Cells and Their Environment
Structure of the Plasma Membrane
 The plasma membrane (cell
membrane) is made of two
groups of organic molecules-phospholipids and proteins.
 A phospholipid is a molecule
shaped like a head with two tails.
The head is polar and the two
tails are nonpolar.
Polar head
Nonpolar tails
 Water molecules form
hydrogen bonds with the
polar heads, but push away
the nonpolar tails, thus forming
a two-layer structure with the
heads facing outward and the
tails facing inward. This is the
plasma membrane.
 The plasma membrane is fluid,
like a soap bubble and has
various proteins stuck in it.
Outside of Cell
Inside of Cell
Channel protein
Receptor protein
Marker protein
Cell Surface Proteins
 The proteins within the plasma
membrane of cells are called
cell surface proteins. There
are several kinds.
 Channel proteins have a
structure that makes them
doughnut shaped, with a hole in
the middle. Polar sugars, amino
acids, ions, and other particles
can pass through these
channels.
 Receptor proteins transmit
information from the world outside
the cell to the cell’s interior. The
outside part of a receptor protein
fits only a particular type of
molecule. If that molecule
attaches to a receptor, the shape
of the other end of the receptor is
changed, thus passing
“information” inside the cell.
Channel
protein
Marker proteins have long
carbohydrate arms attached to
them that identify the cell to other
cells.
•
Receptor Protein
Marker protein with carbohydrate arm
Moving Materials Into and Out
of Cells
 Materials can move through the
cell membrane without using
any of the cell’s energy. This is
called passive transport.
 One kind of passive transport is
diffusion. Particles in a
solution tend to move from an
area of greater concentration
to an area where there are less
of them (a lesser
concentration). This movement
is called diffusion.
Water molecule - will move into the cell
 Water also moves from a greater to
a lesser concentration. When
water travels across a membrane
from a greater to a lesser
concentration of water, this is
called osmosis.
Cell
Solute molecule - will move out of the cell.
Cell’s Environment
•
Water and Solution Concentrations
Water Molecule
Solute Molecule
 Hypotonic Solution- the
environment’s solute
concentration is less than
inside the cell. Water will move
into the cell.
 Hypertonic Solution- the
environment’s solute concentration
is greater than inside the cell.
Water will move out of the cell.
Isotonic Solution- the solute
concentration is the same inside
and outside the cell. Water
movement into the cell equals
movement out of the cell. No net
water movement.
·
Facilitated Diffusion
 Facilitated diffusion is the
movement of specific particles
through a membrane by a
channel protein.
 The molecule that is to be
transferred locks onto the
channel protein on the outer
surface of the cell membrane
and then passes through the
channel to the inside of the cell.
 The plasma membrane is
selectively permeable. It allows
some particles to pass through and
prevents the passage of other
particles.
 Many of the selective protein
channels through the plasma
membrane are two-way channel
proteins called pores. Different
particles fit into different sized
pores.
·
Plasma
Membrane
Interior of Cell
Channel
Protein
Active Transport
 Cells must be able to take in
molecules that are in low
concentration outside of the cell.
To do this, cells need to spend
energy. These processes are
called active transport.
Sodium ions along with food molecules
enter the cell through coupled channels.
Energy from food molecules is used to
drive
Proton pumps that cause the
production of ATP. ATP molecules drive
Sodium-potassium pumps, which
transport sodium ions back outside the
cell.
 Proton pumps carry protons (H+
ions) across membranes to make
ATP, the energy currency of the
cell. The active transport of
protons to make ATP is called
chemiosmosis.
 Sodium-potassium pumps use
some of the ATP produced by the
proton pumps to build up a large
concentration of Na+ ions outside
of the cell.
 Coupled channels carry the
sodium ions back into the cell,
along with some food molecules,
that cannot pass through the
plasma membrane by themselves.
·
Endocytosis and Exocytosis
 Food particles that are too large
to go through protein channels
in the cell membrane enter the
cell by endocytosis.
 In endocytosis, the cell
surrounds and then engulfs the
food particle. This requires
energy. There are two kinds of
endocytosis. If the material
being engulfed is liquid, the
endocytosis is called
pinocytosis. If the material is
solid, the process is called
phagocytosis.
·
 The opposite of endocytosis is
exocytosis. Cells use this
process to get rid of wastes.
Gland cells dump hormones into
the blood by exocytosis.
Endocytosis
1.
2.
3.
Exocytosis
1.
2.
3.
How Cells Communicate
 Your cells communicate with each  Indirect communication between
other directly and indirectly to
cells is done two ways--through the
coordinate growth, development,
endocrine system and the
and other activities.
nervous system.
 Direct communication happens
 The endocrine system
between some cells. These cells
communicates using chemicals
have tiny openings in their cell
called hormones.
membranes called gap
 Nerve cells communicate using
junctions, that let the cytoplasm
chemicals called
of the two cells join together.
neurotransmitters.
 Special proteins on the cell’s
Cell #1 cytoplasm
Gap Junction
surface, called receptors, signal
the cytoplasm when a particular
Cell Membrane
particle has bumped into the cell’s
surface.
Cell Membrane
·
Cell #2 cytoplasm
How Cells Receive
Communications
 The binding of a signal particle
 Some channel proteins can be
to a cell’s surface receptor can
opened or closed. They are called
influence the cell’s cytoplasm in
gated channels. Gated channels
one of three ways:
may be opened either chemically
or electrically.
 The receptor can become an
Signal
enzyme that chemically
molecule
Na+
changes molecules in the cell’s
cytoplasm.
Cell Membrane
 The receptor can create a
Gated channel is closed.
Cell cytoplasm
second messenger which will
Ions cannot enter.
have an effect somewhere else
in the cytoplasm.
Signal
molecule
 The receptor can open a
channel through the plasma
Na+
membrane. Neurotransmitters
do this.
Cell Membrane
·
Signal molecule changes shape of gate, which
opens. Ions pass through gate to cytoplasm.