Revision - Respiration

• A similarity between aerobic and anaerobic respiration is that both processes break down glucose and produce energy (ATP) but much less energy is produced by anaerobic respiration. The oxygen allows as much energy to be removed from glucose. Without oxygen, glucose is changed into lactic acid so only less energy can be produced. Lactic acid is stored in body whereas end products of aerobic respiration (carbon dioxide and water) are released.

• An example of anaerobic respiration is when cheetahs are speeding up to run. Anaerobic respiration moves energy from glucose to ATP quickly so that energy is quickly available to the muscles. This allows the cheetah to run quicke to catch prey. Aerobic respiration cannot supply energy as quickly because there are more chemical changes. But much less energy is made by anaerobic respiration so cheetahs can't run fast for long. With aerobic respiration, the end products of carbon dioxide and water are removed quickly from the muscles, but lactic acid from anaerobic respiration is poison that accumulates in muscles and blood so eventually the body cannot respire. This is another reason why cheetahs can run only for a short time.

Revision- Digestion in small intestine

Organ System	Description
Digestive system	Breaks food into smaller molecules so they can be absorbed by the body
Gas exchange system	Transports air into the body so oxygen can be absorbed into the circulatory system
Circulatory system	Transports material in blood such as food and oxygen to every cell and removes waste such as carbon dioxide

Small intestine 

• When chyme enters the duodenum it's mixed with alkaline bile which neutralises stomach acid and produces an alkaline environment suitable for enzymes from duodenum wall eg lactase and pancreas eg lipase. 
• Lactase digests lactose in milk to give glucose. Lipase digests lipids to give fatty acid and glycerol.
• Lipids are insloluble so bile breaks larger droplets into smaller droplets to increase the surface area for lipase to act so they are digested quickly.
• Protease from the pancreas digests proteins into amino acids.

Structure of the small intestine 

• The inside wall of the small intestine is folded into many finger-like villi that increase the surface area for contact with digested food and the wall to make absorption effective.
• Surface area is further increased by having folds, microvilli, in the membranes of the cell lining the wall. These increase the length of cell membrane in contact with digested food.
• The nutrients are passed into the capillary network and lacteal inside each villus. Soluble material (glucose and amino acids) are transported through the liver to cells that need them.
• amino acids are used to make proteins like enzymes and muscle. Glucose is used to produce energy by respiration.
• Insoluble products in the lacteal (fatty acids and glycerol) are dissolved in lymph and transported to the thoracic duct where they enter the bloodstream and go to cells.
• They can be used as an energy source in cells or stored.

Use of nutrients - Respiration

Glucose is released from the liver into the circulation system to maintain a steady concentration in the blood. The glucose travels to the capillaries in the organs and tissues where it is absorbed by cells and used in respiration.

Aerobic respiration

Aerobic respiration involves oxygen and transfers the energy in glucose into a chemical called ATP. Cells use energy (ATP) for a range of life processes including transporting materials, cell division, and making new cell parts. A lot of energy is formed by aerobic respiration.

Anaerobic respiration

Sometimes low concentrations of oxygen occur inside the mammals body eg muscles working quickly  or for a long time. When oxygen is low anaerobic respiration occurs.

During anaerobic respiration glucose is changed into lactic acid and small amounts of energy (ATP)

Less energy is produced than aerobic respiration as glucose is only partially broken down. Lactic acid made is poisonous, causing cramps and stops mucles working. Oxygen is needed to break lactic acid and is called oxygen debt. Supplying oxygen to remove lactic acid is why high breathing and pulse rates continue after exercise.

Cellular respiration makes other life processes possible because the energy produced can be used for those life processes. eg for growth, the energy can be used to join chemicals together to make the larger molecules for growth. Energy can also be used for the other life processes (MRS GREN)

Transport of nutrients - circulation

• Digested food molecules such as glucose, amino acids, minerals and some vitamins are soluble in water and can be transported by the circulation system.
• The molecules are absorbed by the blood capillaries in the villi and taken directly to the liver in the hepatic portal vein.
• The liver is a sorting area, storing some molecules, changing others and allowing some to continue on in the circulation system.
• Food molecules from the liver travel in veins to the heart, from where the blood is pumped into the lungs to be oxygenated. The blood returns to the heart and is pumped through arteries to all areas of the body where cells are supplied with nutrients and oxygen.
• Insoluble digested food molecules, fatty acids/some vitamins, are absorbed into the lacteals inside the villi and are transported to the thoracic duct of the lymph system. The thoracic duct returns the food molecules and fluid to the circulation system just above the vena cava which leads to the heart.

Revision-Enzymes II

As pH increases or decreases, the shape of the enzyme changes until it is the shape at which it is most active (optimal pH). As pH changes further, the shape changes and the activity lessens. The enzyme denatures (becomes inactive) if the pH is too far from optimal.

The optimal pH for amylase is alkaline (pH 7-8), so amylase is most active in the mouth and duodenum where the pH is in this range. This enzyme is not active in acidic conditions so it is not active in the stomach (pH 1-2). Lipase is also active in the doudenum where the pH is high.

Pepsin has optimal activity in conditions of low pH, so is active in the acidic stomach.

Temperature changes can also alter rate of reaction. Amylase is found in the mouth of humans so it is very active at 37 degrees. At low temperatures, particles involved in the reaction move more slowly so there are fewer collisions between starch and enzyme. At temperatures higher than 37 (eg 50 degrees), the enzyme denatures (changes shape) so it cannot catalyse the breakdown of starch.

Revision- Enzymes

Enzymes break large food molecules into smaller nutrients that can be absorbed (through the intestinal walls). 

Different foods have different chemicals joined together so they need different enzymes, eg some enzymes break protein and some break lipids.

Different areas of the digestive system have different conditions (eg acidic and alkaline) so different enzymes are needed.

Digestion occurs much more quickly with enzymes.

Digestive enzymes are proteins that have special shapes with an active site that allow them to act as catalysts when food is broken down.

Each different enzyme can catalyse only one specific reaction. eg lipase can only break lipids in to glycerol and 3 fatty acids. Amylase can only break starch to maltose.

The enzymes are not used up in the reaction.

Each enzyme has an optimum environmental condition (temperature and pH) in which its activity is the highest rate.

Herbivore and Carnivore digestive systems

A dog is a carnivore with a diet high in easily digested protein so it has a short caecum and large intestine. The caecum is short as very little digestion occurs after the stomach due to the protein rich diet. However, the sheep is a herbivore with a high volume diet of difficult to digest plant material, eg grass containing cellulose, so it has a long large intestine an caecum. Food must stay in the sheep's digestive system for much longer so the micro-organisms in the gut have time to carry out extra cellular digestion of cellulose and allow the maximun nutrients to be absorbed. The caecum contains a large number of different types of bacteria to digest material not broken down in the stomach or small intestine.

The dog has an acidic stomach with a low pH, where enzymes can digest protein that make up a large part of their diet. Whereas the sheep has a four chambered stomach with the first chamber, rumen, having a high pH alkaline environment needed for the microorganisms that do digestion. The sheep has an acidic chamber where proteins in micro-organisms are digested.

The sheep has a much larger large intestine that the dog as plant material is only partially digested so lots of faeces are produced. Dog's food is highly digestible so it has a small large intestine which is mainly involved in the absorption of water. But some bacteria live in the large intestine and caecum,eg to digest some carbohydrates, which increases the efficiency of the dog's digestive system.

Dog                                                      Sheep

Teeth for Life

The following pictures show the skull of a carnivore and the skull of a herbivore. They have different structures to assist with the digestion of the different types of foods eaten.

The carnivore's skull has incisor teeth at the front and premolars and molars along the side of the jaw. In addition, the skull has large pointed canine teeth between the incisors and premolars and a carnassial tooth near the back of the jaw. The canine tooth are important for holding prey and tearing chunks of flesh from the body. The large chunks are sliced into smaller pieces by the thin, blade like molar teeth. The jaw is used in and up and down chopping motion. The small incisor teeth pull small pieces of flesh from the bones. In dogs, the carnassial molars break bones to release marrow. The carnassial teeth can shear flesh and bone.

In the herbivore's skull, the incisor teeth cut plant material from plants for ingestion. The material, like grass, is ground between strong molar teeth, which are wide and may be flat or ridged. The jaw is moved in a circular motion crushing the plant material as the top and bottom molars slide across each other. Herbivore's skull does not have canines as they are not needed to hold food and would make circular chewing difficult.

A similarity between the skulls is that herbivores and carnivores have more than one type of tooth and the teeth have specialised functions.

The differences relate to the food they eat. The cells in plants are tough so physical digestion is important for herbivores. Firstly sharp wide incisors cut plant material. The food must be crushed and ground so the molar teeth are large. There is a large diastema so food can be moved and mixed with saliva. Chewing is a side to side movement to crush plants which increases surface area for enzymes.

Because animal protein is easily digested, physical digestion is less important in carnivores. The teeth and jaws cut lumps of meat to smaller pieces. Long pointed canines hold prey and rip meat from bones, sharp incisors cut them into smaller pieces and thin blade like molars cut meat into small lumps. They have no diastema as the jaw moves up and down to slice meat.

Digestive systems

• Herbivores (eat plants), carnivores (eat animals) and omnivores (eat both plants and animals) have differences in their digestive systems depending on what they eat.

• Humans are omnivores, so they have a digestives system in which all organs, except the appendix, are important for digestion. 

• In horses and rabbits the appendix is very important for digestion. In sheep and cattle, the stomach, especially the rumen section, is very important. 

Food tests

Food tests can be used to find out if a food contains a specific food chemical (e.g. glucose). They can also be used to see if an enzyme has digested a specific food chemical ( e.g. amylase digest starch to glucose).

Enzymes

Enzymes are biological catalysts that make chemical reactions of digestion occur more quickly.

Organ	Enzyme Produced	Digests
Mouth	Amylase	Starch (broken into glucose)
Stomach	Pepsin	Protein (broken into amino acids)
Pancreas (digestion occurs in the small intestine)	Amylase Protease Lipase Nucleases	Starch                       Protein Lipids (broken into glycerol and fatty acids) DNA and RNA
Doudenum (small intestine)	Sucrase Lactase Maltase	Sucrose Lactose Maltose

Different digestive enzymes work on different food chemicals and can require different conditions. e.g. pepsin works on acidic conditions of the stomach but not alkaline conditions of the doudenum.

The following graphs show how temperature and pH affect the activity of digestive enzymes.

Revision - The Intestines

• The small intestine is involved in digestion and absorption of nutrients while the large intestine is involved in the re-absorption of water and other materials and the formation of semi-solid faeces.

• The first part of the small intestine is the duodenum where digestion of lipids, carbohydrates and protein occur. Food enters the stomach as acidic chyme and bile from the gall bladder is added to the chyme. Bile is an alkaline liquid that neutralises the acid in the chyme, making it alkaline for the enzymes from the pancreas to have optimum conditions.

• Bile also contains salts, which help break large particles of fat into small particles, forming an emulsion. The emulsion has a large surface area for enzymes to attach which speeds up digestion. 

• Enzymes released from the pancreas break down food into small molecules that can be absorbed into the blood stream. The muscle movement (peristalsis) of the duodenum walls help mix the food and enzymes enabling rapid digestion as well.

• The end part of the small intestine has villi that stick out, from the inside surface of the gut, into the digested food. Villi create an increase in surface area over which nutrients from food can be absorbed.

• The small intestine is narrow in diameter but the large intestine is much wider. The material in the large intestine is indigestible fibre, water and minerals. The water and minerals are reabsorbed into the walls while the fibre is formed into faeces.

Revision - Comparing the two major types of Digestion

• Physical Digestion is the mechanical breakdown of food from large lumps to smaller lumps where as chemical digestion requires enzymes to break down the chemical bonds in the chemicals that make up food, eg proteins are broken into amino acids.
• The processes differ as physical digestion is acting on lumps of food but chemical digestion is the action of enzymes  on the bonds inside the food.
• Physical digestion takes place in the mouth with chewing and in the stomach where food is mixed with acidic gastric juice until it becomes chyme.  This is needed as it increases the surface area of food available for the enzymes needed for chemical digestion. Without physical digestion, food would be too big for chemical digestion to occur quickly.
• Without chemical digestion, food would not be broken down into particles small enough to pass across the cell membranes of cells lining the gut. Chemical digestion occurs in the mouth (amylase breakdown of starch), Stomach (pepsin to break protein), and small intestine (sucrase, amylase and protease to break carbohydrates and proteins. Lipase to break fat).
• In chemical digestion, the different areas contain different enzymes that require different conditions. eg  pepsin requires acidic conditions of the stomach and won't work in the alkaline conditions of the small intestine.

Absorption by the small Intestine

• The second part of the small intestine is the ileum. 
• The wall of the ileum is covered in millions of tiny bumps called villi.
• Villi increase the area of the wall surface in contact with digested food to allow easier absorption of nutrients into the blood stream.
• Each villus has a blind tube, lacteal, in the centre. Insoluble fatty acids are absorbed there and travel in the lymph vessels to the blood stream.

During absorption, small molecules move from the digestive into the circulatory system. 

The next step is assimilation where small molecules move into the body cells. Some materials like glucose can be used immediately while others must be transported to the liver first and modified.

As undigested material move along the large intestine (colon), water, minerals and other useful materials are absorbed into the bloodstream. Undigested material become firmer to form faeces which is egested (excretion).

Digestion

• In the mouth (Buccal cavity), Food is chewed by molar teeth. The teeth breaks food into smaller lumps. This is physical digestion.

• During chewing, saliva, containing enzymes and mucus is added to the food. Amylase, one enzyme, starts to break down starch. This is the first stage of chemical digestion. Mucus makes food into a smooth paste allowing easy swallowing.

• Food is rolled into a ball and swallowed. It passes down the oesophagus by contraction of muscles (peristalsis).

• In the stomach, hydrochloric acid kills micro-organisms, breaks up food, and provides optimum conditions for stomach enzymes.

• The muscles in the stomach wall contract and mix the food (physical digestion) into a liquid called chyme.

• Pepsin is added to digest proteins.

• small amounts of chyme are released into the first part of the small intestine (duodenum), where bile made in teh liver and stored in the gall bladder, neutralises the acid from the stomach. Bile also breaks fat into small droplets allowing digestion by the enzyme lipase (made in pancreas).

• Sucrase, amylase, and protease enzymes made in the pancreas and intestinal glands are released into the chyme to digest carbohydrates and proteins. In grass eating mammals, a large blind pouch called the caecum contain micro-organisms that produce enzyme cellulase needed to digest cellulose in grass and plants. 

Physical and Chemical Digestion

Physical digestion- Increases the surface area of the food.

• Mechanically tearing up large food items into smaller pieces.
• (No chemical reactions take place.)

• Cutting up food, biting, chewing, churning, emulsification

Chemical digestion

• Using enzymes to break up small items of food into simple, individual molecules.
• (Chemical reactions take place.)

                         enzymes

Polymer ------------------Monomer

Digestion of Food

The main steps of digestion are:

Life processes- an introduction

•  A mammal needs energy to survive. Energy is used for growth, reproduction, excretion, movement, respiration, cell division, sensitivity and active transport of substances across the membranes of cells. 

• To make useful energy the cells undergo respiration. Respiration is  a chemical reaction that happens inside cells and involves burning food to release energy in a controlled manner.

• Cells are tiny, so nutrients must be tiny to get across the cell membrane.This is why food must be digested – to make food particles small and soluble.

• The organ system that breaks down food so that they can be absorbed by blood and moved to cells is the digestive system