Biology 1 · Study notes
Every cell is wrapped in a plasma membrane that decides what enters and what leaves. Membranes separate the cell's chemistry from the outside world, so transport across them controls metabolism, cell volume, and survival. This chapter builds on the cell structure of earlier topics, and its rules about gradients and energy reuse reappear in cellular respiration, nerve function, and plant physiology later in the course.
The plasma membrane is a thin, flexible barrier about 7 nm thick that encloses every cell. Its backbone is a phospholipid bilayer: each phospholipid has a hydrophilic head that faces water and two hydrophobic fatty acid tails that point inward, away from water. Because the interior of the bilayer is oily, the membrane blocks most water-soluble molecules while letting small nonpolar ones slip through.
The fluid mosaic model describes this membrane as a two-dimensional fluid, not a rigid wall. Phospholipids rotate and drift sideways constantly, and so do the proteins embedded among them. Flip-flopping between layers is rare, but lateral movement is fast. Cholesterol, found in animal membranes, regulates fluidity: it stiffens the bilayer at warm temperatures and stops the fatty tails from packing tightly when it is cold. Unsaturated fatty acids, whose double bonds put kinks in the tails, also keep the membrane fluid.
The mosaic part is the crowd of membrane proteins doing different jobs. Integral proteins span the bilayer and act as channels, carriers, pumps, and enzymes. Peripheral proteins attach loosely to one face of the membrane and often anchor it to the cell's internal skeleton. Carbohydrates attached to lipids and proteins form glycolipids and glycoproteins on the outer surface, where they serve as identification tags, for example the markers that give blood its type. The result is selective permeability: the membrane lets some molecules cross freely, moves others with the help of proteins, and excludes the rest.
Common Mistake: Treating the membrane as a rigid, unmoving wall. It is a fluid sheet: lipids and proteins drift sideways constantly, so a membrane can bend, heal small holes, and let its parts mix. Also, facilitated diffusion through a protein is still passive transport, not active transport.
Imagine a pond crowded with lily pads, ducks, and toy boats. The water is the oily middle of the membrane, and the ducks and boats are the proteins. Everything drifts and slides past everything else all day long. That constant sliding is the fluid part, and the mix of different floating objects is the mosaic part.
Transport across the membrane falls into two families. Passive transport moves molecules down their concentration gradient, from high to low concentration, and costs no energy; it is diffusion, sometimes assisted by a protein. Simple diffusion carries small nonpolar molecules such as oxygen, carbon dioxide, and steroid hormones straight through the lipid bilayer. Facilitated diffusion carries polar molecules and ions through proteins without spending energy. Channel proteins form water-filled pores, such as the aquaporins that speed water movement and the ion channels that open briefly to let specific ions pass. Carrier proteins bind one molecule, change shape, and release it on the other side, as the glucose transporter does for sugar entering a red blood cell. Diffusion through channels and carriers is still passive: the gradient does all the work.
For ions, the gradient is electrochemical, combining concentration with the membrane potential, the voltage across the membrane, which in a resting animal cell is roughly minus 70 millivolts. An ion moves toward whichever side is both less concentrated and opposite in charge.
Active transport moves molecules against their gradient, which costs energy. Primary active transport uses ATP directly; the classic example is the sodium-potassium pump, which exports 3 sodium ions and imports 2 potassium ions for every ATP molecule it hydrolyzes, keeping sodium low and potassium high inside the cell. The pump runs continuously and can consume roughly a quarter of a resting animal cell's ATP. Secondary active transport spends no ATP directly; instead it rides the sodium gradient that the pump created. In symport, two molecules travel the same direction, such as glucose entering intestinal cells together with sodium; in antiport, they travel opposite directions.
| Feature | Passive transport | Active transport |
|---|---|---|
| Direction | Down the gradient | Against the gradient |
| Energy | None required | ATP or another gradient |
| Proteins | Channels and carriers | Pumps and cotransporters |
| Example | Glucose entering via a carrier | Sodium potassium pump |
| Result | Tends toward equilibrium | Builds and maintains gradients |
1. Three sodium ions bind to the pump from inside the cell.
2. ATP transfers a phosphate group to the pump, changing its shape.
3. The new shape opens outward and releases the three sodium ions.
4. Two potassium ions bind to the pump from outside the cell.
5. The phosphate group is released and the pump snaps back to its original shape.
6. The original shape opens inward and releases the two potassium ions.Common Mistake: Assuming that any movement through a protein is active transport. Facilitated diffusion also uses channel and carrier proteins, but the molecules still move down their gradient and no energy is spent. The presence of a protein does not make transport active; moving against the gradient does.
A marble rolls downhill by itself, with no one pushing it. That is passive transport, and molecules do the same thing when they drift from a crowded place to an empty one. Rolling a marble uphill takes work, like carrying it up a staircase. That is active transport, and the cell pays for the climb with ATP.
The sodium potassium pump is like a revolving door in the cell wall. Every full turn needs one ticket, and a ticket is one molecule of ATP. One turn throws three sodium ions out of the cell and sweeps two potassium ions in. The door never rests, because the cell must keep its sodium and potassium levels just right.
Osmosis is the diffusion of water across a selectively permeable membrane. Water crosses the phospholipid bilayer slowly and passes quickly through aquaporins, always moving down its own water potential gradient from high to low. Water potential, written with the Greek letter psi, measures how eager water is to move and is the sum of two parts: Ψ = Ψs + Ψp, where Ψs is the solute potential and Ψp is the pressure potential.
Adding dissolved solute always lowers the solute potential, so Ψs is zero in pure water and negative in any solution; the more concentrated the solution, the more negative its Ψs. The pressure potential Ψp is usually positive when a cell pushes against its surroundings, as when a plant cell swells, and it can be negative when water is stretched, as in a tall column of sap. Pure water and open solutions at atmospheric pressure have a pressure potential of zero, so their water potential equals their solute potential. Water always moves toward the side with the more negative total water potential, which means that in osmosis it moves toward the region with more dissolved solute.
A practical consequence follows: a cell placed in pure water, where Ψ is 0, gains water if its own Ψ is negative, and a cell placed in a concentrated solution loses water. These movements are passive; nothing pumps the water, the potential difference pulls it.
Common Mistake: Reversing the direction of osmosis. Water does not move toward the side with more water; it moves toward the side with more dissolved solute, because added solute lowers water potential. Students who remember that water follows salt get the direction right.
Picture two cups of lemonade joined by a straw that only water can pass through. One cup is very sweet and the other is barely sweet. Water crawls through the straw into the sweeter cup to dilute it. Water potential is the measure of how eager water is to move, and it is lower in the sweeter cup, so water flows toward it.
Think of water potential as a score that tells how badly water wants to move. Pure water has the highest possible score, zero. Dissolving sugar in the water lowers the score, and squeezing the water with pressure raises it. Water always flows from the higher score to the lower score, just like air escaping from a stretched balloon.
Tonicity describes what a surrounding solution does to a cell's volume, and it compares the outside solution with the cell interior. In a hypotonic solution, the outside solute concentration is lower than the cell's, so water enters and the cell swells. In a hypertonic solution, the outside solute concentration is higher, so water leaves and the cell shrinks. In an isotonic solution, the concentrations match and there is no net water movement; 0.9 percent sodium chloride is the classic isotonic saline for animal cells.
The response depends on the cell type. Animal cells lack a wall, so a hypotonic environment makes them swell until they may burst, a process called lysis, while a hypertonic environment makes them shrivel, called crenation. Plant cells have a rigid cell wall: in a hypotonic environment the central vacuole fills, pressure builds, and the cell becomes turgid and firm; in a hypertonic environment water leaves, the vacuole shrinks, and the membrane pulls away from the wall in plasmolysis.
| Condition | Outside solute | Animal cell | Plant cell |
|---|---|---|---|
| Hypotonic | Lower than the cell | Swells, may lyse | Turgid, firm |
| Isotonic | Equal to the cell | Normal shape | Flaccid |
| Hypertonic | Higher than the cell | Shrinks, crenates | Plasmolyzed |
Common Mistake: Mixing up the two tonic names. Hypertonic means the outside solution has more solute than the cell, so water leaves the cell; hypotonic means the outside has less solute, so water enters. Anchor the word hyper to more solute outside, and the direction of water follows.
Picture a water balloon in a bathtub. In plain tap water, the balloon swells because water pushes in. In very salty water, it shrivels as water rushes out. Tonicity is just the word for which way that rush goes for a particular cell.
Small molecules cross membranes on their own or through proteins, but large particles, droplets, and whole cells cannot. For those, the membrane itself does the moving, and both processes cost ATP. Endocytosis brings material into the cell, and exocytosis releases material from it. In both, the cargo never actually crosses the lipid bilayer; it stays wrapped in membrane the entire trip.
Endocytosis begins when the membrane folds inward, forming a pocket that pinches off as a vesicle inside the cell. Phagocytosis, cell eating, takes in large particles such as bacteria, and macrophages use it to clear infections. Pinocytosis, cell drinking, takes in droplets of extracellular fluid along with whatever solutes are dissolved in them. Receptor-mediated endocytosis imports specific molecules that bind to protein receptors gathered in coated pits; cells use it to take up LDL cholesterol from the blood.
Exocytosis works in reverse. A vesicle inside the cell moves to the membrane, fuses with it, and spills its contents outside. Cells use exocytosis to secrete digestive enzymes, mucus, and many other products, and every exocytosis event adds lipids and proteins to the surface membrane. Endocytosis removes membrane again, so the two processes keep the surface area roughly constant even while a cell constantly swallows and spits.
Common Mistake: Calling endocytosis and exocytosis forms of diffusion. They are not; both require ATP and physical rearrangement of the membrane, and neither moves material across the lipid bilayer in the usual sense. The cargo rides inside a membrane bubble the whole way.
Endocytosis is like a snake swallowing a meal by wrapping its body around it. The membrane wraps a pocket around the food and pinches the pocket off inside the cell. Exocytosis is the reverse, like a bubble rising to the surface of a puddle and popping open to drop its contents outside. One pulls things in, and the other pushes things out.
High-Yield:
- The sodium potassium pump exports 3 sodium ions and imports 2 potassium ions per ATP, which keeps the cell interior negative.
- Passive transport follows the gradient and costs nothing; any movement against the gradient is active transport.
- Water potential equals solute potential plus pressure potential, and water moves toward the more negative side.
- Tonicity compares the outside solution with the cell interior, so hypertonic means water leaves the cell.
- Membrane fluidity comes from unsaturated fatty acids and cholesterol, and it is what lets membranes bend, heal, and carry out transport.
Key terms are emphasized and defined within the main notes.
See the formulas, procedures, and process blocks in the main notes where applicable.
See the labeled common-mistake callouts in the main notes where present.
Use the quick-review or recap section in the main notes.
5 questions here, of 12 in this lesson’s practice set. Answers stay hidden until you check.
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