B2 Topic 3: common systems

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Fossils and evolution

Fossils are the preserved traces or remains of organisms that lived thousands or millions of years ago.

The collection of fossils identifed from various periods of time that can be interpreted to form a hypothesis about the evolution of life on Earth- fossil records.This shows that these organisms have altered gradually over time (evolution)

Further evidence for evoultion

Internal bone structure of vertebrates are very simliar. even fossil ancestors who are limbless living vertebrates have the same basic five-fingered limb structure- Pentadactyl. So they involved from one common ancestor to form different species and adaptations.  

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Blood is made from 4 substances: plasma, red blood cells, white blood cells and platelets.

Plasma is a yellow substance. It transports dissolved materials e.g. carbon dioxide, food substances and hormones

Red blood cells have a red pigment called haemoglobin, and can combine reversibliy with oxygen.

haemoglobin + oxygen <----> oxyhaemoglobin

Blood (haemoglobin) passing through the lungs combines with oxygen. Oxyhaemoglobin is transported in the red blood cells around the body into the tissues, where the oxygen is then released so that the tissue cells have oxygen for aerobic respiration.

A red blood cell has a biconcave disc shape with dimples which has a large surface area to volume ratio for diffusion in and out of the cell. With no nucleus there is more space for the haemoglobin. 

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Blood continued

White blood cells which are vital for the body's defence against diseases. Some of these cells make antibodies which are proteins which join to the microorganisms that cause disease and destroy them.

Others surrond and destroy any foreign cells which come into the body. All of the white blood cells have a nucleus.

Lastly platelets are tiny pieces of cells (so do not have a nuclei).  They are important as they make blood clot when you have cut or damaged your blood vessels. It dries out and forms a scab to stop microrganisms from entering the body.

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The circulatory system

Blood Vessels are tubes which carry blood around the body. These are

  • arteries
  • capillaries
  • veins


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The circulatory system

Arteries- carry blood away from the heart. Blood has to be under high pressure, so that it can reach all parts of the body, therefore it has stong thick walls.

Capillaries- allows substances to diffuse in and out of the blood into the cells in tissues. In order to help this, they have (very) thin walls.

Veins- wide passage and carry blood to the heart. Wide, because blood flows relatively slowly under low pressure.

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The digestive system

Large insoluble molecules of  foods cannot be abosrbed into the blood. Therefore food is broken down through the process of digestion in the organ called the digestive system.This is made of the alimentary canal - a muscular tube running through the body from mouth to anus. And other organs that makes enzymes for digestion.

1. mouth: Food taken in, while chewing, teeth break down food into small pieces.  This increases the surface area for digestive enzymes.

Tounge helps form chewed food into a ball (bolus). It gets coated in saliva which lubricates it and makes it easier to swallow. Saliva has enzymes called amylase- breaks down carbohydrates.

2. oesophagus- a muscular tube between mouth and the stomach.

The muscular tube contrcats in waves to squeeze food into stomach- peristalsis.

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The digestive system continued

3.stomach- A muscular bag which makes hydrochloric acid and some enzymes. It contains protease which breaks down proteins. The stomach churns up food with these juices by peristalsis to make a thick paste.

4. small intestine- a long, coiled, muscular tube where most of the large insoulble food molecule. It has lots of digestive enzymes (pancreatic juices) formed by the pancreas.

  • protease- protein
  • amylase- carbohydrates
  • lipase- lipids (fats)

The molecules of digested food are absorbed into the blood using finger-shaped projections called villi- contains capillaries

5. Pancreas: makes digestive enzymes and is secreated in the beginning of small intestine.

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The digestive system continued

6. large intestine:  The undigested food is passed through the wide, thin-walled tube. water diffuses back into the blood leaving the waste substances (faeces) towards the end.

7.Anus-  The faeces/ undigested food is passed out of the body.

8. liver-  Once the digested food is absorbed in the blood plasma (in the small intestine), it goes to the liver to be processed. The molcules may be digested even further. Altough some are made into larger molecules again.

This organ makes bile, to help digest fats.

9. gall baldder- small organ which stores bile, made by the liver and is released in the small intestine when needed.

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Breaking down food

3 main types of food molecules:

  • carbohydrtaes
  • proteins
  • fats

Carbohydrates                                                                                                          Foods such as bread and pasta have carbohydrates (carbon,hydrogen and oxygen). Simplest carbohydrtae are sugars and then built into complex ones, e.g. starch.                 

Carbohydrates----> carbohydrase.                                                                                            

Amylase is a carbohydrase------>breaks starch into sugars. Then absorbed into small intestine or broken down into glucose or other carbohydrates.

Amylase is in saliva. And made in the pancreas and released into small intestine.

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Breaking down food continued


Protease are enzymes which digest proteins, breakind them down into shorter chains then into amino acids. Pepsin is a protease and is created in the stomach.

Optimum pH: acidic conditions and breaks down protein quickly. stomach walls make acid with pH 2-3. Small intestine is alkaline so protease released in their works best of about pH 8.

Digesting Fats

Lipase (enzyme) digest fats- breaks down into fatty acids and glycerol.

Bile breaks down large fat globules into tiny droplets forming a watery liquid (emulsion)- which have a larger surface area so lipase can easily break it down.quickly. Bile emulsifies the fat and neutralises acid in stomach. (bile in gall bladder is alkaline)

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