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All organisms use a source of energy for their metabolic activities. Some organisms capture energy from the sun and convert it into chemical energy in food; others use chemical energy in molecules they take in as food ( [link] ).

Levels of organization of living things

Living things are highly organized and structured, following a hierarchy that can be examined on a scale from small to large. The atom    is the smallest and most fundamental unit of matter. It consists of a nucleus surrounded by electrons. Atoms form molecules. A molecule    is a chemical structure consisting of at least two atoms held together by one or more chemical bonds. Many molecules that are biologically important are macromolecules , large molecules that are typically formed by polymerization (a polymer is a large molecule that is made by combining smaller units called monomers, which are simpler than macromolecules). An example of a macromolecule is deoxyribonucleic acid (DNA) ( [link] ), which contains the instructions for the structure and functioning of all living organisms.

Molecular model depicts a DNA molecule, showing its double helix structure.
All molecules, including this DNA molecule, are composed of atoms. (credit: “brian0918”/Wikimedia Commons)

Watch this video that animates the three-dimensional structure of the DNA molecule shown in [link] .

Some cells contain aggregates of macromolecules surrounded by membranes; these are called organelles . Organelles are small structures that exist within cells. Examples of organelles include mitochondria and chloroplasts, which carry out indispensable functions: mitochondria produce energy to power the cell, while chloroplasts enable green plants to utilize the energy in sunlight to make sugars. All living things are made of cells; the cell    itself is the smallest fundamental unit of structure and function in living organisms. (This requirement is why viruses are not considered living: they are not made of cells. To make new viruses, they have to invade and hijack the reproductive mechanism of a living cell; only then can they obtain the materials they need to reproduce.) Some organisms consist of a single cell and others are multicellular. Cells are classified as prokaryotic or eukaryotic. Prokaryotes are single-celled or colonial organisms that do not have membrane-bound nuclei; in contrast, the cells of eukaryotes do have membrane-bound organelles and a membrane-bound nucleus.

In larger organisms, cells combine to make tissues , which are groups of similar cells carrying out similar or related functions. Organs are collections of tissues grouped together performing a common function. Organs are present not only in animals but also in plants. An organ system    is a higher level of organization that consists of functionally related organs. Mammals have many organ systems. For instance, the circulatory system transports blood through the body and to and from the lungs; it includes organs such as the heart and blood vessels. Organisms are individual living entities. For example, each tree in a forest is an organism. Single-celled prokaryotes and single-celled eukaryotes are also considered organisms and are typically referred to as microorganisms.

Questions & Answers

Give me Examples of living thing which have 2 or more flagella?
Mahesh Reply
insect and plants
reproduction it's full meaning
Gift Reply
full meaning of ATP
A life process in which living things increase their population through sexual or non sexual intercouse
please explaination
Gifty ATP means Adenosine tri phosphate
reproduction is the process where living organisms producess their offspring
jerry Reply
what is reproduction
Nmesoma Reply
why some kinds of students are failed
Ahmadi Reply
lack of concentration
lack of guidance and counseling
What is the meaning of optic
Kisaky Reply
Giving a specific section of the alimentary canal,describe 3 ways in which physical digestion occurs.
mouth when chewing
what is population
Ivy Reply
total number of people living in an area
a number of people lives in one catigorize area or named area
what is a cell
Chiko Reply
basic and functional unit of life
cell is tissues that makes up functional life in human or un animal.
is the smallest basic unit of life.
Is the smallest baic unit. o
why cell is very important to human body
what is diffusion
diffusion is a process of mix of particles from higher concentration to the lower one,to make the body functional normal
what is soil
Is the finely divided material covering the earth crust.
is the upper moist of layer of the earth surface
what is reducing sugar
Erica Reply
in genetics which disease is also termed as the queen disease
Phinihas Reply
what are the types of cell
Teye Reply
prokaryote ,eukaryote, akaryotes
what is akaryotes ?
multicellular and unicellular
akaryotes are organisms that function as eukaryotes and prokaryotes
akaryotes are cell with no nucleus
biology is the study of living organisms
bonney Reply
what are the important of cells in the body
Nharnhar Reply
what is biology
Saidu Reply
Is the study of living things
it is the scientific study of living organisms
ls the study of life and living organisms
what are the impotance of proteins
what is the standArd deviation
is the study of living organisms

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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