Ecology: Energy and Photosynthesis

Types of Energy

• Radiant energy refers to sunlight.
• Chemical energy is stored in the bonds of food molecules.
• Kinetic energy is the movement of molecules and is measured as temperature.

The Importance of Light

• Light is the energy source for photosynthesis in all ecosystems.
• Light provides a cue that environmental conditions are changing.

Light and Photosynthesis

• PAR (photosynthetically active radiation) is the solar radiation wavelengths providing energy for photosynthesis.
• PAR includes all visible light wavelengths, except green.
• Light response curves are graphs showing the relationship between light levels and photosynthesis rate.
• The compensation point is the minimum PAR required for positive net photosynthesis.
• The compensation point is when CO2 uptake for photosynthesis is balanced by CO2 loss from cellular respiration.
• The saturation point is the maximum PAR that plants can use for increased rates of photosynthesis.
• Photoinhibition is reduced photosynthesis rates at very high light levels due to damage caused by excessive solar radiation.
• Too much light is generally not a problem for most plants.

Shade-Tolerant Plants

• Shade-tolerant plants can maintain positive net photosynthesis even in very low light conditions.
• These plants are found in forest understories where most light is blocked by the canopy.
• Adaptations of shade-tolerant plants include a low compensation point and a low saturation point.
• Shade-tolerant plants are susceptible to photoinhibition.
• Mechanisms of shade tolerance include allocation of more resources to the light reactions than to the Calvin cycle
• There is also allocation of more resources to the leaves than to the stems or roots.
• Shade-tolerant plants will typically have thinner but wider leaves.

Photosynthetic Response to Temperature

• Graphs show how photosynthetic rates vary with tissue temperature for different plant species.
• Growth temperature influences the photosynthetic rates of plants from desert and coastal populations.

Effects of Oxygen Concentration on Photosynthesis

• Graphs depict how net CO2 uptake varies with O2 concentration at different CO2 levels.

Photorespiration

• Graphs show the electron transport capacity over time under enhanced, control, and reduced photorespiration conditions.

Circadian Rhythms

• Circadian rhythms are daily patterns of activity corresponding to a 24-hour cycle of light and dark.
• These rhythms are best demonstrated by diurnal and nocturnal organisms.
• The adaptive value of circadian rhythms can be physical or biological.
• Physical adaptations include insects being diurnal to "soak up the sun" as ectotherms.
• Biological adaptations include plants that are diurnal opening flowers during the day when their pollinating insects are active.
• Rabbits are nocturnal to avoid the heat, which is a physical adaption.
• Owls are nocturnal because their prey are active at night which is a biological adaptation.
• Photoperiodism is a response to changing day lengths throughout the year, not seen in equatorial organisms.
• Changes in light levels are the most reliable cues of upcoming seasonal changes.

How Organisms Obtain Nutrients

• Autotrophs synthesize their organic nutrients by photosynthesis or chemosynthesis.
• Heterotrophs must consume autotrophs or other heterotrophs to obtain their organic nutrients.

How Organisms Obtain Energy

• Energy comes from organic nutrients through glycolysis, the citric acid cycle, and the electron transport chain.
• Glycolysis nets 2 ATP, citric acid cycle nets an addition 2 ATP, and the electron transport chain nets 28 ATP per glucose.
• This results in a total of 32 ATP per glucose.

How Plants Obtain Inorganic Nutrients

• Nutrients are taken up from the soil.
• Soil particles are classified by size as sand (largest), silt, and clay (smallest).
• Soil also contains humus, which is decaying organic matter.
• Optimal soil particles: sand has large pore spaces, which leads to water and minerals being leached/drained
• Large pore spaces in sand do enable efficient diffusion of oxygen to roots.
• Silt and clay have small pore spaces, that allow availability of water and minerals to plant roots.
• Silt and clay's small pore spaces cause water to drains slowly, saturating the soil, depleting oxygen.
• The most productive soil contains an approximately equal mixture of sand, silt, and clay, termed loam.
• Optimal soil humus: it is the main source of mineral nutrients in the soil.
• Humus provides an increased surface area for the adhesion of water and minerals.
• Optimal soil organisms: live organisms are necessary for decomposition and soil mixing by various species
• Bacteria, fungi, arthropods, worms, and the plant roots themselves live within and shape the soil.
• Optimal soil water: a water deficiency is harmful
• Minerals are dissolved in solution.
• Very dry soil therefore leads to a lack of both water and nutrients.
• Optimal soil pH: soil particles are negatively charged.
• The majority of a plant's essential nutrients are cations, which adhere to negatively charged particles.
• Acidic soil has a high hydrogen ion (H+) concentration while hydrogen ions adhere to soil particles, displacing essential cations.
• Cations are then leached through the soil and become unavailable to plant roots.
• Neutral or slightly basic soil is the optimal pH for plant nutrient acquisition.
• Plants increase the surface area of their roots for increased nutrient absorption by way of millions of tiny root hairs.
• Plant roots are also involved in mutualistic relationships with mycorrhizae and nitrogen-fixing bacteria.
• Mycorrhizae: mutualism between plant roots and fungi; fungi wrap around plant roots, increasing the surface area for nutrient contact and absorption.
• Fungi receive photosynthates from plants.
• Nitrogen-fixing bacteria: bacteria that live mutualistically within root nodules, converting nitrogen gas (N2) into usable forms of nitrogen.
• These bacteria are present only in legumes, receiving photosynthates and shelter from the plant.
• Parasitic plants obtain organic and inorganic nutrients from a host plant, harming the host.
• Hemiparasites are parasitic plants capable of photosynthesis.
• Holoparasites are parasitic plants cannot photosynthesize.
• Carnivorous plants are found in nitrogen-poor soil and supplement their nitrogen supply by digesting insects/small animals.

How Animals Obtain Nutrients

• Animals take in nutrients by ingesting other organisms (though oxygen is obtained from the atmosphere).
• Herbivores eat plants or algae.
• Carnivores eat other animals.
• Omnivores eat both plants and animals.

Advantages of Herbivory

• More energy is available to herbivores feeding lower on food chains.
• Herbivores expend significantly less energy obtaining food because they do not need to hunt.
• Challenges of herbivory are chemical defenses (toxins) and physical defenses (spines, thorns).
• Plants have a high carbon-to-nitrogen ratio, while animals have a low one.
• Herbivores must eat lots of plants to obtain sufficient nitrogen.
• Plant proteins are "incomplete", lacking one or more of an animal's essential amino acids.
• Sodium is an essential nutrient for animals but not for plants.
• Herbivores must supplement their diets with natural salt licks.
• Plants contain cellulose and other indigestible material.
• Mutualistic microorganisms (bacteria, protozoans) within an herbivore's digestive tract break down cellulose.
• This occurs in ruminants with a four-chambered stomach, and in rabbits via coprophagy.