Matter, Elements, and Compounds in Biology

Organisms are composed of matter, defined as anything that occupies space and possesses mass.
Matter manifests in various forms, including rocks, metals, oils, gases, and living organisms.
The distinction between mass and weight is significant:
- Mass is the specific amount of matter contained within an object.
- Weight is a measure of how strongly that mass is pulled by gravity.
Elements are the fundamental units of matter:
- An element is a substance that cannot be broken down into other substances through chemical reactions.
- There are 92 elements recognized as occurring naturally in the environment today.
- Examples of natural elements include gold, copper, carbon, and oxygen.
Chemical Symbols:
- Each element is represented by a symbol, typically the first one or two letters of its name.
- Some symbols originate from Latin or German names. For instance, the symbol for sodium is $Na$ , derived from the Latin word natrium.

A compound is a substance consisting of two or more different elements combined in a fixed ratio.
Emergent Properties of Compounds:
- A compound possesses organized matter having characteristics different from those of its constituent elements.
Example: Sodium Chloride ( $NaCl$ ):
- Consists of the elements sodium ( $Na$ ) and chlorine ( $Cl$ ) in a $1:1$ ratio.
- Pure sodium is a metal, while pure chlorine is a poisonous gas.
- When chemically combined, they form an edible compound (table salt).
Example: Water ( $H_2O$ ):
- Consists of the elements hydrogen ( $H$ ) and oxygen ( $O$ ) in a $2:1$ ratio.

Essential elements are those required by an organism to live a healthy life and reproduce.
Approximately $20-25\%$ of the 92 natural elements are classified as essential elements.
Variations in essential element requirements exist between species:
- Humans require 25 distinct elements.
- Plants require only 17 distinct elements.
Major Elements:
- Just four elements make up $96\%$ of living matter:
  - Oxygen ( $O$ )
  - Carbon ( $C$ )
  - Hydrogen ( $H$ )
  - Nitrogen ( $N$ )
Minor Elements:
- Calcium ( $Ca$ ), phosphorus ( $P$ ), potassium ( $K$ ), sulfur ( $S$ ), and a few others account for most of the remaining $4\%$ of an organism's mass.

Trace elements are required by organisms in only minute quantities, specifically accounting for less than $0.01\%$ of an organism's mass.
Universal vs. Species-Specific Trace Elements:
- Iron ( $Fe$ ) is a trace element required by all forms of life.
- Other elements are required only by certain species.
The role of Iodine ( $I$ ) in vertebrates:
- In animals with backbones, iodine is an essential ingredient for a hormone produced by the thyroid gland.
- A daily intake of just $0.15\,\text{mg}$ of iodine is adequate for the normal activity of the human thyroid.
- An iodine deficiency causes the thyroid gland to grow to an abnormal size, a condition known as goiter.
- Goiter incidence can be reduced by eating seafood or using iodized salt.
Comprehensive list of trace elements (less than $0.01\%$ of mass):
- Boron ( $B$ )
- Chromium ( $Cr$ )
- Cobalt ( $Co$ )
- Copper ( $Cu$ )
- Fluorine ( $F$ )
- Iodine ( $I$ )
- Iron ( $Fe$ )
- Manganese ( $Mn$ )
- Molybdenum ( $Mo$ )
- Selenium ( $Se$ )
- Silicon ( $Si$ )
- Tin ( $Sn$ )
- Vanadium ( $V$ )
- Zinc ( $Zn$ )

Some naturally occurring elements are toxic to organisms.
Example: Arsenic ( $As$ ):
- Exposure to arsenic is linked to numerous diseases and can be lethal to humans.
- Arsenic occurs naturally in some areas and can contaminate groundwater.
- Contamination often results from using water sourced from drilled wells in these regions.