4.1.2Unit 4: Energy, Environment, Microbiology and Immunity
Electron Microscopy of Plant Cells — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 4 parts Lesson 9 of 28 Unit quiz
Section 1 of 4Identifying Features
Electron micrographs show internal detail, so each organelle can be identified by its outline and membranes.
Chloroplast
- An oval organelle with a double membrane and stacks of thylakoids, larger than a mitochondrion, which marks the cell as a plant cell.
Nucleus
- A roughly spherical structure with a nuclear envelope and a darker nucleolus inside it.
Vacuole
- A large space that usually appears pale or white in the micrograph, which also indicates a plant cell.
Cell wall
- A band around the outside of the cell, which marks the perimeter of the plant cell.
Mitochondrion
- A roughly oval organelle with a double membrane, and often with visible folds of the inner membrane called cristae.
Section 2 of 4Membrane and Wall
These two outer layers lie close together and are easily confused.
Membrane and Wall
Plasma membrane
- A thin dark line pressed against the inside of the wall, enclosing the cytoplasm and all the cell contents.
Low magnification
- The membrane and wall run parallel and very close together, so they can be hard to tell apart.
Wall position
- The wall lies outside the membrane, so the outermost boundary of the cell in the image is the wall.
Evidence for plant cell
- A cell wall, large vacuole and chloroplasts are all visible clues that the cell belongs to a plant.
Section 3 of 4Interpreting Micrographs
Use a consistent approach when asked to name structures in an image.
1
Look first for large obvious features such as a big pale vacuole or a thick outer wall.
2
Check for organelles with double membranes and compare their sizes, since chloroplasts are larger than mitochondria.
3
Look for internal detail: thylakoid stacks indicate a chloroplast, while cristae indicate a mitochondrion.
4
Use the scale bar or magnification to estimate sizes, and state the evidence you used for your identification.
Section 4 of 4Exam Checklist & Tips
4.1.2 – Key Points for the Exam
- Electron micrograph An image produced by an electron microscope showing fine cell detail.
- Thylakoid A membrane-bound flattened sac inside a chloroplast containing chlorophyll.
- Cristae Folds of the inner mitochondrial membrane.
- Nucleolus A dense region inside the nucleus where ribosomal components are made.
Common Mistakes
- Confusing chloroplasts with mitochondria; look for thylakoid stacks rather than just a double membrane.
- Calling the white area of a vacuole empty space in the cell, when it holds cell sap.
- Identifying the cell membrane as the outermost boundary, when the cell wall lies outside it.
- Giving an identification without stating the visible evidence that supports it.
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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.