Summary
Highlights
Cells are the smallest living units of an organism. All cells share three fundamental components: a cell membrane, cytoplasm, and DNA. Cells are broadly categorized into two types: eukaryotic cells, which are advanced and complex with organelles like a nucleus (found in plants and animals), and prokaryotic cells, which are simpler, typically unicellular organisms like bacteria, lacking a nucleus and membrane-bound organelles.
Organelles are specialized cellular components, with the nucleus acting as the cell's control center. It contains DNA (genetic material) in the form of chromatin, which condenses into chromosomes during cell division. The nucleolus within the nucleus is responsible for producing ribosomes, which then exit the nucleus to synthesize proteins.
Ribosomes, along with other organelles, are suspended in the cytoplasm. Some ribosomes are free, while others attach to the endoplasmic reticulum (ER). There are two types of ER: rough ER (with ribosomes) and smooth ER (without ribosomes). The ER is a passageway for transporting materials, including proteins synthesized by ribosomes. These materials then move to the Golgi apparatus (or Golgi body) in vesicles, where proteins are customized, folded, or modified with lipids or carbohydrates for cellular use.
Vacuoles are sac-like structures for storing various materials, such as water in plant cells. Lysosomes, found in animal cells, act as garbage collectors, breaking down damaged or worn-out cell parts with enzymes. Mitochondria are the powerhouses of both animal and plant cells, generating ATP molecules through cellular respiration to provide energy for all cell activities. Cells requiring more energy have more mitochondria.
The cytoskeleton, composed of microfilaments and microtubules, helps maintain cell shape. Photoautotrophic organisms like plants have chloroplasts, where photosynthesis occurs due to the green pigment chlorophyll. Plant cells also possess a cell wall outside their cell membrane, providing shape, support, and protection; animal cells lack a cell wall.
Some cells have unique structures. For example, ciliated cells in the human respiratory tract have hair-like projections called cilia that move in waves to trap and expel inhaled particles. Flagella are tail-like structures that help cells move; some bacteria have flagella, and the only human cell with a flagellum is a sperm cell.
In summary, eukaryotic cells (plant and animal) have a nucleus and membrane-bound organelles, while prokaryotic cells (unicellular organisms) lack these. All cells share a cell membrane, cytoplasm, and genetic material. While only plant cells have chloroplasts, both plant and animal cells contain mitochondria.