AP®︎/College Environmental science
Review your understanding of the carbon cycle with this free article aligned to NGSS and AP standards.
|Carbon||An essential element that forms the base of all organic matter, including the bodies of living organisms, and is a key component of fossil fuels|
|Carbon dioxide ||A compound in the atmosphere that is taken in by photosynthetic organisms to make organic molecules and is later released back into the atmosphere through processes such as respiration and decomposition|
|Photosynthesis||The process by which energy from sunlight is used to transform carbon dioxide from the atmosphere into the organic matter that fuels food webs|
|Cellular respiration||The process by which living organisms break down organic compounds to produce usable energy, releasing carbon dioxide into the atmosphere as a by-product|
|Carbon cycle||The complex flow of carbon between inorganic and organic sources within the environment|
The carbon cycle and carbon reservoirs
The carbon cycle describes the continuous flow of carbon between organic and inorganic carbon reservoirs, or areas of Earth where large amounts of carbon are stored. Most of Earth’s carbon is found in inorganic reservoirs such as rocks, water, and sediments. Only a small portion is stored in organic reservoirs, such as in the bodies of living organisms.
The carbon cycle is represented in the following diagram:
To help us understand the carbon cycle, we can think of it as two interconnected subcycles: a biological carbon cycle and a geological carbon cycle.
Biological carbon cycle
The biological carbon cycle occurs through biological processes and describes the relatively quick flow of carbon between the atmosphere and Earth’s ecosystems.
Carbon is introduced into food webs by photosynthetic organisms, which convert gaseous
from the atmosphere into biomass. As organisms carry out cellular respiration, they break and re-form molecular bonds, producing usable energy and cycling back into the atmosphere. The decomposition of dead organisms and other nonliving organic matter also returns to the atmosphere.
Organic matter that is not immediately broken down accumulates and is buried. Given enough time and pressure, this matter turns into fossil fuel deposits and rock, which brings us to the geological carbon cycle.
Geological carbon cycle
The geological carbon cycle occurs by geologic processes and describes the much slower flow of carbon between Earth’s nonliving carbon reservoirs.
During the geological carbon cycle, carbon moves from rocks on land into the oceans via weathering and rainwater runoff. Carbon also enters oceans from the atmosphere as carbon dioxide dissolves into the water, forming carbonic acid and bicarbonate ions.
Bicarbonate is part of the shells of many marine creatures, and these shells and other sunken organic materials ultimately form the sediment and rocks lining the ocean floor. Carbon may remain stored deep in sediments and rocks for millions of years until an event such as a volcanic eruption returns it to the surface.
Human impacts on the carbon cycle
The amount of carbon in the atmosphere has been increasing rapidly over the past century, and this increase is largely the result of human activity. For example, the burning of fossil fuels releases a significant amount of carbon into the atmosphere, and deforestation removes photosynthetic organisms that would normally help trap excess
Although photosynthetic organisms and oceans absorb some
they simply can’t keep up. As a result, carbon is entering the atmosphere faster than it can cycle back into reservoirs for long-term storage. This excess atmospheric contributes to global warming and, ultimately, climate change.
Want to join the conversation?
- What is carbon fixation(5 votes)
- Carbon Fixation is the process where inorganic forms of Carbon, like CO2, are converted or "fixed" by photosynthetic organisms into organic and biologically accessible molecules(10 votes)
- Sometimes this is done photosynthetically — for example cyanobacteria, which are the free-living relatives of what became the chloroplast.
There are also prokaryotes that fix CO₂ without performing photosynthesis — for example some bacteria known as methanogens cary out this reaction:
CO₂ + 4H₂ → CH₄ + 2H₂O(5 votes)
- hello everyone
I have recently joined the Khan Academy. I'm confused between HS BIOLOGY AND HS BIOLOGY NGSS. Which one should I take?. Can anyone please help me with that?
- The lessons in NGSS are going to be aligned to certain learning standards that may or may not be used at your school, whereas the other one is going to be more general. Both are certainly useful! I would recommend looking through some of the lessons on each to decide which one aligns more closely with what you are looking for.(3 votes)
- with organic matter, surely the carbon dioxide and the organic matter itself would have been feasted upon by fungi? explain(3 votes)
- what's the carbon cycle(2 votes)
- How does carbon form shells?(1 vote)
- when carbon enters the oceans, it becomes carbonate. Calcium carbonate is what shells are made of.(3 votes)
- Which organisms are responsible for removing carbon (in the form of carbon dioxide) fro the atmosphere?(1 vote)
- Wouldn't Abiotic be a better word replacement for inorganic when referring to rocks and sediment?(1 vote)
- While rocks and sediment are a part of the abiotic environment, the term inorganic here refers to the fact that the carbon in the rocks have to be extracted, whereas are living plants and animals are made of carbon. Organic compounds have Carbon-Hydrogen interactions which isn't really present in inorganic compounds.(1 vote)