Plant and animal cells are both eukaryotes, meaning they possess a true nucleus and other internal compartments known as membrane-bound organelles. Despite sharing this complex architecture, these two cellular designs have evolved distinct features. These differences reflect the vastly different lifestyles of plants and animals.
Components Shared by Both
Both plant and animal cells are encased by a flexible plasma membrane, a barrier composed of a phospholipid bilayer. This membrane controls the selective passage of substances into and out of the cell. Within this boundary, the cytoplasm acts as a jelly-like substance where metabolic reactions occur, housing all the organelles and the supporting cytoskeleton.
The nucleus serves as the cell’s command center, enclosing the DNA, which contains the instructions for building and operating the organism. Mitochondria are also present in both cell types, functioning as the sites of cellular respiration. These organelles convert chemical energy from organic molecules into adenosine triphosphate (ATP), the primary energy currency used to power most cellular activities.
Protein synthesis is carried out by ribosomes found free in the cytoplasm or attached to the endoplasmic reticulum (ER). The ER is a network of membranes involved in synthesizing lipids and folding proteins, which are then often transported to the Golgi apparatus. The Golgi complex modifies, sorts, and packages these proteins and lipids into vesicles for transport to their final destinations within or outside the cell.
Structures Exclusive to Plant Cells
Plant cells possess three major structures not found in animal cells, reflecting the sessile, self-sustaining nature of plant life. The cell wall is a rigid outer layer that surrounds the plasma membrane. Composed primarily of cellulose microfibrils, this wall provides mechanical strength, a fixed shape, and structural support to the cell and the entire plant.
The cell wall is instrumental in regulating turgor pressure, preventing the cell from bursting when placed in a hypotonic environment. This structural integrity allows plants to remain upright against the force of gravity. The rigid nature of this outer layer restricts the cell’s ability to change shape or migrate.
Chloroplasts are the second unique structure, containing the green pigment chlorophyll and functioning as the site of photosynthesis. These organelles capture light energy and convert it into chemical energy stored in glucose. The internal structure of the chloroplast includes stacks of flattened sacs called thylakoids, where the light-dependent reactions of photosynthesis occur.
Photosynthesis uses carbon dioxide and water to produce oxygen and sugar. This ability to generate their own food source differentiates plants as autotrophs, contrasting with the heterotrophic nature of animals.
The largest organelle in a mature plant cell is often the large central vacuole, which can occupy up to ninety percent of the cell’s volume. This single membrane-bound sac maintains turgor pressure against the cell wall, providing internal support. The central vacuole also functions as a storage compartment for water, nutrients, and waste products, and sometimes performs the digestive functions that lysosomes handle in animal cells.
Structures Exclusive to Animal Cells
Animal cells lack the rigid cell wall, resulting in a highly flexible and often irregular shape. This flexibility allows animal cells to move, engulf materials, and change their morphology as they differentiate. Instead of a cell wall, animal cells rely on a complex extracellular matrix and an internal cytoskeleton for structure and support.
Animal cells contain specialized organelles called lysosomes, which function as the cell’s digestive and waste disposal system. Lysosomes contain hydrolytic enzymes that break down worn-out cellular components, invading bacteria, and large macromolecules like proteins and lipids. These enzymes operate best in the lysosome’s interior, which is maintained at a lower, more acidic pH than the surrounding cytoplasm.
Centrioles are typically organized as a pair within the centrosome near the nucleus. Each centriole is a cylindrical structure composed of nine triplets of microtubules. During cell division, the centrosome replicates. The centrioles play a role in organizing the mitotic spindle, the structure responsible for separating chromosomes into the two new daughter cells.