Chapter 2: Importance of water Flashcards
Describe and explain the molecular structure of water
- Shared negative hydrogen electrons are pulled towards the oxygen atom
- The other side of the hydrogen atom is left with a slight negative charge
- The unshared negative electrons on the oxygen atom give it a slight negative charge
- This makes water molecule polar
- Slight negatively-charged oxygen atoms attract the slightly positively-charged hydrogen atoms in adjacent water molecules
- i.e. hydrogen bonds form as a result of dipoles formed by electronegativity
Name the intermolecular bonds that occur between water molecules
hydrogen bonds
Explain the formation of hydrogen bonds
In water (H20) the oxygen atom attracts the electrons more and so the oxygen atom becomes more electronegative and is said to have a slight negative charge
In comparison the hydrogen atoms become slightly short of electrons and become slightly electropositive
Draw a diagram to show the intermolecular forces that exist between water molecules
Explain the biological importance of the high specific heat capacity of water
- SHC = energy required to raise temperature of 1g water by 10 oC
- Hydrogen bonds between H2O molecules absorb high amount of energy
- i.e. takes a lot of energy to raise the temperature of large volumes of H2O
- Important in keeping aquatic habitats thermally stable
- Important in stabilising internal body temperature as the environment changes
Explain the biological importance of the transparency of water
- Allows light to pass through it
- Important for light dependent stage of PHS
Explain the biological importance of the high heat of latent evaporation of water
- LHE = energy required to break hydrogen bonds between H2O molecules
- i.e. energy required to convert H2O from liquid state to gaseous state
- Important in thermoregulation of mammals: sweating, panting
- Evaporation of water removes heat from surface of skin/tongue
Explain the biological importance of the high cohesion between water molecules
- Cohesion = attraction between H2O molecules
- Important in transpiration stream & long continuous columns of H2O form in the xylem vessels
- Also important in transporting and circulating molecules e.g. plasma, translocation
- Provides supportive role:
- hydrostatic skeleton (e.g. earthworm),
- turgor pressure (plants),
- amniotic fluid (supports & protects foetus),
- supports aquatic organisms
- Enables motility of aquatic organism: as they thrust against it → forward motion
Explain the biological importance of water as a universal solvent
- Carries polar molecules and ions
- Ions and ionic molecules are surrounded by water molecules → hence they dissolve
- Important in transporting ions, proteins etc in plasma
- Important in removing waste e.g. urea in urine
- Important in allowing chemical reactions to take place inside cells e.g. hydrolysis of macromolecules (proteins & lipids), respiration, protein synthesis
- Metabolic functions: reactant in PHS, hydrolysis reactions & medium for all biochemical reactions
Explain the biological importance of water becoming less dense when it freezes
- Density decreases when it freezes → floats on surface of water in liquid state
- Important as insulates water beneath the ice
- Also forms surface which acts as habitats for some organisms e.g. polar bear
- Changes in density produce circulation currents in large bodies of water → aids nutrient cycling
Explain the biological importance of the adhesive properties of water molecules
- Acts as a lubricant
- e.g. formation of pleural fluid (minimizes friction between lungs & ribcage)
- mucus (to allow passage of faeces from large intestine to rectum and out of anus)
- synovial fluid (reduces friction and resistance in joints)
Explain the difference between intercellular and extracellular fluids and give examples of each
a) Intracellular:
- fluids within cells
- e.g. cytosol, cell sap, vacuole contents, fluid in xylem vessels & phloem sieve tube elements
b) Extracellular:
- fluid which is outside of the cells and bathes the cells
- e.g. interstitial fluid, tissue fluid, plasma, lymph, serum
Outline how tissue fluid is formed
- At start of capillary bed nearest arteries there is hydrostatic pressure inside capillaries is greater than the hydrostatic pressure in the TF
- Hydrostatic pressure gradient forces fluid out of plasma through fenestrations
- Fluid filled spaces between cells = TF
- As fluid leaves capillaries the hydrostatic pressure inside capillary decreases
- At venous end of capillary bed the hydrostatic pressure inside capillaries is lower than the hydrostatic pressure in the TF
- Instead the reduction of fluid in the plasma raises the oncotic pressure (due to presence of plasma proteins in plasma → reduce water potential of plasma)
- Water potential of plasma < water potential of TF so water re-enters the capillaries by osmosis
Describe the composition of tissue fluid
- Plasma which has escaped from blood capillaries through fenestrations to surround cells
- Contains oxygen, water and nutrients
- Doesn’t contain RBC
- Doesn’t contain large proteins (as too large to pass through fenestrations) but will contain small short chain proteins with low molecular mass
- May contain some WBC that can squeeze through the fenestrations e.g. macrophages
Outline the formation of lymph
- ~10% TF (i.e. excess TF) drains back into the blood circulatory system via a network of lymph vessels
- Once fluid is inside lymph vessels = called lymph
- It is similar to tissue fluid but contains more leucocytes
- Lymph drains back into the circulatory system via the subclavian veins (under the collar bone) & then passes to the vena cava before passing to the heart
- Valves in lymph vessels prevent backflow of lymph to TF
Describe the composition of lymph
- Cells = lymphocytes (multiplied and stored in lymph nodes) (NOT macrophages!)
- Proteins = some small proteins, antibodies, hormones
- Lipids = higher concentration than blood plasma as fatty acids and glycerol are absorbed from the lacteals in the small intestine
- Glucose = lower levels than plasma and tissue fluid
- Amino acids = lower levels than plasma and tissue fluid
- Oxygen = lower levels than plasma and tissue fluid
- Carbon dioxide = higher levels than plasma and tissue fluid
Explain the role of lymph
- Circulatory role
- Important role in immune response:
- lymph nodes are primary sites where pathogens and other foreign substances are filtered from the lymph fluid and engulfed (phagocytosis) and destroyed
Outline the formation of urine
- Formed by in kidney Involves ultrafiltration of blood followed by selective reabsorption
- Urine formed from
- breakdown of excess amino acids and proteins (can’t be stored)
- in the liver
- produces ammonia (highly toxic)
- so converted to urea
- which dissolves in water to form urine
Explain the function of urine
To remove:
- Nitrogenous waste (urea)
- Soluble waste products
- Excess water
- Excess ions
Describe the composition of serum
- Plasma with clotting factors removed
- Contains electrolytes, antibodies (Ig), antigens, hormones and soluble proteins which are not involved in clotting
Outline how serum can be obtained from whole blood
- Leave whole blood at room temperature & allow it to clot (~15-30 minutes)
- Remove clots by centrifuging (~10 minutes)
- Remove serum and store at 2-8oC
- Used in blood typing and diagnostic testing for IgG toxomplasma antibodies
State the 3 main components of plasma
- Water
- Mineral ions
- Plasma proteins
Explain the role of water as a component of plasma
- Transports dissolved substances
- Provides body cells with water
- Distributes heat & has important role in thermoregulation
- Regulation of water content helps regulate blood pressure and blood volume
Explain the role of mineral ions as a component of plasma
- Maintain osmotic balance
- pH buffering
- regulation of membrane permeability
- Ion specific roles e.g. calcium is important in blood clotting
State the function of a named plasma protein
Serum albumin
- Regulates osmotic balances,
- pH buffering
- calcium transport
Fibrinogen & prothrombin
- Role in blood clotting
Immunoglobulins
- Antibodies involved in immune response
Enzymes
- Regulate and take part in metabolic activities
State 4 examples of non-cellular substances transported in plasma
- Products of digestion (e.g. glucose, fatty acids, glycerol & amino acids)
- excretory products (e.g. urea)
- hormones (e.g. insulin, oestrogen, testosterone)
- vitamins (e.g. Vit A and B12)
Explain the difference between a macromolecule and a polymer and give an example of each
Macromolecule
- Large organic molecule
- E.g. lipid, phopholipid
Polymer
- A specialised type of macromolecule made up from repeating smaller subunits called monomers
- E.g. starch, glycogen, proteins, DNA, RNA
State the 5 main functions of carbohydrates
Hint: SCEEM
- Structural e.g. cellulose
- Cell markers e.g. joined to proteins or lipids → glycoproteins/glycolipids, cell surface markers, receptors for hormones, antigens
- Energy source: immediate source of energy e.g. glc
- Energy store: long term energy store e.g. glycogen, starch
- Macromolecules: used to make other macromolecules e.g. antibodies (specialised glycoproteins)
Draw a molecule of alpha glucose
Explain how alpha glucose differs from beta glucose
- Look at carbon 1
- The OH group points above the ring in beta glucose, but in alpha glucose it is below the ring
ABBA: alpha below, beta above
Explain how 2 monosaccharides are joined together
- requires condensation reaction
- makes a glycosidic bond (type = covalent)
- removes an OH off one monomer and an H off the adjacent monomer
- produces molecule of water as waste product
Explain how a disaccharide is split
- requires hydrolysis reaction
- breaks a glycosidic bond (type = covalent)
- adds an OH to one monomer and an H to the adjacent monomer
- involves the addition of a molecule of water
State definition of a condensation and hydrolysis reaction
Condensation reaction
- joining of two monomers
- by the removal of H2O
- to form a new covalent bond
Hydrolysis reaction
- splitting of a dimer/polymer to remove a monomer
- by the addition of H2O
- to break an existing covalent bond
Draw a diagram to show how 2 monosaccharides can be joined together to form a disaccharide
Write the word equation to produce maltose, sucrose and lactose
alpha-glc + alpha-glc → maltose + H2O
alpha-glc + fructose → sucrose + H2O
alpha-glc + galactose → lactose + H2O
State 2 location in humans where glycogen is stored
- Liver cells
- Skeletal muscle cells
State and describe the process forms glycogen
- Glycogenesis
- Thousands of condensation reactions are used to join thousands of alpha glucose molecules together to form a polysaccharide
Describe the structure of glycogen
- Branched molecule
- At branching points there are alpha 1-6 glycosidic bonds
- Within branches there are alpha 1-4 glycosidic bonds
Describe the properties of glycogen that make it useful as an energy storage molecule
- hydrophillic but due to large size of the macromolecule makes it insoluble
- doesn’t affect water potential of the cell i.e. osmotically inert
- compact so the cell can store large quantities of glc in one location
Explain why it is advantageous to the cell to store energy in the form of glycogen
- Many terminal ends for enzyme attachment to add or to remove glc
- Glc can be stored quickly
- Glc can be mobilisd quickly
- More compact so it takes up less space within the cell
State 3 places where starch is stored in plants
- As starch grains in most plant cells
- Seeds
- Storage organs e.g. tubers
Name the 2 polysaccharides that form starch
- amylose
- amylopectin
Describe the differences between amylose (AL) and amylopectin (AP)
- AL linear; AP highly branched
- AL smaller (300-3000 glc); AP larger (2,000-200000 glc)
- AL less easily digested; AP more easily digested
- AL more compact; AP less compact
- AL insoluble; AP soluble
- AL~30% starch; AP ~70% starch
Explain how the Benedict’s test works to show the presence of reducing sugars
- Sugars with the ability to donate an electron to another molecule
- i.e. can reduce Cu2+ to Cu+
- each disaccharide has one reducing end (C not involved in the glycosidic bond) and one non-reducing end
State 4 examples of reducing sugars
- Glucose
- Fructose
- Galactose
- Ribose
Describe how to determine the relative quantity of reducing sugar in a sample
- Add an equal volume of Benedict’s reagent (copper sulphate solution) and the solution to be tested for the presence of reducing sugars to a boiling tube.
- Place the boiling tube in a water bath at 90 oC for 5-10 minutes.
- Observe the colour of the precipitate formed to estimate the quantity of reducing sugar present.
Write an equation to represent the biochemical reaction that takes place in a Benedict’s test
Reducing sugar (reduced) + Cu2+(aq)
→ reducing sugar (oxidised) + Cu+(s)
Precipitate = copper oxide (CuO)
Describe how to determine the absolute quantity of reducing sugar in a sample
- React analyte (substance to be tested for presence of glc) with excess Benedict’s solution
- Filter solution to remove ppt (thoroughly rinse test tube with distilled water to ensure all ppt removed from test tube)
- Air dry ppt (ensuring no air currents to blow away dried ppt)
- Weigh to constant mass (to ensure all water has evaporated)
- Mass of ppt is proportional to amount of reducing sugar present
Suggest 3 errors with the filtration Benedict’s test
- Not removing all the precipitate from the test tube
- Some of the precipitate may be lost from the filter paper due to air currents
- Some of the precipitate may be lost through the pores of the filter paper
Suggest how to improve the reliability of of the filtration Benedict’s test
- Use finer graded filter paper (smaller pores → less will be lost during filtration process)
- Avoid spills
- Wash the test tube and filter more than once
- Dry to a constant mass
- Take repeat readings and calculate a mean
Describe how to use the colorimeter to determine the precise quantity of reducing sugar present in a sample
- Zero the colorimeter at the start using a ‘blank’ i.e. a cuvette containing distilled water (to ensure the meter is reading 0% absorbance and 100% transmission)
- Add sample of analyte to cuvette
- Handle cuvette carefully = to prevent grease from fingerprints would scatter the light ⇒ produce inaccurate readings
- Select red filter: to ensure greater accuracy of the readings (filter absorbs blue light and transmits red light → if the solution is less concentrated less blue light is absorbed)
- Light beam is then passed through the filter and the sample in the cuvette
- The amount of light that passes through the sample is detected b the photoelectric cell
- This is converted into a electrical signal
- This is displayed on the monitor
Describe the difference between the % transmission and absorbance values obtained from a colorimeter
Absorbance (AU) = amount of light that is absorbed by the solution
% transmission = amount of light that passes through the solution
Describe how to generate a calibration curve to determine the concentration of reducing sugar in a sample
- Use a range of known concentrations of reducing sugars (‘standards’) using a serial diltion
- Carry out a Benedict’s test on each standard (solution of known concentration)
- Filter out the precipitate (or centrifuge to separate the supernatant)
- Use a colorimeter to determine the amount of light that passes through the solution
- Plot the values and construct a line graph (calibration curve) with [reducing sugar] on x-axis, %T on y axis
- Carry out the test on the unknown solution and read the value from the calibration curve
Define the term biosensor
Analytical device that is used to detect the presence of a chemical molecule ( the ‘analyte’) by combining the chemical with a biological component
Explain why biosensors are used in the medical field
- Can be use to determine the presence and/or the concentration of the analyte e.g. glucose
- If the analyte is present and binds to the biological component then
- a colour change occurs or
- an electrical change occurs
State the 4 main components of a biosensory and for each component describe its function
- Molecular recognition (bioreceptor):
- A protein (usually an enzyme or antibody) or single strand of DNA (ssDNA) is immobilised to a surface e.g. glucose test strip
- This will interact with or bind to the analyte
- Transduction
- Reacts with the analyte to cause a change in the transducer e.g. change in pH
- Converts the chemical change into an electrical current (signal)
- i.e. the transducer produces a response
- Amplifier
- Increases the strength of the signal
- Size of the signal is dependent on the concentration of the analyte
- Display (processor)
- The processor then converts the electrical signal into a produces a visible reading or measurement e.g. a particular colour on a test strip
- This can be qualitative or quantitative e.g. a numerical reading on a test machine
Name 2 different commercial biosensors that can be used to test for the presence of glucose in urine. For each biosensor state the colour change that would be observed if glucose levels were elevated
Clinistix
light pink = negative normal level
dark ‘pink’ colour indicates abnormally raised levels of glucose
Diastix:
light blue = negative normal level
dark ‘brown’ colour indicates abnormally raised levels of glucose
Outline how to determine the glucose content of a urine sample using a biosensor
- Test strip is dipped into the urine
- Enzyme, glucose oxidase, is immobilised & attached to a test strip
- Active site of glucose oxidase is complementary to glucose i.e. test is specific for glucose
- Strip has immobilised glucose oxidase & 2nd enzyme called peroxidase on test pad
- Glucose oxidase catalyses the reaction between glucose and oxygen
- To produce gluconic acid and hydrogen peroxide
- Peroxidase breaks down the hydrogen peroxide to water & oxygen
- The oxygen then oxidises a colour dye (a chromogen) on the test pad
- Intensity of the colour change on the pad reflects the amount of glucose present in the urine
- Colour on the test strip compared to colour charts (i.e. standards) after a specified period of time
- Results = qualitative estimate on the concentration of glucose
Write a flow diagram to show the main chemical reactions that occur when using a biosensor to determine the glucose content of urine
Explain why clinistix and diastix can not be used on blood samples
To red colour of the blood would mask any colour change on the test pad
Outline how a glucometer can be used to determine the glucose level in a blood sample
- Sterile lancet used to pierce the skin on a warm finger pad
- Single drop of blood is placed & smeared on to the test strip
- Test strip is placed in a portable meter
- Test strip is impregnated with the enzyme glucose dehydrogenase
- Enzyme is specific for glucose and converts it to gluconolactone
- Reaction produces a signal that is converted to an electrical current (potential) by the electrode (transducer) on the test strip
- Size of the electrical current is dependent on the glucose concentration
- Quantitative result is displayed on the screen within 25-30 seconds
State 5 advantages of a glucometer compared to clinistix
- Simple to use & interpret
- Is not subjective ⇒ generates numerical data
- Gives specific absolute reading (not an estimate within a range)
- Numerical data can be stored over a period of time by the electronic device
- Electronic device can be reused (clinistrips are single use only)
Explain how a colorimeter can be used to determine the glucose concentration of a blood sample
- Beam of light is shone through the analyte solution
- Device measures absorbance or transmission of a particular wavelength of light
- Photoelectric cell detects light that has passed through solution → generates % transmission reading
- Photoelectric cell detects light that has passed through solution → calculates % absorbance reading
Give 4 advantages of using any biosensor rather than standard lab practicals
- specificity the biosensor only measures one specific chemical
- quantitative result obtained
- only a small sample (of blood or urine) is required
- results are obtained rapidly
Define the term ‘non-reducing sugar’ and give an exaple of a NRS
- Sugars without the ability to donate an electron to another molecule
- i.e. can’t reduce Cu2+ to Cu+
- E.g. sucrose
Outline how to carry our a test for a NRS
- Test the solution for the presence of reducing sugar
- If this test gives a negative test result then continue to test for the presence of a non reducing sugar:
- Add an equal volume of hydrochloric acid to the solution to be tested for the presence of a non reducing sugar in a boiling tube.
- Place the boiling tube in a water bath at 100oC for 5 minutes: HCl hydrolyses glycosidic bond within disaccharide e.g. sucrose (NRS) → glc & frc (both RS)
- Remove the boiling tube from the water bath and very slowly add sodium hydrogen carbonate solution to the boiling tube (to neutralise the acid as Benedict’s reagent is alkaline (~pH10) so no reaction will occur at low pH)
- Using a pipette to remove a few drops of the solution, test the contents of the boiling tube with pH paper to determine when the solution has reached pH7 (i.e. continue to add sodium hydrogen carbonate until the pH paper shows pH7)
- Re-test the remaining solution in the boiling tube with Benedict’s reagent.
Outline the biochemical test used to detect the presence of starch
- Add two drops of starch solution to a well in the spotting tile using a dropping pipette.
- Add two drops of potassium iodide (iodine) solution to the well using a clean dropping pipette
- Observe and record the colour.
Outline how to carry out a control test for the starch test
- Repeat the test replacing the two drops of starch with two drops of the distilled water, again using a clean pipette.
- Observe the colour and compare with the colour of the starch–iodine complex.
- Water is used a control test as it will not form a complex with the KI and hence the colour remains yellow-brown (unless there is contamination) – this confirms a negative reaction
State the term used to describe an abnormally high level of protein in the urine
proteinuria
State 2 reasons why protein may appear in the urine
- Kidney damage
- Kidney disease
Explain how testing for the presence of proteins in urine can be used both as a screening test and a diagnostic test
Diagnostic test:
- If there is only albumin in the urine = albuminuria
Screening tests:
- Low levels of proteinuria is a risk indicator for developing heart disease and blood vessel disease
- Low levels of albuminuria is a risk indicator for developing heart disease and blood vessel disease
Explain why urine may still contain some proteins in the urine even if the kidney is working correctly
- Small proteins may be filtered out of the plasma in the glomerulus in the kidney
- Some proteins are actually produced by cells that line the genitourinary tract
Outline how to test for the presence of protein in a sample
- Using a syringe, add 2 cm3 of the solution to be tested to a test tube
- Using a clean syringe, add 2 cm3 of 10% sodium hydroxide (irritant) and shake to mix
- Add few drops of dilute (0.05%) copper (II) sulfate (low hazard) drop by drop, shaking between each addition
- Check for any colour change after 10 minutes (Do not get confused with the blue colour of the copper sulphate!)
- Repeat the test using a clean test tube and syringes and substituting distilled water for the protein suspension
State the 2 chemicals that are used to produce Biuret reagent
Sodium hydroxide Copper (II) sulfate
Describe the observations that could occur when testing a sample for the presence of proteins
Presence of peptide bonds: Lilac solution Absence of peptide bonds: Blue solution
Explain why a solution of 5% glycine would not give a positive Biuret test
Glycine is only a single amino acid Hence there are no peptide bonds present So the Biuret reagent will remain blue
Outline how to carry out a quantitative Biuret test
Prepare a range of 1cm3 solutions of known protein concentrations (standards) using a serial dilution Add an equal volume of Biuret reagent Leave to stand for 10 minutes Place into a cuvette and set the filter at 550nm (red) Record % transmission (or absorbance) Plot calibration curve Test unknown and read concentration of unknown protein solution from
Define osmosis
Passive movement of water molecules from an area of high water potential to an area of low water potential down a water potential gradient through a partially permeable membrane (ppm)
State the water potential of pure water and describe what happens to the water potential as solute is added to produce a solution
Pure water has a WP of zero As more solute is added the WP becomes more negative
State and define the 3 types of solution that can exist:
Hypertonic = solution has a higher number of solute molecules i.e. low WP Isotonic = two solutions either side of the ppm have the same WP Hypotonic = solution has a lower number of solute molecules i.e. high WP
Describe and explain what would happen to an erythrocyte placed in a solution which has a higher WP (hypotonic solution)
Direction of water movement: Into cell State of erythrocytes: haemolysed/cytolysed Explanation: Water moves into the cell via osmosis Water moves down the water potential gradient The volume of the cytosol increases The mass of the cell increases A pressure is exerted on the cell surface membrane The increased pressure causes the cell surface membrane to rupture The cell contents are released into the surrounding solution
Describe and explain what would happen to an erythrocyte placed in a solution which the same WP (isotonic solution)
Direction of water movement: No net movement State of erythrocytes: No change Explanation: For each water molecule that moves into the cell via osmosis one water molecule leaves the cell There is no water potential gradient Water molecules move randomly due their kinetic energy and may randomly cross the cell surface membrane The volume of the cytosol remains the same The mass of the cell remains the same
Describe and explain what would happen to an erythrocyte placed in a solution which has a lower WP (hypertonic solution)
Direction of water movement: Out of the cell State of erythrocytes: crenated Explanation: Water moves out of the cell via osmosis Water moves down the water potential gradient The volume of the cytosol decreases The mass of the cell decreases A pressure inside the cell is reduced The cell loses shape i.e. shrivels and shrinks The cell becomes crenated
State the term for animal cells (other than erythrocytes) that have gained water by osmosis and ruptured
Cytolysed
State the 3 parts of the plant cell that determine which molecules enter and leave the plant cell
Cell wall Cell surface membrane Tonoplast
State the permeability of each of the cell wall, protoplast and central vacuole
Cell wall: freely permeable, made of cellulose, tough & inelastic Protoplast: consists of cell surface membrane & the cytoplasm (i.e. includes the vacuole & tonoplast) Central vacuole: contains sugars, salts & organic acids in solution
Describe and explain what happens to a plant cell that is placed into a solution with a SAME water potential
Direction of water movement: No net movement State of plant cell: Incipient plasmolysis Explanation: Protoplast is not applying any pressure on the cell wall – at this point is called incipient plasmolysis
Describe and explain what happens to a plant cell that is placed into a solution with a LOWER water potential
Direction of water movement: Out of cell State of plant cell: plasmolysed Explanation: Water leaves the cell by osmosis Down the water potential gradient The volume of the cytoplasm & vacuole decreases Water continues to leave the cell taking it past the point of incipient plasmolysis The protoplasm continues to decrease in volume and shrinks further as more water leaves The protoplast pulls away from the cell wall
Describe and explain what happens to a plant cell that is placed into a solution with a HIGHER water potential
Direction of water movement: Into cell State of plant cell: Turgid Explanation: Water enters the cell by osmosis Down the water potential gradient The volume of the cytoplasm & vacuole increases Water continues to enter the cell exerting a force on the cell wall The protoplasm continues to increase in volume The force exerted on the cell wall increases The cell wall resists the pressure but does not rupture (as the cellulose is tough and inelastic) The increased pressure prevents further water molecules from entering the cell
When investigating the change in mass of potato cylinders using a top pan balance, suggest 3 limitations that are likely to occur
Variations between different potatoes (unlikely to get all cylinders from one potato): differences in ages, type etc SA of cylinder may differ between cylinders due to angle the ends are cut It is assumed the cylinders have got to the point that no more osmosis is occurring Blotting technique not standardised
When investigating the change in mass of potato cylinders using a top pan balance, state and explain 3 errors that are likely to occur
top pan balance reading above zero and not calibrated masses heavier than they should be top pan balance wet and not calibrated masses heavier than they should be cylinder(s) not blotted properly prior to weighing under blotting will leave liquid on the cylinder making masses higher than they should be over blotting will draw water out of the cells and hence make masses lower than they should be incorrect sucrose solution added to specimen tube more or less osmosis occurs and cells gain or lose more mass than expected
When investigating the change in mass of potato cylinders using a top pan balance, state 2 improvements that are likely to be possible
Weigh cylinders to a constant mass Repeat investigation using sucrose solutions in a narrower range e.g. 0.25, 0.30. 0.35, 0.40, 0.45 mol dm-3 once the water potential is estimated
