Cell structures
What you'll learn
- What plant, animal and bacterial cells contain, and how each structure is linked to its job.
- How light microscopes, slides, cover slips and stains are used to view cells.
- How to calculate magnification and handle cell sizes using micrometres and nanometres.
- Why electron microscopes changed our understanding of tiny cell structures.
Cells are the basic units of life
A cell is the smallest unit of an organism that can carry out life processes. Organisms may be unicellular, meaning made of one cell, or multicellular, meaning made of many cells.
A cell is not just a flat circle like it often looks in a textbook diagram. It is a tiny three-dimensional object, with structures arranged inside it.
Sub-cellular structure
A sub-cellular structure is a part inside a cell that has a particular job, such as a nucleus, mitochondrion, chloroplast or ribosome. Some people also call these organelles, especially in eukaryotic cells.
Cells are 3D
Do not think of cells as flat shapes. A microscope image is usually a thin view through a cell or a view of cells pressed onto a slide, so diagrams simplify a real three-dimensional structure.
Eukaryotic and prokaryotic cells
Cells can be grouped into two major types.
Eukaryotic cells
A eukaryotic cell is a cell with genetic material enclosed inside a nucleus. Animal and plant cells are eukaryotic.
Prokaryotic cells
A prokaryotic cell is a cell without a nucleus. Bacteria are prokaryotes. Their genetic material is found loose in the cytoplasm, usually as a loop of DNA, and they may also contain small rings of DNA called plasmids.
Genetic material
Genetic material is DNA, the chemical that carries instructions for making proteins and controlling cell activities. In eukaryotic cells, DNA is organised into chromosomes inside the nucleus.
Big difference
Eukaryotic cells have a nucleus. Prokaryotic cells do not have a nucleus.
Identifying a cell type
A cell has a cell wall, ribosomes, cytoplasm, a loop of genetic material and plasmids, but no nucleus. What type of cell is it?
- Compare the genetic material with the main cell groups: eukaryotic cells keep DNA inside a nucleus, while prokaryotic cells do not.
- Use the presence of plasmids as supporting evidence, because plasmids are small rings of DNA commonly found in bacterial cells.
- Conclude that the cell is prokaryotic, so it is a bacterial cell.
Main cell structures and their functions
Cell membrane
The cell membrane is a thin boundary around the cell. It forms a selective barrier, meaning it controls which substances enter and leave the cell.
The cell membrane also contains receptor molecules. These can detect specific chemicals outside the cell, helping cells respond to their environment.
Cytoplasm
The cytoplasm is the jelly-like material inside the cell where many chemical reactions happen. It also holds the cell’s structures in place.
Nucleus, chromosomes and plasmids
The nucleus controls the activities of a eukaryotic cell because it contains chromosomes.
A chromosome is a long molecule of DNA, carrying many genes. A gene is a section of DNA with instructions for a characteristic or for making a protein.
A plasmid is a small circular piece of DNA found in many bacterial cells. Plasmids may carry useful genes, such as genes for antibiotic resistance.
Mitochondria
Mitochondria are sub-cellular structures where cellular respiration happens. Cellular respiration is the process that releases energy from glucose. Mitochondria contain enzymes needed for this process.
Enzyme
An enzyme is a biological catalyst: it speeds up a chemical reaction in living organisms without being used up.
Ribosomes
Ribosomes are the site of protein synthesis, which means making proteins. Proteins are needed for growth, repair, enzymes, cell structures and many cell processes.
Chloroplasts
Chloroplasts are found in plant cells and some algae. They contain chlorophyll, the green pigment that absorbs light energy for photosynthesis.
Plant-only structures you should recognise
Plant cells usually also have a cell wall, which strengthens and supports the cell. They often contain a large permanent vacuole, which holds cell sap and helps keep the cell firm.
Structure fits function
Every cell structure has a job. In exam answers, link the structure to its function, not just its name: for example, mitochondria contain enzymes for respiration, while ribosomes make proteins.
Using a light microscope to view cells
A light microscope uses visible light and lenses to produce a magnified image of a specimen.
The main parts you need to know are:
- Eyepiece lens: the lens you look through.
- Objective lenses: lenses near the specimen, often with different magnifications.
- Stage: platform that supports the slide.
- Lamp or light source: shines light through the specimen.
- Slide: thin glass rectangle that holds the specimen.
- Cover slip: thin glass square placed over the specimen.
Preparing and viewing a slide
To view cells, you usually place a thin specimen on a slide, add a drop of liquid or stain, lower a cover slip onto it, then view it under the microscope.
A stain is a coloured chemical used to make colourless specimens easier to see or to highlight particular tissues or cell structures. For example, a stain may make nuclei stand out more clearly.
A typical method is:
- Place a thin specimen, such as onion epidermis or cheek cells, onto a slide.
- Add a drop of water or stain.
- Lower the cover slip carefully at an angle to reduce air bubbles.
- Start with the lowest-power objective lens.
- Use the coarse focus knob to bring the specimen roughly into view.
- Use the fine focus knob to sharpen the image.
- Increase magnification if needed, then refocus carefully.
Low power first
Always begin on the lowest magnification. It gives you a wider field of view, so it is much easier to find the cells before zooming in.
Magnification and real size
Magnification tells you how many times larger an image is than the real object.
The key equation is:
magnification=image sizereal size\text{magnification} = \frac{\text{image size}}{\text{real size}}magnification=real sizeimage sizeYou can rearrange it:
real size=image sizemagnification\text{real size} = \frac{\text{image size}}{\text{magnification}}real size=magnificationimage sizeCell sizes are often measured in micrometres (µm). Smaller structures, such as ribosomes and membranes, may be measured in nanometres (nm).
Useful conversions:
- 1 millimetre = 1000 micrometres
- 1 micrometre = 1000 nanometres
This bit is Higher Tier only: you may be expected to calculate with numbers in standard form, such as 1.2×10−6 m1.2 \times 10^{-6}\ \text{m}1.2×10−6 m.
Calculating magnification
A plant cell is 80 µm long in real life. In a drawing, it is 40 mm long. Calculate the magnification.
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Convert the drawing size into the same unit as the real cell size: 40 mm = 40 000 µm.
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Substitute into the magnification equation:
magnification=40000 μm80 μm\text{magnification} = \frac{40000\ \mu\text{m}}{80\ \mu\text{m}}magnification=80 μm40000 μm -
Divide the two values:
magnification=500\text{magnification} = 500magnification=500 -
State the answer as a magnification: the drawing is magnified 500 times.
Mixing units
Do not put millimetres and micrometres into the same magnification calculation without converting first. The units must match before you divide.
Estimating cell size
Sometimes you are asked to estimate rather than measure exactly. An estimate is a sensible approximate value, useful when cells are irregular, slightly blurred, or you are counting across a field of view.
For example, if about 10 similar cells fit across a field of view that is 500 µm wide, each cell is roughly 50 µm wide. This is not exact, but it is a reasonable estimate.
Estimating cell width
A microscope field of view is 600 µm across. About 12 cells fit across its diameter. Estimate the width of one cell.
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Treat the cells as roughly equal in width, because the question asks for an estimate rather than an exact measurement.
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Divide the field of view by the number of cells:
cell width=600 μm12\text{cell width} = \frac{600\ \mu\text{m}}{12}cell width=12600 μm -
Calculate the estimate:
cell width=50 μm\text{cell width} = 50\ \mu\text{m}cell width=50 μm
Electron microscopy
A light microscope is very useful for viewing whole cells, but it cannot show every tiny sub-cellular structure clearly.
Resolution
Resolution is the ability to distinguish two points as separate. A microscope with higher resolution can show finer detail.
An electron microscope uses electrons instead of light. A transmission electron microscope, often called a TEM, has much higher resolution than a light microscope. This means it can reveal much smaller structures inside cells, such as ribosomes and details inside mitochondria and chloroplasts.
Electron microscopy increased our understanding of cell structure because scientists could see details that were previously invisible with light microscopes.
Magnification is not the same as resolution
Magnification makes an image bigger. Resolution makes fine detail clearer. A very big blurry image is not very useful.
Choosing the better microscope
A scientist wants to study ribosomes inside a cell. Should they use a light microscope or a transmission electron microscope?
- Decide what needs to be seen: ribosomes are extremely small sub-cellular structures.
- Compare the microscopes: a light microscope can show many cells and some larger structures, but a TEM has much higher resolution.
- Choose the TEM because its increased resolution allows smaller internal structures to be seen in more detail.
In the exam
- When asked for a function, link the structure to the process: mitochondria to respiration, ribosomes to protein synthesis, chloroplasts to photosynthesis.
- For magnification calculations, convert units first, then use magnification=image sizereal size\text{magnification} = \frac{\text{image size}}{\text{real size}}magnification=real sizeimage size.
- If comparing microscopes, use the word resolution when explaining why electron microscopes show more detail.
Check yourself
- Which structures would you expect in a plant cell but not an animal cell?
- Why does a bacterial cell count as prokaryotic?
- A cell is 25 µm wide and its image is 50 mm wide. How would you calculate the magnification?
