Biology AS AQA
-
1-1-biological-molecules-carbohydrates11 主题
-
1-1-1-biological-molecules-key-terms
-
1-1-2-biological-molecules-reactions
-
1-1-3-monosaccharides
-
1-1-4-glucose
-
1-1-5-the-glycosidic-bond
-
1-1-6-chromatography-monosaccharides
-
1-1-7-disaccharides
-
1-1-8-starch-and-glycogen
-
1-1-9-cellulose
-
1-1-10-biochemical-tests-sugars-and-starch
-
1-1-11-finding-the-concentration-of-glucose
-
1-1-1-biological-molecules-key-terms
-
1-2-biological-molecules-lipids3 主题
-
1-3-biological-molecules-proteins5 主题
-
1-4-proteins-enzymes12 主题
-
1-4-1-many-proteins-are-enzymes
-
1-4-2-enzyme-specificity
-
1-4-3-how-enzymes-work
-
1-4-4-required-practical-measuring-enzyme-activity
-
1-4-5-drawing-a-graph-for-enzyme-rate-experiments
-
1-4-6-using-a-tangent-to-find-initial-rate-of-reaction
-
1-4-7-limiting-factors-affecting-enzymes-temperature
-
1-4-8-limiting-factors-affecting-enzymes-ph
-
1-4-10-limiting-factors-affecting-enzymes-enzyme-concentration
-
1-4-11-limiting-factors-affecting-enzymes-substrate-concentration
-
1-4-12-limiting-factors-affecting-enzymes-inhibitors
-
1-4-14-control-of-variables-and-uncertainty
-
1-4-1-many-proteins-are-enzymes
-
1-5-nucleic-acids-structure-and-dna-replication8 主题
-
1-5-2-nucleotide-structure-and-the-phosphodiester-bond
-
1-5-3-dna-structure-and-function
-
1-5-4-rna-structure-and-function
-
1-5-5-ribosomes
-
1-5-6-the-origins-of-research-on-the-genetic-code
-
1-5-8-the-process-of-semi-conservative-replication
-
1-5-9-calculating-the-frequency-of-nucleotide-bases
-
1-5-10-the-watson-crick-model
-
1-5-2-nucleotide-structure-and-the-phosphodiester-bond
-
1-6-atp-water-and-inorganic-ions4 主题
-
2-1-cell-structure7 主题
-
2-2-the-microscope-in-cell-studies4 主题
-
2-3-cell-division-in-eukaryotic-and-prokaryotic-cells8 主题
-
2-4-cell-membranes-and-transport9 主题
-
2-4-1-the-structure-of-cell-membranes
-
2-4-3-the-cell-surface-membrane
-
2-4-4-diffusion
-
2-4-5-osmosis
-
2-4-7-osmosis-in-animal-cells
-
2-4-9-required-practical-investigating-water-potential
-
2-4-10-active-transport-and-co-transport
-
2-4-11-adaptations-for-rapid-transport
-
2-4-13-required-practical-factors-affecting-membrane-permeability
-
2-4-1-the-structure-of-cell-membranes
-
2-5-cell-recognition-and-the-immune-system7 主题
-
2-6-vaccines-disease-and-monoclonal-antibodies6 主题
-
3-1-adaptations-for-gas-exchange6 主题
-
3-2-human-gas-exchange14 主题
-
3-2-5-the-alveolar-epithelium
-
3-2-1-the-human-gas-exchange-system
-
3-2-2-dissecting-the-gas-exchange-system
-
3-2-3-microscopy-and-gas-exchange-surfaces
-
3-2-4-investigating-gas-exchange
-
3-5-5-investigating-heart-rate
-
3-5-6-blood-vessels
-
3-5-7-capillaries-and-tissue-fluid
-
3-5-8-cardiovascular-disease-data
-
3-2-10-risk-factor-data
-
3-2-11-correlations-and-causal-relationships
-
3-2-6-ventilation-and-gas-exchange
-
3-2-8-the-effects-of-lung-disease
-
3-2-9-pollution-and-smoking-data
-
3-2-5-the-alveolar-epithelium
-
3-3-digestion-and-absorption5 主题
-
3-4-mass-transport-in-animals6 主题
-
3-5-the-circulatory-system-in-animals4 主题
-
3-6-mass-transport-in-plants6 主题
-
4-1-dna-genes-and-chromosomes10 主题
-
4-2-dna-and-protein-synthesis3 主题
-
4-3-genetic-diversity-mutations-and-meiosis7 主题
-
4-4-genetic-diversity-and-adaptation6 主题
-
4-5-species-and-taxonomy4 主题
-
4-6-biodiversity9 主题
3-6-7-evaluating-the-mass-flow-hypothesis
Exam code:7401
The mass flow hypothesis: evaluating evidence
-
While the mass flow hypothesis is widely accepted among biologists, it is a theory for which evidence is still being collected
-
There is evidence both for and against the mass flow hypothesis:
Evidence for the mass flow hypothesis
|
Observation |
How this supports the hypothesis |
|---|---|
|
When the phloem sieve tube is punctured phloem sap oozes out |
The contents of the phloem exert pressure on the phloem walls |
|
Phloem sap extracted near a source has a higher sucrose concentration than sap extracted near a sink |
Water would move into the phloem by osmosis near a source and out of the phloem by osmosis near a sink |
|
Metabolic inhibitors stop translocation |
ATP is required for mass flow to occur; it is an active process |
|
Removal of a ring of bark from trees results in a bulge above the ring, and fluid from the bulging region has a higher sugar concentration than fluid from below the ring |
Removal of the phloem tissue (which is just below the bark) prevents the passage of sugars |
Evidence against the mass flow hypothesis
|
Observation |
How this contradicts the hypothesis |
|---|---|
|
Amino acids appear to travel more slowly than sucrose in the phloem |
The mass flow hypothesis states should be flowing at the same rate |
|
Experiments have detected different substances, within the same sieve tube, moving in opposite directions |
The mass flow hypothesis states everything should be flowing in one direction |
|
Sieve plates are present in the phloem |
These create a barrier to mass flow, so there is no reason for them to have evolved |
Examiner Tips and Tricks
Questions on the mass flow hypothesis often use the command word evaluate. Many students miss marks by describing only supporting evidence; be sure to also include evidence that challenges the hypothesis, such as different solutes moving at different rates or in opposite directions.
Responses