Summary
Highlights
All cells, the smallest living units of an organism, share three common features: a cell membrane, cytoplasm, and DNA. Cells are broadly categorized into two types: eukaryotic cells, which are more complex and possess organelles including a nucleus (found in plants and animals), and prokaryotic cells, which are always unicellular, lack a nucleus and membrane-enclosed organelles (like bacteria), but still contain genetic material.
Organelles are specialized cell parts with unique functions. The nucleus, the cell's control center, houses DNA (genetic material) in the form of chromatin. When a cell divides, DNA condenses into chromosomes. The nucleolus, located within the nucleus, is responsible for making ribosomes, which are crucial for synthesizing proteins both freely in the cytoplasm and when attached to the endoplasmic reticulum.
The endoplasmic reticulum (ER) acts as a membrane-enclosed passageway for transporting materials, especially proteins synthesized by ribosomes. There are two types: rough ER with ribosomes and smooth ER without. Proteins and other materials emerge from the ER in vesicles and are then received by the Golgi apparatus (or Golgi body), which customizes them into usable forms by folding or adding other substances like lipids or carbohydrates.
Vacuoles are sac-like structures storing various materials; for example, plant cells have a central vacuole for water storage. Lysosomes, found in animal cells, are the 'garbage collectors' that break down damaged or worn-out cell parts using enzymes. Mitochondria are the 'powerhouses' for both animal and plant cells, generating ATP molecules through cellular respiration to provide energy for all cell activities. Cells needing more energy have more mitochondria.
The cytoskeleton maintains the cell's shape, comprising microfilaments (protein threads) and microtubules (thin hollow tubes). Plant cells, being photoautotrophic, contain chloroplasts where photosynthesis occurs, utilizing chlorophyll to capture sunlight. Plant cells also possess a cell wall outside their cell membrane for shape, support, and protection, a feature absent in animal cells.
Some cells have unique structures serving specific functions. Cilia, microscopic hair-like projections, line the human respiratory tract to trap and expel inhaled particles. Flagella, tail-like structures, enable cell movement; the only human cell with a flagellum is a sperm cell.
In summary, eukaryotic cells (plant and animal) have a nucleus and membrane-enclosed organelles, while prokaryotic cells (unicellular organisms) do not. All cells share a cell membrane, cytoplasm, and genetic material. Although only plant cells possess chloroplasts, both plant and animal cells contain mitochondria.