Archive for February 17th, 2013

Diffusion and Osmosis


-the process by which molecules spread from areas of high concentratiion, to areas of low concentration. When the molecules are even throughout a space – it is called EQUILIBRIUM


Concentration gradient – a difference between concentrations in a space.

Molecules will always move down the concentration gradient, toward areas of lesser concentration. Think of food coloring that spreads out in a glass of water, or air freshener sprayed in a room.

Selectively Permeable – membranes that allow some things through, the cell membrane is selectively permeable, water and oxygen move freely across the cell’s membrane, by diffusion

Osmosis – the diffusion of water (across a membrane)

Water will move in the direction where there is a high concentration of solute (and hence a lower concentration of water.

A simple rule to remember is: salt sucks

Salt is a solute, when it is concentrated inside or outside the cell, it will draw the water in its direction. This is also why you get thirsty after eating something salty.


Type of Solutions

Isotonic Solutions

If the concentration of solute (salt) is equal on both sides, the water will move back in forth but it won’t have any result on the overall amount of water on either side.

ISO” means the same


Hypotonic Solutions

The word “HYPO” means less, in this case there are less solute (salt) molecules outside the cell, since salt sucks, water will move into the cell.

The cell will gain water and grow larger. In plant cells, the central vacuoles will fill and the plant becomes stiff and rigid, the cell wall keeps the plant from bursting

In animal cells, the cell may be in danger of bursting, organelles calledCONTRACTILE VACUOLES will pump water out of the cell to prevent this.


Hypertonic Solutions

The word “HYPER” means more, in this case there are more solute (salt) molecules outside the cell, which causes the water to be sucked in that direction.

In plant cells, the central vacuole loses water and the cells shrink, causing wilting.

In animal cells, the cells also shrink.

In both cases, the cell may die.

This is why it is dangerous to drink sea water – its a myth that drinking sea water will cause you to go insane, but people marooned at sea will speed up dehydration (and death) by drinking sea water.

This is also why “salting fields” was a common tactic during war, it would kill the crops in the field, thus causing food shortages.


Diffusion and Osmosis are both types of PASSIVE TRANSPORT – that is, no energy is required for the molecules to move into or out of the cell.

Sometimes, large molecules cannot cross the plasma membrane, and are “helped” across by carrier proteins – this process is called facilitated diffusion.



The Theory of Evolution

In science, theories are statements or models that have been tested and confirmed many times.

Theories have some important properties:

  • They explain a wide variety of data and observations
  • They can be used to make predictions
  • They are not absolute, they serve as a model of understanding the world and can be changed as the world view changes

In science, the term “Theory” does not express doubt.

In science, the term theory is used to represent ideas and explanations that have been confirmed through tests and observations

The theory of evolution remains one of the most useful theories in biology because it explains many questions and observations.

Some questions that can be answered by evolution.

  • Why do so many different animals have the same structures, the arm bones in a human are the same bones as a flipper in a whale?
  • Why do organisms have structures they no longer use, like the appendix in a human? Non functioning wings in penguins
  • Why are there bones and fossil evidence of creatures that no longer exist? What happened to these creatures?
  • Why do so many organisms’ morphology and anatomy follow the same plan?
  • Why is the sequence of DNA very similar in some groups of organisms but not in others?
  • Why do the embryos of animals look very similar at an early stage?
The Theory of Evolution is considered a Unifying Theory of Biology, because it answers many of these questions and offers and explanation for the data.

Lamarke’s Theory of Acquired Characteristics

Some thought that you would gain or lose features if you overused or didn’t use them, and you could pass these new traits onto your offspring.

This was known as the Inheritance of Acquired Characteristics

  • A lizard that didn’t use it legs would eventually not have legs and its offspring wouldn’t have legs
  • A giraffe stretched its neck to reach higher leaves, and this stretched neck would be a trait inherited by its offspring

Lamarke’s Theory was eventually discarded – PROVEN TO BE WRONG!

Why? Logically it doesn’t work. Imagine if you were in a car accident and had a leg amputed. This does not mean that your children will only have one leg. Features gained during life are not passed on to children.

Darwin’s Theory of Evolution by Natural Selection

Darwin was a naturalist who observed many species. He is famous for his trips to the Galapagos Islands, his observations of the finches (and other animals) and the book he wrote: “The Origin of Species:

1. Variation exists among individuals in a species.
2. Individuals of species will compete for resources (food and space)
3. Some competition would lead to the death of some individuals while others would survive
4. Individuals that had advantageous variations are more likely to survive and reproduce.

This process he describes came to be known as Natural Selection
The favorable variations are called Adaptations

Darwin’s Finches: 

Darwin noted that all the finches on the galapagos island looked about the same except for the shape of their beak. His observations lead to the conclusion that all the finches were descendents of the same original population. The shape of the beaks were adaptations for eating a particular type of food (Ex. long beaks were used for eating insects, short for seeds)

Evidence of Evolution

1. Fossil Evidence

  • If today’s species came from ancient species, the we should be able to find remains of those species that no longer exist.
  • We have tons of fossils of creatures that no longer exist but bear striking resemblance to creatures that do exist today.
  • Carbon dating–gives an age of a sample based on the amount of radioactive carbon is in a sample.
  • Fossil record-–creates a geologic time scale.

2. Evidence from Living Organism

  • Evidence of Common Ancestry –Hawaiin Honeycreeper
  • Homologous Structures–structures that are embryologically similar, but have different functions, the wing of a bird and the forearm of a human
  • Vestigial Organs–seemingly functionless parts, snakes have tiny pelvic and limb bones, humans have a tail bone
  • Biochemistry and DNA
  • Embryological development–Embryos of different species develop almost identically
  • Observation of species change (wolves/dogs, peppered moths)