Edexcel IAL Biology
Topic 2 | Membranes, Protein, DNA and Gene
Expression
Specification Points 2.1 - 2.18
, EDEXCEL IAS-LEVEL BIOLOGY//WBI11 EDEXCEL IAS-LEVEL BIOLOGY//WBI11
between muscle and fat; therefore, it can be o Low-Density Lipoproteins (LDL)
misleading. ▪ They are considered bad cholesterol, must be
𝑊𝑒𝑖𝑔ℎ𝑡 (𝑘𝑔) maintained in low proportions
𝐵𝑀𝐼 =
(𝐻𝑒𝑖𝑔ℎ𝑡)2 (𝑚) ▪ Formed from saturated fats, protein, and cholesterol
▪ An increase in intake of saturated fats causes an
o Waist-to-Hip ratio: It is a scale that’s showing active increase in blood cholesterol, due to the rise in LDLs
association with the increased risk of CVDs. It is the produced. Therefore, there is a causal link between
waist circumference measured in cm divided by the hip LDL levels and the risk of CVD.
circumference also measured in cm. • Dietary Antioxidants: They help reduce the risk of CVDs • Cytosis:
TREATMENT MECHANISM SIDE EFFECTS by oxidising free radicals formed by oxidative stress. o Exocytosis: The bulk movement of substances from
Diuretics: Increase Dizziness, Vitamin C, an antioxidant, ensures a firm attachment of inside the cell to the outside using vesicles. Vesicles
the volume of urine nausea, the endothelium lining. fuse with the cell membrane and release their
produced, causing impotence contents.
the blood volume to
2. MEMBRANES, PROTEINS, DNA, AND GENE • Phospholipid bilayer: This provides the basic structure of o Endocytosis: The bulk movement of substances from
decrease therefore
EXPRESSION membranes; it is selectively permeable and acts as a outside the cell to the inside using vesicles formed by
the blood pressure
Antihypertensives barrier to most water-soluble substances. the cell membrane.
falls
Beta-Blockers: 2.1.1 Cell Membranes o The more unsaturated the tails, the more fluid the
Blocks hormones • Fluid mosaic model: Two scientists, Singer and Nicolson, membrane as unsaturated fatty acid tails are bent and 2.1.3 Osmosis: A special case of diffusion
like adrenaline to put together a cell membrane model based on the therefore fit together more loosely. • Osmosis: The diffusion of water molecules down a water
prevent strong evidence available. The term ‘fluid’ describes how o The longer the tail, the less fluid the membrane. potential gradient through a permeable membrane.
contractions individual phospholipid and protein molecules move • Micelle: phospholipid molecules that • Isotonic Solution: The concentration of solute on both
They work by Liver around within their monolayer. The word ‘mosaic’ arrange themselves in a spherical form in sides is equal.
inhibiting the damage,
describes the pattern produced by scattered protein aqueous solutions. • Hypotonic Solution: The concentration of solute in the
function of the kidney
Statins molecules when the surface of the membrane is viewed • Cholesterol: regulates the fluidity of membranes. Its solvent is less than the cell.
enzyme in the liver failure,
from above. hydrophobic region prevents polar molecules from • Hypertonic Solution: The concentration of solute in the
responsible for nausea
passing through the membrane. solvent is more than the cell.
producing LDLs
Prevents blood clot Could result • Osmosis in animal cells:
formation in the in 2.1.2 Cell Transport and Diffusion
artery uncontrolled • Passive transport:
Anticoagulants
bleeding if o Simple Diffusion: The net movement of molecules or
(warfarin)
dosage is ions from a region of high concentration to a region of
not low concentration, down the concentration gradient.
monitored o Facilitated diffusion: The net movement of molecules
They work by Rashes, down the concentration gradient. Molecules go
reducing the diarrhoea • Osmosis in Plant Cells:
• Phospholipid: The hydrophilic head contains a through channel proteins or carrier proteins instead of
stickiness of o Protoplast: the living part of the cell inside the cell
Platelet inhibitory phosphate group and glycerol, while the hydrophobic passing through phospholipids. This allows for the
platelets, which wall
drugs (aspirin) tail contains 2 fatty acid chains. This is due to the partial passage of large polar ions and molecules e.g. glucose,
reduces the chance o In pure water: water enters the cell by osmosis, and
of blood clot negative charge on the phosphate group that gets amino acids, Na+
the cell wall pushes back against the expanding
formation attracted to the partial positive charge on the hydrogen o Passive transport requires no ATP energy
protoplast, building up pressure rapidly, becoming
• Lipoproteins: They can be divided into: atom of the water molecule. o 𝐹𝑖𝑐𝑘 ′ 𝑠 𝐿𝑎𝑤 𝑜𝑓 𝐷𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 =
𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝐴𝑟𝑒𝑎 ×𝐷𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑖𝑛 𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 turgid.
o High-Density Lipoprotein (HDL) 𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑓 𝐸𝑥𝑐ℎ𝑎𝑛𝑔𝑒 𝑆𝑢𝑟𝑓𝑎𝑐𝑒 o In concentrated solution: water will leave the cell by
▪ They are considered good cholesterol, must be osmosis. The protoplast gradually shrinks until it is
maintained in high proportions • Active transport: Involves the use of carrier proteins to exerting no pressure on the cell wall. The protoplast
▪ Formed from unsaturated fats, protein, and move substances against the concentration gradient continues to shrink and pull away from the cell wall, so
cholesterol using ATP energy. the cell is plasmolysed. The point at which plasmolysis
▪ They have a higher percentage of protein compared is about to occur is referred to as incipient plasmolysis.
to LDLs
PAGE 9 OF 16 PAGE 10 OF 16
, EDEXCEL IAS-LEVEL BIOLOGY//WBI11 EDEXCEL IAS-LEVEL BIOLOGY//WBI11
of these tiny airways due to the absence of • Activation energy is the energy required in any chemical the secondary and tertiary structure of the enzyme.
cartilage. During exercise, the muscles relax to reaction to break the bonds in reactant molecules so This changes the shape of the enzyme and it’s active
allow a greater flow of air to the alveoli. that new bonds are formed to make the product. An site and causes the substrate to no longer fit. The
• Cartilage: It is a connective tissue. It keeps airways in enzyme lowers the activation energy required for the enzyme is denatured.
trachea and bronchi open and air resistance low and reaction. However, the overall energy released during
prevents them from collapsing or bursting as the air the reaction is maintained.
• How plasmolysis occurs: pressure changes during breathing. • Lock-and-key theory: the shape of the active site is exact
• Epithelium: Air flows down the lungs through trachea and substrates that are not complementary to the shape
and bronchi, which are lined by cells adapted to remove of the active site cannot bind. The enzyme-substrate
particles from the air before it reaches the lungs. These complexes formed to enable the reaction to take place
cells make up a tissue called epithelium. There are two more easily.
main kinds:
o Ciliated cells: They are responsible for the
continual movement of mucus towards the larynx.
o pH: Any change in the pH value of the medium around
o Goblet cells: Secretes mucus that traps pathogens, the enzyme will cause the bonds to be damaged, this
which are then moved out with the help of cilia. will change the 3-D shape of the enzyme and deform
the active site. Therefore, the substrate will not be able
ADAPTATIONS FOR EFFICIENT GAS EXCHANGE to fit into active site thus the reaction slows down or
LARGE SURFACE A large number of alveoli is present to
stops. The enzyme gets denatured.
AREA increase the surface area
The wall of the alveolus is very thin • Induced fit theory: the enzyme’s active site is not
2.1.4 The Mammalian Gas Exchange System (single layer of squamous initially an exact fit to the substrate molecule. However,
• Gross structure of lungs: lungs are in chest cavity THIN SURFACE epithelium) to decrease the the enzyme molecules are more flexible and can change
surrounded by the pleural membranes, which enclose an diffusion distance. Also, the walls shape slightly as the substrate enters the enzyme. This
of the capillaries are thin. means that the enzyme molecule will undergo
airtight space. This space contains a small quantity of
Deoxygenated blood is brought to conformational changes as the substrate combines with
fluid to allow friction-free movement as the lungs are STEEP
the alveoli which provides a steep the enzyme’s active site, forming the enzyme-substrate
ventilated by the movement of the diaphragm and ribs. CONCENTRATION o Enzyme concentration: As the concentration of
concentration gradient for complex.
GRADIENT enzymes is increased, there are more available active
diffusion
MOIST SURFACE Allowing dissolving of gases sites for substrates to fit into. More enzyme-substrate
Replacing alveolar air with air from complexes are formed, more products are formed and
VENTILATION outside the body. This helps the rate of reaction is increased. At the beginning, the
maintain the diffusion gradient. limiting factor is the enzyme concentration. Once all
substrates have formed enzyme-substrate complexes,
2.2.1 Enzymes a further increase in enzyme concentration will have
• An enzyme is a biological catalyst that accelerates no effect on the rate of reaction. At this point, the
metabolic reactions. Enzymes are globular proteins as limiting factor is the substrate concentration.
they have a roughly spherical shape and are water- o Substrate concentration: As the concentration of
• Factors Affecting Enzyme Action: substrate increases, the greater the chance of collision
soluble. Enzymes functioning inside a cell are
o Temperature: As the temperature increases, the with enzymes, the more enzyme-substrate complexes
intracellular, but those that are secreted by cells and
kinetic energy and the enzyme activity increases as formed, the more products formed and the greater the
catalyse reactions outside cells are described as
there’s more collisions until optimal temperature is rate of reaction. At the beginning, the limiting factor is
extracellular.
reached (usually 40C). At optimum temperature, the substrate concentration. Once all the enzymes are
• Enzymes have specific active sites that are
. maximum rate of reaction is achieved. If the occupied and working at their maximum rate (vmax), a
complementary to the shape of the substrate. The
• Bronchi: branch to form smaller bronchioles. temperature continues to increase beyond the further increase in substrate concentration will have no
substrate is held in place at the active site by weak
o Bronchioles are surrounded by smooth muscle, optimum temperature, the rate of the reaction begins effect on the rate of reaction. At this point, the limiting
hydrogen and ionic bonds. The combined structure is
which can contract or relax to adjust the diameter to decrease as more kinetic energy breaks the bonds in factor is the enzyme concentration.
called the enzyme-substrate complex.
PAGE 11 OF 16 PAGE 12 OF 16
, EDEXCEL IAS-LEVEL BIOLOGY//WBI11 EDEXCEL IAS-LEVEL BIOLOGY//WBI11
• Why do we measure the initial rate? o Complementary base pairing: between Adenine and 2.2.3 DNA Replication o The 2nd generation formed two bands, one at the
o There are enough substrate molecules present to thymine (or Adenine and uracil in RNA) and between • Semi-conservative replication of DNA: The DNA centre and one up the top, of equal thicknesses
ensure that substrate concentration is not a limiting guanine and cytosine. Because of complementary base separates into two strands, and each strand acts as a o The 3rd generation also formed two bands similar to
reactant pairing, the order of the bases in one strand template. Each new DNA molecule consists of one old the 2nd generation, but the top band was thicker than
o The substrate concentration is only known at the determines the order of the bases in the other strand. strand and a complementary new strand. the middle
beginning of the reaction Therefore, the strands are complementary to each o A template is a length of polynucleotide chain on to o If conservative replication were taking place, the 1st
other as well. which nucleotides attach to start the synthesis of a generation would contain 2 bars upon centrifugation.
2.2.2 The Structure of DNA and RNA ▪ Purines are nitrogenous bases with double ring complementary sequence
• Nucleic Acids: structures (Guanine and Adenine). o The DNA double helix unwinds and ‘unzips’ as the
o Made up of Carbon, Hydrogen, Oxygen, Nitrogen, and ▪ Pyrimidines are nitrogenous bases with single ring hydrogen bonds between the bases break by DNA
Phosphorus structures (Thymine, Uracil, and Cytosine). helicase.
o Monomers called nucleotides make up nucleic acids ▪ Adenine and Thymine form 2 hydrogen bonds, o In the nucleus, there are free nucleotides that pair up
o They are made up of a pentose sugar, organic nitrogen Guanine and Cytosine, form 3. by complementary base pairing on each of the old DNA
base, and phosphoric acid o DNA polymerase links adjacent nucleotides to each
o DNA molecules are double-stranded, RNA molecules other by catalysing phosphodiester bonds.
are single-stranded o Hydrogen bonds form within the structure, allowing it
to coil up into a double helix
• Polynucleotide: many nucleotides are linked together
into a long chain-forming polynucleotides DNA/RNA. 2.2.4 Protein Synthesis
o Phosphodiester bonds formed by condensation • Nature of genetic code:
reactions link the 5-carbon of one sugar molecule and o Made up of a Triplet code: There are 20 different
the 3-carbon of the next. The polynucleotide strand is • RNA: a single stranded polynucleotide chain present in amino acids, and 4 Nitrogen Bases. Therefore minimum
said to have 3΄ and 5΄ ends the nucleus, cytoplasm and ribosome. It contains a code length must be 3. 42 = 16 | 43 = 64
pentose sugar (ribose) and has 4 nitrogenous bases: • Meselson and Stahl’s experiment: o Non-overlapping: e.g. AGGTCA will be read as AGG and
Adenine, Uracil, Guanine and Cytosine. The different o Stahl and Meselson devised an experiment to prove TCA only and not AGG, GGT, GTC, TCA
types of RNA include: the theory of semi-conservative replication o Degenerate: Multiple Triplet codes code for 1 amino
o mRNA (messenger RNA): carries the genetic o They grew a culture of bacteria in a medium of Heavy acid
information in the form of a template from the nucleus
Nitrogen (15N) only • Gene: A sequence of bases on a DNA molecule coding
to the ribosome for translation. for a sequence of amino acids that control body
o The bacteria culture used the heavy Nitrogen to
o tRNA (transfer RNA): has a specific amino acid at one characteristics of an organism.
produce DNA and different proteins, and after many
end and an anticodon at the other end. It fits onto the
• Pentose Sugars: generations, all of the DNA within the bacteria • Codon: A sequence of 3 bases in DNA or mRNA.
mRNA at ribosomes at complementary mRNA codon
o Ribose (C5H10O5) present in RNA nucleotides contained heavy Nitrogen only • Sense Strand: The DNA strand carrying the code for the
for protein synthesis.
o Deoxyribose (C5H10O4) present in DNA nucleotides o Bacteria were then moved into a medium of Normal protein being coded
• DNA is made up of two antiparallel polynucleotide DNA RNA Nitrogen (14N) • Antisense Strand: The DNA strand acting as a template
STRANDS Double-Stranded Single Stranded o Bacteria were grouped and left to divided for one for the mRNA molecule
strands lying side by side, held together by hydrogen
bonds. These strands are arranged into a ladder-like NITROGEN Contains Thymine Contains Uracil generation, two generations, and three generations • Transcription: in the nucleus the following happens:
structure called a double helix. The phosphate and the BASES o Bacterial DNA was extracted and placed in a centrifuge o DNA Helicase breaks the Hydrogen bonds in the gene
STRUCTURE Helical Structure Linear Structure o In a centrifuge, the heavier the item, the lower its forming the sense and antisense strands
deoxyribose sugar form the backbone of the DNA
molecule while the base pairs form the rings. It has 4 PENTOSE Contains Contains Ribose Sugar position is o RNA nucleotides line up with their complimentary base
SUGAR Deoxyribose sugar o The original bacteria formed a single band of DNA at on the Antisense strand.
nitrogenous bases: Adenine, thymine, guanine, and
the bottom o Phosphodiester bonds formed between sugar
cytosine.
o The 1st generation formed a single band at the middle phosphate groups with the help of RNA polymerase
PAGE 13 OF 16 PAGE 14 OF 16
