RADIOLABELLING & FIELD BIOLOGY

L14: Radiolabelling & Field Biology
CSIR-NET Life Sciences | Methods in Biology

Radiolabelling and field biology are important experimental approaches used to understand biological processes at the molecular, cellular, physiological and ecological levels. Radiolabelling uses radioactive isotopes as detectable tracers, whereas field biology involves studying organisms and their interactions under natural or semi-natural environmental conditions.

These notes cover the major topics shown in the L14 lecture:

• Introduction to radioactivity
• Detection and measurement of radiation
• Incorporation of radioisotopes into biological tissues
• Molecular imaging of radioactive materials
• Radiation safety guidelines
• Field biology and field-based experimental approaches

CSIR-NET GATE Biotechnology DBT-JRF ICMR-JRF Methods in Biology Biotechnology

📚 Index / Table of Contents

1. Introduction to Radioactivity
2. Radioisotopes and Isotopic Tracers
3. Radioactive Decay
4. Half-Life and Decay Kinetics
5. Types of Radiation
6. Detection and Measurement of Radiation
7. Geiger-Müller Counter
8. Scintillation Counting
9. Autoradiography
10. Incorporation of Radioisotopes in Biological Tissues
11. Radioisotopes as Biological Tracers
12. Molecular Imaging of Radioactive Materials
13. Radiation Safety Guidelines
14. Field Biology
15. Field Sampling and Experimental Design
16. Field Measurements
17. Laboratory Biology vs Field Biology
18. CSIR-NET Exam-Focused Points
19. 10 Practice MCQs

1. Introduction to Radioactivity

Definition: Radioactivity is the spontaneous disintegration of unstable atomic nuclei with the emission of radiation in the form of particles or electromagnetic energy.

• Atoms contain a nucleus composed mainly of protons and neutrons.
• The stability of a nucleus depends on factors such as the neutron-to-proton ratio and nuclear forces.
• An unstable nucleus undergoes radioactive decay to reach a more stable state.
• The parent radioactive nucleus transforms into a daughter nucleus.
• Radioactive decay is generally a spontaneous process.
• The exact decay of an individual radioactive atom is unpredictable.
• However, the behavior of a large population of radioactive atoms can be described statistically.
• Radioactive isotopes are useful in biological research because radiation can be detected even when the amount of labelled material is extremely small.

The major advantage of radioisotopes as biological tracers is that the radioactive label can follow a molecule through a biochemical pathway without necessarily requiring a large quantity of the molecule. For example, radioactive phosphorus can be used to study nucleic acid metabolism because phosphorus is an important component of nucleotides.

Basic concept of radioactive decay: Parent → Daughter + Radiation

2. Radioisotopes and Isotopic Tracers

• Isotopes are atoms of the same element having the same atomic number but different mass numbers.
• Stable isotopes do not undergo spontaneous radioactive decay.
• Radioisotopes are unstable isotopes that undergo radioactive decay.
• A radioactive isotope can be incorporated into a biological molecule as a tracer.
• The chemical behavior of an isotope is largely determined by its electronic structure.
• Therefore, isotopic substitution can allow researchers to follow metabolic pathways.

Common radioisotopes used in biological research

Radioisotope	Major biological application	Important feature
³H (Tritium)	DNA, RNA, protein and ligand labelling	Low-energy beta emitter
¹⁴C	Metabolic pathway and carbon tracing studies	Beta emitter
³²P	DNA/RNA labelling, phosphorylation studies	High-energy beta emitter
³⁵S	Protein labelling and cysteine/methionine studies	Beta emitter
¹²⁵I	Protein, peptide and receptor-related assays	Low-energy gamma/X-ray emissions associated with decay

Exam point: ³²P is particularly useful for nucleic-acid work because phosphate groups are present in nucleotides and nucleic acids. ³⁵S is useful for protein labelling because sulfur occurs in amino acids such as cysteine and methionine.

3. Radioactive Decay

Radioactive decay describes the transformation of an unstable nucleus into another nuclear state. The rate of radioactive decay depends on the number of radioactive atoms present.

-dN/dt = λN

• N = number of radioactive atoms remaining.
• t = time.
• λ = decay constant.
• The negative sign indicates that the number of radioactive atoms decreases with time.

The integrated decay equation is:

N = N₀e^-λt

where N₀ is the initial number of radioactive atoms.

Activity

• Activity represents the rate of radioactive decay.
• It is commonly represented by A.
• A = λN.
• The SI unit of activity is the becquerel (Bq).
• 1 Bq corresponds to one nuclear disintegration per second.
• The historical unit curie (Ci) is also used in some contexts.

4. Half-Life and Decay Kinetics

Half-life (t½) is the time required for half of the radioactive atoms in a sample to undergo decay.

t½ = 0.693 / λ

• After one half-life → 50% remains.
• After two half-lives → 25% remains.
• After three half-lives → 12.5% remains.
• After four half-lives → 6.25% remains.
• After five half-lives → 3.125% remains.

Number of half-lives	Fraction remaining	Percentage remaining
0	1	100%
1	1/2	50%
2	1/4	25%
3	1/8	12.5%
4	1/16	6.25%
5	1/32	3.125%

CSIR-NET shortcut: If 12.5% of the original radioactive material remains, three half-lives have passed.

5. Types of Radiation

Alpha radiation (α)

• Alpha particles consist of two protons and two neutrons.
• They are essentially helium nuclei.
• They have relatively high mass and charge.
• They have high ionizing ability.
• Their penetration ability is relatively low.
• They can generally be stopped by materials such as paper or the outer dead layer of skin.
• However, alpha-emitting substances can be hazardous if they enter the body.

Beta radiation (β)

• Beta radiation consists of high-energy electrons or positrons depending on the decay process.
• Beta particles have greater penetration than alpha particles.
• Many commonly used biological radioisotopes are beta emitters.
• ³H, ¹⁴C, ³²P and ³⁵S are important examples.

Gamma radiation (γ)

• Gamma radiation is electromagnetic radiation.
• It has no mass and no electrical charge.
• It has high penetration ability.
• Dense materials are commonly used for shielding gamma radiation.

Radiation	Nature	Charge	Penetration	Ionization
Alpha	Helium nucleus	+2	Low	High
Beta	Electron/positron	-1/+1	Moderate	Moderate
Gamma	Electromagnetic wave	0	High	Lower per interaction than alpha

6. Detection and Measurement of Radiation

Radioactive material cannot always be detected visually. Specialized instruments detect the ionizing radiation emitted during radioactive decay.

• Geiger-Müller counters are widely used for detecting ionizing radiation.
• Scintillation counters measure light generated when radiation interacts with a scintillating material.
• Gamma counters are designed for gamma-emitting samples.
• Autoradiography provides a spatial image of radioactivity in a biological sample.
• Dosimeters are used to monitor radiation exposure received by personnel.

7. Geiger-Müller Counter

• A Geiger-Müller counter contains a gas-filled detector tube.
• Incoming radiation ionizes the gas inside the tube.
• The resulting electrons and ions produce an electrical pulse.
• The instrument counts these pulses.
• It is useful for detecting the presence of radiation and monitoring contamination.
• A Geiger counter generally does not provide the same type of quantitative energy information as more sophisticated spectrometric instruments.

Remember: A Geiger counter is primarily a radiation detection/counting instrument. Do not confuse it with a spectrophotometer, which measures absorption of electromagnetic radiation by molecules.

8. Scintillation Counting

Scintillation counting is an important technique for measuring radioactivity, especially for low-energy beta emitters.

• A radioactive particle interacts with a scintillation medium.
• The interaction produces a small flash of light called a scintillation.
• The light is detected by a photodetector.
• The detected signal is converted into an electrical signal.
• The number of detected events can be related to radioactive activity.

Liquid scintillation counting

• The radioactive sample is mixed with a liquid scintillation cocktail.
• Energy released by radioactive decay produces light in the scintillation medium.
• The emitted light is detected by photomultiplier tubes.
• It is especially useful for low-energy beta emitters such as tritium.

9. Autoradiography

Autoradiography is a technique in which radiation emitted by a radioactive sample is used to produce an image showing the location or distribution of the radioactive material.

• A biological sample contains a radioactive label.
• The sample is positioned close to a radiation-sensitive detector or imaging medium.
• Radiation emitted from the sample exposes the imaging medium.
• After processing, a pattern representing the distribution of radioactivity is obtained.
• Autoradiography can be used to locate labelled DNA, RNA, proteins or metabolites.

10. Incorporation of Radioisotopes in Biological Tissues

A radioactive isotope becomes biologically useful when it can be incorporated into a molecule or metabolic pool without completely disrupting normal biological processes.

Important principles

• The isotope should be chemically compatible with the molecule being studied.
• The labelled compound should enter the appropriate metabolic pathway.
• The label should remain detectable for the duration of the experiment.
• The radioactive signal should be distinguishable from background radiation.
• The biological effect of the isotope and radiation must be considered.

Examples

• ³²P: useful for studying nucleic acids and phosphorylation.
• ³⁵S: useful for studying protein synthesis and sulfur-containing amino acids.
• ¹⁴C: useful for carbon metabolism and tracing carbon-containing compounds.
• ³H: useful in biochemical and receptor-binding studies.
• ¹²⁵I: widely used in certain immunoassays and molecular studies.

11. Radioisotopes as Biological Tracers

A tracer is a detectable form of a molecule that can be followed through a biological system. Radioactive tracers are especially powerful because their signal can be detected at very low concentrations.

• Radioactive tracers can be used to study metabolic pathways.
• They can reveal the movement of molecules between tissues.
• They can help determine precursor-product relationships.
• They can be used to measure rates of biochemical processes.
• They can be used to investigate receptor-ligand interactions.
• They can help determine the localization of nucleic acids and proteins.

Conceptual question: If a labelled precursor is converted into a labelled product, radioactivity in the product provides evidence that the precursor contributes to the biosynthetic pathway.

12. Molecular Imaging of Radioactive Materials

Molecular imaging aims to visualize the distribution, movement or activity of molecules inside biological systems.

• Radioactive tracers can be linked to molecules that participate in biological processes.
• The radioactive signal can then be detected by appropriate imaging systems.
• Autoradiography is particularly useful for spatial localization in tissue sections or biological samples.
• In medical imaging, radiotracers can be detected using specialized imaging technologies.
• Imaging can provide both spatial and, depending on the method, temporal information.

Important imaging concepts

• Spatial resolution: ability to distinguish two nearby structures.
• Sensitivity: ability to detect low levels of radioactivity.
• Specificity: ability to associate the signal with the intended biological target.
• Background: unwanted signal that reduces the ability to distinguish the target.

13. Radiation Safety Guidelines

Important: Radioisotope work must be performed only in appropriately authorized facilities and according to institutional radiation-safety procedures. The principles below are educational concepts, not a substitute for institutional rules or regulatory guidance.

Three fundamental principles

• Time: minimize the time spent near a radiation source.
• Distance: maximize distance from the radiation source whenever practical.
• Shielding: use appropriate shielding between the worker and the radiation source.

Good laboratory practices

• Follow approved institutional protocols.
• Use appropriate personal protective equipment.
• Prevent contamination of work surfaces and equipment.
• Clearly label radioactive materials according to institutional requirements.
• Use designated areas for radioactive work.
• Monitor work areas according to laboratory procedures.
• Dispose of radioactive waste through approved procedures.
• Never eat, drink or store food in a radioactive work area.
• Wash hands appropriately after completing work.
• Maintain accurate records of radioactive material according to institutional requirements.

14. Field Biology

Field biology is the study of organisms, populations, communities and ecological processes in their natural or naturally occurring environments.

Unlike highly controlled laboratory experiments, field biology attempts to understand biological phenomena under environmental conditions that may vary in space and time.

• Field biology can involve plants, animals, microorganisms and entire ecosystems.
• It can involve observation, sampling, experimentation and environmental monitoring.
• Field studies can provide information that cannot always be obtained from laboratory experiments.
• Environmental factors such as temperature, humidity, light, soil, water chemistry and nutrient availability may vary naturally.
• Field studies are particularly important in ecology, conservation biology, biodiversity studies, environmental biotechnology and wildlife biology.

15. Field Sampling and Experimental Design

Sampling

• Sampling is the process of selecting a representative subset from a larger population or area.
• A good sampling strategy should minimize bias.
• The sampling method should match the biological question.
• Sample size affects the statistical power and reliability of conclusions.
• Replicates are important for estimating natural variation.

Common field sampling approaches

• Random sampling: sampling units are selected randomly.
• Systematic sampling: samples are collected at predetermined intervals.
• Stratified sampling: the study area is divided into meaningful strata before sampling.
• Quadrat sampling: frequently used for plants or sessile organisms.
• Transect sampling: organisms or environmental variables are recorded along a line or belt.

Quadrat method

• A quadrat is a defined area used to sample organisms.
• It is particularly useful for plants and other organisms that do not move substantially.
• Quadrats can be placed randomly or systematically depending on the experimental design.
• Data may include abundance, frequency, density and percentage cover.

Transect method

• A transect is a line or defined path across a habitat.
• Observations are recorded at predetermined points or intervals.
• Transects are useful for studying changes in species distribution across environmental gradients.
• For example, vegetation may be studied along a gradient from dry to wet habitat.

16. Important Field Measurements

Field biology frequently combines biological observations with measurements of physical and chemical environmental parameters.

• Temperature
• pH
• Dissolved oxygen
• Light intensity
• Humidity
• Soil moisture
• Electrical conductivity
• Nutrient concentration
• Water quality parameters
• Species abundance and diversity

Field notes

• Record date and time of sampling.
• Record the sampling location using an appropriate location system.
• Record environmental conditions.
• Record sample identification codes.
• Record unusual observations.
• Use consistent measurement procedures.
• Maintain a clear field notebook or electronic record.

17. Field Experiment vs Field Observation

Field observation

• The investigator observes naturally occurring phenomena.
• Variables are not necessarily manipulated.
• It is useful for discovering patterns and ecological relationships.
• It can provide high ecological realism.

Field experiment

• The investigator deliberately manipulates one or more variables in the field.
• Experimental and control treatments can be compared.
• Replication is essential.
• Randomization can reduce systematic bias.
• Field experiments can provide stronger evidence for causal relationships than simple observations, provided the design is appropriate.

18. Laboratory Biology vs Field Biology

Feature	Laboratory study	Field study
Environmental control	Usually high	Usually lower
Natural conditions	May be simplified	Usually high
Experimental manipulation	Often easier	Can be more difficult
Environmental variability	Can often be controlled	Often substantial
Ecological realism	May be lower	Often higher
Replication	Generally easier	Can be logistically challenging

19. Integration of Radiolabelling and Biological Research

Radiolabelling and field biology represent different methodological approaches, but both can contribute to understanding biological processes.

• Radiolabelling provides a sensitive method for tracing molecules and biochemical processes.
• Field biology provides information about biological processes under real environmental conditions.
• Tracer studies can be used in ecological research to investigate nutrient movement and biological uptake.
• Radioactive tracers can help determine the movement of elements through biological systems when properly authorized and safely controlled.
• Field sampling can provide material for subsequent laboratory radiolabelling or molecular analysis.

20. CSIR-NET / GATE Exam-Focused Points

• Radioactivity is the spontaneous decay of unstable nuclei.
• Radioactive decay follows first-order kinetics.
• Decay constant is represented by λ.
• Activity is A = λN.
• Half-life is t½ = 0.693/λ.
• After one half-life, 50% radioactive material remains.
• After two half-lives, 25% remains.
• ³²P is an important isotope for nucleic-acid and phosphorylation studies.
• ³⁵S is useful for protein labelling involving sulfur-containing amino acids.
• ³H is a low-energy beta emitter.
• ¹⁴C is widely used for metabolic tracing.
• Alpha particles have high ionizing power but relatively low penetration.
• Gamma radiation has high penetrating ability.
• Geiger-Müller counters detect ionizing radiation through gas ionization.
• Scintillation counters detect light generated by radiation interactions with scintillating material.
• Liquid scintillation counting is particularly useful for low-energy beta emitters.
• Autoradiography gives spatial information about radioactive material.
• Time, distance and shielding are fundamental radiation-protection principles.
• Quadrats are commonly used for sampling plants and sessile organisms.
• Transects are useful for studying distribution across environmental gradients.
• Randomization helps reduce systematic sampling bias.
• Replication helps estimate variation and improves reliability.

21. Quick Revision Table

Term	Remember
Radioactivity	Spontaneous nuclear decay
Half-life	Time required for 50% of radioactive atoms to decay
Activity	Rate of radioactive disintegration
Becquerel	SI unit of activity
³²P	Nucleic acids / phosphorylation
³⁵S	Protein labelling
¹⁴C	Carbon metabolism
³H	Low-energy beta emitter
Geiger counter	Radiation detection/counting
Scintillation counter	Measures scintillation events
Autoradiography	Spatial localization of radioactivity
Quadrat	Area-based ecological sampling
Transect	Sampling along a line/gradient
Radiation safety	Time + Distance + Shielding

22. Practice MCQs – Radiolabelling & Field Biology

Instructions: Select one answer for each question and click Submit Answers. The correct answers will remain hidden until submission.

Q1. Which statement best describes radioactivity? A. Absorption of visible light by molecules B. Spontaneous transformation of an unstable atomic nucleus C. Replication of radioactive DNA D. Formation of proteins from amino acids

Q2. The half-life of a radioactive isotope is the time required for: A. All radioactive atoms to disappear B. The activity to become zero C. Half of the radioactive atoms to undergo decay D. The isotope to become twice as radioactive

Q3. Which isotope is especially useful for nucleic-acid labelling? A. ³²P B. ³⁵S C. ¹²C D. ¹H

Q4. Which instrument is based on gas ionization and electrical pulse detection? A. UV spectrophotometer B. Geiger-Müller counter C. Centrifuge D. PCR machine

Q5. Which technique provides a spatial image of radioactive material in a biological sample? A. Autoradiography B. SDS-PAGE C. Western blotting D. ELISA

Q6. Which combination represents the basic radiation-protection principles? A. Temperature, pH and pressure B. Time, distance and shielding C. Speed, mass and volume D. Concentration, pH and temperature

Q7. Which radioisotope is commonly associated with protein labelling through sulfur-containing amino acids? A. ³⁵S B. ³²P C. ¹⁴C D. ¹⁶O

Q8. Which field-sampling method uses a defined area for sampling organisms? A. Quadrat B. Western blot C. PCR D. Chromatography

Q9. Which statement about radioactive decay is correct? A. Radioactive decay always stops after exactly one half-life B. The decay constant is related to the probability of radioactive decay B. Radioactive decay is controlled by environmental temperature D. Half-life increases every time an atom decays

Q10. Transect sampling is particularly useful for: A. Studying distribution across an environmental gradient B. Measuring DNA melting temperature only C. Protein sequencing D. Measuring enzyme molecular weight

23. One-Minute Final Revision

• Radioactivity: spontaneous decay of unstable nuclei.
• Activity: rate of radioactive disintegration.
• SI unit of activity: becquerel (Bq).
• Half-life: t½ = 0.693/λ.
• ³²P: nucleic acids and phosphorylation.
• ³⁵S: protein labelling.
• ¹⁴C: carbon metabolic tracing.
• ³H: low-energy beta emitter.
• Geiger counter: radiation detection.
• Scintillation counter: detects scintillation events.
• Autoradiography: spatial localization of radioactivity.
• Radiation safety: time, distance and shielding.
• Quadrat: defined area for ecological sampling.
• Transect: sampling along a line or environmental gradient.
• Field biology: study of organisms and biological processes under natural conditions.
• Randomization: reduces systematic bias.
• Replication: helps estimate natural and experimental variation.

🎯 CSIR-NET Memory Trick

“32P = Phosphate, 35S = Sulfur/Protein, 14C = Carbon, 3H = Hydrogen.”

Radiation safety = “TDS” → Time + Distance + Shielding.

Field sampling = “QT” → Quadrat + Transect.

End of L14 Notes – Radiolabelling & Field Biology
CSIR-NET Life Sciences | Methods in Biology