# Cell Biology/Introduction/What is a cell

Cells are structural units that make up plants and animals; also, there are many single celled organisms. What all living cells have in common is that they are small 'sacks' composed mostly of water. The 'sacks' are made from a phospholipid bilayer membrane. This membrane is semi-permeable (allowing some things to pass in or out of the cell while blocking others). There exist other methods of transport across this membrane that we will get into later.
So what is in a cell? Cells are 90% fluid (called cytoplasm) which consists of free amino acids, proteins, carbohydrates, fats, and numerous other molecules. The cell environment (i.e., the contents of the cytoplasm and the nucleus, as well as the way the DNA is packed) affect gene expression/regulation, and thus are VERY important aspects of inheritance. Below are approximations of other components (each component will be discussed in more detail later):

### Elements

* 59% Hydrogen (H)
* 24% Oxygen (O)
* 11% Carbon (C)
* 4% Nitrogen (N)
* 2% Others - Phosphorus (P), Sulphur (S), etc.

### Molecules

* 50% protein
* 15% nucleic acid
* 15% carbohydrates
* 10% lipids
* 10% Other

### Components of cytoplasm

The following is optional reading, as all cell components will be discussed in subsequent chapters.
* Cytosol - contains mainly water and numerous molecules floating in it- all except the organelles.
* Organelles (which also have membranes) in 'higher' eukaryotic organisms:
** Nucleus (in eukaryotes) - where genetic material (DNA) is located, RNA is transcribed.
** Endoplasmic Reticulum (ER) - Important for protein synthesis. It is a transport network for molecules destined for specific modifications and locations. There are two types:
*** Rough ER - Has ribosomes, and tends to be more in 'sheets'.
*** Smooth ER - Does not have ribosomes and tends to be more of a tubular network.
** Ribosomes - half are on the Endoplasmic Reticulum, the other half are 'free' in the cytosol, this is where the RNA goes for translation into proteins.
** Golgi Apparatus - important for glycosylation, secretion. The Golgi Apparatus is the "UPS" of the cell. Here, proteins and other molecules are prepared for shipping outside of the cell.
** Lysosomes - Digestive sacks found only in animal cells; the main point of digestion.
** Peroxisomes - Use oxygen to carry out catabolic reactions, in both plant and animals. In this organelle, an enzyme called catalase is used to break down hydrogen peroxide into water and oxygen gas.
** Microtubules - made from tubulin, and make up centrioles,cilia,etc.
** Cytoskeleton - Microtubules, actin and intermediate filaments.
** Mitochondria - convert foods into usable energy. (ATP production) A mitochondrion does this through aerobic respiration. They have 2 membranes, the inner membranes shapes differ between different types of cells, but they form projections called cristae. The mitochondrion is about the size of a bacteria, and it carries its own genetic material and ribosomes.
** Vacuoles - More commonly associated with plants. Plants commonly have large vacuoles.
* Organelles found in plant cells and not in animal cells:
** Plastids - membrane bound organelles used in storage and food production. These are similar to entire prokaryotic cells - for example, like mitochondria they contain their own DNA and self-replicate. They include:
*** Chloroplasts - convert light/food into usable energy. (ATP production)
*** Leucoplasts - store starch, proteins and lipids.
*** Chromoplasts - contain pigments. (E.g. providing colors to flowers)
** Cell Wall - found in prokaryotic and plant cells; provides structural support and protection.