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Cells and Microscopy

Compare eukaryotic and prokaryotic cells, identify subcellular structures and their functions, link specialised cell adaptations to their jobs, use light microscopes, calculate magnification and standard form, and culture bacteria using aseptic technique. AQA GCSE Biology Higher, specification references 4.1.1.1, 4.1.1.2, 4.1.1.3, 4.1.1.4, 4.1.1.5, 4.1.1.6, 4.4 and 1.2.
  • GCSE
  • AQA
  • Biology
  • Higher
  • Spec 4.1.1.1 · 4.1.1.2 · 4.1.1.3 · 4.1.1.4 · 4.1.1.5 · 4.1.1.6 · 4.4 · 1.2
  • 9 min read
  • 10 revision boxes

Introduction

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2 min read

Eukaryotic and prokaryotic cells

All living things are made of cells, and every cell is one of two types: eukaryotic or prokaryotic.
Eukaryotic cells are complex, and all animal and plant cells are eukaryotic. An organism made up of eukaryotic cells is called a eukaryote. Whether they come from a plant or an animal, these cells share three defining features: a cell membrane, cytoplasm and genetic material enclosed in a nucleus.
Prokaryotic cells are smaller and simpler. Bacteria are prokaryotes, and a prokaryote is a single-celled organism. A bacterial cell has cytoplasm and a cell membrane surrounded by a cell wall.
It is easy to assume every cell keeps its genetic material in a nucleus, but bacterial cells have no true nucleus. Instead they have a single circular strand of DNA that floats freely in the cytoplasm, and they may also have one or more small rings of DNA called plasmids. Bacterial cells have no chloroplasts and no mitochondria.
A eukaryotic animal cell with a cell membrane, cytoplasm and a nucleus holding its genetic material, next to a much smaller prokaryotic bacterial cell with a cell wall, cell membrane, cytoplasm, a freely floating loop of DNA instead of a nucleus, and small plasmid rings.

Learning

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2 min read

Subcellular structures and their functions

The different parts inside a cell are called subcellular structures, and each one does a particular job.
Cross-sections of an animal cell and a plant cell with subcellular structures labelled, showing that the plant cell also has a cell wall, permanent vacuole and chloroplasts.
A plant cell has all of the parts an animal cell has, plus chloroplasts, a permanent vacuole and a cell wall. Bacterial cells have cytoplasm, a cell membrane and a cell wall, and may also carry plasmids.

  • Nucleus

    Contains the genetic material, which controls the activities of the cell.

  • Cytoplasm

    A gel-like substance where most of the chemical reactions in the cell happen. It contains the enzymes that control those reactions.

  • Cell membrane

    Holds the cell together and controls what goes in and out of it.

  • Mitochondria

    Where most of the reactions for aerobic respiration take place. Respiration transfers the energy the cell needs to work.

  • Ribosomes

    Where proteins are made in the cell.

  • Chloroplasts (plant cells)

    Where photosynthesis happens, making food for the plant. They contain a green substance called chlorophyll, which absorbs the light needed for photosynthesis.

  • Permanent vacuole (plant cells)

    Contains cell sap, a weak solution of sugar and salts.

  • Cell wall (plant and algal cells)

    A rigid wall made of cellulose that supports the cell and strengthens it. Algae such as seaweed also have a rigid cell wall and chloroplasts.

  • Plasmids (bacterial cells)

    Small rings of DNA found in bacterial cells in addition to the single circular strand of DNA.
2 min read

Specialised cells and differentiation

A specialised cell is one that has a structure suited to carrying out a particular function. In animals, sperm cells, nerve cells and muscle cells are all specialised. In plants, root hair cells, xylem cells and phloem cells are specialised.
Cells become specialised by differentiation. As an organism develops, its cells differentiate to form different types of cell. As a cell differentiates it acquires different subcellular structures that let it carry out a certain function, and once it has done that, it has become a specialised cell.
Timing differs between animals and plants. Most types of animal cell differentiate at an early stage, so in mature animals cell division is mainly restricted to repair and replacement. Many types of plant cell keep the ability to differentiate throughout their life.
If you are given information about a cell, use it to link structure to job: the features a cell has should explain how it does its work in a tissue, an organ, an organ system or the whole organism.
1 min read

Light and electron microscopes

Microscopes let you see things far too small for the naked eye. As technology and knowledge have improved, microscopy techniques have developed, and each improvement has shown biologists more of what is inside a cell.
Light microscopes use light and lenses to form a magnified image of a specimen. They are good enough to show you individual cells and large subcellular structures such as nuclei.
Electron microscopes use electrons instead of light to form an image. They have a much higher magnification than light microscopes, and also a higher resolution. Resolution is how well a microscope can distinguish between two very close points, so a higher resolution gives a sharper image.
That combination lets electron microscopes show cells in much finer detail: the internal structure of mitochondria and chloroplasts, and even tinier things like ribosomes and plasmids. Because of this, biologists have been able to see and understand many more subcellular structures than light microscopy alone ever revealed.
"\"5 steps\""

Magnification calculations and standard form

The magnification of an image is worked out from two lengths: how big the image looks and how big the object really is.
magnification=image sizereal size\text{magnification} = \frac{\text{image size}}{\text{real size}}
The formula triangle lets you rearrange it without much thought: cover the quantity you want, and what is left showing is the calculation you do.
A formula triangle with image size on top, and magnification multiplied by real size beneath it.
Image size and real size must be in the same units or the equation will not work. To convert micrometres (μm) to millimetres (mm), divide by 1000. Cell sizes are usually written with the prefixes centi, milli, micro and nano.
  1. To write a number in standard form, move the decimal point until the first number is between 1 and 10.
  2. Count how many places the decimal point moved. That count is the power of 10.
  3. Make the power positive if the point moved left, negative if it moved right. So 0.0025 becomes 2.5×10−32.5 \times 10^{-3}.
  4. Give an answer in standard form whenever the number is very big or very small, which is common with cells.
  5. To estimate the area of a subcellular structure, treat it as the regular shape it is closest to. If it is close to a rectangle, use area = length × width.
image size=magnification×real size\text{image size} = \text{magnification} \times \text{real size}
real size=image sizemagnification\text{real size} = \frac{\text{image size}}{\text{magnification}}
TODO — content team to supply
"\"3 stages\""

Required practical 1: using a light microscope

Draw with a sharp pencil, using clear, unbroken lines. Make it large enough, with no colouring or shading. Include a title and label features with straight, uncrossed lines. Draw subcellular structures in proportion.
Include a magnification scale: magnification=length of drawing of cellreal length of cell\text{magnification} = \frac{\text{length of drawing of cell}}{\text{real length of cell}}. For example, 33 mm ÷ 0.3 mm = × 110. Find real cell size by counting cells in 1 mm of the sample.
  1. Add a drop of water to a clean slide.
  2. Use tweezers to peel epidermal tissue from the bottom of an onion layer and place it in the water.
  3. Add iodine solution, a stain that adds colour to highlight objects.
  4. Stand a cover slip upright beside the droplet, then tilt and lower it to avoid air bubbles, which obstruct the view.
    A microscope slide with a stained circular specimen under a square cover slip, labelled to show the slide, the water and iodine drop, the stained specimen and the cover slip.
  5. Clip the slide onto the stage and select the lowest-powered objective lens.
  6. Use the coarse adjustment knob to move the stage up to just below the objective lens.
  7. Look down the eyepiece and use the coarse adjustment knob to move the stage down until roughly in focus.
  8. Use the fine adjustment knob until the image is clear.
  9. Swap to a higher-powered objective lens and refocus if needed.
33 mm ÷ 0.3 mm = × 110
TODO — content team to supply
"Key idea"

Growing bacteria and aseptic technique

Bacteria multiply by simple cell division, called binary fission. With enough nutrients and a suitable temperature, this can happen as often as once every 20 minutes. Grow bacteria in a nutrient broth solution or as colonies on an agar gel plate.
To investigate disinfectants and antibiotics, you need an uncontaminated culture, so only the microorganism being studied grows. Use aseptic technique.
Grow bacteria on an agar plate using aseptic technique, apply antiseptics or antibiotics, then incubate the plate. Clear zones of inhibition appear around treatments. Measure the radius and calculate the cross-sectional area using πr2\pi r^2. Use the same formula for a colony.
Divide the total time by the mean division time to find the number of divisions. The population doubles with each division, so calculate starting population × 2number of divisions2^{\text{number of divisions}}. Express large answers in standard form.
  1. Sterilise Petri dishes and culture media before use to kill unwanted microorganisms.
  2. Sterilise the inoculating loop in a flame before transferring microorganisms to the media.
  3. Secure the lid with adhesive tape to prevent microorganisms entering or escaping.
  4. Store the dish upside down so condensation does not fall onto the agar.
  5. In school laboratories, incubate cultures at 25°C because harmful pathogens are more likely to grow at higher temperatures.
For a zone with radius 4 mm: area = πr2\pi r^2 = π×42\pi \times 4^2 = 50.3 mm².
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Examples

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Find the image size of a 50 μm specimen at × 100

A specimen is 50 μm wide. Calculate the width of the image of the specimen under a magnification of × 100. Give your answer in mm.
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Calculate the real width of a cheek cell in μm

A cheek cell is viewed under a microscope at × 40 magnification. The image of the cell is 2.4 mm wide. Calculate the real width of the cheek cell. Give your answer in μm.
2 marks

Give two structural differences between prokaryotic and eukaryotic cells

Give two differences in structure between prokaryotic and eukaryotic cells. [2 marks]

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Common questions

Do bacterial cells have a nucleus?
No. Bacterial cells have no true nucleus, so their single circular strand of DNA floats freely in the cytoplasm. They may also contain small rings of DNA called plasmids.
Why are electron microscopes better for seeing cell structures?
Electron microscopes have much higher magnification and resolution than light microscopes. This allows them to show finer details, including mitochondria, chloroplasts, ribosomes and plasmids.
Why is iodine solution added when viewing onion cells?
Iodine solution is a stain that adds colour to highlight objects in the onion tissue, making them easier to see through the microscope.