Antibiotics | Biology A-Level Revision

• In normal cells water enters by osmosis - too much water, the cells burst (osmotic lysis)
• Bacterial cells have a tough cell wall which prevents the cell from expanding
• Antibiotics stop the bacterial cell wall from forming correctly so the wall is weaker. 
• This weakened cell wall means that the cell can burst due to osmotic lysis so the bacteria doesn't spread

Although there's a new problem, antibiotic resistance:

• Shortly after antibiotics were discovered, small groups of people were developing resistance due to a chance mutation. 
• The mutation caused the production of a new enzyme which broke down the penicillin. 
• The gene that coded for this mutation then is passed to the next generation (through vertical gene transmission) 
• The antibiotic resistance gene carries plasmids so it can pass onto other bacterial species via horizontal gene transmission. 

Why do bacteria need to adapt?

Similarly to what Darwin said, only the fittest survive, so to increase their chances of survival, they need to adjust to suit their environment. What this does mean however is that antibiotics can quickly become useless as the bacteria adapt to become resistant to them.

Conjugation

1. A donor cell produces a thin conjugation tube between its cell and the recipient cell. 
2. The donor cell replicates its plasmids
3. The plasmid is unwound so it's now straight and can fit through the conjugation tube
4. The replicated plasmid moves through the conjugation tube and enters the recipient cell
5. The plasmid which has just entered the recipient cell now returns to a circular shape
6. The bacteria has now replicated. 

Key terms:

1. Mutations: A random change in the quantity or structure of DNA (one base is added,deleted or replaced)
2. Conjugation: The fastest method of evolution where DNA of existing individuals is combined - horizontal gene transmission
3. Plasmids: A circular piece of DNA

Vertical Gene Transmission - DNA passed to the next generation of the SAME species

Horizontal Gene Transmission - DNA passed to different species via CONJUGATION

What are Fertilisers?

I'm sure we've all either used fertilisers or been around your parents when they've been using fertilisers. Fertilisers provide crops with chemicals needed for growth, like nitrates. This is because crops use up minerals as they grow and the soil can't usually supply enough minerals to fulfil demand.

There are two main types of fertilisers - natural and artificial. Natural fertilisers are made up of organic matter - like manure and sewage sludge. These don't have a set combination of minerals - there's no 'recipe' pretty much just because they're natural.

Artificial fertilisers are man made using inorganic material - unlike natural fertilisers there is a 'recipe' and the recipe depends purely on what you're growing and where you're growing it. These contain pure chemicals like ammonium nitrate in the form of powders or pellets.

Whether they're artificial or natural, they provide crops with the minerals they need for growth, like nitrates. Over time, this level needs to be replenished because they use up the minerals as they grow. The job then falls to the farmers who decide how much to use, too much is too expensive, too little and it would get washed away before it had an affect.  

When fertilisers are washed off of fields (leached), this causes the problem of Eutrophication. Excess fertiliser causes an algal bloom, this blocks light, in turn, it reduces photosynthesis. This means that submerged plants can't carry out photosynthesis, they can only respire so die. Saprobiotic microorganisms aerobically respire, over time the oxygen supply is depleted and fish also die.

This article specifically relates to AQA A2 Biology Unit 4

AS Physics - Electron Diffraction

(for AQA AS Physics)

• Electron diffraction shows the wave nature of electrons (how electrons can behave like waves)
• Diffraction patterns are observed when accelerated electrons in a vacuum tube interact with the spaces in a graphite crystal - this confirms electrons show wavelike properties
• According to the wave theory - the spread of the lines in the diffraction pattern increases if the wavelength of the wave is greater
• In experiments: a smaller accelerating voltage (the electrons are slower which gives widely spaced rings
• Increase in the electron speed - the diffraction pattern circles squash together towards the middle. (fits with the de Broglie equation  - if velocity is higher the wavelength is shorter and the spread of the lines smaller)

Wavelength for electrons accelerated in a vacuum tube is about the same size as electromagnetic waves in the X-ray part of the spectrum

A level Physics: Wave Particle Duality

• Light is part of the electromagnetic spectrum
• The theory of the electromagnetic waves predicted the existence of waves beyond visible light (before we knew about x-rays and so on)
• The discovery of x rays and radio waves confirmed their existence (and their predictions)
• Many scientists in the late 19th Century thought that all aspects of physics could be explained through Newton's laws of motion. 

Interference and Diffraction show light as a wave

• Light produces interference and diffraction patterns - these are alternating bands of dark and light
• Can only be explained using waves interfering constructively or interfering destructively

Constructive Interference - when two waves overlap in phase

Destructive interference - when two waves are out of phase

Although - Particles DON'T show wave-like properties all the time

Diffraction only happens if a particle interacts with an object about the same size as the de Broglie wavelength

de Broglie came up with the wave-particle duality theory

shorter wavelength = less diffraction effects (this fact is used in electron microscopes)
Diffraction effects blur detail on an image 

(specifically for AQA A-level physics specification A)

A-Level Revision - The Cell Surface Membrane

These revision notes refer to AQA AS Biology Chapter 3.5

 The cell surface membrane is another name for the cell membrane or plasma membrane.
- It surround the cell boundary between the cell cytoplasm and the environment (this is where there is different conditions inside and outside of the cell).

The Fluid Mosaic Model

Why is this called the fluid-mosaic model?
Fluid - The Molecules move relative to one another
Mosaic - means being made up of different substances, in this case proteins and phospholipids.

How do substances pass through a cell membrane?
(a very common exam question)

- the phospholipids form a bilayer
- the phospholipids are arranged with hydrophilic heads on the outside and hydrophobic tails on the inside
- There are two types of proteins, some are passing through and some are confined to one layer
- There are other molecules present, for example: cholesterol or glycoproteins
- substances are moved DOWN the concentration gradient (from a high to low concentration)
- water and ions move through the channel proteins
- lipid soluble molecules are passed between the phospholipids
- carrier proteins are involved in facilitated diffusion.