Chapter 5 Membranes transport processes membranes that line

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?

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

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++

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 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.