Cells can be thought of as tiny packages that contain minute factories, warehouses, transport systems, and power plants.
They function on their own, creating their own energy and self-replicating — the cell is the smallest unit of life that can replicate.
However, cells also communicate with each other and connect to create a solid, well stuck-together animal. Cells build tissues, which form organs; and organs work together to keep the organism alive. Robert Hook first discovered cells in Different cell types can look wildly different, and carry out very different roles within the body. For instance, a sperm cell resembles a tadpole, a female egg cell is spherical, and nerve cells are essentially thin tubes.
Despite their differences, they often share certain structures; these are referred to as organelles mini-organs. Below are some of the most important:. There is normally one nucleus per cell, but this is not always the case, skeletal muscle cells, for instance, have two.
The nucleus sends out messages to tell the cell to grow, divide, or die. The nucleus is separated from the rest of the cell by a membrane called the nuclear envelope; nuclear pores within the membrane allow through small molecules and ions, while larger molecules need transport proteins to help them through.
To ensure each cell remains separate from its neighbor, it is enveloped in a special membrane known as the plasma membrane. This membrane is predominantly made of phospholipids, which prevent water-based substances from entering the cell. The plasma membrane contains a range of receptors, which carry out a number of tasks, including being:. The cytoplasm is the interior of the cell that surrounds the nucleus and is around 80 percent water; it includes the organelles and a jelly-like fluid called the cytosol.
Many of the important reactions that take place in the cell occur in the cytoplasm. Both lysosomes and peroxisomes are essentially bags of enzymes. Lysosomes contain enzymes that break down large molecules, including old parts of the cells and foreign material. Peroxisomes contain enzymes that destroy toxic materials, including peroxide. The cytoskeleton can be considered the scaffolding of the cell. It helps it maintain the correct shape.
However, unlike regular scaffolding, the cytoskeleton is flexible; it plays a role in cell division and cell motility — the ability of some cells to move, such as sperm cells, for instance. The cytoskeleton also helps in cell signaling through its involvement in the uptake of material from outside the cell endocytosis and is involved in moving materials around within the cell. The endoplasmic reticulum ER processes molecules within the cell and helps transport them to their final destinations.
In particular, it synthesizes, folds, modifies, and transports proteins. The ER is made up of elongated sacs, called cisternae, held together by the cytoskeleton. There are two types: rough ER and smooth ER. Once molecules have been processed by the ER, they travel to the Golgi apparatus. The Golgi apparatus is sometimes considered the post office of the cell, where items are packaged and labeled.
Once materials leave, they may be used within the cell or taken outside of the cell for use elsewhere. Often referred to as the powerhouse of the cell, mitochondria help turn energy from the food that we eat into energy that the cell can use — adenosine triphosphate ATP. From Genetics Home Reference.
Human cells contain the following major parts, listed in alphabetical order: Cytoplasm Within cells, the cytoplasm is made up of a jelly-like fluid called the cytosol and other structures that surround the nucleus. Endoplasmic reticulum ER This organelle helps process molecules created by the cell. Golgi apparatus The Golgi apparatus packages molecules processed by the endoplasmic reticulum to be transported out of the cell.
Lysosomes and peroxisomes These organelles are the recycling center of the cell. Mitochondria Mitochondria are complex organelles that convert energy from food into a form that the cell can use. Plasma membrane The plasma membrane is the outer lining of the cell. What is DNA?
What is a gene? What is a chromosome? How many chromosomes do people have? What is noncoding DNA? Interphase is the period between successive cell divisions. It is the longest part of the cell cycle. The successive stages of mitosis are prophase , metaphase , anaphase , and telophase. Cytokinesis, division of the cytoplasm, occurs during telophase. Meiosis is a special type of cell division that occurs in the production of the gametes, or eggs and sperm.
These cells have only 23 chromosomes, one-half the number found in somatic cells, so that when fertilization takes place the resulting cell will again have 46 chromosomes, 23 from the egg and 23 from the sperm.
Proteins that are synthesized in the cytoplasm function as structural materials, enzymes that regulate chemical reactions, hormones, and other vital substances. DNA in the nucleus directs protein synthesis in the cytoplasm. A gene is the portion of a DNA molecule that controls the synthesis of one specific protein molecule.
Messenger RNA carries the genetic information from the DNA in the nucleus to the sites of protein synthesis in the cytoplasm.
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