Biology AS OCR
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1-1-practical-skills-written-assessment AS7 主题
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1-2-practical-skills-endorsement-assessment AS16 主题
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1-2-1-practical-ethical-use-of-organisms as
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1-2-2-practical-aseptic-techniques as
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1-2-3-practical-dissection-of-gas-exchange-surfaces-in-fish-and-insects as
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1-2-4-drawing-cells-from-blood-smears as
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1-2-5-practical-investigating-biodiversity-using-sampling as
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1-2-6-practical-data-loggers-and-computer-modelling as
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1-2-7-practical-investigating-the-rate-of-diffusion as
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1-2-8-practical-investigating-water-potential as
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1-2-9-practical-factors-affecting-membrane-structure-and-permeability as
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1-2-10-biochemical-tests-reducing-sugars-and-starch as
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1-2-11-biochemical-tests-lipids as
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1-2-12-biochemical-tests-proteins as
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1-2-13-chromatography as
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1-2-14-serial-dilutions as
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1-2-15-practical-investigating-the-rate-of-transpiration as
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1-2-16-practical-using-a-light-microscope as
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1-2-1-practical-ethical-use-of-organisms as
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2-1-cell-structure AS9 主题
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2-1-2-using-a-microscope as
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2-1-3-drawing-cells as
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2-1-4-magnification-and-resolution as
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2-1-5-eukaryotic-cells as
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2-1-6-eukaryotic-cells-under-the-microscope as
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2-1-7-organelles-and-the-production-of-proteins as
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2-1-8-the-cytoskeleton as
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2-1-9-prokaryotic-and-eukaryotic-cells as
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2-1-1-studying-cells as
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2-1-2-using-a-microscope as
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2-2-biological-molecules AS17 主题
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2-2-1-properties-of-water as
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2-2-2-monomers-and-polymers as
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2-2-3-monosaccharides as
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2-2-4-the-glycosidic-bond as
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2-2-5-polysaccharides as
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2-2-6-biochemical-tests-reducing-sugars-and-starch as
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2-2-7-lipids-and-ester-bonds as
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2-2-8-lipids-structure-and-function as
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2-2-9-biochemical-tests-lipids as
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2-2-10-amino-acids-and-peptide-bonds as
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2-2-11-protein-structure as
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2-2-12-globular-proteins as
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2-2-13-fibrous-proteins as
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2-2-14-inorganic-ions as
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2-2-15-biochemical-tests-proteins as
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2-2-16-finding-the-concentration-of-a-substance as
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2-2-17-chromatography as
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2-2-1-properties-of-water as
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2-3-nucleotides-and-nucleic-acids AS8 主题
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2-4-enzymes AS9 主题
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2-4-1-the-role-of-enzymes as
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2-4-2-enzyme-action as
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2-4-3-enzyme-activity-ph as
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2-4-4-enzyme-activity-temperature as
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2-4-5-enzyme-activity-enzyme-concentration as
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2-4-6-enzyme-activity-substrate-concentration as
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2-4-7-enzyme-activity-enzyme-inhibitors as
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2-4-8-coenzymes-cofactors-and-prosthetic-groups as
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2-4-9-practical-measuring-enzyme-activity as
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2-4-1-the-role-of-enzymes as
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2-5-biological-membranes AS9 主题
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2-5-1-the-cell-surface-membrane as
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2-5-2-membrane-structure-and-permeability as
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2-5-3-diffusion-and-facilitated-diffusion as
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2-5-4-practical-investigating-the-rate-of-diffusion as
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2-5-5-active-transport as
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2-5-6-endocytosis-and-exocytosis as
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2-5-7-osmosis as
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2-5-8-osmosis-in-animal-and-plant-cells as
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2-5-9-practical-investigating-water-potential as
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2-5-1-the-cell-surface-membrane as
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2-6-cell-division-cell-diversity-and-cellular-organisation AS11 主题
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2-6-1-the-cell-cycle as
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2-6-2-the-stages-of-mitosis as
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2-6-3-identifying-mitosis-in-plant-cells as
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2-6-4-the-significance-of-mitosis as
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2-6-5-the-stages-of-meiosis as
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2-6-6-the-significance-of-meiosis as
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2-6-7-specialised-cells as
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2-6-8-the-organisation-of-cells as
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2-6-9-stem-cells as
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2-6-10-stem-cells-in-animals-and-plants as
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2-6-11-the-use-of-stem-cells as
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2-6-1-the-cell-cycle as
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3-1-exchange-surfaces AS7 主题
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3-2-transport-in-animals AS12 主题
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3-2-1-the-need-for-transport-systems-in-animals as
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3-2-2-circulatory-systems as
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3-2-3-blood-vessels as
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3-2-4-tissue-fluid as
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3-2-5-the-mammalian-heart as
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3-2-6-practical-mammalian-heart-dissection as
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3-2-7-the-cardiac-cycle as
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3-2-8-cardiac-output as
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3-2-9-heart-action-initiation-and-control as
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3-2-10-electrocardiograms-ecgs as
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3-2-11-the-role-of-haemoglobin as
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3-2-12-adult-and-fetal-haemoglobin as
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3-2-1-the-need-for-transport-systems-in-animals as
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3-3-transport-in-plants AS11 主题
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3-3-1-the-need-for-transport-systems-in-plants as
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3-3-2-the-xylem-and-phloem as
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3-3-3-the-xylem as
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3-3-4-the-phloem as
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3-3-5-transverse-sections-stems-roots-and-leaves as
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3-3-6-the-process-of-transpiration as
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3-3-7-transpiration-in-plants as
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3-3-8-practical-investigating-the-rate-of-transpiration as
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3-3-9-translocation as
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3-3-10-the-mass-flow-hypothesis as
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3-3-11-the-adaptations-of-xerophytic-and-hydrophytic-plants as
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3-3-1-the-need-for-transport-systems-in-plants as
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4-1-communicable-diseases-disease-prevention-and-the-immune-system AS16 主题
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4-1-1-common-pathogens-and-communicable-diseases as
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4-1-2-transmission-of-communicable-pathogens as
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4-1-3-plant-defences-against-pathogens as
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4-1-4-non-specific-immune-responses as
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4-1-5-phagocytes as
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4-1-6-blood-cells as
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4-1-7-the-t-lymphocyte-response as
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4-1-8-the-b-lymphocyte-response as
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4-1-9-primary-and-secondary-immune-responses as
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4-1-10-antibodies as
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4-1-11-opsonins-agglutinins-and-anti-toxins as
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4-1-12-types-of-immunity as
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4-1-13-autoimmune-diseases as
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4-1-14-principles-of-vaccination as
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4-1-15-sources-of-medicine as
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4-1-16-antibiotics as
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4-1-1-common-pathogens-and-communicable-diseases as
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4-2-biodiversity AS10 主题
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4-2-1-biodiversity as
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4-2-2-sampling-to-determine-biodiversity as
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4-2-3-practical-investigating-biodiversity-using-sampling as
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4-2-4-measuring-species-richness-and-species-evenness as
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4-2-5-simpsons-index as
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4-2-6-genetic-diversity as
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4-2-7-factors-affecting-biodiversity as
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4-2-8-reasons-for-maintaining-biodiversity as
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4-2-9-methods-of-maintaining-biodiversity as
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4-2-10-conservation-agreements as
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4-2-1-biodiversity as
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4-3-classification-and-evolution AS15 主题
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4-3-1-classification-of-species as
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4-3-2-binomial-system as
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4-3-3-classification-of-the-three-domains as
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4-3-4-classification-of-the-five-kingdoms as
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4-3-5-classification-and-phylogeny as
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4-3-6-evidence-of-evolution as
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4-3-7-types-of-variation as
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4-3-8-standard-deviation as
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4-3-9-variation-t-test-method as
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4-3-10-variation-t-test-worked-example as
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4-3-11-spearmans-rank-correlation as
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4-3-12-adaptation as
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4-3-13-natural-selection as
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4-3-14-evolution-of-resistance as
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4-3-15-consequences-of-resistance as
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4-3-1-classification-of-species as
3-3-10-the-mass-flow-hypothesis as
Exam code:H020
Mass flow in the phloem
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Phloem sap, containing sucrose and other organic solutes, moves by mass flow up and down the plant
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Carbohydrates are generally transported in plants in the form of sucrose because:
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sucrose is a disaccharide and therefore contains more energy than a monosaccharide
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sucrose is less reactive than glucose as it is a non-reducing sugar
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Mass flow occurs due to the presence of a hydrostatic pressure gradient
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The pressure gradient is generated by actively loading sucrose into the sieve elements at the source; this lowers the water potential in the sieve tube
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Water moves into the sieve elements by osmosis; this increases the hydrostatic pressure at the source
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At the same time, solutes are unloaded from the sieve elements at the sink, causing water to follow by osmosis; this lowers the hydrostatic pressure at the sink
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The difference in hydrostatic pressure between the source and the sink creates a hydrostatic pressure gradient
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The pressure difference between the source and the sink results in the mass flow of phloem sap from the high hydrostatic pressure area to the low hydrostatic pressure area
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Mass flow means that solutes move faster than they could move by diffusion alone
Examiner Tips and Tricks
Remember that the source is not always the leaves and the sink is not always in the roots; phloem sap moves either up or down the plant depending on the time of year.
The hydrostatic pressure gradient is dependent on water moving in and out of the xylem vessels by osmosis.
Responses