Both plant and animal cells are eukaryotic, meaning their genetic material is enclosed within a nuclear envelope. They also share many organelles in common, including a cell membrane, mitochondria, ribosomes, endoplasmic reticulum, lysosomes, and Golgi apparatus. Both animal and plant cells are aerobic, meaning they depend on oxygen to break down fuel molecules into the energy carrier ATP (adenosine triphosphate).
The major differences between animal and plant cells include the presence of a cell wall, chloroplasts, and large vacuoles in plant cells but not in animal cells. One organelle present in animal cells but absent in virtually all plant cells is the centriole. Each difference will now be discussed in detail.
Plant cells share this feature in common with many species of fungi and bacteria. The cell wall forms a rigid envelope around the cell, but small molecules (water, sugars, amino acids) can still diffuse across it. In plant cells, the cell wall is composed of cellulose, a polymer of glucose better known as wood. Fungal cell walls contain chitin (a polymer of glucosamine also found in crustacean shells). Bacterial cell walls are made of a variety of carbohydrate or protein polymers.
This organelle contains chlorophyll, a pigment which allows plant cells to perform photosynthesis; no animal cell is capable of this feat. Briefly, photosynthesis is a series of chemical reactions in which the energy of sunlight is harnessed to convert water and carbon dioxide into sugar. During photosynthesis, oxygen is released into the atmosphere. Consequently, all aerobic organisms depend on photosynthesis for their survival.
Chloroplasts contain their own DNA and ribosomes and replicate independently of the cell’s nucleus. In this respect, chloroplasts are similar to mitochondria. According to the Margulis hypothesis, both organelles originated as free living prokaryotes which were engulfed by primitive eukaryotic cells over 1 billion years ago. Since chloroplasts are confined to plant cells and certain species of protozoa, this event must have happened after animal and plant cells diverged evolutionarily.
In plant cells, vacuoles are used to store sugar, starch, water, or waste. Vacuoles sometimes occupy a large part of a plant cell’s interior. Some specialized animal cells such as macrophages phagocytose bacteria into a vacuole-like structure then fuse this structure with a lysosome to destroy the bacteria. Other animal cells have small vacuoles or may lack vacuoles entirely.
These organelles are found in animal cells but are virtually absent in plant cells. Centrioles are composed of triplet arrays of microtubules arranged in a cylinder and linked by dynein arms. In animal cells, each centriole replicates prior to mitotic cell division. The daughter centrioles move to opposite poles of the cell and act as so called microtubule organizing centers (MTOC).
Microtubules, which are cytoskeletal proteins made of tubulin polymers, align to form the spindle fiber during prophase of mitosis. One end of each microtubule appears anchored near the centriole while the other end attaches to the kinetochore, a structure located on a chromosome’s centromere. When chromosomes separate after metaphase, each one moves along its respective set of microtubules toward the MTOC.
Plant cells also form a mitotic spindle made of microtubules, but their MTOCs consist of a dense area of poorly characterized proteins and generally lack centrioles. Since plant cells undergo mitosis as efficiently as their animal cell counterparts, it appears that centrioles are not necessary for eukaryotic cell division. Their disappearance from plant cells may simply reflect the ongoing divergence of animals and plants over long spans evolutionary time.