Processes of Life

Characteristics of Life

All living organisms share several fundamental characteristics that distinguish them from non-living matter. These include:

• Growth: Increase in size and/or number of cells.

• Reproduction: Ability to produce new cells or organisms, either sexually or asexually.

• Responsiveness: Ability to respond to environmental stimuli.

• Metabolism: Collection of controlled chemical reactions that occur within cells, enabling energy production and utilization.

Note: Movement is not a universal characteristic of all living things.

Cell Structure and Function

Prokaryotic and Eukaryotic Cells: An Overview

Cells are classified into two main types: prokaryotic and eukaryotic. These types differ in structure, complexity, and the presence of internal compartments.

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles; DNA is located in the nucleoid region. Examples include bacteria and archaea.

• Eukaryotic Cells: Possess a nucleus and various membrane-bound organelles. Examples include algae, protozoa, fungi, animals, and plants.

Cell Size Comparison

Microbial cells vary greatly in size, from viruses (0.3 µm) to protozoa (14 µm) and even larger eukaryotic cells.

Prokaryotic Cell Structure

Prokaryotic cells have a simple structure, lacking internal membrane-bound compartments. Key components include:

• Cytoplasmic membrane

• Cell wall

• Glycocalyx

• Nucleoid (DNA region)

• Ribosomes

• Flagella (for movement)

• Inclusions (storage granules)

Eukaryotic Cell Structure

Eukaryotic cells are more complex, containing numerous organelles that perform specialized functions:

• Nucleus: Contains DNA, nucleolus, and nuclear envelope.

• Mitochondria: Site of ATP production.

• Endoplasmic reticulum (ER): Smooth and rough types, involved in lipid and protein synthesis.

• Golgi body: Processes and packages molecules for export.

• Lysosomes, peroxisomes, vacuoles: Storage and degradation of substances.

• Cytoskeleton: Provides structural support and facilitates movement.

• Cilia and flagella: Motility structures.

External Structures of Prokaryotic Cells

Glycocalyces

The glycocalyx is a gelatinous, sticky layer surrounding the cell, composed of polysaccharides, polypeptides, or both. It protects cells from desiccation and aids in attachment to surfaces.

• Capsule: Organized, firmly attached; may prevent recognition by host immune system.

• Slime layer: Loosely attached, water-soluble; facilitates surface attachment.

Flagella

Flagella are long, whip-like structures responsible for motility. They can be arranged in various patterns and are not present in all prokaryotes.

• Function: Propels bacteria through environment; movement is in response to stimuli (taxis).

• Mechanism: Runs (counterclockwise rotation) and tumbles (clockwise rotation).

Spirochetes and Axial Filaments

Spirochetes possess unique motility structures called axial filaments (endoflagella), which allow corkscrew movement.

• Axial filaments: Located between cell membrane and outer membrane; rotation causes cell to move forward.

Fimbriae and Pili

Fimbriae are short, bristle-like projections used for adhesion, especially in biofilms. Pili (conjugation pili) are longer and facilitate DNA transfer between cells.

• Fimbriae: Hundreds per cell; important in biofilm formation.

• Pili: 1-2 per cell; mediate conjugation (horizontal gene transfer).

Archaeal External Structures

Archaea possess unique external structures, including glycocalyces, flagella, fimbriae, and hami (grappling hook-like structures for attachment).

Biofilms

Biofilms are organized communities of microbes attached to surfaces, often using fimbriae. They exhibit unique behaviors, gene expression, and antibiotic sensitivities.

• Medical relevance: 2/3 of infections are biofilm-related.

• Industrial relevance: Cause problems but also used in waste treatment.

Prokaryotic Cell Walls

Bacterial Cell Walls

Bacterial cell walls provide structural support and protection. Most are composed of peptidoglycan, a polymer of sugars and amino acids.

• NAG: N-acetylglucosamine

• NAM: N-acetylmuramic acid

• Chains are linked by tetrapeptide crossbridges.

Gram-Positive Cell Walls

Gram-positive bacteria have thick peptidoglycan layers with teichoic acids and may contain mycolic acid (in acid-fast bacteria).

• Gram stain: Retain crystal violet dye; appear purple.

• Teichoic acids: Contribute to cell wall stability and anchoring.

Gram-Negative Cell Walls

Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane containing lipopolysaccharide (LPS), phospholipids, and proteins.

• Gram stain: Appear pink.

• LPS: Can be toxic (endotoxin); impedes treatment.

Bacteria Without Cell Walls

Some bacteria lack cell walls and are often mistaken for viruses due to their small size. They retain other prokaryotic features such as ribosomes.

Archaeal Cell Walls

Archaeal cell walls lack peptidoglycan and are composed of specialized polysaccharides and proteins. Gram-positive and Gram-negative archaea stain similarly to bacteria but have different chemistry.

Prokaryotic Cytoplasmic Membranes

Structure

The cytoplasmic membrane is a phospholipid bilayer with embedded proteins, forming a fluid mosaic. Proteins serve as recognition molecules, enzymes, receptors, carriers, or channels.

Function

Membranes are involved in energy storage, selective permeability, and maintaining concentration and electrical gradients.

Transport Processes

Substances cross membranes via passive or active processes:

• Passive: Diffusion, facilitated diffusion, osmosis.

• Active: Active transport (uniport, antiport, symport), often using ATP.

Cytoplasm of Prokaryotes

Components

• Cytosol: Liquid portion containing dissolved substances.

• Inclusions: Storage granules (e.g., PHB).

• Endospores: Defensive structures produced by some bacteria.

• Ribosomes: Sites of protein synthesis.

• Cytoskeleton: Protein fibrils for structural support.

Eukaryotic Cell Walls and Cytoplasmic Membranes

Cell Walls

Fungi, algae, plants, and some protozoa have cell walls composed of various polysaccharides:

• Cellulose: Plant cell walls.

• Chitin, glucomannan: Fungal cell walls.

• Agar, carrageenan, silicates, algin, calcium carbonate: Algal cell walls.

Cytoplasmic Membranes

Eukaryotic membranes are fluid mosaics containing steroid lipids (e.g., cholesterol) for fluidity. They regulate movement via diffusion, facilitated diffusion, osmosis, and active transport. Eukaryotes also perform endocytosis (phagocytosis and pinocytosis) and exocytosis.

Eukaryotic Organelles

Nucleus

The nucleus is the largest organelle, containing most of the cell's DNA. It is surrounded by a double membrane (nuclear envelope) with nuclear pores.

Endoplasmic Reticulum

The ER is a network of tubules continuous with the nuclear envelope. SER is involved in lipid synthesis; RER transports proteins produced by ribosomes.

Golgi Apparatus

The Golgi body processes and packages molecules for export, using secretory vesicles.

Lysosomes, Peroxisomes, Vacuoles, and Vesicles

These organelles store and transfer chemicals, nutrients, and enzymes within the cell. Lysosomes contain catabolic enzymes; peroxisomes degrade poisonous wastes.

Mitochondria

Mitochondria are double-membraned organelles responsible for ATP production. They contain 70S ribosomes and circular DNA, supporting the endosymbiotic theory.

Chloroplasts

Chloroplasts are light-harvesting organelles in photosynthetic eukaryotes, with two membranes, DNA, and 70S ribosomes.

Endosymbiotic Theory

This theory proposes that eukaryotes originated from a symbiotic relationship between aerobic and anaerobic prokaryotes. Mitochondria and chloroplasts are believed to have evolved from these internalized prokaryotes.