Cell Membranes & Transport

Fluid Mosaic Model

The model states that a membrane is a fluid structure with a mosaic of various embedded proteins.

Phospholipids

Amphipathic molecules that make up the phospholipid bilayer which is the foundation of the plasma membrane.

Amphipathic

Having both hydrophilic and hydrophobic properties.

ex. phospholipids have a hydrophilic head and hydrophobic tails.

What do carbohydrates allow?

Cell recognition.

What is the relation between cell activity and the type of molecules they are abundant in?

• More active cell = more proteins

• Less active cell = more phospholipids

How do phospholipids move in the plasma membrane?

Very fast moving laterally (side to side), but rarely moves transversely (flip-flop).

Flippase

Enzyme that allows an outer phospholipid leaflet to move to the inner phospholipid leaflet.

Floppase

Enzyme that allows an inner phospholipid leaflet to move to the outer phospholipid leaflet.

Outer phospholipid leaflet

Made of carbohydrates.

Inner phospholipid leaflet

Attaches to the cytoskeleton.

Are the inner and outer phospholipid leaflets equal to the other?

No.

How does temperature affect the fluidity of the membrane?

• Cooler temps = membranes become more solid

• Hotter temps = membranes become more fluid

How does saturation affect the fluidity of the membrane?

• Unsaturated = more fluid

• Saturated = more solid

How does tail length affect the fluidity of the membrane?

• Long tail = less fluid

• Short tail = more fluid

Desaturase

Enzyme that catalyzes the formation of double bonds in the tails of membrane phospholipids. Converts saturated fatty acids into unsaturated fatty acids.

How does desaturase activity related to the fluidity of a membrane?

• Higher desaturase activity = more unsaturated acids, therefore more fluid.

• Lower desaturase activity = less unsaturated acids, therefore less fluid.

How does cholesterol affect the fluidity of a membrane?

• In hotter environments = restrains phospholipid movement, therefore less fluid.

• In colder environments - maintains fluidity by preventing packing, therefore more fluid.

Is it better to have higher or lower cholesterol in the plasma membrane?

Higher, at all times, regardless of temperature.

For a cell in an incredibly hot environment, what can be assumed about it’s plasma membrane?

• Longer phospholipid tails

• More saturated fatty acids

• Lower desaturase activity

• High cholesterol

For a cell in an incredibly cold environment, what can be assumed about it’s plasma membrane?

• Shorter phospholipid tails

• More unsaturated fatty acids

• Higher desaturase activity

• High cholesterol

Peripheral proteins

Proteins in the plasma membrane that are not embedded into the membrane; they are sitting either on top or bottom.

Integral proteins

Proteins that are embedded into the membrane.

Transmembrane proteins

A type of integral protein where they are embedded fully into the membrane, coming through either side.

What can be assumed about integral proteins if they penetrate the hydrophobic core of the plasma membrane?

That the integral proteins must also have a hydrophobic core, and if they are transmembrane, then they will have a hydrophilic outer.

What are the 6 major functions of membrane proteins?

1. Transport

2. Enzymatic activity

3. Signal transduction

4. Cell-cell recognition

5. intercellular joining

6. Attachment to the cytoskeleton & ECM

Passive transport

The diffusion of a substance across a membrane with no energy investment.

What are the 2 kinds of passive transport?

• Diffusion

• Facilitated diffusion

Diffusion

The tendency for molecules to spread out evenly into the available space.

What kind of movement does diffusion exhibit?

It may exhibit net movement in one direction, allowing it to achieve dynamic equilibrium.

How do substances move when in diffusion?

Down the concentration gradient, going from a high to a low concentration with no energy used. Therefore, no work is done.

Osmosis

The diffusion of water across a selectively permeable membrane.

How is the direction of osmosis determined?

It is determined by the difference in total solute concentration. Water will diffuse into the region with a higher solute concentration.

Tonicity

The ability of a solution to cause a cell to gain or lose water.

Isotonic solution

The solute concentration is the same in and out. There is no net water movement.

Hypertonic solution

The solute concentration is higher outside the cell; the cell will lose water (it will plasmolyze).

Hypotonic solution

The solution concentration is lower outside of the cell; the cell will gain water and swell, potentially bursting depending on what kind of cell it is.

Plasmolysis

The shrinking of the cytoplasm away from the cell wall due to the loss of water in osmosis.

Osmoregulation

The control of water balance.

What will happen to plant cells during osmosis?

Water will diffuse into the vacuole, placing pressure on the cell wall. This is normal for the cell.

Facilitated diffusion

This is passive transport that is aided by proteins. Transport proteins speed the movement of molecules across the plasma membrane.

Channel proteins

Provide corridors that allow a specific molecule / ion to cross the membrane.

Carrier proteins

Undergo subtle changes in shape which translocate the solute-binding site across the membrane.

Active transport

Transport where substances are moving against their concentration gradient, going from lower to higher concentrations. Cells are actively participating in this because it requires energy in the form of ATP.

How is active transport performed?

It is orchestrated by proteins that are embedded in the membranes (integral proteins).

What are the 3 types of transporters in active transport?

• Uniporter

• Symporter

• Antiporter (exchanger / countertransporter)

Uniporter

Moves one molecule across the cell membrane and is independent of other molecules.

Symporter

Moves two molecules in the same direction through protein channels.

Antiporter

Moves 2 molecules in opposite directions through protein channels.

Cotransport

When active transport of a solute indirectly drives transport of another solute.

→ ex. secondary active transport.

What are the two types of active transport?

• Primary (direct) active transport

• Secondary (indirect) active transport

Primary active transport

Energy is derived directly from the breakdown of ATP.

Secondary active transport

Uses an electrochemical gradient created by the primary active transport.

→ most likely will be transporting ions that have a charge, therefore electrochemical gradient.

How do small molecules and water enter / leave the cell?

Through a lipid bilayer or transport proteins.

How do larger molecules cross the membrane?

Via vesicle-mediated transport.

Exocytosis

An active process of moving materials from within a cell to the exterior of a cell using exosomes. Transport vesicles migrate to the membrane and merge with it, releasing their contents.

What increases the surface area of a cell?

Exocytosis, since vesicles will contribute and merge to the plasma membrane once their contents are released.

What kind of transport is exocytosis?

Active transport

Endocytosis

An active cellular process where substances are brought within the cell. Vesicles are formed with the plasma membrane to bring in macromolecules.

What are the different types of endocytosis?

• Phagocytosis

• Pinocytosis / Bulk phase endocytosis

• Receptor mediated endocytosis

Phagocytosis

Where the cell engulfs particles in a vacuole.

Pinocytosis / Bulk phase endocytosis

Extracellular fluids or small particles are ingested by the cell/

Receptor-mediated endocytosis

The binding of ligands to receptors which triggers vesicle formation.

What kind of transport is endocytosis?

Active transport