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Summary A level Biology Edexcel A - Topic 6
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Notes for Edexcel Biology Topic 6. Detailed and Concise Notes that will get you an A*. They are well laid out and easy to learn
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Topic 6: Immunity, Infection and ForensicsThis topic starts by looking at how forensic pathologists use a wide variety of
analytical techniques to determine identity and the time and cause of death of an
organism, including humans. It then considers how bacteria and viruses use a
variety of routes into their hosts and how hosts have evolved barriers and internal
mechanisms to combat infections. These protections are not always successful and
many people in the world still die from infectious diseases. This topic also
investigates the evolutionary battles that take place between invading pathogens
and their hosts. The topic ends by looking at hospital acquired infections, their
prevention and control.
Students should be encouraged to carry out a range of practical experiments
related to this topic in order to develop their practical skills. In addition to the core
practicals detailed below, possible experiments include amplification of DNA using
the polymerase chain reaction, and staining of bacteria.
Opportunities for developing mathematical skills within this topic include calculating
areas of circumferences and areas of circles, surface areas and volumes of
cylinders; using calculators to find and use power, exponential and logarithmic
functions, and recognising and using expressions in decimal and standard form.
(Please see Appendix 6: Mathematical skills and exemplifications for further
information.)
Students should:
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 micro-organisms in the decomposition of organic matter and
the recycling of carbon.
6.3 Know how DNA profiling is used for identification and determining genetic
relationships between organisms (plants and animals).
6.4 Know how DNA can be amplified using the polymerase chain reaction (PCR).
CORE PRACTICAL 14:
Use gel electrophoresis to separate DNA fragments of different length.
6.5 Be able to compare the structure of bacteria and viruses.
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.
6.7 Understand the non-specific responses of the body to infection, including
inflammation, lysozyme action, interferon, and phagocytosis.
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.
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.
20 Pearson Edexcel Level 3 Advanced GCE in Biology A (Salters-Nuffield)
Specification – Issue 3 – July 2016 © Pearson Education Limited 2016
,Students should:
6.10 Understand how one gene can give rise to more than one protein through post-
transcriptional changes to messenger RNA (mRNA).
6.11 i) Know the major routes pathogens may take when entering the body.
ii) Understand the role of barriers in protecting the body from infection,
including skin, stomach acid, and gut and skin flora.
6.12 Understand how individuals may develop immunity (natural, artificial, active,
passive).
6.13 Understand how the theory of an ‘evolutionary race’ between pathogens and
their hosts is supported by the evasion mechanisms shown by pathogens.
6.14 Understand the difference between bacteriostatic and bactericidal antibiotics.
CORE PRACTICAL 15:
Investigate the effect of different antibiotics on bacteria.
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.
Pearson Edexcel Level 3 Advanced GCE in Biology A (Salters-Nuffield) 21
Specification – Issue 3 – July 2016 © Pearson Education Limited 2016
, Topic Six Biology
6.1 – Determining the Time of Death of a Mammal
• This is done by examining the extent of
o Decomposition o Forensic Entomology o Degree of Muscle Contraction
o Stage of Succession o Body Temperature (Rigor Mortis)
Body temperature
o Core human body temperature is 36.2- 37.6 oC, and this cools as someone dies due to the absence of heat-
producing chemical reactions (respiration)
o Useful in first 24 hrs after death
o Core body temp is measured via rectum or abdominal stab
(long thermometer needed or electronic temp probe)
▪ The cooling of a body follows a sigmoid curve
• However, many factors affect post-mortem cooling
o Body size
o Body position
o Clothing
o Air Movement
o Humidity
o Temperature of surroundings
Rigor Mortis
o Muscles usually relax then stiffen – joints become fixed
o Their position depends on position of body position at the time of death. They then relax again after a while
• Stages involved:
1. After death muscles oxygen dependent reactions stop
2. Respiration in cells become anaerobic and produce lactic acid
3. PH of cells falls, inhibiting enzymes so inhibiting anaerobic respiration
4. ATP for muscle contraction no longer produced
5. Proteins can no longer move over one another to shorten muscle, fixing muscle and joints
• Complete rigor mortis occurs 6-9 hours after death
• It lasts for shorter period of time if
o environment temperature is high
o person was physically active before death
Decomposition
o autolysis occurs first – body’s own enzymes from digestive tract and lysosomes break cells down
Signs of Decomposition
• first sign = green discolouration of skin of lower abdomen (caused by sulfhaemoglobin build up in blood)
o this spreads across the rest of the body and darken to reddish-green then purple-black colour
o gas or liquid blisters appear on skin because of bacteria gases forming in intestines and tissue
o this results in the body bloating and smelling
o further tissue decomposition releases gas so body deflates
• There is variation in Decomposition Time
o In average temperate climate, decolouration of abdominal wall occurs within 36-72 hours
o Gas formation occurs after a week, but if body temperature remains above 26oC it occurs within 3 days
o Environmental temperatures influence decomposition
▪ Low temperatures slow down decomposition, warm temperatures speed it up
▪ Rate of decomposition is highest between 21-38 oC
▪ Intense heat denatures enzymes for autolysis slowing decomposition down
▪ Body injuries allow bacteria entry so aid decomposition
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