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Cell Structure Full Summary Notes
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Cell Structure Summary NotesIntroduction to the Cell Structure
The purpose of this guide is to provide an overview of the basic structural components of living cells. In
reviewing these structures, we will also discuss their functions.
All living organisms are composed of cells. A cell is a small, membrane-bound compartment that contains
all the chemicals and molecules that help support an organism's life. An understanding of the structure of
cells is one of the first steps in comprehending the complex cellular interactions that direct and produce
life.
Cells can be thought of as building blocks of organisms. Some organisms are composed of a single cell.
Others, like ourselves, are composed of millions of cells that work together to perform the more complex
functions that make us different from bacteria. It is difficult to imagine that humans are descendants of a
single cell, but this is a common belief in the scientific world. Before we can understand how multiple
cells can work together to create complex biological functions, it is necessary to understand what
biological functions single cells are capable of performing on their own to sustain life.
There are different types of cells with individuated structures. Single-celled organisms have different cell
structure than multi-celled organisms and plant cells have different structures from animal cells. These
differences reflect differences in the functions that each of these classes of cells is required to perform.
While the focus of this guide will be on the structures that compose complex multi-cellular organisms, we
will begin our discussion of cell structure with a structure that is universal to all cells, membranes.
Cell Membranes
Introduction
Cell membranes are very important structures to cells because they function as a barrier between the
components of the cell and the outside environment. The cell membrane is not only responsible for
creating a wall between inside and outside the cell, it must also act as a threshold through which select
molecules can enter and exit the cell when necessary. The cell membrane is what defines the cell and
keeps its components separate from outside cells or organisms.
The cell membrane is composed of a double layer of fat cells called a lipid bilayer in which membrane
proteins are embedded. The structure of the lipid bilayer prevents the free passage of most molecules into
and out of the cell. We will begin our discussion of the structure of the cell membrane by discussing the
structure and properties of the lipid bilayer. We will then go on to discuss the role of membrane-bound
proteins, and finally, will discuss membrane transport structures.
The Lipid Bilayer
Lipid Bilayer Structure
The lipid bilayer is a universal component of all cell membranes. Its role is critical because its structural
components provide the barrier that marks the boundaries of a cell. The structure is called a "lipid bilayer"
because it is composed of two layers of fat cells organized in two sheets. The lipid bilayer is typically
about five nanometers thick and surrounds all cells providing the cell membrane structure.
Lipids and Phospholipids
The structure of the lipid bilayer explains its function as a barrier. Lipids are fats, like oil, that are
insoluble in water. There are two important regions of a lipid that provide the structure of the lipid
, bilayer. Each lipid molecule contains a hydrophilic region, also called a polar head region, and a
hydrophobic, or nonpolar tail region.
Figure : Basic Lipid Structure
The hydrophilic region is attracted to aqueous water conditions while the hydrophobic region is repelled
from such conditions. Since a lipid molecule contains regions that are both polar and nonpolar, they are
called amphipathic molecules.
The most abundant class of lipid molecule found in cell membranes is the phospholipid. The phospholipid
molecule's polar head group contains a phosphate group. It also sports two nonpolar fatty acid chain
groups as its tail.
Figure : Phospholipid Structure
The fatty acid tail is composed of a string of carbons and hydrogens. It has a kink in one of the chains
because of its double-bond structure.
The Bilayer
The phospholipids organize themselves in a bilayer to hide their hydrophobic tail regions and expose the
hydrophilic regions to water. This organization is spontaneous, meaning it is a natural process and does
not require energy. This structure forms the layer that is the wall between the inside and outside of the
cell.
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