Chapter 5: Homeostasis and Cell
Transport
Homeostasis
The steady – state physiological
condition of the body of a cell.
Cell membranes help organisms
maintain homeostasis by
controlling what substances may
enter or leave cells.
 Lipid Bilayer with embedded proteins. Phospholipid bilayer
with some proteins extending from one side of the membrane
to another and some proteins are embedded only half-way
 Proteins are utilized for both PASSIVE AND ACTIVE
TRANSPORT
 Carbohydrate chains are located on the outer surface of the
membrane. If they are attached to phospholipids they are
known as GLYCOLIPIDS. If they are attached to proteins they
are called GLYCOPROTEINS.
 Carbohydrate chains help with cell to cell recognition
What determines the permeability of a
substance across the cell membrane?
o Size
o Polarity
o Hydrophobic vs. hydrophilic
o Charge
 The cell membrane is described as selectively
permeable because it allows some substances to pass
in or out but not others.
 The size, charge, and polarity of a substance
determines its permeability.
 The cell membrane is permeable to gases such as
carbon dioxide and oxygen since gases are small.
 The cell membrane is permeable to water and other
small, polar molecules.
Homeostasis
CELL MEMBRANES help
organisms maintain
HOMEOSTASIS by
controlling what
substances may enter or
leave the cell.
Some substances can cross the
cell membrane without any input
of energy by the cell in a process
called PASSIVE TRANSPORT.
The simplest type of PASSIVE
TRANSPORT is DIFFUSION.
Diffusion is the movement of
molecules from an area of higher
concentration to an area of lower
concentration
The difference in the
concentration of molecules
across a distance or across a
membrane is known as a
Concentration Gradient.
In the absence of other influences,
diffusion will eventually cause the
molecules to be in equilibrium.
Equilibrium – the concentration of
molecules will be the same
throughout the space the molecules
occupy.
 How does the cell membrane help the cell to maintain
homeostasis?
 Why is the cell described as selectively permeable?
What characteristics of a substance can determine its
permeability?
 Draw and label a cross section of the cell membrane.
Make sure to include the phospholipids, embedded
carrier proteins, ion channel proteins, carbohydrate
chains as recognition antennas.