36.1 Nutritional Requirements of Plants

• Biologists define an essential nutrient as an element or compound at is required for normal growth and reproduction means that the plant cannot complete its life cycle without it.

  • Essential nutrients cannot be synthesized by the organism.

• Plants need relatively large quantities of certain elements in the soil; these elements are called macronutrients.

  • Some of them are major components of nucleic acids, proteins, and phospholipids, all of which are plentiful in plants.

• Among the macronutrients, nitrogen (N), phosphorus (P), and potassium (K) are particularly important because they often act as limiting nutrients, meaning their availability limits plant growth.

  • If N, P, and/or K are added in appropriate quantities to the soil as fertilizer, plant growth usually increases.

  • This outcome explains why the leading ingredients in virtually every commercial fertilizer are N, P, and K.

• In contrast to macronutrients, plants require micronutrients in small quantities.

• These elements represent mobile nutrients-meaning ey are readily transported from older leaves to younger leaves when they are in short supply-so older leaves deteriorate first when these elements are scarce.

• Immobile nutrients such as iron or calcium, in contrast, stay tied up in older leaves.

6.2 Soil: A dynamic Mixture of Living and Nonliving Components

• The ability to diagnose nutrient deficiencies is based largely on studies involving hydroponic grow systems.

  • Hydroponic growth takes place in liquid cultures, without soil, so researchers can precisely control the availability of each nutrient.

• This decaying organic matter is called humus.

• Techniques that maintain long-term soil quality and productivity are the basis of sustainable agriculture and sustainable forestry.

• The loss of nutrients via the movement of water through soil is called leaching.

• The presence of protons in soil water can cause the release of cations that are bound to soil particles.

  • The process responsible is called cation exchange.

    • Cation exchange occurs when protons or other soluble cations bind to negative charges on soil particles and cause bound cations.

36.3 Nutrient Uptake

• Most nutrient uptake occurs just above the growing root tip, in the region called the zone of maturation.

• These transport proteins cannot import the ions that the plant needs completely on their own.

  • They often function together with other proteins, specifically proton pumps, or H+-ATPases.

    • Proton pumps aids in the absorption of certain ions by establishing a relatively high concentration of protons on the outside of root epidermal cells.

• One mechanism for coping with toxic concentrations of metals involves small proteins called metallothioneins, and short peptides called phytochelatins that are synthesized by special enzymes.

• 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 membrane allow plants to actively remove toxic substances from the cytosol and store them in the vacuole.

• A transport protein that functions as an antiporter then uses this gradient to move sodium ions into the vacuole.

  • The antiporter accomplishes this by carrying sodium ions into the vacuole as protons diffuse out of the vacuole.

  • As a result, sodium ions are transported against their concentration gradient.

36.4 Nitrogen Fixation

• Leghemoglobin is related to the hemoglobin at carries oxygen in your blood.

  • Like hemoglobin, leghemoglobin binds oxygen.

  • Leghemoglobin is important because nitrogenase is poisoned by the presence of oxygen.

• When rhizobia contact the flavonoids, the bacteria respond by producing sugar-containing molecules called Nod factors.

  • Nod factors, in turn, bind to signaling proteins on the membrane surface of root hairs.

36.5 Nutritional Adaptations of Plants

• Some parasitic plants are non- hotosynthetic and obtain all of their nutrition by tapping into the vascular tissue of the host individual they are heterotrophs, organisms that obtain food by consuming other organisms.

• Carnivorous plants trap insects and other animals, kill them, and then digest the prey to absorb their nutrients.