2 Organelles and Cellular Organization
Learn how eukaryotic organelles specialize cellular tasks and coordinate the processing, transport, uptake, and recycling of materials.
How organelles divide cellular work
Eukaryotic cells divide work among specialized structures called organelles. By concentrating particular activities in particular places, cells can carry out different chemical processes in organized compartments. Some organelles are enclosed by membranes, while others, such as ribosomes, are not.
The nucleus houses most of the cell’s DNA. Its double membrane, the nuclear envelope, contains pores that regulate exchange with the cytosol and is continuous with the endoplasmic reticulum. Inside the nucleus, the nucleolus makes components of ribosomes.
Ribosomes build proteins by translating messenger RNA. Their location helps determine the usual destination of the proteins they make: free ribosomes typically make proteins used in the cytosol, while ribosomes attached to the make many proteins for membranes, organelles, or secretion.
Other organelles have specialized roles:
Smooth ER: Makes lipids and contributes to detoxification and calcium storage.
Vacuoles: Store water and dissolved substances and serve other functions. Plant cells commonly have a large central vacuole that helps maintain internal pressure.
: Convert energy from food molecules into ATP through cellular respiration.
Chloroplasts: In plants and algae, capture light energy to make sugars by photosynthesis.
Peroxisomes: Carry out specialized oxidation reactions, including the breakdown of fatty acids, and help detoxify hydrogen peroxide.
The key idea is specialization: each structure contributes particular functions to the cell, and some organelles coordinate their activities through shared transport pathways.
The
The is a coordinated network for making, modifying, sorting, and transporting proteins and lipids. It includes the nuclear envelope, endoplasmic reticulum, , vesicles, endosomes, lysosomes, and the plasma membrane through its exchange of material. and chloroplasts are excluded because their membranes do not participate in this trafficking network.
The endoplasmic reticulum (ER) is a connected network of membrane-bound sacs and tubules. Its two regions have different roles:
has ribosomes attached to its surface. It makes and begins processing many proteins.
Smooth ER lacks ribosomes and contributes to lipid synthesis, detoxification, and calcium storage.
The further modifies, sorts, and packages proteins and lipids received from the ER. Its cis side receives much of this material, while its trans side sorts it toward destinations such as the plasma membrane, lysosomes, or outside the cell.
Vesicles link these compartments. They form by budding from a membrane and deliver material by fusing with another membrane. Endosomes sort material brought into the cell, while lysosomes break down some of the material delivered to them. This organization allows the cell to move materials while preserving the specialized functions of its compartments.
Following a protein through the cell
A protein destined for secretion illustrates how several parts of the work together:
A ribosome attached to the makes the protein, which enters the ER as it is produced.
The ER folds and begins modifying the protein, then packages it into a transport .
The delivers the protein to the , where it is further modified and sorted.
A new carries the protein to the plasma membrane. When the fuses with the membrane, the protein is released outside the cell.
Other routes send proteins to lysosomes or insert them into the plasma membrane. The route a protein takes depends on its destination, but the general pattern is movement between compartments followed by sorting and delivery.
Uptake, recycling, and the big picture
The also supports uptake and recycling. When a region of the plasma membrane bends inward and buds off, it forms a containing material from outside the cell. That can deliver its contents to an endosome, where the material is sorted. Some is recycled; other material is sent to lysosomes for breakdown.
Together, secretion and uptake move materials in different directions through the cell. Vesicles connect compartments, while sorting determines whether material is delivered, recycled, or broken down. and chloroplasts perform important energy-related functions but do not belong to this membrane-transport network.
Takeaway: Organelles specialize cellular work, and the coordinates the processing and movement of many proteins and lipids.