Published by Patrick Mutisya · 8 days ago
Understanding the structure and function of organelles in eukaryotic cells is essential for grasping how cells maintain life processes. The following notes cover the major organelles and cell structures required for the Cambridge A‑Level Biology (9700) syllabus.
The cell surface membrane (plasma membrane) is a flexible, semi‑permeable barrier composed of a phospholipid bilayer with embedded proteins. It regulates the movement of substances in and out of the cell and facilitates communication via receptors.
The nucleus houses the cell’s genetic material (DNA). It is surrounded by a double‑layered nuclear envelope containing nuclear pores that control molecular traffic. The nucleolus, located within the nucleus, is the site of ribosomal RNA synthesis and ribosome assembly.
The RER is a network of flattened sacs studded with ribosomes on its cytoplasmic surface. It synthesises proteins destined for secretion, insertion into membranes, or lysosomal targeting.
The SER consists of tubular membranes lacking ribosomes. Functions include lipid synthesis, detoxification of drugs and poisons, and calcium ion storage.
The Golgi apparatus is a series of stacked membrane‑bound cisternae. It modifies, sorts, and packages proteins and lipids received from the ER for delivery to their final destinations.
Mitochondria are double‑membrane organelles that generate ATP through oxidative phosphorylation. They contain their own small circular DNA, allowing limited autonomous protein synthesis.
Ribosomes are the sites of protein synthesis. Cytoplasmic ribosomes are 80S (composed of 60S large and 40S small subunits). Ribosomes within chloroplasts and mitochondria are 70S, reflecting their prokaryotic ancestry.
Lysosomes are membrane‑bound vesicles containing hydrolytic enzymes that degrade macromolecules, old organelles, and foreign material.
Centrioles are cylindrical structures composed of nine triplets of microtubules. They organise the mitotic spindle during cell division and serve as basal bodies for cilia and flagella formation.
Cilia are hair‑like projections of the plasma membrane supported by a core of microtubules (the axoneme). They beat rhythmically to move fluids over the cell surface or propel single‑celled organisms.
Microvilli are finger‑like extensions of the plasma membrane that increase surface area for absorption, especially in intestinal epithelial cells. Each microvillus contains a core of actin filaments.
Chloroplasts are double‑membrane organelles found in plant cells and algae. They contain thylakoid stacks (grana) where photosynthesis occurs and possess their own small circular DNA, enabling limited autonomous protein synthesis.
The plant cell wall is a rigid structure composed mainly of cellulose, hemicellulose, and pectin. It provides mechanical support, determines cell shape, and protects against osmotic stress.
Plasmodesmata are microscopic channels that traverse the cell wall, linking the cytoplasm of adjacent plant cells. They allow the transport of ions, metabolites, and signalling molecules.
The large central vacuole occupies up to 90 % of a mature plant cell’s volume. It stores water, ions, pigments, and waste products. The tonoplast is the vacuolar membrane that regulates transport into and out of the vacuole.
| Organelle / Structure | Key Structural Features | Primary Functions |
|---|---|---|
| Cell Surface Membrane | Phospholipid bilayer with embedded proteins | Selective permeability, signalling, transport |
| Nucleus (incl. envelope & nucleolus) | Double membrane with pores; nucleolus inside | DNA storage, RNA synthesis, ribosome assembly |
| Rough ER | Flattened sacs with ribosomes on cytoplasmic side | Protein synthesis for secretion/membranes |
| Smooth ER | Tubular network, no ribosomes | Lipid synthesis, detoxification, Ca²⁺ storage |
| Golgi Apparatus | Stacked cisternae | Protein modification, sorting, packaging |
| Mitochondria | Double membrane, inner cristae, circular DNA | ATP production via oxidative phosphorylation |
| Ribosomes (80S / 70S) | RNA‑protein complexes; 80S in cytoplasm, 70S in organelles | Protein synthesis |
| Lysosomes | Membrane‑bound vesicles with hydrolytic enzymes | Digest macromolecules, recycle organelles |
| Centrioles & Microtubules | 9 triplet microtubule cylinders; spindle fibers | Cell division, cilia/flagella basal bodies |
| Cilia | Membrane‑bound projections with axoneme (9+2 microtubules) | Move fluids, locomotion |
| Microvilli | Actin‑core supported membrane protrusions | Increase absorptive surface area |
| Chloroplasts | Double membrane, thylakoid stacks, circular DNA | Photosynthesis (light & dark reactions) |
| Cell Wall | Cellulose‑rich rigid layer external to plasma membrane | Structural support, protection, osmotic regulation |
| Plasmodesmata | Channels linking adjacent plant cells | Intercellular transport of solutes and signals |
| Large \cdot acuole & Tonoplast | Central fluid‑filled sac; tonoplast membrane | Storage, turgor maintenance, waste sequestration |
These notes provide a concise reference for the required organelles and structures. Students should be able to recognise each organelle in diagrams, describe its main structural components, and explain its role in the cell.