Ecology and Nitrogen Cycle Quiz

Humans and Agriculture

• 14,000 years ago, humans transitioned from foraging to farming
• Wheat cultivation began approximately 10,000 years ago
• Artificial selection was used to improve wheat varieties
• Resources like water, nutrients (fertilizers introduced in 1940), and space were crucial for cultivation
• Plant breeding led to the Green Revolution, increasing yield to meet population demands
• Monoculture (growing the same crop for 10,000 years) has disadvantages
• Genetic engineering and high-tech agriculture are being developed to meet future food needs
• Current global population is 7.8-8 billion, and projected to be 10 billion by 2050

Homeostasis

• Homeostasis is the dynamic process of regulating the internal environment to maintain a stable state.
• It involves compensating or adjusting for changes in internal and external conditions.
• Environmental factors like temperature, pH, solutes, and water pressure are regulated.
• Metabolism plays a vital role in maintaining homeostasis

Plant Nutrient Uptake and Transport

• Plants maintain balance through water and nutrient uptake from the soil.
• Crucial soil factors include types and amounts of minerals available and their form
• Essential plant nutrients include elements needed for growth (carbon, hydrogen, oxygen), and minerals essential in large quantities + those essential in small

Nitrogen Limitations

• Nitrogen (N) is abundant in the air (78%), yet is often the most limiting nutrient for plant growth.
• The triple bond in atmospheric nitrogen requires specific enzymes for conversion.
• The nitrogen cycle is crucial for providing soil nitrogen

Nitrogen Cycling

• Nitrogen fixation converts atmospheric nitrogen (N₂) into ammonia (NH₄⁺) usable by plants.
• This process is performed by nitrogen-fixing bacteria.
• Bacterial ammonification breaks down decaying organic nitrogen compounds into ammonia (NH₄⁺).
• Bacterial nitrification converts ammonia (NH₄⁺) into nitrate (NO₃⁻); plants prefer the nitrate form.
• In this cycle, plants use nitrate to produce organic compounds

Soil Characteristics

• Soil is the "living skin" of the Earth
• It's a complex mixture of mineral particles, organic matter (humus), water, air, and organisms.
• Particle size (sand, silt, clay) influences soil properties and relates to water & nutrient availability
• Soil properties like water and mineral availability are influenced by the relative amount of soil particles

Soil Solution

• Soil solution is a combination of water and dissolved substances
• It coats soil particles and partially fills pore spaces
• This solution is available for plant uptake after gravity drainage.
• Negatively charged clay and humus particles attract water molecules

Mineral Availability

• Dissolved minerals in water are readily available to plants
• Plant roots selectively absorb minerals via ion-specific transport proteins.
• Cations and anions (e.g., NO₃⁻, SO₄²⁻, PO₄³⁻) are present, but not all are equally available.

Cation Exchange

• Cation exchange is a process where mineral cations replace hydrogen ions (H⁺) adhered to soil particles
• This process occurs within the root zone
• Plant roots release H⁺ ions, which displace mineral cations from the clay.

Mineral Availability (Anions)

• Anions (e.g., NO₃⁻, SO₄²⁻, PO₄³⁻) are weakly bound to soil
• They move freely and are easily leached out (lost) by excess water. Soils with high pH (alkaline) favour anion availability.

Passive and Active Transport

• Passive transport moves substances across a cell membrane with no energy necessary
• Substances move down the concentration/electrochemical gradient
• Active transport moves substances against the concentration/electrochemical gradient. Energy (ATP) is required.

Mechanisms to Increase Uptake

• Root hairs increase the surface area for absorbing water and minerals.
• Mycorrhizae (fungus-plant roots symbiotic association), increases nutrient uptake (particularly Phosphorous uptake)

Hydroponics

• Analyzing plant ashes helps determine essential elements.
• Hydroponic culture allows growing plants in a solution containing essential nutrients.
• By removing one nutrient at a time, observe growth effects to identify plant needs.

Mineral Deficiencies

• Mineral deficiencies can cause visual symptoms, including chlorosis (yellowing of leaves)
• Different deficiencies display particular leaf and stem behaviours.

Uptake by Root Systems

• Extensive root systems are adaptations to conserve essential mineral nutrients.
• Roots can comprise 20-50% of total plant mass.
• Active growth of root systems continues as long as the plant lives.

Cation Exchange

• Mineral cations (Mg²⁺, Ca²⁺, K⁺) are attached to negatively charged soil particles (e.g., clay).
• Roots pump H⁺ ions, displacing mineral cations for uptake.
• This process (cation exchange) allows uptake/exchange of cations in the soil

Stomata

• Stomata are tiny pores in the epidermis of leaves, mostly on the lower epidermis
• They consist of guard cells that surround the stoma.
• Guard cells regulate the opening and closing of stomata.
• Stomata, a gateway to CO2 essential for photosynthesis, and water loss through transpiration

Gas Exchange

• Plants need to take up CO2 for photosynthesis and release O2 as a waste product
• Leaves maximise diffusion by increasing surface area and by having gas-filled spaces inside leaves
• Stomata facilitate gas exchange through opening and closing

Transportation of Water

• Water travels through the xylem and phloem in a plant
• Water moves from the roots to the leaves, against gravity.
• Osmosis, Transpiration, and Root pressure all play a role
• Cohesion and tension in the xylem water transport mechanism
• Cohesion –water molecules stick to each other
• Tension –water molecules are pulled up the xylem to replace water that is being lost through the stomata
• Water movement in the xylem is regulated and influenced by external factors like humidity, air temperature, and air movement

Translocation (Pressure Flow)

• The pressure flow mechanism describes how organic compounds like sugars are transported through the phloem.
• The phloem sap contains more than just sugars; other components like amino acids, organic acids and hormones are included
• The translocation process is driven by pressure differences between source and sink regions inside the plant

Sources and Sinks in a Plant

• Source: The region where organic compounds (e.g., sugars) are generated (like the leaves from photosynthesis) and loaded into the phloem
• Sinks: The regions in the plant where sugars are unloaded / needed (e.g. growing roots, fruits seeds, buds)

Programmed Cell Death (PCD) in Sieve Tubes

• Sieve elements/sieve tube undergo a form of programmed cell death.
• this contributes to the phloem network function in plant physiology.

Plant Hormone Control

• Plant hormones such as auxins and gibberellins affect plant growth, development, and response to environmental stimuli.
• They can stimulate or inhibit growth in various plant parts

Leaf Anatomy and Photosynthesis

• Leaf anatomy is critical for efficient gas exchange
• Leaves have features (short cell distances to xylem + large surface area) that facilitate the efficient movement of these compounds for photosynthesis
• Leaf structure like stomata and the cuticle-covered epidermis play crucial roles

Stomata and Water Loss Regulation

• Stomata control water loss through transpiration and allow for CO2 uptake for photosynthesis.
• Stomata open and close in response to various environmental stimuli; these include signals like light, water, ABA.

Plant Defenses

• Plants have defenses against pathogens, insects, and other stressors
• These include physical and chemical deterrents.
• Methods can include using chemical substances or attracting insect allies to fight for them

Potato Blight

• This is a disease caused by Phytophthora infestans; which can decimate potato crops
• One million Irish people died, and nearly two million emigrated during the 1845-52 potato famine as a result
• The monoculture was a contributing factor to the severity of the disease

Systemic Acquired Resistance (SAR)

• A plant's developed immune response to pathogens
• The plant defends itself to protect from future attacks; signaling molecules stimulate this response
• A hypersensitive response (like a localized killing of infected cells) in the infected region signals to other parts of the plant and initiates SAR

Mechanisms of Sugar Loading

• Sugars are actively transported into the phloem (sieve tubes).
• Sucrose is the most usual form transported, and is mobilized by sieve-tube cells
• Mechanisms include apoplastic and symplastic routes; these involve transporting sugars to companion cells and from there via plasmodesmata into the sieve tubes

Pressure/Flow Mechanism

• The pressure-flow mechanism describes how phloem transport is driven by an osmotic gradient.
• More sucrose is loaded into the sieve tubes, decreasing their water potential in the phloem
• Water flows from the xylem into the phloem, increasing turgor pressure and driving phloem sap movement from source to sink régions