Cytoplasm and membrane-bound organelles

Biology · The cell. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

The cytoplasm of a eukaryotic cell contains organelles, each with its own job. Many of them work as one system. Proteins made on the ribosomes of the rough endoplasmic reticulum pass to the Golgi apparatus, where they are modified and packed into vesicles. Some vesicles leave the cell as secretions; others become lysosomes, the bags of digestive enzymes.

Mitochondria release energy as ATP, and chloroplasts trap light energy. Both have a double membrane, their own DNA and their own 70S ribosomes, so they are called self-replicating or semi-autonomous organelles.

The cytoskeleton gives the cell its shape and moves things inside it.

Rules to remember

Rough ER has ribosomes and makes and transports proteins; smooth ER has none and deals with lipid (steroid) metabolism and detoxification of drugs.
Ribosomes: rRNA + protein, no membrane. Eukaryotic 80S = 60S + 40S subunits; prokaryotic 70S = 50S + 30S. A polysome is a group of ribosomes on one mRNA.
Golgi apparatus: stacks of flattened sacs (cisternae); modifies, packages and sorts proteins for secretion; forms lysosomes.
Lysosomes: single membrane, hydrolytic enzymes; digest food particles and worn-out organelles (autophagy). Storage diseases occur when one lysosomal enzyme is missing, e.g. glycogenosis type II and Tay-Sachs disease.
Peroxisomes: oxidative enzymes that form H₂O₂, and catalase that breaks it down. Glyoxysomes: in germinating fatty seeds, change fats into carbohydrates.
Mitochondria: inner membrane folded into cristae, matrix inside. Chloroplasts: stroma with grana, each granum a pile of thylakoids. Plastids: chloroplasts (green), chromoplasts (other colours), leucoplasts (colourless, storage).
Microtubules are made of tubulin, microfilaments of actin. Centriole = 9 triplets of microtubules; cilia and flagella = 9 + 2 (nine doublets around two single microtubules).

Common mistake

Students give lysosomes and peroxisomes the same job. Lysosomes hold hydrolytic (digestive) enzymes; peroxisomes hold oxidative enzymes and catalase, which deal with hydrogen peroxide.

Worked example

A gland cell makes a digestive enzyme and releases it outside the cell. In what order does the enzyme pass through the cell structures?

The enzyme is a protein, so it is made on ribosomes attached to the rough ER.
It moves through the channels of the rough ER and is carried in vesicles to the Golgi apparatus.
The Golgi apparatus modifies it and packs it into secretory vesicles.
The vesicles fuse with the plasma membrane and release the enzyme by exocytosis.

Answer: Ribosomes of rough ER → rough ER → Golgi apparatus → secretory vesicles → plasma membrane.

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Chapter 4: The Cell

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