Structure of Bacteria and Endospore Formation: PECTAA Class 11 Biology, Punjab Board

A bacterium has no nucleus, no mitochondria and no endoplasmic reticulum, yet it copies its DNA, makes proteins and survives conditions that kill most cells. The secret is a compact set of structures, each doing several jobs, plus a survival trick: the endospore.

Chapter 2 of the PECTAA Biology 11 book deals with bacteria. Sections 2.1 and 2.2 describe the structure of a bacterial cell and endospore formation. The previous 1st Year note covered species and speciation from Chapter 1. This note is for FSc Part 1 (1st year, Class 11) students in Lahore, Faisalabad, Sargodha, Rawalpindi, Multan, Gujranwala and the other Punjab boards. Gujrat district comes under the Gujranwala board. PECTAA writes the books. Your own board sets your paper.

What to know

Bacteria are prokaryotic: they lack a true nucleus and membrane-bound organelles. Their parts, from outside in:

Cell wall. Made of peptidoglycan (murein): glycan chains cross-linked by short peptides. It gives shape and stops the cell bursting. The Gram stain, developed by Hans Christian Gram, sorts bacteria into two groups:

The wall also contains lipids linked to the peptidoglycan, and the amount of peptidoglycan differs between bacteria. In Gram-negative bacteria the protein porin acts like a pore that lets specific molecules through the outer membrane. The book's figure of the two wall types also shows the periplasmic space between the outer membrane and the cell membrane of Gram-negative bacteria. Remember the link: thick wall, Gram-positive; thin wall with an outer membrane, Gram-negative.

Capsule. A gelatinous, sticky layer outside the wall in many bacteria. It gives bacterial colonies their sticky character.

Cell membrane. A phospholipid bilayer with proteins, like other membranes, but without sterols. In bacteria that lack a wall, such as Mycoplasmas, it is the outermost layer. Infoldings called mesosomes are linked with DNA replication, cell division and respiration.

Cytoplasm. Has no cytoskeleton and no membrane-bound organelles. It contains dissolved substances and larger structures such as the nucleoid, ribosomes and mesosomes.

Ribosomes. 70S, made of a 50S and a 30S subunit (smaller than the 80S ribosomes of eukaryotes). Many float unattached in the cytoplasm; some are loosely attached to the cell membrane. They are the sites of protein synthesis.

Nucleoid. The region holding the bacterial chromosome: a single, circular, double-stranded DNA molecule without histone proteins and without a nuclear membrane.

Plasmids. Small, circular, self-replicating DNA molecules outside the nucleoid. They can help a bacterium survive unfavourable conditions such as antibiotics. They are also used as vectors in genetic engineering, carrying selected genes into bacteria for cloning or for making specific proteins.

Endospore formation (section 2.2). Under harsh conditions some bacteria form a resting endospore inside the cell. The book's sequence:

1. The DNA replicates.
2. A septum (membrane) forms, cutting off one copy of DNA with a little cytoplasm.
3. The membrane of the larger part surrounds this portion, giving a forespore with a double membrane.
4. The forespore dehydrates.
5. A peptidoglycan layer forms between the membranes.
6. A tough spore coat forms outside.
7. The mother cell breaks down and the endospore is released.
8. The endospore stays dormant and germinates into a new bacterial cell when conditions become favourable.

Key points

• Wall = peptidoglycan (murein).
• Gram-negative: thin wall + outer membrane.
• Ribosomes 70S = 50S + 30S.
• Nucleoid DNA: single, circular, no histones.
• Endospore = survival, not reproduction.

Worked examples

Example 1. A bacterium stains red in the Gram stain. Predict its wall structure.

1. Red means it lost the crystal violet and took the counterstain: Gram-negative.
2. So it has a thin peptidoglycan layer and an outer membrane of lipopolysaccharides and lipoproteins.

Example 2. Why are many Gram-negative bacteria harder to kill with antibiotics?

1. Their outer membrane acts as an extra barrier.
2. Many drugs cannot cross it easily, so they do not reach their targets.

Example 3. Place in order: spore coat forms; DNA replicates; forespore dehydrates; septum forms.

1. DNA replicates → septum forms → forespore dehydrates → spore coat forms.

Example 4. A student writes: "Bacterial ribosomes are 80S and the DNA is wrapped around histones." Correct both errors.

1. Bacterial ribosomes are 70S, with a 50S and a 30S subunit.
2. Bacterial DNA is a single circular molecule with no histones attached.

Common mistakes in the exam

1. Calling the endospore a means of reproduction. One cell makes one spore; it is for survival.
2. Giving bacteria 80S ribosomes. Theirs are 70S.
3. Saying bacterial DNA has histones. It does not.
4. Mixing up which group has the outer membrane. It is Gram-negative.
5. Putting sterols in the bacterial membrane. The book notes they are absent.

Exam technique

• For differences, use a two-column table as above.
• Number the endospore steps in order.
• Name the wall chemical twice: peptidoglycan (murein).
• Check your board's notice for the sections in your paper.

Practice questions

1. Name the subunits of a bacterial ribosome. Answer: 50S and 30S, forming 70S.
2. Which bacteria have the cell membrane as their outermost layer? Answer: Mycoplasmas, which lack a cell wall.
3. Give one use of plasmids. Answer: As vectors in genetic engineering.
4. List two functions linked with mesosomes. Answer: Any two of DNA replication, cell division and respiration.
5. When does an endospore germinate? Answer: When conditions become favourable again.

Ready to practise this? Continue in 1st Year Biology.