Cellular respiration and photosynthesis for the HESI A2 Biology section

Is cellular respiration on the HESI A2?

Odessa College's HESI A2 guide lists Biology topics as basics of biology, water, molecules, metabolism, cells, cellular respiration, photosynthesis, cellular reproduction, genetics, DNA and related areas. Respiration and photosynthesis sit next to each other on that list because questions often compare them.

Biology is a section programs choose to include, so read your program's admissions page before you spend a week on it. Our guide to HESI A2 biology and chemistry topics covers the rest of the Biology list; this page goes deep on the energy pathways only.

The stages of cellular respiration in order

Glycolysis comes first and happens in the cytoplasm. It splits one glucose into two pyruvate molecules and gives a net gain of two ATP and two NADH, without using oxygen directly. Next, each pyruvate is converted to an acetyl group carried by coenzyme A, and one carbon dioxide is released per pyruvate.

The citric acid cycle, also called the Krebs cycle, then removes high-energy electrons and carbon dioxide, storing the electrons in NADH and FADH2. Last, the electron transport chain in the inner mitochondrial membrane passes those electrons along, oxygen accepts them at the end and water forms, and ATP synthase makes ATP as hydrogen ions flow back through it.

Photosynthesis and how it mirrors respiration

Photosynthesis takes six carbon dioxide and six water and, using light energy, produces one sugar and six oxygen. Its light-dependent reactions happen in the thylakoid membrane, where chlorophyll captures light and makes ATP and NADPH. The Calvin cycle then uses that chemical energy in the stroma to build sugar from carbon dioxide.

Read the two equations side by side and they run in opposite directions: photosynthesis uses carbon dioxide and water to store energy in glucose, and respiration breaks glucose down with oxygen to release carbon dioxide, water and usable ATP. Plants do both. Plants do both, and photosynthesis happens in chloroplasts while respiration happens in mitochondria, so keep those locations straight.

Without oxygen: fermentation

When oxygen is not available, the electron transport chain cannot run, and cells rely on glycolysis alone. Fermentation regenerates NAD+ from NADH so that glycolysis can keep going. OpenStax notes that this regeneration step itself does not produce ATP; the small ATP gain still comes from glycolysis.

Two kinds come up in study lists. In lactic acid fermentation, used by human muscle during hard exercise, pyruvate becomes lactic acid. In alcohol fermentation, used by yeast, pyruvate loses carbon dioxide and is reduced to ethanol. Learn which kind happens in muscle and which in yeast.

A quick self-check before test day

Cover your notes and answer from memory: where does glycolysis happen, what does it make, which stage releases carbon dioxide, what is the final electron acceptor, and where does the Calvin cycle run. If you can answer all five in under a minute, move on to genetics or cell structure.

If you miss one, draw the whole pathway on a single page with a box for each stage, its location, what enters and what leaves. Redraw it from a blank page the next day. This is faster than rereading a chapter and shows you exactly which arrow you keep forgetting.

Sources

Odessa College Library: HESI A2 guide. Biology topic list incl. metabolism, cellular respiration and photosynthesis

OpenStax Biology 2e, Chapter 7 Summary. Glycolysis in cytoplasm, net 2 ATP and 2 NADH; acetyl CoA and CO2 release; citric acid cycle removes electrons and CO2 into NADH and FADH2; fermentation regenerates NAD+ without ATP production

OpenStax Biology 2e, 7.4 Oxidative Phosphorylation. Electron transport chain in the inner mitochondrial membrane; oxygen as final electron acceptor forming water; ATP synthase driven by hydrogen ion flow

OpenStax Biology 2e, 7.5 Metabolism without Oxygen. Lactic acid fermentation in muscle during intense exercise; alcohol fermentation in yeast via acetaldehyde to ethanol

OpenStax Biology 2e, 8.1 Overview of Photosynthesis. Photosynthesis equation; light-dependent reactions in the thylakoid membrane make ATP and NADPH; Calvin cycle in the stroma