Subject – BIOLOGY
As part of your preparation for your A Level Biology course, you will need to complete a
research report on The Structures and Functions of Eukaryotic Cells.
Your report will be awarded an attitude score (Outstanding, Motivated, Coasting or
Unacceptable). Please see the green box below for the descriptors for O, M, C and U.
Your Biology teacher(s) will use the following success criteria to assess your attitude.
Success Criteria:
 Report contains ideas that relate to the report title and does not contain plagiarised
content
 Report contains content relevant to the A level Biology specification (the specification
can be found here: http://www.aqa.org.uk/subjects/science/as-and-a-level/biology7401-7402)
 Report is word-processed, Arial font size 12, 1.5 line spacing with margins with normal
widths (2.54 cm)
 The word limit for the report is 1500 words ± 10%
 Correct use of spelling, punctuation and grammar
 Use of annotated diagrams to illustrate key points where relevant
 Diagrams or illustrations should be referenced where applicable
 Inclusion of at least 4 references which may be websites (include full web address)
Outstanding:
You meet all of the success criteria as described above.
It is evident you have a deep understanding of the topic.
After receiving your ‘next steps’, you have made all
improvements to a high standard.
Attitude grade awarded:
Motivated:
You meet most of the success criteria as described above.
It is evident you have a good understanding of the topic.
After receiving your ‘next steps’, you have made most of
the improvements to a high standard.
Coasting:
You meet some of the success criteria as described above.
It is evident you have a moderate understanding of the
topic.
After receiving your ‘next steps’, you have made some of
the improvements but they are not consistently to a high
standard.
Unacceptable:
You meet none of the success criteria as described above.
It is evident you have a weak understanding of the topic.
After receiving your ‘next steps’, you have not made
improvements to a high standard.
Signed (teacher): ………………………………………..
Date: …………………………..
Please see the exemplar report below to help clarify minimum expectations.
Date: 1st June, 2017
Name: A N Other
The Structure and Importance of Biological Molecules
The biological molecules include carbohydrates, lipids and proteins. These molecules are
vitally important as they make up cells and cells are the building blocks of life. In this report,
I will consider each type of biological molecule in turn and will explain their importance.
Carbohydrates
All carbohydrates contain the elements carbon (C), hydrogen (H) and oxygen (O).
Carbohydrates are commonly grouped into the sugars (monosaccharides and disaccharides)
and the polysaccharides (Figure 1).
Figure 1
The monosaccharides are the monomers that form the larger, polymer carbohydrates. An
example of a monosaccharide is glucose, which is a 6 carbon hexose sugar. There are two
isomers of glucose; α- and β-glucose (Figure 2).
The only difference between the
two isomers is the orientation of
the circled groups above and
below the plane of ring of the
hexose.
Figure 2: http://adashofscience.com/2013/05/31/macro-nutrients-part-4-carbohydrates/
Monosaccharides are soluble and usually taste sweet. Glucose is one of the most important
monosaccharides as it is needed for aerobic and anaerobic respiration. Therefore, it is an
important source of energy. A disaccharide is formed when two monosaccharides join
together in a condensation reaction. A glycosidic bond forms between the two
monosaccharides and a molecule of water is released (Figure 3). Table 1 summarises the
formation of some common disaccharides.
Figure 3: http://www.slideshare.net/petersbiology/03-lecturepresentation-26147278
Table 1
Monosaccharide(s)
Disaccharide
Glucose + glucose
Maltose
Glucose + fructose
Sucrose
Glucose + galactose
Lactose
Disaccharides can be broken down into their constituent monosaccharides in hydrolysis
reactions (Figure 4).
Maltose
Glucose
Glucose
Figure 4: https://www.boundless.com/biology/textbooks/boundless-biologytextbook/biological-macromolecules-3/synthesis-of-biological-macromolecules53/hydrolysis-295-11428/
As you can see from Figures 3 and 4, condensation and hydrolysis are reversible reactions,
and as with any chemical reaction in the body, they are catalysed by specific enzymes. The
digestive enzyme maltase catalyses the hydrolysis of maltose into glucose.
A polysaccharide is formed when more than two monosaccharides join together in a
condensation reaction. Important examples of polysaccharides include starch, glycogen and
cellulose.
Plants store the glucose not needed for respiration as starch. Starch is a mixture of two
polysaccharides of α-glucose; amylose and amylopectin. Amylose is a long, unbranched
chain of α-glucose. Due to the angle at which the glycosidic bonds form, the amylose chains
can coil tightly to form a compact structure which means lots of starch molecules can be
stored in a small space (Figure 5). Amylopectin is a long, branched chain of α-glucose. The
‘side branches’ allow easy access to the glycosidic bonds by enzymes. This means that starch
is easily hydrolysed and the glucose is released quickly (Figure 5). Starch is insoluble in water
and, therefore, doesn’t cause water to enter the cells by osmosis which would make them
swell. This makes starch a good storage molecule for glucose, and therefore, a good energy
store in plants.
Amylose
Amylopectin
Figure 5: https://thescienceofnutrition.wordpress.com/tag/amylopectin/
Animals store excess α-glucose as glycogen. Its structure is similar to amylopectin, with
more ‘side branches’. This means that glucose can be released quickly when needed for
energy. Glycogen is a compact, insoluble molecule which makes it suitable as a glucose
storage, and therefore, an energy storage molecule.
Cellulose is made of long, unbranched chains of β-glucose. The chains are linked together by
hydrogen bonds and form strong fibres called microfibrils. These strong cellulose fibres
provide support and rigidity to plant cell walls. Cellulose, is therefore, a structural
polysaccharide rather than an energy store molecule.
Lipids
Lipids are sometimes called fats or oils. There are two main types of lipids; triglycerides and
phospholipids.
Triglycerides contain one molecule of glycerol with three fatty acid chains attached to it. The
fatty acid molecules have long ‘tails’ made of hydrocarbons. The tails are hydrophobic which
means they repel water molecules and explains why lipids are insoluble in water. There are
two types of fatty acid – saturated and unsaturated. Saturated fatty acids do not contain
any double bond between the carbon atoms as the fatty acid is ‘saturated’ with hydrogen
(Figure 6). Unsaturated fatty acids contain double bonds between the carbon atoms which
causes the chain to kink (Figure 6).
Figure 6: http://homepage.smc.edu/wissmann_paul/humanbiology/lipids.html
Triglycerides are formed from condensation reactions between the fatty acid chains and the
glycerol molecule (Figure 7). Ester bonds form with the release of water molecules.
ester bond
Figure 7: http://study.com/academy/lesson/triacylglycerol-structure-function.html
Triglycerides are mainly used as energy storage molecules as their long fatty acid chains
contain a large store of chemical energy that can be released when the chains are
hydrolysed.
Phospholipids are similar in structure to triglycerides apart from one of the fatty acid chains
is replaced with a phosphate group. The phosphate group is hydrophilic and so attracts
water whereas the fatty acid chains are hydrophobic. This is important as it allows the
phospholipids to form a bilayer in cell membranes (Figure 8). The phospholipid bilayer,
together with the proteins associated with it, control the entry and exit of substances from
cells.
Figure 8: http://biology.stackexchange.com/questions/34679/why-dont-phospholipidbilayers-dissolve
Both types of lipid are insoluble in water and so do not cause cells to swell due to water
moving into cells by osmosis.
Proteins
Proteins contain polypeptides which are polymers of the monomers, amino acids. All amino
acids have the same basic structure; a carboxyl group (-COOH), an amino group (-NH2) and a
R group which is variable (Figure 9).
Figure 9:
http://www.personal.psu.edu/staff/m/b/mbt102/bisci4online/chemistry/chemistry8.htm
Before polypeptides are formed, two amino acids bond together during a condensation
reaction to form a dipeptide with the release of a water molecule. The bond that is formed
is called a peptide bond. The bond is broken during a hydrolysis reaction (Figure 10).
Figure 10: http://study.com/academy/lesson/dipeptide-definition-formation-structure.html
Proteins are large, complicated molecules and their structures can be described in four
levels: the primary, secondary, tertiary and quaternary structures (Figure 11).
The primary structure is the sequence of amino acids in the polypeptide chain. The
secondary structure is the coiling and twisting of the primary structure into either a α-helix
or β-pleated sheet. The tertiary structure is formed when the secondary structure is coiled
and folded further into a unique 3D structure. Hydrogen bonds and ionic bonds form
between different amino acids due to the attraction between negative and positive charges.
Disulfide bridges form when two molecules of the amino acid, cysteine, are close together
as the sulfur atom in one cysteine molecule bonds to the sulfur atom in the other cysteine
molecule. These bonds are important as they help to maintain each protein’s specific
structure, it’s a protein’s structure that allows the protein to function effectively. Some
proteins are fully functional with a tertiary structure, however, some proteins require a
quaternary structure to be fully functional. This occurs when two or more polypeptide
chains are held together by bonds to form the final 3D structure. Examples of quaternary
structure proteins are haemoglobin, insulin and collagen. Table 2 describes the most
common examples of proteins.
Figure 11:
http://biowiki.ucdavis.edu/TextMaps/OpenStax_Biology/1%3A_The_Chemistry_of_Life/3%
3A_Biological_Macromolecules/3.4%3A_Proteins
Table 2
Type of protein
Description
Enzymes
Due the folding of the polypeptide chains, these proteins are
usually spherical in shape. They are soluble and have many
roles in metabolism (e.g. digestive enzymes such as proteases,
lipases and carbohydrases) and the synthesis of other
molecules. Enzymes are biological catalysts which increase the
rate of biochemical reactions in organisms. Without enzymes,
the rate of reactions would be too slow resulting in cell death
and organ failure.
Antibodies
These proteins are made of four polypeptide chains and are
an important part of the immune response. Without
antibodies, we would not be able to defend our bodies from
pathogens effectively.
Transport Proteins
These include channel proteins which transport molecules or
ions across the cell membrane. The channel proteins contain
hydrophobic and hydrophilic regions which cause the protein
to fold up in a particular way and form a channel. An example
is a glucose channel protein. Without transport proteins, vital
molecules and ions would not be able to exit or enter cells.
Structural Proteins
These proteins consist of long polypeptide chains that are
arranged side by side in a parallel fashion with cross-links
between them. This arrangement makes these proteins
physically strong. Structural proteins include keratin (found in
hair and nails) and collagen (found in connective tissue).
As my report shows, carbohydrates, lipids and proteins have very unique structures and
they are essential for a variety of roles within organisms.
Word count: 1395 (excluding reference list below).
References
All websites were accessed for research purposes between 28th May – 2nd June, 2017.
http://adashofscience.com/2013/05/31/macro-nutrients-part-4-carbohydrates/
http://www.slideshare.net/petersbiology/03-lecturepresentation-26147278#
https://www.boundless.com/biology/textbooks/boundless-biology-textbook/biologicalmacromolecules-3/synthesis-of-biological-macromolecules-53/hydrolysis-295-11428/
https://thescienceofnutrition.wordpress.com/tag/amylopectin/
http://homepage.smc.edu/wissmann_paul/humanbiology/lipids.html
http://study.com/academy/lesson/triacylglycerol-structure-function.html
http://biology.stackexchange.com/questions/34679/why-dont-phospholipid-bilayersdissolve
http://www.personal.psu.edu/staff/m/b/mbt102/bisci4online/chemistry/chemistry8.htm
http://study.com/academy/lesson/dipeptide-definition-formation-structure.html
http://biowiki.ucdavis.edu/TextMaps/OpenStax_Biology/1%3A_The_Chemistry_of_Life/3%
3A_Biological_Macromolecules/3.4%3A_Proteins
http://www.s-cool.co.uk/a-level/biology/biological-molecules-and-enzymes
C Burrows, C Lindle, C McGarry, S Pattinson, C Plowman, R Rogers & H Thompson (2015). A
Level Biology. The Complete Course for AQA. Coordination Group Publications. First Edition,
Newcastle Upon Tyne.
G Toole & S Toole (2015). AQA Biology A level Student Book. Oxford University Press.
Second Edition, Oxford.