Biology AS AQA
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1-1-biological-molecules-carbohydrates11 主题
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1-1-1-biological-molecules-key-terms
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1-1-2-biological-molecules-reactions
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1-1-3-monosaccharides
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1-1-4-glucose
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1-1-5-the-glycosidic-bond
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1-1-6-chromatography-monosaccharides
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1-1-7-disaccharides
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1-1-8-starch-and-glycogen
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1-1-9-cellulose
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1-1-10-biochemical-tests-sugars-and-starch
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1-1-11-finding-the-concentration-of-glucose
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1-1-1-biological-molecules-key-terms
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1-2-biological-molecules-lipids3 主题
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1-3-biological-molecules-proteins5 主题
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1-4-proteins-enzymes12 主题
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1-4-1-many-proteins-are-enzymes
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1-4-2-enzyme-specificity
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1-4-3-how-enzymes-work
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1-4-4-required-practical-measuring-enzyme-activity
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1-4-5-drawing-a-graph-for-enzyme-rate-experiments
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1-4-6-using-a-tangent-to-find-initial-rate-of-reaction
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1-4-7-limiting-factors-affecting-enzymes-temperature
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1-4-8-limiting-factors-affecting-enzymes-ph
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1-4-10-limiting-factors-affecting-enzymes-enzyme-concentration
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1-4-11-limiting-factors-affecting-enzymes-substrate-concentration
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1-4-12-limiting-factors-affecting-enzymes-inhibitors
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1-4-14-control-of-variables-and-uncertainty
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1-4-1-many-proteins-are-enzymes
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1-5-nucleic-acids-structure-and-dna-replication8 主题
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1-5-2-nucleotide-structure-and-the-phosphodiester-bond
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1-5-3-dna-structure-and-function
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1-5-4-rna-structure-and-function
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1-5-5-ribosomes
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1-5-6-the-origins-of-research-on-the-genetic-code
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1-5-8-the-process-of-semi-conservative-replication
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1-5-9-calculating-the-frequency-of-nucleotide-bases
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1-5-10-the-watson-crick-model
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1-5-2-nucleotide-structure-and-the-phosphodiester-bond
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1-6-atp-water-and-inorganic-ions4 主题
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2-1-cell-structure7 主题
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2-2-the-microscope-in-cell-studies4 主题
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2-3-cell-division-in-eukaryotic-and-prokaryotic-cells8 主题
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2-4-cell-membranes-and-transport9 主题
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2-4-1-the-structure-of-cell-membranes
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2-4-3-the-cell-surface-membrane
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2-4-4-diffusion
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2-4-5-osmosis
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2-4-7-osmosis-in-animal-cells
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2-4-9-required-practical-investigating-water-potential
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2-4-10-active-transport-and-co-transport
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2-4-11-adaptations-for-rapid-transport
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2-4-13-required-practical-factors-affecting-membrane-permeability
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2-4-1-the-structure-of-cell-membranes
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2-5-cell-recognition-and-the-immune-system7 主题
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2-6-vaccines-disease-and-monoclonal-antibodies6 主题
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3-1-adaptations-for-gas-exchange6 主题
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3-2-human-gas-exchange14 主题
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3-2-5-the-alveolar-epithelium
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3-2-1-the-human-gas-exchange-system
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3-2-2-dissecting-the-gas-exchange-system
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3-2-3-microscopy-and-gas-exchange-surfaces
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3-2-4-investigating-gas-exchange
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3-5-5-investigating-heart-rate
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3-5-6-blood-vessels
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3-5-7-capillaries-and-tissue-fluid
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3-5-8-cardiovascular-disease-data
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3-2-10-risk-factor-data
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3-2-11-correlations-and-causal-relationships
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3-2-6-ventilation-and-gas-exchange
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3-2-8-the-effects-of-lung-disease
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3-2-9-pollution-and-smoking-data
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3-2-5-the-alveolar-epithelium
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3-3-digestion-and-absorption5 主题
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3-4-mass-transport-in-animals6 主题
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3-5-the-circulatory-system-in-animals4 主题
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3-6-mass-transport-in-plants6 主题
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4-1-dna-genes-and-chromosomes10 主题
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4-2-dna-and-protein-synthesis3 主题
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4-3-genetic-diversity-mutations-and-meiosis7 主题
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4-4-genetic-diversity-and-adaptation6 主题
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4-5-species-and-taxonomy4 主题
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4-6-biodiversity9 主题
1-4-8-limiting-factors-affecting-enzymes-ph
Exam code:7401
The effect of pH
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All enzymes have an optimum pH
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Enzymes are denatured at extremes of pH
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Hydrogen and ionic bonds hold the tertiary structure of the protein (i.e. the enzyme) together
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Below and above the optimum pH of an enzyme, solutions with an excess of H+ ions (acidic solutions) and OH– ions (alkaline solutions) can cause these bonds to break
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This alters the shape of the active site, which means enzyme-substrate complexes form less easily
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Eventually, enzyme-substrate complexes can no longer form at all
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At this point, complete denaturation of the enzyme has occurred
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Where an enzyme functions can be an indicator of its optimal environment:
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E.g. pepsin is found in the stomach, an acidic environment at pH 2
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Pepsin’s optimum pH is therefore pH 2
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Buffer solutions
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When investigating the effect of pH on the rate of an enzyme-catalysed reaction, you can use buffer solutions to measure the rate of reaction at different pH values:
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Buffer solutions each have a specific pH
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Buffer solutions maintain this specific pH, even if the reaction taking place would otherwise cause the pH of the reaction mixture to change
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Examiner Tips and Tricks
Temperature affects both the movement of molecules (increasing collision rate) and can denature enzymes at high levels. pH does not affect collision rate, but changes in pH can disrupt substrate binding, reducing successful collisions until the active site is no longer functional.
Calculating pH
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If the hydrogen ion (H+) concentration of a solution is known, the pH can be calculated using the equation:
pH = -log₁₀ [H⁺]
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You can find the ‘log’ function on your calculator (‘log’ is the same as ‘log10’ so don’t worry if the calculator doesn’t say ‘log10’)
Worked Example
The hydrogen ion concentration of a solution is 1.6 x 10-4 mol dm-3. Find the pH of this solution.
The pH of the solution is:
pH = -log₁₀ [H⁺]
pH = -log₁₀ 1.6 x 10-4 = 3.796
pH = 3.8
Worked Example
The hydrogen ion concentration of a solution of sodium hydroxide is 3.5 x 10-11 mol dm-3. Find the pH of this solution.
The pH of the solution is:
pH = -log₁₀ [H⁺]
pH = -log₁₀ 3.5 x 10-11 = 10.456
pH = 10.5
Worked Example
Ethanoic acid (also known as acetic acid) is a weak acid produced by wood ants that they can spray at predators as a defence mechanism. The hydrogen ion concentration of a sample of ethanoic acid taken from some wood ants was 8.39 x 10-6 mol dm-3. Find the pH of the ethanoic acid produced by wood ants.
The pH of the solution is:
pH = -log₁₀ [H⁺]
pH = -log₁₀ 8.39 x 10-6 = 5.076
pH = 5.08
Examiner Tips and Tricks
Don’t forget the minus sign in the formula: pH = –log₁₀ [H⁺] as this is a common exam mistake. pH values should be between 0 and 14. If your answer falls outside this range, check your working. Use clues in the question to estimate if your answer is reasonable (e.g. alkalis should have high pH, weak acids around pH 4–6).
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