Process
OCR A-Level Biology B H422
The Developing Cell: Cell Division and Cell Differentiation
The Developing Individual: Meiosis, Growth and Development
The Development of Species: Evolution and Classification
Cells and Microscopy
Topic overviewMicroscopy and the cell theoryBlood smears, staining and microscope drawingsBlood cells and their structureMagnification and measuring cellsHaemocytometer cell counts and dilutionsFlow cytometry in blood analysisUltrastructure of the animal cellPlant and prokaryotic cellsMembrane lipids and the fluid mosaic modelMovement of molecules across membranesThe roles of membranes and the secretion of proteins
Water and Its Importance in Plants and Animals
Proteins and Enzymes
Nucleic Acids
Patterns of Inheritance
Population Genetics and Epigenetics
Gene Technologies
The Principles and Importance of Homeostasis
The Hormonal Control of Blood Glucose and the Management of Diabetes
Kidney Functions and Malfunctions
Topic overviewExcretion and the gross structure of the kidneyThe nephron: ultrafiltration and selective reabsorptionInvestigating the composition of urine, plasma and filtrateOsmoregulation and the action of ADHThe kidney as an endocrine gland and kidney failureDialysis, transplantation and the future of transplant surgery
The Cellular Basis of Cancer and Treatment
Respiratory Diseases and Treatment
Cellular Respiration
Metabolism and Exercise
Topic overviewThe consequences of exercise on the bodyAerobic fitness and VO₂ maxTraining, F.I.T.T. factors and investigating fitnessEnhancing athletic performanceHaemoglobin and the transport of oxygenOxygen dissociation curves of different respiratory pigmentsOxygen deficit and oxygen debtThe structure of skeletal muscleThe sliding filament theory of muscle contraction
Pathogenic Microorganisms
The Immune System
Controlling Communicable Diseases
Photosynthesis, Food Production and Management of the Environment
Topic overviewChloroplast ultrastructure and photosynthetic pigmentsThe light-dependent stage of photosynthesisThe Calvin cycleInvestigating the factors affecting photosynthesisThe Hill reaction and DCPIPUsing TP and GP to build carbohydrates, lipids and amino acidsThe compensation point and crop productionMicroorganisms, the nitrogen cycle and biosensorsBiomass transfer and efficiency in food productionRuminants in the human food chainFarms as ecosystems and the conflict with conservationDeflected succession and measuring biodiversity
The Impact of Population Increase
Planning and Implementing Practical Work
Analysing and Evaluating Practical Results
Practical Skills, Apparatus and Techniques
Topic overviewInvestigative approaches and working safelyFollowing instructions, recording and presenting dataResearch, referencing and processing data with softwareMeasuring mass, volume, time, temperature, length and pHColorimeters and potometersLaboratory glassware and serial dilutionsLight microscopy, graticules and biological drawingQualitative reagents for biological moleculesChromatography and electrophoresisAseptic technique and dissectionWhole-organism responses, fieldwork sampling and data logging
Fertility and Assisted Reproduction
The Effects of Ageing on the Reproductive System
Plant Reproduction
The Nervous System and the Identification and Consequences of Damage
Topic overviewOrganisation of the nervous system and nervous tissueThe structure and function of the human brainThe structure of motor, sensory and relay neuronesResting potential and the action potentialSynapses and the integration of responsesReflexes, reaction times and their investigationBrain scans and the consequences of nervous system damageDrugs, brain function and dependency
Monitoring Visual Function
The Effect of Ageing on the Nervous System
The Heart and Monitoring Heart Function
Transport Systems in Mammals
Gas Exchange in Mammals and Plants
Pathogenic Microorganisms
Identifying bacteria: Gram staining and culture
- 3 things to know
- Components 01, 02 and 03
Overview
Recall questions on the steps of Gram staining and why Gram-positive and Gram-negative bacteria differ in colour, iodine as a mordant, Staphylococcus aureus and Escherichia coli, acid-fast staining of Mycobacterium tuberculosis, cell shapes and colony features, streak plates and safe incubation.
What you need to know
Answer each one before you reveal it.
Your recallNot started
Process
State the four main steps of the Gram staining procedure in order.
Key term
Why do Gram-negative bacteria lose the crystal violet stain?
Questions from this subtopic
Each one with the model answer underneath.
- 1
State the four main steps of the Gram staining procedure in order.
Model answer
The heat-fixed smear is flooded with crystal violet, treated with iodine solution, decolourised briefly with alcohol, then counterstained with safranin.
- 2
What colour is a Gram-positive bacterium after Gram staining?
Model answer
It appears purple or blue-black.
- 3
What colour is a Gram-negative bacterium after Gram staining?
Model answer
It appears red or pink.
- 4
Why do Gram-positive bacteria keep the crystal violet stain when alcohol is added?
Model answer
Their thick peptidoglycan layer traps the crystal violet–iodine complex, which is too large to be washed out of the wall by the alcohol.
- 5
Why do Gram-negative bacteria lose the crystal violet stain?
Model answer
The alcohol dissolves their outer lipopolysaccharide membrane and their peptidoglycan layer is thin, so the crystal violet–iodine complex washes out and the cells then take up the counterstain.
- 6
What is the function of the iodine solution in the Gram stain?
Model answer
It acts as a mordant, forming a large insoluble complex with the crystal violet inside the cell.
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