GCSE Biology Edexcel B2 Topic 3

I made these revision notes myself. I got an A* in Biology GCSE and 77 UMS in the B2 exam using only these notes.

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B2 Topic 3 Common systems
Fossils and evolution
Fossils are the preserved traces of organisms that lived millions of years ago.
Fossils are formed in 3 ways:
o A buried organism in soft material such as clay decays and leaves a cast/imprint of the organism.
Footprints can be preserved this way.
o When no decay happens due to preservation such as in peat bogs (too acidic), glaciers (too cold) and
sticky tree resin which traps organisms (no oxygen and water). Bacteria cannot live in these
conditions and therefore no decomposition.
o A gradual replacement by minerals in bones, teeth and shells, forming rock like substance. Fossil
remains in the rock.
Fossils in rock layers tell us three things:
o What the creatures/plants looked like
o How long ago they existed: deeper the rock, the older the fossil
o How the creature has evolved. From studying the similarities and differences in differently aged
rocks we can see how species have changed and developed over billions of years.
Fossil record is incomplete
There are many gaps in the fossil record due to a number of reasons:
o Soft tissues decay so soft-bodied organisms leave little fossil evidence behind.
o Other organisms did not form fossils because the hard parts were destroyed.
o Not all fossils have been found.
This means scientists have to interpret how organisms have changed over time using incomplete data.
An example of the effect of this is the Megalosaurus fossils. In 1954 scientists believed it to have a huge
head and walked on four legs. This is because not all of the bones were found.
In our modern interpretation scientists have concluded that it had a small head and walked on two legs. This
was due to better fossil evidence.
The pentadactyl limb
A pentadactyl limb has five digits. Interestingly this bone structure has been found in many species.
The limb on the outside may look different (and have a different function) but on the inside the bone
structure is remarkably similar.
These bones are called homologous structures.
This similarity suggests a common ancestor, many millions of years ago. If these animals had evolved from
different ancestors, it would be highly unlikely they would share the same bone structure.
Growth
Growth in an organism is the increase in the number of cells and cell size. There are three ways growth can be
measured:
o Size: height, width, length, circumference
o Wet mass: the mass of the organism including all the water in the body. It can vary from day to day.
o Dry mass: Mass of an organism with no water in its body. It can only be measured once the organism
is dead. It can be dried out in a hot oven and weighed.

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Percentile charts
Growth charts are used to assess a child's growth over time so that an overall pattern in development can be
seen and any problems highlighted (e.g. obesity, malnutrition, dwarfism etc.)
For example, a baby's growth is monitored after birth to check they are growing normally. They are plotted
onto percentile charts so they can be compared with other babies.
For example a baby in the 75th percentile means 75% of babies his age are lighter than him and 25% are
heavier.…read more

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These help fight disease.
Some types can engulf and digest pathogens (phagocytes). Others (lymphocytes) produce antibodies
(proteins that bind to pathogens to destroy then) and antitoxins to neutralise any toxins produced byt he
microorganisms.
A low WBC count could increase risk of infection while a high count could mean you have an infection or even
leukaemia.
Platelets
Fragments of cells that help with clotting of the blood.
Blood clots at wounds stop blood pouring out and stop microorganisms getting in.…read more

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Here's how the blood flows through the heart:
1. Vena cava brings deoxygenated blood from the whole body into the right atrium.
2. The right atrium contracts and forces the blood into the right ventricle, through the valves. Valves
prevent the back flow of blood.
3. When the right ventricle is full of blood the muscles in its wall contract, forcing the blood through
more valves through the pulmonary artery. This carries deoxygenated blood to the lungs where it
picks up oxygen.
4.…read more

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Oesophagus: a muscular tube between the mouth and the stomach. The muscles contract in waves to
squeeze the food down; this is called peristalsis.
3. Stomach: muscular bag that produces acid and enzymes (mostly protease). It churns up food with these
juices by peristalsis to make a thick paste.
4. Small intestine: this is where most of the large insoluble food molecules are broken down into smaller
soluble ones. It contains many digestive enzymes made by the pancreas and also makes its own enzyme.…read more

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Circular muscles which surround the gut
o Longitudinal muscles running down the length of the gut.
The muscles squeeze food along which is called peristalsis.
Waves of circular muscle contractions push food along the gut.
Waves of longitudinal muscle contractions run slightly ahead to keep food in a ball.
Villi
These absorb the soluble molecules of food.
There are millions of villi covering the interior of the small intestine.…read more

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Probiotics
These contain live bacteria such as Lactobacillus and Bifidobacteria which are `friendly'. They are similar to
those found naturally in the gut and produce lactic acid.
They are added to foods such as yoghurt, soya milk and dietary supplements.
They are claimed to improve your digestive system and makes your immune system strong.
However not clear evidence they are helpful: out of 180 health claims made 10 were rejected completely
and there was not enough evidence to support the other 170 claims.…read more

Comments

Swallowtail

A very detailed set of notes with some nice diagrams. The diagram of the heart with the blank labels would be a useful learning tool. The notes cover the digestive system, cells, tissues and organs,blood, evolution and growth so would be useful for a number of different specifications.

SMARTREVISE

Sick

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