Edexcel A Biology A level core practical Write ups. All include an introduction, hypothesis, variables, method, results, evaluations...
CP3 - Investigate membrane structure, including the effect of alcohol concentration or temperature on membrane permeability
Core practical 3 - Investigate membrane structure including the effect of alcohol
concentration or temperature on membrane permeability
Aim: to investigate the effect of alcohol concentration on membrane structure
Introduction
Red beetroot (Beta vulgaris) has a large and fleshy root that is edible. It belongs to the
Chenopodiaceae family and is rich in betalains . Betalains are water-soluble nitrogen
pigments present in vacuoles of plants (Sadowska-Bartosz and Bartosz, 2021). Betalamic
acid can either condense with imino compounds (cyclo-DOPA and/or its glucosyl derivatives)
to form violet betacyanins (e.g. betanin), or with amines and their derivatives to form yellow
betaxanthins which contains different amino acids or amine residue (Prieto-Santiago et al.,
2020). Betalain make up to ~ 70–100% of phenolic composition of beetroot, limited to 0.8–
1.3 g/L of fresh beetroot juice (Mirmiran et al., 2020) (about 75-95% betacyanins and 5-25%
betaxanthins). The content of these pigments is the highest on the surface (peel) of the
beetroot and steadily decreases to the interior of the root. (Sadowska-Bartosz and Bartosz,
2021).
In 1972, the fluid mosaic model for membrane structure was proposed based on
thermodynamic principles of organisation of membrane lipids and proteins and available
evidence of asymmetry and lateral mobility within the membrane matrix. It was proposed by
S.J. Singer and G.L Nicolson and remains relevant with new information being used to build
on the original model. The model depicts biological membrane as a matrix made up of a
mostly fluid bilayer of phospholipids with globular integral membrane proteins and
glycoproteins that were intercalated into the fluid lipid bilayer. (Nicolson, 2014).
A phospholipid is made up of 2 fatty acid tails and a phosphate group head. Fatty acids are
long chains that are mostly made up of hydrogen and carbon, while phosphate groups
consist of a phosphorus molecule with four oxygen molecules attached. These two
components are connected via a third molecule, glycerol (BD Editors, 2017).The phosphate
head of the molecule is polar; one end is slightly positive and the rest is negative. This
makes the phosphate head attract other polar molecules like water and is therefore
hydrophilic. The fatty acid tails are nonpolar and therefore hydrophobic. Thus the membrane
lipids, mainly phospholipids, self-assemble with their hydrophobic tails excluding water to
form bilayers (University Of York Science Education Group. Nuffield Curriculum Centre,
2015).
The most fluid membranes, including those that are low in cholesterol, are the most easily
disordered by ethanol. Ethanol is able to form hydrogen bonds with the lipids in the bilayer,
and these hydrogen bonds reduce the order parameter of the lipid hydrocarbon chains. The
combination of these aspects results in an easy penetration of ethanol through the bilayer.At
high concentrations, alcohols reduce bilayer stability and break down the lipid bilayer barrier
properties, causing increased ion permeability (Philip, 2021). The cells of beetroot have
red pigment in the vacuoles. When the membranes of the vacuole and the cell membrane
are damaged by ethanol, a kind of alcohol, pigment will leak out (Heath, 2019).
, Core Practical 3 - 23/03/2022
Hypothesis : as the alcohol concentration increases, the absorbance value on the
colorimeter will increase as ethanol damages the cell surface membrane causing more
pigment to leak out.
Variables
Independent variable : alcohol concentration ranging from 0% to 70% that the samples of
beetroot are placed in
Dependent variable : Permeability of the beetroot cell membrane which is inferred from
light absorbance, transmission of light (Au)
Control variables :
- Keep the surface area and mass of the beetroot pieces as similar as possible using
ruler, scalpel and the same size cork borer. As a larger surface area would have
more cells which will get damaged and release more pigment than a smaller piece.
- Perform the entire practical in one go to maintain a regular temperature as a higher
temperature would affect the strength of the membrane.
- Should be put in distilled water for the same amount of time and left in the solutions
for the same amount of time. The longer the beetroot is left in the alcohol solution,
the more the cell membrane will be damaged and the more pigmented the solution
will become.
- Using the same filter of the same colour, which is green in our experiment as
different coloured filter would absorb and transmit different colours
- The same colorimeter should be used for all the solutions which are calibrated using
distilled water. This makes sure all the values are relative to the water and changing
colorimeter would need to be recalibrated each time and may give different results
- Do not touch the clear part of the cuvette as this will leave fingerprints. Fingerprints
absorb and scatter light slightly despite not being readily visible which would affect
the absorbance reading.
- After waiting 30 minutes, shake the solutions for the pigment and alcohol solutions to
be evenly mixed. This will prevent any zones of concentrated pigment. Shake every
tube the same amount and as little as possible. Shaking may cause damage to the
beetroot and cause extra colour to leak out not due to the alcohol which would affect
the validity of the results.
- Size of cuvette for colorimeter and the same type of the cuvette is the same used for
all trials since different types of cuvette are made with different materials which may
affect the experiment when using the colorimeter as they perform differently. The
cuvette was calibrated at the beginning.
Control
Keeping a beetroot piece in 5cm³ of distilled water at room temperature can provide control
results. Water should not affect the permeability of the membrane which can be used to
compare the effect of the alcohol solutions.
Risk assessment
- Cork borer and scalpel are sharp and may result in cuts → cut away from
fingers, use forceps to hold the sample while cutting with the scalpel and
put away immediately after use
- Alcohol is highly flammable and an irritant → wear safety goggles and
keep away from ignition sources
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