Cell Membranes

1
Cell Membranes

• Chapter 5 Purves
• Reading and Vocabulary
• Diagrams Study
• Laboratory Osmosis and Diffusion
• Laboratory Purple Onion

2
Membrane Functions

• Cell membranes are bilayered, dynamic structures
  that
• Perform vital physiological roles
• Form boundaries between cells and their
  environments
• Regulate movement of molecules into and out of
  cells
• Lipids, proteins, and carbohydrates in various
  combinations make these tasks possible.

3
Membrane Structure and Character

• The lipid portion of a cellular membrane provides
  a barrier for water-soluble molecules.
• Lipids are like the water of a lake in which
  proteins float. This general design is called
  the fluid mosaic model.
• Membrane proteins are embedded in the lipid
  bilayer.
• Carbohydrates attach to lipid or protein
  molecules on the membrane, generally on the outer
  surface.

4
Membrane Structure

• All biological membranes contain proteins.
• The ratio of protein to phospholipid molecules
  varies depending on membrane function.
• Many membrane proteins have hydrophilic and
  hydrophobic regions.
• The association of protein molecules with lipid
  molecules is not covalent both are free to move
  around laterally, according to the fluid mosaic
  model.

5
Figure 5.1 The Fluid Mosaic Model
Outside of cell
Inside of cell
6
Proteins and Lipid Bilayer
7
Fluid Mosaic

• Although all biological membranes are
  structurally similar, some have quite different
  compositions of lipids and proteins.
• Cholesterol may increase or decrease fluidity
  depending on other factors, such as the fatty
  acid composition of the other lipids found in the
  membrane.
• For any given membrane, fluidity also decreases
  with declining temperature. The membranes of
  cells that live at low temperatures tend to be
  high in unsaturated and short-chain fatty acids.

8
Fluid Mosaic

• Although all biological membranes are
  structurally similar, some have quite different
  compositions of lipids and proteins.
• Cholesterol may increase or decrease fluidity
  depending on other factors, such as the fatty
  acid composition of the other lipids found in the
  membrane.
• For any given membrane, fluidity also decreases
  with declining temperature. The membranes of
  cells that live at low temperatures tend to be
  high in unsaturated and short-chain fatty acids.

9
Cell recognition

• Molecules are located on the exterior cells.
  These are primarily proteins
• Cell are identified as self vs. non self
• They are referred to as antigens
• HLA antigens are on all of human tissues
• Blood antigens are on Red blood cells

10
Channels

• Channels are formed by proteins.
• They may be gated or non gated
• Non gated channels allow ions and small molecules
  to enter
• Gated are highly selective
• Pumps are one type of channel

11
Cell Junctions

• Specialized cell junctions form between cells in
  a tissue.
• Animals have three types of cell junctions tight
  junctions, desmosomes, and gap junctions.

12
Receptors

• Receptors are located on the exterior of cell
  membranes for interactions with other cells
• Hormones
• Neurotransmitters
• Messengers

13
Types of Junctions
14
Tight Junctions
15
Tight Junctions

• Tight junctions are specialized structures at the
  plasma membrane that link adjacent epithelial
  cells.
• They have two primary functions
• To restrict the migration of membrane proteins
  and phospholipids from one region of the cell to
  another
• To prevent substances from moving through the
  intercellular space

16
Gap Junctions
17
Gap Junctions

• Gap junctions are connections that facilitate
  communication between cells.
• Gap junctions are made up of specialized protein
  channels called connexons.
• Connexons span the plasma membranes of two
  adjacent cells and protrude from them slightly.
• Connexons are made of proteins called connexins,
  which snap together to generate a pore

18
Desmosomes

• Link adjacent cells
• Permit materials to move through the
  intercellular space
• They are anchored in dense plaques
• Cell adhesion molecules cross the space and bind
  cells together
• Keratin is an importan molecule in this process

19
Passive transport

• Biological membranes are selectively permeable.
  They allow some substances to pass, while others
  are restricted.
• Some substances can move by simple diffusion
  through the phospholipid bilayer.
• Some must travel through proteins to get in, but
  the driving force is still diffusion. This
  process is called facilitated diffusion.

20
Diffusion

• Diffusion is the process of random movement
  toward the state of equilibrium.
• Although individual particles move randomly, in
  diffusion the net movement is directional, from
  regions of greater concentrations to regions of
  lesser concentrations, until equilibrium is
  reached.

21
Diffusion
22

• Isotonic solutions have equal solute
  concentrations.
• A hypertonic solution has a greater total solute
  concentration than the solution to which it is
  being compared.
• A hypotonic solution has a lower total solute
  concentration than the solution to which it is
  compared.

23
Osmosis
24
(No Transcript)