Topic 2
2 Explain 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.
Davson Danielli model proposed membrane consisted of phospholipid
bilayer surrounded by 2 layers of protein. This was incorrect
because it was unclear how proteins would permit membrane to change
shape without bonds being broken Proteins are largely hydrophobic
so it shouldnt be in contact with aqueous cytoplasm Number and type
of proteins vary greatly between different cells. Evidence from
freeze fracturing - split cell membranes between 2 lipid layers
gives 3D image of surface Shows smooth surface with bumps sticking
out Phospholipid bilayer is the structure accepted which consists
of: Glycolipid - Carbohydrate chain attached to phospholipid
bilayer. Involved in cell signaling and cell attachment
Glycoprotein - Peripheral or Integral protein which carbohydrate
chain attached. Involved in cell-to-cell recognition Cholesterol -
Type of lipid. C27H46O. Controls fluidity of membrane. More
cholesterol = less fluidity = less permeable. Keeps membranes
stable at room temp, otherwise they would burst open. Integral
Protein - Span whole width of the membrane. Many are glycoproteins
Peripheral Proteins - Confined to inner and outer surface of
membrane. If on extracellular side, mainly used for receptors to
hormones and neurotransmitters and cell recognition . If on
cytosolic side, used for cell signaling and chemical reactions. Can
dissociate into the cytoplasm Channel Protein - Integral protein
which helps diffuse substances that pass through the membrane alone
but are vital for the cell Receptor Protein - Integral and
peripheral proteins with binding sites for hormones and
neurotransmitters. Can be used for enzymes catalysing reactions.
Filamental Cytoskeleton - structure within the cytoplasm that helps
it to keep its structure, protects the cell and allows locomotion
Phospholipid - Main component of bilayer. Consists of a phosphate
head and fatty acid tail. Phosphate head is hydrophilic and fatty
acid tail is hydrophobic, therefore aqueous cytoplasm is kept
outside the cell. Have a polar nature
3 Explain 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). Osmosis is the
net movement of free water molecules from area of high water
concentration to area of low water concentration via a partially
permeable membrane. Process by which cells exchange water with
their environment Net movement is dependent on difference in water
potential Isotonic - No net movement of water Hypertonic - Net
movement of water out of a cell. In plants, cytoplasm pulls away
from cell wall, cell becomes plasmolysed. In animals, cytoplasm
pulls away from cell membrane and cell becomes crenated Hypotonic -
Net movement of water into a cell. In plants, cytoplasm is pushing
against cell wall, so the cell becomes turgid In animals, cytoplasm
pushes against cell membrane and causes the cell to burst by lysis.
This is because there is no protective cell wall, unlike
plants.
4 Explain what is meant by passive transport (diffusion,
facilitated diffusion), active transport (including the role of
ATP), endocytosis and exocytosis and describe the involvement of
carrier and channel proteins in membranetransport. Small and
non-polar molecules (e.g. O2 and CO2) pass through easily As the
molecules become larger, and increase in polarity, the membrane
becomes less permeable Passive transport - The transport of
substances without the use of energy in the form of ATP Active
Transport - The transport of substances with the use of energy in
the form of ATP Diffusion - The net movement of particles from an
area of high concentration to an area of low concentration
Facilitated diffusion - The net movement of particles (diffusion)
down a concentration gradient via a channel or carrier protein.
Specifically for ions. Active Transport - The net movement of
particles (diffusion) against a concentration gradient (from low to
high) with the use of ATP. For organic molecules and ions
Endocytosis and Exocytosis involves the transport of substances
within vesicles (bulk transport for large substances) In
endocytosis, the vesicle will fuse with the cell surface membrane
In exocytosis, the vesicle will pinch off the cell surface
membrane
Give two differences between active transport and diffusion.
1. active transport is {against /eq}concentration gradient /eq
;2. active transport requires ATP /eq ;3. ref to involvement of
(membrane) proteins inactive transport ;
5 Describe how membrane structure can be investigated
practically, eg by the effect of alcohol concentration or
temperature on membrane permeability.
1. Cut piece of beetroot into 1cm3 cubes2. Place in distilled
water overnight to remove any dye released on preparation. Then
wash and blot dry. 3. Place 8 different boiling tubes of distilled
water into 8 different water baths of varying temperatures 4. When
water reaches temperature of corresponding water bath, place a 1cm3
into each boiling tube 5. Leave all test tubes for 30 mins6. Remove
beetroot cube from each tube after and place dye solution from each
boiling tubes in separate cuvettes7. Place each cuvette in
colorimeter to measure % absorbance8. Use absorbance to calculate %
transmission ( 100 - absorbance.)9. Increase in temp increases
transmission/ absorbance due to membrane molecules gaining more
heat energy.10. Vibrations caused large gaps in membrane due to
denaturing of integral and peripheral proteins in membrane, leaving
large pores for dye to leak out of. 11. Possible issues: Cutting
accurately sized beetroot/ error in reading from colorimeter/
difficulty in maintaining temperature
Variables that both of these students must keep the same if
their results are to be compared.
1. reference to pre-treatment e.g. rinsing method ; 2. {size /
mass / surface area / volume / shape} of beetroot ; 3. beetroot
storage conditions / eq ; 4. {same / type / species / eq} beetroot
; 5. {age of beetroot / storage time} ; 6. (incubation) time / eq ;
7. {volume / concentration / eq} of {water / solution}(added to
beetroot) ; 8. pH ;
6 Describe the properties of gas exchange surfaces in living
organisms (large surface area to volume ratio, thickness of
surface, difference in concentration) and explain how the structure
of the mammalian lung is adapted for rapid gaseous exchange.
Rate of diffusion is increased with: Temperature (more kinetic
energy) Steep Concentration gradient (more collisions)
Stirring/moving (more movement) Large Surface area to volume ratio
Short diffusion pathway (distance) E.g site of gas exchange in
mammals are the lungs Alveolar epithelium O2 diffuses out of the
alveoli across the alveolar and capillary epithelium into the blood
CO2 diffuses into the alveoli from the blood and is breathed
out
suggest how gas exchange occurs in an amoeba.
1. (gas exchange) occurs through the {cell membrane /
phospholipid bilayer} ; 2. idea that the membrane is thin ; 3.
oxygen enters cell (from water) / eq ; 4. carbon dioxide leaves
cell (into water) / eq ; 5. {O2 / oxygen / CO2 / carbon dioxide}
are {small / non-polar} (molecules) ; 6. reference to diffusion ;
7. {reference to / description} (suitable) concentration gradient ;
8. reference to large surface area (to volume ratio) ;
7 Describe the basic structure of an amino acid (structures of
specific amino acids are not required) and the formation of
polypeptides and proteins (as amino acid monomers linked by peptide
bonds in condensation reactions) and explain thesignificance of a
proteins primary structure in determining its three-dimensional
structure and properties (globular and fibrous proteins and types
of bonds involved in three dimensional structure).
Residual group [R] varies within each amino acid Amino acids are
joined together by peptide bonds These can formed through
condensation reactions (removal of water) This produces a
polypeptide chain This sequence of amino acids is the primary
structure of a protein Amino acids are linked by hydrogen bonds,
forming alpha- helices or eta pleated sheets and are the secondary
structures The residual group in the amino acids, and the sequence
of amino acids in the primary structure determine the bonding and
folding of the 3D structure of a protein The 3D structure has
hydrogen and ionic bonding with disulfide bridges It also has
hydrophilic regions (on the outside) and hydrophobic regions (on
the inside) Type of amino acid determines types of Residual group
Globular proteins have a relatively high number of R groups
Quaternary protein structures form when there is more than one
polypeptide chain bonded together 3D structure of a protein
determines its properties There are two types of protein: Globular
and Fibrous
Globular Round and compact Chains are coiled so hydrophilic
regions face outwards and hydrophobic regions face inwards Soluble
-> easily transported in fluids e.g Haemoglobin in the blood
Fibrous Long coiled polypeptide chains forming a rope shape Held
together by bonds (e.g. hydrogen, disulfide, ionic) makes it
strong. Often found in supportive tissue e.g. Collagen, Keratin
8 Explain the mechanism of action and specificity of enzymes in
terms of their three-dimensional structure and explain that enzymes
are biological catalysts that reduce activation energy, catalysing
a wide range of intracellular andextracellular reactions.
Enzymes speed up biological reactions The more concentrated a
system is, the more collisions occur between molecules or ions of
reacting substances The energy between these collisions has the
potential to cause a metabolic reaction [Metabolic reaction - A
molecule is combined with something else, split into smaller parts
or change its shape] Metabolic reactions are reversible therefore
they tend to run spontaneously towards a dynamic equilibrium
Enzymes are globular proteins which speed up the rate at which the
dynamic equilibrium in a reversible reaction is attained Enzymes
are not used up in the reaction Enzymes either work for the forward
or backward reaction Within a metabolic reaction, there is a
minimum amount of energy required for the reaction to spontaneously
take place. This is the activation energy Enzymes lower the
activation energy
Enzymes have an active site specific to a particular substrate
that it can chemically recognise, bind to and modify Early
scientists believed in the lock and key model - where a substrate
directly fits into the active site, forms an enzyme substrate
complex, and the products detach from the active site However this
was not accurate, since evidence showed that the substrate changed
shape slightly to fit into the active site The Induced Fit Model
showed that the substrate does not have to fit into the active
site. The initial contact strains the bonds in the substrate, which
makes them easier to break in the formation of new products In
order for the reaction to be catalysed, the substrate must change
its shape correctly at the right orientation for the reaction to
occur
An enzyme's active site is determined by its 3D structure Each
enzyme has a different 3D structure, therefore a different active
site
Describe the structure of an enzyme.
1. ref to an enzyme as a protein ;2. ref to {3D / tertiary /
globular} structure ;3. ref. to named bonds (holding structure
inplace) ;4. between the R groups ;5. ref to active site ;6. idea
of specificity of active site ;
Describe the three-dimensional (tertiary) structure of an
enzyme
1. globular / eq ; 2. reference to active site ;3. reference to
specific shape of active site ; 4. reference to {bonds /named bond
/ interaction / eq} between R groups ; 5. credit correctly named
{bond/interaction} e.g. disulphide bond, hydrogen bonds,
hydrophobic interactions (between R groups) ; Explain how the
primary structure of an enzyme determines its three-dimensional
(tertiary) structure and its properties.
1. (primary structure) {position / sequence / order /eq} of the
{amino acids / R groups} / eq ; 2. idea that this determines the
{positioning / type} of the {bonds / folding / eq} ; 3. determining
the {shape / properties} of the active site / eq ; 4. idea of
interaction of active sites and substrates e.g. enzyme substrate
complex forms ; 5. idea of {polar / hydrophilic} on the outside of
enzymes / {non polar / hydrophobic} on the inside / eq ; 6.
reference to solubility ;
9 Describe how enzyme concentrations can affect the rates of
reactions and how this can be investigated practically by measuring
the initial rate of reaction.1. Set up a reaction where there is
effervescence of a gas as a product (e.g. Hydrogen Peroxide and
catalase)2. Therefore you can measure the volume of oxygen give
off. 3. Connect a delivery tube connected to a bung of the solution
of hydrogen peroxide and catalase to an upside down measuring
cylinder4. This measuring cylinder must be upside down in water (to
collect the oxygen)5. For the first minute, record the vol produced
and divide by the time to give the initial rate of reaction 6.
Repeat by increasing the concentration of catalase7. Controls: Vol
of hydrogen peroxide, temperature, pH 8. Errors: Oxygen escapes
from measuring cylinder whilst placing upside down, some oxygen
diffuses into the water9. Explanation: As enzyme concentration
increases, rate of reaction increases. 10. Increase in
concentration, increases the number of active sites and probability
of enzyme substrate complexes forming11. This lowers the activation
energy of the reaction and speeds up the rate
Describe how this apparatus could be used to compare the
catalase activity
1. reference to measuring volume of oxygen ; 2. suitable
reference to time e.g. oxygen produced in unit time, time taken to
produce same volume of oxygen ; 3. idea of measuring the initial
rate of reaction ; 4. reference to controlled variable in relation
to the mussel e.g. age, part of mussel, mass, surface area ; 5.
reference to a controlled variable in relation to the experiment
e.g. volume of hydrogen peroxide, temperature, concentration, pH ;
6. suitable reference to repeats ;
10 Describe the basic structure of mononucleotides (as a
deoxyribose or ribose linked to a phosphate and a base, ie thymine,
uracil, cytosine, adenine or guanine) and the structures of DNA and
RNA (as polynucleotides composed ofmononucleotides linked through
condensation reactions) and describe how complementary base pairing
and the hydrogen bonding between two complementary strands are
involved in the formation of the DNA double helix.
Hydrogen bonds occur between the nitrogenous bases In DNA,
Adenine to Thymine and Guanine to Cytosine This is called
Complementary base pairing In RNA, Uracil replaces Thymine
Mononucleotides are linked by condensation reactions This forms a
DNA double helix
11 Describe DNA replication (including the role of DNA
polymerase), and explain how Meselson and Stahls classic experiment
provided new data that supported the accepted theory of replication
of DNA and refuted competing theories.
DNA Replication Double helix unzips forming 2 separate strands
Mononucleotides join to the exposed strands by complementary base
pairing This forms 2 identical DNA strands
Three types of replication proposed before Meselson Stahl
Experiment: Conservative - DNA strands were wrapped around histone
proteins which allowed it to replicate Semi conservative - DNA
helix could unzip and new templates could be copied from each side
Dispersive - Split itself into multiple segments and then
replicates itself
Meselson and Stahl Grew E.coli in Heavy Nitrogen (N15) and spun
in centrifuge N15 would sink to the bottom of the test tube Grew
E.coli in Light Nitrogen (N14) Combined with light nitrogen DNA of
first generation, bacteria were hybrid half N15 and N14 Second
generation formed 2 strands of N14 This shows that semi
conservative replication is the method of DNA replication
12 Explain the nature of the genetic code (triplet code only;
nonoverlappingand degenerate not required at AS).
Triplet code - 3 bases code for one amino acid 4 x 4 x 4 = 64
amino acids which meets the requirement of 20 amino acids that need
to be coded for
13 Describe a gene as being a sequence of bases on a DNA
molecule coding for a sequence of amino acids in a polypeptide
chain.
Gene is a sequence of bases on a DNA molecule coding for a
sequence of amino acids on a polypeptide chain
14 Outline the process of protein synthesis, including the role
of transcription, translation, messenger RNA, transfer RNA and the
template (antisense) DNA strand (details of the mechanism of
protein synthesis on ribosomes are notrequired at AS).
Transcription Hydrogen bonds break between bases/ DNA Helix
unwinds This is catalysed by DNA Helicase Forms 2 separate strands
Free floating mono nucleotides containing ribose sugars attach to
the exposed bases on the template strand Formation of
phosphodiester bonds by condensation reaction During this Thymine
replaces Uracil Catalysed by DNA ligase, the mRNA strand detaches
from the template strand This mRNA strand is modified then leaves
the nucleus via the nuclear envelope.
Translation The mRNA strand attaches to the ribosome binding
site tRNA molecules carrying an anti codon and amino acid, join to
the codon on the mRNA strand (by complementary base pairing) When
the tRNA molecule detaches it leaves an amino acid behind. As the
mRNA strand passes through the ribosome, more amino acids join
together by peptide bonds This forms a polypeptide chain (primary
structure of a protein) The start codon initiates translation The
stop codon stops translation (UUG)
15 Explain how errors in DNA replication can give rise to
mutations and explain how cystic fibrosis results from one of a
number of possible gene mutations. Mutation is a change in the base
sequence of DNA Point mutation - Change specific nucleotides
(bases) Chromosomal mutation - Change in position of whole genes
within a chromosome Whole chromosome - Chromosome is missing or
duplicated (deletion, insertion, replication)
16 Explain the terms gene, allele, genotype, phenotype,
recessive, dominant, homozygote and heterozygote, and explain
monohybrid inheritance, including the interpretation of genetic
pedigree diagrams, in the context of traits such ascystic fibrosis,
albinism, thalassaemia, garden pea height and seed morphology.
Allele - Version of a gene Genotype - Set of genes in an organism
Phenotype - Set of characteristics of an organism as a result of
its genotype and its interaction with the environment Recessive -
Allele that does not affect the phenotype in the presence of
dominant allele (requires two copies of allele) Dominant - Allele
that always affects the phenotype (only require one copy of allele)
Homozygote - Individual which possesses two copies of the same
allele Heterozygote - Individual which possesses two different
alleles Monohybrid Inheritance - The inheritance of one
characteristic
1) Cystic Fibrosis Recessive Mutation in the CFTR protein
Affects the lungs, digestive and reproductive systems
2) Albinism Recessive Mutation in protein for melanin synthesis
Stops melanocytes from producing melanin
3) Thalassaemia Recessive Mutation in gene coding for alpha haem
Causes slow haemoglobin production
4) Garden Pea Height Controlled by a single gene with 2 alleles
Allele for tall plants is dominant (T) Allele for small plants is
recessive (t) 5) Seed Morphology Controlled by a single gene with
two alleles Allele for smooth seeds is dominant (S) Allele for
wrinkled seeds is recessive (s)
17 Explain how the expression of a gene mutation in people with
cystic fibrosis impairs the functioning of the gaseous exchange,
digestive and reproductive systems.
CFTR protein present in apical membranes (in epithelial cells)
Affects reproductive, digestive and respiratory systems
1. CFTR protein is a pump to actively transport Cl- ions out of
the cell 2. This prevents Na+ from entering the cell 3. Water
remains outside of the cell by osmosis
Respiratory Cilia lining the airways cannot move because mucus
is viscous Leads to mucus buildup Gas exchange cannot take place in
the area below the blockage Surface area available is reduced,
reducing the efficiency of gas exchange This causes breathing
problems More prone to lung infections because mucus containing
microbes cannot be removed
Digestive Tube connecting pancreas to intestine becomes blocked
with viscous mucus Prevents digestive enzymes produced by pancreas
from reaching small intestine Reduces ability to digest food fewer
nutrients absorbed Mucus can cause cysts to form in pancreas,
inhibiting the production of enzymes Mucus lining of the small
intestine is abnormally thick
Reproductive In men Vas Deferens becomes blocked or is missing,
so sperm from the testicles cannot reach the penis In women,
cervical mucus prevent sperm from reaching the egg reduces motility
of the sperm
18 Describe the principles of gene therapy and distinguish
between somatic and germ line therapy.
Principles: Cells are removed from the patient In lab, virus is
modified so it cannot reproduce A gene is inserted into the virus
(Genetic engineering) The altered virus is mixed with cells from
the patient The cells from the patent become genetically altered
The altered cells are injected into the patient (e.g bone marrow)
The genetically altered cells produce the desired protein or
hormone Treatment must be repeated Somatic therapy Change alleles
in body cell Added to cells most affected by disorder Germ line
therapy Changes to alleles in sex cells Cells of offspring will be
affected and wont suffer from the disease If disorder is recessive,
add a working dominant allele If disorder is dominant silence allle
by sticking DNA in the middle of it Types of vectors: Liposome
(spheres made of lipid), Plasmids (rings of bacterial DNA, modified
viruses (that can accept human DNA)zx
19 Explain the uses of genetic screening: identification of
carriers, preimplantation genetic diagnosis and prenatal testing
(amniocentesis and chorionic villus sampling) and discuss the
implications of prenatal genetic screening.
Used for identifying a carrier (before pregnancy) Carried out by
a taking a blood sample and looking at the chromosomes to see if
the patient is a carrier
Preimplantation genetic diagnosis requires IVF Remove cell from
zygote before placing in mother Check DNA/genome to see if it
carries the gene for the disease Prenatal screening Amniocentesis -
Withdrawal of a tiny sample of amniotic fluid within the membrane
sac (amnion) surrounding the fetus. (15-16 weeks) Chorionic Villus
Sampling (CVS) - Withdraw fluid containing cells from the membrane
sac (chorionic villi) surrounding the amnion (8-12 weeks)
Implications PGD allows couples to test embryos before implantation
(so only embryos without the allele may be implanted) PGD reduces
chance of babies being born with genetic disorders Prenatal
screening allows parents to make informed decisions and decide
whether they will have the child or have an abortion Prenatal
screening allows parents to prepare for the future care of the
child and arrange any medical treatment which can start as soon as
the child is born.
20 Identify and discuss the social and ethical issues related to
genetic screening from a range of ethical viewpoints
Social Tests arent always 100% accurate - could give false
positives and negatives Other genetic abnormalities may be found
Prenatal testing increases the risk of miscarriage Results of
genetic tests may be given to employers or insurance companies -
leading to genetic discrimination Ethical Unethical to abort a
fetus because it has a genetic disorder Used to find out other
characteristics - concern over people in the future selecting
embryos for desired characteristics (designer babies)