7.2 The Plasma Membrane
Chapter 7
A View of the Cell
Pages 175 - 178
Homeostasis – Maintaining
a Balance
• Cells must keep the proper concentration
of nutrients and water and eliminate
wastes.
• The plasma membrane is selectively
permeable – it will allow some things to
pass through, while blocking other things.
• A cell's contents would be the same as its
surrounds, were it not for selective
permeability.
Structure of the Plasma
Membrane
• Because cells have a watery environment
both inside and outside, the polar ends
of the phospholipids in the plasma
membrane form double layers
• The double layer of the plasma
membrane is called a lipid bilayer
Structure of the Plasma
Membrane
• Lipid bilayer – two sheets of lipids
(phospholipids).
– Found around the cell, the nucleus,
vacuoles, mitochondria, and
chloroplasts.
– Embedded with proteins and
strengthened with cholesterol
molecules.
What is a Phospholipid?
• A phospholipid
is like a fat
molecule but
has
– one less fatty
acid group
– a phosphate
portion.
What’s a Phospholipid?
– Polar (water-soluble) heads face
out and the nonpolar fatty acids
hang inside.
Hydrophobic / Hydrophillic
• Hydrophobic = “afraid of
water”
• Non-polar side
• This is the part that is like
the fats – how does water act
with oil?
• Hydrophillic = “water loving”
• Polar side
• This is the side that is in
contact with the watery
environments.
Membrane Proteins
• Determine what particles can pass
through the membrane – like doors
• Serve as enzymes (may speed
reactions).
• Act as markers that are recognized by
chemicals and molecules from the inside
and the outside of the cell (the immune
system) – like a house number.
Carbohydrate
sticking out
Fluid Mosaic Model
• Because the phospholipid molecules and
some proteins are free to move, the
plasma membrane is said to be a fluid
mosaic.
• The fluid mosaic model describes a
structure with polar layers on the
outside and nonpolar layer on the inside.
Fluid Mosaic Model
• Which of the following pictures below most
likely approximate the motion phospholipids
make in a plasma membrane?
All of these things are true for
plasma membranes:
• Folded membranes increase surface
area for efficiency.
• Endoplasmic reticulum is made up of
folded membranes.
• Ribosomes are sometimes attached to
folded membranes.
Carbohydrate
chains
Membrane protein
Polar head of
phospholipid
Non-polar tails
of phospholipid
Cholesterol
• What structure is analogous to a sewage system?
• Membrane protein
• Where are you least likely to find water?
• Between the non-polar tails of the phospholipids
• What would happen to the plasma membrane if part D is
completely removed?
• It would collapse on itself
Diseases of the plasma membrane
Decreased movement of molecules
entering the cell might be a result of a
disease that causes a thickened plasma
membrane.
Cystic Fibrosis is a disease of the plasma
membrane
Diseases of the plasma membrane
• Cystic Fibrosis, incurable hereditary
disorder that causes the body to
secrete an abnormally thick, sticky
mucus that clogs the pancreas and the
lungs, leading to problems with
breathing and digestion, infection, and
ultimately, death.
Diseases of the plasma membrane
• One of the most common fatal genetic
disorders in the United States, cystic
fibrosis occurs in about one in every
3,900 babies.
Diseases of the plasma membrane
• Cystic fibrosis is caused by a defect in
the gene responsible for making a
protein that controls the flow of
chloride ions into and out of certain
cells through the membrane.