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AQA GCSE Biology 8461

2. Organisation

Organisation in animals

Big organisms are built up in stages. Cells specialise, group into tissues, tissues combine into organs, organs work together as organ systems, and the organ systems together make the whole organism.

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What organisation in animals covers

Every subtopic below has its own questions, mark schemes and notes.

4.2.1Levels of organisation

Big organisms are built up in stages. Cells specialise, group into tissues, tissues combine into organs, organs work together as organ systems, and the organ systems together make the whole organism.

4.2.2.2The heart, lungs and blood vessels

The lungs get oxygen into the blood and carbon dioxide out of it; the heart pumps that blood around two joined circuits; and three types of blood vessel, each built differently, carry it there and back.

4.2.2.3Blood

Blood is a tissue: a straw-coloured liquid called plasma with red blood cells, white blood cells and platelets suspended in it. Each component has a job, and each is built to suit that job.

Key terms

The definitions examiners expect, in the wording that earns the mark.

Tissue — A group of cells with a similar structure and function that work together.
Organ — A group of different tissues working together to perform a specific function.
Organ system — A group of organs working together to perform a particular function.
Differentiation — The process by which a cell becomes specialised for a particular job.
Alveoli — The tiny air sacs at the ends of the smallest airways, where gas exchange happens.
Double circulatory system — Two joined circuits, so blood passes through the heart twice on each trip round the body.
Pacemaker — A group of cells in the right atrium wall that sets the natural resting heart rate.
Lumen — The hole down the middle of a blood vessel, through which the blood flows.
Coronary arteries — Arteries branching off the aorta that supply the heart muscle with oxygenated blood.
Haemoglobin — The red pigment in red blood cells that binds to oxygen.
Oxyhaemoglobin — What haemoglobin becomes when it has bound to oxygen in the lungs.
Phagocytosis — When a white blood cell changes shape to engulf and digest a microorganism.
Antitoxin — A chemical made by white blood cells that neutralises a toxin produced by a microorganism.
Plasma — The pale straw-coloured liquid part of blood, which carries everything else.

Where marks get lost

The mistakes that come up again and again in this topic.

  • Do not define a tissue as 'identical cells' — a tissue can contain more than one cell type.
  • The word that makes something an organ is 'different': different tissues working together.
  • Being mostly one tissue does not make something a tissue. The heart is mostly muscle but is still an organ.
  • When converting sizes, get both lengths into the same unit before you divide.
  • The pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood — an artery is defined by direction, not by content.
  • Heart diagrams are drawn as seen from the front, so the person's left side appears on the right of the page.
  • You are not expected to name the individual heart valves, only to say what valves do.
  • The left ventricle wall is thicker because it must generate higher pressure, not because it holds more blood.
  • An artificial pacemaker corrects the timing of the heartbeat; it does not pump the blood.
  • In rate calculations, convert the time to minutes and the volume to millilitres before dividing.
  • 'Large surface area' on its own is a weak answer — say what it is a large surface area for.
  • Plasma carries carbon dioxide, but oxygen is carried by the red blood cells as oxyhaemoglobin.
  • Platelets are fragments of cells, not whole cells — calling them 'blood cells' can lose the mark.
  • Antibodies act on the microorganism; antitoxins act on the poison it produces.
  • Say what an adaptation is for: 'biconcave, giving a large surface area for absorbing oxygen'.
  • White blood cells have a nucleus; red blood cells and platelets do not.

Other topics

Cell biology Transport in cells Cell division Enzymes and the human digestive system Plant tissues, organs and systems Non-communicable disease Communicable disease and plant defences Preventing and treating disease Monoclonal antibodies Photosynthesis Respiration Homeostasis and the nervous system Hormonal coordination DNA, genes and variation Reproduction Evolution and Classification Adaptations, interdependence and competition Organisation of an ecosystem Humans and biodiversity Working scientifically Practical skills

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