bacteria, viruses and non-specific immunity

What is a bacterium?

A prokaryotic, single-celled organism with no membrane-bound nucleus or membrane-bound organelles.

What structures are commonly found in bacterial cells?

A cell-surface membrane, cytoplasm, 70S ribosomes, a circular DNA molecule and a peptidoglycan cell wall.

Where is the main DNA found in a bacterium?

As a circular DNA molecule free in the cytoplasm because bacteria do not have a nucleus.

What are plasmids?

Small circular pieces of DNA found in some bacteria that replicate independently of the main bacterial DNA.

Why can plasmids be important?

They may carry useful genes, such as genes that confer antibiotic resistance.

What type of ribosomes do bacteria contain?

70S ribosomes.

What is the function of bacterial ribosomes?

Protein synthesis.

What is the bacterial cell wall made from?

Peptidoglycan (murein).

What is the function of the bacterial cell wall?

It supports the cell, maintains its shape and helps prevent the cell from bursting due to osmotic entry of water.

What is a bacterial capsule?

A layer outside the cell wall found in some bacteria that can protect the bacterium and help it attach to surfaces.

What is the function of a bacterial flagellum?

It enables a bacterium to move.

How do bacteria reproduce?

By binary fission.

What happens during binary fission?

The bacterial DNA replicates, the cell grows, the DNA molecules move apart and the cytoplasm divides to form two daughter cells.

What is a pathogen?

A microorganism or other infectious agent that causes disease.

How can pathogenic bacteria cause disease?

They can damage cells directly and/or produce toxins that disrupt normal cell function.

What is a virus?

An acellular infectious particle containing genetic material enclosed in a protein coat that can reproduce only inside a host cell.

Why are viruses described as acellular?

They are not made of cells and lack the normal cellular structures of living organisms.

What genetic material can viruses contain?

Either DNA or RNA, depending on the virus.

What is a capsid?

The protein coat surrounding the genetic material of a virus.

What are capsomeres?

Protein subunits that make up a viral capsid.

Do viruses contain ribosomes?

No. They depend on the host cell's machinery for protein synthesis.

Why can viruses reproduce only inside living cells?

They lack the cellular machinery needed to carry out reproduction independently, so they use the host cell's enzymes, ribosomes and other resources.

What is a viral envelope?

A lipid membrane surrounding some viruses, usually derived from the host cell membrane.

What are attachment proteins?

Viral surface proteins that bind to specific receptors on host cells.

Why can a virus infect only particular types of cells?

Its attachment proteins have complementary shapes to particular receptor molecules on specific host cells.

What happens when a virus infects a host cell?

The virus attaches to the host cell, introduces its genetic material, uses the host's machinery to make viral components, new viruses assemble and are released.

What is the main structural difference between bacteria and viruses?

Bacteria are prokaryotic cells capable of carrying out cellular processes, whereas viruses are acellular particles that reproduce only inside host cells.

Why don't antibiotics treat viral infections?

Antibiotics target structures or processes found in bacteria, which viruses do not possess.

What is an immune response?

The body's response to foreign cells, particles or molecules in order to prevent or limit infection.

What is a non-specific immune response?

A defence mechanism that responds in a similar way to many different pathogens rather than targeting one particular antigen.

What is the first line of defence against pathogens?

Physical and chemical barriers that prevent pathogens from entering the body.

How does the skin act as a defence against infection?

It forms a physical barrier that prevents pathogens from entering body tissues.

How does blood clotting defend against pathogens?

A clot seals a break in the skin, reducing blood loss and preventing pathogens from entering through the wound.

How do mucous membranes protect against pathogens?

They produce mucus that traps microorganisms and other particles.

How do ciliated epithelial cells defend against pathogens?

Cilia move mucus containing trapped pathogens away from the lungs towards the throat, where it can be swallowed.

How does stomach acid act as a defence?

The low pH kills many microorganisms that are swallowed.

What is inflammation?

A non-specific response to tissue damage or infection involving increased blood flow and movement of immune cells into the affected tissue.

What causes inflammation?

Damaged cells and immune cells release signalling chemicals that cause changes to nearby blood vessels.

Why does blood flow increase during inflammation?

It increases the delivery of immune cells and molecules involved in defence and tissue repair to the affected area.

What is phagocytosis?

The process by which a phagocyte surrounds, engulfs and digests a pathogen or foreign particle.

What is a phagocyte?

A white blood cell that carries out phagocytosis.

Give examples of phagocytic cells.

Neutrophils and macrophages.

What are the main stages of phagocytosis?

The phagocyte recognises and attaches to the pathogen, surrounds and engulfs it into a vesicle, lysosomes fuse with the vesicle, and hydrolytic enzymes digest the pathogen.

What happens after a pathogen has been engulfed by a phagocyte?

It is enclosed within a vesicle, which fuses with lysosomes containing hydrolytic enzymes.

What is the function of lysosomes during phagocytosis?

They contain hydrolytic enzymes that digest the engulfed pathogen.

Why is phagocytosis described as non-specific?

Phagocytes can engulf many different types of pathogen rather than recognising one specific antigen as part of a particular immune response.

What is an antigen?

A molecule, usually a protein or glycoprotein, that is recognised as foreign by the immune system and can stimulate an immune response.

Where can antigens be found?

On the surfaces of pathogens, cells, transplanted tissues and other foreign material.

What is the difference between an antigen and an antibody?

An antigen is a molecule recognised as foreign that can trigger an immune response; an antibody is a specific protein produced by B cells/plasma cells that binds to a complementary antigen.

What is the difference between non-specific and specific immunity?

Non-specific immunity acts against many pathogens in broadly the same way, whereas specific immunity recognises particular antigens and produces a targeted response.

What are examples of non-specific defences?

Skin, blood clotting, mucus, cilia, stomach acid, inflammation and phagocytosis.

Why is the non-specific immune response important?

It provides rapid defence against a wide range of pathogens and can limit their spread before a specific immune response develops.

Why is an intact skin barrier important?

Pathogens cannot easily cross the tightly packed cells of intact skin, so damage to the skin increases the risk of infection.

How do mucus and cilia work together in the respiratory system?

Mucus traps pathogens and particles, while cilia move the mucus towards the throat so it can be removed or swallowed.

Bacterium vs virus: which is larger?

Bacteria are generally much larger than viruses.

Bacterium vs virus: which has cytoplasm?

Bacteria have cytoplasm; viruses do not.

Bacterium vs virus: which has ribosomes?

Bacteria have 70S ribosomes; viruses have no ribosomes.

Bacterium vs virus: which can reproduce independently of a host cell?

Bacteria can reproduce by binary fission when conditions are suitable; viruses require a living host cell.

Bacterium vs virus: which contains a cell wall made of peptidoglycan?

Bacteria.

Bacterium vs virus: which contains a capsid?

Viruses.

EXAM TIP: Why can't you say that antibiotics 'kill all pathogens'?

Antibiotics act against bacteria, not viruses, so 'pathogens' is too broad.

EXAM TIP: What key words should you include when describing viral specificity?

Viral attachment proteins have a complementary shape to receptors on particular host cells.

EXAM TIP: What key words should you use when describing phagocytosis?

Recognition/attachment, engulfment, vesicle, lysosome, hydrolytic enzymes and digestion.

EXAM TIP: Is inflammation a specific or non-specific response?

Non-specific.

EXAM TIP: Is phagocytosis a specific or non-specific response?

Non-specific.