Lecture: Cell Structure & Function
Lecture script · about 15–20 minutes spoken · Week 4
Opening
Sprinkle a pinch of salt on a crisp lettuce leaf and walk away. Come back a little later and the leaf is limp and wet. Nothing tore it. Something left its cells, and here's the twist: it wasn't the salt. A nursing student learning why a cell swells or shrinks in the fluid around it, a cook who salts vegetables and watches them sag, and a gardener reviving a wilted houseplant are all looking at the same thing. By the end of this lecture, you'll be able to explain it.
In Week 1, you met the cell as the smallest unit that is unambiguously alive. In Week 2, you met ions and water, the solvent of every cell. In Week 3, you met the four classes of large molecules, including the phospholipid, with its water-loving head and two water-fearing tails. This week we open the cell up. We'll sort cells into two kinds, tour the parts inside, look hard at the border, follow what crosses it, and finish with a puzzle about size you can solve with a few model cubes. Next week, in Week 5, the energy that runs all of this gets its own story.
1. Two kinds of cell
Picture two buildings. One is a single open room with no inner walls. The other is a big house with many rooms, each set up for one kind of work. Cells come in those same two plans.
The open room is a prokaryotic cell. A prokaryotic cell is a cell with no nucleus and no membrane-bound organelles, whose DNA sits in a region called the nucleoid. Bacteria and archaea are prokaryotes, and they're typically about 0.1 to 5.0 micrometers across. The nucleoid is the region of a prokaryotic cell where its DNA sits, with no membrane around it.
The big house is a eukaryotic cell. A eukaryotic cell is a cell with a true nucleus and membrane-bound organelles, typically about 10 to 100 micrometers across. Animals, plants, fungi, and protists are built from eukaryotic cells, and so are you. An organelle is a structure inside a cell that does one particular job.
Whichever plan it follows, every cell has a plasma membrane, cytoplasm, DNA, and ribosomes. A part that every cell has can't tell the two kinds apart.
One more thing. A prokaryote isn't a eukaryote that lost its nucleus. It's an older, distinct kind of cell, built on its own plan.
Pause and try this. A cell has a plasma membrane, cytoplasm, DNA, and ribosomes. Is it prokaryotic or eukaryotic?
Think about which of those parts every cell has.
Here's the answer. You can't tell yet, because every cell has all four of those parts. The clue that decides it is a nucleus wrapped in a membrane. A true nucleus means eukaryotic; DNA loose in a nucleoid means prokaryotic.
2. A tour of the organelles
Think about a busy restaurant kitchen: a recipe office, cooks, a prep line, a packing table, a generator out back, and a cleanup crew. A eukaryotic cell is organized in a similar way, so at each stop I'll name the structure and its job.
The nucleus holds the cell's DNA and directs the making of proteins. It's the recipe office. A ribosome builds proteins. Ribosomes are the cooks, and every cell has them. The rough endoplasmic reticulum is a network of membranes studded with ribosomes that makes and folds proteins the cell will export or place in its membranes. The smooth endoplasmic reticulum is a network of membranes without ribosomes that makes lipids and helps detoxify harmful substances. The Golgi apparatus is a stack of membrane sacs that modifies, sorts, and ships proteins and lipids. It's the packing table.
The mitochondrion is the organelle that makes ATP for the cell. ATP is the cell's energy currency, so the mitochondrion is the generator. Its inner membrane is folded over and over, and those folds are called cristae. The folds add surface for making ATP, which is structure suiting function in one small organelle.
A chloroplast carries out photosynthesis in plants and algae, using light energy to make sugar. A lysosome breaks down waste, large molecules, and worn-out parts. It's the cleanup crew. In a plant cell, the central vacuole is a large water-filled sac that stores water and presses outward to keep the cell firm. That firmness is called turgor. Finally, a cell wall is a rigid layer outside the membrane that gives support and shape to the cells of plants, fungi, and bacteria. Animal cells, including yours, have none.
Follow one exported protein, in order: the instructions stay in the nucleus, a ribosome on the rough endoplasmic reticulum builds the chain, the rough endoplasmic reticulum folds it, the Golgi apparatus ships it, and it leaves through the plasma membrane.
Pause and try this. Which organelle would you call the cell's packing and shipping table, and what does it do?
Picture the station right before the delivery door.
Here's the answer. The Golgi apparatus, because it modifies, sorts, and ships proteins and lipids to where they're needed. It takes a folded protein from the rough endoplasmic reticulum and sends it on its way.
3. Plant cells have both
Picture a houseplant on a sunny windowsill. In daylight it makes sugar from light. At night, with the light gone, it keeps living. It keeps growing roots, moving water, and repairing itself, and all of that work costs energy around the clock. So where does the night-time energy come from?
It comes from the plant's mitochondria. A plant cell has chloroplasts and mitochondria, both. Chloroplasts make sugar, and mitochondria burn it, day and night. I like to say the chloroplast stocks the shelf and the mitochondrion spends from it.
People often assume chloroplasts replace mitochondria. They don't. Every eukaryotic cell you meet this week, animal, plant, or fungus, has mitochondria, because every one needs ATP.
The cell wall has its own trap. A cell wall belongs to plants, fungi, and bacteria. An animal cell has a plasma membrane as its outer border and no wall at all.
Pause and try this. A friend says, "Plant cells don't need mitochondria, because they have chloroplasts." What would you tell them?
Think about the houseplant at night.
Here's the answer. A plant cell has both. Chloroplasts make sugar in the light, and mitochondria burn that sugar for ATP, day and night, so the plant can keep working in the dark.
4. The plasma membrane
Think about a soap bubble: a thin, flexible skin that closes on itself. A cell's border is far more complex, but it shares that fluid, self-sealing quality, and it's built from the molecule you met last week.
Let me describe the membrane as a labeled figure, in words: a sandwich seen from the side. The two slices of bread are rows of phospholipid heads, the water-loving ends. One row faces the watery fluid outside the cell, and the other faces the watery cytoplasm inside. The filling is the phospholipid tails, the water-fearing ends, tucked inward, away from water on both sides. Set into the sandwich, some passing all the way through, are proteins. Some form channels; some act as carriers or pumps. That double layer is a phospholipid bilayer.
The way biologists picture this is called the fluid mosaic model, proposed by Singer and Nicolson in 1972: a fluid phospholipid bilayer with proteins set into it like pieces of a mosaic that can drift sideways. Fluid means the parts aren't locked in place. Mosaic means the surface is a patchwork of different pieces. And it's a model, a simplification you can hold in your head, not a photograph of every membrane.
Here's the key idea. The membrane is selectively permeable, letting some substances cross while holding others back. It's a gate, not a wall.
Pause and try this. Why do the phospholipid tails end up in the middle of the membrane, not on the outside?
Think about where the water is.
Here's the answer. There's watery fluid on both sides of the membrane, and the tails are the water-fearing ends, so they tuck into the middle, away from water. The water-loving heads face the water on each side. The shape follows from the molecule's two ends.
5. Passive transport
Open a bottle of vanilla extract in one corner of a kitchen, and soon someone across the room smells it. Nobody carried the scent over; the molecules spread out on their own.
That spreading is diffusion, the movement of a substance from where it's more concentrated to where it's less concentrated, with no energy spent. The difference that drives it is a concentration gradient. Moving from high to low is moving down the gradient, like a ball rolling downhill.
When that happens across a membrane, it's passive transport: movement down the concentration gradient, with no energy spent by the cell. It comes in three forms. Some substances diffuse straight across the bilayer. In facilitated diffusion, a substance still moves downhill, but through a protein channel or carrier. It's still free; the protein only opens a path.
The third form is the one that explains the lettuce. Osmosis is the diffusion of water across a selectively permeable membrane toward the side with more dissolved solute. A solute is a substance dissolved in a liquid, like salt or sugar in water. So water moves toward the side with more dissolved stuff. And here's the line I want you to say every time: osmosis moves water, not the salt.
Pause and try this. Two salt solutions sit on either side of a membrane that lets water through but not salt. Side A has more salt. Which way does the water move?
Find the side with more dissolved stuff.
Here's the answer. Water moves toward side A, the side with more solute. The salt stays where it is, because the membrane holds it back. Osmosis moves the water toward the salt, never the salt itself.
6. Reading tonicity from the cell's side
Drop a few raisins into plain water overnight, and by morning they're plump. The salted lettuce went limp. One rule explains both, with one habit: read the outside solution from the cell's point of view.
There are three words. A hypotonic solution has less solute than the inside of the cell, so water moves into the cell, and an animal cell placed in one swells and can burst. A hypertonic solution has more solute than the inside of the cell, so water moves out and the cell shrivels. An isotonic solution has the same solute concentration as the inside of the cell, so water moves in and out equally, and there's no net change.
Now the lettuce. The salty film on the leaf has more solute than the inside of the lettuce cells, so the outside is hypertonic. Water moves out toward the salt. The cells lose water, the central vacuoles lose their push, turgor drops, and the leaf goes limp. The salt stayed outside; the water left.
The raisin runs the other way. Plain water has less solute than the inside of its cells, so it's hypotonic, and water moves in.
A plant cell in a hypotonic setting doesn't burst: the vacuole swells, the wall holds, and the cell turns firm. That's why a watered houseplant stands back up.
Pause and try this. A red blood cell, an animal cell, is placed in fluid with less solute than its inside. Which word names that fluid, and what happens to the cell?
Read it from the cell's side.
Here's the answer. The fluid is hypotonic, because it has less solute than the inside of the cell. Water moves in, and with no cell wall to hold it, the cell swells and can burst.
7. Active transport
Picture pushing a bike up a steep hill. Rolling down took no effort; going up, you pay the whole way. Some substances have to go uphill across a membrane, and the cell pays for that trip.
That's active transport: movement of a substance across a membrane against its concentration gradient, from low to high, using a protein pump paid for with ATP. It's the opposite of passive transport: uphill, and paid.
The best-known pump is the sodium-potassium pump, a membrane protein that uses one ATP in each cycle to move three sodium ions out of the cell and two potassium ions in. Let me count its work for 4 cycles. Each cycle spends 1 ATP, so 4 cycles spend 4 ATP. Sodium out is 4 times 3, which is 12. Potassium in is 4 times 2, which is 8. So in 4 cycles the pump spends 4 ATP and moves 12 sodium ions out and 8 potassium ions in.
Facilitated diffusion and active transport both use a membrane protein, so ask two questions. Which way is it going, and is ATP spent? Passive is downhill and free; active is uphill and paid.
Pause and try this. A substance crosses the membrane through a carrier protein, moving from where it's plentiful to where it's scarce, and no ATP is spent. Is that passive or active?
Ask the two questions.
Here's the answer. It's passive, specifically facilitated diffusion, because it moves downhill and spends no ATP. Using a protein doesn't make it active. Only uphill movement paid for with ATP is active.
8. Why cells stay small
Think about cooling soup. In one deep pot it stays hot for ages; in shallow dishes it cools fast. What changed is how much surface each bit of soup has.
A cell trades with the world through its surface, the plasma membrane, but it has to feed its whole volume. As a cell grows, its surface area grows as the square of its size, and its volume grows as the cube. So volume races ahead. The surface-area-to-volume ratio is a cell's surface area divided by its volume, and it falls as a cell grows.
Let's put numbers on it with the cube cell, a model built for this course, not real data. For a cube, the surface area is six times the side times the side, because it has six square faces. The volume is the side times the side times the side. And the ratio simplifies to 6 divided by the side.
Side 1: surface area 6, volume 1, so the ratio is 6 to 1. Side 2: six times 2 times 2 is 24, and 2 times 2 times 2 is 8. Then 24 divided by 8 is 3, so 3 to 1. Side 3: six times 3 times 3 is 54, and 3 times 3 times 3 is 27. Then 54 divided by 27 is 2, so 2 to 1, never left as 54 to 27. Side 4: six times 4 times 4 is 96, and 4 times 4 times 4 is 64. Then 96 divided by 64 is 1.5, so 1.5 to 1.
The ratio falls from 6 to 3 to 2 to 1.5 as the cube grows. Double the side from 2 to 4, and the surface grows 4 times while the volume grows 8 times, so each bit of the inside gets half as much membrane. A big cell has too little membrane for the volume it must feed. Real cells cope three ways: they divide, they flatten, or they fold their membranes, with tiny folds called microvilli or the cristae you met in the mitochondrion.
Pause and try this. Which cube cell has the higher surface-area-to-volume ratio, side 2 or side 3?
Use the ratios we worked out.
Here's the answer. The side-2 cube, at 3 to 1, beats the side-3 cube at 2 to 1. The smaller cube has more surface for each unit of volume, which is exactly why cells stay small.
Closing
Let me pull it together in the order we built it. Cells come in two kinds: prokaryotic, with no nucleus, and eukaryotic, with a true nucleus and organelles, and every cell shares a membrane, cytoplasm, DNA, and ribosomes. Each organelle's structure suits its job, from the nucleus that holds the DNA to the folded cristae of the mitochondrion. A plant cell has chloroplasts and mitochondria, both, and cell walls belong to plants, fungi, and bacteria, never to animal cells. The membrane is a phospholipid bilayer, pictured by the fluid mosaic model, and it's a gate, not a wall. Passive transport runs downhill for free, and osmosis moves water, not the salt. Read tonicity from the cell's side. Active transport runs uphill and pays in ATP. And because volume outgrows surface, cells stay small.
Next in this module come the slide deck, the Readings & Resources page, the lecture tutorial, the practice set, the quiz, the discussion, and the assignment. Then the Biology Lab & Inquiry has you build the cube-cell table yourself and ask how big a cell can get. In Week 5, Energy, Enzymes & Metabolism, you'll follow the ATP that this week's pump spends.
Built with The Course Maker · samples.thecoursemaker.com