What is the difference between rough ER and smooth ER?

Rough endoplasmic reticulum (RER) has ribosomes attached to its surface, giving it a 'rough' appearance. These ribosomes synthesise proteins that are then processed and transported by the RER. Smooth ER (SER) lacks ribosomes and is involved in lipid synthesis, carbohydrate metabolism, and detoxification. Both are continuous with the nuclear envelope.

Why do mitochondria have a double membrane?

The double membrane of mitochondria is a key adaptation for aerobic respiration. The outer membrane is smooth and permeable to small molecules, while the inner membrane is highly folded into cristae to increase surface area for the electron transport chain and ATP synthase. The intermembrane space allows for a proton gradient to be established, which drives ATP production.

What are the main differences between prokaryotic and eukaryotic cells?

Prokaryotic cells lack a nucleus and membrane-bound organelles, while eukaryotic cells have both. Prokaryotes have a single circular chromosome in a nucleoid region, 70S ribosomes, and a cell wall made of peptidoglycan. Eukaryotes have linear chromosomes enclosed in a nucleus, 80S ribosomes, and may have cell walls made of cellulose (plants) or chitin (fungi). Prokaryotes are generally smaller and simpler.

How do you identify organelles in an electron micrograph?

Look for distinctive shapes and structures: mitochondria appear as oval or bean-shaped with folded inner membranes (cristae); chloroplasts have stacks of thylakoids (grana); the nucleus is large and round with a double membrane and pores; rough ER has ribosomes on its surface; the Golgi apparatus consists of stacked flattened sacs (cisternae). Use scale bars to estimate size.

What is the function of the nucleolus?

The nucleolus is a dense region within the nucleus where ribosomal RNA (rRNA) is synthesised and assembled with proteins to form ribosomal subunits. These subunits are then exported to the cytoplasm to form functional ribosomes. The nucleolus is not membrane-bound and is visible under a light microscope during interphase.

Why are lysosomes important in cells?

Lysosomes are membrane-bound organelles containing hydrolytic enzymes that break down waste materials, old organelles, and pathogens. They play a key role in autophagy (recycling cellular components) and apoptosis (programmed cell death). Their acidic interior (pH ~5) keeps enzymes inactive until needed, preventing damage to the cytoplasm.

Cell structure

OCR

A-Level

This topic explores the fundamental structure and function of cells, which are the basic units of all living organisms. It covers the use of various microscopy techniques to investigate cell ultrastructure, the roles of specific organelles, and the key differences between prokaryotic and eukaryotic cells.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

Topic Overview

Cell structure is a foundational topic in OCR A-Level Biology, covering the detailed anatomy of eukaryotic and prokaryotic cells. You will explore the functions of organelles such as the nucleus, mitochondria, chloroplasts, and ribosomes, and understand how these structures work together to maintain life. This topic also introduces the concept of compartmentalisation, which is key to understanding metabolic efficiency and cell specialisation.

Mastering cell structure is crucial because it underpins almost every other topic in biology, from respiration and photosynthesis to protein synthesis and cell division. By understanding how cells are built, you can better grasp how diseases like cancer arise from structural changes, and how antibiotics target bacterial cell walls. This knowledge is also essential for practical work, such as using microscopes to identify cell types and organelles.

In the wider OCR A-Level syllabus, cell structure connects to topics like transport across membranes, cell recognition and the immune system, and cellular control. It provides the vocabulary and conceptual framework needed for more advanced studies, making it a cornerstone of your biology revision. Expect to see questions that test your ability to compare and contrast different cell types, explain adaptations, and interpret electron micrographs.

Key Concepts

Core ideas you must understand for this topic

→Eukaryotic vs prokaryotic cells: Eukaryotes have a nucleus and membrane-bound organelles; prokaryotes lack these and have a nucleoid, 70S ribosomes, and a cell wall made of peptidoglycan (murein).
→Organelle functions: Know the roles of the nucleus (genetic material), mitochondria (ATP production via aerobic respiration), chloroplasts (photosynthesis), rough ER (protein synthesis and transport), Golgi apparatus (modification and packaging), and lysosomes (digestion).
→Compartmentalisation: Membrane-bound organelles create separate compartments for different metabolic reactions, increasing efficiency and preventing interference (e.g., lysosomal enzymes kept separate from cytoplasm).
→Cell ultrastructure: Be able to identify organelles from electron micrographs and describe their adaptations, such as the folded inner membrane of mitochondria (cristae) for increased surface area.
→Prokaryotic features: Prokaryotes have a cell wall (peptidoglycan), capsule, flagella, pili, and plasmids; they lack membrane-bound organelles but have 70S ribosomes and a single circular chromosome.

What You Need to Demonstrate

Key skills and knowledge for this topic

Distinction between magnification and resolution
Functions of organelles: nucleus, nucleolus, nuclear envelope, rough/smooth ER, Golgi apparatus, ribosomes, mitochondria, lysosomes, chloroplasts, plasma membrane, centrioles, cell wall, flagella, and cilia
Interrelationship between organelles in protein production and secretion
Importance of the cytoskeleton for mechanical strength, transport, and movement
Comparison of prokaryotic and eukaryotic cell structure and ultrastructure
Calculation of magnification using the formula: magnification = size of image / size of real object
Preparation of slides and use of staining in light microscopy
Use of eyepiece graticule and stage micrometer

Marking Points

Key points examiners look for in your answers

Distinction between magnification and resolution
Functions of organelles: nucleus, nucleolus, nuclear envelope, rough/smooth ER, Golgi apparatus, ribosomes, mitochondria, lysosomes, chloroplasts, plasma membrane, centrioles, cell wall, flagella, and cilia
Interrelationship between organelles in protein production and secretion
Importance of the cytoskeleton for mechanical strength, transport, and movement
Comparison of prokaryotic and eukaryotic cell structure and ultrastructure
Calculation of magnification using the formula: magnification = size of image / size of real object
Preparation of slides and use of staining in light microscopy
Use of eyepiece graticule and stage micrometer

Examiner Tips

Expert advice for maximising your marks

💡Always check units before performing magnification calculations; ensure image size and real object size are in the same units.
💡When asked about organelle function, be specific (e.g., mention protein synthesis for ribosomes rather than just 'making things').
💡Practice interpreting electron micrographs (TEM vs SEM) as these are frequently tested.
💡Ensure you can describe the pathway of a protein from synthesis in the rough ER to secretion via the Golgi apparatus.
💡Use clear, sharp pencil lines for scientific drawings and ensure annotations are precise.
💡Use precise terminology: In exam answers, always use the correct terms like 'peptidoglycan' for bacterial cell walls and 'cristae' for mitochondrial inner membrane folds. Avoid vague terms like 'stuff' or 'things'.
💡Link structure to function: When describing an organelle, always explain how its structure helps it perform its function. For example, 'The Golgi apparatus has flattened cisternae to increase surface area for modifying and packaging proteins.'
💡Practice drawing and labelling: You may be asked to draw or interpret diagrams. Ensure your labels are accurate and include key features like the double membrane of mitochondria and the presence of ribosomes on rough ER.

Common Mistakes

Pitfalls to avoid in your exam answers

Confusing magnification with resolution
Failing to correctly convert units (e.g., mm to µm) when using the magnification formula
Inaccurate labeling of cell diagrams or failing to annotate scientific drawings
Misunderstanding the specific roles of the rough ER versus the smooth ER
Incorrectly identifying the differences between prokaryotic and eukaryotic cells (e.g., assuming prokaryotes lack all membranes)
Misconception: All cells have a nucleus. Correction: Only eukaryotic cells have a true nucleus; prokaryotic cells have a nucleoid region with no membrane.
Misconception: Mitochondria and chloroplasts have 80S ribosomes like the cytoplasm. Correction: They have 70S ribosomes, similar to prokaryotes, supporting the endosymbiotic theory.
Misconception: The cell wall is the same in all organisms. Correction: Plant cell walls are made of cellulose, fungal walls of chitin, and bacterial walls of peptidoglycan (murein).

Frequently Asked Questions

Common questions students ask about this topic

Before You Start

Prior knowledge that will help with this topic

•GCSE Biology: Basic cell structure, including the differences between animal and plant cells, and the functions of major organelles.
•Basic microscopy: Understanding of magnification, resolution, and how to use a light microscope.
•Biochemistry basics: Familiarity with the roles of proteins, lipids, and carbohydrates in cell membranes and walls.

Likely Command Words

How questions on this topic are typically asked

Describe

Explain

Calculate

Compare

Contrast

Identify

Outline

Ready to test yourself?

Practice questions tailored to this topic

Cell structure

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in OCR A-Level Biology

Analysis

Biodiversity

Biological membranes

Biological molecules

How to Revise Cell structure — OCR A-Level Biology

Examiner Tips for Cell structure

Common Mistakes in Cell structure

Key Marking Points

Cell structure

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

What is the difference between rough ER and smooth ER?

Why do mitochondria have a double membrane?

What are the main differences between prokaryotic and eukaryotic cells?

How do you identify organelles in an electron micrograph?

What is the function of the nucleolus?

Why are lysosomes important in cells?

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in OCR A-Level Biology

Analysis

Biodiversity

Biological membranes

Biological molecules