Download The Cell Membrane

The Cell Membrane
Cell membranes are composed of two
phospholipid layers.
 The cell membrane, or the
plasma membrane, forms a
boundary between the cell
and the outside
environment.
 It controls the passage of
materials into and out of
the cell.
 The cell consists of a
double layer of
phospholipids interspersed
with a variety of other
molecules.
Phospholipids
 A phospholipid is a molecule
composed of three basic parts:
 A charged phosphate (PO4-)
head.
 Glycerol
 Two fatty acid chains
 Together, the glycerol and the
phosphate group form the
“head” of a phospholipid.
 The fatty acids form the tail.
Polarity of Phospholipids
 The head of the phospholipid is polar because it bears a charge.
 Water molecules are also polar.
 Polar molecules love polar molecules, so water and the polar head of
the phospholipid form a hydrogen bond. Hydrophillic.
Polarity of Phospholipids
 The fatty acid tails are non-polar.
 Polar molecules (like water) and non-polar molecules repel
each other.
 Therefore, non-polar tails are attracted to each other and repel
water. Hydrophobic.
Sandwich Membrane
 The polar (or non-polar) properties
of the phospholipids causes them to
arrange themselves in layers, like a
sandwich.
 Polar heads are like bread.
 They form the outer surfaces of the
membrane, where they interact with the
watery environment both outside and
inside a cell.
 The non-polar tails are like the filling.
 The are sandwiched between the layers of
polar heads, where they are protected
from the watery environments.
Molecules in the membrane.
 Molecules are embedded
throughout the cell
membrane.
 Cholesterol molecules
strengthen the cell membrane.
 Some proteins help materials
cross through them membrane,
others are components of the
cytoskeleton.
 Carbohydrates attached to
membrane proteins serve as
identification tags, enabling
cells to distinguish one cell
type from another.
Fluid Mosaic Model
 The cell membrane is flexible,
not rigid.
 The phospholipids in each layer
can move from side to side and
slide past each other.
 This makes the cell membrane act
like oil on the surface of water.
 The proteins that stud the membrane
act like an arrangement of colorful
tiles with different textures and
patterns.
Selective Permeability
 The cell membrane allows
some, but not all, materials
to cross.
 Selective permeability allows
a cell to maintain
homeostasis.
 Because a cell need to maintain
certain conditions to carry out
its functions, it must control the
import and export of certain
molecules and ions.
Crossing the membrane.
 Molecules cross the membrane in several ways.
 Some methods require the cell to expend energy, some do not.
 How a particular molecule crosses the membrane depends on the molecules
size, polarity, and concentration inside versus outside the cell.
 Small non-polar molecules easily pass through the membrane.
 Small polar molecules are transported via proteins.
 Large molecules are moved in vesicles.
oxygen, carbon
dioxide, and other
small, nonpolar
molecules; some
water molecules
glucose and other large,
polar, water-soluble
molecules; ions (e.g.,
H+, Na+, K+, Ca++,
Cl–); water molecules
Chemical signals are transmitted
across the cell membrane
 A receptor is a protein that
detects a signal molecule and
performs an action in response.
 It recognizes and binds to only
certain molecules, which
ensures that the right cell gets
the right signal at the right time.
 When receptors are activated
(they combine with a hormone
or neurotransmitter), they
change shape.
 This relays information to the cell.
Types of Proteins
 The cell has many different types of proteins
Adhesion
communication
embedded inside it.
 Adhesion proteins-Help the cell adhere to
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neighboring cells.
Communication proteins-chemical and electrical
signals flow from one cell to the next.
Receptor proteins-gets a message from neighboring
cells and communicates with the nucleus.
Recognition proteins-identify the cell as “self ”
Passive transporters-have an open channel through
which material may cross.
Active transporters-have a pump that can pull
substances across against concentration gradients.
receptor
recognition
passive
active
Intracellular Receptor
 Intracellular receptors
are inside the cell.
 These generally react to
hormones, which can
pass through the cell
membrane easily.
 Each intracellular
receptor only responds
to certain chemicals,
they are specific.
Intracellular receptors
Membrane Receptor
 When a molecule cannot pass through the cell membrane, they need
a membrane receptor to help them get the message through.
 When the membrane receptor binds to a signal molecule, the entire
membrane receptor changes shape.
 As a result, it causes the molecules inside the cell to respond.
 Example:

Band 3 protein binds to a membrane receptor. The receptor changes shape. This
triggers processes that carry carbon dioxide from body tissues to the lungs.
Voyage into the Cell Membrane
 Voyage inside the Cell: Membrane -YouTube