Unit 2:
Cells, Membranes and Signaling …
CELL MEMBRANE
Chapter 5 Hillis Textbook
HOW DOES THE LAB RELATE TO THE NEXT CHAPTER?
SURFACE AREA: the entire outer covering of a cell that enables
materials pass.
VOLUME: the inside spare that can be filled with organelles/etc.
CELLS WITH MORE SURFACE AREA AND LESS VOLUME
WILL BE VERY EFFICIENT!!!
You looked at the rate of diffusion
Which cell could diffuse materials quicker and more
efficiently?
HOW DO MOLECULES GET IN AND OUT OF THE CELL?
• MANY different molecules nee to get into the cells…
• For example, after we eat BIOMOLECULES are broken
into monomers and absorbed by the small intestines, into
the blood and traveled all around the body to the CELLS!
• Biomolecules must get into the cells for the cells to use
them
 Ex: Mitochondria need glucose, ribosomes need amino
acids…
 CELL MEMBRANE needs lipids, carbs and proteins
These molecules get in and out of the cell by the CELL
MEMBRANE!
PART ONE: PROPERTIES OF THE CELL MEMBRANE
PHOSOPHOLIPID BILAYER
A membrane’s structure and functions are determined by its constituents:
lipids, proteins, and carbohydrates.
The general structure of membranes is known as the fluid mosaic
model.
Phospholipids form a bilayer which is like a “lake” in which a variety of
proteins “float.”
LIPIDS
Lipids form the hydrophobic core of the membrane.
Most lipid molecules are phospholipids with two
regions:
 Hydrophilic regions
electrically charged “heads”
that associate with water
molecules
 Hydrophobic regions
nonpolar fatty acid “tails” that
do not dissolve in water
BILAYER:
A bilayer is formed when the fatty acid “tails”
associate with each other and the polar “heads”
face the aqueous environment (Water).
Bilayer organization helps membranes fuse during
vesicle formation and phagocytosis.
FLUID MOSAIC:
Membranes are fluid due to the flexible composition and
mosaic due to the patterns that form from lipids and proteins
Two important factors in membrane fluidity:
 Lipid composition—types of fatty acids can increase or
decrease fluidity
 Temperature—membrane fluidity decreases in colder
conditions
Proteins are randomly scattered within the cell membrane to
form a mosaic pattern
PROTEINS:
Biological membranes contain proteins, with varying ratios of phospholipids.
• Peripheral membrane proteins lack hydrophobic groups and are not
embedded in the bilayer.
• Integral membrane proteins are partly embedded in the phospholipid
bilayer.
Anchored membrane proteins have lipid components that anchor them
in the bilayer.
Proteins are asymmetrically distributed on the inner and outer
membrane surfaces.
• A transmembrane protein extends through the bilayer on both sides, and
may have different functions in its external and transmembrane domains.
Some membrane proteins can move within the phosopholipid bilayer,
while others are restricted.
Proteins inside the cell can restrict movement of membrane proteins,
as can attachments to the cytoskeleton.
CARBOHYDRATES:
Plasma membrane carbohydrates are
located on the outer membrane and can
serve as recognition sites.
• Glycolipid—a carbohydrate bonded to a
lipid
• Glycoprotein—a carbohydrate bonded
to a protein
SELECTIVELY PERMEABLE (SEMI-PERMEABLE):
Biological membranes allow some
substances, and not others, to pass.
This is known as selective
permeability.
Two processes of transport:
• Passive transport does not require
metabolic energy.
• Active transport requires input of
metabolic energy.