TAKS Science Exit Exam Review Objective Two:

The Organization of Living Systems

(Biology)

Cells: The Basic Structure of an Organism

• Prokaryotes: organisms whose cells lack a nucleus (single-celled organism—EX: bacteria)

• Eukaryote: organisms whose cells contain a nucleus (multi-celled organism—EX: animal cells/plant cells)

• Homeostasis: the stable, internal state of an organism in which cells are kept alive despite changing external conditions (EX: humans maintain the internal temperature of 98.6°F despite changing external temperatures around them)

• All living things are made up of cell

• All cells come from other cells

Organelles: structures within cells that perform a specific functionCellular Components

Plant Cells• Chloroplast: uses the energy in sunlight to

make glucose (a sugar that stores energy)

Animal and Plant Cells• Nucleus: command center of the cell

and controls it’s activities—houses DNA• Mitochondria: breaks down food

molecules (like the sugar glucose) to release the energy stored

• Ribosomes: where proteins are made

DNA: Deoxyribonucleic Acid• Found in the cell of all organisms • Contains the genetic information that

controls what a cell can do and what types of molecules it can make

• Made of of million of units called Nucleotides, which are arranged into two strands (double-helix) with the sugar/phosphate group making up the ladder and the nitrogen bases making up the rungs• The four nitrogen bases in DNA are:

• Guanine (G) and Cytosine (C)• Adenine (A) and Thymine (T)

• The bases are complimentary:• C pairs with G and A pairs with T

DNA ReplicationReplication: The process of cell division—results in two identical copies of a DNA molecule• Enzymes unwind two strands of

DNA and pairs new complimentary nitrogen bases together to form a new strand

• Each strand contains the information needed to make the new strand

• The cell now has a complete set of the organism’s genetic information and can replicate again

• This is how all new cells are created

RNA: Ribonucleic acid (single-strand)• A cell can make a copy of part of it’s genetic

code that tells it how to make a protein• This copy is called a mRNA (Messenger RNA)

—carries that information from the DNA to other parts of the cell• Proteins: the “machines” of the cell that

perform a specific task—Ex: red blood cells contain the protein hemoglobin

• The four nitrogen bases for RNA are• Guanine (G) and Cytosine (C)• Adenine (A) and Uracil (U)

• Transcription: The process of copying the genetic information in DNA to mRNA

• tRNA (Transfer RNA): reads the instructions from mRNA and strings amino acids together in the correct order to form a protein chain

• Translation: The process in which tRNA produces a protein from mRNA

Codons: three-letter blocks of nitrogen bases of the mRNA that serve as the “words” of the message about what protein the cell will make• Each codon

codes for a specific amino acid in the protein chain

Mutation• A mistake or change in the DNA sequence—a change in DNA can lead

to a change in mRNA that can lead to a change in the protein created• Mutations DO NOT usually have an effect on an organism HOWEVER

a mutation affects an organism if/when it makes it more or less likely to survive—ONLY mutations in sex cells can affect offspring

• EX: Cancer can be caused by a mutation in which cancerous cells are unable to stop dividing and thus grow uncontrollably

• Cancer cells create tumors which may invade or impair other parts of the body

Tissue: a group of cells that carry out a specific function• EX: skin tissue protects the

body from microorganisms, injury and fluid loss

Organs: a collection of tissues joined together to perform a common function• EX: the heart is a

collection of muscle, nerve, blood and connective tissue that works to pump blood throughout the body

Tissues & Organs

Organ SystemsOrgan System: a group of organs that work together to perform a specific function• EX: The respiratory system includes the nasal passage, airways,

the lungs, other respiratory muscles, etc., and works together to bring oxygen into the body as well as expel carbon dioxide from the body (i.e. breathing)

Organ systems work together: This table outlines how the circulatory, respiratory, and muscular system works with other organ systems in the body