AQA A-Level Biology Topic 2: Cells, Microscopy, Cell Division, Membrane Transport, and Immunology

Comprehensive vocabulary review flashcards for AQA A-Level Biology Topic 2 (Cell structure, microscopy, cell cycle and mitosis, transport across cell membranes, cell recognition, and immunology) based on Miss Estruch's lecture.

Tonoplast

The single membrane that surrounds the permanent vacuole in plant cells.

Nuclear pores

Holes within the double nuclear envelope that allow substances, such as mRNA, to move into and out of the nucleus.

Nucleoplasm

The jelly-like fluid or granular material that fills the nucleus and contains the chromatin.

Nucleolus

A dense spherical region inside the nucleus that is the site of RNA production and assembles ribosomes.

Linear DNA

Protein-bound DNA associated with histone proteins to form chromosomes, characteristic of eukaryotic cells.

Rough endoplasmic reticulum (RER)

A system of folded membranes (cisternae) studded with ribosomes on its outer surface, serving as the site of protein synthesis.

Smooth endoplasmic reticulum (SER)

A network of folded membranes lacking ribosomes that synthesizes, stores, and transports lipids and carbohydrates.

Cisternae

The flattened, folded membrane sheets or sacs that make up the endoplasmic reticulum and Golgi apparatus.

Golgi apparatus

A series of curved, folded membrane sacs that modifies proteins and lipids, packages them, labels them with their destination, and forms lysosomes.

Secretory vesicles

Membrane-bound sacs that pinch off from the Golgi cisternae to transport finished products to the cell surface membrane for release.

Lysosomes

Membrane-bound bags of digestive enzymes (such as lysozymes) that hydrolyze ingested pathogens, digest worn-out organelles, and break down dead cells.

Cristae

The folded projections of the inner mitochondrial membrane that provide a large surface area for enzymes involved in aerobic respiration.

Mitochondrial matrix

The fluid-filled central space of a mitochondrion containing proteins, lipids, respiration enzymes, loops of DNA, and $70\text{S}$ ribosomes.

Ribosomes

Small, non-membrane-bound organelles composed of two subunits of protein and ribosomal RNA, which function as the site of protein synthesis.

Thylakoids

Folded, disc-shaped internal membranes found inside chloroplasts where the light-dependent stage of photosynthesis takes place.

Grana

Stacks of disc-shaped thylakoid membranes within a chloroplast (singular: granum).

Stroma

The fluid-filled matrix of a chloroplast surrounding the grana where the light-independent reactions of photosynthesis occur.

Cellulose cell wall

A rigid layer surrounding plant cells composed of cellulose microfibrils that provides structural strength and prevents osmotic bursting.

Chitin cell wall

The rigid outer wall found in fungal cells made of the structural polysaccharide chitin, providing cellular strength.

Murein

A glycoprotein (peptidoglycan) that forms the structural cell wall of prokaryotic bacterial cells.

Plasmids

Small circular loops of DNA in bacteria that replicate independently and typically carry genes for traits such as antibiotic resistance.

Bacterial capsule

A mucilaginous slimy outer layer surrounding the cell wall of some bacteria that prevents desiccation and masks antigens to evade white blood cells.

Flagella

Long, whip-like protein structures present in some bacteria used for locomotion and movement.

Acellular

Describing entities, such as viruses, that are not composed of cells and do not possess a cell surface membrane.

Non-living (viral classification)

The classification given to viruses because they do not carry out metabolic reactions, cannot respire, cannot move, and cannot replicate without a host cell.

Capsid

The protective outer protein coat of a virus particle that encloses its genetic material.

Attachment proteins

Specific proteins on the exterior of a virus particle that bind to complementary receptors on the host cell surface.

Lipid envelope

An outer phospholipid membrane layer acquired by certain viruses, such as HIV, from the membrane of their host cell.

Magnification

The number of times larger an image appears compared to the actual size of the object being viewed ($\text{Image size} / \text{Actual size}$).

Resolution

The minimum distance between two points at which they can still be clearly distinguished as separate entities.

Optical microscope

A microscope that condenses visible light using glass lenses to view specimens, producing 2D colored images at lower magnification and poorer resolution.

Transmission electron microscope (TEM)

An electron microscope where an electron beam transmits through an extremely thin specimen in a vacuum, forming high-resolution 2D images showing internal organelles.

Scanning electron microscope (SEM)

An electron microscope that beams electrons across the surface of a specimen causing them to scatter back, producing a 3D image of the specimen's contours.

Eyepiece graticule

A transparent glass disc etched with an uncalibrated scale (resembling a ruler) placed inside the eyepiece of a microscope.

Stage micrometer

A glass slide etched with a known, precise scale (typically $2\,\text{mm}$ long with $10\,\mu\text{m}$ subdivisions) used to calibrate the eyepiece graticule.

Cell fractionation

The laboratory process of breaking open cells and separating out their individual organelles based on density so their structures and functions can be studied.

Homogenization

The initial step of cell fractionation in which cells are broken open (e.g., using a blender) in an ice-cold, isotonic, buffered solution.

Ultracentrifugation

The process of spinning a filtered homogenate at progressively increasing speeds in a centrifuge to separate organelles by their differing densities.

Pellet

The solid sediment formed at the bottom of a centrifuge tube containing the densest organelles separated at a particular spinning speed.

Supernatant

The liquid remaining above the organelle pellet after centrifugation, which is poured off and spun at a higher speed.

Binary fission

The method of asexual replication in prokaryotes where circular DNA and plasmids replicate, and the cytoplasm divides into two daughter cells.

Interphase

The non-dividing phase accounting for approximately $90\%$ of the cell cycle, during which cellular growth, organelle replication, and DNA replication take place.

$G_1$ phase

The first growth phase of interphase during which the cell grows in size and doubles many of its organelles.

$\text{S}$ phase

The synthesis phase of interphase during which the cell's nuclear DNA undergoes semi-conservative replication.

$G_2$ phase

The second growth phase of interphase where the cell continues to grow, checks for DNA replication errors, and prepares for mitosis.

Mitosis

A type of nuclear division that produces two genetically identical diploid daughter nuclei with identical DNA, utilized for growth and tissue repair.

Cytokinesis

The division of the cytoplasm and cell membrane at the end of nuclear division to produce two separate daughter cells.

Prophase

The first stage of mitosis in which chromosomes condense and become visible, centrioles migrate to opposite poles, and spindle fibers begin to develop.

Centrioles

Paired cylindrical structures in animal cells that migrate to opposite cellular poles during prophase and synthesize spindle fibers.

Spindle fibers

Protein filaments synthesized by centrioles that attach to chromosome centromeres to align and separate chromatids.

Centromere

The constricted region of a replicated chromosome that holds two sister chromatids together and serves as the attachment point for spindle fibers.

Metaphase

The second stage of mitosis in which chromosomes line up in single file along the cell equator with spindle fibers attached to their centromeres.

Anaphase

The third stage of mitosis in which centromeres divide and spindle fibers retract, pulling sister chromatids to opposite poles using ATP.

Telophase

The final stage of mitosis where separated chromosomes arrive at opposite poles, uncoil into long and thin chromatin, and nuclear envelopes reform.

Mitotic index

The proportion of cells in a microscopic field of view undergoing mitosis, calculated as the number of cells in mitosis divided by the total number of cells.

Tumor

An abnormal mass of cells resulting from uncontrolled, unregulated cell division caused by disruptions in the control of mitosis.

Fluid mosaic model

The model of membrane structure featuring a phospholipid bilayer with lateral mobility ('fluid') embedded with various proteins, carbohydrates, and lipids ('mosaic').

Phospholipid bilayer

The fundamental two-layered arrangement of phospholipid molecules that makes up the basic structure of all biological membranes.

Hydrophilic head

The polar, negatively charged phosphate portion of a phospholipid that interacts favorably with aqueous solutions on either side of the membrane.

Hydrophobic tail

The nonpolar, uncharged fatty acid chains of a phospholipid that repel water and point inward toward the center of the bilayer.

Cholesterol (membrane)

A lipid embedded within the phospholipid bilayer that restricts the lateral movement of phospholipids, making membranes less fluid at high temperatures.

Integral proteins

Transmembrane proteins that span across the entire phospholipid bilayer, functioning primarily as carrier proteins or channel proteins.

Protein channels

Hollow, water-filled tube-like membrane proteins that selectively allow water-soluble polar ions to cross the membrane by facilitated diffusion.

Carrier proteins

Membrane-spanning proteins that bind specific complementary molecules, undergo a conformational shape change, and release the molecules on the opposite side.

Simple diffusion

The passive net movement of lipid-soluble and small nonpolar molecules down their concentration gradient across a membrane without ATP.

Facilitated diffusion

The passive movement of polar, charged, or large water-soluble molecules down their concentration gradient via protein channels or carrier proteins without ATP.

Osmosis

The net passive movement of water molecules from an area of higher water potential to an area of lower water potential across a partially permeable membrane.

Water potential ($\Psi$)

The pressure exerted by free water molecules in a solution, measured in kilopascals ($\text{kPa}$), where pure water has a maximum value of $0\,\text{kPa}$.

Hypotonic solution

A solution with a higher (less negative / closer to zero) water potential than inside the cell, causing water to enter the cell by osmosis.

Hypertonic solution

A solution with a lower (more negative) water potential than inside the cell, causing water to leave the cell by osmosis and make it shrivel.

Active transport

The movement of substances from a low to high concentration (against a concentration gradient) via carrier proteins using metabolic energy from ATP hydrolysis.

Co-transport

A coupled active transport mechanism where the movement of sodium ions down their concentration gradient facilitates the uptake of glucose or amino acids against theirs.

Non-self cells

Cells detected as foreign by the immune system, including pathogens, abnormal body cells (cancer), transplanted tissues, or toxins.

Antigen

A foreign protein or glycoprotein located on the surface of a cell that stimulates an immune response by lymphocytes.

Antigen variability

Genetic mutations in a pathogen that change the shape of its surface antigens, rendering existing memory cells ineffective.

Phagocyte

A type of white blood cell (such as a macrophage) responsible for detecting, engulfing, and hydrolyzing non-self cells during non-specific immunity.

Phagocytosis

The non-specific immune mechanism in which a phagocyte engulfs a foreign pathogen and digests it inside a phagosome using lysozymes.

Phagosome

The intracellular vesicle formed around an engulfed pathogen within the cytoplasm of a phagocyte.

Lysozymes

Digestive, lytic enzymes found inside lysosomes that hydrolyze bacterial cell walls and break down engulfed pathogens.

Antigen-presenting cell (APC)

Any cell (such as a macrophage, infected body cell, or B cell) that presents foreign antigens on its cell surface membrane to activate T cells.

Helper T cells

T lymphocytes with receptors complementary to specific antigens on APCs that, when activated, divide by mitosis to stimulate B cells, phagocytes, and cytotoxic T cells.

Clonal expansion

The rapid mitotic division of an activated lymphocyte to produce large numbers of genetically identical clone cells bearing identical antigen-specific receptors.

Cytotoxic T cells

T lymphocytes that destroy infected host cells or abnormal body cells by releasing perforin to punch holes in the target cell membrane.

Perforin

A cytolytic protein released by cytotoxic T cells that creates pores in the cell surface membrane of infected host cells, causing osmotic lysis.

B lymphocytes (B cells)

White blood cells that mature in the bone marrow and drive the humoral immune response by taking in foreign antigens and producing antibodies.

Humoral response

The branch of the immune response mediated by B cells and antibodies circulating freely in blood and tissue fluids.

Plasma cells

Short-lived differentiated B cells that synthesize and secrete large quantities of monoclonal antibodies into the blood.

Memory B cells

Long-lived lymphocytes that circulate for decades and rapidly divide into plasma cells upon re-infection with the same pathogen.

Antibody

A quaternary globular protein composed of four polypeptide chains (two heavy, two light) with specific antigen-binding sites, synthesized by plasma cells.

Variable region (antibody)

The antigen-binding site at the ends of an antibody whose unique tertiary structure is complementary in shape to a specific antigen.

ELISA test (Enzyme-linked immunosorbent assay)

A diagnostic biochemical assay using monoclonal antibodies attached to enzymes to detect and quantify the presence of antigens or antibodies via a color change.

Agglutination

The clumping together of pathogens caused by antibodies binding to multiple antigens at once, facilitating more efficient detection and phagocytosis.

Passive immunity

Short-term immunity resulting from the introduction of external antibodies without direct exposure to the pathogen or antigen, producing no memory cells.

Active immunity

Long-term immunity developed when an individual's own immune system produces antibodies and memory B cells in response to exposure to an antigen.

Vaccine

A biological preparation containing dead, weakened, or isolated antigens administered to stimulate an active immune response and form memory cells.

Herd immunity

Indirect protection from disease that occurs when a sufficiently large proportion of the population is vaccinated, preventing pathogen transmission to unimmunized individuals.

Reverse transcriptase

An enzyme carried by retroviruses like HIV that synthesizes a complementary DNA copy from the viral RNA template inside a host helper T cell.

Human Immunodeficiency Virus (HIV)

A retrovirus that infects and progressively destroys helper T cells by binding to CD4 surface receptors, crippling both humoral and cell-mediated immunity.

Acquired Immune Deficiency Syndrome (AIDS)

The advanced disease state resulting from HIV infection characterized by a critically low helper T cell count, leading to severe opportunistic infections and cancers.

Monoclonal antibodies

Identical antibodies produced from a single clone of identical B cells that bind specifically to one complementary antigen.