recognise organelles and other cell structures found in eukaryotic cells and outline their structures and functions, limited to: cell surface membrane, nucleus, nuclear envelope and nucleolus, rough endoplasmic reticulum, smooth endoplasmic reticulum

Cambridge A‑Level Biology – Topic 1: Cell Structure

These notes cover everything required for the Cambridge 9700 syllabus on cell structure. They are organised to help you meet the assessment objectives:

• AO1 – Knowledge: recognise and describe all required organelles and cell structures.
• AO2 – Handling information: calculate magnifications, interpret scale bars, compare cell types and produce accurate labelled drawings.

1. Microscopy Basics (AO2)

1.1 Preparing a temporary (wet) mount

1. Place a small drop of water (or appropriate medium) on a clean glass slide.
2. Transfer the specimen (e.g., onion epidermis, cheek cells) into the drop.
3. Lower a cover‑slip at an angle; slide it gently to avoid air bubbles.
4. Secure the slide on the stage, bring it into focus with the coarse knob, then fine‑tune with the fine knob.

1.2 Magnification and resolution

• Magnification (M) = objective power × eyepiece power.
• Example: 40× objective + 10× eyepiece = 400× total magnification.
• Resolution is the smallest distance that can be distinguished. For light microscopy the limit is ≈ 0.2 µm.
• Common mistake: forgetting that resolution is a property of the instrument, not of the specimen.

1.3 Using a stage micrometer

1. Place the stage micrometer (e.g., 1 mm = 100 µm) on the stage and bring it into focus.
2. Count how many eyepiece‑graticule divisions span a known distance on the micrometer.
3. Calculate the scale:
   

   Scale (µm per division) = (known distance in µm) ÷ (number of divisions).

Example calculation: 50 divisions of the eyepiece graticule span 100 µm → 1 division = 2 µm.

Tip: always write the units when you calculate; converting µm to mm later is a frequent source of error.

1.4 Interpreting scale bars

• The scale bar on a photomicrograph is labelled (e.g., 10 µm).
• Measure the feature of interest with a ruler or the software’s measurement tool.
• Calculate the real size:
  

  Real size = (measured length ÷ scale‑bar length) × scale‑bar value.

Example: a nucleus measures 5 mm on the screen, the scale bar is 2 mm and represents 10 µm → nucleus ≈ (5 ÷ 2) × 10 µm = 25 µm.

1.5 Electron microscopy (EM) – a brief overview

• Resolution: up to 0.1 nm (far below light microscopy), allowing visualization of membranes, ribosomes and viral particles.
• Typical magnifications: 10 000× – 1 000 000×.
• Two main types:

  • Transmission EM (TEM) – shows internal ultrastructure of thin sections.
  • Scanning EM (SEM) – shows surface topography.

• Practice activity: examine a supplied TEM image of a plant cell, identify the cell wall, chloroplast thylakoids and mitochondria, and note the scale bar (e.g., 0.5 µm).

2. Recognising Key Eukaryotic Organelles (AO1)

2.1 Cell Surface Membrane (Plasma Membrane)

• Structure: fluid phospholipid bilayer; embedded integral & peripheral proteins; cholesterol (animals); carbohydrate chains (glycoproteins/glycolipids) on the outer leaflet.
• Functions:

  • Selective permeability – controls entry/exit of ions, nutrients and waste.
  • Signal transduction – receptors bind hormones, neurotransmitters.
  • Cell‑cell recognition – glycoproteins/glycolipids act as markers.
  • Anchoring of cytoskeletal elements.

• Relevance to key concepts: enables the cell to maintain homeostasis and to communicate with its environment.

2.2 Nucleus, Nuclear Envelope & Nucleolus

• Nucleus – largest organelle; contains DNA organised into chromosomes.
• Nuclear envelope – double membrane continuous with the ER; perforated by nuclear pores (≈30–50 nm) that regulate traffic of RNA, proteins and ribosomal subunits.
• Nucleolus – dense sub‑structure; site of rRNA transcription, processing and ribosome subunit assembly.
• Functions:

  • DNA storage and protection.
  • Transcription of mRNA, rRNA and snRNA.
  • Export of mRNA to cytoplasm; import of proteins (e.g., transcription factors).
  • Ribosome biogenesis in the nucleolus.

• Key concept link: the nucleus is the control centre of the cell, directing protein synthesis.

2.3 Rough Endoplasmic Reticulum (RER)

• Structure: network of flattened sacs (cisternae) studded with ribosomes on the cytoplasmic face.
• Functions:

  • Synthesis of proteins destined for secretion, insertion into membranes, or delivery to lysosomes.
  • Initial folding and N‑linked glycosylation of nascent polypeptides.
  • Transport of newly made proteins to the Golgi apparatus via vesicles.

• Concept link: connects gene expression (nucleus) with the secretory pathway.

2.4 Smooth Endoplasmic Reticulum (SER)

• Structure: tubular network lacking ribosomes; often more abundant in cells that synthesise lipids.
• Functions:

  • Lipid synthesis – phospholipids, cholesterol and steroid hormones.
  • Detoxification – metabolism of drugs, alcohol and xenobiotics (especially in liver cells).
  • Storage and regulated release of Ca²⁺ ions (important in muscle cells and signal transduction).

• Concept link: provides the cell with essential lipids and helps maintain intracellular calcium balance.

2.5 Mitochondrion

• Structure: double membrane; inner membrane folded into cristae, enclosing the matrix; own circular DNA.
• Functions:

  • Production of ATP by aerobic respiration (oxidative phosphorylation).
  • Regulation of apoptosis and calcium homeostasis.

• Concept link: the “powerhouse” of the cell – supplies energy for all active processes.

2.6 Golgi apparatus

• Structure: stacked, flattened cisternae (cis‑face receives vesicles from ER, trans‑face dispatches vesicles).
• Functions:

  • Modification of proteins and lipids (e.g., glycosylation, phosphorylation).
  • Sorting and packaging into secretory vesicles or lysosomes.

• Concept link: final processing centre of the secretory pathway.

2.7 Lysosome

• Structure: membrane‑bound vesicle containing hydrolytic enzymes (acid hydrolases) active at pH ≈ 5.
• Functions:

  • Intracellular digestion of macromolecules, worn‑out organelles (autophagy) and material taken up by endocytosis.
  • Role in programmed cell death (apoptosis).

• Concept link: maintains cellular cleanliness and recycles components.

2.8 Vacuole

• Structure: large, membrane‑bound sac; in plant cells a central vacuole occupies up to 90 % of cell volume; in animal cells usually smaller, often called vesicles.
• Functions:

  • Storage of water, ions, sugars, pigments and waste products.
  • In plants, generates turgor pressure that maintains rigidity.

• Concept link: contributes to cell volume regulation and, in plants, to growth.

2.9 Cell Wall (plant cells)

• Structure: rigid layer outside the plasma membrane; mainly cellulose fibres embedded in a matrix of hemicellulose and pectin.
• Functions:

  • Provides mechanical support and defines cell shape.
  • Prevents excessive water uptake (osmotic protection).

• Concept link: essential for plant form and for resisting turgor pressure.

2.10 Cilia and Microvilli

• Cilia – short, hair‑like projections (9 + 2 microtubule arrangement) that beat rhythmically to move fluid or the cell itself.
• Microvilli – finger‑like extensions of the plasma membrane supported by actin filaments; increase surface area for absorption.
• Functions:

  • Cilia: locomotion (e.g., protozoa) or moving mucus in respiratory epithelium.
  • Microvilli: nutrient absorption in intestinal epithelium.

• Concept link: adaptations that modify the cell’s interaction with its environment.

2.11 Centrioles (animal cells)

• Structure: a pair of orthogonal cylinders composed of nine triplets of microtubules.
• Functions:

  • Organise the mitotic spindle during cell division.
  • Form the basal bodies that nucleate cilia and flagella.

• Concept link: crucial for accurate chromosome segregation.

2.12 Chloroplast (plant & algae)

• Structure: double membrane; internal thylakoid stacks (grana) surrounded by stroma; own circular DNA.
• Functions:

  • Photosynthesis – conversion of light energy into chemical energy (glucose) via the light‑dependent and light‑independent reactions.
  • Synthesis of fatty acids and amino acids.

• Concept link: the site of primary energy capture in autotrophic eukaryotes.

2.13 Virus (non‑cellular entity)

3. Prokaryotes vs. Eukaryotes (AO1)

4. Drawing & Interpreting Photomicrographs (AO2)

Interpreting an EM image (advanced AO2): Identify the double membrane of mitochondria, the stacked thylakoids of a chloroplast, and the ribosome‑studded RER. Use the provided scale bar to estimate dimensions (e.g., cristae spacing ≈ 15 nm).

5. Summary Table of All Required Organelles

6. Suggested Diagram

7. Learning Outcomes

• Identify each organelle in labelled diagrams and real micrographs (light and electron).
• Describe the main structural components and at least two key functions for every organelle listed in the syllabus.
• Perform microscopy calculations: magnification, resolution, scale‑bar conversions, and avoid common unit‑conversion errors.
• Compare prokaryotic and eukaryotic cells, explaining why certain structures are absent in prokaryotes.
• Produce accurate, labelled drawings of plant and animal cells, including a scale bar and magnification.
• Explain how each organelle contributes to the broader concepts of “cells as the basic unit of life” and “cellular metabolism”.