IB Biology SL Practice Flashcards

Comprehensive vocabulary flashcards covering the major concepts across the six primary topics of the IB Biology SL curriculum for May 2026.

Cell Theory

The scientific theory consisting of three statements: 1. All living organisms are composed of one or more cells. 2. The cell is the basic unit of life. 3. All cells come from pre-existing cells.

Acetabularia

A genus of giant algae that consists of only one giant cell, serving as an exception to the standard cell theory.

Prokaryote

Simple unicellular organisms (like bacteria) characterized by a lack of a membrane-bound nucleus and having small $70S$ ribosomes.

Peptidoglycan

The material that makes up the cell wall of bacteria in Domain Bacteria.

Phospholipid Bilayer

A fluid structure composed of hydrophilic (water-loving) heads facing outward and hydrophobic (water-fearing) tails facing inward.

Cholesterol

A component of the cell membrane that sits between phospholipids to stabilize fluidity, preventing the membrane from becoming too rigid in cold or too fluid in heat.

Osmosis

The movement of water molecules across a selectively permeable membrane from a region of higher water potential to a region of lower water potential.

Active Transport

The movement of ions or molecules from a region of low concentration to high concentration (against the gradient) requiring energy in the form of ATP and carrier proteins.

Metaphase

The second stage of mitosis where chromosomes align at the equator (metaphase plate) and spindle fibers attach to the centromeres.

Cytokinesis

The process during telophase where the cytoplasm divides to form two distinct daughter cells.

Purines

A category of nitrogenous bases (Adenine and Guanine) characterized by a double ring structure.

Helicase

The enzyme responsible for unwinding and unzipping the DNA double helix by breaking hydrogen bonds between base pairs.

DNA Polymerase

The enzyme that reads the DNA template strand and adds complementary free nucleotides to the new strand in the $5' \rightarrow 3'$ direction.

Okazaki Fragments

Short fragments of DNA synthesized on the lagging strand that are eventually joined by the enzyme ligase.

Degenerate Code

A property of the genetic code where multiple different codons can code for the same amino acid, which helps reduce the effect of mutations.

Induced Fit Model

An enzyme model describing how the active site changes shape slightly to mould around the substrate when it binds.

Non-competitive Inhibitor

A molecule that binds to the allosteric site of an enzyme, changing the shape of the active site so the substrate cannot bind.

Chemiosmosis

The process where ATP synthase uses the $H^+$ ion gradient across the inner mitochondrial membrane to generate ATP during aerobic respiration.

Gene

A heritable factor that controls a specific characteristic, consisting of a sequence of DNA that codes for a polypeptide.

Locus

The specific, fixed position of a gene on a chromosome.

Codominance

A genetic scenario where both alleles are expressed simultaneously in the heterozygote, such as in AB blood groups.

Crossing Over

The process in Prophase I of meiosis where homologous chromosomes exchange segments at chiasmata, leading to genetic variation.

Non-disjunction

The failure of homologous chromosomes or sister chromatids to separate during meiosis, which can lead to conditions like Trisomy 21 (Down Syndrome).

Species

A group of organisms that can interbreed and produce fertile offspring.

Detritivore

An organism that ingests dead organic matter and digests it internally, such as an earthworm.

Saprotroph

An organism, like a fungus or bacteria, that secretes digestive enzymes externally onto dead matter and then absorbs the products.

Nitrogen Fixation

The process performed by bacteria like Rhizobium and Azotobacter that converts atmospheric $N_2$ into ammonium ($NH_4^+$).

Nitrification

The conversion of ammonium ($NH_4^+$) into nitrites ($NO_2^-$) and then into nitrates ($NO_3^-$) by nitrifying bacteria.

Carrying Capacity ($K$)

The maximum population size of a species that an environment can sustain indefinitely.

Homologous Structures

Anatomically similar structures (like the pentadactyl limb) in different species that suggest a common ancestor despite different functions.

Allopatric Speciation

Speciation that occurs when a geographic barrier separates a population into two isolated groups that eventually diverge genetically.

Pepsin

An enzyme produced by the chief cells of the stomach wall that breaks down proteins into polypeptides at an optimal pH of approximately $2$.

Villi

Finger-like projections in the small intestine that increase surface area for nutrient absorption and contain blood capillaries and lacteals.

Sinoatrial (SA) Node

The natural pacemaker of the heart that generates electrical impulses to initiate each heartbeat.

Phagocytes

White blood cells (neutrophils and macrophages) that provide a non-specific immune response by engulfing and destroying pathogens.

Memory Cells

Lymphocytes produced during a primary immune response that persist in the body to enable a faster and stronger secondary response to the same antigen.

Surfactant

A substance that coats the inner surface of alveoli to prevent their walls from sticking together during exhalation.

Synaptic Cleft

The gap between two neurons across which neurotransmitters diffuse chemically to transmit a signal.

Insulin

A hormone produced by beta cells of the pancreas that lowers blood glucose by promoting glycogenesis in the liver and muscle cells.

Glycogenolysis

The process of breaking down glycogen into glucose, stimulated by the hormone glucagon when blood glucose levels are low.