Cells and Food

Specific Learning Outcomes

By the end of this unit you should be able to

recall the seven characteristics of living things (MRSGREN) and identify each of them in a particular living organism
give actual examples of the seven life processes and state what each one is for
identify the parts of a cell (range: cell wall, cell membrane, chloroplast, cytoplasm, vacuole, nucleus, mitochondria)
apply the term 'organelle' to parts of the cell
give the purpose of each cell organelle specified above
distinguish between plant and animal cells and identify the parts of the cell unique to plants
recall that multicellular organisms consist of cells organised into tissue, organs and organ systems
label a diagram of the parts of a microscope and state the purpose of each part
be able to work out the magnification of a microscope given the power of the eyepiece and objective lenses
prepare and view a wet-mount microscope slide
give examples of why living organisms need food (energy, growth and repair, carrying out life functions)
explain why foods must be digested before an organism can use them
explain the role of enzymes in digestion
distinguish between internal and external digestion
identify the parts of the human digestive system and give the purpose of each part (range: mouth, teeth, esophagus, stomach, liver, small intestine, large intestine, rectum)
state the major food groups (carbohydrates, proteins and fats) and identify why each one is needed by living organisms
give examples of foods which are rich in each of the three food groups
carry out a lab test for protein, starch and simple (reducing) sugars in a liquid food

Additional text material:

Cells Food (note - these are quite large PDF files (about 9 Mb each) . They are only available if you are logged on to your SHC Google account and are not available to viewers outside SHC for copyright reasons.

Living things

Living things are called organisms. All living things on Earth are thought to be descendants of a single, common ancestor - the great-grandmother of us all. Scientists are not entirely sure when this first organism appeared but it was probably between 3.5 and 4 billion years ago. This first organism was probably very small and single celled, something like the most primitive bacteria we find today. There may have been others types of organism, but they have left no descendants.

What is life?

Europa, is a moon of Jupiter. It is thought to be one of the likelier places in the Solar System for extraterrestrial life, because there is a deep ocean of water under its icy surface. Life could possibly be found around underwater hot springs that are quite likely to exist at the bottom of this ocean .

But if Europa has life, how could we recognise it?

Scientists think that there are 7 characteristics that together are unique to things that are living. We use the mnemonic MRS GREN for this:

Movement - e.g. a leaf turning towards the light
Respiration - converting food into energy
Sensitivity- the ability to respond to signals from the environment in an organised way
Growth - all living things must start out small and grow bigger
Reproduction - nothing lives forever, so all organisms must reproduce for their kind to survive
Excretion - means the giving off of waste
Nutrition (or feeding)

Some texts add 'C' for cells to make it MRS C GREN. Although all life on Earth is composed of one or more cells, we cannot be sure that this characteristic is universal to other life forms. The other 7 MRSGREN features should be universal. We call these life functions.

Non-living things may have some of the "significant seven" but not all of them. For example, viruses don't respire (but they do reproduce, so they are sometimes termed "semi-living"). If ever we develop self-replicating machines, we may need to think about our definition of life again

Note that there can be exceptions - worker bees cannot reproduce even though they are alive. This is because the bee colony acts as a 'super-organism' whose reproductive function is carried out by the queen. There are other similar examples.

Dead things are things that were once alive but are no longer alive, whereas non-living thngs were never alive.

Cells

All living things are made of cells. For the first two billion years of life on earth, all organisms were made of just one cell, and were tiny. Such living things are termed single celled, or unicellular.

The first cells had no separate nucleus. Such cells belong to a group called Prokaryotic. All other organisms, unicellular and multicellular, are called Eukaryota and have a nucleus.

About 600 million years ago, groups of cells started to live together and do different jobs. These were the first animals and plants. Organisms made of multiple cells are termed multicellular.

Parts of a cell are called organelles. There are many of these, but below is a list of ones you need to know about:

Both animal and plant cells have -

cell membrane: a thin layer that controls all the substances that go in and out of the cell
cytoplasm - the 'soup' of chemicals that supply the cell's parts with the things they need, and where cellular chemical reactions take place.
nucleus: the 'control' centre of the cell, containing the DNA. It is also very important in cells making new cells (called replication) and directing the function of other parts of the cell.
mitochondria (not shown in diagram): these are small organelles which carry out respiration for the cell - they convert food into the cell's energy. Mitochondria are rather interesting and the link takes you to a Simple Wikipedia article about them

The first living things did not have a nucleus. They belong to the kingdom of prokaryotes, which are single celled organisms that don't have a nucleus. The prokaryotes you will be most familiar with are bacteria.

Plants

In addition. to the three parts above, plant cells ALSO have these extra parts:

Chloroplasts: the green parts of the cell (containing chlorophyll) where photosynthesis takes place
Cell wall: a thicker layer on the outside of the cell membrane, which give the cell shape. It is made of cellulose in plants (fungi also have cell walls, but they are made of a different carbohydrate called chitin).

Microscopes

Cells are very tiny. To view them, you will need to use a microscope such as the one shown on the right. You will also need to prepare a wet mount slide of some cells. One simple example of this is a slide of onion skin.

A series of steps to do this is illustrated below. You can use this for a variety of things, provided they are thin.

The magnification of a microscope is given by multiplying the eyepiece power by the objective power.

Some types of slide require use of a stain to make the specimen easier to see.

For onion skin, use iodine instead of water. This will stain the starch and highlight cell structures.

Cheek cells would be stained with methylene blue. However, due to the paranoia of the Ministry of Education we are not allowed to do cheeik cells any more.

How multicellular organisms are put together

Multicellular organisms have their structures organised into a hierarchy of systems from individual cells to the whole organism. This hierarchy goes from cells to tissue to organs to organ systems and the organ systems together make up the organism.

A group of cells of the same type growing together is called tissue e.g. muscle tissue.

A set of tissues working together to carry out a function is called an organ. For example, the heart is an organ containing muscle tissue, connective tissue, blood vessels and so on.

A group of organs working together in the body is called an organ system. The heart is part of the circulatory system. The organ system we are going to look at in more detail is the digestive system, which supplies the body with energy and nutrients then passes out the stuff it doesn't need.

Digestion

Digestion is the process of breaking down food to absorb it. Most foods are made of large molecules such as starches and other carbohydrates, proteins, fats and other lipids. Most of these are insoluble and cannot be absorbed into cells.

The purpose of digestion is to break the large molecules down into small, simple molecules which can pass through cell walls. This breakdown is carried out by chemicals called enzymes. Enzymes are chemicals produced by living organisms to carry out chemical reactions, such as chopping a long molecule into short bits or controlling chemical reactions in cells.

Single celled organisms release their enzymes through their cell membranes and absorb the broken down food after the enzymes have done their work. Fungi do the same thing, even though they are multicellular. This is called extracellular digestion or external digestion.

On the right is a picture of a fungus (bread mould) feeding on bread:.

Another example of external digestive is the ripening of a soft chees such as a brie. It is the digestive enzymes of the white mould growing on the outside that is responsible for the creamy texture of the brie cheese in this photo. This is because they have broken long protein molecules down into shorter peptides.

The middle of the cheese has not yet been fully digested by the enzymes which were released by the white mould on the outside. This makes the inner part of the cheese less runny than the outer part. It is also a different colour.

We like ripe cheese because the digestive enzymes act on the milk curd to produce tasty compounds (peptides). Milk curd by itself is rather flavourless. A fully 'digested' cheese is termed 'ripe'.

Internal digestion

One of the defining features of the animal kingdom is the presence of a digestive tract, where food can pass through a tube or bag where it is digested. All higher animals, including us, have a tube for food. Food passes in one end (ingestion), breaks down and is absorbed (digestion) then leftover undigested material passes out the other end (egestion).

A very simple animal, such as a worm, is a tube with the digestive system running down the middle and surrounded by muscles to move it around.

More complex animals like humans have more complex digestive systems.

The human digestive system

Mouth: this is where ingestion takes place. Many animals have teeth to chop the food up, so it has more surface area for the enzymes to act on. Humans and many animals add some enzymes to the food with the saliva, to start the digestion process off. The main enzyme in human saliva is called amylase and breaks some starches down into glucose.

Oesophagus: also spelt esophagus, this is a muscular tube that transports food from the mouth to the stomach.

Since the mouth is also used for breathing in humans, it is necessary that the airways are closed off when swallowing; food lodged in the airways causes choking.

Stomach: this is where most of the digestive enzymes are added, together with some hydrochloric acid. The food is sloshed around with the acid and enzymes to help it break down. It is then passed through into the small intestine so the dissolved food can be absorbed.

Small intestine: it is termed "small" because its diameter is less than the large intestine, but it is longer. The small intestine is surrounded by blood vessels. Digested molecules pass through the walls of the small intestine and into these blood vessels. The blood is then transported to the liver.

Liver: this is where some of the food molecules are dealt with e.g. glucose can be converted into glycogen for storage. The liver also breaks down poisons and produces enzymes to break down fat.

Large intestine:also called the colon. A lot of water is added to the food during digestion, and if you didn't have a way to remove it again there would be some consequences. First of all, the waste that passes out your anus (feces) would be very watery and even more unpleasant to deal with than usual. Second, you would be at a big risk of dehydration.

The large intestine re-absorbs added water back into the bloodstream. If it doesn't do this, you get diarrhoea. Severe cases of diarrhoea can cause death through dehydration, particularly in children and the elderly.

If waste moves too slowly through the large intestine, too much water is absorbed and the feces become hard and difficult to pass. This is called constipation. Quite a few drugs, particularly most strong painkillers, slow down the muscle contractions that move food along the gut and can cause constipation. Eating fibre can help, as it adds bulk and 'hangs on' to the water.

Rectum: this is where feces are held ready to be expelled. If the feces are not passed out after arrival, they are moved back to the last part of the colon. This causes more water to be absorbed. This is why 'holding on' for too long is a bad idea, as it can cause constipation.

Food

Food is needed to provide energy and building materials for your body.

There are three types of food:

Proteins
Fats (which includes oils and a range of chemicals called "fatty acids")
Carbohydrates (which include sugars, starches and fibre)

Water is not considered a food, but is necessary for your body to work properly.

Vitamins and minerals are not considered a major food group because you only need very small quantities of them, and you can't use them for energy (which you can do with all the three food groups). Vitamins and minerals are chemicals needed for your body to work properly.

For example, you need small amounts of the element iodine (classed as a mineral) in your food. You can get this from chemicals dissolved in water (e.g. sodium iodide, found in small quantities in seawater and in some streams or springs) or from fish, seaweed or some vegetables. If you don't get enough iodine, your thyroid gland can't make a hormone needed for normal growth. Because iodine is often short in the diet of people who don't eat much fresh seafood, a small amount of sodium iodide is often added to our salt (as it doesn't change the flavour). The sodium in the salt is also a mineral your body needs.

About the main food groups

Protein

Protein is found in many foods. However, some foods which are mostly sugar and/or fat, such as fruit juice, soft drinks or chocolate, contain very little protein. Other foods, such as potatoes or rice, contain some protein but not enough to supply all you need.

The richest sources of protein ar meat, fish, milk products and eggs. Rich non-animal sources of protein include legumes (beans, peas, soya), nuts and products made from seeds such as tofu and tempeh.

Proteins are made of long chains of chemicals called amino acids. Several amino acids grouped together make peptides, so proteins are also called polypeptides. Important chemicals in your body, such as DNA, collagen (in your muscles) and keratin (hair and nails) are made of proteins. Protein is very important for body growth and repair, but can be used for energy.

Whey protein is a form of soluble protein found in milk.

Because whey is soluble, it is easily absorbed. It also contains a good balance of the amino acids your body needs. Sugar-free whey protein drinks are therefore very useful if you are trying to build muscle mass without getting fat. Protein extracted from soy or other seeds can be used in a similar way for a vegan equivalent.

Foods high in protein are better at preventing hunger with fewer calories, so are useful for those who are trying to lose weight or keep trim.

Fppds that have been partly digested by microbes can contain peptides which give them flavour. Examples are fermented meats (salami etc), fermented fish (anchovies etc.), fermented soybeans (soy sauce, miso etc.), cheeses. We presumably evolved a liking for these flavours because they signal a source of easily digestible and useful protein.

Carbohydrate

The name carbohydrate comes from "carbon" and "hydrate" meaning water. Carbohydrates are built up from carbon atoms linked together via water molecules. They are used mainly as sources of energy, although some carbohydrates can be converted to fat which is then used for body structures.

The simplest carbohydrates are chemicals called sugars. Your body uses a simple sugar called glucose for energy, and some foods contain glucose.

Starch consists of a lot of glucose molecules joined together in a long chain. Enzymes in your saliva and stomach break starch down into glucose. This takes some time, so starch is good for releasing energy over a longer period of time without raising your blood sugar too much. There are different types of starch and some are better at this than others

How quickly a carbohydrate makes your blood sugar go up is called the glycemic index, or GI. Low GI carbohydrates do not raise your blood sugar as quickly.

There are other sugars besides glucose.

Fructose is a sugar found in table sugar, honey and some fruit. It is sweeter than glucose and is popular with sweet food and drink manufacturers for that reason. Although fructose is lower GI than glucose, the way your body breaks it down makes it more likely to be converted to fat and contribute to health problems such as obesity and diabetes
Sucrose is table sugar. It breaks down into glucose and fructose in your stomach.
Lactose is found in milk. Baby mammals break it down into glucose and another sugar, galactose. Many humans can also do this as adults, but those who are lactose intolerant can't do this. The lactose passes undigested into the colon, where bacteria break it down and can generate a lot of gas and irritants which can cause diarrhea. Lactose isn't sweet.

All carbohydrates can be converted to fat if they are eaten in excess..

Sugar names end in "-ose", such as lactose, maltose and cellulose.

Fibre is a term we use for carbohydrates that our body's digestive enzymes cannot break down. They include cellulose from plant cell walls, and starch-like molecules made from chains of sugars other than glucose or joined together in a way our enzymes can't work with. Fibre is important for several reasons. It adds bulk to the material in your large intestine and help it move along. Some fibre, particularly soluble fibres, help to feed the "good" bacteria that live in your colon. This has quite a number of beneficial health effects. However, if you increase the intake of soluble fibre very suddenly you can provide a bit of a feast for some of the gas-generating bacteria. This produces a lot of gas, which has to find its way out.

A fibre called dextrin can turn red in the iodine test. Dextrin is a soluble fibre added to some foods (e.g. Up and Go) to increase the fibre content. Inulin is another soluble fibre. It is found in chicory and jerusalem artichokes; it is used as a food additive to boost fibre but can cause gas.

Sugar alcohols are natural sweeteners that your body can't digest, and are used as 'low calory sweeteners' e.g. in sugar free gum. Glycerol, xylitol and erythritol are examples.

Fats (lipids)

Lipids are chemicals made of units called fatty acids joined onto a sugar alcohol called glycerol, or glycerine. We call liquid lipids oils, and ones that are solid on a warm day are called fats. The melting point of the lipid depends on the fatty acids that make it up. Fats and oils do not dissolve in water, but can disperse in water as tiny droplets. We call such an oil-water mix an emulsion. Cream and mayonnaise are examples of emulsions. Butter is an emulsion of tiny water droplets dispersed in solid butterfat.

Lipids are the most concentrated form of energy in food. We tend to like the 'mouth-feel' of fat in our food because we evolved to seek out energy rich foods; however, the easy availability of such foods in modern society means that it is easy to eat more than you need. Your body stores extra food as body fat, and too much of this can lead to poor health. Foods rich in sugar and lipid have the greatest risk of this.

There are fatty acids your body cannot make and must be obtained from food. These are called essential fatty acids. An example is the "omega-3" fatty acids found in many vegetable oils.

Food tests

We need to be able to test for simple foods - simple sugars, starch and soluble protein.

Testing for simple sugars

What to do: Place a small amount of the liquid food to be tested (about 1 mL)to a test tube (add a little water if it is ‘thick’). Add a few drops of Benedict’s Solution – enough to make it a very pale blue colour.

HEAT GENTLY on a blue flame until the liquid JUST STARTS to boil. DO NOT boil it. Alternatively, heat in a water bath of boiling water.

Observations: if glucose is present it will go orange (or red, or green depending on the type of sugar and how much is present). If there are no simple sugars, it will stay a blue colour.

Benedict's test showing from left: no glucose, a little, quite a bit and lots (this is termed semi-quantitative).

Testing for protein

Place 1 mL of the food to be tested into a test tube

Add 1 mL of sodium hydroxide

Add 2 or 3 drops of copper sulfate.

Observations: if protein is present it goes a purple colour. If there is no protein, it will stay a pale blue. The purple colour of a positive test is illustrated on the right.

This test is known as the Biuret test.

Testing for starch

Place 1 mL of the food to be tested into a test tube.

Add two drops of iodine.

Observations: if starch is present it will go dark blue or black. If there is no starch it will stay a yellow-brown colour. Some types of soluble fibre will go a red colour.

Note: some preservatives and fats can react with iodine to remove the colour. You will need to add extra iodine if this happens.

Test for fats

This is more difficult, and won't be part of your standard testing.

You can test a solid food for fat by rubbing it on some paper; if it leaves a 'greasy' mark it contains fat or oil.

Liquid foods need to be dried (usually freeze dried). A liquid that dissolves fat and oil, such as hexane, is mixed with the food then it is filtered. The liquid is evaporated away, and the oil is left behind. This can be used to work out the amount of fat. It is also the method used to extract the last of the oil from major oil sources such as olives or palm oil kernel.

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