Organelle Structure and Function
A thorough understanding of organelle ultrastructure and function is essential for A-Level Biology. You need to know how each organelle's structure is related to its function, how organelles work together within a cell, and how to interpret electron micrographs. This lesson covers the organelles required by AQA specification 3.2.1 in detail.
Key Definition: An organelle is a distinct, membrane-bound or non-membrane-bound structure within a cell that performs a specific function. The term literally means 'little organ'.
The Nucleus
The nucleus is the largest organelle in most eukaryotic cells and serves as the control centre.
Structure
- Surrounded by a nuclear envelope — a double membrane perforated by nuclear pores (approximately 3000 per nucleus).
- Nuclear pores are about 100 nm in diameter and are lined by specific pore proteins that regulate transport of large molecules (mRNA, ribosomal subunits out; histones, DNA polymerase in).
- Contains chromatin — a complex of DNA and histone proteins. During cell division, chromatin condenses into visible chromosomes.
- One or more nucleoli (singular: nucleolus) are present. The nucleolus is a dense region where ribosomal RNA (rRNA) is transcribed and ribosomal subunits are assembled before being exported through nuclear pores.
- The nucleoplasm (nuclear sap) is the fluid matrix inside the nucleus, containing dissolved enzymes, nucleotides, and ions.
Functions
- Contains the cell's genetic information (DNA) which directs protein synthesis.
- DNA is transcribed into mRNA in the nucleus, which then passes through nuclear pores to ribosomes for translation.
- The nucleolus produces rRNA, essential for ribosome assembly.
- Retains the chromosomes within a protected environment, separated from the cytoplasmic enzymes that might damage DNA.
Exam Tip: Be precise about the nuclear envelope — it is a double membrane, not a single membrane. Each nuclear pore is a complex structure, not simply a hole.
Mitochondria
Mitochondria are the sites of aerobic respiration — the process that produces the majority of a cell's ATP.
Structure
- Double membrane: the outer membrane is smooth; the inner membrane is folded into cristae (singular: crista).
- The cristae greatly increase the surface area of the inner membrane, providing more space for the electron transport chain and ATP synthase — the enzymes involved in oxidative phosphorylation.
- The fluid-filled interior is called the matrix. It contains enzymes for the Krebs cycle (also known as the citric acid cycle or tricarboxylic acid cycle), as well as the link reaction.
- Mitochondria contain their own circular DNA and 70S ribosomes, allowing them to synthesise some of their own proteins.
- Typically 1–10 µm in length and 0.5–1 µm in diameter. The number per cell varies: metabolically active cells such as muscle cells and liver hepatocytes contain many thousands, whereas red blood cells contain none.
Functions
- Carry out the link reaction and Krebs cycle in the matrix.
- Perform oxidative phosphorylation on the inner membrane (cristae), producing the majority of the cell's ATP via chemiosmosis.
- Involved in apoptosis (programmed cell death) through the release of cytochrome c.
Structure–Function Link: Cells with high energy demands (e.g., sperm cells, cardiac muscle fibres, active transport epithelial cells) contain large numbers of mitochondria. The extensive cristae maximise ATP production.
Endoplasmic Reticulum (ER)
The endoplasmic reticulum is an extensive system of membrane-bound flattened sacs (cisternae) and tubules continuous with the nuclear envelope.
Rough Endoplasmic Reticulum (RER)
- Studded with 80S ribosomes on its cytoplasmic surface.
- Synthesises proteins destined for secretion (e.g., digestive enzymes, antibodies, hormones such as insulin), for the cell surface membrane, or for lysosomes.
- Proteins are threaded into the RER lumen during translation, where they may be folded and undergo initial modifications (e.g., glycosylation — the addition of carbohydrate groups).
- Transport vesicles bud off from the RER and carry proteins to the Golgi apparatus for further processing.
- Cells specialised for protein secretion (e.g., B lymphocytes producing antibodies, pancreatic acinar cells producing digestive enzymes) have extensive RER.
Smooth Endoplasmic Reticulum (SER)
- Lacks ribosomes on its surface.
- Synthesises lipids, including phospholipids and steroids (e.g., cholesterol, oestrogen, testosterone).
- Involved in detoxification of drugs and alcohol in liver cells (hepatocytes).
- Stores and releases calcium ions in muscle cells (sarcoplasmic reticulum), which is essential for muscle contraction.
- Cells in the adrenal cortex and ovaries/testes have extensive SER due to steroid hormone production.
Golgi Apparatus (Golgi Complex)