Energy, Environment, Microbiology and Immunity
Unit 4 moves into A2 Biology and connects energy transfer, photosynthesis, ecosystems, environmental change, microorganisms, disease, immunity and forensic techniques.
Topic 5 — Energy Flow, Ecosystems and the Environment
Topic 6 — Microbiology, Immunity and Forensics
What you need to master
- Photosynthesis
- ATP and energy transfer
- Energy flow through ecosystems
- Productivity
- Population and community ecology
- Sampling techniques
- Climate change
- Natural selection and speciation
- Microbial growth and culturing
- Pathogens and disease
- Innate and adaptive immunity
- Antibiotics and resistance
- Forensic DNA analysis
Photosynthesis
Photosynthesis converts light energy into chemical energy stored in organic molecules.
Light energy is required.
Chloroplast structure
Grana
Stacks of thylakoid membranes containing photosynthetic pigments and electron carriers.
Stroma
Fluid region containing enzymes involved in carbon fixation and the Calvin cycle.
Thylakoid membrane
Site of the light-dependent reactions.
Starch grain
Storage form of carbohydrate produced from photosynthetic products.
Light-dependent reactions
- Chlorophyll absorbs light energy.
- Electrons become excited.
- Water undergoes photolysis.
- Oxygen is released.
- ATP is produced by photophosphorylation.
- NADP is reduced to reduced NADP.
H₂O → 2H⁺ + 2e⁻ + ½O₂
Cyclic vs non-cyclic photophosphorylation
| Feature | Cyclic | Non-cyclic |
|---|---|---|
| ATP produced | Yes | Yes |
| Reduced NADP | No | Yes |
| Photolysis | No | Yes |
| Oxygen released | No | Yes |
Calvin cycle
The Calvin cycle uses ATP and reduced NADP from the light-dependent reactions to reduce carbon dioxide and produce carbohydrate.
Light-dependent reactions →
ATP + reduced NADP
Calvin cycle →
uses ATP + reduced NADP
ATP and Energy Transfer
ATP acts as an immediate source of energy for cellular processes.
Why ATP is useful
- Releases manageable amounts of energy.
- Can be rapidly regenerated.
- Couples energy-releasing reactions to energy-requiring processes.
Respiratory substrates
Carbohydrate
Readily available and commonly used respiratory substrate.
Lipid
High energy value per unit mass.
Protein
Can be used when other substrates are limited.
Energy values
Lipids contain more chemical energy per gram than carbohydrates or proteins.
Ecosystems and Energy Flow
Population
Members of the same species living in an area at the same time.
Community
Populations of different species living and interacting in an area.
Habitat
The place where an organism or population normally lives.
Ecosystem
A community and its abiotic environment interacting together.
Energy transfer
Energy enters most ecosystems as light and is converted into chemical energy by producers.
Why energy is lost
- Respiration releases energy as heat.
- Not all material is eaten.
- Some material is indigestible.
- Energy is lost through waste.
Productivity
NPP = GPP − R
where R = energy lost through respiration.
Sampling Populations
Quadrat sampling
Quadrats are used to investigate organisms that are relatively stationary, such as plants.
Random coordinates can be generated so that every area has an appropriate chance of being selected.
Transects
Transects can be used to investigate how the distribution of organisms changes along an environmental gradient.
Line transect
Records organisms touching or occurring along a line.
Belt transect
Uses a strip of defined width to sample organisms across a gradient.
Sampling reliability
- Increase sample size.
- Use random sampling where appropriate.
- Repeat sampling.
- Sample different areas.
Climate Change
Earth's climate changes naturally, but human activities have significantly altered atmospheric concentrations of greenhouse gases.
Greenhouse gases
- Carbon dioxide
- Methane
- Nitrous oxide
Major human sources
- Burning fossil fuels
- Deforestation
- Agriculture
- Industrial activity
Evidence
Evidence for climate change can include long-term temperature records, atmospheric carbon dioxide measurements, ice-core data, sea-level changes and changes in species distributions.
Natural Selection and Speciation
Natural selection
- Individuals in a population show variation.
- Some variation is heritable.
- There is competition for resources.
- Individuals with advantageous characteristics are more likely to survive and reproduce.
- Their alleles become more common in the population over generations.
Speciation
Speciation occurs when populations become reproductively isolated and diverge genetically.
Allopatric speciation
Geographic separation prevents gene flow between populations.
Sympatric speciation
Reproductive isolation develops without geographic separation.
Variation → selection pressure → differential reproductive success → allele-frequency change → divergence → reproductive isolation
Microbiology and Microbial Growth
Aseptic technique
Aseptic technique reduces contamination of cultures and prevents microorganisms from being transferred between cultures, people and the environment.
- Sterilise equipment where appropriate.
- Disinfect work surfaces.
- Minimise exposure of cultures.
- Use sterile apparatus.
- Follow appropriate laboratory safety procedures.
Microbial growth
Lag phase
Cells adapt to the new environment.
Log phase
Rapid cell division and exponential growth.
Stationary phase
Growth rate slows as resources become limiting.
Factors affecting growth
- Temperature
- pH
- Nutrient availability
- Oxygen availability
- Waste-product accumulation
Pathogens and Infectious Disease
Bacteria
Prokaryotic organisms. Some cause disease by producing toxins or damaging tissues.
Viruses
Acellular infectious particles that replicate inside host cells.
Fungi
Eukaryotic organisms that can cause infections in humans.
Protoctists
Some protoctists are pathogenic and can cause serious disease.
Transmission
- Direct contact
- Droplets
- Contaminated food or water
- Vectors
- Contaminated surfaces
The Immune Response
Non-specific defenses
- Skin
- Mucus
- Ciliated epithelial cells
- Stomach acid
- Lysozyme
- Phagocytic cells
Phagocytosis
- Phagocyte detects the pathogen.
- Pathogen is engulfed.
- A phagosome forms.
- Lysosomes fuse with the vesicle.
- Digestive enzymes destroy the pathogen.
Specific immunity
Lymphocytes recognise specific antigens and produce an immune response.
B lymphocytes
Produce antibodies and form memory cells.
T lymphocytes
Participate in cell-mediated immune responses and help coordinate immunity.
Antibodies
Antibodies have specific binding sites that recognise complementary antigens.
Vaccination
Vaccination exposes the immune system to an antigen so that memory cells can develop. On later exposure to the pathogen, the secondary immune response is faster and stronger.
Secondary response: Faster and larger due to memory cells.
Antibiotics and Resistance
Antibiotics target bacterial structures or processes and therefore do not directly treat viral infections.
Antibiotic resistance
- Random genetic variation produces a resistant bacterium.
- Antibiotic exposure kills susceptible bacteria.
- Resistant bacteria survive.
- Resistant bacteria reproduce.
- The resistance allele becomes more common.
Selected population: Resistant bacteria
Reducing resistance
- Use antibiotics only when appropriate.
- Avoid unnecessary prescriptions.
- Use appropriate treatment regimes.
- Develop new antimicrobial drugs.
- Reduce transmission of resistant organisms.
Microorganisms and Nutrient Cycling
Microorganisms play major roles as decomposers, breaking down organic material and returning nutrients to ecosystems.
Decomposition
- Microorganisms release extracellular enzymes.
- Large molecules are hydrolysed into smaller soluble molecules.
- Soluble molecules are absorbed.
- Microorganisms respire and grow.
- Carbon and mineral ions are returned to the environment.
DNA Analysis and Forensic Biology
DNA profiling
DNA profiling can compare DNA samples by analysing differences in DNA fragments.
Gel electrophoresis
DNA fragments can be separated according to size using an electric field through a gel.
DNA sample → fragments → electrical field → separation → banding pattern → comparison
Why do smaller fragments move further?
Smaller DNA fragments move more easily through the pores in the gel and therefore travel further in the same period of time.
Unit 4 Practical Skills
Core Practical 10
Investigate the effects of light intensity, wavelength, temperature and carbon dioxide availability on the rate of photosynthesis using a suitable aquatic plant.
Core Practical 11
Carry out an ecological study of a habitat, using techniques such as quadrats and transects.
Microbiology
Apply aseptic technique when culturing microorganisms and interpreting growth.
Forensic analysis
Understand how biological evidence such as DNA fragments can be separated and compared.
Know the independent variable, dependent variable, controls, apparatus, method, expected pattern, sources of error and realistic improvements.
Calculations and Data Analysis
Percentage change
Rate
Efficiency
Population estimate
Interpreting graphs
- Identify the overall trend.
- Identify anomalies.
- Compare gradients.
- Quote data to support conclusions.
- Distinguish correlation from causation.
Unit 4 Practice
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ATP and reduced NADP.
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Water undergoes photolysis during the light-dependent reactions, producing oxygen as a product.
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Energy is lost through respiration and released as heat. Some biomass is also uneaten or not assimilated.
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Plants are generally stationary, so quadrats provide a defined area in which abundance or distribution can be measured.
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Genetic variation produces resistant bacteria. Antibiotic treatment kills susceptible bacteria, while resistant bacteria survive and reproduce. Their resistance alleles therefore become more common in the population.
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Memory cells remain after the initial immune response and allow a faster and stronger response if the same antigen is encountered again.
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Smaller fragments pass more easily through the pores in the gel and therefore migrate further in the same time.
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Temperature, oxygen availability, moisture and pH are acceptable examples.
How to Attack a Unit 4 Question
Photosynthesis? Ecology? Immunity? Microbiology? Evolution?
State, describe, explain, calculate, compare, evaluate or suggest.
Avoid vague phrases such as "energy disappears." Say that energy is transferred and eventually dissipated as heat.
Especially in six-mark explanations, connect each biological step logically.
When a graph or table is provided, quote relevant values and describe patterns.