Structures for Survival

Have you ever watched a plant grow from a tiny seed into a full plant with flowers? This lesson uses special plants called Fast Plants, which grow so quickly that you can watch their whole life in just a few weeks. As the plants grow, you get to be a scientist. You look closely at each part of the plant, like the roots, stems, leaves, and flowers, and figure out what job each part does. Roots hold the plant in the soil and drink up water. Leaves catch sunlight to make food. Flowers help the plant make seeds so new plants can grow. Then you build a model and make an argument, using what you observed, to show how these inside and outside parts help the plant survive, grow, and reproduce.

Roots may hide under the soil, but they do three big jobs that keep a plant alive. First, roots anchor the plant, which means they hold it firmly in the ground so wind and rain cannot knock it over. Second, roots soak up water and minerals from the soil and send them up to the rest of the plant. Third, some roots store extra food that the plant made, so the plant has energy saved for later. Plants have two main kinds of root systems. A taproot system has one thick main root that grows straight down, like a carrot. A fibrous root system has many thin roots that spread out, like grass roots. Some plants even have special changed roots, like the prop roots that hold up corn plants. Each kind of root structure helps the plant survive in its own home.

Leaves are the food factories of a plant. Inside a leaf, a green material called chlorophyll catches sunlight. The leaf uses that sunlight, plus water and air, to make sugar, which is the plant's food. This process is called photosynthesis. A typical leaf has a wide flat part called the blade, which is shaped to catch as much sunlight as possible, and a stalk called the petiole that connects the leaf to the stem. Tiny openings on the leaf let air in and water vapor out. Some leaves have changed shape to do special jobs. A cactus has leaves shaped like sharp spines, which protect it from hungry animals and save water in the dry desert. The shape and structure of each leaf helps the plant survive where it lives.

All animals reproduce, which means they make new animals like themselves. There are two main ways this happens. In asexual reproduction, one parent makes offspring all by itself, and the young ones are exact copies of the parent. Some sea creatures can do this by splitting in two or growing a small bud that breaks off into a new animal. In sexual reproduction, two parents each share some of their genetic material, and the offspring is a mix of both. This mixing is helpful because it makes young animals a little different from each other, so some may be better at surviving when the environment changes. Fertilization, the joining of egg and sperm, can happen outside the body, like when fish release eggs and sperm into the water, or inside the body, like in dogs and birds. Each way of reproducing has trade offs, and the structure of the animal's body matches the way it reproduces.

For a new animal to start growing, a sperm cell and an egg cell must join together. This joining is called fertilization, and animals have different body structures to make it happen. Some animals, like many fish and frogs, use external fertilization. The female lays her eggs in water, and the male adds sperm over them outside the body. Water keeps the eggs from drying out, and many eggs are released at once so at least some survive. Other animals, like birds, reptiles, and mammals, use internal fertilization. The sperm joins the egg inside the female's body. This protects the delicate cells and lets the offspring start developing in a safe place, like an egg with a shell or the mother's body. These body structures are adaptations, special features that help animals reproduce successfully in the places where they live.