Surrounding every cell is a permeable membrane made of two layers of phospholipids. This membrane controls the transport of substances into and out of the cell. Water and other substances move naturally from high concentration to low concentration across the membrane by osmosis. However, along the membrane there are proteins that act as pumps to move materials from low concentration to high concentration, a process known as active transport. In plant cells, the cell membrane is surrounded by a rigid cell wall, which helps the cell keep its form and structure.
Inside the cellular membrane is the fluid of the cell, which is known as the cytoplasm. This is where all the organelles reside, and it has the consistency of a soft gel. Held together by the cell membrane, the cytoplasm acts as a medium for transportation and also to give the cell structure and form.
Within every cell is the nucleus, which is the center of control in the cell. The nucleus contains the DNA of the organism, and this is where the body's function get its directions and information. Encased within a nuclear membrane, DNA will come apart to build proteins and other substances the body needs. It is also in charge of reproduction, which leads to cellular division and the replication of new cells. Associated with the nucleus are centrioles, which are proteins that are used to pull apart the nucleus during cell replication.
Everything needs a place to store materials, and vacuoles serve this purpose for cells. Vacuoles are large fluid sacs within the cell where food or waste can be stored for later use or disposal. In animal cells, vacuoles are small and scattered. In plant cells, they are larger because plants tend to store larger amounts of food. Similar structures called vesicles are smaller than vacuoles and are used for transportation of substances within the cell.
Lysosomes are membraned structures within a cell that aid in the breakdown of materials. Lysosomes contain a strong digestive solution to achieve this. They can break down larger food molecules into smaller ones to be utilized by the cell. They can also be used to break down old parts of the cell that are no longer useful as well as harmful intruders such as bacteria.
Ribosomes are small pieces of protein and RNA, which is similar to DNA but used to transport information to make substances. They are used to build larger molecules such as proteins by transcribing the genetic information in the cell. These ribosomes are relatively small but high in numbers. They attach to the endoplasmic reticulum to produce the proteins.
The endoplasmic reticulum, commonly referred to as ER, is a large, tubular organelle that produces large molecules in the cell. The ER also stores and transports the materials it produces There are two main types of endoplasmic reticulum. The smooth ER produces molecules of fatty acids, steroids and lipids. The rough ER produces molecules of proteins. It is said to be rough because ribosomes attach to it with information on protein synthesis, which gives the organelle the appearance of being rough or bumpy.
The golgi apparatus, also known as the golgi body, is a series of tubular structures that are interconnected. When small vesicles transport a protein to the gogli apparatus from the ER, that substance is taken inside and surrounded with a membrane. This allows the substance to be transported to a different location of the cell or even transported out of the cell.
The mitochondria are often called the powerhouse of the cell. This is where energy is produced from sugar molecules. Mitochondria are oval-shaped and have their own membranes, as well as their own DNA inside.
Plant cells contain organelles called chloroplasts. They are similar in size to mitochondria but serve the opposite function. Chloroplasts are essential to photosynthesis because they contain chlorophyll, which is what converts sunlight into stored energy.