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Summary of AQA AS-Level Biology

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Comprehensive Biology notes covering everything students need for the exam and get top grades: -Key concepts and their applications- examples are given -Core praticals, their variable, methods, and results -Annotated diagrams -Exam practice and tips -Specification points

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1.1.1 Blood

(a) Blood Samples
Sterile equipment and disposable gloves are used. Skin is swabbed with alcohol. Tourniquet is
put on upper arm to make the veins stand out more. The needle is inserted into a vein and
blood is slowly drawn. Place a swab over wound as soon as needle is removed. Cover with a
sterile dressing once bleeding has stopped.

(b) Blood Smears
Place a very small drop of blood near the end of a clean microscope slide, Place the end of
another slide on the sample slide and hold it at about 30o and push it along the slide, spreading
the blood. Allow the slide to dry in the air and fix with alcohol. Stain the slide using
Romanowsky stain, such as Wright’s or Leishman’s stain. Pour over and leave for 2 minutes and
wash off with water.

(c) Haemocytometer
A haemocytometer is a chamber designed for counting blood cells. The cells have to be diluted
using Dacie’s fluid. Red blood cells are diluted 1:200 and white blood cells 1:20. Place the
diluted cells on the lined squares and count using the northwest rule; discount the cells lying on
the east or south boundary of the squares.

(d) Different Cell Types
● Lymphocyte- Large nucleus, small rim of cytoplasm, makes antibodies
● Neutrophil- Granular toxins, multilobed nucleus, engulfs cells by phagocytosis
● Monocyte- Horseshoe nucleus, used in immunity
● Lymphocyte- Single round nucleus in the lymphatic system

(e) Platelets
Platelets are fragments of giant cells called megakaryocytes which are a very important part of
blood clotting.

(f) Cell Organelles
● Golgi modify and repackage vesicles.
● RER is where proteins are made.
● SER is where lipids are synthesised.
● Lysosomes are vesicles with enzymes in.
● Vesicles are small membrane sacks.
● Mitochondria are where ATP is made.
● Chloroplasts are involved in photosynthesis.

, ● Cytoskeleton provide stability to the cell.
● Nucleus controls cell functions.
● Nucleolus is a concentrated part of the nucleus.

(g) Magnification
Magnification = size of structure in picture
real size of the structure

(h) Fluid Mosaic Model

(i) Membranes
The cell surface membrane separates the cell from its environment. Membranes are also found
inside the cell in the form of membrane bound organelles. These membranes allow complex
processes to happen separately from others. Such as, only the enzymes needed for respiration
are found in mitochondria.

(j) Proteins
Proteins line both sides of the cell bilayer. Proteins that span the whole layer are known as
intrinsic proteins, and proteins that are only on one side of the bilayer are intrinsic.
Glycoproteins on the bilayer are used in cell recognition and cell adhesion.

,1.1.2 Molecules

(a) Amino Acids

(b) Peptide Bond Formation

(c) Proteins
1. Primary structure – the sequence that particular amino acids are linked together is called the primary
structure of a protein. The positioning and chemical behaviour of these amino acids ultimately determines
how the polypeptide chain folds up and the final function of that protein. Altering the amino acid types
and/or sequence will change the shape and nature of the protein that forms.
2. Secondary structure – localised folding within a polypeptide chain form recognisable, organised
shapes. Alpha helix is a spiral shape which is stabilised by hydrogen bonding that occurs between every
fourth peptide bond in that part of the polypeptide chain. Globular proteins like haemoglobin and
enzymes possess a high degree of this type of secondary structure. Beta pleated sheet forms stiffer
pleated‘ sections where again hydrogen bonding stabilises the shape.

, 3. Tertiary structure – the overall 3D shape one polypeptide chain folds into. It is stabilised by several
types of chemical interaction : hydrogen bonding, ionic bonds, hydrophobic / hydrophilic interactions
and disulphide bridges - strong covalent bonds that form between two cysteine amino acids R groups
4. Quaternary structure – more than one polypeptide chain can interact to form a final protein complex eg
haemoglobin.

(d) Haemoglobin
Haemoglobin is a globular protein that carries oxygen in the blood. It is comprised of four polypeptide
chains, each with a haem group attached to the end.
Each haem group has a iron ion in the centre, which can carry one oxygen molecule. This means that
haemoglobin can carry 4 oxygen molecules at once.

(e) Blood Products
Plasma- The liquid part of blood which contains antibodies, ions, hormones, food substances, oxygen,
urea and heat
Serum- When the blood clotting protein has been removed from blood plasma you are left with serum
Tissue Fluid- Formed when blood passes through the capillaries
Lymph- When tissue fluid drains into lymphatic capillaries it is called lymph

(f) Blood Plasma
Blood plasma is the liquid part of blood and depending on where it is in the body it is made of different
things.

● Proteins like fibrinogen for blood clotting

● Ions such as sodium, potassium and calcium to maintain osmotic balance

● Hormones such as insulin and oestrogen

● Dissolved food substances like glucose and amino acids

● Oxygen is dissolved in small amounts

● Waste such as urea and carbon dioxide

● Heat is transferred around the body

(g) Water as Transport
● Solvent- Allows water to act as a transport medium for polar molecules
● Lubricant- Water’s cohesive and adhesive properties mean that it is viscous

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