All living organisms on our planet are comprised of the microscopic building blocks of life called cells. A cell is the smallest unit of an organism that can carry on the activities of life. These activities include maintaining homeostasis, regulating metabolism, reproduction and heredity. Cells are divided into two subcategories: animal and plant cells, which are similar but simultaneously unique to each other. Both cell types have small organelles within them that each carry out a specific duty to ensure the organism thrives. An organelle is a membrane-bound structure that is responsible for a specific cell function.
Cell Membrane, Cell Wall and Cytoplasm
The cell membrane (aka plasma membrane or lipid bilayer) is shaped by the interaction between the phospholipids of the cell membrane and the water found inside and outside the cell. The plasma membrane performs two basic functions: it acts as a selective barrier that allows only certain substances through, and it provides a flexible shape to the cell. Although not an organelle, the cell wall, just above the cell membrane, makes up the outermost surface layer of a plant cell. It is made of cellulose, and its rigid structure gives support to the plant cell. The cell contains cytoplasm, in which all organelles float.
Nucleus and Nucleolus
The nucleus oversees all cell activity by storing and copying genetic information. The nucleolus is an organelle found inside the nucleus and is responsible for making RNA (ribonucleic acids) and proteins.
Microtubules, Microfilaments and Ribosomes
Microtubules and microfilaments are straws of proteins that make up the cytoskeleton of the cell, which is responsible for organizing and maintaining a cell's shape. There are two classes of ribosomes: those attached to the endoplasmic reticulum and those that float freely. The ribosomes attached to the endoplasmic reticulum produce proteins to be inserted into the cell membrane or exported from the cell to use in other parts of the organism. Contrastingly, free floating ribosomes float freely in the cytoplasm and produce proteins that are used in the cytosol (the liquid portion of the cytoplasm).
The endoplasmic reticulum is a system of membranous sacs and tubules that function in biosynthesis, modification and transportation. It is divided into two subcategories: the smooth and the rough endoplasmic reticulum. The smooth ER is involved in the production of lipids, whereas the rough ER contains ribosomes and is involved in the production of proteins.
The mitochondria is an organelle specific to animal cells and is the site of cellular respiration or the organism's power production center. It transforms nutrients (digested into glucose) and oxygen into energy-storing molecules using sunlight, a reaction that yields the byproducts carbon dioxide and water. Contrastingly, the chloroplast belongs to only plant cells and is the site of photosynthesis. It converts sunlight, water and carbon dioxide into glucose and oxygen, which is the opposite chemical reaction of cellular respiration.
Golgi Body and Lysosomes
The golgi body processes, packages and secretes substances from the cell. Lysosomes are only found in animal cells, protists and fungi; they contain hydrolytic enzymes that are responsible for the breakdown (digestion) of organic molecules (proteins, carbohydrates, lipids, nucleic acids) and old organelles in the cell as well as bacteria and viruses that have entered the cell.
Centrioles and Vacuoles
Centrioles are unique to animal cells and aid in the separation of genetic material to the proper cells during cell division. Vacuoles mainly store water in the cells.