But what moves the particles in the first place? As discussed before, particles just move, each with its own tiny kinetic energy - speeding them up or slowing them down is associated with changes in temperature, which is a measure of the average particle speeds in the measured medium. And particles, when placed as a clump into a medium through which they can move, will tend to spread out, randomly distributing themselves from a crowded area to available less crowded areas. This tendency to spread out, to appear to move from areas of high concentration to areas of low concentration, has a name: diffusion. Weve all seen it at work: a drop of ink or food coloring, or tea from a tea bag, spreading through the water around it, spreading through the glass whether we help it by stirring or not. When there is a permeable barrier with an imbalance of particles on each side, there will be a net diffusion movement through the barrier - a movement called flux - until the concentrations are equal on both sides. At this point, the material is in equilibrium, which means particles keep moving through the barrier but for each one going one way, theres one coming back. As the exchange reaches equilibrium, flux slows down.
The barrier that will be the main focus of this chapter is the cell membrane, a lipid-based barrier with water-based (aqueous) solutions on either side. Flux through a barrier can be increased in several ways, some of which apply easily to cells and some of which dont. For instance, the size of the particles affect the flux - smaller particles move faster - but living things cant usually change particle size in order to move something more quickly. It should be obvious that speeding the particles up - raising the temperature - will speed the flux, but that would have other effects on biological systems, so few cells or systems speed diffusion by warming themselves on demand.
But there are approaches that cells can use to increase flux. One way to speed flux is to increase the concentration gradient, which keeps the net movement of material speeding from a crowded area into a much emptier one. Cells do this by either processing particles just as theyve crossed, or carrying them off, as blood systems do with cellular wastes, or sequestering them in other less-accessible parts of the cell that they cant go back from. Sometimes, by being permeable in just one direction, the rate of entry slows as the amounts entering drop, but particles are not lost in a return flux. Another feature of flux through a barrier is the limitations presented by the surface to be penetrated - the more surface area, the more particles can move through. Cells commonly increase surface area (for example, with microvilli) to increase particle movement in or out.
A particular type of diffusion concerns a type of particle that follows all of the diffusion rules we just discussed, but which may be ignored because it is the solvent rather than the solute. Osmosis is the diffusion of water, moving from highly dilute areas to areas less so. Typical cell membranes are permeable to water, and if the dilution of cell contents and that of cell surroundings are not in balance, there will be osmosis. It should be no surprise that many organisms in the oceans have water concentrations inside that match the water concentrations outside (although the solutes may be different), so no net flux of water occurs.
Solutions are labelled according to their particle concentrations - balanced solutions are isotonic. If surroundings get too many solutes, as happens in some inland salt-water lakes, they are hypertonic to cytoplasm, and cells can lose so much water by osmosis that cell chemistry is affected. If surroundings are too dilute, or hypotonic, water can flood into a cell, swelling it to the breaking point. Organisms exposed to fresh water must somehow deal with osmosis, or they will die. The flow of water inward can be so strong that it exerts an osmotic pressure. In small plants, the flow from groundwater into the roots - root pressure - can be strong enough to push water up the plant against the pull of gravity (usually not very far, maybe half a meter). Plants will also use water pressure (turgor pressure) in a central vacuole to provide a stiffness to their structure. This is done by pumping particles, mostly potassium ions, into the vacuole, so that water diffuses in.
ADAPTATIONS TO DEAL WITH INCOMING OSMOSIS - In small fresh-water organisms, a few mechanisms will stem the flux. One is a structure like a cell wall - a rigid retainer that will not let the cell expand beyond a certain size. Many plants and bacteria need no other way to keep themselves from filling with water. But animal cells, which lack cell walls, have a problem. For animals, two basic approaches work: first, water can be pumped out as it enters, such as with a contractile vacuole; second, in multicellular systems, the outer surfaces can be largely waterproofed. In animals such as fish or crustaceans, only a few outer surfaces, such as respiratory or digestive linings, are not waterproofed. Any extra water that enters by osmosis across those surfaces is removed by structures like kidneys.
Adaptations to fresh water or the changing dilutions of tidal pools would have "set up" a capability to deal with no water at all - a fish or crustacean waterproofed against osmosis in fresh water would also be resistant to drying out in the air. For this reason, animal life on land almost certainly moved up from tidal and / or fresh water environments.
The lipid part of a membrane is actually two layers, and each layer is made up of phospholipid molecules. In these phospholipids, two carbons on the glycerol hold hydrophobic fatty acids, which tend to extend in the same direction, while the hydrophilic phosphate groups on the middle carbon face outward toward the surrounding water. In that layer that is the membrane, the inside and outside surfaces are phosphate groups, and the middle of the membrane is two layers of fatty acids. Just the phobic reaction to water tends to form the membrane overall into a bubble shape - the molecules do not need to be bonded to each other, and in fact they flow around each other like floating balls in a very crowded barrel. In the molecular model of membrane structure, the fluid mosaic model, these free-floating lipids are the "fluid" part. The fatty acid components, which may be "kinked," can determine how tightly the molecules cluster and how permeable the basic membrane is. The flow and looseness of the phospholipids can also be changed by cholesterol molecules floating in the hydrophobic layers.
But a membrane cannot be simply phospholipids, because it would be too impermeable - very few solutes could penetrate such a barrier. Some very small, uncharged particles, like oxygen, carbon dioxide, even the occasional molecule can slip through a pure phospholipid membrane, and particles that are themselves lipids, such as steroid hormones, can dissolve through, but there are many other things that need to get in or out, and many ways that a cell needs to interact with the environment other than serving as gatekeeper. Those additional needs are served by a wide variety of proteins embedded in the membrane. These proteins almost always have hydrophobic domains which fit in the middle layer. Some of the proteins float on a surface, while many penetrate through from one side to the other - they are the "mosaic" part of the fluid mosaic model, presenting a meaningful pattern out of small bits on the surface. Embedded proteins may be involved in transport, as pores, or gates, or carriers, or pumps, etc; they may be receptors, picking up and relaying messages to the other side; they may be markers, allowing cells to tell one another apart; they may be connectors in multicellular systems; and the list goes on an on.
Pores, also called channels, are essentially small holes through the membrane, but the edges often have charged domains that exclude many ions - for life that evolved in an environment full of sodium and chloride ions, that would have been a must - so only small uncharged or slightly charged particles pass through easily. These particles would follow a diffusion flux - oxygen and other nutrients flow in to be used (which removes them and preserves the gradient), wastes flow out and spread or get carried away.
Gates are pores, but often less restrictive than regular pores, that can be opened or closed as necessary. A nerve impulse travels down a nerve as a wave of gates is opening and closing, letting an imbalance of ions suddenly but temporarily diffuse through and change the overall charge of the membrane. Muscles activate in a similar way.
Carriers are used to pass materials that cant get through the pores. Carriers tend to be specific for particular materials, and in fact are in many ways like enzymes. What would be an active site on an enzyme is called a binding site on a carrier or receptor, and the substrates are often referred to with the more general term ligands. Such molecules also follow Michaelis - Menten Kinetic patterns. Some carriers are single-step passers, while others may involve a sequence of surface and intramembrane molecules. If a material flows with such help in the direction diffusion should move it, the process is called facilitated diffusion and, although it may require some energy to accomplish, is generally considered passive transport, like all diffusion-oriented movement through the membrane. Some carriers do double duty as enzymes, changing the materials as they get passed through - this process is called group translocation.
Pumps move materials through a membrane opposite to the direction the materials would diffuse; driving them against the normal flux takes energy, and this process is called active transport. Sometimes pumps are linked to other carriers, in either the same or opposite direction, which may supply some pumping power from their own flux, or pumps are linked and help each other work - these cooperative systems are called coupled channels.
Endocytosis (and the reverse, exocytosis) is a form of active transport that forms vacuoles from cell membrane around something that has no dedicated carriers. It does require cellular energy to accomplish.
Well, sometimes they dont. In multicelled organisms, not all cell make new cells. In humans, for instance, few or no new nerve or muscle cells are made after infancy.
But in cells that will reproduce, the requirement is that the root of all functions, the information center from which everything can be built, the DNA, must be copied, and complete copies must be distributed properly to the new cells. The process of DNA copying is called replication.
Replicating DNA follows a process called semiconservative, which means that each "copy" is half new and half original. Enzymes start the process - topoisomerase allows most of the heavy unravelling of DNA from its coiled form, and helicase straightens the helical structure and causes the strands to separate. This can all be done on a short length of a chromosome at a time. Primase attaches small RNA priming sequences on each strand. DNA polymerase, starting at the primers, puts a new strand across from the old strand. Replication can move in only one direction along the strands, which means that one side can be replicated from the primer along the entire length of the chromosome (areas open ahead of it and close behind it), but the other side must be made in bits and pieces the other way on the open bits, going "backwards" with multiple primers. Those bits are called Okazaki fragments. Eventually, a second DNA polymerase replaces the RNA primers with DNA bits, telomerase (if active) makes sure the chromosome ends are complete, and two chromosomes exist where one was. These two chromosomes remain attached to one another at the centromere.
PROPHASE - This will set up the cell so that a complete complement of chromosomes can be placed on each side of the division plane, an area called the cell equator. As prophase progresses, the DNA of the cell winds up tighter and tighter, condensing first down to visible-in-a-light-microscope strands which were the first objects to be called chromatin (now the term applies to the DNA-protein complex thats always there, visible or not), and eventually becoming visible as separate, double-stranded chromosomes, each one with two chromatids connected at a centromere. At the "ends" of the cell away from the equator, the cell poles, microtubule-organizing centers called centrioles, inside a complex known as a centrosome, have moved into position (the cell had started with one from the last division, and a copy has been made during interphase) and begin to grow microtubules, called spindle fibers, in all directions. Some of the spindle will attach to the cell membrane and hold the centrioles in place - others will attach to structures on the chromosomes centromeres called kinetochores. But there is something in the way - the nuclear envelope is between the microtubules and the chromosomes, and nuclear structures such as the nucleolus and the nuclear matrix surround them. All of these structures will "disappear" during prophase, although what exactly happens to them isnt clear - are they broken into bits and dispersed within the cell or broken down entirely? Probably both, to varying degrees in different cells. But once these obstructions are removed, the microtubules attach to the chromosomes and start to pull on them - the longer the microtubule, the stronger the pull, and the ensuing "tug of war" will go on until each chromosome has equal-length microtubules on each side, putting the chromosomes on the equator of the cell.
METAPHASE - Once the chromosomes are close to centered in a cell (in a disc, although it looks like a line from the side), the cell is considered to be in metaphase. This phase lasts until the cell signals the chromatids to separate. The centromeres "pop" apart; now each single-stranded chromosome is attached to microtubules hauling it toward a particular pole, and once they are separate and moving away from each other, its the next phase...
ANAPHASE - During this stage the separate groups of chromosomes are drawn toward the poles, away from the equatorial division line. In plant cells, the beginnings of the cell wall that will separate the two new cells, the cell plate, begins to form.
TELOPHASE - This phase puts things back the way they were before prophase, so much of what happens is like the details of prophase, in reverse. The spindle detaches from the chromosomes and disassembles. The nuclear envelope reforms around the chromosomes, which loosen up into chromatin and seem under a light microscope to disappear, while the nucleolus reappears. Eventually, conditions are back in interphase.
CYTOKINESIS - Actual division of the cell may not take place during mitosis, or it may occur in anaphase, telophase, or the next interphase. Some cells use mitosis, which is technically just production of new nuclei, to produce a multi-nucleated cell that may or may not divide at some point. When cells divide, microfilaments usually pinch the membrane until separate cells are made. If the division is in two and equal daughter cells are made, its called binary fission; if unequal (as in yeast), budding; if multiple cells are produced all at once, fragmentation. The last two are also forms of asexual reproduction in multicelled organisms, producing multicelled offspring but following the same patterns.
where there is only one chromosome,
division follows a different course. DNA is replicated,
producing two separate looped chromosomes, but each then
attaches to the
cell membrane well away from where the cell will divide by binary
fission. After division, the chromosome releases into the new
Meiosis begins like mitosis, with an interphase that, among other things, replicates DNA. This means that the cell undergoing meiosis starts with its two sets of chromosomes doubled - its going to have to make four nuclei if each will have only one set each. Meiosis gets to four nuclei by going through two division stages - one to two cells, then another to four - that are called Meiosis I and Meiosis II. Both divisions go through the prophase, metaphase, anaphase, telophase order, but there are some differences compared to mitosis.
MEIOSIS I - During prophase, chromosomes are not just hauled to the equator, but pairs of homologous chromosomes are set up next to one another (synapsis), where they may swap pieces by crossing over. Crossing over separates linked alleles. The odds that two linked alleles will separate depends on how far apart they are - separation rates can be used to map locations of genes on chromosomes. During metaphase, the chromosomes do not separate into single strands - homologous chromosomes are pulled away from each other and eventually wind up in different nuclei. These nuclei are haploid (one set), but the chromosomes are double-stranded. A short interphase follows, just a bridge to the next division.
MEIOSIS II - This looks like a typical mitosis, except that theres only one set of chromosomes present - at metaphase, single strands pop apart and are pulled away from each other. Four nuclei (usually in 4 cells), each with a single set of single-stranded chromosomes, are produced.
FUSION happens when two nuclei from meiosis merge together, mixing their chromosomes. This can happen when nuclei are exchanged between protozoans, when non-gendered cells cross fungal bridging fibers, or when sperm cells enter egg cells.
SEX WITH GENDER
- Most organisms
that reproduce sexually also do it through the use of genders.
But what makes males male and females female is not what you probably
think it is. Remember, maleness and femaleness are terms that have
to apply not just to people and dogs but to trees, and flower parts, and
organ systems in animals that are male and female simultaneously (monoecious,
what used to be called hermaphroditic, as opposed to
separate-gendered dioecious species). And dont assume it
always involves "X" and "Y" chromosomes, which is just one way of
several to set up gender. Simply put, male
and female are defined by the gametes, the sex cells,
that are made -
everything else is peripheral to that. Many of the
"regular" differences between males and females can easily be
traced to the differences in their gametes.
When organisms moved from a water-based environment up onto "dry" land, it seems that the hardest change to adapt to was the lack of a support medium that sperm could swim through to reach the egg cell. It was the last problem definitively solved in the evolution of land animals (female internal fluids substitute for outside water) and land plants (sperm were sealed into pollen).
A page with many animations showing simplified versions of molecular activity.
Online Introduction to Biology (Advanced)
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