Animal Biology: Nutrition, Respiration, and Reproduction

Animal Nutrition and Digestion

Animals are heterotrophs that obtain nutrients through feeding. Different feeding methods include absorption directly from the environment, filtration of food present in water, and ingestion through the mouth. Digestion is the process where food is transformed into nutrients.

Digestive Mechanisms

• Sponges: Filter food particles from water entering through pores.
• Mechanical Digestion: Breaking food into smaller pieces.
• Chemical Digestion: Separating nutrients from food.
• Absorption: Blood absorbs nutrients for the body.
• Excretion: Expulsion of waste from the body.

Digestive Structures

• Gastrovascular Cavities: Sac-shaped structures with one opening for food intake and waste expulsion.
• Digestive Tracts: Tubes with two openings: the mouth and the anus.

Gas Exchange and Respiration

Gas exchange allows organisms to capture oxygen and excrete carbon dioxide.

• Simple Animals: Most cells are in contact with the environment, allowing gas exchange by diffusion across cell membranes.
• Complex Animals: Cells are not in direct contact with the environment, requiring specialized structures:

Specialized Respiratory Organs

• Trachea: Tubes linking to animal tissues.
• Cutaneous Respiration: Exchange through the surface of the body; requires thin, moist skin.
• Gills: Layers of tissue where oxygen-rich water flows in contact with blood vessels.
• Lungs: A series of organs formed by tubes and cavities through which air travels.

Circulatory Systems

The circulatory system transports nutrients to cells and collects waste. Substances flow through tubes in blood pumped by the heart.

• Open Circulation: Circulatory fluid leaves vessels and flows through tissues; exchange occurs when fluid contacts cells. Common in many invertebrates.
• Closed Circulation: Fluid flows inside vessels at all times. Vessels branch into capillaries with thin walls for efficient substance exchange.

Excretion and Sensory Systems

Cell activity produces waste products. In simple animals, cells excrete waste directly. In complex animals, waste is collected by specialized excretory organs.

Sensory Organs

Receptors send information to the brain:

• Photoreceptors: Eyes that perceive light (simple, compound, or camera eyes).
• Mechanoreceptors: Detect vibration, sound (ears), touch, smell (nose), and taste (mouth).
• Thermoreceptors: Detect temperature changes.

Animal Reproduction

Asexual Reproduction

• Budding: A new individual grows from a bud on the parent.
• Fragmentation: The parent divides into fragments that develop into new individuals.

Sexual Reproduction

In most animals, reproduction involves gametes (male testicles and female ovaries).

• Unisexual: Each parent has a distinct reproductive system.
• Hermaphroditic: One individual possesses both male and female reproductive systems.
• Fertilization: The union of male and female gametes to form a zygote.
• External Fertilization: Takes place outside the female's body.
• Internal Fertilization: Takes place inside the female's body.