Transport in organisms for N(A)-Level Science (Biology): the components of blood, the human circulatory system and double circulation, and transport in plants by xylem and phloem

What this module covers

Transport in organisms is about how substances are carried around a body too large for diffusion alone. You need to know what blood is made of and what each part does, how the heart and blood vessels form a double circulation, and how plants move water in the xylem and food in the phloem, including the transpiration that pulls water up.

This overview links every dot point in the module. Work through them in order, then check yourself with the questions at the end. See the full set at /sg-n-level/biology/syllabus.

The blood and its functions

Start with the transport fluid itself. The page on the blood and its functions describes the four main components of blood and what each does: red blood cells carry oxygen, white blood cells fight disease, platelets help clotting, and plasma carries everything else in solution. It also explains how the red blood cell is adapted to carry oxygen.

The human circulatory system

Next, the pump and the pipes. The page on the human circulatory system describes the structure of the heart, the three types of blood vessel (arteries, veins and capillaries), and how the double circulation carries blood to the lungs and to the rest of the body.

Transport in plants

Finally, how plants move substances without a heart. The page on transport in plants describes the roles of the xylem (water and mineral salts up) and the phloem (food around the plant), explains the transpiration stream, and lists the factors that change how fast a plant loses water.

How this module is examined

At N(A) level there is no standalone pure Biology paper. Biology is taken as part of a combined Science subject, either Science (Physics, Biology) syllabus 5106 or Science (Chemistry, Biology) syllabus 5107, so this module is examined within whichever combination your school offers.

• Match component to job. Blood questions reward pairing red cells with oxygen, white cells with defence, platelets with clotting and plasma with transport.
• Use direction for vessels. Arteries carry blood away from the heart, veins towards it, whatever the oxygen content.
• Compare xylem and phloem. State that xylem carries water upwards and phloem carries food in any direction.

Check your knowledge

A mix of recall, comparison and application questions. Attempt them, then check against the solutions.

1. State the function of red blood cells and of platelets. (2 marks)
2. Explain what is meant by a double circulation. (2 marks)
3. State one difference between the xylem and the phloem. (1 mark)
4. State two conditions that increase the rate of transpiration. (2 marks)