What is the purpose of the Gram stain?

It divides bacteria into two groups according to the structure of their cell walls, which makes the cells visible and helps in identifying them.

What is the function of the iodine in the Gram stain?

It acts as a mordant, combining with the crystal violet to form a large complex inside the cell that cannot easily escape.

What does the alcohol do during the Gram stain?

It dissolves the outer lipid membrane of a Gram-negative cell and washes the dye complex out through its thin peptidoglycan layer, while a thick peptidoglycan layer holds the complex in.

Eduqas A-Level Biology A400QS

Biological Molecules

Topic overview Inorganic ions and the properties of water Monosaccharides and disaccharides Polysaccharides, storage and structure Lipids, triglycerides and phospholipids Amino acids and protein structure Condensation, hydrolysis and the food tests

Cell Structure and Organisation

Topic overview The nucleus and the endomembrane system Mitochondria, chloroplasts and other organelles Prokaryotic cells and viruses Cell theory, microscopy and measuring cells Comparing cells and the levels of organisation

Cell Membranes and Transport

Topic overview The fluid-mosaic model Factors affecting membrane permeability Diffusion and facilitated diffusion Osmosis and water potential Active transport and bulk transport Determining water potential in practice

Enzymes

Topic overview Enzymes as biological catalysts The active site and induced fit Temperature, pH and buffers Substrate and enzyme concentration Inhibition and immobilised enzymes

Nucleic Acids and Their Functions

Topic overview Nucleotides and the structure of ATP ATP as the energy currency of the cell The structure of DNA Comparing DNA and RNA Semi-conservative replication The genetic code Transcription Translation and the fate of the polypeptide Extracting DNA from living material

Importance of ATP

Topic overview ATP synthetase and the synthesis of ATP Chemiosmosis and the electrochemical gradient Proton pumps and the electron transport chain Membranes of mitochondria and chloroplasts compared Investigating dehydrogenase activity

Photosynthesis

Topic overview Chloroplast structure and the distribution of chloroplasts Photosynthetic pigments and chromatography Photosystems and light harvesting Non-cyclic and cyclic photophosphorylation The light-independent stage Limiting factors and the rate of photosynthesis Nitrogen, magnesium and plant growth

Respiration

Topic overview The need for respiration and glycolysis The link reaction and the Krebs cycle Reduced NAD, FAD and oxidative phosphorylation The energy budget of aerobic and anaerobic respiration Respiratory substrates and investigating respiration in yeast

Microbiology

Topic overview Bacterial shape and cell wall structure The Gram stain Culturing microorganisms in the laboratory Conditions for growth and aseptic technique Monitoring population growth and the milk investigation

Population Size and Ecosystems

Topic overview Population growth and its measurement The population growth curve and carrying capacity Density dependent and density independent factors Sampling abundance and distribution Ecosystems, habitats and communities Energy transfer and pyramids of biomass Succession Decomposition and the carbon cycle The nitrogen cycle and human intervention Eutrophication and the effects of drainage

Human Impact on the Environment

Topic overview Endangered species and extinction Conserving gene pools in the wild and in captivity Deforestation and overfishing Sustainability and informed decision making Planetary boundaries

Classification and Biodiversity

Topic overview The need for classification and the taxonomic hierarchy The three domain and five kingdom systems Characteristic features of the five kingdoms Assessing relatedness by physical features and biochemistry The species concept and the binomial system Biodiversity, adaptation and natural selection Measuring biodiversity in a habitat Genetic and molecular measures of biodiversity

Mitosis and Meiosis

Topic overview The cell cycle and interphase The stages of mitosis Cytokinesis and the significance of mitosis The stages of meiosis Comparing mitosis with meiosis, and seeing them on prepared slides

Sexual Reproduction in Humans

Topic overview The reproductive systems and the histology of testis and ovary Spermatogenesis and oogenesis Intercourse, fertilisation and implantation The menstrual cycle and its hormonal control The placenta, birth and lactation

Sexual Reproduction in Plants

Topic overview Flower structure and the contrast between wind and insect pollination The development of pollen grains and ovules Pollination and double fertilisation The formation and structure of seed and fruit Germination of Vicia faba and the action of gibberellin

Inheritance

Topic overview Alleles, monohybrid inheritance and codominance Dihybrid inheritance and linkage The chi squared test and testing genetic ratios Sex linkage, haemophilia and Duchenne muscular dystrophy Mutation, mutagens and epigenetics

Variation and Evolution

Topic overview Genetic and environmental sources of variation Investigating continuous variation and the t test Competition and selective agencies Gene pools, genetic drift and selection The Hardy-Weinberg principle Isolation, speciation and Darwin's theory of evolution

Application of Reproduction and Genetics

Topic overview The Human Genome Project, other sequenced genomes and genomics in healthcare Screening embryos, stem cells and the ethical issues Genetic fingerprinting, PCR and electrophoresis in forensic work Recombinant DNA and the production of human insulin Genetically modified crops and gene therapy

Adaptations for Gas Exchange

Topic overview Surface area to volume ratio and gas exchange in small organisms Specialised respiratory surfaces and the need for ventilation Gills and counter current flow in bony fish The human breathing system and its histology Ventilation and gas exchange in humans The insect tracheal system The angiosperm leaf, stomata and gas exchange

Adaptations for Transport

Topic overview Vascular systems compared across animal groups The mammalian heart and blood vessels The cardiac cycle and its control Blood, haemoglobin and dissociation curves The Bohr effect, chloride shift and tissue fluid The root and the uptake of water The endodermis and the entry to the xylem Xylem, transpiration and cohesion-tension The potometer and xerophytes and hydrophytes Phloem and translocation

Adaptations for Nutrition

Topic overview Modes of nutrition Saprotrophic nutrition Holozoic nutrition and the unicellular gut From a sac-like gut to a tube gut The human gut and the effect of diet Herbivores, carnivores and parasites

Homeostasis and the Kidney

Topic overview Homeostasis and feedback The structure of the kidney and the nephron Ultrafiltration and selective reabsorption Water regulation and antidiuretic hormone Excretory products, adaptation and kidney failure

The Nervous System

Topic overview The components of a nervous response The spinal cord and spinal nerves The reflex arc and nerve nets The structure of a motor neurone The nerve impulse Oscilloscope traces and conduction speed The synapse and synaptic transmission Chemicals affecting transmission

Immunology and Disease: Disease and Its TransmissionImmunology

Topic overview The language of infection and disease The human body as a host Cholera and tuberculosis Smallpox, influenza and viral pathogenicity Malaria and its transmission

Immunology and Disease: AntibioticsImmunology

Topic overview Bacteriostatic and bactericidal antibiotics Broad and narrow spectrum antibiotics Penicillin and the bacterial cell wall Tetracycline and the Gram negative barrier The spread of antibiotic resistance

Immunology and Disease: The Immune ResponseImmunology

Topic overview Natural barriers to infection Inflammation, clotting and phagocytosis Antigens, B lymphocytes and the humoral response Cell-mediated immunity and T lymphocytes Active and passive immunity Vaccination programmes and their ethics

Human Musculoskeletal Anatomy: Skeletal TissuesMusculoskeletal

Topic overview Cartilage and its three types The composition and functions of compact bone Bone remodelling and the Haversian system Rickets, osteomalacia, osteoporosis and brittle bone disease The structure of skeletal muscle The sliding filament theory Fast and slow twitch fibres, and energy for contraction

Human Musculoskeletal Anatomy: Structure and Function of the SkeletonMusculoskeletal

Topic overview The axial and appendicular skeleton The girdles and the bones of the limbs The vertebral column and its vertebrae Fractures and postural deformities The functions of the skeleton

Human Musculoskeletal Anatomy: JointsMusculoskeletal

Topic overview Types of joint in the human body The structure of a synovial joint Osteoarthritis and rheumatoid arthritis Joints as levers Antagonistic muscle action in the forelimb

Neurobiology and Behaviour: The BrainNeurobiology

Topic overview The structure of the human brain The functions of the cerebrum, cerebellum and medulla oblongata The hypothalamus and the control of the endocrine system The sympathetic and parasympathetic nervous systems Sensory and motor areas, the homunculus and language

Neurobiology and Behaviour: NeuroscienceNeurobiology

Topic overview Non-invasive study of the brain: CT and MRI Functional imaging: fMRI, PET and EEG Brain development and critical periods Neuroplasticity Gene expression, brain development and mental illness

Neurobiology and Behaviour: BehaviourNeurobiology

Topic overview Innate behaviour: escape reflexes, kineses and taxes Habituation and imprinting Classical and operant conditioning Primate societies and the costs and benefits of group living Insect castes, dominance hierarchies, territory and courtship

Microbiology

The Gram stain

3 things to know
Component 1

Overview

Purpose of the Gram stain, how a smear is prepared for staining and function of the iodine in the Gram stain.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Describe how a smear is prepared for staining.

Key term

Why is the smear heat-fixed?

Process

Give the four staining steps of the Gram technique in order.

Questions from this subtopic

Each one with the model answer underneath.

1
What is the purpose of the Gram stain?
Model answer
It divides bacteria into two groups according to the structure of their cell walls, which makes the cells visible and helps in identifying them.
2
Describe how a smear is prepared for staining.
Model answer
A loopful of culture is spread thinly over a clean slide, allowed to air dry, and then passed briefly through a Bunsen flame.
3
Why is the smear heat-fixed?
Model answer
Heat kills the bacteria and makes them adhere to the slide, so they are not washed off during the staining and rinsing steps.
4
Give the four staining steps of the Gram technique in order.
Model answer
Flood with crystal violet and rinse, apply iodine solution and rinse, decolourise briefly with alcohol, then counterstain with safranin.
5
What is the function of the iodine in the Gram stain?
Model answer
It acts as a mordant, combining with the crystal violet to form a large complex inside the cell that cannot easily escape.
6
What does the alcohol do during the Gram stain?
Model answer
It dissolves the outer lipid membrane of a Gram-negative cell and washes the dye complex out through its thin peptidoglycan layer, while a thick peptidoglycan layer holds the complex in.

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