Skip to main content
MassachusettsBiologySyllabus dot point

How do photosynthesis and cellular respiration move carbon between organisms and the environment, and how do cells build large molecules from sugars?

Develop a model of the role of photosynthesis and cellular respiration in cycling carbon, and explain how cells combine atoms from sugars into amino acids and other large carbon-based molecules (MA STE HS-LS1-6, HS-LS2-5).

A standard-level answer on carbon cycling and matter in organisms for the Massachusetts High School Biology MCAS: how photosynthesis and respiration move carbon, and how cells build amino acids and large molecules from sugars under HS-LS1-6 and HS-LS2-5.

Generated by Claude Opus 4.811 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Jump to a section
  1. What this topic is asking
  2. Photosynthesis and respiration as a carbon cycle
  3. How cells build large molecules from sugars
  4. Conservation of matter: atoms are reused
  5. Try this

What this topic is asking

Two Massachusetts STE standards meet here. HS-LS1-6 asks you to explain how cells combine carbon, hydrogen, and oxygen from sugars to build amino acids and other large molecules. HS-LS2-5 asks you to develop a model of the role of photosynthesis and cellular respiration in cycling carbon among organisms and the environment. On the High School Biology MCAS, this topic is usually tested with a model or diagram of carbon moving between the air, plants, and animals, and you are asked to name the processes and trace an atom. The crosscutting concept is energy and matter, with a focus on the conservation of matter.

Photosynthesis and respiration as a carbon cycle

The two processes you met in this module are opposite halves of a loop:

  • Photosynthesis takes carbon dioxide out of the air and uses it (with water and light energy) to build glucose. Carbon moves from the air into living things.
  • Cellular respiration breaks glucose back down and releases carbon dioxide to the air. Carbon moves from living things back to the environment.

Because plants and animals both respire, and plants also photosynthesize, carbon is constantly moving between the atmosphere and organisms. This short loop is the heart of the carbon cycle, which you meet at ecosystem scale in cycling of matter in ecosystems.

How cells build large molecules from sugars

The glucose made in photosynthesis is more than a fuel; it is also a source of atoms for building other molecules. The carbon, hydrogen, and oxygen in glucose can be rearranged, and combined with elements such as nitrogen, to make:

  • Amino acids (and then proteins), by adding nitrogen to carbon-based building blocks;
  • Lipids, by joining glycerol and fatty acids built from sugar-derived carbon;
  • other carbohydrates and the carbon skeletons of nucleotides.

This is the meaning of standard HS-LS1-6: the carbon, hydrogen, and oxygen from sugar molecules combine with other elements to form the large carbon-based molecules of life. The atoms are not created; they are reorganized, which links straight back to the chemistry of life.

Conservation of matter: atoms are reused

The big idea the MCAS tests here is the conservation of matter: atoms are neither created nor destroyed in biological processes, only rearranged. A carbon atom can move from carbon dioxide in the air, into glucose by photosynthesis, into a protein in a plant, into an animal that eats the plant, and back to the air by respiration, all without a single carbon atom being made or lost. Tracing an atom through this cycle is a favorite question, because it tests whether you really understand that matter cycles while energy flows through and is lost as heat.

Try this

Q1. Name the two processes that move carbon between the air and living things. [2]

  • Cue. Photosynthesis (air to organism) and cellular respiration (organism to air).

Q2. Explain what the conservation of matter means for a carbon atom in your body. [2]

  • Cue. The atom was not created; it was rearranged from carbon dioxide a plant captured, so atoms cycle through organisms rather than being made or destroyed.

Exam-style practice questions

Practice questions written in the style of MA DESE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

HS Biology MCAS (style)3 marksA model shows carbon moving between the air, plants, and animals. (a) Name the process that moves carbon from the air into plants. (b) Name the process that returns carbon to the air from both plants and animals. (c) Explain how a carbon atom in the air could end up in a protein in an animal.
Show worked answer →

A 3-point item on developing and using models, with the energy and matter concept.

(a) 1 point: photosynthesis (carbon dioxide from the air is taken into plants).
(b) 1 point: cellular respiration (both plants and animals release carbon dioxide).
(c) 1 point: a plant uses photosynthesis to fix the carbon into glucose; the carbon atoms in glucose are rearranged to build amino acids and proteins; an animal eats the plant and uses those atoms to build its own proteins. Markers reward tracing the atom through photosynthesis, sugar, and into a protein.

HS Biology MCAS (style)2 marksA standard says cells build amino acids and other large molecules using atoms from sugars. Explain where the carbon, hydrogen, and oxygen in these molecules originally come from.
Show worked answer →

A 2-point item on the conservation of matter.

1 point: the carbon, hydrogen, and oxygen come from sugars (such as glucose) that the cell already has.
1 point: those sugar atoms came originally from carbon dioxide and water that a plant combined during photosynthesis, so the atoms are rearranged, not created. Markers reward tracing the atoms back to photosynthesis and noting matter is conserved.

Related dot points

Sources & how we know this