Cells - Structure of Prokaryotes and Viruses

Question 1

What are the two main types of cells?

Answer 1

• Eukaryotic Cells: are larger and have a nucleus bounded by nuclear membranes (nuclear envelope).
• Prokaryotic Cells: are smaller and have no nucleus or nuclear envelope.

Question 2

What are the features of prokaryotic cells?

Answer 2

• no true nucleus, only an area where DNA is found
• (pro) DNA is not associated with proteins
• some DNA may be in the form of circular strands called plasmids free in the cytoplasm (one or more)
• no membrane-bounded organelles
• no chloroplasts, only bacterial chlorophyll associated with the cell-surface membrane in some bacteria
• ribosomes are smaller (70S)
• cell wall made of murein (peptidoglycan)
• may have an outer mucilaginous layer called a capsule
• one or more flagella

Question 3

What are the features of eukaryotic cells?

Answer 3

• distinct nucleus, with a nuclear envelope
• DNA is associated with proteins called histones
• there are no plasmids and DNA is linear
• membrane-bounded organelles, such as mitochondria, are present
• chloroplasts present in plants and algae
• ribosomes are larger (80S)
• where present, cell wall is made mostly of cellulose (or chitin in fungi)
• no capsule

Question 4

Why are bacteria so successful?

Answer 4

Bacteria occur in every habitat in the world - they are versatile, adaptable and very successful. Much of their success is a result of their small size.

Question 5

Describe the structure of a bacterial cell.

Answer 5

• cellular structure is relatively simple
• all bacteria possess a cell wall (made up of murein, a polymer of polysaccharides and peptides)
• many protect themselves by secreting a capsule of mucilaginous slime around this wall
• inside cell wall is cell-surface membrane, containing cytoplasm that has 70S ribosomes (smaller than those in the cytoplasm of eukaryotic cells (80S), but nevertheless still synthesis proteins)
• bacteria store food reserves as glycogen granules and oil droplets
• genetic material in bacteria is in the form of a circular strand of DNA
• separate from this are smaller circular pieces of DNA, called plasmids (can reproduce themselves independently and may give the bacterium resistance to harmful chemicals, such as antibiotics)
• plasmids are used extensively as vector carriers of genetic information in genetic engineering

Question 6

What is the role of the cell wall found in a bacterial cell?

Answer 6

Physical barrier that excludes certain substances and protects against mechanical damage and osmotic lysis (water potential changes).

Question 7

What is the role of the capsule found in a bacterial cell?

Answer 7

It’s a slime layer composed of carbohydrates that lies outside the cell wall of some prokaryotic cells and protects the cell from immune responses and desiccation. It also protects bacterium from other cells and helps groups of bacteria to stick together for further protection.

Question 8

What is the role of the cell-surface membrane found in a bacterial cell?

Answer 8

Acts as a differentially permeable layer, which controls the entry and exit of chemicals.

Question 9

What is the role of the circular DNA found in a bacterial cell?

Answer 9

Possesses the genetic information for the replication of bacterial cells.

Question 10

What is the role of the plasmid found in a bacterial cell?

Answer 10

It’s a small, circular piece of DNA found in prokaryotic cells, chloroplasts and mitochondria. Possesses genes that may aid the survival of bacteria in adverse conditions, e.g. produces enzymes that break down antibiotics.

Question 11

Describe the structure of viruses.

Answer 11

• a virus is an infectious, highly specialised intracellular parasite
• they are acellular and non-living
• smaller than bacteria
• contain nucleic acids such as DNA and RNA as genetic material but can only multiply inside living host cells
• nucleic acid is enclosed within a protein coat called the capsid
• some viruses, like the human immunodeficiency virus, are further surrounded by a lipid envelope
• lipid envelope, or if this is not present, the capsid, have attachment proteins which are essential to allow the virus to identify and attach to a host cell

Question 12

What are the features of a generalised bacterial cell?

Answer 12

• flagellum: (there may be more than one) used for locomotion (only certain species)
• capsule: a mucilaginous layer of slime (only certain species) that prevents desiccation and chemical attack
• cell wall: made of murein
• cell-surface (plasmid) membrane: exchange of substances with environment
• plasmid: small, circular piece of DNA (only certain species)
• ribosomes: smaller than in eukaryotic cells, protein synthesis
• cytoplasm: jelly-like, contains enzymes and other soluble materials, site of many reactions
• genetic material (chromosomal DNA): comprises a large circle of DNA, located in nucleoid
• pilus: used to exchange genetic material during a type of reproduction called conjugation
• fimbriae: used by bacteria to attach to a host cell

Question 13

What are the features of the human immunodeficiency virus (HIV)?

Answer 13

• genetic material (RNA)
• attachment proteins
• capsid
• lipid envelope
• matrix
• reverse transcriptase (enzyme)

Question 14

What does acellular mean?

Answer 14

Not consisting of cells.

Question 15

What is an attachment protein?

Answer 15

Protein found on the external surface of a cell or virus that enables it to attach to other structures.

Question 16

What is bacteria?

Answer 16

A type of prokaryotic cell.

Question 17

What is a capsid?

Answer 17

The protein shell of a virus.

Question 18

What is a capsule?

Answer 18

Slime layer composed of carbohydrate that lies outside the cell wall of some prokaryotic cells and protects the cell from immune responses and desiccation. It also protects prokaryotes fom being detected by other cells. The waxy capsule allows the cells to stick together, increasing the chance of survival.

Question 19

What is flagella?

Answer 19

Long structure composed of protein found on sperm and some prokaryotic cells that enables motility.

Question 20

What is murein (peptidoglycan)?

Answer 20

A polymer consisting of sugars and amino acids that forms bacterial cell walls. It is made of polypeptides and polysaccharides.

Question 21

What is a plasmid?

Answer 21

Small, circular piece of DNA found in prokaryotic cells, chloroplasts and mitochondria.

Question 22

What is a virus?

Answer 22

Acellular, infectious, non-living structures that are smaller than bacteria. They contain DNA or RNA but can only multiply inside living host cells.

Question 23

What structures are found in all prokaryotes?

Answer 23

cell membrane, cell wall, DNA

Question 24

What are the non-universal features of prokaryotes?

Answer 24

flagellum, waxy capsule, pili, mesosome

Question 25

What are mesosomes?

Answer 25

Mesosomes are infoldings of the prokaryotic cell membrane. They carry out aerobic respiration, having a similar role to the mitochondrial cristae.

Question 26

What are some similarities between eukaryotes and prokaryotes?

Answer 26

Prokaryotes and eukaryotes originate from the same common ancestor - so they have some features in common.

They both have cytoplasm, a cell membrane and ribosomes.

Question 27

What are the differences between prokaryotes and eukaryotes?

Answer 27

Although prokaryotes and eukaryotes originally come from the same ancestor, they have evolved independently for a long time.

• All prokaryotes have a cell wall, while only some eukaryotes have a cell wall.
• Prokaryotes are smaller than eukaryotes.
• While both have ribosomes, prokaryotic ribosomes have 70S ribosomes whereas eukaryotes have 80S ribosomes.
• Eukaryotes have membrane bound organelles such as mitochondrion or nucleus - these aren’t present in prokaryotes.
• Some prokaryotes have a waxy capsule whereas eukaryotes don’t.
• While some eukaryotic cells have tails (like the human sperm cell), none have flagella.
• Prokaryotes have a cytoskeleton which is used in division - it is simpler in structure than the eukaryotic cytoskeleton.
• Prokaryotes have pili which are different from the similar-looking cilia in eukaryotes.

Question 28

How does DNA in prokaryotes and eukaryotes differ?

Answer 28

• Prokaryotic and eukaryotic DNA differs in how it is arranged.
• Eukaryotic DNA is contained in a membrane-bound nucleus while prokaryotic DNA is free in the cytoplasm.
• Inside the nucleus of eukaryotes, the DNA is associated with histone proteins whereas in the prokaryotes, the DNA is naked.
• The prokaryotic DNA is also circular - whereas eukaryotic DNA is linear.
• Eukaryotes don’t have plasmids - while some prokaryotes do.

Question 29

Why are viruses not organisms?

Answer 29

Viruses are metabolically inert until they are inside the host cell and hijacking its metabolic machinery to make new viral particles. However, they are often called microorganisms.

Question 30

What happens when viral replication is complete?

Answer 30

When viral replication is complete, the new viral particles leave the host cell to infect more cells. In enveloped viruses, this is achieved by budding from the host cell. Each viral particle takes some of the host’s cell membrane to create the viral envelope. This also helps the virus (e.g. HIV and influenza virus) avoid detection by the host’s immune system.