Nutrition Mode in Living Organisms

Nutritional Mode

• Animals differ from plants and fungi in their mode of nutrition, as they cannot construct their own organic molecules and instead ingest them.
• Fungi are heterotrophs that feed by absorption, whereas most animals feed by ingesting their food and then using enzymes to digest it within their bodies.

Cell Structure and Specialization

• Animals are eukaryotes, multicellular, and lack structural support of cell walls, instead relying on external proteins like collagen for structural support.
• Animal cells are organized into tissues, groups of similar cells that act as a functional unit, such as muscle tissue and nervous tissue.
• Muscle and nerve cells are central to the animal lifestyle, enabling movement and nerve impulses.

Reproduction and Development

• Most animals reproduce sexually, with a dominant diploid stage in their life cycle.
• Sperm and egg cells are produced directly by meiotic division, and the zygote undergoes cleavage, forming a multicellular embryonic stage called a blastula.
• The process of gastrulation produces layers of embryonic tissues that develop into adult body parts, resulting in a gastrula.
• Many animals go through a larval stage, which eventually undergoes metamorphosis to become a juvenile resembling an adult.

History of Animals

• The history of animals spans over half a billion years, with molecular analyses suggesting that the common ancestor of all extant animal species lived about 770 million years ago.

Origin of Multicellular Animals

• Choanoflagellates are the closest living relatives of animals, providing clues to the origin of multicellularity.
• The evolution of multicellularity required new ways for cells to adhere and signal to each other.
• DNA sequence analyses show that animal cadherin proteins share domains with a cadherin-like protein in choanoflagellates.

Symmetry

• A basic feature of animal bodies is their type of symmetry, which can be radial or bilateral.
• Radially symmetrical animals, like flowerpots, have equal parts around a central axis.
• Bilaterally symmetrical animals, like shovels, have two sides, a dorsal and ventral side, and an anterior and posterior end.
• Bilateral animals typically move actively from place to place, with a central nervous system in the head.

Tissues

• Tissues are collections of specialized cells that act as a functional unit, although some groups, like sponges, lack tissues.
• In animals with tissues, the embryo becomes layered during gastrulation, forming ectoderm, endoderm, and mesoderm germ layers.
• These germ layers give rise to various tissues and organs, with ectoderm forming the outer covering and central nervous system, and endoderm forming the lining of the digestive tract and organs.

Nutritional Mode

• Animals differ from plants and fungi in their mode of nutrition, as they cannot construct all their own organic molecules.
• Animals ingest organic molecules by eating other living organisms or nonliving organic material.
• Unlike fungi, most animals feed by ingesting their food and then using enzymes to digest it within their bodies.

Cell Structure and Specialization

• Animals are eukaryotes and multicellular, but lack cell walls.
• Proteins external to the cell membrane provide structural support to animal cells and connect them to one another.
• Collagen is the most abundant of these proteins, found only in animals.
• Animal cells are organized into tissues, groups of similar cells that act as a functional unit.
• Muscle tissue and nervous tissue are responsible for moving the body and conducting nerve impulses, respectively.

Reproduction and Development

• Most animals reproduce sexually, and the diploid stage usually dominates the life cycle.
• In most animal species, a small, flagellated sperm fertilizes a larger, nonmotile egg, forming a diploid zygote.
• The zygote undergoes cleavage, a succession of mitotic cell divisions without cell growth between the divisions.
• During development, cleavage leads to the formation of a multicellular embryonic stage called a blastula.
• The process of gastrulation produces the layers of embryonic tissues that will develop into adult body parts.
• The resulting developmental stage is called a gastrula.
• Many animals develop through a larval stage, which undergoes metamorphosis to become a juvenile.

History of Animals

• The history of animals spans over 770 million years.
• Molecular analyses suggest that the common ancestor of all extant animal species lived about 770 million years ago.
• Choanoflagellates are the closest living relatives of animals.
• The origin of multicellularity requires the evolution of new ways for cells to adhere and signal to each other.

Symmetry

• A basic feature of animal bodies is their type of symmetry—or absence of symmetry.
• Radial symmetry is found in some animals, such as starfish and sea urchins.
• Bilateral symmetry is found in animals with two axes of orientation: front to back and top to bottom.
• Bilateral animals have a dorsal (top) side and a ventral (bottom) side, a left side and a right side, and an anterior (front) end and a posterior (back) end.
• Nearly all animals with a bilaterally symmetrical body plan have sensory equipment concentrated at their anterior end.

Tissues

• Tissues are collections of specialized cells that act as a functional unit.
• Sponges and a few other groups lack tissues.
• In all other animals, the embryo becomes layered during gastrulation, forming the various tissues and organs of the body.
• Ectoderm, the germ layer covering the surface of the embryo, gives rise to the outer covering of the animal and, in some phyla, to the central nervous system.
• Endoderm, the innermost germ layer, lines the pouch that forms during gastrulation and gives rise to the lining of the digestive tract and organs.
• Cnidarians and a few other animal groups that have only two germ layers are said to be diploblastic.
• All bilaterally symmetrical animals have a third germ layer, called the mesoderm, which fills much of the space between the ectoderm and endoderm.