Explain how the organelles of eukaryotic cells work together as a system, compare eukaryotic and prokaryotic cells, and predict how substances cross a selectively permeable membrane

Learning goal: Describe the cell as a functioning biochemical system rather than a list of parts: explain the structure, function and interrelatedness of cell organelles, compare eukaryotic cells and prokaryotic cells, and predict which substances cross the cell membrane freely, which need a protein and which need an input of energy, all in service of homeostasis.

Two kinds of cells. Every known organism is made of cells, and every cell falls into one of two broad groups. Prokaryotic cells, which make up bacteria and archaea, are usually small (most are one to ten micrometers across), have no nucleus and no membrane-bound organelles. Their DNA lies in a region of the cytoplasm called the nucleoid, they have ribosomes, a plasma membrane and usually a cell wall, and, although many also have simpler protein filaments and some specialized internal structures, that is nearly the whole inventory of a typical prokaryote. Eukaryotic cells, which make up protists, fungi, plants and animals, are typically ten to a hundred micrometers across and are divided by internal membranes into compartments called organelles, including a nucleus that encloses the DNA. Prokaryotes are the earliest cells in the fossil record, appearing well over three billion years ago; eukaryotic cells appear much later, and complex multicellular organisms descended from eukaryotic ancestors later still. The difference is one of organization, not of importance: a prokaryotic cell carries out every process needed for life inside one compartment.

Organelles as a system. It is tempting to memorize organelles as isolated parts with one job each. Ohio's course asks for something harder and more useful: how the parts depend on one another. Here is the inventory, stated by function. The nucleus stores the DNA and is where genes are copied into messenger RNA. Ribosomes, either free in the cytoplasm or attached to the endoplasmic reticulum, read messenger RNA and build proteins. The rough endoplasmic reticulum (rough ER), studded with ribosomes, folds and modifies proteins that will be secreted or embedded in membranes; the smooth ER makes lipids and helps break down some toxins. The Golgi apparatus receives proteins from the ER, modifies and sorts them and packages them into vesicles. Vesicles are small membrane bubbles that carry cargo between compartments and to the cell surface. Lysosomes contain digestive enzymes that break down worn-out parts and material taken in from outside; they are the cell's waste-disposal system. Mitochondria carry out most of cellular respiration, transforming the chemical energy of food into the usable energy currency ATP. Chloroplasts, found in plants and algae, carry out photosynthesis. The cytoskeleton, a complex network of protein fibers, gives the cell shape, organizes the organelles and drives movement; every eukaryotic cell has one, and prokaryotes have simpler versions of some of its protein filaments. The central vacuole of plant cells stores water and solutes and keeps the cell firm. A cell wall, found in plants, fungi and most bacteria but not in animal cells, gives mechanical support outside the membrane.

A worked tour: making and releasing insulin. Consider a beta cell in the pancreas, which secretes the hormone insulin. The insulin gene in the nucleus is transcribed into messenger RNA, which leaves through pores in the nuclear envelope. Translation begins on a free ribosome in the cytoplasm; the first stretch of the new chain is a signal sequence that steers the ribosome to the rough ER, where it attaches and the growing chain threads into the ER. There the signal sequence is removed and the chain folds into its working shape, held by chemical cross-links. The folded precursor travels in vesicles to the Golgi apparatus, which sorts and packs it into secretory vesicles; inside those vesicles enzymes trim the precursor to finished insulin, and the vesicles wait near the membrane. When blood glucose rises, the cell detects the change, the vesicles fuse with the plasma membrane and insulin is released outside. Count the organelles involved: nucleus, ribosome, rough ER, vesicles, Golgi apparatus, plasma membrane, and, powering every step, ATP, most of it supplied by mitochondria. Remove any one and the hormone is never delivered. That interdependence is what "the cell is a system" means.

Why many reactions happen only in some cells. All the body cells of one organism carry essentially the same DNA, yet a beta cell makes insulin and a nerve cell does not. Different cells use different parts of the genetic instructions, so many chemical reactions that occur in some cells of a multicellular organism do not occur in most of its other cells. Cell specialization is therefore a matter of which genes are switched on, a theme the heredity lessons develop.

The membrane and homeostasis. Every cell produces a plasma membrane, a double layer of phospholipids in which proteins float. Each phospholipid has a water-attracting (hydrophilic) head and two water-avoiding (hydrophobic) tails; the tails point inward, forming an oily core that most water-soluble substances cannot cross on their own. The membrane is selectively permeable: what passes depends on the molecular properties and concentration of the substance. Small nonpolar molecules such as oxygen and carbon dioxide dissolve through the core and diffuse freely from where they are more concentrated to where they are less concentrated. Water crosses by osmosis, partly through the bilayer and much faster through water-channel proteins. Ions and larger polar molecules such as glucose cannot cross the core; they need channel or carrier proteins. Movement down a gradient through such a protein is facilitated diffusion and costs the cell no energy (for an uncharged molecule such as glucose the gradient is one of concentration; for an ion it is the combined effect of concentration and electrical charge across the membrane, called the electrochemical gradient). Movement against a gradient is active transport and requires energy, either directly from ATP or indirectly from another gradient that ATP built; the sodium-potassium pump, which moves sodium out and potassium in against both gradients, is the standard example. Very large loads enter by endocytosis, where the membrane folds inward around them, and leave by exocytosis, as the insulin vesicles did. Through this differential passage the cell keeps its internal conditions steady while the outside changes: that is homeostasis, and a cell that loses control of its membrane dies.

Predicting osmosis. Osmosis is the net movement of water across a selectively permeable membrane toward the side with the higher concentration of dissolved particles that cannot cross the membrane (a solute that crosses freely soon evens out and does not drive osmosis). A cell in a solution with the same solute concentration as its interior gains and loses water equally. In a more concentrated (hypertonic) solution the cell loses water and shrinks (in a plant cell the membrane pulls away from the wall, which is called plasmolysis); in a less concentrated (hypotonic) solution it gains water. An animal cell in pure water can swell until it bursts; a plant cell does not, because its wall resists and the cell simply becomes firm. The wall pushes back: as the cell swells, pressure builds inside until it stops any further net entry of water, even though the cell contents are still more concentrated than the water outside. So the concentration rule gives the direction only while no pressure difference opposes it; the fuller description used in later courses, water potential, combines solute concentration and pressure. This is why the same experiment gives different results for red blood cells and for potato cells, and why "cells burst in water" is an over-general rule.

Why some organelles have double membranes. Mitochondria and chloroplasts are each wrapped in two membranes, and the nucleus is enclosed by a double-membraned envelope. In mitochondria the inner membrane is folded into cristae and carries the proteins that transform energy, so more inner membrane means more capacity in the same volume; the two membranes create separate compartments (the space between them and the matrix inside) in which concentration differences are built up and used. In chloroplasts the energy-transforming proteins sit on a third membrane system inside, the stacked thylakoids, which again multiplies surface area and creates a separate inner compartment. The double membrane is also consistent with the widely accepted hypothesis that these organelles descend from free-living prokaryotes taken inside an ancestral cell; both organelles still carry their own small DNA and ribosomes, which is what that hypothesis predicts.

Misconceptions. "Prokaryotes are primitive and cannot do much" is wrong: they carry out photosynthesis, chemosynthesis, respiration and every other essential process, mostly without membrane-bound compartments. "Diffusion needs energy from the cell" is wrong; diffusion is driven by the random motion of particles, and it is moving against a gradient that costs ATP. "Osmosis moves water toward more water" confuses the direction; water moves toward more dissolved particles, which is the side with proportionally less water.

Investigation you can do. Cut eight potato cylinders of equal size with a cork borer or knife, blot and weigh each to 0.1 g, and place two in each of four salt solutions: 0 percent, 1 percent, 5 percent and 10 percent sodium chloride by mass, all at room temperature. After 30 minutes, blot and reweigh. Record initial mass, final mass and percent change for each cylinder, then average the pairs. A result in which cylinders gain mass in the weakest solutions and lose mass in the strongest, with the change reversing sign somewhere between, supports the claim that water crosses potato cell membranes by osmosis toward the higher solute concentration, and the crossover, where there is no net water movement, gives a rough estimate of the cells' own solute concentration (rough because the pressure of the cell walls also affects water movement). What the model cannot show: it does not reveal the membrane's proteins, it says nothing about active transport, and because potato cells have walls it cannot show what happens to a wall-less animal cell.

Worked prediction, a red blood cell in two solutions: A 0.9 percent sodium chloride solution (normal saline) has about the same total concentration of dissolved particles as the inside of a red blood cell, so a red blood cell placed in it has no net water movement and keeps its shape. Placed in 3 percent saline, the outside has more dissolved particles than the inside, so water leaves the cell by osmosis and the cell shrinks and crinkles, which is called crenation. Placed in pure water, water enters, and because an animal cell has no wall it swells and can burst. For solutes that cannot cross the membrane, and with no pressure difference across it (an animal cell has no wall to build one up), the direction is toward the higher solute concentration, and the size of the effect depends on how different the two concentrations are.

Worked classification of transport: (a) Oxygen entering a muscle cell from the blood: oxygen is small and nonpolar and moves from high to low concentration, so this is simple diffusion, no protein, no energy. (b) Glucose entering the same cell: glucose is polar and too large for the core, but it moves down its gradient, so this is facilitated diffusion through a carrier protein, no energy. (c) Iodide ions being concentrated inside a thyroid cell to levels far above the blood: this is movement against a gradient, so it is active transport and needs energy; here the energy comes indirectly from ATP, through the sodium gradient that the ATP-driven sodium-potassium pump maintains. (d) A white blood cell engulfing a bacterium: the load is enormous, so the membrane wraps it in; this is endocytosis.

Worked organelle chain for a secreted digestive enzyme: A cell lining the stomach releases a protein-digesting enzyme. Path: gene in the nucleus → messenger RNA through nuclear pores → translation starts on a free ribosome, and the enzyme's signal sequence moves the ribosome onto the rough ER → protein folded and modified in the ER → vesicle to the Golgi apparatus → sorted into a secretory vesicle → exocytosis at the membrane. The enzyme is made in an inactive form and activated only outside the cell, which protects the cell that made it; ATP, mostly from mitochondria, powers every step. If the Golgi apparatus were blocked, enzyme would pile up inside the cell and none would reach the stomach, which shows that the organelles form one pathway, not separate stations.

Worked data, potato osmosis: Hypothetical averaged results after 30 minutes: 0 percent salt, mass change +8 percent; 1 percent, +2 percent; 5 percent, minus 6 percent; 10 percent, minus 12 percent. Mass rose where the solution was less concentrated than the cell contents and fell where it was more concentrated, so the crossover lies between 1 and 5 percent; a linear estimate between the two nearest points gives about 2 percent salt. In this hypothetical data set, that is the salt concentration at which the cylinders neither gain nor lose water overall; it matches the concentration of the cells' contents only approximately, because in plant tissue the pressure of the cell walls also affects water movement; a real class should expect its own crossover, which may be lower, and should report its measured value rather than this one. The pattern supports osmosis toward the higher solute concentration; it does not identify which solutes are inside the cells.

Supported record: Draw a labeled diagram of a eukaryotic animal cell and, beside it, a prokaryotic cell at the same scale ratio (mark which is roughly ten times larger). Then make a table with columns Organelle, Function, Depends on, Supplies to. Fill it for the nucleus, ribosome, rough ER, Golgi apparatus, vesicle, lysosome, mitochondrion and plasma membrane so that every row names at least one other organelle in the last two columns.

Independent performance: Carry out the potato osmosis investigation with four salt concentrations and two cylinders each, then write a laboratory report. Success criteria: (a) a data table records initial mass, final mass and percent change for all eight cylinders with correct arithmetic; (b) a graph plots average percent change against salt concentration and the estimated crossover concentration is read from it; (c) the explanation states the direction of osmosis in terms of solute concentration, not "toward more water"; (d) it classifies the water movement as passive and explains why no ATP was needed; (e) it names two things the model cannot show, including why the result would differ for an animal cell; (f) it identifies one variable that was controlled and one source of error.