Summary Edexcel Biology A Level (Salters Nuffield A) Topic 6 - Immunity, Infection & Forensics - Full Notes
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Topic 6 (9BN001)
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Detailed and comprehensive notes on topic 6 (immunity, infection & forensics) of Edexcel biology A Level. Covers estimating time of death, DNA profiling, bacteria, viruses, barriers to infection, the immune response, immunity, post-transcriptional modifications, the evolutionary race, antibiotics, ...
Forensics .............................................................................................................................................. 3-7
Estimating Time of Death ........................................................................................................................................................................... 3-5
Body Temperature .............................................................................................................................................................................................. 3
Degree of Muscle Contraction ........................................................................................................................................................................ 3
Extent of Decomposition .................................................................................................................................................................................. 4
Forensic Entemology.......................................................................................................................................................................................... 5
Stage of Succession ............................................................................................................................................................................................. 5
DNA Profiling ................................................................................................................................................................................................... 6-7
Polymerase Chain Reaction.................................................................................................................................................................... 6-7
Gel Electrophoresis ........................................................................................................................................................................................ 7
Core Practical 14 ............................................................................................................................................................................................ 7
Bacteria and Viruses ............................................................................................................................8-14
Bacteria ............................................................................................................................................................................................................... 8-9
Viruses .......................................................................................................................................................................................................... 10-11
HIV .................................................................................................................................................................................................................. 12-13
Tuberculosis ...................................................................................................................................................................................................... 14
Infection ............................................................................................................................................ 15-19
Pathogens and Barriers to Infection ....................................................................................................................................................... 15
The Immune Response ................................................................................................................................................................................. 16
The Non-Specific Immune Response ...................................................................................................................................................... 17
The Specific Immune Response ......................................................................................................................................................... 18-19
,Specification
6.1 Understand how to determine the time of death of a mammal by examining the extent of
decomposition, stage of succession, forensic entomology, body temperature and degree of muscle
contraction. (P3-5)
6.2 Know the role of microorganisms in the decomposition of organic matter and the recycling of
carbon. (P4)
6.3 Know how DNA profiling is used for identification and determining genetic relationships between
organisms (plants and animals). (P6)
6.4 Know how DNA can be amplified using the polymerase chain reaction (PCR). (P6-7)
6.5 Be able to compare the structure of bacteria and viruses. (P8-10)
6.6 Understand how Mycobacterium tuberculosis (TB) and Human Immunodeficiency Virus (HIV)
infect human cells, causing a sequence of symptoms that may result in death. (P12-14)
6.7 Understand the non-specific responses of the body to infection, including inflammation, lysozyme
action, interferon, and phagocytosis. (P17)
6.8 Understand the roles of antigens and antibodies in the body’s immune response including the
involvement of plasma cells, macrophages and antigen-presenting cells. (P19)
6.9 Understand the differences between the roles of B cells (B memory and B effector cells) and T cells
(T helper, T killer and T memory cells) in the body’s immune response. (P18)
6.10 Understand how one gene can give rise to more than one protein through post- transcriptional
changes to messenger RNA (mRNA). (P21)
6.11 i) Know the major routes pathogens may take when entering the body. (P15)
ii) Understand the role of barriers in protecting the body from infection, including skin, stomach
acid, and gut and skin flora. (P15)
6.12 Understand how individuals may develop immunity (natural, artificial, active, passive). (P20)
6.13 Understand how the theory of an ‘evolutionary race’ between pathogens and their hosts is supported
by the evasion mechanisms shown by pathogens. (P22)
6.14 Understand the difference between bacteriostatic and bactericidal antibiotics. (P22)
6.15 Know how an understanding of the contributory causes of hospital acquired infections have led to
codes of practice regarding antibiotic prescription and hospital practice that relate to infection
prevention and control. (P24)
CORE PRACTICAL 14:
Use gel electrophoresis to separate DNA fragments of different length. (P7)
CORE PRACTICAL 15:
Investigate the effect of different antibiotics on bacteria. (P23)
2
,Estimating Time of Death
6.1 Understand how to determine the time of death of a mammal by examining the extent of
decomposition, stage of succession, forensic entomology, body temperature and degree of muscle
contraction.
Post-Mortem Stages of Death
1. Pallor Mortis – Paleness of the Skin
2. Algor Mortis – Cooling of the Body
3. Rigor Mortis – Stiffening of Body Muscles
4. Livor Mortis – Settling of the Blood
Body Temperature
• All mammals produce heat from metabolic reactions, e.g. respiration.
• The human body has a core body temperature of around 37°C.
• From the time of death, metabolic reactions slow down and eventually stop, causing the body
temperature to fall until it equals the ambient temperature (algor mortis).
• Core body temperature decreases in a stereotypical way, so forensic scientists can estimate the time
of death based on how much the body has cooled.
• Various conditions may increase the rate of cooling of the body:
o Body Size – higher surface area: volume ratio.
o Body Position – higher surface area exposed to air.
o Clothing – more (insulating) clothing.
o Air Movement – greater air movement.
o Humidity – higher humidity.
o Injury – greater extent of injury.
o Ambient Temperature – lower ambient temperature relative to body.
Degree of Muscle Contraction
• After death, the muscles in the body start to contract and become stiff (rigor mortis).
1. Anaerobic respiration still occurs, causing a build-up of lactic acid.
2. This decreases the pH of the cells, inhibiting the enzymes that produce ATP.
3. Without ATP, the bonds between the myosin and actin in the muscle cells become fixed.
• Rigor mortis only lasts for a few hours.
• Decomposition by the action of bacteria/fungi break down the biological molecules, including actin
and myosin.
• Sarcomeres and muscle fibres deteriorate, softening muscle tissue.
• There is a progression in the development of rigor mortis, with smaller muscles stiffening before
larger ones.
• Rigor mortis will set in more quickly an last for a shorter period of time if:
o The environmental temperature is high.
o The person had been physically active before death.
3
, Estimating Time of Death
6.1 Understand how to determine the time of death of a mammal by examining the extent of
decomposition, stage of succession, forensic entomology, body temperature and degree of muscle
contraction.
6.2 Know the role of microorganisms in the decomposition of organic matter and the recycling of
carbon.
The Role of Microorganisms in Decomposition
• Microorganisms, e.g. bacteria and fungi, are an important part of the carbon cycle.
• When plants and animals die, microorganisms on and in them secrete enzymes that decompose the
dead organic matter into smaller molecules that they can respire.
• When the microorganisms respire these small molecules, methane and carbon dioxide are released.
• This recycles carbon back into the atmosphere.
Extent of Decomposition
Autolysis:
• After death, tissues start to break down due to enzymes from the digestive tract and lysosomes.
• Bacteria from the gut or gaseous exchange systems rapidly invade the tissues after death, releasing
enzymes that result in further decomposition.
• The loss of oxygen in tissues favours the growth of anaerobic bacteria.
Discolouration of the Skin:
• The formation of sulfhaemoglobin in the blood initially causes a greenish discolouration of the skin
of the lower abdomen.
• This will spread across the rest of the body, darken to reddish-green and then turn purple-black.
• In average conditions, the discolouration of the abdominal wall will occur between 36-72 hours after
death.
Gas Formation:
• Due to bacteria, gases such as CO2, H2S, CH4, NH3 and H2 form in the intestine and body tissues.
• This causes the body to smell and become bloated.
• As the tissues further decompose, the gas is released and the body deflates.
• When the fluid associated with putrefaction (decomposition) drains away, the soft tissues shrink and
the decay rate is reduced.
• Gas formation typically occurs after 1 week of death, but if the temperature of the body remains high
enough it may occur within 3 days.
Influence of Ambient Conditions:
• Low temperatures slow down decomposition.
• High temperatures speed up decomposition.
• Intense heats denature the enzymes involved in autolysis, delaying the start of decay.
• Injuries to the body allow for the entry of bacteria that aid decomposition.
4
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