Bio - cells etc.

Movement through the Membrane

Hydrophobic (nonpolar) molecules are able to dissolve in the lipid bilayer of a membrane and move through it freely. By contrast, hydrophilic molecules such as ions and polar molecules are impeded in their movement through the membrane by the hydrophobic core; their passage is slow.

hypotonic

If the solution surrounding a cell contains non-penetrating solutes at lower concentrations than in the cell, the solution is said to be hypotonic. When a cell is in a hypotonic solution, water enters by osmosis and the cell tends to swell.

Hypertonic solution

If the solution that surrounds a cell contains non-penetrating solutes at a higher concentrations than the cell, the outside solution is said to be hypertonic to the cells. When a cell is in a hypertonic solution, water leaves by osmosis.

Isotonic

In animals, ions, proteins, and other molecules are concentrated in extracellular fluids, as well as inside cells so that the concentration of water inside and outside cells is usually equal or isotonic. To keep fluids on either side of the plasma membrane isotonic, animal cells must constantly use energy to pump Na+ from inside to outside by active transport, otherwise water move inward and cause cells to burst.

Carrier Proteins continued

In performing the transport step, the carrier protein undergoes conformation changes that progressively move the solute binding site from one side of the membrane to the other, thereby transporting the solute. This is what distinguishes carriers from channels.

Plasmolysis

In plants, the shrinkage and loss of internal osmotic pressure under these conditions caused stems and leaves to wilt. In extreme cases, plant cells shrink so much that they retract from their walls, a condition known as plasmolysis.

Active Vs Passive Transport

In short, active transport usually establishes differences in solute concentrations or voltage across membranes that are important for cell or organelle function. By contrast, passive diffusion and facilitated diffusion act mostly to move substances across membranes in the direction toward equalizing their concentrations on each side. Active transport also depends on membrane transport proteins, is specific, and can be saturated. The carrier proteins also chan their conformation as they function.

3 functions of active transport

1. Uptake of essential nutrients from the fluid surrounding cells even when their concentrations are lower than in cells 2. Removal of secretory or waste materials from cells or organelles even when the concentration of those materials is higher outside the cells or organelles. 3. Maintenance of essentially contains intracellular concentrations of H+, Na+, K+, and Ca+. In short, active transport usually establishes differences in solute concentrations or voltage across membranes that are important for cell or organelle function.

Simple Diffusion

A few small substance diffuse through the lipid part of a membrane. With one major exception - water - these substance are nonpolar inorganic gases such as O2, N2, and CO2, and nonpolar inorganic molecules such as steroid hormones. Depends solely on molecular size and lipid solubility is simple diffusion.

Diffusion

A form of passive transport. The net movement of ions or molecules from a region of higher concentration to a region of lower concentration. Diffusion depends on the constant motion of ions or molecules at temperatures above absolute zero (-273C). The constant motion gradually mixes the dye molecules and water molecules until they are distributed uniformly.

Water and Simple Diffusion

Are strongly polar molecules, nevertheless are small enough to slip through momentary spaces created between the hydrocarbon tails of phospholipid molecules as they flex and move in a fluid bilayer. Relatively slow.

Membrane Potential

Because ions are charged molecules, active transport of ions may contribute to voltage - an electrical potential difference - across the plasma membrane called a membrane potential. The unequal distribution of ions across the membrane created by passive transport also contributes to the voltage.

Specificity of facilitated diffusion/Protein regulation

Because the proteins that perform facilitated diffusion are specific, cells can control the kinds of molecules and ions that pass through their membranes by regulating the types of transport proteins in their membranes. As a result, each type of cellular membrane, and each type of cell, has its own group of transport proteins and passes a characteristic group of substances by facilitated diffusion. The kinds of transport proteins present in a cell ultimately depend on the activity of genes in the cell nucleus.

Selective Permeability

Charged atoms and molecules are mostly blocked from moving through the membrane because of the hydrophobic core.

Passive Transport

Depends on concentration differences on the two sides of a membrane. Ions and molecules move across the membrane from the side with the higher concentration to the side with the lower concentration (that is the gradient). The difference in concentration provides the energy for this form of transport.

Facilitated Diffusion and the Concentration Gradient

Facilitated Diffusion is also dependent on concentration gradients. Proteins aid the transport of polar and charged molecules through membranes, but a favorable concentration gradient provides the energy for transport. Transport stops if the gradient falls to zero.

Saturated carrier proteins

Facilitated diffusion by carrier proteins can become saturated when there are too few transport proteins to handle all the solute molecules. For example, if glucose is added at higher and higher concentrations to the solution that surrounds an animal cell, the rate at which it passes through the membrane at first increases proportionately with the increase in concentration, however at some point, as the glucose concentration is increased still further, the increase in the rate of transport slows. Eventually further increases in concentration cause no additional rise in the rate of transport.

Aquaporins

Facilitated diffusion of water through membranes occurs through specialized water channels called aquaporins. A billion molecules of water per second can move through an aquaporin channel. Each water molecule is severed from its hydrogen bonded neighbors as it is handed off to a succession of hydrogen bonding sites on the aquaporin protein channel.

Osmosis continued

For osmosis to occur, the selectively permeable membrane must allow water molecules, but not molecules of the solute, to pass. Because osmosis occurs in response to a concentration gradient, it releases free energy and can accomplish work.

Channel Proteins

Form hydrophilic channels in the membrane through which water and ions can pass. The channel "facilitates" the diffusion of molecules through the membrane by providing an avenue.

Transport

Is the controlled movement of ions and molecules from one side of a membrane to the other. Membrane proteins are the molecules responsible for transport. Critical to the ionic and molecular organization of cells, and with it, the maintenance of cellular life.

Simple Diffusion (Chart)

Lipids and Water. There is no binding of transported substance. Concentration gradient provides energy source. Direction of transport is with gradient of transported substance. This mode of transportation is nonspecific and does not become saturated.

Facilitated Diffusion

Many polar and charged molecules such as water, amino acids, sugars, and ions diffuse across membranes with the help of transport proteins. This is facilitated diffusion. The transport proteins enable polar and charged molecules to avoid interaction with the hydrophobic bilayer.

Active Transport

Moves ions or molecules against the concentration gradient; that is from the side with the lower concentration to the side with the higher concentration. Uses energy directly or indirectly obtained by breaking down ATP.

Concentration

Number of molecules or ions per unit volume.

Gated Channels

Other type of channel protein which facilitates the transport of ions such as sodium Na+, potassium K+, Calcium C+, and chlorine (Cl-). Occur in all eukaryotes. Gated channels switch between open, closed or intermediate states. The gates may open or close in response to changes in voltage across the membrane or by binding signal molecules. The opening or closing involves changes in the protein's 3D shape. They perform functions that are vital to survival.

Osmotic Pressure

Pressure created by the weight of the raised solution that exactly balances the tendency of water molecules to move from the beaker into the tube in response to the concentration gradient. This causes the solution in the tube to stop rising. At this point, the system is in a state of dynamic equilibrium and no further net movement of water molecules occurs.

Facilitated Diffusion (Chart)

Proteins and Water. There is binding of transported substance. Concentration gradient provides energy source. Direction of transport is with gradient of transported substance. This mode of transportation is specific to molecules or molecular classes and does become saturated at high concentrations of transported molecules.

Active Transport (Chart)

Proteins. There is binding of transported substance. ATP hydrolysis or concentration gradients provide energy source. Direction of transport is against the gradient of transported substance. This mode of transportation is specific to molecules or molecular classes and does become saturated at high concentrations of transported molecules.

Directional Transport

Some ions and molecules consistently move into cells, whereas others move out of cells.

Plants and Hypotonic solutions

Strong walls prevent the cells from bursting in a hypotonic solution. In most land plants, the cells at the surface of roots are surrounded by almost pure water, which is hypotonic to the cells and tissues of the root. As a result water flows from the surrounding soil into the root cells by osmosis.

Specific Transport

That is; only certain ions and molecules move directionally across membranes.

Carrier proteins

The 2nd type of transport proteins. They also form passage ways through the lipid bilayer. Carrier proteins each bind a specific single solute, such as glucose or an amino acid, and transport it across the lipid bilayer. Because a single solute is transferred in this carrier mediated fashion, the transfer is called uniport transport.

concentration gradient

The concentration difference that drives diffusion. It is a form of potential energy. Initial state, when molecules are more concentrated in one region of a solution, the molecules are highly organized and at a state of minimum entropy. As the distribution proceeds to the state of maximum disorder, the molecules release free energy that can accomplish work.

Facilitated Diffusion is Specific

The membrane proteins involved transport certain polar and charged molecules, but not others.

Osmosis

The net diffusion of water molecules through a selectively permeable membrane in response to a solute concentration gradient. The net movement of water molecules across a selectively permeable membrane by passive diffusion, from a solution of lesser concentration to a solution of greater solute concentration.

Turgor Pressure

The osmotic pressure developed by the inward flow contributes part of the force required to raise water from the roots to the leaves of the plant. Osmosis also drives water into cells of the stems and leaves of plants. The resulting turgor (osmotic) pressure, pushes the cells tightly against their walls and supports the softer tissues against the force of gravity.

Two kinds of Transport Proteins

The proteins that carry out facilitated diffusion are integral membrane proteins that extend entirely through the membrane. There are two types: channel proteins and carrier proteins.

Primary Active Transport

The same protein that transports a substance also hydrolyzes ATP to power the transport directly. They all move positively charged ions - H+, Ca+, Na+, and K+ - across membranes. The gradient of positive ions established by primary active transport pumps underlie functions that are absolutely essential for cellular life.

Secondary Active Transport

The transport is indirectly driven by ATP hydrolysis. That is the proteins do not break down ATP; instead, the transporters use a favorable concentration gradient of ions, built up by primary active transport, as their energy source for active transport of a different ion or molecule.

free water concentration

The water available to move across the membrane.

Passive transport of Water

Water diffuses both directly through the membranes and through aquaporins. This passive transport occurs instantly in living cells. Inward or outward movement of water by osmosis develops forces that can cause cells to swell and burst or shrink and shrivel up. Much of the energy budget of many cell types, particularly in animals, is spent counteracting the inward or outward movement of water by osmosis.

dynamic equilibrium

When molecules or ions exist in a concentration gradient, more of them move from the are of higher concentration to areas of lower concentration than in the opposite direction. Even after their concentration is the same in all regions, there is still constant movement of molecules or ions from one space to another, but there is no net change in concentration on either side.