2.1Unit 2: Cells, Development, Biodiversity and Conservation
SNAB Cell Structure & Organisation — IAL Edexcel Modular Revision Notes & Topical Questions
5 min read 12 parts Lesson 3 of 8 1 quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
2.1 Cell Structure and Organisation — IAL EDEXCEL MODULAR notes: Describe eukaryotic and prokaryotic cell structure, the levels of organisation of cells, and the use of light and electron microscopes.Cell theory — Schleiden and Schwann proposed in 1837 that all living organisms a…
- Common features
- Differences
- Ultrastructure
- Eukaryotic cells
- Cell surface membrane
- Mitochondria
Part 1 of 10Cell theory
Cell theory
Cell theory
- Schleiden and Schwann proposed in 1837 that all living organisms are made of cells.
- All living organisms consist of one or more cells.
- Cells are the basic functional unit of living organisms.
- New cells are produced from pre-existing cells.
Common features
- Common features all cells have a cell surface membrane, cytoplasm, DNA and ribosomes.
Differences
- Differences prokaryotes have no internal membranes and smaller ribosomes, whereas eukaryotes have membrane-bound organelles and larger ribosomes.
Ultrastructure
Ultrastructure the internal structures of a cell seen under a microscope.
Part 2 of 10Eukaryotic cells: surface membrane, nucleus and mitochondria
Eukaryotic cells
- Eukaryotic cells they have membrane-bound organelles including a nucleus and 80S ribosomes.
- Animal cells have centrioles and sometimes microvilli, which increase surface area for absorption.
- Plant cells have a cellulose cell wall, a large permanent vacuole and chloroplasts.
Cell surface membrane
- Cell surface membrane a partially permeable phospholipid bilayer about 10 nm wide that controls exchange between the cell and its environment.
Nucleus
- Nucleus it is surrounded by a double membrane, the nuclear envelope, with pores.
- Pores let mRNA and ribosomes out and enzymes (such as DNA polymerase) and signalling molecules in.
- It contains chromatin, made of linear DNA wound around histone proteins.
- The nucleolus is a dark region where ribosomes are produced.
Mitochondria
- Mitochondria they are the site of aerobic respiration and are just visible with a light microscope.
- A double membrane with an inner membrane folded into cristae surrounds a matrix containing respiratory enzymes.
- The matrix also holds circular mitochondrial DNA and ribosomes needed for replication of mitochondria.
Part 3 of 10Ribosomes, endoplasmic reticulum, Golgi, lysosomes and centrioles
1Ribosomes
- Ribosomes they have no membrane and are made of rRNA and protein in almost equal amounts, made in the nucleolus.
- They are the site of translation.
- 80S ribosomes occur in eukaryotes, and 70S ribosomes in prokaryotes, mitochondria and chloroplasts.
2Rough endoplasmic reticulum (RER)
- Rough endoplasmic reticulum (RER) folded membrane continuous with the nuclear envelope and covered in ribosomes that processes proteins made on them.
3Smooth endoplasmic reticulum (SER)
- Smooth endoplasmic reticulum (SER) folded membrane without ribosomes that produces, processes and stores lipids, carbohydrates and steroids.
4Golgi apparatus
- Golgi apparatus a regular stack of flattened membrane sacs that modifies proteins and lipids and packages them into vesicles.
- The cis face lies near the RER and the trans face near the cell surface membrane.
5Lysosomes
- Lysosomes vesicles containing hydrolytic enzymes that break down waste materials and worn-out organelles.
- They are used in immune cells and in programmed cell death (apoptosis).
6Centrioles
- Centrioles hollow microtubule structures, absent from plants and fungi, with two at right angles forming a centrosome that organises spindle fibres.
Part 4 of 10Protein production and secretion
1
Transcription of the gene occurs in the nucleus.
2
Ribosomes on the RER make proteins by translation, and the proteins pass into the RER lumen.
3
The protein is folded and processed in the RER, then carried in a vesicle to the Golgi apparatus.
4
The Golgi modifies the protein and packages it into vesicles.
5
Vesicles carry the protein to the cell surface membrane for secretion, for example insulin, or to lysosomes or other organelles.
- Free ribosomes
- they make proteins that stay in the cytoplasm.
Part 5 of 10Prokaryotic cells
Prokaryotic cells
Prokaryotes
- Prokaryotes bacteria are much smaller than eukaryotic cells.
- The cytoplasm has no membrane-bound organelles.
- Ribosomes are smaller (70S).
- There is no nucleus, only a single circular chromosome free in the cytoplasm and not associated with proteins.
- The cell wall contains murein (peptidoglycan).
Optional structures
- Optional structures plasmids (DNA loops), capsules, flagella, pili and mesosomes.
- (folds of the cell membrane)
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | Absent | Present |
| DNA | Circular, free, no histones | Linear, with histones |
| Ribosomes | 70S | 80S |
| Organelles | No membrane-bound organelles | Membrane-bound organelles |
| Cell wall | Murein | Cellulose (plants) or chitin (fungi); none in animals |
Part 6 of 10Levels of organisation
Specialised cells
- Specialised cells cells become adapted for a function,
- such as intestinal epithelial cells for absorption, red blood cells for oxygen transport and xylem cells for water transport.
Tissue
Tissue a group of cells working together to perform a particular function, such as epithelial or muscle tissue.
Organ
Organ a group of tissues working together, such as the heart or a leaf (palisade mesophyll, spongy mesophyll and vascular tissue).
Organ system
Organ system a group of organs working together, such as the circulatory system or the digestive system.
Part 7 of 10Light microscopy and preparing slides
Light microscope
- Light microscope light passes through a thin specimen on a slide and is focused by objective and eyepiece lenses.
- Parts are the eyepiece lens, objective lenses, stage, light source and coarse and fine focus.
Liquid specimen
- Liquid specimen place drops on the slide and lower a coverslip gently to avoid air bubbles.
Solid specimen
- Solid specimen cut a small piece, peel a very thin layer so light passes through, stain it and add a coverslip.
- Some tissues are fixed with formaldehyde, dehydrated in ethanol, impregnated with paraffin or resin, sliced thinly, then de-waxed and stained.
Using the microscope
- Using the microscope always start on the low power objective to find the specimen and avoid damaging the lens.
- A drop of water prevents the specimen drying out.
- A blurry image may need the low power lens, coarse focus or a thinner specimen.
Limitations
- Limitations structures look different in different planes of section, light microscopes cannot see small structures and preparation may alter cells.
Part 8 of 10Staining and biological drawings
- Stains
- most colours in light micrographs come from stains, except green chloroplasts.
- Differential staining uses several dyes to show different tissues.
| Stain | What it colours |
|---|---|
| Methylene blue | Animal cell nuclei blue |
| Iodine | Starch blue-black |
| Toluidine blue | Tissues with DNA and RNA blue |
| Phloroglucinol | Lignin red or pink |
Drawing conventions
- Drawing conventions a title, the magnification, a sharp pencil on plain white paper, single clear lines, no shading and no sketching.
- The drawing fills the page, shows well-defined structures in proper proportion and draws only what is visible.
- Label lines are ruled, do not cross, have no arrowheads and touch the part labelled.
Plan drawings
- Plan drawings low-power drawings of tissues that never show individual cells, unlike cell drawings made at high power.
Part 9 of 10Magnification and measuring size
Magnification
Magnification image size ÷ actual size, with both measured in the same units.
- Total magnification equals eyepiece magnification (often ×10) multiplied by objective magnification.
- Convert mm to µm by multiplying by 1000.
Eyepiece graticule
- Eyepiece graticule an engraved ruler in the eyepiece with no fixed units that must be calibrated for each objective lens.
- A stage micrometer with divisions of 0.01 mm (10 µm) is used to find the size of one graticule unit.
- Once calibrated, the graticule measures objects in the field of view.
Part 10 of 10Electron microscopes
- Resolution = the ability to distinguish two separate points; magnification is limited by resolution.
- Light microscope resolution is limited by the long wavelength of light.
- Electrons have a much smaller wavelength, so electron microscopes resolve closer points and show organelles, viruses and DNA.
- Electron microscopy needs dead specimens in a vacuum, giving a snapshot such as chromosomes at stages of mitosis.
| Feature | TEM | SEM |
|---|---|---|
| Beam | Transmitted through a thin specimen | Scanned across the surface |
| Image | 2D, shows internal structures; denser parts appear darker | 3D, shows surface |
| Resolution | Very high | Lower than TEM |
| Specimen | Must be thin | Need not be thin |
Key terms
| Term | Meaning |
|---|---|
| Ultrastructure | The internal structure of a cell seen with an electron microscope. |
| Resolution | The ability to distinguish two separate points. |
| Tissue | A group of cells working together to perform a function. |
| Organelle | A specialised part of a cell with a particular function. |
| Murein | Glycoprotein (peptidoglycan) forming the bacterial cell wall. |
| Graticule | An engraved scale in the eyepiece used with a stage micrometer to measure size. |
| Transmission electron microscope | Microscope that sends electrons through a thin specimen to show internal structure. |
| Scanning electron microscope | Microscope that scans electrons over a surface to give a 3D image. |
Avoid these mistakes
1
Including size as a structural difference between prokaryotic and eukaryotic cells — size is not a structural feature.
2
Confusing magnification and resolution — resolution is how clearly two points are separated.
3
Forgetting to convert units before calculating magnification — convert so image and actual size use the same unit.
4
Calling the cell surface membrane the cell wall — a wall is a separate outer layer in plants, fungi and bacteria.
Marked practiceQuizzes for this lesson
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Cambridge 9700 AS Biology
Core Formula
Magnification Triangle & Unit Conversion
Formula: M = I / A ⇔ A = I / M ⇔ I = M × A
Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.