A-Level Biology – Cells, Viruses & Reproduction

Vocabulary flashcards covering key terms from A-Level Biology Unit 2 on cell structure, microscopy, prokaryotes, antibiotics, viruses, the cell cycle, mitosis, meiosis, gametogenesis, fertilisation and plant reproduction.

Cell theory

Concept stating that cells are the fundamental unit of structure, function and organisation in all living organisms.

Eukaryotic cell

Cell type with a membrane-bound nucleus and organelles such as mitochondria and ER.

Prokaryotic cell

Cell lacking a true nucleus and membrane-bound organelles; includes bacteria and archaea.

Cytoplasm

Gel-like fluid inside a cell where metabolic reactions occur and organelles are suspended.

Protoplasm

Collective term for the cytoplasm and all cellular organelles.

Plasma membrane (cell membrane)

Partially permeable phospholipid bilayer controlling substance movement into and out of the cell.

Nucleus

Largest organelle; stores DNA and controls cellular activities.

Nuclear envelope

Double membrane surrounding the nucleus; regulates material exchange with cytoplasm.

Nucleolus

Dense spherical region in the nucleus that synthesises rRNA and assembles ribosomal subunits.

Nuclear pore

Protein channel in the nuclear envelope allowing passage of mRNA, ribosomal subunits and nucleotides.

Mitochondrion

Double-membraned organelle that produces ATP by aerobic respiration.

Cristae

Infoldings of the inner mitochondrial membrane increasing surface area for respiratory enzymes.

Matrix (mitochondria)

Semi-rigid fluid in mitochondria containing enzymes, 70S ribosomes and circular DNA.

Centrioles

Pair of 9-triplet microtubule cylinders involved in spindle formation during cell division.

Rough endoplasmic reticulum (RER)

Membranous network studded with ribosomes; site of protein synthesis and transport.

Smooth endoplasmic reticulum (SER)

ER lacking ribosomes; synthesises, stores and transports lipids and carbohydrates.

Ribosome

RNA-protein complex that assembles polypeptides; 80S in eukaryotes, 70S in prokaryotes.

80S ribosome

Larger ribosome (~25 nm) found in eukaryotic cytoplasm.

70S ribosome

Smaller ribosome found in prokaryotes, mitochondria and chloroplasts.

Golgi apparatus

Stack of flattened cisternae that modifies, sorts and packages proteins and lipids into vesicles.

Cisternae (Golgi / ER)

Flattened membrane-bound sacs comprising ER or Golgi stacks.

Lysosome

Vesicle formed from Golgi containing hydrolytic enzymes for intracellular digestion and apoptosis.

Cell wall (plant)

Rigid layer of cellulose fibres providing strength, shape and preventing osmotic bursting.

Tonoplast

Membrane surrounding the plant vacuole; regulates ion and metabolite movement.

Permanent vacuole

Large fluid-filled sac in plants storing cell sap and maintaining turgor pressure.

Chloroplast

Double-membraned plastid where photosynthesis occurs.

Thylakoid

Flattened membrane disc in chloroplast containing chlorophyll; site of light-dependent reactions.

Granum

Stack of thylakoids within a chloroplast.

Stroma

Fluid matrix of chloroplast where light-independent (Calvin cycle) reactions occur.

Peptidoglycan (murein)

Polysaccharide-peptide polymer forming the bacterial cell wall.

Slime capsule

Polysaccharide layer outside some bacteria; protects against phagocytosis and desiccation.

Pili

Hair-like protein projections on bacteria used for attachment and conjugation.

Flagellum (bacterial)

Long helix of flagellin rotating to propel prokaryotic cells.

Plasmid

Small circular DNA molecule in bacteria carrying accessory genes, e.g., antibiotic resistance.

Nucleoid

Region in prokaryotes containing the circular bacterial chromosome.

Gram-positive bacterium

Bacterium with thick peptidoglycan and teichoic acid; stains purple with Gram stain.

Gram-negative bacterium

Bacterium with thin peptidoglycan and outer lipopolysaccharide membrane; stains red/pink.

Teichoic acid

Acidic polymer in Gram-positive cell walls anchoring peptidoglycan.

Lipopolysaccharide (LPS)

Outer membrane component of Gram-negative bacteria contributing to endotoxin activity.

Beta-lactam antibiotic

Drug class (e.g., penicillin) inhibiting peptidoglycan cross-linking; effective mainly on Gram-positive bacteria.

Glycopeptide antibiotic

Large polar antibiotic (e.g., vancomycin) effective against Gram-positive bacteria by blocking cell-wall synthesis.

Polypeptide antibiotic

Antibiotic group (e.g., polymyxins) disrupting Gram-negative outer membranes; high toxicity.

Magnification

Number of times an image is enlarged compared with the actual specimen.

Resolution

Minimum distance between two points that can be distinguished as separate.

Light microscope

Instrument using visible light and glass lenses; max resolution ~200 nm.

Electron microscope

Microscope using electron beams; higher resolution due to shorter electron wavelength.

Transmission electron microscope (TEM)

Electron microscope where electrons pass through thin specimen, giving 2-D ultrastructure images.

Scanning electron microscope (SEM)

Electron microscope scanning specimen surface, producing 3-D images of exterior topology.

Interphase

Phase of the cell cycle (G1, S, G2) where cell grows and DNA replicates.

G1 phase

First interphase stage; cell grows and synthesises new organelles and proteins.

S phase

Interphase stage where DNA replication occurs.

G2 phase

Interphase stage preparing for mitosis; protein synthesis and organelle production continue.

M phase

Mitotic phase involving nuclear division (mitosis) and cytokinesis.

Mitosis

Nuclear division producing two genetically identical diploid daughter cells.

Prophase

First mitosis stage: chromosomes condense, nucleolus disappears, spindle forms, nuclear envelope breaks down.

Metaphase

Mitotic stage where chromosomes align on cell equator attached to spindle via centromeres.

Anaphase

Stage where sister chromatids separate at centromeres and migrate to opposite poles.

Telophase

Final mitotic stage: chromosomes decondense, nuclear membranes reform, cytokinesis begins.

Cytokinesis

Division of cytoplasm forming two separate daughter cells after mitosis or meiosis.

Spindle fibres

Microtubules that attach to centromeres and pull chromatids/chromosomes during division.

Centromere

DNA region linking sister chromatids; attachment point for spindle fibres.

Chromatid

One of the two identical DNA strands of a duplicated chromosome.

Meristem

Region of plant with actively dividing, undifferentiated cells (e.g., root tip).

Mitotic index

Ratio of dividing cells to total cells in a tissue; used to gauge proliferation.

Meiosis

Two-stage nuclear division producing four genetically varied haploid gametes.

Homologous chromosomes

Chromosome pair with same gene loci, one maternal, one paternal.

Bivalent

Pair of homologous chromosomes physically associated during prophase I of meiosis.

Crossing over

Exchange of genetic material between nonsister chromatids during prophase I.

Chiasma (plural chiasmata)

Visible crossover point where chromatids remain joined after crossing over.

Independent assortment

Random orientation of homologous pairs on metaphase I plate, generating allele variation.

Non-disjunction

Failure of chromosome pairs or chromatids to separate during meiosis, causing aneuploidy.

Down’s syndrome (Trisomy 21)

Condition caused by non-disjunction leading to three copies of chromosome 21.

Turner’s syndrome (XO)

Monosomy of X chromosome in females, leading to infertility and lack of puberty.

Translocation (chromosome)

Mutation where a chromosome segment detaches and reattaches to a non-homologous chromosome.

Polyploidy

Condition of having more than two complete sets of chromosomes; common in plants.

Spermatogenesis

Process in testes producing spermatozoa from spermatogonia.

Spermatogonium

Diploid stem cell in seminiferous tubules dividing mitotically to start spermatogenesis.

Primary spermatocyte

Diploid cell that undergoes meiosis I during spermatogenesis.

Secondary spermatocyte

Haploid cell produced after meiosis I; enters meiosis II to form spermatids.

Spermatid

Haploid cell resulting from meiosis II; differentiates into spermatozoon.

Spermatozoon (sperm)

Motile male gamete specialised for fertilising an egg.

Oogenesis

Formation of ova in ovaries from oogonia.

Oogonium

Diploid germ cell in ovary dividing mitotically during fetal development.

Primary oocyte

Diploid cell arrested in prophase I until puberty.

Secondary oocyte

Haploid cell arrested in metaphase II; released at ovulation.

Polar body

Small haploid cell produced during oogenesis that generally degenerates.

Ovum

Mature female gamete formed after completion of meiosis II at fertilisation.

Capacitation

Biochemical changes in sperm within female tract that increase motility and membrane fragility.

Acrosome reaction

Release of hydrolytic enzymes from sperm acrosome enabling penetration of zona pellucida.

Cortical reaction

Exocytosis of cortical granules after sperm entry, hardening zona pellucida to block polyspermy.

Zygote

Diploid cell formed by fusion of sperm and egg nuclei.

Morula

Solid ball of embryonic cells resulting from early mitotic divisions of the zygote.

Blastocyst

Hollow ball of cells (with inner cell mass) formed ~5 days post-fertilisation; implants in uterus.

Pluripotent stem cell

Cell capable of differentiating into almost all body cell types; found in inner blastocyst.

Pollen grain

Haploid male gametophyte of flowering plants containing tube and generative nuclei.

Microspore mother cell

Diploid cell in anther that undergoes meiosis to form microspores.

Tube nucleus

Nucleus in pollen grain that controls growth of the pollen tube.

Generative nucleus

Pollen nucleus that divides to form two male gametes for double fertilisation.

Pollen tube

Structure that grows from pollen grain to ovule, delivering male nuclei.

Megaspore mother cell

Diploid ovule cell undergoing meiosis to produce megaspores.