9u3.0 - Introduction to the unit

Pre-unit test: Quizlet live is fun as a way of finding out prior knowledge on the objectives above.

Starter options:

• 1. Ask them to list the organs involved in digestion.
• 2. Ask them to label a blank diagram to see what they recognise before the lesson.
• 3. Ask students to draw and label the digestive system on a piece of blank paper or the miniwhiteboards before you show them the diagram.

9u3.1 - The digestive system

Digestion

This is the passage of food through our body and the conduits (tubes) and organs that we need to digest the food, i.e. break it down into smaller chemical structures for further use.

Mouth

The process of digestion begins in the mouth with mechanical breakdown of food into smaller pieces. Salivary glands add liquid that contains enzymes to speed up the chemical breakdown.

The food is then swallowed and passes down the oesophagus into the stomach.

Stomach

The stomach is an organ that mixes the food with dilute hydrochloric acid and more enzymes. The acid has a low pH so the enzymes must be able to act efficiently in this acidic environment.

The main macronutrients that must be broken down are carbohydrates and proteins. Both of these macronutrients have chemical bonds that are broken by the acidic environment. The enzymes speed this process of hydrolysis and ensure that the complex carbohydrates get broken down into simpler sugars, such as glucose, while the protein gets hydrolyses into amino acids.

The partially digested food passes from the stomach into the small intestine.

Small intestine

The name is not really appropriate, as the small intestine measures more than 5 metres! However, it is coiled and folded many times to fit into the abdomen cavity. The small intestine completes the digestion process and absorbs the nutrients through the intestine wall, which is lined with millions of small protrubences, called villi. The villi increase the surface area of the intestines making the food absorption more efficient. At the end of the small intestine the remaining undigested food passes into the large intestine

Large intestine

Also called the colon, this is a wide tubular structure whose function is to extract the water remaining in the digested food and to compact the remains into faeces for expulsion.

9u3.2 - Diffusion

Mechanisms of digestion

Diffusion is the movement of particles from a higher density region to one of lower density. It is the reason why gases spread out, and why you can smell perfume from a distance.

The process of digestion breaks food down into smaller substances that must then find their way into the bloodstream. This happens via a process of diffusion.

As mentioned in the previous section, digested food diffuses through the walls of the intestines into the blood stream. The walls of the small intestine is lined with villi and microvilli that increase the surface area of the small intestine by a factor of 600.

9u3.3 - Nutrients

A very complex machine

The body is a very complex machine that needs several different kinds of "fuel" to be able to maintain health. We call the essential dietary components nutrients and sub-divide them into micronutrients and macronutrients.

Several substances found in organisms are polymers. This means that they are large molecules made up of many repeating units. The adjective of polymer is polymeric.

Macronutrients

These are substances of which we need to eat a large amount, or at least have a regular supply.

• Carbohydrates
• Proteins
• Fats

Carbohydrates

These are molecules made of the elements carbon, hydrogen and oxygen in the ratio C_x(H₂O)_y. They are essential for energy production in the body.

The most common foods are rice, bread, potatoes and pasta. These are all said to be staples in the diet as they are essential for energy. The body breaks down the complex polymeric molecules of starch into glucose that can then be absorbed and used in the muscles for energy.

This is the job of the mitochondria that we will meet in the section on cells.

Proteins

These are complex polymeric molecules made up of amino acids all linked together making very large structures with complex shapes. These molecules are responsible for many things in the body such as regulating the rate of chemical reactions (enzymes), transport of oxygen to cells, making structures such as skin and hair etc.

Fats

These are also called "Lipids". They are molecules made up of long hydrocarbon (containing hydrogen and carbon) chains joined to other hydrocarbon chains by ester linkages (-COO-). They are used for energy stores when the body consumes more carbohydrate than it needs. They also provide other essential functions in the body, such as insulation.

Micronutrients

These are also essential components of our diet, but they are needed in only tiny amounts. They are the specific chemical compounds (vitamins) that our bodies cannot make for themselves, but which are essential to the healthy functioning of our chemical processes. They are also the essential elements that our body needs to make compounds essential to health, such as metals sodium, potassium, calcium, iron, copper, zinc and cobalt and non-metals such as phosphorus, sulfur and selenium.

There are far too many to discuss, so we will just look at a couple of the most common examples.

Vitamins

• vitamin A - essential for good vision and healthy skin.
• vitamin B - this covers a range of compounds with many essential purposes.
• vitamin C - essential for collagen that holds cells together.
• vitamin D - essential for intestinal absorption of other nutrients, bones, teeth, immune system, brain.

Minerals

• Calcium - essential for healthy bones and teeth.
• Iron - essential for making red blood cells for oxygen transport.
• Nitrogen - needed to make amino acids for protein manufacture and growth.
• Phosphorus - essential for DNA synthesis and cell division.
• Iodine - makes the thyroid hormones (chemical messengers) that regulate the speed of metabolic processes.

9u3.4 - Deficiency

Deficiency means that the organism does not obtain enough of the nutrients it needs for a healthy existence.

9u3.5 - Food tests

Testing for starch

Starch is a polymer formed by joining together many glucose molecules together.

Humans can digest starch and break it down to the simple glucose molecules to be used for energy production in the cells. Many of our staple foods contain starch, such as bread, rice, pasta, potatoes etc.

Testing for glucose

Glucose is the monosaccharide formed when starch is hydrolysed (broken apart by water). The word "monosaccharide" means a simple sugar with only one sugar unit. Starch is a polysaccharide, meaning that it is made up of many monosaccharide units all joined together.

nC₆H₁₂O₆ → (C₆H₁₀O₅)_n + nH₂O

When starch is broken down to glucose by hydrolysis the reverse equation occurs.

(C₆H₁₀O₅)_n + nH₂O → nC₆H₁₂O₆

Where "n" just means a very large number.

Testing for protein - The Biuret test

In this test a few drops of biuret solution is added to the test sample. If the sample turns purple, this indicates the presence of at least 2 peptide bonds, i.e. a protein.

9u3.6 - Energy from food

The staples in the diet

The staples are those foods that provide us with carbohydrates for energy. As explained previously, the sources of carbohydrates are tubers, grains, and nuts. These are usually processed to provide bread, cereals, pasta etc..

Carbohydrates in the diet are sugars and starch. It is thought that eating sugars directly may be damaging to health, as the body must deal with the rapid arrival of sugars in the bloodstream, while starch is digested more slowly.

Excess sugars in the diet are stored as fat for survival during times of food shortage. Clearly, if such food shortages do not arise then this can give rise to obesity and diseases linked to obesity, such as diabetes and coronary heart problems.

9u3.7 - Photosynthesis

Glucose

Photosynthesis is the process whereby plants use the energy of sunlight to drive a reaction that joins together water and oxygen forming glucose.

Glucose is a large carbohydrate molecule with the formula C₆H₁₂O₆. Carbohydrates are molecules that are made up of carbon, hydrogen and oxygen which have the general formula C_x(H₂O)_y.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Starch

Plants do not use all the glucose produced by photosynthesis immediately as an energy store. Instead they save it as starch and cellulose. The starch becomes an energy store, while the cellulose is used by plants for the cell walls.

9u3.8 - Animal and plant cells

Plant cells

The cell structure comprises individual components with specific functions essential to carry out life’s processes. These components include- cell wall, cell membrane, cytoplasm, nucleus, and cell organelles.

Cell Membrane

• The cell membrane supports and protects the cell. It controls the movement of substances in and out of the cells. It separates the cell from the external environment. The cell membrane is present in all the cells.
• The cell membrane is the outer covering of a cell within which all other organelles, such as the cytoplasm and nucleus, are enclosed. It is also referred to as the plasma membrane.
• By structure, it is a porous membrane (with pores) which permits the movement of selective substances in and out of the cell. Besides this, the cell membrane also protects the cellular component from damage and leakage.
• It forms the wall-like structure between two cells as well as between the cell and its surroundings.
• Plants are immobile, so their cell structures are well-adapted to protect them from external factors. The cell wall helps to reinforce this function.

Cell Wall

• The cell wall is the most prominent part of the plant’s cell structure. It is made up of cellulose, hemicellulose, and pectin.
• The cell wall is present exclusively in plant cells. It protects the plasma membrane and other cellular components. The cell wall is also the outermost layer of plant cells.
• It is a rigid and stiff structure surrounding the cell membrane.
• It provides shape and support to the cells and protects them from mechanical shocks and injuries.

Cytoplasm

• The cytoplasm is a thick, clear, jelly-like substance present inside the cell membrane.
• Most of the chemical reactions within a cell take place in this cytoplasm.
• The cell organelles such as endoplasmic reticulum, vacuoles, mitochondria, ribosomes, are suspended in this cytoplasm.

Nucleus

• The nucleus contains the hereditary material of the cell, the DNA.
• It sends signals to the cells to grow, mature, divide and die.
• The nucleus is surrounded by the nuclear envelope that separates the DNA from the rest of the cell.
• The nucleus protects the DNA and is an integral component of a plant’s cell structure.

Animal cells

Animal cells do not require the rigid cell wall as the structure is taken up by the skeleton. However, many of the other parts of the cell perform the same functions.

Animals do not photosynthesise and so have no need for chloroplasts.

9u3.9 - Respiration

Respiration is a process by which living organisms exchange gases with their environment, primarily taking in oxygen and releasing carbon dioxide. In cellular respiration, which occurs within the cells, oxygen is used to break down glucose and other organic molecules, producing energy in the form of ATP.

Respiration in plants

Plants undergo respiration just like animals, but they also photosynthesise, producing their own food. During respiration, plants take in oxygen and release carbon dioxide, which is the opposite of photosynthesis.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Respiration occurs inside the mitochondria of the plant cells, where glucose and oxygen are used to produce energy, carbon dioxide, and water. This process, which occurs in the mitochondria, provides the energy cells need for growth, reproduction, and other cellular activities.

Similar processes occur in both animals and plants.

Respiration in our bodies

First, the glucose from your food enters the cells. There, it goes through a process called glycolysis, which breaks the glucose in half, making smaller molecules called pyruvate. This process happens in the cytoplasm (the jelly-like substance inside the cell) and releases a little bit of energy.

Using Oxygen

Next, if there’s oxygen available, the pyruvate moves into a part of the cell called the mitochondria (often called the powerhouse of the cell). Here, it goes through a process called the Krebs cycle (also known as the citric acid cycle), which breaks down the pyruvate even more and produces some more energy, along with carbon dioxide (which animals breathe out).

Making Lots of Energy

After the Krebs cycle, the cell uses another process called the electron transport chain. This part uses the oxygen you breathe and creates water and a lot of ATP (adenosine triphosphate) – this is the main energy currency of the cell. It's like the money that the cell uses to do everything it needs to do, like growing, dividing, and repairing itself.

So, in simple terms, cellular respiration is the process where cells take in food and oxygen, break it down to release energy, and then use that energy to live and grow. This process is crucial because without it, cells wouldn’t be able to get the energy they need to function.

9u3.10 - Unit test - Summative