Table of Contents
Overview
Cells contain many different structures, but a few basic types show up again and again across life: thin membranes, thread‑like fibrils, and small grain‑like grana. These are not specific to one organelle or cell type; instead, they are “construction principles” from which many cellular components are built.
In this chapter, you will learn what is meant by membrane, fibril, and granum as basic structural types, how they are organized, and why cells so often rely on these recurring designs.
Membranes
General Features of Biological Membranes
A membrane in biology is a very thin, flexible boundary layer that separates two spaces containing water‑based solutions. Almost all membranes in cells share a common basic structure:
- They are made mainly of lipids (especially phospholipids) and proteins.
- They are typically about 7–10 nm thick (far below what can be seen with a light microscope).
- They form closed surfaces: there is no free “edge” in a healthy cell membrane; it always encloses something.
Membranes create compartments: separated reaction spaces in which different conditions (pH, ion concentrations, and so on) can be maintained.
The Fluid Mosaic Concept (Structural Principle)
Biological membranes are often described as a fluid mosaic:
- Fluid: Lipid molecules can move sideways within the layer. Proteins can also float and move within this “sea” of lipids. The membrane is not a rigid shell but has a certain flexibility and self‑healing ability (small tears can reseal).
- Mosaic: Different proteins and lipids are distributed in a patchy pattern. Various types of proteins are embedded in the membrane at specific positions.
At its simplest, the membrane consists of a phospholipid bilayer:
- Hydrophilic (“water‑loving”) heads face the watery environment inside and outside the cell.
- Hydrophobic (“water‑repelling”) tails point inward, away from the water, facing each other.
This arrangement is thermodynamically stable in water and explains why membranes form spontaneously around cells and organelles.
Functions as a Basic Structure
As a basic structural type, a membrane provides several generic functions:
- Barrier: It separates inside from outside and different internal compartments from one another.
- Selective exchange: Only certain substances pass easily (for example, small nonpolar molecules). Others require special proteins.
- Interface for reactions: Many reactions in cells take place on or in membranes. Membranes can be densely packed with enzymes and other proteins.
- Surface for electrical and chemical gradients: Differences in ion concentrations and electrical charge are often maintained across membranes. This is crucial, for example, for signal transmission and energy conversion.
The exact details of how these functions are used in specific organelles (e.g., mitochondria, chloroplasts, endoplasmic reticulum) are covered in their respective chapters. Here the focus is: membranes are the universal thin boundary and reaction surface of cells.
Types of Membrane‑Related Structures
From the basic membrane layer, cells can build many forms:
- Flat sheets or sacs (cisternae)
- Tubes and tubules
- Vesicles (small membrane spheres)
- Stacks of membrane sacs
- Invaginations and folds that increase surface area
The next basic structure, the granum, is an example of a stacked membrane structure.
Fibrils
What Is a Fibril?
A fibril (from Latin fibra = fiber) is a long, thin, often thread‑like structure in the cell. The term is used somewhat broadly in biology, but as a basic structure type, it refers to elongated, mostly filamentous assemblies of proteins.
Fibrils often:
- Have a diameter in the nanometer range (not visible in a light microscope as individual strands).
- Are much longer than they are wide.
- Are formed by the regular association (polymerization) of many identical or similar protein subunits.
Fibrils as Building Elements of the Cytoskeleton
A key role of fibrils is in the cytoskeleton, the internal “scaffolding” of the cell. Three major fibrillar systems are typical for eukaryotic cells:
- Microfilaments (actin filaments)
- Very thin, flexible filaments made of actin protein.
- Important for cell shape changes, cell movement, and contractile structures.
- Intermediate filaments
- Medium thickness.
- Provide mechanical stability; help cells resist stretching.
- Microtubules
- Hollow tubes made of tubulin protein.
- Organize the cell interior, form “tracks” for transport, and are central to the mitotic spindle.
Even though each of these filament types has its own detailed functions (covered elsewhere), they share a fibrillar construction principle: many small protein subunits form long, linear fibers.
Extracellular and Other Fibrils
Fibrils are not limited to the cell’s interior:
- Collagen fibrils in connective tissue (between cells) form tough, rope‑like structures that give tissues strength.
- Cell wall fibrils in plants (for example, cellulose microfibrils) strengthen walls and define cell shape.
- Amyloid fibrils are misfolded protein aggregates that form long fibers; they occur in some diseases.
In each case, the fibril serves as a mechanical element:
- To bear tension or compression.
- To transmit forces.
- To define shape and spatial structure.
Generic Properties of Fibrillar Structures
As a basic structural motif, fibrils have several general properties:
- Polarity (in many cases): the two ends of a fibril can be different (plus/minus ends), which is important for directed assembly and transport.
- Dynamic assembly: fibrils can be built up and broken down relatively quickly by adding or removing subunits.
- Bundling and networking: fibrils can form thicker fibers by bundling or meshworks by cross‑linking, giving rise to higher‑order structures such as networks, cables, and layers.
Thus, fibrils function as a modular construction kit for mechanical and organizational tasks in and around cells.
Grana (Singular: Granum)
Granum as a Structural Type
A granum (plural grana, Latin for “grain”) is a small stack of membrane sacs found specifically in the chloroplasts of plants and some algae. Although grana are characteristic of chloroplasts, they illustrate a more general structural idea: densely stacked membranes forming a compact, grain‑like unit.
In terms of basic structure, a granum is:
- A column of flattened membrane sacs (thylakoids) piled on top of each other.
- Surrounded by the internal fluid of the chloroplast (stroma).
- Connected to other membrane sacs by thinner membrane channels (stroma lamellae).
Under the light microscope, grana appear as tiny dark spots or grains inside chloroplasts; under the electron microscope, the stack of membranes becomes visible.
Organization of Grana
Some key structural features that define grana:
- Flattened sacs: Each “disc” in the stack is a thylakoid, a flattened vesicle enclosed by a membrane.
- Regular spacing: The sacs are separated by very narrow gaps filled with a thin layer of fluid.
- Stacking: Several to dozens of thylakoids can be stacked to form a single granum.
- Lateral connections: Grana are not isolated; unstacked thylakoid membranes (stroma lamellae) interconnect the stacks, forming a continuous internal membrane system.
The granular appearance results from the repeated layering of membranes in a defined, tightly packed arrangement.
Why Stacking Is Structurally Important
The granum illustrates a general design principle: increasing surface area in a limited volume by stacking membranes. This brings several structural advantages:
- High density of membrane surface: Many membrane‑bound protein complexes can be accommodated in a small space.
- Orderly arrangement: Proteins and pigments are organized in specific patterns within the stacked membranes.
- Functional separation: The stack can have slightly different composition and functions from surrounding unstacked regions of the same membrane system.
While the detailed function of grana is linked to photosynthesis and is treated in the respective chapter, here the key idea is that the granum is a specialized, stacked membrane unit—a three‑dimensional arrangement built from the basic structural element “membrane.”
Grana as an Example of Higher‑Order Membrane Structures
From a construction point of view, the granum shows how basic structures can combine:
- Start with a membrane bilayer (basic membrane).
- Shape it into flattened sacs (thylakoids).
- Arrange many sacs into a stack (granum).
- Connect multiple stacks with unstacked membrane regions into a large, integrated membrane system.
This hierarchical organization—membrane → sac → stack → interconnected network—is a common strategy in cells to build complex organelles from simple basic units.
Comparison and Interaction of the Three Structural Types
Membranes, fibrils, and grana represent three different but complementary structural principles used by cells:
- Membrane
- Form: thin, flexible sheet or closed surface.
- Main function: separation of spaces; controlled exchange; reaction surface.
- Typical composition: lipid bilayer with embedded proteins.
- Fibril
- Form: elongated, thread‑like fiber.
- Main function: mechanical support, shape, and movement; internal organization.
- Typical composition: long chains of protein subunits (e.g., actin, tubulin, collagen).
- Granum
- Form: compact stack of flattened membrane sacs (a higher‑order membrane arrangement).
- Main role: high‑density packing of membrane surfaces in a limited volume (in chloroplasts).
Although they differ in shape and composition, they interact:
- Membranes often need fibrillar supports (for example, underlying cytoskeletal fibrils that maintain cell shape or help position membrane‑bound organelles).
- Fibrils can be anchored to membranes, coupling mechanical forces to membrane surfaces.
- Structures like grana are built from membranes but are positioned and sometimes influenced by underlying cytoskeletal fibrils within the chloroplast.
By reusing a small set of basic structures—membranes, fibrils, and stacked membrane units such as grana—cells build a wide variety of complex, specialized components while relying on a limited number of fundamental construction principles.