It only takes one biological cell to create an organism. In fact, there are countless species of single celled organisms, and indeed multi-cellular organisms like ourselves.
A single cell is able to keep itself functional by owning a series of 'miniature machines' known as organelles. The following list looks at some of these organelles and other characteristics typical of a fully functioning cell. The italic links for each lead to an extra description in the dictionary, as do all similar links in the tutorials;
An important cell organelle involved in respiration
A fluid surrounding the contents of a cell and forms a vacuole
- Golgi Apparatus
The processing area for the creation of a glycoprotein
- Endoplasmic Reticulum
An important organelle heavily involved in protein synthesis.
Packages of substances that are to be used in the cell or secreted by it.
The "brain" of a cell containing genetic information that determines every natural process within an organism.
- Cell Membrane
Also known as a plasma membrane, this outer layer of a cell assists in the movement of molecules in and out the cell plays both a structural and protective role
Membranous sacs that contain digestive enzymes
A structure that characteristically is found in plants and prokaryotes and not animals that plays a structural and protective role.
Cells can become specialised to perform a particular function within an organism, usually as part of a larger tissue consisting of many of the same cells working in tandem, for example;
- Nerve cells to operate as part of the nervous system to send messages back and forth via the brain at the centre of the nerve system.
- Skin cells for waterproof protection and protection against pathogens in the open air environment.
- Xylem tubes to transport water around plants and to provide structural support for the plant as a whole.
Cells combine their efforts in these tissue types to perform a common cause. The task of the specialised cell will determine in what way it is going to be specialised, because different cells are suited to different purposes, as illustrated in the above list and below example;
- Muscle cells are long and smooth in structure and their elastic nature allows these cells to perform flexible movements, just as they do in our own body's.
- Some white blood cells contain powerful digestive enzymes to eliminate pathogens by breaking them down to the molecular level.
- Cells at the back of the eye are sensitive to light stimuli, and thus can interpret differences in light intensity which can in turn be interpreted by our nervous system and brain.
Many of these cells contain organelles, though after some cells are specialised, they do not possess particular characteristics as they do not require them to be there. i.e. efficiency is the key, no resources are wasted and the resources available are put to their idyllic optimum.
The Cell Membrane
The cell membrane, otherwise known as the plasma membrane is a semi-permeable structure consisting mainly of phospholipid (fat) molecules and proteins. They are structured in a fluid mosaic model, where a double layer of phospholipid molecules provide a barrier accompanied by proteins.
It is present round the circumference of a cell to acts as a barrier, keeping foreign entities out the cell and its contents (like cytoplasm) firmly inside the cell.
The plasma membrane allows only selected materials to pass in and out of a cell, and is thus known as a selectively permeable membrane. There are a number of methods that allow the exchange of materials in and out the cell possible, mentioned below.
There are three methods in which ions are transported through the cell membrane into the cell,
- Active Transport - Active transport is the transport of molecules with the active assistance of a carrier that can transport the material against a natural concentration gradient.
- Passive Transport (Diffusion) - The movement of molecules from areas of high concentration (i.e. outside a cell) to areas of low concentration (i.e. within a cell) via a carrier. This process does not require energy.
- Simple Diffusion - The movement of molecules from areas of high concentration to areas of low concentration in a free state. Osmosis of water involves this type of diffusion through a selectively permeable membrane (i.e. plasma membrane)
The Breakdown of Materials in a Cell
In cells, sometimes it is required to breakdown more complex molecules into more simple molecules, which can then be 're-built' into what is needed by the body with these new raw materials.
'Pinocytosis' where to contents of a structure (such as bacteria) are drank, essentially by breaking down molecules into a drinkable form.
'Phagocytosis' where contents are 'eaten'. See cell defence for more information in regards to this.
Absorption and Secretion
Absorption is the uptake of materials from a cells' external environment. Secretion is the ejection of material.
This page is designed to give you an introductory overview of a single cell. The continuing cell biology tutorial elaborates on the concepts mentioned here, and will give you a fuller understanding of the biological cell at work.