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Summary A level Biology Edexcel A - Topic 2
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Notes for Edexcel Biology Topic 2. Detailed and Concise Notes that will get you an A*. They are well laid out and easy to learn
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Topic 2: Genes and HealthThis topic considers the following biological principles through the context of the
genetic disease cystic fibrosis: the properties of and transport of materials, across
cell membranes and gas exchange surfaces, DNA structure and replication, protein
synthesis, enzymes and monohybrid inheritance through the context of the genetic
disease cystic fibrosis. The topic also allows for discussion of the social and ethical
issues surrounding the genetic screening for genetic conditions.
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 investigation of the effect of
surface area to volume ratio on uptake by diffusion, examination of slides of alveoli
to observe the features that aid diffusion into the bloodstream, investigation of
osmosis and diffusion across membranes, and investigation of inheritance using,
for example, corn ears.
Opportunities for developing mathematical skills within this topic include calculating
areas of circumferences and areas of circles, surface areas and volumes of
rectangular blocks and spheres, using ratios, fractions and percentages, plotting
two variables from experimental or other data, determining the slope and
intercepts of a linear graph, understand that y=mx+c represents a linear
relationship, drawing and using the slope of a tangent to a curve as a measurement
of rate of change, understanding simple probability and completing a statistical
test. (Please see Appendix 6: Mathematical skills and exemplifications for further
information.)
Students should:
2.1 i) Know the properties of gas exchange surfaces in living organisms (large
surface area to volume ratio, thickness of surface, difference in concentration).
ii) Understand how the rate of diffusion is dependent on these properties and
can be calculated using Fick’s Law of Diffusion.
iii) Understand how the structure of the mammalian lung is adapted for rapid
gaseous exchange.
2.2 i) Know the structure and properties of cell membranes.
ii) Understand how models such as the fluid mosaic model of cell membranes
are interpretations of data used to develop scientific explanations of the
structure and properties of cell membranes.
CORE PRACTICAL 3:
Investigate membrane structure, including the effect of alcohol concentration
or temperature on membrane permeability.
2.3 Understand what is meant by osmosis in terms of the movement of free water
molecules through a partially permeable membrane (consideration of water
potential is not required).
2.4 i) Understand what is meant by passive transport (diffusion, facilitated
diffusion), active transport (including the role of ATP as an immediate source of
energy), endocytosis and exocytosis.
ii) Understand the involvement of carrier and channel proteins in membrane
transport.
Pearson Edexcel Level 3 Advanced GCE in Biology A (Salters-Nuffield) 9
Specification – Issue 3 – July 2016 © Pearson Education Limited 2016
,Students should:
2.5 i) Know the basic structure of mononucleotides (deoxyribose or ribose linked to
a phosphate and a base, including thymine, uracil, cytosine, adenine or
guanine) and the structures of DNA and RNA (polynucleotides composed of
mononucleotides linked through condensation reactions).
ii) Know how complementary base pairing and the hydrogen bonding between
two complementary strands are involved in the formation of the DNA double
helix.
2.6 i) Understand the process of protein synthesis (transcription) including the role
of RNA polymerase, translation, messenger RNA, transfer RNA, ribosomes and
the role of start and stop codons.
ii) Understand the roles of the DNA template (antisense) strand in
transcription, codons on messenger RNA and anticodons on transfer RNA.
2.7 Understand the nature of the genetic code (triplet code, non-overlapping and
degenerate).
2.8 Know that a gene is a sequence of bases on a DNA molecule that codes for a
sequence of amino acids in a polypeptide chain.
2.9 i) Know the basic structure of an amino acid (structures of specific amino acids
are not required).
ii) Understand the formation of polypeptides and proteins (amino acid
monomers linked by peptide bonds in condensation reactions).
iii) Understand the significance of a protein’s primary structure in determining
its three-dimensional structure and properties (globular and fibrous proteins
and the types of bonds involved in its three-dimensional structure).
iv) Know the molecular structure of a globular protein and a fibrous protein and
understand how their structures relate to their functions (including
haemoglobin and collagen).
2.10 i) Understand the mechanism of action and the specificity of enzymes in terms
of their three-dimensional structure.
ii) Understand that enzymes are biological catalysts that reduce activation
energy.
iii) Know that there are intracellular enzymes catalysing reactions inside cells
and extracellular enzymes produced by cells catalysing reactions outside of
cells.
CORE PRACTICAL 4:
Investigate the effect of enzyme and substrate concentrations on the initial
rates of reactions.
2.11 i) Understand the process of DNA replication, including the role of DNA
polymerase.
ii) Understand how Meselson and Stahl’s classic experiment provided new data
that supported the accepted theory of replication of DNA and refuted competing
theories.
2.12 i) Understand how errors in DNA replication can give rise to mutations.
ii) Understand how cystic fibrosis results from one of a number of possible gene
Students should:
mutations.
2.13 i) Know the meaning of the terms: gene, allele, genotype, phenotype,
recessive, dominant, incomplete dominance, homozygote and heterozygote.
10 ii) Understand patterns of inheritance, including
Pearson Edexcel the interpretation
Level 3 Advanced of genetic
GCE in Biology A (Salters-Nuffield)
pedigree diagrams, in the context of monohybrid
Specification – Issue 3 – July inheritance.
2016 © Pearson Education Limited 2016
2.14 Understand how the expression of a gene mutation in people with cystic fibrosis
impairs the functioning of the gaseous exchange, digestive and reproductive
systems.
2.15 i) Understand the uses of genetic screening, including the identification of
carriers, pre-implantation genetic diagnosis (PGD) and prenatal testing,
including amniocentesis and chorionic villus sampling.
ii) Understand the implications of prenatal genetic screening.
2.16 Be able to identify and discuss the social and ethical issues related to genetic
screening from a range of ethical viewpoints.
,Topic Two Biology
2.1- Properties of Gas Exchange Surfaces in Living Organisms
• Diffusion is the passive movement of particles from a high concentration to a low concentration
• Gas exchange surfaces are adapted for Efficient Diffusion :
o Large surface area to volume ratio e.g. lungs
o Thin (one cell thick)
o Steep concentration gradient
• Larger objects have smaller surface area to volume ratios
• The smaller the surface area to volume ratio the slower the rate of gaseous exchange
• However, gas exchange organs need a large ratio to increase the rate of gaseous exchange
• The lungs are adapted for efficient gaseous exchange as
o Oxygen diffuses out of alveoli across alveolar epithelium & capillary endothelium then into blood
o C02 diffuses into alveoli from blood and is breathed out
Mammalian lungs have the following features increasing gaseous exchange more:
• Lots of alveoli so large surface area
• The alveolar epithelium & capillary endothelium are one cell thick
• Good blood supply maintaining concentration gradient
• Ventilation maintaining concentration gradient
• Moist alveoli
Fick’s law is that the: rate of diffusion Surface area of surface x difference in concentration
Thickness of diffusion surface
2.2 - Structure and Properties of Cell Membranes
• Cell membranes have a fluid mosaic structure
• They are composed of lipids (mainly phospholipids) ,proteins and carbohydrates (attached to proteins or lipids)
The Fluid Mosaic Model - Describes the arrangement of molecules in the membrane
1. Phospholipid molecules have a head and a tail
a. The head contains the phosphate group and is hydrophilic (polar)
b. The tail is made of two fatty acids and is hydrophobic (non polar)
2. This causes the formation of a bilayer with the head facing the water and the hydrophobic tails are on
the inside, making the centre of the bilayer hydrophobic, stopping water soluble substances through it.
The cell surface membrane also contains:
3. glycoprotein - Proteins with a polysaccharide chain attached (for cell recognition and receptors)
4. glycolipids - lipids with a polysaccharide chain attached (for cell recognition and receptors)
5. Cholesterol present in between phospholipids forming bonds making membrane more rigid
• The membrane is partially permeable so small molecules can move through gaps between phospholipids but large molecules & ions pass
through membrane proteins called channel and carrier proteins
Evidence for Fluid Mosaic Model
• Before 1970s it was believed cell membranes had a phospholipid layer between two layers of proteins. As
an electron microscope showed 3 layers in membrane. But this didn’t support the hydrophobic tail in fats
and hydrophilic head
• Experiments showed two types of proteins
o Peripheral proteins – loosely attached on outside surface of membrane
o Integral proteins- fully imbedded within the phospholipids
- Come from freeze thaw fractures with smooth mosaic-like surface interspersed by much larger particles
• Improved electron microscope techniques show bilayer in phospholipids randomly distributed
• Cell membranes was proved to be fluid as they fused mouse cell with human cell and the membrane protein completely intermixed via
diffusion
2.3 - Osmosis
• The diffusion of water molecules from a high concentration to a low concentration through a partially permeable membrane
• If solute molecules present water molecules form hydrogen bonds reducing movement of water molecules
• osmosis will occur till both solutions are equally concentrated/isotonic
, 2.4 – Passive Transport, Active transport, Endocytosis and Exocytosis
• Facilitated diffusion uses carrier and channel proteins
o Larger molecules e.g. glucose and ions don’t diffuse through the
phospholipid bilayer of cell membrane
o Instead they use the carrier and channel proteins
o It occurs from a high concentration to low
• Active transport
o The ions & molecules move across membranes using energy (ATP)
o It’s from a low to high concentration involving carrier proteins not
channel
o Therefore a molecule attaches to carrier protein and changes shape moving molecule to other
side.
o ATP formed by respiration & when broken down in the cell releases energy
• Endocytosis – Enters the cell
o Some substances are way too large to use carrier proteins e.g. Proteins, lipids and
Carbohydrates
o A cell can surround a substance with a section of its cell membrane
o The membrane then pinches off to form a vesicle inside the cell containing ingested substance
o Sometimes white blood cells take in microorganisms too destroy them this way
o It uses ATP for energy
• Exocytosis – Exits the cell
o Some substances produced by the cell (hormones, lipids) need to be released from the cell
o Vesicles containing the substances pinch off from the sacs of the Golgi apparatus (makes proteins) and
moves towards the cell membrane
o The vesicles fuse with cell membrane releasing contents outside
o It uses ATP (adenosine triphosphate) as energy source
2.5 – Structures of Mononucleotide, DNA and RNA
• gene = sequence of bases on a DNA molecule that codes for a sequence
of amino acids in polypeptide chain
• DNA is one type of nucleic acid called deoxyribonucleic acid. It is made
up of a chain of nucleotides or mononucleotides
• A mononucleotide contains 3 molecules linked by condensation reactions:
o Phosphate group
o Deoxyribose (pentose sugar)
o organic base containing nitrogen.
• Mononucleotides link via condensation reactions between the sugar and the phosphate producing a long chain of polynucleotide’s.
• The bond between two nucleotides is a phosphodiester bond
• The nitrogen part of the base has four bases: adenine, thymine, guanine and cytosine.
• The sugars and phosphates form the backbone of DNA and the bases point inwards so the 2 strands are held together by hydrogen bonds.
• The 2 polynucleotide strands are antiparallel
Why bases pair up
• The shape and structure of bases tells how many hydrogen bonds each one can form
• Adenine and Thymine (two hydrogen bonds)
Complementary base pairs
• Guanine and Cytosine (three hydrogen bonds)
How does DNA code for proteins
• the CF gene is on chromosome 7 and instructs the cell to make the CFTR protein
• the sequence of bases in DNA tells the cell which amino acids to link together to make CFTR protein
• every gene is a sequence of bases on a DNA molecule coding for a sequence of amino acids in a polypeptide chain
• this chain twists and folds to form functional protein
From DNA to proteins
• DNA is in the nucleus but proteins are made in cytoplasm
• DNA is too big to pass through membranes around nucleus so protein synthesis occurs
Difference between DNA and RNA
• RNA = single stranded polynucleotide made of ribonucleic acid which have very similar structure to DNA
• RNA has ribose sugar instead of deoxyribose
• The base Uracil replaces Thymine in RNA
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