Edexcel Biology A-level Notes - Topic 5: On the Wild Side
Ecosystems and Succession
Ecosystem: Defined as all organisms living in a particular area, known as the community, alongside all the nonliving elements of that environment, such as climate and nutrient cycles.
Community: Refers to all populations of all organisms living in a particular habitat at a particular time.
Population: Defined as all organisms of a particular species living in a specific habitat at a specific time.
Habitat: The physical place where an organism lives.
Factors Influencing Distribution and Abundance of Organisms:
Biotic Factors: Involves living influences like predators, food availability, parasitism, or disease.
Abiotic Factors: Involves non-living influences such as light, oxygen, moisture levels, and temperature.
Ecological Niche:
Every species has a specific role within its habitat, consisting of both biotic and abiotic interactions with the environment.
Species distribution and abundance are often limited by the number and type of ecological niches present in the habitat.
According to the niche concept, only one species can occupy each niche in a given habitat at a specific time. If two or more species share overlapping niches, the best-adapted species will outcompete others in survival and reproduction.
Succession: The gradual change in species inhabiting an area over time, initiated by changes to the environment caused by the organisms colonizing it.
Primary Succession: Occurs in areas previously devoid of life (e.g., after a volcanic eruption) where soil must be developed before complex organisms can grow.
Secondary Succession: Takes place in areas with existing soil but cleared vegetation (e.g., following a forest fire).
Pioneer Species: The first organisms to colonize disturbed areas, such as lichens, are capable of surviving in harsh conditions.
For example, pioneer species can penetrate rock surfaces and break them down into soil particles, while their roots stabilize loose, shifting sands.
Decomposition and Soil Formation:
As organisms die, decomposers contribute to the formation of humus, which enriches the soil.
Richer soils support a wider variety of larger and more productive organisms, ultimately leading to the establishment of a climax community, characterized by high productivity, self-sustainability, and stability, usually represented by a few dominant species.
Photosynthesis
Stages of Photosynthesis:
Light-dependent Reaction:
Occurs in the thylakoid membranes of chloroplasts where chlorophyll molecules trap light energy.
Photosystems:
Photosystem I (PSI) absorbs light at a wavelength of 700 nm.
Photosystem II (PSII) absorbs light at a wavelength of 680 nm.
Electrons are excited, passed down an electron transport chain, and ATP is generated from ADP and inorganic phosphate through photophosphorylation. The process can be cyclic or non-cyclic.
Final Electron Acceptor: NADP; when it accepts an electron, it forms reduced NADP.
Cyclic Photophosphorylation:
Photon energizes electrons in PSI.
Excited electrons are taken up by an electron acceptor.
Electrons are passed along the chain, releasing energy for ATP synthesis.
Electrons return to PSI.
Non-Cyclic Photophosphorylation:
Photon energizes electrons in PSII, exciting them.
Electrons are taken up by an acceptor and passed to PSI, leading to ATP synthesis.
Photolysis: Light energy splits water molecules into hydrogen and hydroxide ions, replacing electrons lost by PSII and releasing oxygen as a byproduct.
Electrons from PSI are again taken up and passed to NADP, which forms reduced NADP when coupled with H+ ions from water dissociation.
Light-independent Reaction (Calvin Cycle):
Utilizes ATP and reduced NADP to synthesize glucose.
Process:
RuBP (a 5-carbon compound) combines with CO2, catalyzed by the enzyme RUBISCO, forming an unstable 6-carbon intermediate that breaks into two glycerate-3-phosphate (GP) molecules.
GPs are reduced to GALP using ATP and reduced NADP.
One in six GALP molecules will form glucose, which is then transformed into essential organic compounds (e.g., polysaccharides, lipids).
The remaining GALP molecules are used to regenerate RuBP with the assistance of ATP.
Chloroplast Structure:
Grana: Stacks of thylakoid membranes housing chlorophyll pigments, the site of light-dependent reactions.
Stroma: The fluid surrounding grana containing enzymes for light-independent reactions.
Chloroplast Envelope: A double membrane controlling substance movement; supports the endosymbiotic theory.
Starch Granules: They store products derived from photosynthesis.
Energy Transfers
Net Primary Productivity (NPP): The rate at which solar energy is converted into the organic molecules constituting new plant biomass.
Gross Primary Productivity (GPP): The total energy acquired by primary producers, with the relationship defined as:
NPP = GPP - RWhere R represents energy lost through respiration.
Energy Loss in Trophic Levels: Common reasons include:
Undigested matter.
Respiration, which transfers thermal energy to surroundings (an exothermic reaction).
Metabolic waste products (e.g., urea).
Climate Change
Global Warming: Refers to the gradual increase in Earth's average atmospheric and surface temperatures, believed to enact permanent climate changes.
Evidence of Climate Change:
Increasing carbon dioxide levels in the atmosphere (contributing to the greenhouse effect).
Temperature records indicating changes in Earth's climate.
Pollen analysis from peat bogs to infer historical vegetation.
Dendrochronology: The study of tree rings, where size variations reflect past temperatures.
Extrapolated data assists in predicting future climate changes but has limitations in excluding variables like human efforts to reduce greenhouse gases.
Greenhouse Effect:
Short-wavelength UV radiation penetrates the atmosphere and reflects off the Earth's surface as longer wavelength infrared radiation.
Greenhouse gases (e.g., CO2 and methane) trap this longer wavelength radiation, raising surface and atmospheric temperatures.
Effects of Climate Change:
Changes in rainfall patterns and seasonal cycles may lead to:
Altered species distributions as species migrate towards cooler regions (e.g., northward), risking extinction through competition.
Temperature-sensitive sex determination in reptiles could skew sex ratios, posing extinction risks.
Disruption of life cycles and enzyme activities as increased temperatures initially heighten reaction rates due to increased kinetic energy, but subsequently lead to enzyme denaturation above
optimumtemperatures.
Carbon Cycle Knowledge: Can inform strategies to mitigate greenhouse gas levels.
Reduction efforts can include:
Utilizing plants as biofuels, which are deemed carbon neutral since the carbon emitted during combustion balances with the carbon absorbed during growth.
Reforestation to enhance carbon capture through photosynthesis.
Scientific Consensus on Climate Change: Often influenced by the backgrounds and goals of those concluding research; peer-reviewed publications and scientific conferences validate ideas and conclusions.
Natural Selection and Evolution
Evolution: The change in heritable traits of biological populations over successive generations. Driven by variations in allele frequencies influenced by fluctuating selection pressures.
The process of evolution through natural selection includes:
A range of phenotypes exists within a population due to random mutations.
An environmental change occurs altering selection pressures.
Individuals with advantageous alleles possess a selective advantage, improving survival and reproductive chances.
These advantageous alleles get passed to the next generation, subsequently altering the allele frequency within the population.
Speciation: The formation of new species from populations that become reproductively isolated, reducing or halting gene flow.
Speciation occurs when genetic differences accumulate, resulting in populations that can no longer interbreed to create fertile offspring.
Can be further categorized into:
Allopatric Speciation: Geographic isolation leads to speciation.
Sympatric Speciation: Non-geographic factors such as temporal, behavioral, or gametic isolation cause speciation.