Edexcel GCSE Biology: Cell Structure

Cell Structure

Living organisms can be classified into five kingdoms: Animals, Plants, Fungi, Protoctists, and Prokaryotes.
Animals, plants, fungi, and protoctists are eukaryotic organisms, characterized by having cells with a nucleus and distinct membrane.
Eukaryotic cells can be multicellular or single-celled.

Eukaryotic Organisms: Animals & Plants

Animals:
- Multicellular
- Cells contain a nucleus with a distinct membrane
- No cell walls made of cellulose
- No chloroplasts (cannot carry out photosynthesis)
- Feed on organic substances made by other living things
- Store carbohydrates as glycogen
- Have nervous coordination
- Able to move from place to place
Plants:
- Multicellular
- Cells contain a nucleus with a distinct membrane
- Cell walls made of cellulose
- Contain chloroplasts (can carry out photosynthesis)
- Feed by photosynthesis
- Store carbohydrates as starch or sucrose
- No nervous coordination

Eukaryotic Organisms: Fungi & Protoctists

Fungi:
- Usually multicellular, but some single-celled (e.g., yeast)
- Cells contain a nucleus with a distinct membrane
- Cell walls made of chitin
- No chloroplasts (cannot carry out photosynthesis)
- Feed by secreting extracellular digestive enzymes and absorbing digested molecules (saprotrophic nutrition)
- Some are parasitic and feed on living material
- Store carbohydrates as glycogen
- No nervous coordination
Protoctists:
- Very diverse kingdom
- Mainly microscopic and single-celled
- Cells contain a nucleus with a distinct membrane
- Some have features making them more like animal cells (e.g., Plasmodium) or plant cells (e.g., green algae)
- Feed in various ways (photosynthesis or heterotrophy)
- No nervous coordination
- Examples: amoeba, Paramecium, Plasmodium, Chlorella

Prokaryotic Organisms

Prokaryotes:
- Always single-celled
- No nucleus (nuclear material in cytoplasm)
- Much smaller than eukaryotic cells
- No chloroplasts or mitochondria
- Bacteria are prokaryotic organisms
Bacteria:
- Microscopic single-celled organisms
- Possess cell wall (made of peptidoglycan), cell membrane, cytoplasm, and ribosomes
- Lack a nucleus, but have a circular chromosome of DNA in the cytoplasm
- Plasmids are present in prokaryotes, containing extra genes
- Lack mitochondria, chloroplasts, and other membrane-bound organelles
- Examples: Lactobacillus, Pneumococcus### Specialised Cells
Specialised cells have developed characteristics (adaptations) to perform specific functions
Cells specialise through differentiation, which involves developing the necessary structure and characteristics to carry out their functions
Examples of specialised cells in animals include:
- Sperm cells
- Egg cells
- Ciliated epithelial cells
Sperm cells are highly specialised for reproduction, with adaptations to carry DNA to the egg cell
- Adaptations include a tail for propulsion and a special membrane to fuse with the egg cell
Egg cells are also highly specialised for reproduction, with adaptations to be fertilised by a single sperm and develop into an embryo
- Adaptations include a large size and a special membrane to fuse with the sperm cell
Ciliated epithelial cells are highly specialised for wafting bacteria and other particles trapped by mucus up to the throat or down to the stomach
- Adaptations include cilia, which are hair-like projections that can move to 'waft' mucus along

Microscopy

Microscopy techniques have developed over time, increasing our understanding of cell structures and organelles
The first light microscopes were developed in the 17th century, with scientists such as Anton van Leeuwenhoek and Robert Hooke using them to observe cells
- Robert Hooke observed the first cells (of a cork) in 1665 using a light microscope
Light microscopes use light and lenses to form a magnified image of a specimen
- Modern light microscopes can achieve a maximum magnification of 1000-2000x
Electron microscopes use beams of electrons to visualise specimens, with a much higher resolution and magnification than light microscopes
- Electron microscopes can achieve a maximum magnification of approximately 2,000,000x
Electron microscopes have helped biologists understand many more subcellular structures, including mitochondria, chloroplasts, and ribosomes

Magnification Calculations

Magnification is calculated using the equation: Magnification = Drawing size ÷ Actual size
An equation triangle can be used to remember the formula: Magnification = Image size ÷ Actual size
The formula can be rearranged to find actual size, image size, or magnification
Magnification does not have units and is written as 'X' followed by a number (e.g. X 10 or X 5000)

Practical: Microscopy

Many biological structures are too small to be seen by the naked eye, making optical microscopes an essential tool for scientists
The key components of an optical microscope include:
- Eyepiece lens
- Objective lenses
- Stage
- Light source
- Coarse and fine focus
Other apparatus used in microscopy include:
- Forceps
- Scissors
- Scalpel
- Coverslip
- Slides
- Pipette
Specimens must be prepared on a microscope slide to be observed under a light microscope, with careful handling to avoid damaging the specimen and structures within it### Preparing and Using Microscopes
Stains may be required to make structures visible, depending on the type of tissue being examined. Commonly used stains include methylene blue for cheek cells and iodine for onion cells.
When using an optical microscope, always start with the low power objective lens to easily find what you are looking for in the field of view and prevent damage to the lens or coverslip.
To prevent dehydration of tissue, add a drop of water to the specimen (beneath the coverslip) to prevent cell damage.
To obtain clear images, switch to the lower power objective lens and use the coarse focus, and ensure the specimen sample is thin enough for light to pass through.

Measuring Cells and Structures

Use a calibrated graticule to measure cells, cell structures, and organelles.
An eyepiece graticule and stage micrometer are used to measure the size of the object when viewed under a microscope.
The stage micrometer has three lines, and the eyepiece graticule has 100 division markings.

Producing Labelled Scientific Drawings

Produce biological drawings of what you see under the microscope, following scientific rules.
Drawings should be as large as possible, taking up at least half of the space available on the page.
Limitations of biological drawings include inconsistencies in size and structure due to 3D cells being viewed on a 2D slide.

Limitations of Optical Microscopes

Optical microscopes do not have the same magnification power as other types of microscopes, so some structures cannot be seen.
The treatment of specimens when preparing slides can alter the structure of cells.

Using Units and Converting Between Them

Convert units to ensure both measurements have the same unit before doing calculations.
Remember that 1 mm = 1000 µm, and 1 µm = 0.001 mm.
Use standard form to write very big or very small numbers using powers of 10.
Standard form is written as a × 10n, where 1 ≤ a < 10, and n is the power of 10.
Use standard form to convert between units, such as writing 1 metre in millimetres as 1 × 10³ mm.
Examples of using standard form in conversion calculations include stating 45 centimetres in millimetres as 4.5 × 10² mm, and 250 micrometres in millimetres as 2.5 × 10⁻¹ mm.