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Fossils and evolution
- Fossils are the preserved traces of organisms that lived millions of years ago.
- Fossils are formed in 3 ways:
- A buried organism in soft material such as clay decays and leaves a cast/imprint of the organism. Footprints can be preserved this way.
- 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.
- 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:
- What the creatures/plants looked like
- How long ago they existed: deeper the rock, the older the fossil
- 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
- Usually in GCSE Biology curriculum, we need to know that - there are many gaps in the fossil record due to a number of reasons:
- Soft tissues decay so soft-bodied organisms leave little fossil evidence behind.
- Other organisms did not form fossils because the hard parts were destroyed.
- 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
- In GCSE Biology, by definition, growth in an organism is the increase in the number of cells and cell size. There are three ways growth can be measured:
- Size: height, width, length, circumference
- Wet mass: the mass of the organism including all the water in the body. It can vary from day to day.
- 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.
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.
- There is usually no concern unless the baby’s size is above the 98th percentile or below the 2nd percentile.
- Plants grow throughout their lives. There are three processes which occur:
- Cell differentiation: cells change to become specialised for its job.
- Cell division: by mitosis.
- Cell elongation: when a plant cell expands making the cell bigger so the plant can grow (only in plants).
- Plants have meristems (stem cells) behind the tip of their roots and shoots and these can differentiate into any plant cell. These meristems exist throughout the entire plant’s life.
- This is how old trees can keep growing new branches.
Growth in animals
- Animals tend to grow when they’re young and then they reach their full growth and stop growing.
- This means cell division is at a fast rate when they are young but in adults cell division is for repair.
- Adults do have stem cells but they have a limited range that they can turn into: blood cells and skeletal tissue. This is why most animals cannot regrow a damaged limb or body part but plants can grow new shoots, roots and leaves.
Cells make up tissues, organs and systems
- Tissue: a tissue (e.g. muscle tissue) is a group of similar cells that work together to carry out a particular function.
- Organ: An organ is a group of different tissues that work together to perform a particular function.
- Organ system: An organ system is a group of organs working together to perform a particular function.
Blood
- Blood may be a liquid but it is an organ containing many different types of specialised cells that carry out particular functions in the body.
- Blood is made up of four components: plasma (55%), white blood cells and platelets (<1%) and red blood cells (45%).
Plasma
- This is a yellow liquid which keeps the blood fluid and transports many things in GCSE Biology:
- Red and white blood cells and platelets.
- Nutrients like glucose and amino acids. They are absorbed from the gut and taken to body cells.
- Carbon dioxide – transported by the blood from the cells to the lungs where it is removed.
- Urea – blood transports it to the kidneys where it’s removed.
- Hormones – transported from glands to target organs.
- Antibodies and antitoxins produced by the white blood cells.
White blood cells
- 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.
- Lack of platelets can caused excessive bleeding and bruising.
Red blood cells
- The job of red bloods cells is to carry oxygen around the body. It contains the red pigment haemoglobin which reacts with oxygen to make oxyhaemoglobin.
- In cells the reverse happens so the cells can get oxygen.
- Red blood cells are biconcave discs which means they have dimples on both sides to give a large surface area for absorbing oxygen.
- They also don’t have a nucleus to make more room for haemoglobin in GCSE Biology.
- Anaemia is a lack of iron which means the blood doesn’t carry enough oxygen as haemoglobin contains a lot of iron.

The heart
- The heart consists of four chambers and four major blood vessels.
- Deoxygenated blood goes to the right side and oxygenated blood into the left side. The two sides of the heart are completely separated by the muscular septum.
- In GCSE Biology, blood flows through the heart:
- Vena cava brings deoxygenated blood from the whole body into the right atrium.
- The right atrium contracts and forces the blood into the right ventricle, through the valves. Valves prevent the back flow of blood.
- 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.
- Oxygenated blood returns from the lungs to the left atrium of the heart in the pulmonary vein. When the left atrium is full it contracts and squeezes blood through valves into the left ventricle.
- The ventricle walls then contract and push the blood up more valves into the aorta. The aorta is a large blood vessel that carries oxygenated blood around the body. The wall of the left ventricle is thicker than the right as it needs more muscle to pump blood around the whole body whereas the right ventricle needs only pump to the lungs.
Blood vessels
- In GCSE Biology, there are 3 types of blood vessels:
- Arteries –carry blood away from the heart
- Capillaries – are involved in the exchange of materials
- Veins –carry the blood to the heart
Arteries
- The heart pumps the blood out at high pressure so the artery walls are strong and elastic.
- The walls are thick compared to its lumen. They contain thick layers of muscles to make them strong.
Capillaries
- Arteries branch into capillaries.
- Very small with very thin walls to allow substances to diffuse in and out of the blood.
- Carry blood close to cells to exchange substances with them.
- One-cell thick walls.
Veins
- Capillaries join up to form veins.
- The blood is at lower pressure so the walls don’t need to be as thick as artery walls.
- Bigger lumen to help the blood flow despite lower pressure.
- Valves to help keep the blood flowing in the right direction.
The Digestive System
This is commonly asks in the GCSE Biology:
- Food contains a lot of large, insoluble molecules that need to be broken down into smaller soluble molecules. This is so they can be absorbed and used by the body.
- Digestion is the process of breaking down food which takes place in the organ system called the digestive system.
- The digestive system is made up of the alimentary canal and other organs providing chemicals for digestion.
- Mouth: food is ingested here. Mechanical digestion occurs where the teeth break up the food into smaller pieces to increase SA. This is so digestive enzymes have more area to work on. The tongue forms the chewed food into a bolus, which gets lubricated with saliva, making it easier to swallow.
- a muscular tube between the mouth and the stomach. The muscles contract in waves to squeeze the food down; this is called peristalsis.
- muscular bag that produces acid and enzymes (mostly protease). It churns up food with these juices by peristalsis to make a thick paste.
- 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. The molecules of food are absorbed by villi which contain capillaries. Food is moved along by peristalsis.
- produces digestive enzymes and releases than into the duodenum (first part of small intestine).
- Large intestine: undigested food passes into this wide, thin walled tube. Water diffuse back into the blood leaving faeces behind.
- egestion occurs here (undigested food passed out of the body.
- digested food is absorbed and dissolves in blood plasma. Once in the blood it is taken to the liver via the hepatic portal vein to be processed. Some molecules are broken down furthers and others built up again. The liver also makes bile which helps digestion of fats and neutralisation of stomach acid.
- Gall bladder: stores bile and releases when needed.

Enzymes in digestive system
- In GCSE Biology, three main types of digestive enzyme:
- Protease (digests proteins)
- Carbohydrase (digests carbohydrates)
- Lipase (digests fats)
Digesting carbohydrates
- Food like bread and pasta are full of large carbohydrates. They simplest carbohydrates are called sugars (glucose etc.) and they can be built up to more complex carbohydrates such as starch.
- Carbohydrases break down carbohydrates.
- Amylase breaks down starch into sugars (i.e. maltose). Amylase is present in saliva and secreted from the pancreas.
Digesting proteins
- Proteases digest proteins, breaking them down into shorter chains and then amino acids.
- Pepsin is a protease that is produced in the stomach and works best in acidic conditions e.g. pH 2-3. As the stomach walls produce acid this is the optimum place for pepsin.
- However, the contents of the small intestine are alkaline so the proteases released into the small intestine work best around pH 8 (i.e. trypsin).
Digesting fats
- Lipases digest fats into fatty acids and glycerol.
- Fat and water don’t mix so at first the fats form large globules in the watery digestive juices.
- These globules have a very small SA to volume ration so lipases can only digest the fats slowly. Bile helps by physically breaking down the large globules into tiny droplets forming an emulsion. We say bile emulsifies fats.
- As the smaller droplets have a larger SA they can be broken down much more quickly.
- More bile is produced after a meal.
- Bile is alkaline when released so it helps neutralise the acidic contents of the stomach when it enters the small intestine. This is so the protease enzymes have a slightly alkaline environment to work in.
Peristalsis
- There is a muscular tissue all the way down the digestive system:
- Circular muscles which surround the gut
- 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. Villi massively increase the surface area of the small intestine so more diffusion of substances can take place.
- Each villus has a good network of blood capillaries. A steep concentration gradient is created as there is a lower concentration of soluble food molecules in the blood than the small intestine. This means diffusion takes place much faster.
- There is only a single layer of cells between the contents of the small intestine and blood vessels in the villus. There is a very short distance in which molecules need to diffuse so diffusion is quicker.
- All of these things makes the diffusion of food molecules as efficient as possible.
Evidence for importance of villi
- This is common in GCSE Biology that people with coeliac disease lose villi and cannot absorb products of digestion properly. They become very thin as a result.
- This shows villi are essential for efficient absorption.
Probiotics and prebiotics
- In GCSE Biology, functional foods are eaten by people in order to make themselves healthier.
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.
Prebiotics
- “Food” for the beneficial bacteria in the gut. We cannot digest them ourselves.
- Oligosaccharides, a common prebiotic, are found in tomatoes, leek, onions and asparagus. However, they do not contain enough of them to have a significant effect so supplements containing prebiotics are taken.
- These prebiotics help with the growth of good bacteria which in consequence can help improve the health of the digestive system and immune system.
- The evidence is growing that prebiotics increase beneficial bacteria in the gut so maintain good health.
Plant stanol esters
- In GCSE Biology, chemicals that stop the small intestine absorbing cholesterol thus lowering the levels of cholesterol in the blood.
- Stanols occur naturally in plants but in small amounts.
- Stanols are produced commercially by using bacteria to convert sterols into stanols.
- They are added to spread and dairy products. People may consume these if they are worried about getting heart disease (high cholesterol has been linked with a risk of heart disease)
- Clear evidence that they have an effect.
All pictures from: https://getrevising.co.uk/resources/gcse_biology_edexcel_b2_topic
End of the topic!

Drafted by Gina (Biology)