An A-Z of keywords and phrases, all of which are relevant to the post-16 biology curriculum.


Habitable zone

The region around a star where liquid water is stable (usually where the temperature sits between 0 and 100ºC, though pressure can affect this).


Where a population lives.

Haematopoietic stem cells (HSCs)

Multipotent blood cells that can give rise to all other blood cells.


The protein found in red blood cells that is responsible for transporting oxygen in the blood.


A cell containing a single set of unpaired chromosomes.

Hard and soft skills

Hard skills are specific, teachable skills that can be defined and measured, for example, maths or technological skills.

Soft skills are less tangible and harder to quantify, for example, public speaking, research skills or working well in a team.

Hayflick limit

The Hayflick limit describes the number of times most human cells can divide within their lifespan.

Heart disease

A general term that refers to conditions affecting the heart. These can include problems with the heart muscle, its valves, its rhythm and its surrounding vessels, and can lead to heart attacks, chest pain (angina) and stroke.

HeLa cells

The first continuously cultured cells, a human cervical cancer strain taken from a woman called Henrietta Lacks (and named after her).


A parasitic worm, such as a flatworm or roundworm.

Helper T cells

T cells that recognise antigens presented by APCs and stimulate T, B and other immune cells. Also known as CD4+ cells, because of a protein that they express on their cell surface. These are the cells depleted by infection with HIV (human immunodeficiency virus). A type of lymphocyte.


A cell junction involved in cell connections with the extracellular matrix via integrin molecules and intermediate filaments.

Hepatic stellate cells

Hepatic stellate cells are found in the liver and account for 5 to 8 per cent of the cells there. In a healthy liver, these cells are quiescent (dormant) and contain vitamin A droplets. In response to injury, the cells are activated. They lose their vitamin A and proliferate, causing fibrosis, a scarring of the liver.


Passing on of characteristics from one generation to the next.


A six-carbon sugar formed as the product of six turns of the Calvin cycle. Glucose is a hexose.

High-density lipoprotein (HDL) cholesterol

Cholesterol is transported in the blood in complexes called lipoproteins. High-density lipoproteins (HDLs) tend to remove cholesterol from the tissues to the liver for excretion. Low-density lipoproteins (LDLs) are more likely to deposit cholesterol in damaged areas of blood vessels and lead to the formation of atherosclerotic plaques. High levels of LDL cholesterol can lead to heart disease, whereas high levels of HDL cholesterol can protect against heart disease.

High-throughput screening

A technique that can conduct lots and lots of tests very quickly. In a drug development context, it allows scientists to test many potential candidate drugs and measure their effect on the target. However, this technique is also used in genomics.


A part of the brain. Essential for making new memories and finding your way around.


A chemical with a variety of roles in the body. In allergic reactions, it is released when mast cells are stimulated. It causes capillaries to become leaky, releasing fluid and white blood cells into tissues, causing inflammation.

Histone proteins

Specialised proteins inside the nucleus of a eukaryotic cell. These proteins help to regulate and structure DNA by allowing the genetic material to wind or wrap around them.

HIV (human immunodeficiency virus)

HIV is a virus that causes AIDS (acquired immune deficiency syndrome), which is also known as late-stage HIV. HIV is a retrovirus, which means it contains RNA as its genetic material, not DNA.


An organism or cell’s tendency to regulate its internal conditions (such as temperature, chemistry, blood pressure, resting time) in order to stabilise health and functioning despite changes in the environment.


Chemical messengers that carry information from one part of an organism to another to regulate physiology and behaviour.


In symbiosis, an organism that harbours another, providing it with nourishment or shelter.

Human Genome Project

The international effort that set out to sequence the whole human genome.

Humoral immune response

The part of the specific (adaptive) immune response that involves T cells.


A cell formed by the fusion of a plasma B cell and a cancer cell. They are used in the production of monoclonal antibodies.

Hydrogen carrier

A molecule that can accept hydrogen atoms or ions and donate them to another carrier. Hydrogen carriers include NAD and FAD.


A chemical reaction that uses water to break bonds within molecules. A hydrolytic enzyme, or hydrolase, increases the rate of hydrolysis. Proteases are a type of hydrolase that catalyse the hydrolysis of interior peptide bonds in chains of peptides.


Hydrophilic (lit. ‘water loving’) molecules tend to dissolve in water.


Hydrophobic (lit. ‘water fearing’) molecules are insoluble in water.


The enlargement of an organ or tissue caused by an increased in the production of cells.


Also known as high blood pressure, this medical condition is caused by an increase in the blood pressure within arteries. The extra pressure can cause damage to the arteries or the heart. Hypertension can raise the risk of coronary heart disease or stroke.


The enlargement of an organ or tissue caused by an increase in the size of its cells.


A small area at the base of the brain found in all vertebrates, this is the interface between the brain and pituitary gland. It is responsible for many aspects of homeostasis and produces hormones that control a variety of bodily functions, including hunger, thirst, body temperature and sleep patterns.