Explain why viruses are said to challenge cell theory.

They show some characteristics of life such as replication and evolution, yet they are not made of cells and cannot reproduce without a host cell.

Distinguish between the magnification and the resolution of a microscope.

Magnification is how many times larger the image is than the object, while resolution is the smallest distance apart at which two points can still be seen as separate.

What limits the resolution of a light microscope?

The wavelength of visible light, so two points closer together than about 200 nm cannot be separated however much the image is enlarged.

Why can an electron microscope resolve far more detail than a light microscope?

A beam of electrons has a very much shorter wavelength than light, so points only a fraction of a nanometre apart can be distinguished.

State one advantage the light microscope keeps over the electron microscope.

It can be used on living material and shows natural colour, whereas an electron microscope requires a dead, dehydrated specimen in a vacuum.

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

Cell Structure and Organisation

Cell theory, microscopy and measuring cells

3 things to know
Component 1

Overview

Three principles of cell theory, why viruses are said to challenge cell theory and magnification and the resolution of a microscope.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Process

Describe how a light microscope is calibrated with an eyepiece graticule and a stage micrometer.

Key term

Why must the microscope be recalibrated when the objective lens is changed?

Key term

State the three principles of cell theory.

Questions from this subtopic

Each one with the model answer underneath.

1
State the three principles of cell theory.
Model answer
All living organisms are made of one or more cells, the cell is the basic unit of structure and function in life, and all cells arise from pre-existing cells by division.
2
Explain why viruses are said to challenge cell theory.
Model answer
They show some characteristics of life such as replication and evolution, yet they are not made of cells and cannot reproduce without a host cell.
3
Distinguish between the magnification and the resolution of a microscope.
Model answer
Magnification is how many times larger the image is than the object, while resolution is the smallest distance apart at which two points can still be seen as separate.
4
What limits the resolution of a light microscope?
Model answer
The wavelength of visible light, so two points closer together than about 200 nm cannot be separated however much the image is enlarged.
5
Why can an electron microscope resolve far more detail than a light microscope?
Model answer
A beam of electrons has a very much shorter wavelength than light, so points only a fraction of a nanometre apart can be distinguished.
6
State one advantage the light microscope keeps over the electron microscope.
Model answer
It can be used on living material and shows natural colour, whereas an electron microscope requires a dead, dehydrated specimen in a vacuum.

See the rest and revise

Other ways to work this

The same content, four other ways.

What is inside Levo

Six ways to work the same material.

1Closed book

2Open book

3Closed book

The 3-step method, run for you

Answer cold, learn it, then prove it stuck. Levo tracks the jump between your first attempt and your last.

Biological Molecules

Cell Structure and Organisation

Cell Membranes and Transport

Enzymes

Cram a whole paper in one sitting

Sort every topic into what you know and what you do not, then drill only the weak ones.

The cycle.

CalvinKrebsurea

Blank Blitz makes facts automatic

Rapid timed gap-fills on the terms and numbers you need to recall without thinking.

AThe radius is 0.20 mm.

BThe area of the field is π × 0.20² = 0.126 mm².

CThe density is 15 ÷ 0.126 = 119 stomata per mm².

Build It gets the exam shape right

Assemble a full-mark answer from its marking points, in the order an examiner wants them.

A stoma is the pore itself.

TrueFalse

Test It catches misconceptions

Spot the fault in a statement. The traps are the mistakes students actually make.

721941163128510

Brain Breaks between sessions

Short focus games to reset attention, so the next set lands better than the last.

Every question in Cell Structure and Organisation, on a schedule.

Attempt a question from memory, write the correct version, then attempt it again. Levo records the jump between your first attempt and your last, and brings each question back on the day you are due to forget it.

Start revising, £5 a month See how the method works

Or £46 a year, about £3.83 a month. Cancel anytime.