Principles of Biology Fall 2002

Bi 211 John Roden

1. A fluid is a liquid or gas that can move or change shape without breaking apart.
2. The concentration of a substance in a fluid is the number of molecules per unit volume.
3. A gradient is a physical difference between 2 locations such that molecules tend to move from one region to the other.

Molecules in a fluid are in constant motion due to their kinetic energy. The movements are also random even if there is a gradient. There are significant gradients across the plasma membrane (cytoplasm is a concentrated solution of many substances). There are 2 types of transport;

1. Passive transport – where substances move into or out of a cell "down" concentration

2. Active transport – where the cell uses energy to move substance against a concentration gradient. In this case transport proteins are required to control the direction of movement.

Diffusion is the net movement of molecules down a concentration gradient and can occur within a fluid or across a membrane.

1. Recall that that this movement is due to the random motion of molecules.
2. The greater the concentration gradient the faster the rate of diffusion.
3. If no other processes intervene, diffusion will continue until the gradient is eliminated.
4. Diffusion cannot move molecules rapidly over long distances
5. Once equilibrium is achieved, there is no more net movement. Just as many molecules are randomly moving one direction as another direction.
6. Diffusion is a spontaneous process, thus it decreases the free energy (-DG).
7. A substance diffuses down its own concentration gradient and is not affected by the gradient of other substances (Figure 8.10).

Some molecules such as water, lipid soluble molecules (ethyl alcohol), some vitamins and dissolved gases move across membranes by simple diffusion. Channel and carrier proteins provide a way for most water-soluble molecules to move down their concentration gradient. This process is known as facilitated diffusion. Most channel proteins form permanent pores in the lipid bilayer and have specific interior diameters and electrical fields allowing only one type of ion to pass through. Carrier proteins bind specific molecules, undergoes a shape change allowing the molecule to pass through.

Osmosis is a special name for the diffusion of water across membranes. The diffusion of water also proceeds from regions of high to low concentrations of water. Pure water has the highest concentration and if any substance is added to pure water then some of the water molecules are displaced. Solutes reduce the water content of a solution. In addition, some water molecules can form hydration spheres around molecules making those water molecules less likely to move.

If a concentration gradient is imposed across and artificial membrane (Figure 8.11), then water will move, but not the solute (sugar) until equilibrium is reached.

Osmosis depends on the concentration of water in both the cytoplasm and in the liquid that bathes the cell. If two solutions are compared, the solution with the higher concentration of solutes is termed hypertonic and the solution with a lower concentration of solutes is termed hypotonic. If the two solutions are identical then they are termed isotonic.

If a cell is placed in a hypertonic solution then there are more solutes (a lower concentration of water) outside the cell than inside so water move out and the cell shrinks (Figure 8.12). In an isotonic solution there is no gradient for water so there is no net water movement. A cell in a hypotonic solution would take in water by osmosis down its concentration gradient. If this continues a cell might burst.

All cells require the uptake of nutrients that are less concentrated outside the cell than in the cytoplasm. In this instance, diffusion would hinder the movement rather than help. In active transport, membrane proteins use cellular energy (ATP) to move individual molecules across the plasma membrane against their concentration gradient (Figure 8.16). These transport systems are often called pumps since they are analogous to water pumps that move water uphill.

When dealing with ions we must not only discuss the concentration gradient bit also electrical field effects. Proton pumps are a very common active transport system. In cotransport, a membrane protein couples the movement of one molecule with another (Figure 8.18).

Another common way for cells to bring material into the cytoplasm is to engulf particles or fluid by endocytosis. The plasma membrane moves to encircle a particle and pinches off a vesicle (food vacuoles) which is then later digested. Exocytosis moves materials out of the cell (excretion) also using vesicles.

The chemical energy of organic molecules produced by photosynthesis must be extracted by all organisms (even plants) to maintain their precarious, highly organized state. The chemical elements essential for life are recycled, but the energy is not (Figure 9.1).

Energy needed for growth, replication of cells and DNA, maintenance and ion movements in and out of cells all require ATP. Respiration allows the organisms to retrieve the energy stored in carbohydrates and in the process the sugars are modified to form the carbon skeletons that make up the basic building blocks of cell structure.

All higher organisms are aerobic which means that they use oxygen in metabolism and in particular respiration,

[CH₂O]_n +O₂ +H₂O à CO₂ + 2H₂O DG = -2872 kJ mol^-1

Respiration occurs in 3 different stages (1) glycolysis (2) Krebs cycle and (3) electron transport. Gycolysis can occur in either the cytoplasm whereas the Krebs cycle and e^- transport occurs in the mitochondria (Figure 9.6).

Glycolysis is the breakdown of starch or sucrose into a 3-carbon compound pyruvate (Figure 9.8). It is considered an ancient reaction and the summary reaction is,

Glucose + 2NAD + 2ADP + 2Pi à 2 pyruvic acid + 2 NADH + 2ATP

The energy yield in gycolysis comes partly from substrate phosphorylation where an enzyme detaches a phosphorous functional group from a substrate to ADP forming ATP (Figure 9.7). In addition, glycolysis produces NADH an electron shuttle. These electron carriers will also produce ATP by oxidative phosphorylation (in electron transport).

The yield of energy is low by doing just glycolysis, but under anaerobic conditions it can provide sufficient energy for survival. There is an initial investment (Figure 9.8) of 2ATP to get gycolysis started, but the remaining reactions to pyruvate yield 4ATP (so a net of 2 ATP).

The problem with using only gycolysis is that it will stop once NAD is used up. Some organisms get around this by using pyruvate as the final acceptor of e^- and H⁺ regenerating NAD. Lactic acid buildup in muscles and alcoholic fermentation are both examples of anaerobic glycolysis.

The Krebs cycle was named in honor of Hans Krebs (Nobel Prize) who in the 1930’s elucidated this pathway. The whole scheme (Figure 9.11) is a bit daunting, but the summary reactions are,

Pyruvate + 4NAD + FAD + ADP + Pi à 3CO₂ + ATP + 4NAPH + 4H⁺ + FADH₂

The energy from the pyruvate is converted primarily to reducing power. Pyruvate is produced in the cytoplasm and the TCA cycle takes place in the mitochondrial matrix. (Figure 9.10) The first step is to transport pyruvate through both membranes into the matrix.

Each pyruvate is split into CO₂ and a 2-C acetly group which enters the Krebs cycle releasing the remaining 2 carbons as CO₂, producing 1 ATP and donating electrons to the carrier molecules, NAD and FAD (Figure 9.12).

At this point the cell has gained only 4 ATP from the original glucose molecule. However, the cell has captured many energetic electrons in the carrier molecules NADH (10) and FADH₂ (2) in the process.

Oxidation-reduction reactions (redox) are the transfer of electrons from one reactant to another. The loss of an e^- is termed oxidation and the gain of an e^- is reduction. These two always go together since the transfer of e^- always requires a donor and acceptor. Energy is required to pull electrons from atoms and the more electronegative the atom the stronger it holds onto electrons.

To harness the energy released the process must be controlled and performed stepwise. Thus the electrons are transferred in a progression to more electronegative molecules (Figure 9.13). The respiratory electron transport chain has 4 major protein complexes (Figure 9.15). As electrons move through the system protons are pumped from the interior to the exterior. The DpH gradient produced is an energy source used by the coupling factor and ATP synthase to make ATP.

The final acceptor of the electron is oxygen to produce water. If the cell lacks O₂ then the electrons would pile up in the transport system, the H⁺ would not be pumped across the membrane and no ATP produced. ATP is produced as protons move through a coupling factor down their diffusional gradient. The electron transport chain produces about 32-34 molecules of ATP per glucose much higher than the 4 ATP produced prior (Figure 9.16).