Unit 1: Environment- Physical Characteristics of the Terrestrial Environment

The Terrestrial Environment: Physical Characteristics

Introduction

• This lecture focuses on the physical characteristics of the terrestrial environment and the adaptations organisms have developed to live on land.
• It contrasts with the aquatic environment discussed earlier.
• The primary challenge of terrestrial life is the limited availability of water.

The Problem of Drying Out

• The central issue for terrestrial organisms is avoiding desiccation or drying out.
• This applies to plants, animals, and other organisms.
• The lecture will discuss how organisms prevent water loss, the problems they face, and their solutions.

Water Loss in Animals

• Animals primarily lose water through two surfaces:
  • Outer surface (skin)
  • Respiratory surfaces (lungs)
• Respiratory surfaces need to be wet for gas exchange (oxygen in, carbon dioxide out).
• Breathing with lungs leads to moisture loss during exhalation, posing a challenge.

Strategies to Avoid Water Loss in Animals

• Behavioral Adaptations:
  • Avoiding hot, dry parts of the day (middle of the day).
  • Seeking shade, burrows, or forests with leaf cover.
  • These locations are cooler, reducing moisture loss.
• Warm air causes faster evaporation compared to cool air.
• Desert organisms are active early in the morning, at dusk, or at night to conserve moisture.

Obtaining Water

• Eating wet foods and drinking water.
• Metabolic Water:
  • Some organisms, like kangaroo rats, can create water through metabolic processes.
  • They oxidize dry food items like seeds.
  • They break down fats and proteins to obtain hydrogen and oxygen, which they combine to create water ($H_2O$).
  • This allows them to survive in environments without readily available water sources.

Concentrating Waste Products

• Efficient water balance involves concentrating waste products (urine and feces) to extract water.
• Kangaroo rats produce extremely dry feces and highly concentrated urine.
  • Feces are almost 150 times drier than those of a typical lab rat.
  • Urine is 10 to 20 times more concentrated.

Boundary Layer

• The boundary layer is central to minimizing water loss through the outer body surface.
• It's the space between the skin and the outside air.
  • Without a boundary layer, wind velocity decreases linearly towards the skin surface, reaching zero at the skin.
  • Water evaporates from the skin surface at a rate proportional to the dryness of the outside air.
  • Moving air carries away water molecules accumulating near the skin, making the area drier and increasing evaporation.
• Hair, scales, and feathers disrupt wind flow, creating a boundary layer.
  • Wind velocity is reduced more quickly, allowing water molecules to build up.
  • This reduces the difference in water concentration inside and outside the body.
  • Water loss is slowed down.
• Smooth scales (e.g., on Tegu lizards) create a tight boundary layer.
  • They're dry, contrary to common misconceptions about reptiles.
  • Air and gases move in and out of the boundary layer more easily than water.

Insulation

• Boundary layers provide both insulation and moisture retention.
• In the summer, birds lay their feathers flat to hold onto moisture.
• In the winter, birds puff up their feathers to increase the boundary layer thickness and retain heat.

Water Reclamation in the Lungs

• Lungs contribute significantly to water loss.
• Mammals (e.g., camels), some reptiles, and many birds have a water reclamation process involving the sinuses.
• Sinuses have a wavy structure that increases surface area.
• Inhalation:
  • Dry air passes across the wavy structure, picking up moisture and body temperature.
  • Cooling hot air and warming cool air before reaching the lungs.
• Exhalation:
  • Damp breath passes across a different set of turbinates, causing cooling.
  • Cooled air condenses water, which sticks to the surfaces.
  • Drainage systems return the water to the body.
• Camels can reclaim about 90% of the water they would otherwise lose through their sinuses.
• Example: A runny nose on a cold day is often due to water condensing in the sinuses.

Water Loss in Plants

• Plants also face the problem of water loss on land.
• They need to lose some water for proper function (water moves from roots to leaves for photosynthesis and sugar transport).
• However, they need to regulate the amount and location of water loss.

Plant Adaptations to Prevent Water Loss

• Thick, Waxy Cuticle:
  • Covers the leaf surface.
  • Prevents moisture loss.
  • Protects against harsh sunlight.
  • The top cuticle is thicker than the bottom cuticle.
• Boundary Layers:
  • Formed by structures like trichomes (spiky structures on the leaf surface).
  • Trichomes slow down airflow across the leaf surface, reducing water loss.
  • Trichomes can be physical or chemical.
• Stomata:
  • Openings on the undersides of leaves for CO2 intake and water evaporation.
  • Plants can open and close their stomata.
  • Open stomata for photosynthesis and CO2 intake.
  • Close stomata to prevent excess water loss, especially at night.
  • Plants in wetter areas have more exposed stomata, while those in drier areas have stomata flush with the leaf surface to minimize moisture loss.

Photosynthesis Adaptations

• C-3, C-4, and CAM plants are different ways of performing photosynthesis to maximize water efficiency.
• CAM plants collect $CO_2$ at night and close their stomata during the day to prevent water loss while still performing photosynthesis.

Buoyancy

• Buoyancy is another challenge for terrestrial organisms compared to aquatic organisms.
• Water is dense, providing buoyancy.
• Air is much less dense, requiring organisms to support themselves.
• Aquatic mammals have reduced bone structure compared to terrestrial mammals.
• Aquatic plants have less biomass than terrestrial plants since they float easily in water.
• Trees have extensive structures to support their mass and reach the canopy for photosynthesis.

Light Intensity

• Terrestrial ecosystems have high light intensity, but not all light is used for photosynthesis.
• Scientists refer to Photosynthetic Active Radiation (PAR), which is the sunlight plants use for photosynthesis.
• PAR is similar to the visible part of the spectrum.

Photosynthetic Efficiency

• Most terrestrial ecosystems absorb sunlight through plant structures before it hits the soil.
• Photosynthetic efficiency is the percentage of light that strikes a plant and is used for photosynthesis.
• This percentage is very low, typically between 0.3% and 10%, with most studies showing around 1%.
• Reasons for low efficiency:
  • Not all sunlight is PAR (some is UV, gamma rays, or infrared).
  • Not all photons strike chloroplasts.
  • Environmental conditions may not be appropriate for photosynthesis.
• In forest ecosystems:
  • About 79% of incoming PAR is absorbed by the canopy.
  • 7% by mid-story plants.
  • Only 2% reaches the forest floor.

Forest Structure and Light Penetration

• Tropical rainforests are relatively barren on the forest floor due to the lack of light.
• Thick ground cover is found near the edges of the forest where there is exposed sunlight.
• Leaf Area Index (LAI) is the ratio of the surface area of leaves above relative to the surface below.
• A healthy ecosystem usually has an LAI greater than two, as leaves can overlap and still photosynthesize.

Life in the Shade

• Life in the shade involves tolerating low levels of light and being able to photosynthesize.
• Compensation intensity is the value where photosynthesis and respiration are equal.
• At compensation intensity, the production of carbohydrates equals the breakdown of carbohydrates by the plant.

Sun vs. Shade Leaves

• Sun leaves are smaller and thicker, adapted for maximum sunlight exposure.
• Shade leaves are broader and thinner, designed to capture the relatively few photons available.
• Some species have both sun and shade leaves.
• Oak trees may have broader leaves lower down and smaller leaves higher up due to different light conditions.

Shade Tolerance

• Shade-tolerant species can handle a good deal of shade and tend to have more accessory pigments, which allow them to capture a broader range of light.
• They also have a lower compensation intensity, meaning they can photosynthesize at lower light levels.
• Shade-intolerant species require higher light intensity for photosynthesis and have a higher maximum rate of photosynthesis.
• Shade-tolerant plants also tend to have lower respiration rates, as they have evolved to use minimal energy in low-light conditions.

Graph Explanation

• Two lines, A and B, represent different kinds of plants.
• The graph shows light intensity on the x-axis.
• The y-axis shows net carbon dioxide production or absorption, which measures the balance of photosynthesis and respiration.
• Zero represents compensation intensity.
• The positive side indicates photosynthesis is greater than respiration.
• The negative side indicates respiration is greater.
• Saturation intensity is the amount of light at which a plant will not photosynthesize more, even with more light.

Conclusion

• This concludes the lecture on the terrestrial environment.
• The next lecture will cover climate change.
• This also concludes the Unit One lectures.