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Summary A level Biology Edexcel A - Topic 3
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Notes for Edexcel Biology Topic 3. Detailed and Concise Notes that will get you an A*. They are well laid out and easy to learn
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Topic 3: Voice of the GenomeThis topic follows the development of multicellular organisms from single cells to
complex individuals. Cell structure and ultrastructure, cell division, the importance
of fertilisation, the roles of stem cells, gene expression, cell differentiation and
tissue organisation are all considered within this topic, as is the role of the
genotype, epigenetics and the effect of environment on phenotype.
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 examination of animal cells
under the microscope and using electron micrographs, plant tissue culture to
demonstrate the totipotency of plant cells, and demonstration of how a gene can be
switched on by, for example, the induction of β-galactosidase.
Opportunities for developing mathematical skills within this topic include using
ratios, fractions and percentages, making order of magnitude calculations using
decimal and standard form and using the appropriate number of significant figures,
understanding the terms mean, median and mode, constructing and interpreting
frequency tables and diagrams, bar charts and histograms, and completing a
statistical test. (Please see Appendix 6: Mathematical skills and exemplifications for
further information.)
Students should:
3.1 Know that all living organisms are made of cells, sharing some common
features.
3.2 Know the ultrastructure of eukaryotic cells, including nucleus, nucleolus,
ribosomes, rough and smooth endoplasmic reticulum, mitochondria, centrioles,
lysosomes, and Golgi apparatus.
3.3 Understand the role of the rough endoplasmic reticulum (rER) and the Golgi
apparatus in protein transport within cells, including their role in the formation
of extracellular enzymes.
3.4 Know the ultrastructure of prokaryotic cells, including cell wall, capsule,
plasmid, flagellum, pili, ribosomes, mesosomes and circular DNA.
3.5 Be able to recognise the organelles in 3.2 from electron microscope (EM)
images.
3.6 Understand how mammalian gametes are specialised for their functions
(including the acrosome in sperm and the zona pellucida in the egg).
3.7 Know the process of fertilisation in mammals, including the acrosome reaction,
the cortical reaction and the fusion of nuclei.
3.8 i) Know that a locus (plural = loci) is the location of genes on a chromosome.
ii) Understand the linkage of genes on a chromosome and sex linkage.
3.9 Understand the role of meiosis in ensuring genetic variation through the
production of non-identical gametes as a consequence of independent
assortment of chromosomes and crossing over of alleles between chromatids
(details of the stages of meiosis are not required).
3.10 Understand the role of mitosis and the cell cycle in producing identical daughter
cells for growth and asexual reproduction.
12 Pearson Edexcel Level 3 Advanced GCE in Biology A (Salters-Nuffield)
Specification – Issue 3 – July 2016 © Pearson Education Limited 2016
,Students should:
CORE PRACTICAL 5:
Prepare and stain a root tip squash to observe the stages of mitosis.
3.11 i) Understand what is meant by the terms ‘stem cell, pluripotency and
totipotency’.
ii) Be able to discuss the way society uses scientific knowledge to make
decisions about the use of stem cells in medical therapies.
3.12 Understand how cells become specialised through differential gene expression,
producing active mRNA leading to synthesis of proteins, which in turn control
cell processes or determine cell structure in animals and plants, including the
lac operon.
3.13 Understand how the cells of multicellular organisms are organised into tissues,
tissues into organs and organs into systems.
3.14 i) Understand how phenotype is the result of an interaction between genotype
and the environment.
ii) Know how epigenetic changes, including DNA methylation and histone
modification, can modify the activation of certain genes.
iii) Understand how epigenetic changes can be passed on following cell division.
3.15 Understand how some phenotypes are affected by multiple alleles for the same
gene at many loci (polygenic inheritance) as well as the environment and how
this can give rise to phenotypes that show continuous variation.
Pearson Edexcel Level 3 Advanced GCE in Biology A (Salters-Nuffield) 13
Specification – Issue 3 – July 2016 © Pearson Education Limited 2016
, Topic Three Biology
3.1 Living organisms
• Organisms can be prokaryotes or eukaryotes
o All living organisms are made of cells and share some common features
1. Prokaryotic organisms have prokaryotic cells (single celled
organism)
2. Eukaryotic organisms are made of eukaryotic cells
3. Both types of cells contain organelles, which are parts of cells
4. If you examine a cell through electron microscope you can see the
organelles and internal structure of most of them which is known as cell ultrastructure
3.2 Eukaryotic Cells
The organelles found inside a Eukaryotic cell are:
Organelle Description Function
Nucleus • A large organelle surrounded by a nuclear envelope • The pores allow substances to move between the nucleus
(double membrane) and the cytoplasm.
• Contains many pores • Chromosomes made of DNA contain genes controlling
• The nucleus contains chromosomes and nucleolus protein synthesis
• Controls the cells activities
Nucleolus • A dense body within nucleus • Where ribosomes are made
Lysosome • Spherical/Round sacs • Involved in breakdown of unwanted structures within a cell
• has digestive enzymes • Destruction of whole cells when old cells are to be
• bound by a single membrane replaces/ during development
• no clear internal structure
Ribosomes • A very small organelle that floats free in the cytoplasm or • The site where proteins are made
is attached to the rough endoplasmic reticulum.
• It’s made up of proteins and RNA.
• Not surrounded by membrane
Rough • System of interconnected membrane-bound, flattened • folds and processes proteins that have been made at
Endoplasmic sacs ribosome
Reticulum • covered in ribosomes on outer surface
Smooth • they are smooth as they have no attached ribosomes • synthesises and processes lipids and steroids
Endoplasmic
Reticulum
Golgi Apparatus • stacks of flattened, membrane-bound sacs formed by • modifies proteins and lipids and packages them in vesicles
fusion of vesicles from ER for transport
• also makes lysosome
Vesicles • A small fluid-filled sac, which is in the cytoplasm • Transport substances in and out of the cell and between
• It is surrounded by a membrane the organelle.
Centrioles • Every animal cell has one pair of it • Involved in the formation of the spindle during nuclear
• Hollow cylinders (proteins in helix) division
• made of a ring of nine protein microtubules • Transport within the cell cytoplasm
Mitochondria • double bound membrane, • The site of aerobic respiration, where ATP is produced.
• the inner one is folded to form cristae • They are very active and require a lot of energy
• inside that is the matrix.
• It has enzymes used for respiration
Cell Surface • Phospholipid bilayer containing proteins • Controls the substances allowed into and out of the cell
Membrane • Partially permeable
(plasma membrane)
Cell wall • Made of cellulose • Supports the cell and prevents it from changing
• Isn’t in every Eukaryotic cell
, 3.3 Role of Rough Endoplasmic Reticulum & Golgi Apparatus
1. proteins are made at the ribosomes
2. ribosomes on RER make proteins that are excreted/attached to
cell membrane
3. new proteins produced at RER are folded and processed (sugar
chain added) in rough ER
4. transported from ER to golgi in vesicles
5. at golgi undergo further processing (modified)
6. proteins enter more vesicles to be transported around cell by
exocytosis
• extracellular enzymes (digestive) move to cell surface and
are secreted
3.4 Prokaryotic Cells – smaller and similar than eukaryotes e.g. bacteria
Cytoplasm – no membrane bound organelles unlike
eukaryotic cells
Plasmid – small circle of DNA, with genes for antibiotic
resistance but not always present in prokaryotes
Circular DNA – no nucleus the DNA floats in cytoplasm and is
circular with one long coiled up strand
Ribosome – where proteins are made
Pilli – thin, protein tubes allowing bacteria to surfaces
Cell wall -supports the cell preventing it change shape and
contains peptidoglycan that’s a polypeptide+ polysaccharide
Capsule – made of secreted slime for protection of bacteria
and prevents dehydration
Flagellum – helps the cell move not all prokaryotes have it
Mesosomes – infolding of cell surface membrane
(respiration site)
There are many differences between Eukaryotes and prokaryotes,:
Eukaryotes Prokaryotes
Larger cells (2-200um) Extremely small cells less and 2.0um
DNA is linear DNA is circular
Nucleus present-(DNA is inside) No nucleus-DNA is free in cytoplasm
No cell wall in animal cells, cellulose cell wall Cell wall made of polysaccharides but not
in plants and chitin cell wall in fungi cellulose or chitin
Many organelles, mitochondria present Few organelles, no mitochondria
Large ribosomes Smaller ribosomes
Example Human liver cell Example E.Coli bacterium
3.5 Organelles in Eukaryotic Cells from electron microscope
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