Cells are the smallest living units of an organism.All cells have three things in commonno matter what type of cell they are.All cells have a cell membrane which separates the inside the cell from its environment,cytoplasm, which is a jelly-like fluid,and DNA which is the cell's genetic material.There are two broad categories of cells.The first category is eukaryotic cells.They have organelleswhich include the nucleus and other special parts.Eukaryotic cells are more advanced, complex cellssuch as those found in plants and animals.The second category is prokaryotic cells.They don't have a nucleus or membrane enclosed organelles.They do have genetic material but it's not contained within a nucleus.Prokaryotic cells are always one celled,or unicellular organisms, such as bacteria.So what are organelles?Organelle means "little organ."Organelles are the specialized parts of a cell that have unique jobs to perform.Let's start with the nucleus, the control center of the cell.The nucleus contains DNA or genetic material.DNA dictates what the cell is going to doand how it's going to do it.Chromatin the tangled, spread out form of DNA found inside the nuclear membrane.When a cell is ready to divideDNA condenses into structures known as chromosomes.The nucleus also contains a nucleolus,which is a structure where ribosomes are made.After ribosomes leave the nucleusthey will have the important job of "synthesizing",or making, proteins.Outside the nucleus the ribosomes and the rest of the organellesfloat around in cytoplasm, which is the jelly-like substance.Ribosomes may wander freely within the cytoplasmor attach to the endoplasmic reticulum, sometimes abbreviated as ER.There are two types of ER:rough ER has ribosomes attached to itand smooth ER doesn't have ribosomes attached to it.The endoplasmic reticulumis a membrane enclosed passageway for transporting materialssuch as the proteins synthesized by ribosomes.Proteins and other materialsemerge from the endoplasmic reticulumin small vesicleswhere the Golgi apparatus, sometimes called the Golgi bodyreceives them.As proteins move through the Golgi body they're customizedinto forms that the cell can use.The Golgi body does this by folding the proteins into usable shapes.or adding other materials on to themsuch as lipidsor carbohydratesVacuoles are sac-like structures that store different materials.Here, in this plant cell, the central vacuole stores water.Going back to the animal cell,you'll see an organelle called a lysosome.Lysosomes are the garbage collectorsthat take in damaged or worn out cell parts.They are filled with enzymes that break down this cellular debris.The mitochondrion in is an organelle that is the powerhouse for bothanimal and plant cells.During a process called cellular respirationthe mitochondria make ATP moleculesthat provide the energy for all the cells activities.Cells that need more energyhave more mitochondria.Meanwhile the cell maintains its shapethrough a cytoskeleton.The cytoskeleton includes the thread-like microfilamentswhich are made of proteinand microtubules which are thin hollow tubesSome organismssuch as plans that are photoautotrophicmeaning they capture sunlight for energyhave cells with an organelle called a chloroplast.The chloroplast is where photosynthesis happensIt's green because it has a green pigment calledchlorophyll.Plant cells also have a cell wall outside of their cell membranesthat shape, support, and protect the plant cell.Animal cells never have a cell wallThere are many other unique structures that only some cells have.Here are just a few.In humans, for example, the respiratory tract is lined with cells that have cilia.These are microscopic hair-like projectionsthat can move in waves.This feature helps trap inhaled particles in the air and expels them when you cough.Another unique feature in some cells is flagella.Some bacteria have flagella.A flagellum is like a little tail that can help a cell move or propel itself.The only human cell that has a flagellumis a sperm cell.In summary remember:eukaryotic cells are plant and animal cells with a nucleus and membrane-enclosed organellesWhile prokaryotic cells are unicellular organisms without these things.All cells have a cell membrane, cytoplasm and genetic material.And even though only plant cells have chloroplastsboth plant and animal cells have mitochondria.