Oxygen is a colorless, odorless gas that is less dense than air and a poor conductor of heat and electricity. It cycles between the atmosphere, plants, animals, and oceans through photosynthesis and respiration. Photosynthesis produces oxygen as a byproduct in plants, which is then used by animals through respiration and releases carbon dioxide back into the atmosphere. Oxygen levels on Earth have increased over time as photosynthesis evolved and continues to be balanced between the atmosphere and bodies of water.

2. Definition of Oxygen
• Oxygen – a colorless, odorless,
tasteless gas
• Denser than air
• Poor conductor of heat and electricity

4. Step One of Oxygen Cycle
• Plant release oxygen into the
atmosphere as a by-product of
photosynthesis.
oxygen

5. Step Two of Oxygen Cycle
• Animals take in oxygen through the process
of respiration.
• Animals then break down sugars and food.

6. Step Three in Oxygen Cycle
• Carbon dioxide is released by animals
and used in plants in photosynthesis.
• Oxygen is balanced between the
atmosphere and the ocean.

8. History of Oxygen
• Early evolution of Earth, oxygen released
from H2O vapor by UV radiation and
accumulated in the atmosphere as the
hydrogen escaped into the earth's
atmosphere
• Photosynthesis became a source of
oxygen
• Oxygen released as organic carbon and
gets buried in sediments.

10. Photosynthesis
•Definition- process in which green plants use the
energy from the sun to make carbohydrates from
carbon dioxide and water in the presence of
chlorophyll.

11. How is Photosynthesis
Carried Out?
•Photosynthesis only occurs in plants containing
chlorophyll:
•Water is absorbed by the roots and carried to the
leaves by the xylem
•Carbon dioxide is obtained from air that enters the
leaves through the stomata and diffuses to the cells
containing chlorophyll.
•Chlorophyll is uniquely capable of converting the
energy from light into a dormant form that can be
stored and used when needed.

12. Steps in Photosynthesis
•
•
•
•
•
The light energy strikes the leaf, passes into the
leaf and hits a chloroplast inside an individual cell
The light energy, upon entering the chloroplasts, is
captured by the chlorophyll inside a grana.
Inside the grana some of the energy is used to split
water into hydrogen and oxygen.
The oxygen is released into the air.
The hydrogen is taken to the stroma along with the
grana's remaining light energy.

13. Steps Continued:
• Carbon dioxide enters the leaf and passes into the
chloroplast.
• In the stroma the remaining light energy is used to
combine hydrogen and carbon dioxide to make
carbohydrates.
• The energy­rich carbohydrates are carried to the
plant's cells.
• The energy­rich carbohydrates are used by the cells
to drive the plant's life processes.

15. Respiration
• Process by which an organism
exchanges gases with its environment
• Process → oxygen is abstracted from
air, transported to cells for the oxidation
of organic molecules while CO2 and
H2O, the products of oxidation, are
returned to the environment

20. Earth’s Layers
• The lithosphere is Earth's surrounding layer,
composed of solids such as soil and rock.
• The atmosphere is the surrounding thin layer
of gas.
• The hydrosphere refers to liquid
environments such as lakes and oceans that
lie between the lithosphere and atmosphere.
• The biosphere's creation and continuous
existence results from chemical, biological,
and physical processes.

21. Today
The Earth’s atmosphere
consists of:
• 21% Oxygen
The Earth’s lithosphere
consists of:
• 99.5% Oxygen
The Earth’s hydrosphere
consists of:
• 46.60% Oxygen
The Earth’s biosphere
consists of:
• 0.01% Oxygen

22. Biological Importance of
Oxygen
• Humans need it to breathe
• Needed for decomposition of
organic waste
• Water can dissolve oxygen and it is
this dissolved oxygen that supports
aquatic life.

23. Ecological Importance of
Oxygen
•
Without oxygen at the bottom of the water body, anaerobic
bacteria (those that live without oxygen) produce acids. These
acids not only increase acidity, but also cause a massive
release of phosphorus and nitrogen, two major fertilizers, from
the organic sediment and into the water column.
•
These same anaerobic bacteria put toxic gases in the water
including hydrogen sulfide (that rotten egg smell), ammonia,
carbon dioxide and methane. These gases are all toxic to fish,
beneficial bacteria and insects.
•
Lack of bottom oxygen is the cause of odors produced by
anaerobic bacteria.

24. Ecological Importance of Oxygen
Cont.
•
Lack of fish enables disease-hosting mosquitoes to thrive, as
mosquitoes are natural food for fish.
•
Without oxygen at the bottom at all times, beneficial bacteria
and insects cannot biodegrade the organic sediment. Large
accumulations of organic sediment follow.