Membrane Transport

Vesicle

A small sac of membrane with a small amount of particle inside. Vesicles are usually spherical and are usually in eukaryotic cells.

Axons

An axon is part of a neuron (nerve cell) that consist of a tubular membrane with cytoplasm inside. They can be as narrow as a micrometer but they have to be a meter long. Their function is to convey messages rapidly from one part of the body to another in an electrical form called nerve impulse. It involves rapid movement of sodium and potassium ions across the axon membrane.

Facilitated Diffusion of Potassium in Axons

Consist of 4 protein subunits with a narrow pore between them that allows only K+ ions to pass either direction. The width of the channel is very specific - it is only slightly larger than potassium. The potassium is bonded with water molecules (hydrated shell) so when the K+ ions pass through they break the bond with water and bond with the amino acids in the channel. When it passes through the K+ ions bond with water molecules again.

Difference between diffusion and osmosis

Diffusion: -Movement of any substance -Operate in any medium -Applicable to solid, liquid and gases -Does not need to be semi-permeable -Dependent on free energy of the diffusing substance -Is not dependent on the solute -Helps equalize the amount of diffusing solute on both sides Osmosis -Movement of only the solvent, or water -Operates only in liquid -Must be semi-permeable -Dependent on the reduction of free energy of one solvent over another -Dependent on solute -Does not equalize amount of solvent on both sides

Hypertonic

Hypertonic solution is one of two solutions that has a higher concentration of a solute This means high osmolarity because water has to leave the cells through osmosis towards the side with more solute

Hypotonic

Hypotonic solution is one of two solutions that has a lower concentration of a solute. This means low osmolarity solutions because water leaves from this side to hypertonic solution side.

Diffusion

Molecules in a solution are at constant random motion, due to kinetic energy. The motion allows molecules to be evenly distributed in the solution. The molecules move from areas of higher concentration to areas of lower concentration. So diffusion is the net movement of molecules down the concentration gradient. Diffusion is the spreading out of particles in liquid and gases that happen because the particles are in continuous random motion. The particles move from areas of higher concentration to lower concentrated areas. Diffusion does not require energy so it is a passive process (passive transport)

Net movement

Net movement is the overall direction of the movement of a substance. Net movement is measured by the force capacity on one side against the force capacity on the other.

Osmosis example with water

Osmosis can occur because of the differences in concentration of substances dissolved. Ex. water and solute - the solute forms intermolecular bonds with water molecules so the bond restricts the movement of water molecules. Regions with higher concentration of solute have a low concentration of free water molecules and regions with low concentration of solute have a high concentration of free water. This causes a net movement of water molecules from regions of lower solute to regions of high solute concentrations.

Osmosis

Osmosis focuses on the movement of the solvent in a solution, in this case focusing on water molecules. When the movement of water going in and out are the same, there is no "net movement" But, when more molecules of water move in one direction that another, the net movement is not equal and that is called "Osmosis." Osmosis is simply just the movement of water from high concentration (low solute) to low concentration (high solute) through a partially permeable membrane.

Estimation of osmolarity in tissues by bathing sample in hypotonic and hypertonic solutions.

Osmosis is due to solutes that form bonds with water. These solutes are osmotically active (anything that induces osmosis to occur). Ex. glucose, Na+, K+ Cl- ions are osmotically active that is why it is used in a lot of osmosis experiments. Cells contain different osmotically active solutes. The osmolarity of a solution is the total concentration of osmotically active solutes. If a sample tissue is bathed in a hypertonic solution and a hypotonic solution and measurements are taken to find out whether water leaves of enters the tissue, you can deduce what concentration of solution would be isotonic so therefore you can find the osmolarity of the tissue.

Receptor-mediated Endocytosis

Part of endocytosis These transport specific molecules such as low density lipoproteins (LDL) through receptors. The molecules first bind to specific receptors on the plasma membrane. The interior portion of the protein receptor is embedded in the membrane in the area of the pit. When enough molecules are bind to the receptors, the pit deepens and seals off to form a coated vesicle in the cell.

Pinocytosis

Part of endocytosis When the cells take in material that is liquid.

Phagocytosis

Part of endocytosis When the cells take in particles such as bacteria or fragment of organic matter.

Facilitated diffusion

Particles can move across the membrane through facilitated diffusion. Ions and other charged particles that cannot diffuse through the membrane can only pass through channels which have specific diameter in the walls of the protein. This means that only one type of ion/particle can pass through because of the specific diameter.

Active Transport

Particles move across the membrane by active transport. Cells take in substances even if they already have a high concentration inside compared to the outside. This means it is absorbed against the concentration gradient. Cells can also pump substances out, even if there is a high concentration outside although this is less likely. This needs energy in the form of ATP which cells produce regularly itself by cell respiration. Active transport is carried out by globular proteins in the membranes called "pump proteins." This only allows specific particles through to control the content of the cytoplasm.

Diffusion in Membranes

Particles move across the membranes by simple diffusion. In the membrane this involves particles passing between the phospholipid bilayer however only some particles can move through so it is "semi-permeable." Non-polar particles such as oxygen can diffuse easily. If the oxygen concentration is higher outside, and lower inside, it will pass through the membrane by passive diffusion. The center of the membranes are hydrophobic, so ions with a positive or negative charge cannot pass through easily as they are polar. Partial polar molecules can diffuse at slow rates.

Vesicle movement in Secretory Cells and Growth of Plasma membrane

Secretory cells (cells that release chemicals) Proteins are synthesized (created) by ribosome on the endoplasmic reticulum (rER) and accumulate (gather) inside the rER. Vesicles carrying protein carry them to the golgi apparatus to pack the protein in the final form). The vesicle then buds off to the plasma membrane where protein is released. In a growing cell the plasma membrane needs to grow. Phospholipids are synthesized next to the rEr and become inserted into the rER membrane. Ribosomes (protein builder) on the rER synthesize membrane proteins which will also become inserted in the membrane. Vesicles bud off the rER and move to the plasma membrane. They fuse together, increasing a small area.

Polar particles through the membrane

The center of the membrane is hydrophobic so ions with a positive or negative charge cannot pass through easily. Partial polar molecules can diffuse at low rates (very slow) in the membranes. Smaller polar particles can pass through the membrane more easily than large particles. Ex. Urea and ethanol pass faster because they are small

Exocytosis

The fluidity of the membrane allows materials to be released by exocytosis. Vesicles can release materials when it fuses with the plasma membrane and the materials get released outside the cell. This is used to expel waste or unwanted products. This is an active transport as energy is required. Ex. Digestive enzymes are released from glands. The polypeptides in enzymes are synthesized by rER, processed in golgi apparatus and carried to the membrane in vesicles. The release is referred to as "secretion" because a useful substance is being released not waste product.

Endocytosis

The fluidity of the membrane allows materials to be taken into cells by endocytosis. They simply form vesicles that are inside the cell. This is very dynamic (systematic) as they are constructed, moved around and deconstructed. This can happen because of the fluidity of the membrane. For a vesicle to form inside, part of the membrane is taken off. The vesicles will contain material outside the cell. They can contain water and the solute outside the cell, they can also contain large molecules that cannot pass through the plasma membrane. Ex. Placenta in mothers womb, they use endocytosis to obtain proteins from mothers blood, including antibodies which are absorbed into the fetus. This is an active transport as energy is required.

Passive Transport

The movement of a chemical substance across a cell membrane without the expenditure of energy such as diffusion such or osmosis.

Active transport of sodium and potassium in Axons

The movement of this pump is able to occur because of the concentration gradient between the inside and outside of the axons. The concentration is built up by active transport which carry Na+ and K+ ions. 1. The pump is open inside the axon so 3 Na+ ions attach to binding sites 2. ATP transfer one phosphate group (end up with ADP) and allows the protein to change shape and open outside cell 3. Pump releases the Na+ ions 4. 2 K+ ions attach to binding sites from outside 5. Binding of K+ causes phosphate group to release so pump changes back to the same shape and opens inside cell 6. Releases K+ ions inside 7. Cycle repeats

Vesicle movement in Cells

Vesicles are used to move material around inside the cell. In some cases it is the content of the vesicle that needs to be moved. In other cases it is the protein in the membrane that needs to be moved.

Isotonic

When cells are in isotonic solution, movement of water out of the cell is exactly balanced by movement of water into the cell (this is because the amount of solute on both inside and outside are equal so no net movement)