An animal cell is the fundamental unit of life in the animal kingdom. As a eukaryotic cell, it contains a true nucleus that houses its genetic material. These microscopic units perform all processes necessary for survival and organize to form tissues, which in turn assemble into organs. This organization allows for specialized functions throughout the body, from nutrient absorption to the pumping action of the heart.
The Outer Boundary of the Cell
The first structure of an animal cell is the plasma membrane, a flexible outer lining separating the cell’s internal contents from the outside environment. This dynamic boundary is composed of a phospholipid bilayer. Each phospholipid molecule has a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) tail. They arrange into two layers with the tails facing inward and the heads facing the watery environments inside and outside the cell, forming a stable barrier.
This structure’s selective permeability allows it to control what enters and exits. Small, nonpolar molecules like oxygen and carbon dioxide can pass freely, but larger or charged substances cannot. To manage the transport of these other molecules, the membrane is embedded with various proteins. These proteins act as channels and transporters, regulating the passage of materials to maintain the cell’s internal environment.
Cholesterol molecules are also embedded within the animal cell membrane, where they help regulate its fluidity. At high temperatures, cholesterol restricts the movement of phospholipids, preventing the membrane from becoming too fluid. At lower temperatures, it prevents the phospholipids from packing too tightly, ensuring the membrane remains flexible. This regulation helps maintain the integrity of the cell’s protective barrier across different conditions.
Key Components Within the Cell
Contained by the plasma membrane is the cytoplasm, a jelly-like substance that fills the cell and surrounds the nucleus. This fluid matrix, composed of water, salts, and organic molecules, is the site of many metabolic reactions. Suspended within the cytoplasm are specialized structures known as organelles, which perform specific jobs to maintain the life of the cell.
- Nucleus: Serving as the cell’s control center, this organelle is enclosed by a nuclear envelope and contains the cell’s DNA. DNA holds the instructions for building and operating the cell, and the nucleus controls cell growth and reproduction.
- Mitochondria: Often called the “powerhouses” of the cell, these organelles generate energy through cellular respiration. They convert nutrients like glucose into adenosine triphosphate (ATP), the main energy-carrying molecule for cellular functions. Cells with high energy requirements, such as muscle cells, contain many mitochondria.
- Ribosomes: These small structures carry out protein synthesis and can be found floating in the cytoplasm or attached to the endoplasmic reticulum. Following instructions from the nucleus, they assemble amino acids into complex proteins that perform a vast array of tasks.
- Endoplasmic Reticulum (ER): This network of membranes is a manufacturing and transportation system. Rough ER is studded with ribosomes and synthesizes proteins for export, while Smooth ER synthesizes lipids, detoxifies substances, and stores calcium ions.
- Golgi Apparatus: After production by the ER, proteins and lipids are sent to this organelle. The Golgi apparatus modifies, sorts, and packages these molecules into vesicles for delivery, acting like a cellular post office. Vesicles are then transported to their final destinations.
- Lysosomes: These small organelles handle the cell’s recycling and waste disposal. They contain powerful digestive enzymes that break down worn-out organelles, food particles, and foreign invaders into simpler substances that can be reused.
Distinguishing Animal Cells from Plant Cells
While both are eukaryotic, animal and plant cells have distinct features. The primary difference is the absence of a cell wall in animal cells. Animal cells are enclosed only by a flexible plasma membrane, allowing for various shapes, while plant cells have a rigid cell wall for structural support and protection.
Another distinction is how they obtain energy. Animal cells lack chloroplasts, the organelles responsible for photosynthesis in plants. This means animals are heterotrophs that must consume other organisms for energy, while plants are autotrophs that produce their own food.
The vacuoles also differ between these two cell types. Animal cells may have several small, temporary vacuoles for storage and transport, but they lack the large, permanent central vacuole found in mature plant cells. In plants, this vacuole stores water, nutrients, and waste, and maintains turgor pressure to help support the plant.