain membrane proteins that bring nutrients into the cytosol of root cells -Some ions are brought in via proton pumps, which work in tandem with membrane proteins

What factors affect nutrient availability?

1. The elements required for plant growth occur in the soil as ions -Some nutrients are available as elemental ions, such as K+ and Cl- -Other nutrients are present as molecular ions, such as HPO4^2 and NO3- 2. Anions-ions with negative charges-usually dissolve in soil water, because they interact with water molecules via hydrogen bonding -As solutes, anions are readily available to plants for absorption but are easily washed out of the soil by rain -The loss of nutrients via the movement of water through soil is called leaching 3. Cations-ions with positive charges-dissolve in soil water but are no immediately available as anions -In solution, cations interact with negative charges found on two types of soil particles: -Organic matter that is Ruch in negatively charged organic acids -The surfaces of clay particles that are rich in mineral anions

How Nitrogen-Fixing Bacteria infect plant roots

1. The first infection event in a recognition step that occurs between the root hairs of the legume and its symbiotic bacterium -Young roots release compounds called flavonoids -When rhizobia contact the flavonoids, they produce Nod factors, which bind to signaling proteins on the membrane surface of the root hairs

Plants synthesize all of their own nucleic acids, amino acids, enzymes and cofactors, and chlorophylls

1. To synthesize these materials, plants must harvest a wide variety of elements in the form of ions and molecules. 2. Most of these nutrients are found at low-sometimes extremely low-concentrations in soil 3. The one-way flow of water carries these nutrients up the xylem.

In the early 1600s, Jean Baptiste van Helmont performed a classic experiment to answer this question.

1. Van Helmont wanted to know where the mass in a growing plant comes from 2. He used a willow tree as his study organism

Soil: A Dynamic Mixture of Living and Nonliving components

1. Weathering-the forces applied by rain, running water, and wind-continually breaks down solid rock into soil 2. The first particles formed by weathering are gravel, sand, silt, and clay 3. As organisms occupy soil, they add decaying organic matter called humus. 4. Soil texture differs with the proportions of gravel, sand, silt, and clay 5. Soil texture is important for several reasons: -It affects the ability of roots to penetrate soil to obtain water and nutrients and to anchor and support plants -It affects soil's ability to hold water and make it available to plants -It affects the availability of oxygen for cellular respiration

Epiphytic Plants

Epiphytes grow in the absence of soil, often on the leaves or branches of trees -These plant absorb water and nutrients from rainwater, dust, and particles that collect in their tissues -Some epiphytic bromeliads have leaves that grow in rosettes to form "tanks" that collect water and organic debris -Nutrients are actually absorbed through the leaves

Establishing a Proton Gradient

The plasma membranes of root hairs and other epidermal cells contain proton pumps -These pumps move nutrients into the cell against a strong concentration gradient -Root cells use proton pumps to establish an electrochemical gradient, with an excess of protons on the exterior of the plasma membrane relative to the interior -This leads to a charge or voltage across the membrane, creating a membrane potential