1. Diffusion: this is movement of molecules from an area of high concentration to an area of low concentration. Small non-polar molecules (like oxygen and carbon dioxide gases)can diffuse directly across the lipid bilayer. Ions, polar molecules, and large molecules cannot. Each molecule responds to its OWN concentration gradient... so Oxygen gas flows in while carbon dioxide flows out.
2. Osmosis: this is defined as the movement of water across a membrane from an area of high water potential to an area of low water potential. Now, water potential is affected by 2 things: the concentration of dissolved solutes, and pressure. PUSH on water (increase the pressure) and you increase its water potential and it will move to areas under less pressure (low potential). If you increase the amount of solutes in solution, you decrease water potential. In effect, an increase in solute concentration reduces the concentration of water molecules...
- Hypotonic SOLUTION: The solution has a lower (hypo - like hypodermic needle) solute concentrattion than the cell (higher potential), and water will move into the cell, possibly causing it to rupture
- Hypertonic SOLUTION: The solution has a higher solute concentration than the cell (lower potential), so water moves out of the cell into solution andthe cell shrivels.
- ISOtonic SOLUTION: The solute concentration of cell and environment are equal. There is not net movement of water molecules.
3. Facilitated Diffusion: this is movement of molecules from an area of high concentration to an area of low concentration through a protein channel or with a protein carrier.
4. Active Transport: The cell uses energy to pump molecules through protein channels, against their concentration gradient. A common example is the Na -.K pump in animals, or hydrogen pumps in plants and bacteria. The differential transport of these ions creates a charge differential across the membrane (voltage - electric potential energy). When they flow back across in response to their concetration gradient, the energy can be used to do work.... so this is a way to store energy without making bonds.
5. Cotransport: When protons diffuse back across the membrane after a gradient has been established by active transport, they must travel through protein channels. Essentially, the flow of H+ can "open" a channel and other molecules can be coupled to the hydrogen and moved against their concentration gradient. So in a sense, chemical energy is converted to electrical energy by establishing a charge differential by pumping hydrogen across the membrane. The flow of hydrogen in response to their concentration gradient - the "spending" of that electrical energy - can be used to pump sugar against its concentration gradient.
6. Bulk transport:
Phagocytosis: the membrane engulfs a large piece of matter
Pinocytosis: the membrane engulfs environmental fluid and any solutes it contains.
These can both be stimulated by "signal transduction" - the binding of a ligand to a receptor which initiates an internal cellular response... such as the change in orientation of cytoskeletal fibers which causes a change in shape of the membrane.
II. Harvesting Energy
- it transforms radiant energy ('carried' by photons) to chemical energy ('carried' by electrons shared in covalent bonds)
- it requires an electron DONOR to provide electrons that will 'carry' this energy.
- initially, bonds between ADP and P are made to store this energy, creating ATP
- in most photosynthetic organisms, the subsequent catabolism of ATP is coupled to reactions binding carbon dioxide together in glucose, for longer term E storage.
- it is a two step process:
1. the light dependent reaction harvests light energy
as chemical energy storing that energy in the bonds in ATP and NADPH
2. the light independent reaction uses this chemical
energy to link carbon dioxide into glucose for longer-term energy storage.
- Where?
- prokaryotes (bacteria):
involves double cell membrane and cytoplasmic enzymes
- eukaryotes - localized
in chloroplasts
- chloroplasts:
- double membrane
- own DNA, similar to bacterial DNA (and about same size as bacteria)
- molecules nested in inner "thylakoid" membrane in groups called photosystems.
2. Light Dependent Reaction
a. Where? - on the inner membrane of prokaryotes or chloroplasts, involving suites of proteins organized into "photosystems" or "electron transport chains"
b. How?
1. PRIMITIVE SYSTEM: "sulphur bacteria"
- these bacteria have a double membrane (two bilayers)... proteins are nested in the inner membrane
- light hits these groups of proteins arrayed around a "chlorophyll" molecule - with an atom of Magnesium at the center. This group of proteins = Photosystem I
- the Magnesium is large - the electrons in the outer shell are far away and easy to excite... the photon transfers energy to the electron - it is raised to a higher energy state, correlating with moving further from the nucleus...in fact, it is lost completely by the atom and transferred to an 'electron acceptor molecule'.
- This electron acceptor molecule then passes the electron to a series of proteins nested within the inner membrane of this bacterium... the ELECTRON TRANSPORT CHAIN (ETC)
- ESSENTIALLY, each time the electron is transferred, protons follow ('electrostatically') and are 'pumped across the inner membrane into the intramembrane space. This builds up an electrostatic charge differential across the membrane. There are protein channels ('initially closed') that, when opened, allow the H+ to flood through in response to the charge gradient. This electric discharge energy is transferred to ADP, linking it to P to create ATP. This is called 'chemiosmotic synthesis' .
- In this case, the electron that has now lost its energy can be returned to the photosystem.... so this process that has produced ATP is 'cyclic phosphorylation' (the ADP was 'phosphorylated by adding a P).
- Something else can happen, too. Instead of the Electron Acceptor giving the electron to the ETC, it can give the electron to NADP... another 'energy transport molecule' like ADP. When this happens, the NADP gains energy and a negative charge and is NADP-. It reacts with free H+ ions that are always present in aqueous solutions (you should know why...), to make the high energy transport molecule, NADPH. In this case, the electron isn't returned to the Magnesium.... photosynthesis would stop, unless the photosystem can strip electrons from other molecules in solution.
Sulphur bacteria have photosystems that can strip electrons from Hydrogen Sulphide (H2S). This releases 2H+ ions and S as a waste product..... NOT OXYGEN
So, sulphur bacteria, that are still present today, photosynthesize in sulphur springs and do not produce oxygen as a waste product.
This explains an interesting geological pattern: The oldest fossil life on record are photosynthetic bacteria that date to 3.8 billion years old. However, the first evidence of oxygen in the Earth's atmosphere occurs at about 2 billion years ago. So, how can you now explain how there were photosynthetic bacteria present for 1.8 billion years, without any oxygen being produced?
- The problem was, and still is, that these bacteria are limited to living in spots where H2S is abundant - sulphur springs. These are rare. If something evolved a system that could strip electrons from a more abundant source, like water, then these new organisms could exploit a wider range of environments....
Study Questions:
1. What type of material can diffuse across the membrane? How does this occur, and is energy required?
2. What types of molecules require facilitated diffusion to cross into a cell? How does this occur (describe two processes) and is energy required?
3. Why is active transport so important for making the cell different from the environment? How does it work? How does the Na/K pump work?
4. What is a hydrogen pump and how does it work?
5. What happens in endocytosis and pinocytosis?
6. Explain the energetics of energy transfer from photons to electrons... what happens when to the photon when it loses energy, and what happens to the electron when it gains energy?
7. What chemicals are made in the light dependent reaction? What chemical is made in the light independent reaction?
8. Draw what happens in the primitive light reaction of sulphur bacteria, and explain the events that occur.
9. explain chemiosmosis. Include the flow of electrons, the response of H+ ions, the establishment of a charge differential across the membrane, and the production of ATP.