Chapter 5.
Membranes: transport processes
- membranes that line body cavities
- ***the cell membrane
Membranes in the body line a body cavity
or are between two compartments.
Why have a cell membrane?
- isolate the cell  cytosol (intracellular
fluid) from extracellular fluid
- regulate exchange with the environment
 ions, nutrients, wastes, secretions
- cell communicates with the environment
 eg. receptor proteins bind signaling molecules
- structural support  cellular junctions,
anchor cytoskeletal proteins
What are the components of the cell membrane
and what are their functions?
What are the components of the cell membrane
and what are their functions?
Lots of phospholipids.
Integral mb proteins are tightly associated to mb
(eg. membrane spanning).
Associated protein is loosely bound to mb
(eg. enzymes, some structural).
An example of a membrane spanning protein.
Functions of membrane proteins:
1. Structural proteins (junctions, cytoskeleton)
3. Receptors – bind a signaling molecule (ligand)
2. Enzymes – reactions cell membrane (digestion, signaling)
4. Transporters
- channels - water filled pore
- carrier proteins - bind molecule to be carried across
1. Structural proteins:junctions
Functions of membrane proteins:
1. Structural proteins: cytoskeleton
Functions of membrane proteins:
2. Cell membrane receptor protein
3. Enzymes
Functions of membrane proteins:
3. Channel proteins are gated.
Gating of a channel protein:
How are the channels gated (opened)?
Voltage gated, chemically gated, mechanically gated.
4. Facilitated diffusion by a carrier protein:
- never a continuous pore
Facilitated diffusion by a carrier protein:
How can lipids enter a cell?
How can glucose enter a cell?
Facilitated diffusion by a carrier protein:
G
G
G
How can calcium (Ca++) enter a cell?
Calcium enters through a calcium channel
Ca++
How does insulin bind onto a cell?
Insulin receptor protein
Insulin
Insulin
What molecules cross the cell
membrane?
How do these molecules cross the
cell membrane?
*
The phospholipd bilayer is permeable
to gases, water and lipids.
Gases, water and lipids can pass through
the cell mb.
These diffuse through the mb.
*
Characteristics of diffusion:
*molecules are always moving**
• Down a concentration gradient
• Diffusion rate depends on the steepness
of the concentration gradient
• Kinetic energy from molecules that are
moving
• Eventually the molecules distribute
themselves evenly (equilibrium).
• Molecules diffuse short distances quickly.
Characteristics of diffusion:
• Diffusion is directly related to temperature
• Diffusion is inversely related to molecular
size
• Diffusion can occur in an open system
• Diffusion can occur across a barrier if the
molecule can cross the barrier
Diffusion rate of lipophilic molecules across
the cell mb (phospholipid bilayer)
• Diffusion rate depends of solubility in lipid
layer
• Diffusion rate depends on the surface area
of the cell mb.
• Faster diffusion with thinner mb (short
distance to cross)
Ions diffuse through channels down
their concentration gradient.
Ca++
Ca++
Ca++
How do the rules of diffusion apply
to movement by carrier proteins?
Carrier mediated transport into cells:
- net movement as long as there is a
concentration gradient across the
mb  Facilitated Diffusion
Diffusion and carrier proteins
Carrier mediated transport into cells:
Specificity
• Eg. Glucose
transporters move 6
carbon sugars
(hexoses).
Carrier mediated transport into cells:
Specificity
• Glucose transporters
will not move maltose.
Carrier mediated transport into cells:
Competition
• 6 carbon sugars bind to the glucose carrier
• The carrier has a higher affinity for glucose
than for fructose or galactose
• Glucose moves into the cell faster than the
others
Carrier mediated transport into cells:
Competition
Only galactose
Glucose and galactose
Carrier mediated transport into cells:
Competition
• A competing molecule
can block the
transport
Carrier mediated transport into cells
Saturation
• Carriers transporting a substrate reach
their maximal rate.
Diffusion and carrier proteins
Carrier mediated transport into cells
Saturation
Cells need glucose as a source of
ATP for energy.