An egg is not one ingredient. It is two, packaged together, and they do close to opposite things.
The white is mostly water with protein dissolved in it. The yolk is mostly fat, with an emulsifier and its own set of proteins. Put them in a batter and the white is pulling toward structure and dryness while the yolk is pulling toward tenderness and richness. Most of the confusion about eggs in baking dissolves the moment you stop thinking of an egg as a unit.
Once you can see the two halves separately, recipe instructions that looked arbitrary start making sense: the lone extra yolk, the whites-only angel cake, the recipe that wants one whole egg and one yolk.
Allergen note before anything else: egg is a major allergen. This page describes what egg does mechanically. It does not offer substitutions and nothing here should be read as making a recipe safe for someone with an egg allergy.
The white: water, protein and structure
Egg white is very largely water, and the rest is protein. Both matter, and in different ways.
As water, the white is liquid in your recipe like any other liquid. It hydrates the flour, it turns to steam in the oven, and steam is a leavening force in its own right. A recipe with more white in it is a wetter recipe than the ingredient list suggests.
As protein, the white is the strongest structure-builder an egg has to offer. Heat those proteins and they unfold and link to each other, setting into a firm network. That set is what holds a baked good's shape once the starch and the gluten have done their part.
Two practical consequences follow, and they explain a lot of failures.
Too much white makes things tough and dry. An overly firm protein network squeezes moisture out and eats. This is exactly the mechanism at work when a cake comes out dense and heavy for reasons that are not about leavening.
Whites alone can be whipped into a foam. Beating unfolds the proteins and traps air in a wet film, and that foam is the entire structure of a meringue. What makes that foam succeed or collapse is a subject of its own on why a meringue deflates and why a meringue weeps.
The yolk: fat, emulsifier and tenderness
The yolk is the other direction entirely.
As fat, the yolk shortens. Fat coats flour particles and gets between protein strands, and both effects interrupt structure rather than build it. Interrupted structure eats as tender. This is the same mechanism that makes butter a tenderizer, described in what each ingredient in a cake is actually doing.
As an emulsifier, the yolk does something no other common baking ingredient does as well: it holds fat and water together in one stable mixture. A batter is fat and water that would rather separate, and the yolk is what persuades them not to. That emulsifying power is why a yolk improves the texture and the keeping quality of a bake out of proportion to how much of it is in there.
As protein, the yolk sets too, but into a softer, more custard-like set than the white's. That softer set is what a baked custard is made of, and it is why the temperature window for custard is so narrow. The full read on that is on why a custard curdles.
And the yolk carries flavor and color. Fat carries flavor compounds, and the yolk's pigments are why an egg-rich bake looks golden.
Reading a recipe once you know this
Now the odd instructions make sense.
"One egg plus one yolk." The recipe wants the structure of one whole egg and then more richness and more emulsification without adding more water or more setting protein. That is the entire purpose of the extra yolk.
"Whites only." The recipe wants maximum structure with minimum fat, usually because fat would destroy a foam. Any trace of yolk in a bowl of whites is a genuine problem for the same reason.
"Yolks only." The recipe wants richness and emulsification without the drying, toughening effect of the white. Custards, curds and rich cookie doughs live here.
"Room-temperature eggs." Cold egg dropped into creamed butter chills the fat and can break the emulsion you just built. The batter looks curdled and the texture suffers. This is one of the mechanisms behind a broken buttercream as well.
The three jobs at once, and why substitutions are hard
Here is the thing that makes egg the hardest ingredient to replace and the reason substitution gets its own treatment rather than a paragraph here.
In a single recipe, one egg is usually doing three jobs simultaneously: adding liquid, building protein structure, and emulsifying. A replacement that handles one of those does not necessarily handle the other two. Something that supplies moisture supplies no structure. Something that binds may not emulsify. Which of the three the recipe was actually leaning on decides whether a swap works, and that varies recipe by recipe.
This page does not give substitutions and does not evaluate any. It gives you the framework for reading whether a proposed swap has a chance: ask which of the three jobs the egg was doing in that particular recipe, and whether the replacement does that job.
The size question
Recipes are generally written around one standard egg size, which varies by country, and a recipe using several eggs accumulates the difference.
We publish no gram weight and no volume figure for an egg here, because it depends on grading standards that differ between countries and on the individual egg. If a recipe matters to you and your eggs look noticeably large or small, the honest move is to weigh them, and to compare against whatever weight the recipe's own author gives if they give one. A number taken from a general page is not a specification.
What to take away
- White equals water and structure. More white, firmer and drier.
- Yolk equals fat, emulsifier and tenderness. More yolk, richer and softer.
- A whole egg is a compromise between the two, and recipes tilt it deliberately.
- When a bake is tough, ask whether it has too much white. When it is fragile or greasy, ask whether it has too much yolk for the structure available.