Describe the need to transport substances into and out of a range of…

The idea

Every organism must move substances across its body surface or through a transport system. Oxygen enters for aerobic respiration, and carbon dioxide leaves as its waste product. Water moves in for cells and is lost in urine, sweat and transpiration. Dissolved food molecules such as glucose are absorbed for respiration and growth, while mineral ions such as nitrate are taken up by plants for making proteins. Urea is a nitrogenous waste made in the liver from excess amino acids and must be removed by the kidneys. The need applies across a range of organisms: a single-celled amoeba relies on diffusion across its surface, a plant moves water and ions up xylem and sugars in phloem, and a mammal uses blood and a heart. The exam asks you to match each substance to its direction and purpose.

What earns the marks

• Oxygen is transported into organisms because it is needed for aerobic respiration.
• Carbon dioxide is transported out because it is a waste product of respiration.
• Water is transported in for cell processes and lost as a waste or by transpiration.
• Dissolved food molecules such as glucose are absorbed for respiration and growth.
• Mineral ions such as nitrate are taken up by plants to make proteins and other molecules.
• Urea is a nitrogenous waste produced from excess amino acids and removed by the kidneys.
• The need applies to a range of organisms, from single-celled organisms to plants and mammals.

Common mistakes and how to put them right

• Thinking carbon dioxide must be taken in for respiration.

  Put it right by stating carbon dioxide is a waste product removed from the organism.

• Confusing mineral ions with food molecules.

  Put it right by noting mineral ions such as nitrate are used for making proteins, not as an energy source.

• Believing urea is useful.

  Put it right by explaining urea is a toxic waste made in the liver and removed by the kidneys.

Exam tips

• Link each substance to its purpose before choosing an option, for example oxygen to respiration and urea to excretion.
• Check the direction of movement: some substances go in, others go out, and a distractor often reverses this.
• Eliminate options that name a substance but give the wrong reason for its transport.