6.4A UV radiation Notes

The Role of the Sun's Electromagnetic Radiation in the Biosphere

1. The Sun emits electromagnetic (EM) radiation across a continuous spectrum, ranging from low-frequency radio waves to high-frequency gamma rays.
2. Different wavelengths carry different levels of energy, which determines their biological significance and potential to cause harm.
3. The portion of solar radiation reaching Earth depends on atmospheric absorption and reflection.

Types of Solar Radiation Relevant to the Biosphere

1. Infrared radiation warms the Earth’s surface and atmosphere, helping to maintain global temperatures suitable for life.
2. Visible light supports photosynthesis and therefore determines the productivity of terrestrial and aquatic ecosystems.
3. Ultraviolet (UV) radiation can be damaging to living organisms due to its high energy.

UV Categories and Biological Risk

1. UV radiation has shorter wavelengths and higher frequencies, which means it carries more energy than visible or infrared radiation.
2. The higher the frequency, the greater the potential to damage DNA, cells and tissues in living organisms.
3. The UV spectrum is subdivided into three biologically relevant types:

Stratospheric Ozone as a Protective Layer

1. The ozone layer exists in the stratosphere, a region 16–35 km above the Earth’s surface.
2. It plays a crucial role in protecting life by absorbing nearly all UVC radiation and most UVB radiation, which would otherwise reach the surface at dangerous intensities.
3. Ozone is continually created and destroyed naturally, and its concentration varies geographically and seasonally.

Natural Formation and Maintenance of Ozone

1. Ozone forms when oxygen molecules (O₂) are split by solar UV radiation into single oxygen atoms, which then combine with other O₂ molecules to form ozone (O₃).
2. Ozone is constantly broken down and regenerated, creating a natural equilibrium that ensures harmful UV radiation remains controlled.

Biological and Ecological Effects of UV Radiation

Human Health Effects

1. Sunburn results from cell and tissue inflammation after excessive UV exposure.
2. UV radiation triggers mutations in DNA, increasing the risk of skin cancers.
3. Continual UV exposure breaks down collagen and elastin, causing premature wrinkles and skin ageing.
4. UV radiation can damage proteins in the eye, leading to cataracts and long-term vision impairment.
5. UV exposure can weaken the immune system, making individuals more vulnerable to infections and other diseases.

Effects on Terrestrial Productivity

1. High UV levels reduce photosynthetic efficiency in plants, causing lower biomass production.
2. UV exposure disrupts plant metabolism, which can reduce crop yield and growth rates.
3. Increased UV levels in farming regions can reduce food security, especially where crops are already stressed by drought or nutrient limitations.

Effects on Aquatic Productivity

1. Phytoplankton are highly sensitive to UV radiation, particularly UVB.
2. UV radiation:
   1. Damages DNA and cellular proteins in phytoplankton
   2. Reduces photosynthetic rates and growth
   3. Decreases primary productivity in oceans and freshwater ecosystems
3. Reduced phytoplankton biomass impacts the entire aquatic food web:
   1. Zooplankton have less food available
   2. Fish populations decline
   3. Higher predators including mammals and sea birds lose food sources
4. UV effects in regions with ozone thinning (such as Antarctica) have been linked to significant declines in phytoplankton productivity.

Wider Environmental Impacts

1. Decreased plant and phytoplankton productivity reduces global carbon sequestration, worsening climate change.
2. Reduced vegetation increases soil erosion and nutrient loss.
3. UV impacts reduce biodiversity, particularly in ecosystems already stressed by pollution, drought or warming.