In-Depth Notes on Animal Biology

Multicellular: Animals are composed of more than one cell.
Heterotrophic: Animals need to ingest other organisms for nutrition (cannot synthesize their food).
Eukaryotic: Made up of eukaryotic cells.
Overall definition: An animal is defined as a multicellular heterotrophic eukaryote.

All animals develop tissues from embryonic germ layers.
These layers guide the development of the body tissues.
Exceptions exist, e.g., sea sponges are closer to colonies of single cells rather than completely multicellular organisms.

Heterotrophic Feeding: Animals rely on consuming other organisms for nutrition, unlike plants which use photosynthesis.
- Ingestive Heterotrophs: Animals consume their food via ingestion.
- Cellular Respiration: This process is used by animals to extract energy from ingested food.
Comparison with fungi:
- Fungi are absorption heterotrophs and do not consume food by eating.
- Fungi absorb nutrients by breaking down decaying matter, using structures called filaments.

Absence of Cell Wall: Unlike plant and bacterial cells, animal cells do not have a cell wall.
Animal cells cannot perform photosynthesis.
Tissues: Groups of cells working together for a common function (e.g., epithelial, adipose, skeletal muscle tissues).
Proteins: Play a critical role in holding cells and tissues together.

Nervous Tissue: Found in animals, allows for response to environmental changes (not found in plants).
Muscle Tissue: Enables movement, another feature absent in plants.

Sexual Reproduction: Involves blending of genetic material from two parents.
Diploid and Haploid Cells:
- Only the gametes (sperm and egg) are haploid (contain half the number of chromosomes).
- All other cells in the organism are diploid (contain two sets of chromosomes).
Zygote: The fertilized cell formed when the sperm and egg unite, which will develop into a full organism.