Bio 1.3-1.4
1.3 THE CELL
Introduction to Cells
The concept of the cell as an independent unit of life.
A cell is defined as the simplest entity capable of existing independently.
All living organisms are comprised of either a single cell or a combination of multiple cells.
Examples of unicellular organisms include bacteria, yeasts, and algae.
Multicellular organisms include plants and animals, which have billions to trillions of cells functioning together coordinately (see Fig. 1.10).
Figure 1.10: Unicellular and Multicellular Organisms
Living organisms depicted:
(a) Bacteria
(b) Brewer’s yeast
(c) Algae
(d) Cheetahs
(e) Humans
Size and Diversity of Cells
Most cells are microscopic, typically measuring below the threshold of visibility to the naked eye.
Skin cells average about 100 micrometers (μm) or 0.1 mm in diameter, translating to approximately 20 skin cells fitting across the period at the end of this sentence.
Bacteria often measure less than one micrometer in length.
Specialized cells can be significantly larger, such as nerve cells in humans which can extend axons over distances reaching up to a meter.
Additionally, an ostrich egg represents a single, giant cell (see Fig. 1.11).
Figure 1.11: Cell Diversity
Examples of different types of cells presented:
(a) Skin cells
(b) Nerve cells
(c) Ostrich egg
Common Features of All Cells
Despite their differences in appearance and size, cells are structurally similar in several key aspects:
Information Storage: All cells maintain a stable blueprint of information in a molecular format that aids in growth, function, and reproduction.
Boundary Definition: Cells are enclosed by distinct membranes that separate their interior from the external environment.
Energy Utilization: Cells are capable of harnessing materials and energy from their surroundings.
1.4 DNA and Its Role
Nucleic Acids Function
Nucleic acids, such as DNA, serve the crucial role of storing and transmitting information necessary for cellular functions.
DNA’s Structure and Function
DNA is a double-stranded helical molecule composed of sequences of four different nucleotide subunits.
The unique arrangement of these nucleotides forms the genetic code, essential for encoding cellular information.
It directs the synthesis of proteins, the functional and structural molecules responsible for the majority of cellular activities.
Protein Synthesis Process
Information in DNA leads to protein synthesis through a two-step process:
Transcription: DNA's information is transcribed into messenger RNA (mRNA).
Translation: The mRNA is interpreted by ribosomes to assemble amino acids into proteins.
Central Dogma of Molecular Biology
The key flow of information within cells is encapsulated as the Central Dogma of Molecular Biology:
Pathway: DNA → RNA → Protein
It lays the fundamental framework for understanding cellular operation, with certain exceptions.
A gene is defined as a DNA sequence that codes for a specific protein.
1.5 Replication and Mutation in DNA
DNA Replication
DNA replication is the process by which DNA is accurately copied to ensure transmission of genetic information across generations.
Each strand of the double helix acts as a template for producing a new complementary strand.
Replication must be precise to prevent genetic errors that could jeopardize cellular function.
Mutations
Mutations are permanent changes in the DNA sequence that may arise from errors during replication or environmental damage.
They can be detrimental, neutral, or beneficial, influencing the evolutionary trajectory of species.
1.6 Cell Membranes
Plasma Membrane Function
The plasma membrane is essential for delineating cells from their external environments.
It is dynamic, allowing for selective transport of materials in and out of the cell, with the ability to control waste disposal as well as nutrient uptake (see Fig. 1.14).
Internal Membranes
Within cells, internal membranes establish distinct compartments for specialized functions.
The nucleus, for instance, houses genetic material and regulates molecular traffic between its interior and the cytoplasm.
Cell Classification
Cells can be classified into two categories:
Prokaryotes: Cells without a nucleus; examples include bacteria.
Eukaryotes: Cells containing a nucleus; include a wide range of organisms such as fungi, plants, and animals.
Origin of Cells
Prokaryotic cells are believed to be the earliest forms of life, emerging around 4 billion years ago, with descendants including bacteria that play various roles in ecosystems.
Eukaryotic cells evolved later from prokaryotic ancestors around 2 billion years ago and may exhibit cellular specialization in multicellular forms, like humans.
1.7 Energy and Metabolism
Metabolism Defined
Metabolism encompasses chemical reactions enabling cells to convert energy from their environment into usable forms.
Organisms derive energy primarily from two sources: sunlight or chemical compounds.
Energy Transfer
The molecule adenosine triphosphate (ATP) is crucial for energy transfer, allowing cells to perform multiple tasks such as growth, division, and active transport.
Conservation of Metabolic Processes
Certain metabolic pathways are evolutionarily conserved across various organisms, indicative of their fundamental importance in cellular processes.
1.8 Viruses
Characteristics of Viruses
Viruses consist of genetic material encased in a protein coat; however, they lack the ability to metabolize energy independently and depend on host cells for replication.
Viruses infect host cells by inserting their genetic material and utilizing host machinery for reproduction.
Implications of Viral Infection
The host cell often sustains damage, leading to its lysis (breakdown), which releases new viral particles.
Viruses exhibit specificity for bacterial, archaeal, or eukaryotic hosts.
They play crucial roles in biological research and serve as model systems for understanding various biological phenomena.
1.9 Evolution
Unity and Diversity in Life
The overall theme of unity and diversity in life is explored through the lens of evolution by natural selection.
Natural selection operates on variation within populations, favoring traits that enhance reproduction and survival in a specific environment.
Genetic Variation and Evolution
Two primary sources of variation in a population:
Environmental Variation: External factors influencing traits.
Genetic Variation: Differences in DNA among individuals transmitted from parents to offspring, which leads to observable traits and hence evolutionary change.
1.10 Mechanisms of Natural Selection
Principles of Natural Selection
Variants best suited to their environments reproduce more effectively, leading to observable changes in populations over generations.
Evolutionary Changes and Adaptation
Adaptive traits tend to proliferate within a population, while detrimental traits diminish, leading to gradual changes in the genetic composition of populations over time.
Example: Artificial Selection
Historical selection for desirable traits in agriculture and domestication illustrates principles of natural selection evident in contemporary species variations, such as dog breeds (see Fig. 1.15).
Evolution of New Species
New species arise through population divergence from common ancestors, leading to nested similarities among species as depicted in evolutionary trees (see Fig. 1.16).
1.11 Phylogenetic Relationships
Overview of the Tree of Life
The tree of life diagram illustrates evolutionary relationships, showing the branching of species and the descent from common ancestors that trace back billions of years.
It emphasizes that eukaryotes originated from a specific event of symbiosis between archaea and bacteria, leading to vast diversity in modern organisms.
Prediction of Fossil Record
The evolutionary model makes predictions about the order of species appearance in the fossil record, which is corroborated by geological data and continues to be refined by advances in molecular biology.