HESI A2 Biology key terms

The smallest unit of life that can carry out all processes needed to survive and reproduce. Organisms may consist of one cell or many specialized cells working together.

A thin, flexible barrier of phospholipids and proteins surrounding the cell. It controls which substances enter and leave, allowing the cell to keep a stable internal environment.

A specialized structure inside a cell that performs a particular job, such as energy release, protein building, or waste storage. Each organelle is usually enclosed by its own membrane.

The membrane-bound organelle that holds a eukaryotic cell's DNA and directs its activities. It is the site where genetic instructions are stored and where RNA is produced.

An organelle that produces most of a cell's usable energy through aerobic respiration. Its folded inner membrane increases the surface area for the reactions that generate ATP.

A small structure, free in the cytoplasm or attached to the rough endoplasmic reticulum, where proteins are assembled. It reads messenger RNA and links amino acids in the coded order.

The green organelle found in plant cells and some algae where photosynthesis takes place. It contains chlorophyll, which absorbs light energy used to make sugars from carbon dioxide and water.

A single-celled organism whose DNA is not enclosed in a nucleus, such as a bacterium. Prokaryotes lack membrane-bound organelles and are generally much smaller than eukaryotic cells.

An organism whose cells contain a true nucleus and other membrane-bound organelles. This group includes protists, fungi, plants, and animals, all of which may be single-celled or multicellular.

The net movement of particles from an area of higher concentration to an area of lower concentration, driven by random motion. It requires no energy input from the cell.

The diffusion of water across a selectively permeable membrane from a region of higher water concentration to one of lower water concentration. It is a passive process that needs no cellular energy.

Describing a solution with a higher solute concentration than the fluid inside a cell. A cell placed in such a solution tends to lose water and shrink.

Describing a solution with a lower solute concentration than the fluid inside a cell. A cell placed in such a solution tends to take in water and may swell.

Describing a solution whose solute concentration matches that inside a cell. Water moves in and out at equal rates, so the cell keeps its normal shape.

Movement of substances across a membrane against their concentration gradient, from lower to higher concentration. It requires energy, usually from ATP, and often uses carrier proteins.

Adenosine triphosphate, the molecule cells use as their main short-term energy currency. Breaking its terminal phosphate bond releases energy that powers many cellular reactions.

The set of reactions by which cells break down glucose, with or without oxygen, to release energy stored as ATP. Aerobic respiration occurs mainly in the mitochondria.

The first stage of cellular respiration, in which one glucose molecule is split into two pyruvate molecules in the cytoplasm. It yields a small net amount of ATP and does not require oxygen.

An anaerobic pathway that regenerates the molecules needed for glycolysis to continue when oxygen is absent. It produces little ATP and yields products such as lactic acid or ethanol and carbon dioxide.

The process by which plants, algae, and some bacteria use light energy to convert carbon dioxide and water into sugars, releasing oxygen. It occurs in chloroplasts and stores energy in chemical bonds.

The green pigment in chloroplasts that absorbs light energy for photosynthesis. It absorbs mainly red and blue wavelengths and reflects green light, which is why leaves look green.

Deoxyribonucleic acid, the molecule that stores genetic instructions in a sequence of bases. Its two strands are held together by base pairing and wind into a double helix.

Ribonucleic acid, a single-stranded nucleic acid involved in carrying out genetic instructions. Messenger RNA carries codes to ribosomes, and other forms help build proteins.

The repeating building block of nucleic acids, made of a sugar, a phosphate group, and a nitrogenous base. The order of bases along a chain encodes genetic information.

A segment of DNA that carries the instructions for making a functional product, usually a protein. Genes are passed from parents to offspring and can exist in different versions called alleles.

One of the alternative forms of a gene that can occupy the same position on a chromosome. Different alleles may produce different traits, and an organism inherits one from each parent.

The particular combination of alleles an organism carries for a gene or set of genes. It is the genetic makeup that, together with environment, influences observable traits.

The observable characteristics of an organism, such as eye color or height, produced by its genotype interacting with the environment. It is what can be seen or measured.

Describing an organism that carries two identical alleles for a particular gene. A homozygous individual always passes that same allele to its offspring.

Describing an organism that carries two different alleles for a particular gene. In a heterozygous pair, a dominant allele may mask the effect of a recessive one.

Describing an allele whose trait appears in the phenotype whenever it is present, even if only one copy is inherited. It masks the effect of a recessive allele paired with it.

Describing an allele whose trait is expressed only when two copies are inherited. A recessive allele is hidden in the phenotype when paired with a dominant allele.

The process of building proteins from amino acids using instructions carried by RNA. It involves transcription of DNA into messenger RNA and translation of that message at ribosomes.

Cell division that produces two daughter cells with the same chromosome number and genetic content as the parent cell. It is used for growth, repair, and asexual reproduction.

A two-stage form of cell division that produces gametes with half the usual chromosome number. It creates genetic variety through crossing over and independent assortment.

A long, threadlike package of DNA and protein that carries genes. Human body cells normally contain 46 chromosomes, arranged as 23 pairs.

A reproductive cell, such as a sperm or egg, that carries half the usual number of chromosomes. Two gametes join at fertilization to form a complete new organism.

The tendency of a living system to keep its internal conditions, such as temperature and pH, within stable limits. It relies on feedback mechanisms that detect and correct changes.

A control mechanism in which a change in a body condition triggers a response that reverses that change. It is the main way organisms keep variables near a set point.