What are cells?

All living organisms, including plants, animals, bacteria and fungi, are made up of cells. Cells are the smallest parts of all living organisms.

If we look at all the living organisms in the world we see that there are two main types of organisms based on the structures of their cells. The most important difference in structure is the presence of a nucleus. Cells that contain a nucleus are classified as eukaryotic cells, while those without a nucleus are prokaryotic cells. In this chapter we will specifically look at eukaryotic cells that make up organisms such as plants and animals. Examples of organisms with prokaryotic cells are bacteria.

We can say that cells are the basic structural and functional units of all living organisms. You cannot see individual cells with the naked eye, because they are too small - you need to use a microscope to be able to see cells. We say cells are microscopic because they can only be seen under a microscope.

Robert Hooke was the first cytologist to identify cells under his microscope in 1665. He decided to call the microscopic shapes that he saw in a slice of cork "cells" because the shapes reminded him of the cells (rooms) that the monks in the nearby monastery lived in.

Different types of cells

Your body is made up of many different kinds of cells. We say your cells are specialised to perform a specific function. Depending on the function of the cell, it can be specialised by having a different shape or size or may have some components which other cells do not have. Have a look at the differences between nerve cells and red blood cells in the images.

Even though there are many different types of cells, there are components of the cell structure which are common to all cells. There are also some structures which most, but not all, cells have. Let us take a look at this in the next section.

Cell Structure

As we have mentioned before, all cells have some common structures. These are:

• a cell membrane

• cytoplasm; and

• in most eukaryotic cells, a nucleus

Let's now have a look at the structure of these components of the cell, and some of the other organelles common to cells. An organelle is a specialised structure within the cell that performs a function for the cell. Examples of organelles in cells are vacuoles and mitochondria. Look at the diagram which identifies the different components in a simple animal cell.

Cytoplasm

In Natural Sciences we speak of a medium when we talk about a solution in which organelles, cells or organs are grown or suspended. Can you think of other meanings for the word medium?

The cytoplasm is often referred to as the jelly-like substance within cells. This is only partly true as the cytoplasm actually consists of the jelly-like substance (called cytosol) and all the cell organelles (except the nucleus). The term for the combined cytoplasm (cytosol + organelles) and the nucleus (containing nucleoplasm) is protoplasm. You should be aware of this when introducing the cytoplasm to learners so that this misconception can be avoided later on in their school career.

The cytoplasm (cytosol and organelles) and the nucleus together is referred to as protoplasm.

The cytoplasm includes all living parts of the cell within the cell membrane, but excluding the nucleus. The cytoplasm is made up of the cytosol and the cell organelles. The cytosol is a watery, jelly-like medium made of 70%-90% water, and is usually colourless.

The cytosol is a mixture of different substances dissolved in water. Do you remember what a mixture is from Matter and Materials? These substances within the cytosol include salts, various elements, such as sodium and potassium, and more complex molecules, such as proteins.

The cytosol is also where many chemical reactions take place. Next term, in Matter and Materials, we will learn more about chemical reactions.

The cell organelles making up the cytoplasm include mitochondria, chloroplasts and vacuoles. Vacuoles are organelles enclosed by a membrane and contain mostly water with other molecules in solution. The size and number of vacuoles within a cell varies greatly and depends on the type and function of the cell.

In this image of a plant cell, there are several vacuoles present, as is the case in many plant cells. The nucleus, with its dark nucleolus is also very clear. You can come back to this image a bit later to look at the nucleus again. If learners ask what the grey-white structure in the top left hand side is, this is a starch grain. Plants store starch grains within the cytoplasm. You can get your learners to provide labels for the vacuoles and cytoplasm in this image. Also visible is the cell wall of the plant cell - the cell membrane is the inner, more folded boundary, whereas the cell wall is the light grey, more rigid structure on the outside. Faintly visible are several mitochondria, for example the grey oval structures at the bottom left. Come back to this image once you have done these structures to add these labels in.

Nucleus

Plant and animal cells have a nucleus inside the cytoplasm. It controls all the processes and chemical reactions that take place inside the cell. The nucleus also contains the cell's genetic material which is organised into long DNA molecules.

The difference between eukaryotic and prokaryotic cells is that eukaryotic cells have a nucleus which contains the genetic material surrounded by a membrane. Prokaryotic DNA floats in the cytoplasm without a membrane.

The nucleus is structured as follows:

• A double membrane called the nuclear membrane encloses the DNA. This nuclear membrane contains pores (holes) for substances to pass through.

• There is a nucleolus inside the nucleus. This is often seen as a darker area within the nucleus.

• The DNA contains information about inherited characteristics (hereditary), such as whether the person will have blue, brown or green eyes.

Have a look at the micrograph of a nucleus and the diagram of the nucleus.

Studying micrographs is a very important skill to develop for later in Life Sciences. Learners are often only exposed to schematic diagrams of cells which present an idealised view of the cell. They then find it very difficult to identify these structures within a micrograph of an actual cell. Encourage your learners to take note of how the diagram below was drawn and how it differs to the micrograph of the nucleus. Remind your learners that a micrograph is a photograph taken through a microscope, and in this case, a transmission electron microscope.

Researchers are starting to use DNA molecules to store data! In 2012 Harvard scientists were able to store 5.5 petabits (about 700 terabytes) of data on 1 gram of DNA! "DNA USB hard drives" are not available yet, however they might still be possible in your lifetime!

DNA is a very important part of all cells and therefore of all life. DNA contains information that encodes all our inherited traits or characteristics. This refers to characteristics which are passed down in families, such as your skin and eye colour, whether you have allergies and also the likelihood of contracting different types of illnesses.

Identical twins come from the same fertilised egg which splits in two. They have the same DNA. However, they are notexactly the same due to environmental factors that can influence how they develop. Non-identical twins developed from two different eggs and two different sperm.

Every organism has unique DNA. The difference in DNA that occurs between individuals is called variation. Even the slightest difference in DNA might cause significant variations within species and between species. Within species DNA differences or variations can lead to albino animals or the transference of similar illnesses, like sickle cell anemia.

Based on the work of many genetic researchers, we can now say that all humans share 99.9% identical DNA. The differences in the last 0.1% account for all the differences we see in humans.

Learn more about genes http://learn.genetics.utah.edu/content/begin/tour/

Mitochondria

Do you remember that we spoke about food as the energy source for our bodies? Just as wood is burned to use the stored potential energy to make a fire to heat some water, the food that we eat needs to be broken down in order to release the energy so that our bodies can function. Mitochondria are responsible for doing this.

You can use this opportunity to revise what is meant by stored potential energy and the fact that food is seen as a fuel for our bodies.

Singular or Plural? mitochondrion is the singular and mitochondria is the plural form of the word.

Mitochondria are organelles enclosed by a double membrane. Cells that are very active would typically have more mitochondria than cells that are less active. Which type of cell, do you think, will have more mitochondria: a muscle cell or a bone cell?

A muscle cell will contain more mitochondria as muscle cells need a huge supply of energy for movement. You can point this out in the following image of mouse heart muscle tissue. Cardiac muscle tissue is very active and therefore needs a big energy supply, hence the frequency of mitochondria present in the tissue, as seen below.

Once food molecules enter the cells and pass into the mitochondria, they are used by the mitochondria in a process called cellular respiration. During respiration the mitochondrion can combine molecules from food with oxygen to release energy that the cell can use. Carbon dioxide, water and waste materials are by-products of this process.

Mitochondria have their ownmitochondrial DNA that is completely different to the DNA in the nucleus. What do you think we can we deduce from this fact?

With reference to the did you know fact in the margin about mitochondrial DNA, learners might ask what we can deduce from the fact that there is such a thing. One theory is that mitochondria originated from bacteria that invaded ancient eukaryotic cells and started living in a symbiotic relationship with the host, eventually losing its ability to live independently. This idea is still very controversial and not universally accepted. MtDNA seems to have an influence in gene expression and can be used to trace maternal lineage, since we inherit all of it from our mothers, not fathers.

Look at the micrograph of the mitochondrion in the image. What differences can you see between this mitochondrion and the diagram shown previously? In the diagram, it was very clear that the inner membrane folds, whereas in the micrograph it is not as easy to see this. This is because of the way that the cell was sectioned (cut) before it was viewed on the transmission electron microscope. In a diagram, we show an ideal representation of the organelle. But, when we view an actual organelle under a microscope, it may look quite different depending on how it was cut into a very thin section to view.

If you would like to find out how mitochondrial DNA (mtDNA) is responsible for 'old age', read this article by Dr Barry Starr from Stanford University http://genetics.thetech.org/ask/ask165

: Summarise what you have learnt

Now that you've studied the internal structure of a cell, let us summarise what we have learnt so far. Complete this table filling in the main function of each of the cell structures.

Cell Structure	Function(s)
Cell membrane
Cytoplasm
Nucleus
Mitochondrion
Vacuole

Cell Structure	Function(s)
Cell membrane	regulates / controls passage of substances in and out of cell
Cytoplasm	where many of the chemical reactions and processes in the cell takes place
Nucleus	contains DNA (hereditary material) of cell and controls the cell's activity
Mitochondrion	releases energy from food for the cell
Vacuole	stores substances, water, nutrients

Cell wall

All animal and plant cells are enclosed or surrounded by a cell membrane as we learned before. However, as you probably noticed in the previous activity, animal cells often have an irregular shape, whereas plant cells have a much more regular, rigid shape.

Plant cells have an additional layer surrounding the cell on the outside of the cell membrane. This is called the cell wall. This wall provides a protective framework for support and stability for the plant cell.

The cell wall is formed from various compounds, the main one being cellulose. Cellulose helps maintain the shape of the plant cell. This allows the plant to remain rigid and upright even if it grows to great heights.

Chloroplasts

You might remember learning about photosynthesis in previous grades. Can you still remember why photosynthesis is so important to all life on earth?

Did you notice the green organelles present in plant cells which were not there in the animal cells in the previous activity? These are chloroplasts. Chloroplasts are the only cell organelles that can produce food from the sun's energy. Only plants with chloroplasts are able to photosynthesise because they contain the very important green pigment, chlorophyll. Chlorophyll can absorb radiant energy from the sun and convert this to chemical energy that plants and animals can use. Animal cells lack chloroplasts and are not able to photosynthesise.

Vacuoles

Does a plant have a skeleton? Turn to a friend and discuss what could possibly be used in a plant cell as a skeleton. Think for example of a blade of grass or a long stemmed rose.

Vacuoles in plant cells are usually quite large organelles that can occupy as much as 90% of the cell's volume. The liquid in the vacuole, called cell sap, helps to support the plant. The full vacuoles push out against the cell wall and make the cells, and therefore the plant, rigid. We say the cells are turgid in this condition. The opposite to turgid is flaccid.

You can easily see when a plant's vacuoles are full and when they are not. When the vacuoles are full the plant's stem and leaves will be held upright and firm. However, if the leaves and stem are drooping, the vacuoles might have lost a lot of water because the soil is too dry and the cell was forced to use up this water to survive.

Vacuoles are only present in some animal cells and they are typically very small and have a short life-span.

Comparing plant and animal cells

1. Study the two diagrams of plant and animal cells below.
   1. Draw a table of differences between the two cell types in the space provided. Give your table a suitable heading.
   2. Also provide labels for the different cell structures and organelles.

Here are the labels for the animal and plant cell diagrams.

A typical animal cell.

A typical plant cell.

An example of the type of table that learners might produce is given below. Learners might present the information in a slightly different layout, which should be encouraged, if it is logical and legible. For example, they might not have the first column with the characteristic.

Table summarising the differences between plant and animal cells

Characteristic	Animal cells	Plant cells
Shape	Have an irregular shape	Have a rigid, regular shape
Cell wall	Do not have a cell wall, only a cell membrane	Have both a cell membrane and a cell wall.
Vacuoles	Have small vacuoles, which are often temporary or absent	Have large vacuoles
Chloroplasts	Do not have chloroplasts	Have chloroplasts to photosynthesise

Observing cells under a microscope

Have you ever used a microscope before? Microscopes are instruments that are used to look at and study objects that are too small to be seen with the naked eye. Since the days of Hooke's observations, the development of microscopes has come a long way. Today we have incredibly powerful microscopes called electron microscopes which use electrons instead of light to observe very fine detail - even as small as a single column of atoms!

Before we start working with microscopes, let's have a look at the different parts of a basic light microscope and the safety precautions we need to follow when using these pieces of equipment.

A microscope allows you to see detail in specimens that you cannot see with the naked eye. The image you see needs to be:

• well lit with enough light provided to see the specimen
• well focused
• contrasted with its surroundings to clearly see details

The next image explains the different parts of a light microscope and what they are used for.

When you use a microscope, make sure to follow these safety precautions:

1. There is a special way to carry the microscope: one hand supports the base and the other holds the frame of the microscope.
2. Put it down on a stable, horizontal, clear counter.
3. Before using the microscope, clean the lenses with proper lens paper. Do not touch the lenses with your fingers! Make sure the stage and slides are clean.
4. When handling the slides, do not use broken or cracked slides and handle cover slips by the edges.
5. When focusing with the objectives:
   • Focus smoothly and slowly
   • Be careful with the objectives and do not scratch them
6. When you are done:
   • Always turn the lowest magnification objective into place before storing the microscope.
   • Make sure that the stage and slides are clean before putting everything away.
   • Always store the microscope in a box or covered with a dust jacket to avoid dust from settling on the lenses.

To view cells under a microscope, we need to make and prepare something called a specimen on a slide.

A specimen is a small part or slice, or an example of an organism that we want to examine. When we view a specimen under a microscope it needs to let light pass through the specimen so we can see it. Therefore we need to prepare the specimen and cut extremely thin slices of less than 0.5 mm. Specimens are then placed on a glass slide.

We can prepare samples or specimens on a slide using these different techniques:

• wet mount - good for observing living organisms and is especially used for aquatic samples

• dry mount - good for observing hair, feathers, pollen grains or dust

• smears are often made of blood or slime that is smeared over the slide and allowed to dry before observing them.

• stains are added to wet or dry mounts by dropping colouring chemicals onto the specimens, like iodine solution, methylene blue or crystal violet. We use staining to improve the colour contrasts on the slide.

Evaluating microscopic images

This is an extension activity. It is optional and does not need to be done. However, if you do not have microscopes to work with at Gr. 9 level, this does provide an opportunity to get some experience with working with microscopes and troubleshooting problems, without actually using a real one.

INSTRUCTIONS:

Carefully study this image of onion cells that have been stained blue. Evaluate this image in terms of the focus, light and contrast visible in the photo.

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Learners need to explain that the image is sharp and in focus, that there is enough light shone on the specimen and that the contrast is clearly well achieved to show the structures of the cells.

These same onion cells were viewed under a microscope which had not been adjusted properly and the following photos were taken. Identify what is wrong with the photograph compared to the one above.

Image	What is wrong with the image?	How could the image have been adjusted and corrected, using what part of the microscope?

Image	What is wrong with the image?	How could the image have been adjusted and corrected, using what part of the microscope?
	The image is fuzzy.	This image could have been focused using the fine and coarse adjustment screws.
	The image is very dark.	The brightness of the image could have been adjusted by changing the brightness of the lamp or moving the mirror to reflect more light onto the slide. The brightness can also be adjusted using the diaphragm and condenser apertures.
	This image has poor contrast.	The contrast of the image can also be adjusted by changing the intensity of the light and the diaphragm aperture.

Learners might battle with the last image about contrast as this is quite tricky to understand, so you might have to explain the answer to them. The difference between brightness and darkness is that brightness refers to how light or dark an image is, whereas contrast refers to the difference in lighting between different areas of the specimen.

Making a wet mount with onion and cheek cells

There is a very specific way to prepare slides for viewing under a microscope. You will use this technique very often in Life Sciences to study specimens.

This activity will show learners how to prepare onion cells in a step-by-step manner, and will then challenge them to prepare their own cheek cells (using an ice cream stick or your own nail for collection) in order to study it under a microscope.

Very important: Make sure learners use clean, sterile sticks and that they do not re-use them and swop them.

If your class does not have access to a microscope, learners may still practice preparing a wet mount, and then examine the images at the end of the exercise.

MATERIALS:

• onion
• scalpel or knife
• dissecting needle
• forceps
• microscope slides
• coverslips
• dropper
• tissue paper or filter paper
• distilled water
• iodine solution
• light microscope

Risk assessment: Some people are allergic to iodine and/or shellfish. If any learners are allergic to iodine or shellfish DO NOT use the iodine solution to stain your specimens. Methylene blue and crystal violet are harmful and can be irritants. Avoid contact with the skin.

INSTRUCTIONS:

You will need to work quite quickly as the onion cells will dry out!

Step 1: Prepare your microscope and slides as discussed in the safety methods above.

Step 2: Cut the onion into blocks of about 1 cm square with a sharp knife or scalpel.

Step 3: Use forceps to pull or peel a small piece of the very thin membrane-like epidermis lining off one of the inner layers of the onion.

Step 4: Place a drop of iodine solution onto the slide.

Step 5: Place the membrane directly in the drop on the slide.

Step 6: Gently lower a coverslip at an angle onto the onion cells. Hold the coverslip up with a dissection needle and gently lower the slip. This prevents air bubbles from getting trapped under the cover slip.

If you accidentally trapped an air bubble, gently press on the middle of the coverslip to get rid of any trapped air using the dissecting needle or drop some extra fluid right next to the edge of the coverslip.

Step 7: Wipe off excess fluid around the edge of the coverslip with tissue paper or filter paper.

Step 8: Make sure the lowest power objective lens (this is the shortest lens) is in line with the eyepiece. Switch on the lamp or use the mirror to reflect the light onto your stage. Place the prepared slide onto the stage and secure it with the stage clips.

Step 9: While on the low power, look from the side and lower the objective lens to just above the coverslip. Then look through the eyepiece and use the fine focus to focus your image. Viewing your specimen

Step 10: Magnify your cells by swapping the objective lens to a higher powered lens. Only use the fine focus adjustment to focus clearly.

Step 11: Make careful drawings of your observations in the space below and remember to label what you see. Add a heading including the specimen, the stain used and the magnification.

Learner-dependent answer

Did you see something like this?

Now that you have prepared slides of onion cell specimens, use a toothpick to gently scrape the inside of your cheek to collect cheek cells using the side of the toothpick or ice cream stick. Follow the same instructions as above to prepare the cheek cell specimen and to view it under the microscope. Draw and label the cheek cells that you viewed under the microscope in the space below.

Learner-dependent answer

Learners must gently scratch the inside of their cheeks to collect some cheek cells and then wipe the toothpick on the slide and cover with a drop of water. (Please ensure that learners scrape with the broad side of the toothpick, slowly and gently, so that they do not injure themselves!) Alternatively, learners can use old wooden ice cream sticks. The water droplet will most likely contain several cheek cells. It is going to be near impossible to see the cheek cells in water. You should use a stain to colour contrast the cells, namely methylene blue or iodine solution. These cells are MUCH smaller than onion cells and the learners may battle to find them - look for tiny blue/yellow 'flakes' that are not lying on top of each other and magnify a small group of 3-4 cells.

Did you see something like this?

What are some of the differences and similarities you noted between the animal and cheek cells?

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Learners should be able to identify some of the following main differences and similarities between onion and cheek cells as:

• The onion cells have a thick cell wall and a cell membrane. The animal cells only have a cell membrane.
• The onion cells have a regular shape whereas the cheek cells have a irregular shape and seem more flimsy.
• In the onion cells they might notice a large vacuole which might not be as visible in the cheek cells. Cheek cells do not have vacuoles.
• Both onion and cheek cells have a nucleus and nuclear membrane.
• Both cells also have a cytoplasm and some might say that they see organelles inside this.

Cells differ in shape and size

We looked at the basic differences between plant and animal cells. However, not all plant cells and not all animal cells are the same. Cells within an organism need to have different shapes and sizes because they fulfill different functions.

Look at the photo of the rose. Do you think the cells in the roots, stem, leaves, and petals of the rose all look the same?

The cells in the different parts of the rose all have to perform very specific functions and therefore have different sizes and shapes.

Your body contains a great number of specialised cells, meaning they have different functions. They have differences in their structures allowing them to have different functions. We say they have differentiated.

Do you remember we spoke about nerve cells and red blood cells briefly in the beginning of the chapter? Some of them are summarised in the following table.

Specialised cell	Structure	Function
Epithelial cells - they are mostly flat		They cover the surface of the body for protection.
Muscle cells - some are long and spindle shaped		Muscle cells can contract and relax allowing for movement within your body
Nerve cells - the are very long and have branched ends		Nerve cells are specialised to carry messages that coordinate the functions of the body.
Red blood cells - Round and biconcave shape		Red blood cells carry carry oxygen and carbon dioxide throughout the body.

Microscopic and Macroscopic organisms

We have just looked at specialised cells within organisms. The organisms that we discussed, plants and animals, consist of many, many cells. Your body has millions of cells! Did you know that there are some organisms which consist of only a single cell? We have many different specialised cells to perform the different functions within our body whereas in a single-celled organism, all the functions it performs are done in this one cell. We can make a distinction between organisms that are made of one cell (unicellular) and those that are made of many cells (multicellular).

Microscopic organisms

We call one cell organisms that can only be seen with the help of a microscope microscopic organisms. There are many single-celled microscopic organisms. Have a look at the images.

Fresh water amoebae and

Organisation of cells in macroscopic organisms

In microscopic single-celled organisms, the individual cell has to perform all the life processes for that microscopic organism.

So what about the cells in macroscopic organisms that consist of many cells? We have already learnt about specialised cells in macroscopic organisms, so we know that not all cells perform all the processes - they are specialised to perform a specific function.

Specialised cells that perform a specific function, group together to form a tissue. For example, muscle cells will group together to form muscle tissue, epithelial cells will group together to form the skin, and nerve cells will group together to form the brain and nerves.

Groups of tissues that work together form organs. Think of the stomach for example - it is made of many different specialised cells that form muscle tissue to make it contract and epithelial tissue (made from specialised epithelial cells) which lines the inside of the stomach and produces mucus.

When organs work together we say they form systems or organ systems. There are many different systems in your body where specific organs work closely together to make your body function. Have a look at the following diagram which shows how cells are organised into tissues in the stomach which form part of the digestive system in a human (the organism).

All the systems work together to form an organism. We will be looking at some of these systems later on in the term.

Have you noticed the VISIT boxes in the margins which contain links? You simply need to type this whole link into the address bar in your internet browser, either on your PC, tablet or mobile phone, and press enter, like this:

It will direct you to our website where you can watch the video or visit the webpage online. Be curious and discover more online on our website!