Cell Structure, Function, and Metabolism: Study Guide for Biology Students

General Structure of Prokaryotic and Eukaryotic Cells

Cell Theory and Historical Context

The cell theory is foundational to biology, stating that all living organisms are composed of cells, which are the basic structural and functional units of life. Cells arise from preexisting cells, and all life forms are composed of one or more cells.

• Cell: The smallest unit of life, first described by Robert Hooke in 1655.

• Cell Theory: Proposed by Schleiden and Schwann in 1838-1839.

• Major Events in Cell Biology: Key discoveries include the identification of bacteria, development of cell theory, and advances in genetic engineering.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on structural and functional differences.

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles; include bacteria and archaea.

• Eukaryotic Cells: Possess a nucleus and membrane-bound organelles; include plants, animals, fungi, and protists.

Characteristic	Prokaryotic Cell	Eukaryotic Cell
Size	0.2–60 µm	5–100 µm
Chromosome	One	Multiple
Plasma membrane	Yes	Yes
Cell wall	Yes	Yes (plant, algae, fungi)
Nucleus	No	Yes (except red blood cells)
Nucleoid area	Yes	No
Mitochondria	No	Yes
Endoplasmic reticulum	No	Yes
Golgi apparatus	No	Yes
Cytoskeleton	No	Yes
Ribosome	70S	80S
Mode of reproduction	Asexual	Asexual and sexual

Plasma Membrane and Cell Wall

The plasma membrane is a dynamic, fluid-mosaic structure composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. It regulates the entry and exit of substances and provides structural support.

• Integral proteins: Embedded within the bilayer.

• Peripheral proteins: Located on the membrane surface.

• Cell wall: Provides additional protection and shape; composition varies among bacteria, plants, fungi, and algae.

Gram-Positive vs. Gram-Negative Bacterial Cell Walls

Bacterial cell walls are distinguished by Gram staining, which reflects differences in peptidoglycan content and structure.

• Gram-Positive: Thick peptidoglycan layer, contains teichoic acids, minimal periplasmic space.

• Gram-Negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharides, large periplasmic spac

Medical Highlights

Endotoxin Shock: Endotoxins from gram-negative bacteria can cause severe systemic inflammatory responses when released during cell lysis.

Types and Functions

Surface appendages facilitate motility, attachment, absorption, and sensory functions. They are present in both prokaryotic and eukaryotic cells.

Surface Appendage	Cell Type	Composition	Function
Flagella	Eukaryotic	Microtubules (tubulin)	Motility (whiplike action)
Flagella	Prokaryotic	Flagellin subunits	Motility (propeller-like rotation)
Pili	Prokaryotic	Pilin proteins	Adhesion, conjugation
Cilia	Eukaryotic	Microtubules	Movement of fluid, sensory
Microvilli	Eukaryotic	Membrane extensions	Absorption, secretion, motility

Flagella Structure and Types

Flagella are responsible for motility in both prokaryotic and eukaryotic cells, but their structure differs. Bacterial flagella consist of a filament, hook, and basal body.

• Monotrichous: Single flagellum at one end.

• Lophotrichous: Tufts of flagella at one or both ends.

• Amphitrichous: Single flagellum at each end.

• Peritrichous: Flagella distributed over the entire surface.

Pili, Cilia, and Microvilli

Pili are rigid structures in prokaryotes for adhesion and genetic exchange. Cilia are shorter than flagella and move fluids over cell surfaces in eukaryotes. Microvilli are membrane extensions in eukaryotic cells, increasing surface area for absorption.

Biofilms

Formation and Importance

Biofilms are collections of surface-associated microbes enclosed in a polysaccharide matrix. They are important in healthcare due to their resistance to antimicrobial agents and their role in chronic infections.

• Stages: Reversible attachment, irreversible attachment, growth and division, exopolymer production, attachment of other organisms.

• Applications: Bioremediation, biofilters, wastewater treatment.

Cytoplasm and Cytoskeleton

The cytoplasm is a gelatinous matrix where metabolic activities occur. The cytoskeleton provides structural support, enables intracellular transport, and forms the spindle apparatus during cell division.

• Components: Actin filaments, intermediate filaments, microtubules.

Cell Organelles

Organelles are specialized structures within eukaryotic cells, each with distinct functions.

Name	Description	Function
Endoplasmic reticulum (ER)	Network of membranes; rough ER has ribosomes	Protein and lipid synthesis
Ribosomes	RNA and protein bodies	Protein manufacture
Golgi apparatus	Membrane layers	Protein modification and sorting
Mitochondria	Internal folded membranes	ATP production
Lysosomes	Digestive enzyme sacs	Intracellular digestion
Peroxisomes	Enzyme-containing organelles	Breakdown of harmful substances
Proteasomes	Barrel-shaped organelles	Protein degradation
Vesicles	Membrane-bound sacs	Storage and transport
Centrioles	Rod-shaped bodies	Chromosome separation

Fluid Compartments and Membrane Transport Mechanisms

Intracellular and Extracellular Fluid Compartments

Cells maintain homeostasis by regulating internal and external fluid compartments.

• Intracellular Fluid (ICF): Largest compartment, high in potassium, phosphate, magnesium.

• Extracellular Fluid (ECF): Surrounds cells, high in sodium, chloride, bicarbonate.

Membrane Transport Mechanisms

Transport across membranes occurs via passive and active mechanisms.

• Passive Transport: No energy required; includes diffusion, facilitated diffusion, osmosis, and filtration.

• Active Transport: Requires energy (ATP); includes pump transport, endocytosis, and exocytosis.

Cellular Metabolism

Enzymes

Enzymes are biological catalysts that lower activation energy and speed up chemical reactions. They are specific for substrates and can be regulated by temperature, pH, substrate concentration, and inhibitors.

• Enzyme + Substrate = Product + Enzyme

• Types: Oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases.

• Regulation: Competitive and noncompetitive inhibition.

Cellular Respiration and Photosynthesis

Cellular respiration releases energy from nutrients to produce ATP. It includes aerobic (glycolysis, Krebs cycle, electron transport chain) and anaerobic (fermentation) pathways. Photosynthesis converts light energy into chemical energy in plants and algae.

• Aerobic respiration: $\text{Glucose} + 2\text{NAD} + 2\text{ATP} + 2\text{Pi} \rightarrow 2\text{pyruvic acid} + 2\text{NADH} + 2\text{ATP}$

• Photosynthesis: $\text{CO}_2 + \text{H}_2\text{O} + \text{light energy} \rightarrow \text{glucose} + \text{O}_2 + \text{H}_2\text{O}$

Protein Synthesis

Transcription and Translation

Protein synthesis follows the central dogma: DNA is transcribed into RNA, which is translated into protein. Transcription occurs in the nucleus (eukaryotes) or cytoplasm (prokaryotes), and translation occurs at ribosomes.

• Transcription: mRNA is synthesized from DNA template.

• Translation: tRNA brings amino acids to ribosomes, matching codons to anticodons.

DNA Replication

DNA replication produces two identical double strands, each consisting of one old and one new strand. In bacteria, DNA is circular; in eukaryotes, it is a double helix.

Cell Cycle and Mitosis

The cell cycle includes interphase (G1, S, G2) and M phase (mitosis and cytokinesis). Mitosis consists of prophase, metaphase, anaphase, and telophase.

Meiosis

Meiosis involves one chromosome duplication followed by two cell divisions, resulting in haploid cells for sexual reproduction.

Genotype and Phenotype

The genome is the sum of genetic material, existing as chromosomes or plasmids. Genotype refers to the genetic makeup; phenotype refers to observable traits, influenced by genotype and environment.