Bacterial Cell Structure

In brief:Bacteria are unicellular prokaryotic microorganisms (1–5μm). They lack a nucleus and membrane-bound organelles but have unique structures.

Bacteria are unicellular prokaryotic microorganisms, typically 1–5μm in size. They are the most abundant organisms on Earth.

Key features of bacterial cells:
• No membrane-bound nucleus - DNA is loose in the cytoplasm (nucleoid region)
• No membrane-bound organelles (no mitochondria, no ER, no Golgi)
• 70S ribosomes (smaller than eukaryotic 80S ribosomes)
• Chromosomal DNA: single circular chromosome, no histones

Unique bacterial structures:
• Plasmid: small circular DNA separate from chromosomal DNA. Can transfer between cells (conjugation). Often carries antibiotic resistance genes - a major medical concern.
• Cell wall: made of peptidoglycan (not cellulose). Gives shape and prevents bursting.
• Capsule/slime layer: prevents drying out, provides protection from white blood cells (phagocytosis)
• Flagellum: whip-like structure for movement (not all bacteria have one)
• Pili: short hair-like projections for attachment to surfaces and other cells

Three bacterial shapes: Coccus (spherical, e.g. Streptococcus), Bacillus (rod, e.g. E. coli), Spirillum (spiral, e.g. Treponema)

Bacterial (prokaryotic) cell
Wikimedia Commons (public domain / CC)

diagram

Bacterial Cell Structure

CChromosomal DNACircular, in cytoplasm

PPlasmidSmall DNA ring, resistance genes

CCell WallPeptidoglycan, shape & strength

CCapsuleProtection, prevents drying

RRibosomes (70S)Protein synthesis

FFlagellumMovement

table

Three Bacterial Shapes

Shape	Name	Description	Example
Sphere	Coccus	Spherical	Streptococcus, Staphylococcus
	Bacillus	Rod-shaped	E. coli, Bacillus subtilis
Spiral	Spirillum	Spiral/corkscrew	Treponema (syphilis)

Key Points

1Chromosomal DNA: loose in cytoplasm, no associated histone proteins (unlike eukaryotes).
2Plasmid: small circular DNA that can transfer between cells. Often carries antibiotic resistance genes.
3Cell membrane: selectively permeable, controls substances in/out. Made of phospholipids and proteins.
4Cell wall: gives shape and strength. Made of peptidoglycan (not cellulose like plants).
5Capsule/slime layer: prevents drying out, protects from white blood cell phagocytosis. Not in all bacteria.
6Flagellum: hair-like structure for movement. Not in all bacteria.
7Pili: small hair-like structures for attachment to surfaces and other cells.
8Three bacterial shapes: coccus (spherical), bacillus (rod), spirillum (spiral).

Learning Outcomes

Compare the structure of prokaryotic and eukaryotic cells
Distinguish between bacteria and fungi in terms of structure, nutrition, and cellular nature

Fungal Structure

In brief:Fungi can be unicellular (yeast) or multicellular (Rhizopus/bread mould). Fungal cells are eukaryotic with cell walls made of chitin.

Structure of Rhizopus (bread mould) with labelled parts — Structure of Rhizopus (bread mould) showing hyphae, stolon, rhizoids, sporangiophore and sporangium
Wikimedia Commons (public domain / CC)

Fungal (yeast) cell — Fungal cell (yeast)
Wikimedia Commons (public domain / CC)

Key Points

1Fungal cells are 10–100μm - larger than bacteria. Eukaryotic with membrane-bound nucleus.
2Cell walls made of chitin (not peptidoglycan like bacteria, not cellulose like plants).
3Multicellular fungi made of filamentous structures called hyphae.
4Mycelium: the network of hyphae that forms the body of a fungus.
5Rhizoids: hyphae that anchor the fungus and absorb nutrients from the substrate.
6Sporangiophore: aerial hyphae supporting the sporangium.
7Sporangium: structure where spores for reproduction are formed.
8Stolon: aerial hyphae that allow the fungus to spread/colonise more substrate.

Learning Outcomes

Distinguish between bacteria and fungi in terms of structure, nutrition, and cellular nature

Microbial Nutrition

In brief:Microbes are classified by how they obtain food. Bacteria can be autotrophic or heterotrophic. All fungi are heterotrophic.

Key Points

1Autotrophic: can make own food. Two types in bacteria:
2 • Photosynthetic bacteria: use sunlight energy (e.g. green sulphur bacteria).
3 • Chemosynthetic bacteria: use energy from chemical reactions (e.g. nitrifying bacteria).
4Heterotrophic: cannot make own food, must take in food from other organisms.
5 • Saprophytic: feed on dead organic matter by secreting enzymes externally (e.g. decomposer bacteria, most fungi).
6 • Parasitic: feed on living organisms causing them harm (e.g. Streptococcus, parasitic fungi).
7Fungi store carbohydrate as glycogen (like animals), not starch (like plants).
8Lichens: symbiotic mutualistic relationship between fungi and algae/cyanobacteria. Used as air quality indicators.

Learning Outcomes

Distinguish between bacteria and fungi in terms of structure, nutrition, and cellular nature
Outline the concept of a microbiome; explore the role of microbiomes in promoting human health and nutrient cycling in soils

Microbial Reproduction

In brief:Bacteria reproduce asexually by binary fission. Fungi reproduce asexually by budding or spore formation, and sexually through zygospore formation.

Key Points

1Binary fission: DNA replicates → cell elongates → membrane/wall divide → two identical daughter cells.
2Under ideal conditions, one bacterium can produce over 1 billion cells in 12 hours (dividing every 20 mins).
3Yeast budding: small bud forms → nucleus divides by mitosis → one nucleus enters bud → bud pinches off.
4Sporulation (fungi): cells in sporangium divide by mitosis → haploid spores → released when sporangium bursts.
5Sexual reproduction in fungi (Rhizopus): (+) and (−) hyphae form gametangia → nuclei fuse → diploid zygospore → germinates by meiosis.

Learning Outcomes

Distinguish between bacteria and fungi in terms of structure, nutrition, and cellular nature
Investigate factors affecting the growth of microorganisms, use primary and secondary data to support conclusions

Microbial Growth Curve

In brief:In a closed system (e.g. petri dish), microbial population growth follows a predictable pattern of four phases.

When microorganisms are grown in a closed system (e.g. petri dish, sealed flask), their population follows a predictable four-phase growth curve:

1. Lag Phase: Bacteria adapt to new environment, synthesise enzymes and prepare for division. Little or no increase in cell numbers.
2. Log (Exponential) Phase: Rapid cell division - population doubles at regular intervals. Maximum growth rate. Nutrients abundant, space available.
3. Stationary Phase: Growth rate = death rate. Population levels off. Nutrients depleted, waste products accumulate, space limited.
4. Death (Decline) Phase: Death rate exceeds growth rate. Nutrients exhausted, toxic waste products kill cells. Population decreases.

Factors affecting microbial growth:
• Temperature: each species has an optimum (mesophiles ~20-40°C, thermophiles >45°C)
• pH: most bacteria prefer neutral (~7), fungi tolerate more acidic conditions
• Oxygen: obligate aerobes (need O₂), obligate anaerobes (killed by O₂), facultative anaerobes (either)
• Nutrients: carbon source, nitrogen, minerals, vitamins
• Water: essential for all metabolic reactions

diagram

Bacterial Growth Curve (Four Phases)

process

Microbial Growth Curve - Four Phases

Lag Phase
Adapting, enzymes made

→

Log Phase
Rapid doubling

→

Stationary
Growth = Death

→

Death Phase
Nutrients gone, toxins

table

Oxygen Requirements of Microbes

Type	Oxygen Requirement	Example
Obligate Aerobe	Must have O₂	Mycobacterium tuberculosis
Obligate Anaerobe	Killed by O₂	Clostridium botulinum
Facultative Anaerobe	With or without O₂	E. coli, Saccharomyces (yeast)

Key Points

1Lag phase: bacteria adapt to environment, synthesise enzymes. Little or no increase in numbers.
2Log (exponential) phase: rapid cell division, population doubles at regular intervals. Maximum growth rate.
3Stationary phase: growth rate = death rate. Nutrients depleted, waste products accumulate.
4Death/decline phase: death rate exceeds growth rate. Nutrients exhausted, toxic waste builds up.
5Factors affecting growth: temperature, pH, nutrients, water, oxygen, external solute concentration, antibacterial chemicals.
6Obligate aerobes: need oxygen. Obligate anaerobes: killed by oxygen. Facultative anaerobes: can grow with or without oxygen.

Learning Outcomes

Investigate factors affecting the growth of microorganisms, use primary and secondary data to support conclusions
Discuss the importance of microorganisms in industries