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DNA and RNA - reproduction and heredity

Cells and Membranes


A cell is the basic unit of life as we know it. It is the smallest unit capable of independent reproduction. Robert Hooke suggested the name 'cell' in 1665, from the Latin cell a meaning storeroom or chamber, after using a very early microscope to look at a piece of cork.

It is also said that he thought that the rectangular chambers looked like the cells in some monasteries.

What is a cell

The simplest answer is that a cell is a container, like a box or a bottle or a jar. It has an inside and an outside, and a flexible 'bag' in between that sustains the structure and integrity of the cells content. This is called a cell membrane.

The cell theory, put forth in the middle of the 19th century, states that:

This theory still holds true, with the minor exception that viruses, that lack a cell wall are only alive while infecting a cell and they paratise a cell's machinery for reproduction. They are incapable of independent existence.

Most people believe that viruses and also prions are inert, and non living, because they don't metabolize or reproduce when they're outside their host organisms.

Physically cells always have a boundary membrane - the cell membrane, a little like a phospholipid 'plastic' bag enclosing contents within it. Inside the space limited by the membrane there is a controlled and ordered environment.

All living things are made up of one or more cells. Organisms that exist as single cells are called unicellular and organisms that are made up of groups of cells working together are called multicellular.

There are two groups of unicellular organisms (Archaea and Bacteria), and three groups of multicellular organisms (Animals, Fungi and Plants), and one group which contains a mixture of both unicellular and multicellular organisms (the Protista).

All living things are divided into two major groups depending on how their cells are set up, these two groups are the Prokaryotes, and the Eukaryotes.

Prokaryotes do not have a nucleus, mitochondria or any other membrane bound organelles. In other words neither their DNA nor any other of their metabolic functions are collected together in a discrete membrane enclosed area. Instead everything is openly accessible within the cell, though some bacteria have internal membranes as sites of metabolic activity these membranes do not enclose a separate area of the cytoplasm.

Eukaryotes have areas inside the cell separated off from the rest of the cell by membranes, like the cell membrane. These areas include the nucleus, numerous mitochondria and other organelles such as the golgi body, and or chloroplasts within each of their cells.

These areas are made distinct from the main mass of the cells cytoplasm by their own membrane in order to allow them to be more specialised. The nucleus contains all the cell's DNA, the mitochondria are where energy is generated, chloroplasts are where plants trap the suns energy in photosynthesis.

It has been proposed that Chloroplasts and Mitochondria were derived from primitive prokaryotes that were captured by other cells in a symbiotic relationship [add link].

In a sense living things are bags within bags within bags. Organelles are bags within the bag of the cell. Cells within cells if the chloroplasts and mitochondria once had an independent existence. Cells are bags within multicellular structures. Blood and other body fluids that bathe the cells of animals has a very similar composition to the seas from which life originated.

All the Prokaryotes (Bacteria and Archaea) are unicellular, only Eukaryotes:- the Protista, some Fungi and some Plants are multicellular.


All cells have a cell membrane. It is the cell membrane that maintains the integrity of the interior. It has 'pores' or openings through which particular molecules and ions are selectively allowed to pass.

Recent research has shown have that a seemingly ordinary protein called YidC found within the membranes of bacteria serves as a gatekeeper of sorts, allowing into the membrane other proteins essential for the bacteria to live. When YidC isn't present, the bacteria die. This finding surprised scientists who long believed that certain "independent" proteins were able to pass into the membrane on their own.

The new discovery, reported in journal Nature, may suggest a completely new pathway for the translocation of proteins within basic biological units.

Even more startling was the discovery that several other proteins that are remarkably similar to YidC may play similar roles inside mitochondria and in chloroplasts as well. The discovery suggests that bacteria, chloroplasts and mitochondria may all have evolved from a common ancestor.

The membranes of the cell carry out a diverse multiplicity of functions that are essential for life.

Membranes compartmentalize cells and form barriers between different environments. They also move molecules from one part of the cell to another by recognizing elements of molecular structure that indicate where in the cell the molecule belongs.

In addition, membranes play a key role in energy transformations, taking light or chemical energy and converting it into other forms that can be used by the cell. The lipid molecules that make up most of membranes have an affinity for both oil and water; that is, they have both hydrophobic and hydrophilic groups.

They naturally tend to line up in a bilayer with their hydrophilic groups on the outside and their hydrophobic groups on the inside. Embedded in this barrier are complex proteins that serve as molecular ports, allowing certain kinds of molecules to cross the barrier, but not others.

The interconversion of different forms of energy is essential for life and this interconversion usually involves membranes. For example, energy from the sun is transformed by plants into chemical energy that is stored as carbohydrates, which in turn are consumed by animals and transformed into the energy needed for development, motion and thought.

The ability of membranes to change energy from one form to another depends on their unique structure.

Proteins designed to transport molecules are aligned in the membrane so that they can generate concentration gradients; if the molecules are ions, then a trans-membrane electrical gradient is also generated.

The trans-membrane electrical potential difference, referred to as a membrane potential, can be used to perform various tasks.

Energy released by discharging an ionic or chemical gradient can be used to synthesize new compounds or to drive cellular processes.

At the same time, the gradient is continually regenerated through respiration or the input of light or chemical energy, just as a battery can be continually recharged. Often the energy released when a gradient is broken down is stored as chemical energy by the high energy molecule, ATP, while the generation of an ionic gradient is driven by the breakdown of ATP.

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