Plasma Membrane & Molecules Transportation

Membrane phospholipids form a bilayer

Lipids, mainly phospholipids are the main structural components of membranes.

A structure of phospholipids has a phosphate group and only two fatty acids instead of three.

The head, with its charged phosphate group, is hydrophilic.

The fatty acid double tail is non polar and hydrophobic.

Membrane phospholipids form a bilayer

Thus, the tail end of a phospholipids is pushed away by water, while the head is attracted to water.

The structure of phospholipids molecules is well suited to their role in membranes.

Phospholipids form a two layer sheet called a phospholipids bilayer.

Fluid Mosaic Model

The membrane is a fluid mosaic of phospholipids and proteins Cellular membranes it is commonly described as a fluid mosaic.

The word mosaic denotes a surface made of small fragments.

A membrane is a mosaic in having diverse protein molecules embedded in a frameworks of phospholipids.

The membrane mosaic is fluid in that most of the individual proteins and phospholipids molecules can drift laterally in the membrane.

The membrane is a fluid mosaic of phospholipids and proteins

Steroid cholesterol, helps stabilize the phospholipids at body temperature and keep the membrane fluid at lower temperature.

The outside surface of the plasma membrane has carbohydrate, bonded to proteins and lipids in the membrane.

Carbohydrate + proteins = glycoprotein

Carbohydrate + lipid = glycolipid

Plasma Membrane

Membrane Proteins

Membrane Proteins

Proteins make the membrane a mosaic of function

The word mosaic as applied to a membrane can refer not only to the positioning of proteins in the phospholipids bilayer but also to the varied activities of these proteins.

Proteins perform most of the functions of the membrane.

More than 50 different kinds of proteins have been found in the plasma membrane of human red blood cells.

Many membrane proteins are enzymes which may function in catalytic teams for molecular assembly lines.

Some membranes protein, function as receptor for molecular messengers from other cells. eg: hormone.

Other membrane proteins act like glue, forming cell junctions where adjacent cell stick together.

Proteins make the membrane a mosaic of function

Some proteins that project into the cytoplasm attach to the cytoskeleton and help maintain cell shape.

Transport of Small Molecules

Diffusion

Diffusion is the tendency for particles of any kind to spread out evenly in an available space, moving from where they are more concentrated to regions where they are less concentrated.

Diffusion requires no work or energy, results from the random motion (kinetic energy) of atoms and molecules.

The diffusion of a substance across a biological membrane is called passive transport. WHY ?

Diffusion

A figure showed molecules moves randomly, there will be a net movement.

Molecules diffuses down its concentration gradient until equilibrium is reached.

At equilibrium, molecules continue to move back and forth, but there is no net change in concentration on either side of the membrane.

Diffusion is extremely important to all cell. eg: in our lung, diffusion driven by concentration gradients is the sole means by which oxygen enters red blood cells and carbon dioxide passes out of them.

Osmosis

Diffusion of water molecule in response to a water concentration gradient between two regions separated by a selectively permeable membrane.

Passive transport.

The term tonicity describes the tendency of a cell in a given solution to lose or gain water.

Isotonic solution

Hypotonic solution (The solution with the lower solute concentration)

Hypertonic solution (The solution with a higher concentration of solutes).

Water balance between cells and their surroundings

Osmoregulation : The control of water balance for an animal to survive in a hypertonic or hypotonic environment.

Eg: marine animals and freshwater animals.

To prevent excessive uptake or excessive loss of water.

Some aquatic animals that live in the sea have body fluids with a solute concentration equal to that of seawater and called osmoconformers.

Animals do not undergo a net gain or loss of water because equal amounts of water move back and forth between two solutions with equal solute concentrations. Eg: jellyfish, lobsters.

All freshwater animals, all land animals and most marine vertebrate, have body fluids whose solute concentration is different from that of their environment, called osmoregulators.

Water balance between cells and their surroundings (osmoregulation)

Water balance between cells and their surroundings (osmoregulation)

Facilitated diffusion

Net movement of molecules or ions down a concentration gradient by physically binding to carrier protein.

Numerous substances do not diffuse freely across membranes because of their size, polarity or charge.

Passive transport.

Does not require energy.

Driving force is the concentration gradient.

Sugars, amino acids, ions and even water.

Passive diffusion & facilitated diffusion

Active transport

Active transport requires energy to move molecules across a membrane against the solute’s concentration gradient.

Active transport allows a cell to maintain concentrations of small molecules that are different from the concentration of it’s surroundings.

Eg : Na+ – K+ pump.

Active transport: Na+ – K+ pump. Animal cells have a low internal concentration of Na+, relative to their

surroundings, and a high internal concentration of K+.

They maintain these concentration differences by actively pumping Na+ out of the cell and K+ in.

The remarkable protein that transports these two ions across the cell membrane is known as the sodium-potassium pump.

The important characteristics of the sodium-potassium pump is that it is an active transport process, transporting Na+ and K+ from areas of low concentration to areas of high concentration.

Active transport: Na+ – K+ pump. The sodium-potassium pump works through the following series of conformational

changes in the trans-membrane protein:

Step 1: Three sodium ions bind to the cytoplasmic side of the protein, causing the protein to change its conformation.

Step 2: In its new conformation, the protein binds a molecule of ATP and cleaves it into adenosine diphosphate and phosphate. ADP is released, but the phosphate group remains bound to the protein. The protein is now phosphorylated.

Active transport: Na+ – K+ pump. Step 3: The phosphorylation of the protein induces a second conformational

change in the protein. This change translocates the three Na+ across the membrane, so they now face the exterior. In this new conformation, the protein has a low affinity for Na+ and the three bound Na+ dissociate from the protein and diffuse into the extracellular fluid.

Step 4: The new conformation has a high affinity for K+, two of which bind to the extracellular side of the protein as soon as it is free of the Na+.

Active transport: Na+ – K+ pump. Step 5: The binding of K+ causes another conformational change in the protein,

this time resulting in the dissociation of the bound phosphate group.

Step 6: Freed of the phosphate group, the protein reverts to its original conformation, exposing the two K+ to the cytoplasm. This conformation has a low affinity for K+, so the two bound K+ dissociate from the protein and diffuse into the interior of the cell. The original conformation has a high affinity for Na+; when these ions bind, they initiate another cycle.

Active transport: Na+ – K+ pump.

Active transport: Coupled transport. Some molecules are moved up their concentration gradient by using

the energy stored in a gradient of a different molecule.

In this process, the energy released as one molecule moves down its concentration gradient is captured and used to move a different molecule against its gradient.

The energy stored in ATP molecules can be used to create a gradient of Na+ and K+ ions across the membrane.

These gradients can then be used to power the transport of other molecules across the membrane. Eg: The transport of glucose.

Active transport: Coupled transport. The transport of glucose across the membrane in animal cells

requires energy for two reasons.

First, glucose is a large polar molecule that cannot move through the membrane by itself.

Second, the concentration of glucose inside the cell is frequently higher than the concentration outside the cell.

The glucose transporter uses the Na+ gradient produced by the sodium-potassium pump as a source of energy to power the movement of glucose into the cell.

In this system, both glucose and Na+ bind to the transport protein, which allows Na+ to pass down its concentration gradient, capturing the energy and using it to move glucose into the cell.

In this kind of co-transport, both molecules are moving in the same direction across the membrane, and hence it is called symport.

Passive transport & Active transport

Transport of large molecules

Exocytosis

A cell uses the process of exocytosis (exo, outside and kytos, cell) to export bulky materials.

Formation of vesicle.

A membrane-enclosed vesicle filled with macromolecules moves to the plasma membrane.

The vesicle fuses with the plasma membrane, and the vesicle’s contents spill out of the cell.

Endocytosis (endo, inside and kytos, cell)

Phagocytosis

Cellular eating.

Pseudopods engulf the particle and trap it within an internalized vesicles.

Lysosomes fuses with the vesicle membrane and released it hydrolytic enzymes.

Enzymes break down engulf material into reusable raw ingredients.

Endocytosis (endo, inside and kytos, cell)

Pinocytosis

Cellular drinking.

Small droplet of extracellular fluid is internalized.

Pinocytosis provided a mean to retrieve extra plasma membrane that has been added to cell surface during exocytosis.

Destroy invaders in extracellular fluid.