Cell Structures - OCR GCSE Biology A (Gateway) Revision Notes

Light Microscopes

Microscope Equipment

• Biological structures often too small to see with the naked eye.
• Optical microscopes are crucial for studying tissues, cells, and organelles.
• Light passes through biological material mounted on a glass slide, focused by lenses to produce visible images.

Key Components

• Eyepiece lens
• Objective lenses
• Stage and light source
• Coarse and fine focus

Additional Apparatus

• Forceps, Scissors, Scalpel
• Coverslip, Slides, Pipette

Calibration, Sample Prep & Setup

Preparing Specimens

• Must carefully prepare specimens on slides to avoid damage.
• Common specimens: cheek cells (animal), onion cells (plant).
• Liquid specimens: Add drops to slide, cover with coverslip.
• Solid specimens: Use scissors to cut sample, prepare thin cell layer.
• Stains: e.g., methylene blue for cheek cells, iodine for onion cells.

Viewing Specimen

• Start with low power objective lens.
• Clear images require proper focus and thin enough specimens.

Using Graticule

• Graticule: Scale for measuring cells, requiring calibration for each lens.
• Limitations: Varying cell sizes, 2D slides from 3D structures.
• Conversion of units (mm, μm, nm).

Calculation of Magnification

• Formula: Magnification = Image Size / Actual Size.
• Total magnification = Eyepiece magnification x Objective magnification.
• Standard form for unit conversions.

Cell Structure

Overview

• Organisms made of cells, classified as eukaryotes or prokaryotes.
• Eukaryotes: Unicellular or multicellular, e.g., animals, plants.
• Prokaryotes: Always unicellular, e.g., bacteria, simpler structure.

Prokaryote Cell Structure

• Smaller size, cytoplasm for chemical reactions.
• Cell membrane: selective barrier.
• Cell wall: Provides strength, shape.
• DNA: Circular chromosome in cytoplasm, plus plasmids.

Eukaryote Cell Structure

Animal Cells

• Cytoplasm and cell membrane for holding cell together and reactions.
• Nucleus with DNA, mitochondria for energy from food.

Plant Cells

• Same structures as animal cells plus cellulose cell wall, chloroplasts for photosynthesis.

Electron Microscopes

Increased Resolution

• Electron microscopes use electron beams.
• Higher resolution and magnification than light microscopes.
• Developed in the 1930s.

Transmission Electron Microscopy

• Higher magnification (up to 50 million times).
• Detailed study of cell structures like nucleus and membrane.
• Not typically available in schools.

Comparing Microscopes

• Light Microscopes: Stained images, lower resolution.
• Electron Microscopes: Higher resolution, black and white images.
• Images show detailed cellular structures.

This guide covers essential concepts needed to understand cell structures and the use of different types of microscopes in biological studies, focusing on the differences between light and electron microscopes, and the preparation and viewing of specimens.