These organelles manage cellular breakdown, storage, and energy capture. Understanding their membranes, internal compartments, and core functions helps explain how eukaryotic cells maintain stability, recycle materials, and convert energy into usable chemical forms.

Lysosomes

Structure and enzymes

Lysosomes are prominent in many animal cells and are part of cellular “cleanup” and recycling. Their single membrane isolates digestive enzymes from the cytosol, preventing uncontrolled degradation of cellular components.

Key features and requirements:

• Acidic lumen (low pH) supports maximal activity of many hydrolytic enzymes.

• Hydrolytic enzymes break down proteins, nucleic acids, lipids, and polysaccharides.

• Selective targeting delivers specific substrates to lysosomes rather than random cytosolic digestion.

Digestion and recycling roles

Lysosomes digest materials that come from both outside and inside the cell.

This diagram shows the sequence of lysosomal digestion, from uptake of material into a vesicle to fusion with a lysosome and enzymatic breakdown of contents. It reinforces how hydrolytic enzymes are compartmentalized within a membrane-bound organelle to prevent uncontrolled digestion of the cytosol. The final step highlights recycling, where digested products can re-enter cellular metabolism. Source

• Endocytosed material: macromolecules or particles internalised into vesicles can fuse with lysosomes for breakdown.

• Damaged organelles/components: worn-out cell parts can be delivered for degradation, allowing reuse of monomers (e.g., amino acids, nucleotides).

• Pathogen destruction: many immune cells use lysosomal digestion to help eliminate engulfed microbes.

This recycling supports cellular efficiency by returning building blocks to biosynthetic pathways rather than relying solely on new uptake.

Lysosomes and apoptosis

Lysosomes can contribute to apoptosis by releasing degradative enzymes that help dismantle cellular structures once death pathways are activated. This supports development, tissue maintenance, and removal of damaged cells. Controlled digestion limits harm to neighbouring cells, contrasting with uncontrolled cell rupture.

Vacuoles

General functions and diversity

Vacuoles are especially important in plant cells, where a large central vacuole often occupies much of the cell volume. In other eukaryotes, smaller vacuoles may store nutrients or sequester substances.

Core roles emphasised in AP Biology:

• Storage of water, ions, pigments, nutrients, or wastes

• Regulation of internal conditions by controlling solute and water content

• Structural support in plants through pressure-based mechanisms

Turgor pressure and plant support

The central vacuole helps maintain turgor pressure by accumulating water.

This diagram compares plant cells under different osmotic conditions to show how water entry or loss changes vacuole volume and pressure against the cell wall. It visually connects vacuolar water storage to mechanical support (turgid cells) versus wilting/flaccidity when water leaves the vacuole. The labeled arrows emphasize that turgor is an emergent property of osmosis acting on a large central vacuole. Source

When water enters the vacuole, it expands and presses the cytoplasm against the cell wall, helping the cell resist compression and maintain shape. If water is lost, turgor drops and cells become flaccid, reducing support in non-woody tissues.

Functional implications in plant cells:

• Maintains upright growth in leaves and young stems

• Contributes to cell expansion as vacuoles enlarge

• Stores solutes that influence water movement into or out of the vacuole

Chloroplasts

Role in photosynthesis and overall organisation

Chloroplasts perform photosynthesis and are characterised by a complex membrane system. Their architecture creates specialised regions that support different reaction stages and efficient energy conversion.

Double membranes and internal membranes

Chloroplasts have:

• An outer membrane and inner membrane (a double-membrane envelope) that separate the organelle from the cytosol and regulate exchange.

• Extensive internal membranes called thylakoid membranes, which are often stacked into grana and interconnected.

These membranes create distinct compartments:

This labeled chloroplast illustration highlights the double-membrane envelope and the internal thylakoid membrane system organized into grana. It helps students map each compartment to its role in photosynthesis: the stroma as the enzyme-rich interior and the thylakoid/lumen as the membrane-bound space that supports energy conversion. The diagram reinforces how membrane organization increases surface area and separates conditions needed for photosynthetic reactions. Source

• Intermembrane space: between outer and inner membranes

• Stroma: fluid interior containing enzymes, DNA, and ribosomes

• Thylakoid lumen: interior space enclosed by thylakoid membranes

The presence of double membranes and internal membranes increases surface area and enables separation of chemical conditions needed for photosynthetic energy conversion, including the organisation of pigment-protein complexes and electron transport components within thylakoid membranes.

Functional significance of compartmentalisation

By structuring photosynthesis across compartments, chloroplasts:

• Localise light-dependent processes to thylakoid membranes

• Maintain distinct chemical environments between stroma and thylakoid lumen

• Improve efficiency by concentrating key molecules and membrane complexes where they are needed

This internal membrane architecture is central to chloroplast function, enabling the conversion of light energy into chemical energy stored in carbohydrates.