Lipids & Membranes

BCH 100 — Introductory Biochemistry · Dr. Radi

build Jul 17 · 15:06 · CC BY-NC-SA 4.0 · owned figures (RDKit / matplotlib / PyMOL)
Dr. Radi

By the end of this unit, you can…

  • Describe the roles of fats and the structure and naming of fatty acids (systematic, common, ω), and distinguish saturated, mono-, and polyunsaturated
  • Identify the major lipid classes (triglycerides, phospholipids, glycolipids, steroids) and the trans-fat problem
  • Explain the fluid mosaic model, what controls membrane fluidity, and the types of membrane protein
  • Distinguish passive and active transport, and the roles of channels, pumps, and transporters
Dr. Radi

Today's route 🗺️

  1. Fatty Acids
  2. The Lipid Families
  3. Membranes: Fluid & Mosaic
  4. Crossing the Membrane
Dr. Radi

1 · Fatty Acids

"Meet the fatty acid — a greasy tail with an acid head. One little kink is the difference between butter and olive oil."

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Why fat gets a bad rap it half-deserves

Fat is the body's best energy store — it packs more than twice the energy of carbs or protein, gram for gram. It also insulates you, cushions your organs, is the raw material for steroid hormones, and builds every cell membrane you own. Here's fat doing its main job: adipocytes, each a cell nearly filled by a single oil droplet.

Adipose tissue histology: 3ryen, CC0 (Wikimedia Commons)
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A fatty acid: greasy tail, acid head

Strip a fat down and you get fatty acids: a long hydrocarbon chain (greasy, water-hating) capped by a carboxyl group (–COOH, the "acid"). That's it — a tail and a head. The chain is where the energy is stored; the head is the reactive handle the cell grabs.

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One kink changes everything

A saturated fatty acid (stearic, 18:0) has no double bonds — it's a straight rod. Add one double bond and you get a monounsaturated one (oleic, 18:1); add two or more and it's polyunsaturated (linoleic 18:2, α-linolenic 18:3). Each double bond puts a permanent kink in the chain. Saturated = straight; unsaturated (mono or poly) = kinked. That bend is about to matter a lot.

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Butter vs. olive oil

Straight saturated chains stack neatly and pack tight — so they're solid at room temperature (butter, lard). Kinked unsaturated chains can't pack — they stay loose and liquid (olive oil, right there in the bottle). Same idea in your cells: more unsaturated fat = a more fluid membrane. The kink is why.

Olive oil: margenauer, CC0 (Wikimedia Commons)
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Naming a fatty acid

Fatty acids get named a few ways. Systematic and common names you memorize (oleic = 18:1). But the useful one is the omega (ω) system: count from the CH₃ end to the first double bond. ω-3, ω-6, ω-9 — that number names the family, and (as you'll see) it's what your body cares about.

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The fats you MUST eat

Your body can make most fatty acids — but not the ω-6 (linoleic) and ω-3 (α-linolenic) ones. Those are essential: you have to eat them. They build membranes and become signaling molecules. Oily fish are loaded with ω-3s — which is why "eat more fish" is on every heart-health list.

Oily fish (an ω-3 source): Earth'sbuddy, CC BY-SA 3.0 (Wikimedia Commons)
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2 · The Lipid Families

"Four families of grease, four totally different jobs — plus the one man-made fat your arteries can't forgive."

Dr. Radi

One word, four very different molecules

"Lipid" isn't a shape — it's an attitude: water-hating. That's the only thing these four families share. Triglycerides store energy, phospholipids build membranes, glycolipids flag the cell surface, and steroids are rigid signaling rings. Same club, wildly different jobs — so let's meet them one at a time.

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Triglyceride: the fat you store

Take glycerol (a 3-carbon backbone) and hang three fatty acids off it with ester bonds — that's a triglyceride (a triacylglycerol, TAG). This is the fat in your adipose tissue, in butter, in oil. Three greasy tails, zero polar groups: it's pure energy storage, packed away water-free. Snip those ester bonds (a lipase does it) and the fatty acids are freed to burn.

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Phospholipid: the two-faced membrane builder

Swap one fatty acid for a phosphate head and everything changes. Now the molecule is amphipathic — a water-loving head and two water-hating tails on the same body. Drop a crowd of them in water and they self-assemble: heads face out, tails hide in. That sheet — the lipid bilayer — is the wall around every cell you own. Membranes exist because phospholipids can't decide which they are.

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Steroids: four rings, big consequences

No tails here at all. Steroids are four fused carbon rings — and the parent of the family is cholesterol. It wedges into membranes to tune their stiffness, and it's the raw material your body carves into steroid hormones (testosterone, estrogen, cortisol) and bile salts. Same four-ring core, redecorated. (Glycolipids, the fourth class, are just a lipid wearing a sugar name-tag on the cell surface.)

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Two kinds of fat tissue: savings vs furnace

Not all adipose is equal. White fat (WAT) is the savings account — each cell is one big oil droplet, holding energy, insulating, and cushioning. Brown fat (BAT) is a furnace — many small droplets crammed with mitochondria whose protein UCP1 (thermogenin) short-circuits the proton gradient to make heat instead of ATP. It's how newborns (and hibernators) stay warm, and it's why BAT is a hot target in metabolic research.

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THE PROBLEM: trans fats

Nature makes unsaturated fats cis — kinked and fluid. Industry partially hydrogenates vegetable oil to harden it, and accidentally flips some double bonds to trans, which straightens the chain back out so it acts like a saturated fat your body never evolved with. Trans fats raise LDL, lower HDL, and drive atherosclerosis — bad enough they're now banned from the food supply in much of the world. One flipped bond, one public-health disaster.

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3 · Membranes: Fluid & Mosaic

"The cell membrane isn't a wall — it's a living, drifting oil slick with proteins bobbing in it. Here's what keeps it just fluid enough."

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The fluid mosaic model

The membrane around your cells is two things at once. It's fluid — the phospholipids and proteins aren't nailed down; they drift sideways like boats on a sea. And it's a mosaic — that lipid sea is studded with a patchwork of proteins, cholesterol, and sugar tags. Put those together and you have the fluid mosaic model, biology's picture of every membrane you own.

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What keeps it just fluid enough

A membrane has to stay soft enough to work but not fall apart — and three things tune that. Saturation: straight saturated tails pack tight (less fluid); kinked unsaturated tails can't pack (more fluid). Chain length: shorter tails = more fluid. Temperature: heat loosens it, cold gels it. Same trick as butter vs. oil — now doing a job inside your cells.

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Cholesterol: the membrane's thermostat

Cholesterol wedges in among the phospholipid tails and does something clever — it works both directions. When the membrane gets hot and too runny, cholesterol reins in the flailing tails and firms it up. When it gets cold and starts to solidify, cholesterol wedges between the tails and blocks them from packing — keeping it from freezing. One molecule, a built-in fluidity buffer.

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The proteins in the mosaic

The membrane's real machinery is its proteins, and they ride in three ways. Integral (transmembrane) proteins pass all the way through the bilayer — these are your channels, pumps, and receptors. Peripheral proteins cling loosely to one surface and lift off easily. Lipid-anchored proteins float above the membrane, tied down by a lipid tail tucked into one leaflet. Different mountings, different jobs.

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Why "fluid" isn't optional

Here's fluidity earning its keep. A red blood cell is about 8 µm across — but it has to squeeze single-file through capillaries half that width, millions of times, without bursting. Only a fluid, flexible membrane can fold, deform, and spring back like that. Make the membrane too stiff (too much cholesterol, too little unsaturation) and cells get rigid and fragile — the membrane's softness is a feature, not a flaw.

Red blood cells: Arek Socha, CC0 (Wikimedia Commons)
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4 · Crossing the Membrane

"The membrane keeps almost everything out — so how does anything get in? Two answers: coast downhill for free, or pay ATP to climb."

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The membrane is a picky gatekeeper

That greasy bilayer is a great wall — which is a problem, because cells need to let things in and out. What crosses on its own is short: gases (O₂, CO₂) and small nonpolar molecules slip right through; water trickles through slowly. But ions (Na⁺, K⁺, Cl⁻), glucose, and anything large or charged are stuck at the door. They need a protein to let them through — and that's what the rest of this lecture is about.

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Two ways across: free vs paid

Every crossing is one of two kinds. Passive transport goes down the concentration gradient — from crowded to empty — so it's free, no energy needed (that covers simple diffusion, osmosis, and facilitated diffusion through a protein). Active transport goes the other way, up the gradient from empty to crowded — which nature never does for free, so it costs ATP. Downhill is free; uphill you pay.

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Osmosis: when water is the thing that moves

Osmosis is just diffusion of water — and water moves toward the side with more solute (trying to dilute it). This is why the fluid around a cell matters. In a hypotonic solution (less solute outside), water rushes in and the cell swells — and can burst. In a hypertonic one (more solute outside), water leaves and the cell shrivels. Only an isotonic solution leaves it happily unchanged.

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Paying to pump: the Na⁺/K⁺ pump

The star of active transport sits in every one of your cells. The Na⁺/K⁺ pump grabs 3 Na⁺ and pushes them out, grabs 2 K⁺ and pulls them in — both against their gradients — and it burns one ATP each cycle to do it. That relentless pumping builds the ion gradient your nerve and muscle cells discharge to fire. It's so important it eats a big slice of your resting energy budget.

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The three doormen: channels, carriers, pumps

The proteins that move things come in three flavors. A channel is an open (often gated) pore — ions stream through fast, passively. A carrier (transporter) binds its passenger and flips its shape to ferry it across — slower, and either passive or active. A pump is a carrier that spends ATP to force cargo uphill. Channels and passive carriers coast down the gradient; pumps push against it.

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The pump that runs your mind

Why should you care that a pump moves 3 Na⁺ for 2 K⁺? Because that tiny imbalance is the battery behind every thought, heartbeat, and muscle twitch. Neurons spend up to a fifth to a third of your resting ATP just running Na⁺/K⁺ pumps to keep that battery charged. Stop the pumps — as the poison ouabain or a failing blood supply does — and the gradient collapses, cells swell, and excitable tissue goes silent.

Neuron: Doctor Jana, CC BY 4.0 (Wikimedia Commons)
Dr. Radi

Can you…?

  • ☐ describe the roles of fats and the structure and naming of fatty acids (systematic, common, ω), and distinguish saturated, mono-, and polyunsaturated?
  • ☐ identify the major lipid classes (triglycerides, phospholipids, glycolipids, steroids) and the trans-fat problem?
  • ☐ explain the fluid mosaic model, what controls membrane fluidity, and the types of membrane protein?
  • ☐ distinguish passive and active transport, and the roles of channels, pumps, and transporters?

If any box stays empty, the practice site has a drill for it. 🧪

Dr. Radi