Biology: Cells and Cellular Structure

The Fundamental Units of Life: Cells

• Cells are the basic units of life, similar to atoms in chemistry, and are essential to understanding biological systems.
• Many different types of cells work together in an organism, each with unique functions, such as muscle cells that help move eyes and nerve cells that transmit signals to the brain.

Cellular Structure and Function

• Cells are the simplest units of matter that can be considered living entities, and all organisms are composed of cells.
• Single-celled organisms, such as Paramecium, exist, while larger organisms are multicellular, consisting of many specialized cells that work together.

Evolutionary History of Cells

• All cells share a common ancestry, having descended from earlier cells.
• Over time, cells have evolved and diversified into different types, despite sharing common features.

Cellular Components and Organization

• Cells are organized into higher levels, such as tissues and organs, but remain the basic unit of structure and function in an organism.
• The cell's components and functions will be explored in this chapter.

Microscopy

• Biologists use microscopes to study cells, which are too small to be seen by the unaided eye.
• Microscopes were invented in 1590 and further refined during the 1600s.
• Cell walls were first seen by Robert Hooke in 1665 using a microscope.
• Antoni van Leeuwenhoek was able to visualize living cells using his crafted lenses.
• Light microscopes (LM) use visible light passed through the specimen and then through glass lenses to magnify the image.
• Three important parameters in microscopy are magnification, resolution, and contrast.
• Magnification is the ratio of an object's image size to its real size, up to 1,000 times the actual size of the specimen.
• Resolution is a measure of the clarity of the image, with a minimum distance of 0.2 micrometer (µm) or 200 nanometers (nm) for light microscopes.
• Contrast is the difference in brightness between the light and dark areas of an image, and can be enhanced through staining or labeling cell components.

Electron Microscopy

• The electron microscope (EM) was introduced to biology in the 1950s to study organelles in eukaryotic cells.
• Electron microscopes focus a beam of electrons through the specimen or onto its surface.
• The resolution of electron microscopes is inversely related to the wavelength of the electrons used, achieving a theoretical resolution of about 0.002 nm.
• In practice, electron microscopes can resolve structures as small as 2 nm across, a 100-fold improvement over standard light microscopes.
• The scanning electron microscope (SEM) is especially useful for detailed study of the topography of cells.

Light Microscopy

• Brightfield microscopy: light passes directly through the specimen, but image has little contrast unless the cell is naturally pigmented or artificially stained.
• Staining with dyes enhances contrast, but most staining procedures require cell fixation, which kills the cells.
• Phase-contrast microscopy: variations in density within the specimen are amplified to enhance contrast in unstained cells.
• Differential interference contrast (Nomarski): optical modifications are used to exaggerate differences in density, creating a 3-D image.
• Fluorescence microscopy: specific molecules in the cell are revealed by labeling with fluorescent dyes or antibodies; fluorescent substances absorb ultraviolet radiation and emit visible light.

Confocal and Deconvolution Microscopy

• Confocal microscopy: uses a laser to eliminate out-of-focus light, creating a single plane of fluorescence in the image.
• Deconvolution: digitally removes out-of-focus light and reassigns it to its source, creating a sharper 3-D image.

Super-Resolution Microscopy

• Super-resolution microscopy: uses sophisticated equipment to light up individual fluorescent molecules and record their position, allowing for resolution beyond the 200-nm limit.

Electron Microscopy

• Scanning electron microscopy (SEM): produces 3-D images of the surface of a specimen.
• Transmission electron microscopy (TEM): profiles a thin section of a specimen, revealing its internal structure.

Electron Microscopy

• Scanning Electron Microscope (SEM) is used for detailed study of specimen topography, producing a 3D image of the surface.
• SEM operates by scanning the surface of a gold-coated sample with an electron beam, which excites electrons that are detected and translated into an electronic signal.
• Transmission Electron Microscope (TEM) is used to study internal structure of cells by aiming an electron beam through a thin section of the specimen.
• TEM uses heavy metal atoms to stain certain cellular structures, enhancing electron density and producing an image displaying the pattern of transmitted electrons.

Electron Microscopy vs. Light Microscopy

• Electron microscopes have revealed many subcellular structures that were impossible to resolve with light microscopy.
• However, light microscopy offers advantages in studying living cells.
• Electron microscopy requires specimen preparation methods that kill the cells, whereas light microscopy allows for observation of living cells.

Advances in Light Microscopy

• Labeling individual cellular molecules or structures with fluorescent markers has made it possible to see structures with increasing detail.
• Confocal and deconvolution microscopy have produced sharper images of three-dimensional tissues and cells.
• Super-resolution microscopy has enabled researchers to distinguish subcellular structures as small as 10-20 nm across.

Cell Fractionation

• Cell fractionation is a technique used to study cell structure and function by separating major organelles and subcellular structures from one another.
• The centrifuge is used to spin test tubes holding mixtures of disrupted cells at increasing speeds, causing cell components to settle to the bottom of the tube.
• Cell fractionation enables researchers to prepare specific cell components in bulk and identify their functions, correlating cell function with structure.

Integration of Cytology and Biochemistry

• Cytology is the study of cell structure, while biochemistry is the study of the chemical processes (metabolism) of cells.
• Understanding cell function requires the integration of cytology and biochemistry.
• Cell fractionation and electron microscopy have helped biologists determine the functions of specific cell components, such as mitochondria being the sites of cellular respiration.

Cell Structure and Function

• Cells are the basic structural and functional units of every organism.
• There are two distinct types of cells: prokaryotic and eukaryotic.

Characteristics of Cells

• All cells are bounded by a selective barrier called the plasma membrane (or cell membrane).
• Cells contain a semifluid, jellylike substance called cytosol, where subcellular components are suspended.
• All cells have chromosomes, which carry genes in the form of DNA.
• Cells have ribosomes, which make proteins according to instructions from the genes.

Comparison of Prokaryotic and Eukaryotic Cells

• A major difference between prokaryotic and eukaryotic cells is the location of their DNA.
• In eukaryotic cells, most of the DNA is in the nucleus, which is bounded by a double membrane.
• In prokaryotic cells, the DNA is concentrated in a region called the nucleoid, which is not membrane-enclosed.

Organisms and Their Cell Types

• Organisms of the domains Bacteria and Archaea consist of prokaryotic cells.
• Protists, fungi, animals, and plants all consist of eukaryotic cells.
• Protists are a diverse group of mostly unicellular eukaryotes.