A Tour of the Cell: Structure and Function of Eukaryotic Cells
The Fundamental Units of Life
All living organisms are composed of cells.
The cell represents the simplest collection of matter that can be considered alive.
All cells are related by their shared evolutionary descent from earlier cells.
Despite their diversity, all cells possess common fundamental features.
The internal organization of eukaryotic cells enables them to perform the essential functions of life:
Energy and Matter Transformations:
A system of internal membranes synthesizes and modifies proteins, lipids, and carbohydrates.
Chloroplasts are specialized organelles that convert light energy into chemical energy.
Mitochondria are responsible for breaking down molecules, generating ATP (adenosine triphosphate) for cellular energy.
Internal membranes compartmentalize the cell, creating distinct regions where specific chemical reactions can occur efficiently.
Genetic Information Storage and Transmission:
DNA, located within the nucleus, contains the instructions required for making proteins.
Ribosomes serve as the sites where protein synthesis takes place, translating the genetic instructions.
Interactions with the Environment:
The plasma membrane acts as a selective barrier, controlling the movement of substances (e.g., oxygen, nutrients, waste) into and out of the cell.
Plant cells possess an additional protective layer, the cell wall.
Studying Cells: Microscopes and Biochemistry
Cells are typically microscopic and cannot be observed with the naked eye.
Microscopy
Microscopes are indispensable tools used to visualize cells and their components.
Light Microscope (LM):
Operates by passing visible light through a specimen and then through glass lenses.
Lenses refract (bend) the light, which magnifies the image of the specimen.
Three Important Parameters of Microscopy:
Magnification: Defined as the ratio of an object’s image size to its real size.
Resolution: A measure of the clarity of the image, representing the minimum distance between two distinguishable points. The resolution of standard light microscopy is approximately .
* Contrast: Refers to the visible differences in brightness between various parts of a sample, enhancing the ability to distinguish structures.
* Light microscopes can effectively magnify specimens up to approximately times their actual size.
* Various techniques are employed to enhance contrast and permit the staining or labeling of specific cell components.
* However, the resolution of standard light microscopy is too low to adequately study organelles, which are the membrane-enclosed structures characteristic of eukaryotic cells.Electron Microscopes (EMs): Used to study subcellular structures with much higher resolution than LMs.
Scanning Electron Microscopes (SEMs):
Focus a beam of electrons onto the surface of a specimen.
Produce images that provide a detailed three-dimensional (3-D) appearance of the specimen's surface.
Transmission Electron Microscopes (TEMs):
Focus a beam of electrons through a specimen.
Are primarily used to investigate the internal structure of cells, revealing organelles and subcellular details.
Recent Advances in Light Microscopy:
Confocal microscopy and deconvolution microscopy are sophisticated techniques that provide sharper, more detailed images of three-dimensional tissues and cells, overcoming some limitations of conventional 2D imaging.
New techniques for labeling cells have led to super-resolution microscopy, which significantly improves resolution beyond the theoretical limits of traditional light microscopy.
Cryo-electron microscopy (Cryo-EM):
A method that allows the preservation of biological specimens at extremely low temperatures.
This technique enables the visualization of structures in their native cellular environment, eliminating the need for chemical fixatives or stains that could alter their natural state.
Cryo-EM is often used to complement X-ray crystallography in elucidating the structures of protein complexes and subcellular structures.
Microscopes are essential tools for cytology, the scientific discipline focused on the study of cell structure.
Cell Fractionation
Cell fractionation is a process that disassembles cells and separates their major organelles from one another.
Centrifuges are the primary instruments used to fractionate cells into their component parts through differential centrifugation.
This process involves subjecting a cell homogenate to progressively higher centrifugal forces and durations.
Example sequence:
for yields a pellet rich in nuclei and cellular debris.
The supernatant is then spun at for , yielding a pellet rich in mitochondria and chloroplasts.
Further centrifugation at for produces a pellet rich in “microsomes” (fragments of ER and other membranes).
Finally, spinning at for yields a pellet rich in ribosomes.
Purpose: Cell fractionation allows scientists to isolate and individually study the functions of specific organelles.
Interdisciplinary approach: Biochemistry and cytology are integrated to correlate the observed structure of cellular components with their biochemical functions.
Eukaryotic Cells: Internal Membranes and Compartmentalization
The basic structural and functional unit of every organism is either a prokaryotic or a eukaryotic cell.
Prokaryotic Cells: Organisms belonging to the domains Bacteria and Archaea are composed solely of prokaryotic cells.
Eukaryotic Cells: Protists, fungi, animals, and plants are all comprised of eukaryotic cells.
Comparing Prokaryotic and Eukaryotic Cells
Basic Features Common to All Cells:
Plasma membrane: The outer boundary that encloses the cell.
Cytosol: A semifluid, jellylike substance found inside the plasma membrane, in which subcellular components are suspended.
Chromosomes: Structures carrying genetic information in the form of DNA (genes).
Ribosomes: Molecular complexes responsible for protein synthesis.
Prokaryotic Cell Characteristics:
Lack a true, membrane-bound nucleus.
Their DNA is concentrated in an unbound region known as the nucleoid.
Do not possess membrane-bound organelles within their cytoplasm.
Their cytoplasm is directly enclosed by the plasma membrane.
May feature structures such as fimbriae, a cell wall, a capsule, a bacterial chromosome, and flagella (as seen in, for example, the rod-shaped bacterium Bacillus coagulans).
Eukaryotic Cell Characteristics:
Possess DNA housed within a nucleus, which is bounded by a membranous nuclear envelope.
Contain various membrane-bound organelles that perform specialized functions.
Their cytoplasm refers specifically to the region located between the plasma membrane and the nucleus.
Generally significantly larger than prokaryotic cells.
The Plasma Membrane
The plasma membrane functions as a selective barrier, regulating the passage of essential substances—such as oxygen, nutrients, and waste products—to adequately service the volume of every cell.
Structurally, it is a double layer (bilayer) of phospholipids with associated proteins and carbohydrate side chains (forming glycolipids and glycoproteins).
Cell Size Limits: Surface Area to Volume Ratio
Metabolic requirements impose critical upper limits on the size that cells can attain.
The surface area to volume ratio of a cell is a critical determinant of its efficiency.
As a cell's size increases, its volume grows proportionally much more rapidly than its surface area (volume increases by while surface area increases by ).
This relationship means that larger cells have a smaller surface area to volume ratio, which can limit the rate at which substances can enter or leave the cell relative to its metabolic needs.
For example: Consider cubes with different side lengths. A single cube with a side length of units has a surface area of units and a volume of units, resulting in an S-to-V ratio of . In contrast, smaller cubes, each with a side length of unit, would collectively have a total surface area of units and a total volume of units, yielding a much higher S-to-V ratio of . This demonstrates how smaller units, or cells with extensive membrane folds (e.g., microvilli), maintain a high surface area for efficient exchange.
Panoramic View of the Eukaryotic Cell
A eukaryotic cell is characterized by internal membranes that partition the cell into various specialized compartments called organelles.
The fundamental structural component of most biological membranes is a double layer of phospholipids along with other lipids.
Plant and animal cells typically share most of the same organelles, though some differences exist (e.g., chloroplasts and cell walls in plants; lysosomes in animals).
Genetic Instructions: Nucleus and Ribosomes
The Nucleus: Information Central
The nucleus contains the vast majority of the DNA (genes) in a eukaryotic cell and is usually the most conspicuous organelle.
Nuclear Envelope:
Encloses the nucleus, physically separating its contents from the cytoplasm.
It is a double membrane, with each membrane consisting of a lipid bilayer.
The inner surface of the nuclear envelope is lined by the nuclear lamina, a meshwork of proteins that helps maintain the shape of the nucleus.
Nuclear Pores: These perforations in the nuclear envelope regulate the specific entry and exit of molecules (e.g., proteins, RNA) between the nucleus and the cytoplasm.
DNA Organization:
Within the nucleus, DNA is organized into discrete units known as chromosomes.
Each chromosome is composed of a single DNA molecule intimately associated with a variety of proteins, primarily histones.
This complex of DNA and associated proteins is collectively referred to as chromatin.
Chromatin condenses and coils tightly to form visible, discrete chromosomes as a cell prepares to undergo division.
Nucleolus:
A prominent, non-membranous structure located within the nucleus.
It is the primary site of ribosomal RNA (rRNA) synthesis.
Also, in the nucleolus, rRNA combines with proteins imported from the cytoplasm to assemble ribosomal subunits, which are then exported to the cytoplasm.
Ribosomes: Protein Factories
Ribosomes are molecular complexes meticulously constructed from ribosomal RNA (rRNA) and various proteins.
Their fundamental role is to carry out protein synthesis (translation) in two distinct locations within the cell:
In the cytosol (Free Ribosomes): These ribosomes synthesize proteins that are destined to function within the cytoplasm itself (e.g., enzymes involved in glycolysis).
On the outside of the endoplasmic reticulum or the nuclear envelope (Bound Ribosomes): These ribosomes typically synthesize proteins destined for insertion into membranes, for secretion from the cell, or for specific organelles within the endomembrane system.
The Endomembrane System: Protein Traffic and Metabolic Functions
The endomembrane system is an intricate network of membranes within eukaryotic cells that work together to synthesize, modify, and transport proteins and lipids, detoxify poisons, and carry out various metabolic functions.
Its components are either continuous with one another or are connected functionally via the transfer of membrane-bound sacs called vesicles.
Components of the Endomembrane System:
Nuclear envelope
Endoplasmic reticulum (ER)
Golgi apparatus
Lysosomes
Vacuoles
Plasma membrane
The Endoplasmic Reticulum (ER): Biosynthetic Factory
The endoplasmic reticulum (ER) constitutes over half of the total membrane content in many eukaryotic cells, forming an extensive network.
The ER membrane is physically continuous with the outer membrane of the nuclear envelope.
It consists of a network of membranous tubules and sacs called cisternae, enclosing an internal compartment called the ER lumen or cisternal space.
There are two functionally and structurally distinct regions of the ER:
Smooth ER: Characterized by its lack of ribosomes on its surface, giving it a smooth appearance.
Rough ER: Distinguished by the presence of ribosomes studded on its outer surface, making it appear