CHAPTER 3: CELLS: THE LIVING UNITS
COMPONENTS OF A EUKARYOTIC CELL
Eukaryotic cell: A eukaryotic cell is any cell that contains a nucleus and membrane-bound organelles.
EX: Somatic cells (i.e. epithelial cells, bone cells, white blood cells) and gametes (i.e. egg and sperm)
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, CHAPTER 3: CELLS: THE LIVING UNITS
CELL (PLASMA) MEMBRANE
Lipid-bilayer structure: The cell membrane is composed of two layers of phospholipids with other lipid-molecules and
various proteins composing this structure. The lipid bilayer provides the basic structural framework of the cell membra ne.
Membrane lipids: Lipid-like molecules that compose the cell membrane include phospholipids, cholesterol, and
glycolipids.
Phospholipids: Phospholipids compose about 30% of the cell membrane, and they are organized in a
bilayer arrangement due to their unique chemistry. Phospholipids are composed of a phosphate group,
which is the polar “head” region, bonded to two fatty acid chains, which is the nonpolar “tail” region. The
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, CHAPTER 3: CELLS: THE LIVING UNITS
polar heads of the molecules orient themselves toward the fluid-filled intracellular and extracellular
environments of the cell, where water is present. The nonpolar tails of the molecules orient themselves
away from the watery, fluid environments as they are repelled by the polarity of the water. When
observing the orientation of the phospholipids in the bilayer structure of the cell membrane, it’s important
to note that the polar heads make contact with the intracellular and extracellular environments, whereas
the nonpolar tails make contact with each other in the center of the membrane.
Cholesterol: Cholesterol is a steroid molecule that exhibits lipid-like properties (i.e. nonpolar,
hydrophobic, and insoluble in water) and functions to provide rigidity (structure) to the cell membrane.
Glycolipids: Glycolipids are lipid-like molecules that have a carbohydrate bonded to them, and they are
located in the superficial layer of the cell membrane. The carbohydrate portion of the molecule projects
extracellularly, and it functions to provide the cell with its identification markers.
Membrane proteins: Membrane proteins compose about 50% of the cell membrane, and they include a variety of
integral proteins, peripheral proteins, and glycoproteins.
Integral proteins: Integral proteins partly transverse or completely transverse the cell membrane and are
firmly embedded in the membrane. These proteins are composed of hydrophobic and hydrophilic
regions, which allow them to interact with the lipid-like molecules in the membrane.
Transmembrane proteins: Transmembrane proteins are integral proteins that completely
transverse the cell membrane; the ends of these proteins extend intracellularly and
extracellularly. EX: receptors, ion channels, and transporters
Peripheral proteins: Peripheral proteins are located on the intracellular and extracellular surfaces of the
cell membrane, and they interact with integral proteins in the membrane. They are not firmly embedded
in the membrane and are capable of detaching themselves from the membrane to perform specific
functions. EX: filaments that provide structural support to cell, G proteins involved in hormonal responses
Glycoproteins: Glycoproteins are proteins with carbohydrate molecules bonded to them. The
carbohydrate portion of the molecule extends extracellularly and creates a glycocalyx on the surface of
the cell. These proteins function as cell-surface identification markers.
Glycocalyx: A glycocalyx is the extracellular sticky covering of the cell membrane created by
glycoproteins and glycolipids. The unique patterns of carbohydrate molecules in the glycocalyx
function as cell identification markers.
Functions of membrane proteins: Membrane proteins have various functions, and some basic examples
are provided below:
Receptors – membrane proteins that detect specific stimuli in the intracellular and extracellular
environments of the cell
Ion channels – membrane proteins that act as “gateways” or openings to allow ions into and out
of the cell
Transporters – membrane proteins that transport molecules into and out of the cell
Cell-identity markers – membrane proteins that provide the identity of the cell
Enzymes – specialized proteins that act as biological catalysts (increase the rate of metabolic
reactions)
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COMPONENTS OF A EUKARYOTIC CELL
Eukaryotic cell: A eukaryotic cell is any cell that contains a nucleus and membrane-bound organelles.
EX: Somatic cells (i.e. epithelial cells, bone cells, white blood cells) and gametes (i.e. egg and sperm)
Page 1
, CHAPTER 3: CELLS: THE LIVING UNITS
CELL (PLASMA) MEMBRANE
Lipid-bilayer structure: The cell membrane is composed of two layers of phospholipids with other lipid-molecules and
various proteins composing this structure. The lipid bilayer provides the basic structural framework of the cell membra ne.
Membrane lipids: Lipid-like molecules that compose the cell membrane include phospholipids, cholesterol, and
glycolipids.
Phospholipids: Phospholipids compose about 30% of the cell membrane, and they are organized in a
bilayer arrangement due to their unique chemistry. Phospholipids are composed of a phosphate group,
which is the polar “head” region, bonded to two fatty acid chains, which is the nonpolar “tail” region. The
Page 2
, CHAPTER 3: CELLS: THE LIVING UNITS
polar heads of the molecules orient themselves toward the fluid-filled intracellular and extracellular
environments of the cell, where water is present. The nonpolar tails of the molecules orient themselves
away from the watery, fluid environments as they are repelled by the polarity of the water. When
observing the orientation of the phospholipids in the bilayer structure of the cell membrane, it’s important
to note that the polar heads make contact with the intracellular and extracellular environments, whereas
the nonpolar tails make contact with each other in the center of the membrane.
Cholesterol: Cholesterol is a steroid molecule that exhibits lipid-like properties (i.e. nonpolar,
hydrophobic, and insoluble in water) and functions to provide rigidity (structure) to the cell membrane.
Glycolipids: Glycolipids are lipid-like molecules that have a carbohydrate bonded to them, and they are
located in the superficial layer of the cell membrane. The carbohydrate portion of the molecule projects
extracellularly, and it functions to provide the cell with its identification markers.
Membrane proteins: Membrane proteins compose about 50% of the cell membrane, and they include a variety of
integral proteins, peripheral proteins, and glycoproteins.
Integral proteins: Integral proteins partly transverse or completely transverse the cell membrane and are
firmly embedded in the membrane. These proteins are composed of hydrophobic and hydrophilic
regions, which allow them to interact with the lipid-like molecules in the membrane.
Transmembrane proteins: Transmembrane proteins are integral proteins that completely
transverse the cell membrane; the ends of these proteins extend intracellularly and
extracellularly. EX: receptors, ion channels, and transporters
Peripheral proteins: Peripheral proteins are located on the intracellular and extracellular surfaces of the
cell membrane, and they interact with integral proteins in the membrane. They are not firmly embedded
in the membrane and are capable of detaching themselves from the membrane to perform specific
functions. EX: filaments that provide structural support to cell, G proteins involved in hormonal responses
Glycoproteins: Glycoproteins are proteins with carbohydrate molecules bonded to them. The
carbohydrate portion of the molecule extends extracellularly and creates a glycocalyx on the surface of
the cell. These proteins function as cell-surface identification markers.
Glycocalyx: A glycocalyx is the extracellular sticky covering of the cell membrane created by
glycoproteins and glycolipids. The unique patterns of carbohydrate molecules in the glycocalyx
function as cell identification markers.
Functions of membrane proteins: Membrane proteins have various functions, and some basic examples
are provided below:
Receptors – membrane proteins that detect specific stimuli in the intracellular and extracellular
environments of the cell
Ion channels – membrane proteins that act as “gateways” or openings to allow ions into and out
of the cell
Transporters – membrane proteins that transport molecules into and out of the cell
Cell-identity markers – membrane proteins that provide the identity of the cell
Enzymes – specialized proteins that act as biological catalysts (increase the rate of metabolic
reactions)
Page 3
