The Cell Essay

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Pages: 9
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A Tour of the Cell

Overview: The Fundamental Units of Life

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All organisms are made of cells

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The cell is the simplest collection of matter

that can live

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Cell structure is correlated to cellular function

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All cells are related by their descent from earlier cells

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LMs can magnify effectively to about 1,000 times the size of

the actual specimen

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Various techniques enhance contrast and enable cell

components to be stained or labeled

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Most subcellular structures, including organelles (membraneenclosed

compartments), are too small to be resolved by an

LM

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Two basic types of electron microscopes (EMs) are used to

study subcellular structures

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Scanning electron microscopes (SEMs) focus a beam of

electrons onto the surface of a specimen, providing images

that look 3-D

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Transmission electron microscopes (TEMs) focus a beam

of electrons through a specimen

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TEMs are used mainly to study the internal structure of cells

Concept 6.2: Eukaryotic cells have internal membranes that

compartmentalize their functions

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The basic structural and functional unit of every organism is

one of two types of cells: prokaryotic or eukaryotic

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Only organisms of the domains Bacteria and Archaea consist

of prokaryotic cells

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Protists, fungi, animals, and plants all consist of eukaryotic

cells

Comparing Prokaryotic and Eukaryotic Cells

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Basic features of all cells:

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Plasma membrane

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Semifluid substance called cytosol

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Chromosomes (carry genes)

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Ribosomes (make proteins)

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Prokaryotic cells are characterized by having

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No nucleus

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DNA in an unbound region called the nucleoid

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No membrane-bound organelles

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Cytoplasm bound by the plasma membrane

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Eukaryotic cells are characterized by having

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DNA in a nucleus that is bounded by a membranous nuclear

envelope

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Membrane-bound organelles

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Cytoplasm in the region between the plasma membrane and

nucleus

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Eukaryotic cells are generally much larger than prokaryotic

cells

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The plasma membrane is a selective barrier that allows

sufficient passage of oxygen, nutrients, and waste to service

the volume of every cell

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The general structure of a biological membrane is a double

layer of phospholipids

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The logistics of carrying out cellular metabolism sets limits on

the size of cells

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The surface area to volume ratio of a cell is critical

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As the surface area increases by a factor of n2, the volume

increases by a factor of n3

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Small cells have a greater surface area relative to volume

A Panoramic View of the Eukaryotic Cell

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A eukaryotic cell has internal membranes that partition the cell

into organelles

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Plant and animal cells have most of the same organelles

Concept 6.3: The eukaryotic cellÕs genetic instructions are housed

in the nucleus and carried out by the ribosomes

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The nucleus contains most of the DNA in a eukaryotic cell

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Ribosomes use the information from the DNA to make proteins

The Nucleus: Information Central

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The nucleus contains most of the cellÕs genes and is usually

the most conspicuous organelle

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The nuclear envelope encloses the nucleus, separating it

from the cytoplasm

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The nuclear membrane is a double membrane; each

membrane consists of a lipid bilayer

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Pores regulate the entry and exit of molecules from the

nucleus

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The shape of the nucleus is maintained by the nuclear

lamina, which is composed of protein

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In the nucleus, DNA and proteins form genetic material called

chromatin

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Chromatin condenses to form discrete chromosomes

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The nucleolus is located within the nucleus and is the site of

ribosomal RNA (rRNA) synthesis

Ribosomes:
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