Preheat oven to 350 degrees F. Coat a 12-cup Bundt pan, preferably nonstick, with cooking spray and dust with flour (or use cooking spray with flour).

Whisk whole-wheat flour, all-purpose flour, poppy seeds, baking powder, baking soda and salt in a medium bowl. Combine buttermilk, oil, vanilla, lemon zest and lemon juice in a glass measuring cup.

Beat eggs, egg whites and sugar in a large bowl with an electric mixer on high speed until thickened and pale, about 5 minutes.

Fold the dry ingredients into the egg mixture with a rubber spatula, a third at a time, alternating with 2 additions of the buttermilk mixture. Scrape the batter into the prepared pan, spreading evenly.

Bake the cake until the top springs back when touched lightly and a toothpick inserted in the center comes out clean, 35 to 40 minutes. Let cool in the pan for 5 minutes, then turn out onto a wire rack.

To prepare glaze: Sift 3/4 cup confectioners' sugar into a small bowl; mix with lemon juice and water to create a thin glaze. Poke 1-inch-deep holes all over the cake with a skewer. Coat the warm cake with the glaze using a pastry brush. Let cool completely. To serve, set the cake on a serving plate and dust with confectioners' sugar.

Note: Whole-wheat pastry flour is milled from soft wheat. It contains less gluten forming potential than regular whole-wheat flour and helps ensure a tender result in delicate baked goods while providing the nutritional benefits of whole grains. Store in the freezer. Sources: Available in large supermarkets and in natural-foods stores, and from King Arthur Flour, (800) 827-6836, bakerscatalogue.com, and Bob's Red Mill, (800) 349-2173, bobsredmill.com.

Tips: Toast poppy seeds in a small dry skillet over medium heat, stirring constantly, until fragrant, 3 to 4 minutes. Transfer to a plate to cool.

No buttermilk? You can use buttermilk powder prepared according to package directions. Or make "sour milk" the ratio is 1 tablespoon lemon juice or vinegar to 1 cup milk.

To bring cold eggs to room temperature quickly, place in a mixing bowl and set it in a larger bowl of warm water for a few minutes; the eggs will beat to a greater volume.

To separate eggs safely: Use an egg separator, an inexpensive gadget found in cookware stores; separating eggs by passing the yolk back and forth between pieces of eggshell or your hands can expose the eggs to bacteria.

MAKE AHEAD TIP: Equipment: 10-inch (12-cup) Bundt pan, preferably nonstick