IB Biology - 1.4 Membrane Transport

What is a vesicle?

-A vesicle is a small sac of membrane with a droplet of fluid inside. -Vesicles are spherical and are normally present in eukaryotic cells. -They are constructed, moved around and then deconstructed. This can happen because of the fluidity of membranes, which allows structures surrounded by a membrane to change shape and move.

Active transport of sodium and potassium in axons.

-An axon is part of a nerve cell and their function is to convey messages rapidly from one part of the body to another in an electrical form called a nerve impulse Facilitated diffusion through sodium and potassium channels allows the rapid movements of sodium and potassium ions across the axon membrane (aka nerve pulse) -They occur because of concentration gradients between the inside and outside of the axon, the concentration gradients are built up by active transport and carried out by a sodium-potassium pump protein Sodium-potassium pump cycle: *The interior of the pump is open to the inside of the axon; 3 sodium ions enter the pump and attach to their binding sites. * ATP transfers a phosphate group from itself to the pump; causing the pump to change shape and the interior to close. *The interior of the pump opens to the outside of the axon and the 3 sodium ions are released. *2 potassium ions from outside can then enter and attach to their binding sites *Binding of potassium causes release of the phosphate group; this causes the pump to change shape again so that it's only open to the inside of the axon *The interior of the pump opens to the inside of the axon and the two potassium ions are released **See image on page 40

What is an example of moving the vesicle contents?

-An example of moving the vesicle contents occurs in secretory cells. -Protein is synthesized by ribosomes on the rough endoplasmic reticulum and accumulates inside the rER. -Vesicles containing the proteins bud off the rER and carry them to the Golgi apparatus. -The vesicles fuse with the Golgi apparatus, which processes the protein into its final form. -When this has been done, vesicles bud off the Golgi apparatus and move to the plasma membrane, where the protein is secreted.

What is active transport?

-Cells sometimes take in substances even if there is already a higher concentration inside than outside. The substance is absorbed against the concentration gradient. Cells sometimes pump substances out even if there is already a larger concentration outside -This type of movement across the membrane requires -energy (in this case ATP) and is called active transport -Active transport is carried out by pump proteins in membranes. Cell membranes contain many different pump proteins allowing the cell to control the content of its cytoplasm -The molecule or ion enters the pump protein as far as the central chamber. A conformational change to the protein happens using energy from ATP. The ion or molecule can pass to the opposite side of the membrane and the pump protein returns to its original conformation

How are digestive enzymes released?

-Digestive enzymes are released from gland cells by exocytosis. -The polypeptides in the enzymes are synthesized by the rER, processed in the Golgi apparatus and then carried to the membrane in vesicles for exocytosis. -In this case the release is referred to as secretion, because a useful substance is being released, not a waste product.

What can exocytosis be used to do?

-Exocytosis can be used to expel waste products or unwanted materials. -An example is the removal of excess water from the cells of unicellular organisms. -The water is loaded into a vesicle, sometimes called a contractile vacuole, which is then moved to the plasma membrane for expulsion by exocytosis.

What is facilitated diffusion?

-Facilitated diffusion is one of the four methods of moving particles across membranes. -Ions and other particles that cannot diffuse between phospholipids can pass into our out of cells if there are channels for them through the plasma membrane. -These channels are holes with a very narrow diameter. The walls of the channel consist of protein. -The diameter and chemical properties of the channel ensure that only one type of particle passes through, for example sodium ions, or potassium ions, but not both. -Because these channels help particles to pass through the membrane, form a higher concentration to slower concentration, the process is called facilitated diffusion. -Cells can control which types of channel are synthesized and placed in the plasma membrane and in this way they can control which substances diffuse in and out. *See image on page 37

What needs to happen in a growing cell?

-In a growing cell, the area of the plasma membrane needs to increase. -Phospholipids are synthesized next to the rER and become inserted into the rER membrane. -Ribosomes on the rER synthesize membrane proteins which also become inserted into the membrane. -Vesicles bud off the rER and move to the plasma membrane. -They fuse with it, each increasing the are of the plasma membrane by a very small amount. -This method can also be used to increase the size of organelles in the cytoplasm such as lysosomes and mitochondria. *See image on page 35

Preventing osmosis in excised tissues and organs

-In a solution with higher osmolarity water leaves the cells by osmosis so their cytoplasm shrinks in volume. The area of plasma membrane doesn't change so it develops indentations (crenellations). -In a solution with lower osmolarity the cells take in water by osmosis and swell up. They may eventually burst leaving ruptured plasma membranes called red cell ghosts. -Hypertonic and hypotonic solutions therefore damage human cells it is therefore important for any human tissues and organs to be bathed in an isotonic solution (usually an isotonic sodium chloride solution) during medical procedures

What is osmosis?

-Osmosis is one of the four methods of moving particles across membranes. -Water is able to move in and out of most cells freely. -Sometimes the number of water molecules moving in and out is the same number and there is no net movement, but at other times more molecules move in one direction or the other. This net movement is osmosis. -Osmosis is due to differences in the concentration of substances dissolved in water (solutes). -Substances dissolve by forming intermolecular bonds with water molecules. These bonds restrict the movement of the water molecules. -Regions with a higher solute concentration therefore have a lower concentration of water molecules free to move than regions with a lower solute concentration. -Because of this there is ante movement of water form regions of lower solute concentration to regions with higher solute concentration. -This movement is passive because no energy has to be expended directly to make it occur. -Osmosis can happen in all cells because water molecules, despite being hydrophilic, are small enough to pass through the phospholipid bilayer. -Some cells have water channels called aquaporins, which greatly increase membrane permeability to water. -Examples are kidney cells that reabsorb water and root hair cells that absorb water form the soil -At its narrowest point, the channel in an aquaporin is only slightly wider than water molecules, which therefore pass through in single file. Positive charges at his point in the channel prevent protons form passing through.

Secretion vs. Excretion.

-Secretion is when a useful substance is being released. -Excretion is when a waste product is being released.

What is simple diffusion?

-Simple diffusion is one of the four methods of moving particles across membranes. -Diffusion is the spreading out of particles in liquids and gases that happens because the particles are in continuous random motion. -More particles move form an area of higher concentration to an area of lower concentration than move in the opposite direction. -There is therefore a net movement from the higher to the lower concentration - a movement down the concentration gradient. -Living organisms do not have to sue energy to make diffusion occur so it is a passive process. -Simple diffusion across membranes involves particles passing between the phospholipids in the membrane. -it can only happen if the phospholipid bilayer is permeable to the particles. -Non-polar particles such as oxygen can diffuse through easily. If the oxygen concentration inside a cell is reduced due to aerobic respiration and the concentration outside is higher, oxygen will pass into the cell through the plasma membrane by passive diffusion. *See image on 36

What is the significance of the centre of membranes being hydrophobic?

-The centre of membranes is hydrophobic, so ions with positive or negative charges cannot easily pass through. -Polar molecules, which have partial positive and negative charges over their surface, can diffuse at low rates between the phospholipids of the membrane. -Small polar particles such as urea or ethanol pass through more easily than large particles *see image page 36

How do vesicles form?

-To form a vesicle, a small region of membrane is pulled form the rest of the membrane and is pinched off. -Proteins in the membrane carry out this process, using energy form ATP. -Vesicles can be formed by pinching off a small piece of the plasma membrane of cells. -The vesicle is formed on the inside of the plasma membrane. -It contains material that was outside the cell, so this is a method of taking materials into the cell (endocytosis).

Vesicle movement in cells.

-Vesicles can be used to move materials around inside cells. -In some cases it is the contents of the vesicle that need to be moved. -In other cases it is proteins in the membrane of the vesicle that are the reason for vesicle movement.

What is exocytosis?

-Vesicles can be used to release materials form cells. -If a vesicle fuses with the plasma membrane, the contents are then outside the membrane and therefore outside the cell. This process is called exocytosis.

What do vesicles taken in by endocytosis contain?

-Vesicles taken in by endocytosis contain water and solutes from outside the cell but they also often contain larger molecules needed by the cell that cannot pass across the plasma membrane. -For example, in the placenta, proteins form the mother's blood are absorbed into the fetus by endocytosis -Some cells take in large undigested food particles by endocytosis -Some types of white blood cells take in pathogens including bacteria and viruses by endocytosis and then kill them, as part of the body's response to infection.

What is a passive process?

A process that does not need energy to make happen. For example, simple diffusion.

What is endocytosis?

Endocytosis is the method that uses vesicles to take material that was outside the cell into the cell.