Cell structure

Eukaryotes and prokaryotesSpec 4.1.1.1

Plant and animal cells are eukaryotic cells. They have a cell membrane, cytoplasm and genetic material enclosed in a nucleus.

Bacterial cells are prokaryotic cells. They are much smaller in comparison. They have cytoplasm and a cell membrane surrounded by a cell wall. The genetic material is not enclosed in a nucleus. It is a single DNA loop, and there may be one or more small rings of DNA called plasmids.

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Scale and size of cells

You need to understand the scale and size of cells and be able to make order of magnitude calculations. An order of magnitude is a power of ten. A cell that is 100 times wider than another is two orders of magnitude bigger, because 100 = 10².

To change units: 1 mm = 1000 µm and 1 µm = 1000 nm. Standard form is a number between 1 and 10 multiplied by a power of ten, for example 0.000 004 m = 4 × 10⁻⁶ m.

Animal and plant cellsSpec 4.1.1.2

Most animal cells have: a nucleus, cytoplasm, a cell membrane, mitochondria and ribosomes. In addition, plant cells often have: chloroplasts and a permanent vacuole filled with cell sap. Plant and algal cells also have a cell wall made of cellulose, which strengthens the cell.

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Estimating size and area

You should be able to use estimations to judge the relative size or area of sub-cellular structures. Round the measurements to easy numbers and compare them. Use an estimate when you only need to know roughly how much bigger one structure is than another, or to check that a calculated answer is sensible.

Required practical 1: using a light microscopeSpec 4.1.1.2

1. Place a thin piece of tissue (for example onion epidermis, or cheek cells) on a clean microscope slide. Add a drop of water or a stain such as iodine solution.
2. Lower a cover slip onto the specimen at an angle, using a mounting needle, so that no air bubbles are trapped.
3. Clip the slide onto the stage. Select the lowest-power objective lens and turn the coarse focusing knob until the specimen is roughly in focus.
4. Use the fine focusing knob to get a sharp image. Then switch to a higher-power objective lens and refocus with the fine focusing knob only.
5. Draw what you see using a sharp pencil. Use clear, unbroken lines, no shading, and make the drawing large. Draw only a few cells accurately rather than many badly.
6. Label the sub-cellular structures with straight label lines that do not cross. Add a title and the magnification.

To find the total magnification of a light microscope, multiply the magnification of the eyepiece lens by the magnification of the objective lens. The magnification of your drawing is found with the equation magnification = size of image ÷ size of real object, using the width of your drawing and the real width of the cell.

Safety: carry the microscope with two hands, handle glass slides and cover slips carefully, and take care with the stain, which can mark skin and clothes. Start with the lowest-power lens so that the objective lens does not hit the slide.

Cell specialisationSpec 4.1.1.3

Cells may be specialised to carry out a particular function. You should be able to explain how the structure of different types of cell relate to their function in a tissue, an organ or organ system, or the whole organism, when you are given appropriate information.

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Cell differentiationSpec 4.1.1.4

As an organism develops, cells differentiate to form different types of cells. As a cell differentiates it acquires different sub-cellular structures to enable it to carry out a certain function. It has become a specialised cell.

• Most types of animal cell differentiate at an early stage.
• Many types of plant cell retain the ability to differentiate throughout life.
• In mature animals, cell division is mainly restricted to repair and replacement.

Differentiation is important because it allows cells to become specialised, with the sub-cellular structures they need for a particular job. A multicellular organism made of specialised cells can carry out many different functions efficiently, as the specialised cells work together in tissues, organs and organ systems.

MicroscopySpec 4.1.1.5

Microscopy techniques have developed over time. The first microscopes used light and lenses to magnify a specimen, and these light microscopes were improved with better lenses and the use of stains. Later, electron microscopes were developed. These use electrons instead of light.

An electron microscope has much higher magnification and resolving power than a light microscope. This means that it can be used to study cells in much finer detail. This has enabled biologists to see and understand many more sub-cellular structures.

Magnification

How many times larger the image is than the real object.

Resolving power

The ability to distinguish between two points that are close together as separate points. Higher resolving power gives a sharper, more detailed image.

Magnification calculations

Rearranged: real size = image size ÷ magnification, and image size = real size × magnification. Make sure the image and real object are in the same units before you divide, then convert at the end. Give the answer in standard form if appropriate.

Culturing microorganismsSpec 4.1.1.6Triple only

Bacteria multiply by simple cell division called binary fission, as often as once every 20 minutes if they have enough nutrients and a suitable temperature.

Bacteria can be grown in a nutrient broth solution or as colonies on an agar gel plate. Uncontaminated cultures of microorganisms are required for investigating the action of disinfectants and antibiotics, so that the only microorganisms present are the ones being tested.

Preparing an uncontaminated culture using aseptic technique

1. Sterilise the Petri dish and the agar culture medium before use (for example by heating), to kill any unwanted microorganisms.
2. Sterilise the inoculating loop by passing it through a flame.
3. Use the sterile loop to transfer the microorganisms to the agar. Keep the lid of the Petri dish open for as short a time as possible.
4. Secure the lid of the Petri dish with adhesive tape and store it upside down.
5. Incubate the plate at 25 °C.

Culture calculationsSpec 4.1.1.6Triple only

Number of bacteria after a certain time

You should be able to calculate the number of bacteria in a population after a certain time if you are given the mean division time. Each division doubles the population.

1. Work out the number of divisions: total time ÷ mean division time (use the same units).
2. Number of bacteria = starting number × 2 to the power of the number of divisions.

Areas of colonies and clear areas

You should be able to calculate the cross-sectional area of colonies, or of clear areas around colonies, using πr². The radius is half of the diameter.

Required practical 2: antiseptics and antibioticsSpec 4.1.1.6Triple only

1. Using aseptic technique, spread a sample of bacteria evenly over the surface of a sterile agar plate to make a bacterial 'lawn'.
2. Using sterile forceps, place sterile paper discs soaked in different antiseptics or antibiotics (or different concentrations of one) onto the agar. Include a control disc soaked in sterile water.
3. Secure the lid with adhesive tape, label the base, store the plate upside down and incubate at 25 °C for about two days.
4. Measure the diameter of the clear area (zone of inhibition) around each disc and calculate its area using πr².

Where the antiseptic or antibiotic has killed or stopped the growth of the bacteria, there is a clear zone around the disc. The larger the zone of inhibition, the more effective the substance is against that bacterium. The control disc should have no clear zone, which shows that the water itself does not kill the bacteria.

• Independent variable: the type or concentration of antiseptic or antibiotic.
• Dependent variable: the size (diameter or area) of the zone of inhibition.
• Control variables: type and amount of bacteria, type and depth of agar, size of disc, volume of solution on the disc, temperature, incubation time.

Safety: the cultures may contain harmful microorganisms. Use aseptic technique, wash hands, keep the plate taped shut, never open it after incubation, and disinfect the work area and dispose of plates safely.