A-level Biology/Biology Foundation/cell structure

Cell theory

The cell theory in biology states three things:

1. All living organisms are composed of smaller subunits called cells.
2. All cells come from prexisting cells.
3. The cell is the smallest organizational unit of a living organism.

The microscope and cells

The term cell was coined in the 1600's by a man by the name of Robert Hooke, who had observed small 'box like' sub units under a simple microscope. These first microscopes were very basic, but allowed for cells to be visible for the first time.

The microscope

The microscope, is a device which allows things not visible by the naked eye to be seen. Over time, microscope technology developed and more complex types emerged. These include; stereo and electron microscopes.

How a light microscope works

A light microscope works by using a series of lenses to maginify an image. Most modern microscopes have an inbuilt lamp or a reflective mirror which provides light for the microscope.

For a better explanation look here: Microscope

Objective lenses

Most modern microscopes contain several objective lenses, which when selected allow for a different maginification. The most common objectives are 4x, 10x and 40x, although some variation may exist. The final objective lense is the eyepeice, most eye peices are either 10x or 12x.

The small 'x' indicates times by, for example 4x means that the detail level is 4 times normal eyesight. This also inidcates that the area is $\frac{1}{4}$ of the total objective area.

Calculating magnification

To calculate the maginfication of the image, it is a simple process of mutiplying the objective lenses together.

For example, an objective lense of 4x and a 10x eyepiece would give a total maginification of 4 x 10 = 40x maginification.

Electron Microscopes: Have a closer look

One major improvement in microscopy was the invention of the electron microscope. These used a stream of electrons to magnify an image and allowed a much higher resolution to be viewed. Most electron microscopes can view objects at 10 000 times magnification or higher. One down side related to this type of microscope is that they can only be used to view dead structures.

Types of Cells

There are two very broad categories of cells used in biology.

Prokaryotes

Are the most simple cells, with no distinct nucleus or organelles. Kingdom Monera (bacteria) are the only prokaryotes in existance.

Eukaryotes

Eukaryotes are more complex cells than prokaryotes, and contain a distinct nucleus and organelles. All the other four kingdoms in biology (Protista, Fungi, Animalia and Plantae) contain eukaryotic cells. They are much larger (in volume)than prokaryotic cells, typically around 1000 to 10000 times larger. Scientists now believe that eukaryotic cells evolved from prokaryotic cells millions of years ago.

Cell organelles

All cells have three common elements:

A plasma membrane, a cytoplasm and genetic information.

Plasma membrane

The plasma membrane (or cell surface membrane) is one of the most important structures in the entire cell. It is described as a ""partially permeable membrane"" It allows for products to enter and exit the cell, via diffusion, osmosis and active transport. This membrane consists of two layers of lipid molecules in a distinct pattern called a phospholipid bilayer. This layer also contains cholesterol, receptor molecules and transport proteins.

Cholesterol

Regulates the fluidity of the cell membrane by 'plugging' in between the phospholipids.

Receptor molecules

Essentially glycoprotein molecules which act as the 'antennae' of cells. They may bind to extracellular chemicals and also plays a role in cell adhesion during tissue formation.

Channel proteins

Enable the passage of hydrophilic and polar molecules across the plasma membrane.

Carrier proteins

Enable larger molecules to travel across the plasma membrane travel through facilitated transport or active transport.

The cytoplasm

The cytoplasm is the jelly-like substance found within cells. This acts both as a medium of transport within the cell and also helps the cell maintain its structural integrity. Note: this is technically not an organelle, but will be listed here anyhow, for ease.

Genetic information

Usually DNA, in Eukaryotic cells, the information is contained in the Nucleus, in Prokaryotic cells the DNA lies in circular strands in the cytoplasm.

---

Some other elements are not found in prokaryotic cells however, these may include:

Mitochondria

Mitochondria carry out the later stages of aerobic respiration in cells. they produce ATP (the basic energy unit of cells) as a result of the respiration process. Mitochondria also carry out lipid synthesis.

Golgi apparatus (or Golgi body)

The golgi apparatus collects, sorts and processes molecules. These are then transported by the golgi vesicles to other parts of the cell or extcreted out of the cell (exocytosis).

Rough endoplasmic recticulum (RER)

Have ribosomes attached to the cytosolic side of their membrane. Found in cells that are making proteins for export (enzymes, hormones, structural proteins, antibodies). Involved in protein synthesise. Modifies proteins by the addition of carbohydrates, removal of signal sequences. Phospholipid synthesis and assembly of polypeptides

Smooth endoplasmic reticulum (SER)

Have no ribosomes attached and often appear more tubular than the rough endoplasmic reticulum. Necessary for steroid synthesis, metabolism and detoxification, lipid synthesis. Numerous in the liver

Prokaryotes	Eukaryotes
unicellular	multicellular
May have cell wall (made of peptidoglycan)	Cell walls present in plant cells(made of cellulose) fungi (made of chitin)
no distinct nucleus, circular DNA free in cytoplasm	distinct nucleus, linear DNA
no membrane bound organelles	membrane bound organelles present

Drawing biological diagrams

There are a few key features of all biological drawings:

1. They contain no shading
2. They contain bold and distinct lines
3. They include a magnification
4. They are drawn in the correct proportions