Download Cell Membrane

Cell Membrane
Selectively Permeable
Basic Structure
• Double layer of
phospholipids
• Referred to a bilayer
• A phospholipid has a
head and two tails
• The phospholipids
face away from each
other in the bilayer
Phospholipids
• Phosphate “heads”
are water-soluble
• They like water and
are attracted to water
• Fatty acid “tails” are
not soluble in water
• They repel water and
want to stay away
from it
Permeability
• Small molecules that
dissolve in lipids can
pass right through the
phospholipid bilayer
• Oxygen, carbon
dioxide and some
water can also pass
through
Permeability
• Glucose, amino
acids, and other
large molecules that
do not dissolve in
lipids cannot pass
through
• Most water
molecules, and ions
such as H+, NA+,
K+, and Ca+ cannot
pass through
Some molecules need help
• We use proteins
embedded in the
membrane to help get
these molecules
across the cell
membrane
Phospholipid
bilayer plus
Notice that
there are
many
structures
stuck in the
phospholipid
bilayer
Proteins scattered throughout
A Gallery of Membrane Proteins
• Adhesion proteins:
Help cells of the same
type stick together in
tissues
• Cell to cell
communication
proteins: match up
with another cell to
pass on a signal
A Gallery of Membrane Proteins
• Receptor proteins:
stick out of the
membrane to bind
hormones.
• Recognition proteins:
Identify each person’s
cells as their own
• Must be matched in
transplants, blood
transfusions, etc.
A Gallery of Membrane Proteins
• Passive Transporter
proteins: allow
molecules to move
through them without
requiring energy
• Active transporters:
use energy to pump
molecules “uphill”
across the membrane
Transport Across Cell
Membranes
Passive or Active
Movement
• Is either passive or active
• Passive does not require cell energy
• Active uses cell energy
Passive Mechanisms of Transport
• Diffusion: molecules
spread or scatter from
regions of higher
concentration toward
regions of lower
concentration
• Molecules will travel
down the
concentration
gradient
Passive Mechanisms of Transport
• Facilitated Diffusion:
molecules cannot
pass through the lipid
bilayer without the
help of a membrane
protein. The
molecule being
transported combines
with a carrier
molecule which
moves it to other side
Passive Mechanisms of Transport
• Osmosis: the diffusion
of water. Water
moves from a region
of higher
concentration to a
region of lower
concentration
• In this example, the
membrane is NOT
permeable to the
solute
Passive Mechanisms of Transport
• Filtration: forces
molecules through
membranes
• Often used to
separate solids from
liquids
• Kidneys filter blood,
the force coming from
blood pressure
Active Mechanisms of Transport
• Active transport: uses
energy and carrier
proteins to move
molecules from areas
of LOW concentration
to areas of HIGH
concentration
• Against the
concentration
gradient; a pump
Active Mechanisms of Transport
• Endocytosis: uses
cellular energy to
form a vesicle from a
section of cell
membrane to bring IN
substances
• Exocytosis: uses
energy to secrete a
substance stored in a
vesicle