By Momina Shehzadi | BS Zoology Graduate, Pakistan In this article, we will learn Fungi - Morphology, Reproduction, Physiology and Importance - in simple words with diagram. This topic is very important for BS Zoology and Microbiology students. Fungi are found everywhere around us. They are present in soil, in water, in air, on dead plants, on dead animals, on our food, on our skin and even inside our body. When bread becomes old, green or black spots appear on it. These spots are fungi. When we see mushroom growing after rain, that is also a fungus. When yeast is used to make bread soft, that is also a fungus. Fungi are very different from bacteria and plants. They are eukaryotic organisms. It means their cells have a true nucleus and other membrane bound organelles. They do not have chlorophyll, so they cannot make their own food like plants. They get their food from other living or dead things. That is why they are called heterotrophic organisms. The study of fungi is called mycolog...
By Momina Shehzadi | BS Zoology Graduate, Pakistan
In this article, we will learn Structure of Bacteria - Flagella, Pili, Capsule, Cell Wall and Cell Membrane - in simple words with diagram. This topic is very important for BS Zoology and Microbiology students.
INTRODUCTION
Bacteria are prokaryotic microorganisms which do not have true nucleus and membrane bound organelles. The bacterial cell structure is very simple as compared to eukaryotic cell but it has many important external and internal structures which help bacteria to survive in different environments.
Understanding structure of bacteria is very important for BS Zoology students because it helps to understand how bacteria cause diseases, how antibiotics work and how Gram staining differentiates bacteria.
A typical bacterial cell consists of outer structures and inner structures. Outer structures include flagella, pili, fimbriae and capsule. Inner structures include cell wall, cell membrane, cytoplasm, nucleoid, ribosomes, plasmids and inclusions.
EXTERNAL STRUCTURES OF BACTERIA
1. FLAGELLA
Flagella are long, thin, hair like appendages that help bacteria in motility or movement. Flagella are not present in all bacteria. Bacteria without flagella are called atrichous and bacteria with flagella are called trichous.
Structure of Flagella: Each flagellum consists of three parts. Filament, Hook and Basal Body. Filament is the outermost long part made of protein called flagellin. Hook connects filament to basal body. Basal body is embedded in cell wall and membrane and acts as motor which rotates flagellum.
Types of Flagella on Basis of Arrangement:
a) Monotrichous: Single flagellum at one pole. Example Vibrio cholerae.
b) Amphitrichous: Single flagellum at both poles. Example Alcaligenes.
c) Lophotrichous: Tuft of flagella at one pole. Example Pseudomonas.
d) Peritrichous: Flagella all around the body. Example Escherichia coli and Salmonella.
Function: Main function is motility. Bacteria move towards nutrients and away from harmful chemicals. This movement is called chemotaxis. Flagella also helps bacteria to cause infection by moving through mucus.
2. PILI AND FIMBRIAE
Pili are short, hair like appendages on bacterial surface. They are shorter and thinner than flagella. Fimbriae and pili are often used as same term but there is difference. Fimbriae are numerous and help in attachment while pili are few and longer.
Structure: Pili are made of protein called pilin. They originate from cytoplasmic membrane.
Types of Pili:
a) Common Pili or Fimbriae: Help bacteria to attach to surfaces and host tissues. For example, E. coli in urinary tract attaches with help of fimbriae.
b) Sex Pili: Longer and only 1 to 4 in number. They help in conjugation which is transfer of DNA from one bacterium to another. Sex pili are made by bacteria containing F plasmid.
Function: Attachment to host cells, formation of biofilms and transfer of genetic material during conjugation.
3. CAPSULE AND SLIME LAYER
Capsule is outermost thick layer present outside cell wall in some bacteria. It is made of polysaccharides and sometimes proteins. When this layer is thick and well organized it is called capsule. When it is thin and loosely attached it is called slime layer.
Examples: Capsule is present in Streptococcus pneumoniae, Klebsiella pneumoniae and Bacillus anthracis.
Functions of Capsule:
a) Protection: Capsule protects bacteria from drying, from white blood cells of host and from bacteriophages.
b) Virulence: Capsulated bacteria cause more severe diseases because phagocytes cannot engulf them easily.
c) Attachment: Capsule helps bacteria to attach to surfaces.
d) Prevents from Antibiotics: Some capsules prevent entry of antibiotics.
Capsule is important in identification of bacteria by Capsule staining and Quellung reaction.
INTERNAL STRUCTURES OF BACTERIA
4. CELL WALL
Cell wall is rigid layer present outside cell membrane. It gives shape and rigidity to bacterial cell. Bacterial cell wall is made of peptidoglycan or murein. Peptidoglycan consists of N-acetyl glucosamine and N-acetyl muramic acid with peptide cross links.
On basis of cell wall composition, bacteria are divided into Gram Positive and Gram Negative by Gram staining which you have already studied.
Functions: Gives shape, prevents from bursting, provides site for bacteriophage attachment and contains antigens.
5. CELL MEMBRANE OR PLASMA MEMBRANE
Cell membrane is thin layer present inside cell wall. It is made of phospholipids and proteins. It is selectively permeable. Unlike eukaryotic cells, bacterial membrane does not contain sterols except Mycoplasma.
Functions: Controls entry and exit of substances, site for respiration because electron transport chain is present in membrane in bacteria, helps in DNA replication and cell wall synthesis. Mesosomes are invaginations of cell membrane which help in respiration and cell division.
6. CYTOPLASM
Cytoplasm is gel like substance inside cell membrane. It contains water, proteins, lipids, nucleic acids and many inclusions.
Cytoplasm contains:
a) Nucleoid: Bacterial DNA is present as single circular chromosome in central region called nucleoid. It is not surrounded by nuclear membrane. The DNA is double stranded, circular and haploid.
b) Plasmids: Extra-chromosomal small circular DNA molecules. They give extra characters like antibiotic resistance and toxin production. Plasmids are very important in genetic engineering.
c) Ribosomes: Bacterial ribosomes are 70S type consisting of 50S and 30S subunits. They are site of protein synthesis. Because bacterial and human ribosomes are different, antibiotics like streptomycin can target bacterial ribosomes without harming human cells.
d) Inclusion Bodies: Reserve materials like glycogen, polyphosphate and sulfur granules are stored in cytoplasm as inclusion bodies. Gas vacuoles are present in aquatic bacteria for buoyancy.
Conclusion
Structure of bacteria is simple but each part has important function. Flagella for motility, pili for attachment and conjugation, capsule for protection, cell wall for shape and cell membrane for control of transport. For BS Zoology students, it is essential to learn this chapter with diagrams.
FAQs for Students - Structure of Bacteria
Q1: What is difference between flagella and pili?
Ans: Flagella are long, few in number and help in motility. Pili are short, numerous and help in attachment and conjugation. Flagella have basal body motor but pili do not.
Q2: What is function of capsule in bacteria?
Ans: Capsule protects bacteria from phagocytosis by WBCs, from desiccation, from bacteriophages and increases virulence of bacteria.
Q3: What is nucleoid?
Ans: Nucleoid is region in bacterial cytoplasm where single circular chromosome is present. It is not true nucleus because it has no nuclear membrane.
Q4: What are plasmids and their importance?
Ans: Plasmids are small extra circular DNA molecules in cytoplasm which carry genes for antibiotic resistance, toxin production and fertility. They are used as vectors in genetic engineering.
Q5: Why bacterial cell wall is important for antibiotics?
Ans: Because human cells do not have peptidoglycan cell wall. So antibiotics like penicillin which destroy peptidoglycan can kill bacteria without harming human cells.
Q6: What are mesosomes?
Ans: Mesosomes are invaginations of cell membrane inside cytoplasm. They increase surface area for respiration and help in cell division.
Also Read: Bacterial Growth Curve - Lag, Log, Stationary and Death Phase
Written by Momina Shehzadi, BS Zoology Graduate from Pakistan.
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