Cell Biology: Topic 1.2 Ultrastructure of cells

Question 1

Define prokaryotes

Answer 1

Prokaryotes are organisms whose cells lack a nucleus

Question 2

Prokaryotes belong to the kingdom Monera and have been further classified into two distinct domains:

Answer 2

Archaebacteria – found in extreme environments like high temperatures, salt concentrations or pH (i.e. extremophiles)
Eubacteria – traditional bacteria including most known pathogenic forms (e.g. E. coli, S. aureus, etc.)

Question 3

Prokaryotic cells will typically contain the following cellular components (9 features)

Answer 3

Cytoplasm – internal fluid component of the cell
Nucleoid – region of the cytoplasm where the DNA is located (DNA strand is circular and called a genophore)
Plasmids – autonomous circular DNA molecules that may be transferred between bacteria (horizontal gene transfer)
Ribosomes – complexes of RNA and protein that are responsible for polypeptide synthesis (prokaryote ribosome = 70S)
Cell membrane – Semi-permeable and selective barrier surrounding the cell
Cell wall – rigid outer covering made of peptidoglycan; maintains shape and prevents bursting (lysis)
Slime capsule – a thick polysaccharide layer used for protection against dessication (drying out) and phagocytosis
Flagella – Long, slender projections containing a motor protein that enables movement (singular: flagellum)
Pili – Hair-like extensions that enable adherence to surfaces (attachment pili) or mediate bacterial conjugation (sex pili)

Question 4

Define eukaryotes (3 points)

Answer 4

Eukaryotes are organisms whose cells contain a nucleus

They have a more complex structure and are believed to have evolved from prokaryotic cells (via endosymbiosis)

Eukaryotic cells are compartmentalised by membrane-bound structures (organelles) that perform specific roles which can have single or double membranes

Question 5

Eukaryotes can be divided into four distinct kingdoms:

Answer 5

Protista – unicellular organisms; or multicellular organisms without specialised tissue
Fungi – have a cell wall made of chitin and obtain nutrition via heterotrophic absorption
Plantae – have a cell wall made of cellulose and obtain nutrition autotrophically (via photosynthesis)
Animalia – no cell wall and obtain nutrition via heterotrophic ingestion

Question 6

What are the advantages of a compartmentalised structure in eukaryotes?

Answer 6

1. Enzymes and substrates for a particular process can be much more concentrated than if they were spread throughout the cytoplasm.
2. Substances that could cause damage to the cell can be kept inside the membrane of an organelle. For example, the digestive enzymes of a lysosome could digest and kill a cell, if they were not safely stored
   inside the lysosome membrane.
3. Conditions such as pH can be maintained at an ideal level for a particular process, which may be different to the levels needed for other processes in a cell.
4. Organelles with their contents can be moved around within the cell

Question 7

State the structure and function of the nucleus

Answer 7

The nuclear membrane is double and has pores through it. The nucleus contains the chromosomes consisting of DNA associated with histone proteins. Uncoiled chromosomes are spread through the nucleus and are called chromatin. There are often densely staining areas of chromatin around the edge of the nucleus. The nucleus is where DNA is replicated and transcribed to form mRNA, which is exported via the nuclear pores to the cytoplasm.

Question 8

State the structure and function of the Rough Endoplasmic Reticulum

Answer 8

The rER consists of fattened membrane sacs, called cisternae. Attached to the outside of these cisternae are ribosomes. They are larger than in prokaryotes and are classified as 80S. The main function of the rER is to
synthesize protein for secretion from the cell. Protein synthesized by the ribosomes of the rER passes into
its cisternae and is then carried by vesicles, which bud off and are moved to the Golgi apparatus.

Question 9

State the structure and function of the Golgi apparatus

Answer 9

This organelle consists of fattened membrane sacs called cisternae, like rER. However the cisternae are not as long, are often curved, do not have attached ribosomes, found near the plasma membrane and have many vesicles nearby. The Golgi apparatus is Involved in the sorting, storing, modification and export (packed in vesicles) of secretory products.

Question 10

State the structure and function of lysosomes

Answer 10

These are approximately spherical with a single membrane. They are formed from Golgi vesicles. They
contain high concentrations of protein, which makes them densely staining in electron micrographs. They contain digestive enzymes, which can be used to break down ingested food in vesicles or break down organelles in the cell or even the whole cell.

Question 11

State the structure and function of mitochondrion

Answer 11

A double membrane surrounds mitochondria, with the inner of these membranes invaginated to form structures called cristae. The fluid inside is called the matrix. The shape of mitochondria is variable but is usually spherical or ovoid. They produce ATP for the cell by aerobic cell respiration. Fat is digested here if it is being used as an energy source in the cell.

Question 12

State the structure and function of free ribosomes

Answer 12

These appear as dark granules in the cytoplasm and are not surrounded by a membrane. Ribosomes are two subunits made of RNA and protein. They are 80s ribosomes in eukaryotic cells and 70s ribosomes in prokaryotic cells. They are the site of polypeptide synthesis (this process is called translation). Nucleolus is the site of ribosome assembly

Question 13

State the structure and function of vesicles

Answer 13

Very small in size with and has a single membrane with fluid inside. Used to transport materials inside of the cell.

Question 14

State the structure and function of vacuoles

Answer 14

Single membrane with fluids inside. In plant cells, vacuoles are large and permanent, often occupying majority of the cell’s volume with the function of maintaining the cell’s hydrostatic pressure. In animal cells, vacuoles are small and temporary and used for various reasons.

Question 15

State the structure and function of microtubules

Answer 15

In the cytoplasm of the cells there are small cylindrical fibres called microtubules that have a variety of roles,
including moving chromosomes during cell division.

Question 16

State the structure and function of centrioles

Answer 16

Animal cells have structures called centrioles, which consist of two groups of nine triple microtubules. Centrioles form an anchor point for microtubules during cell division and also for microtubules inside cilia and flagella.

Question 17

State the structure and function of flagellum and cilia

Answer 17

These are whip-like structures projecting from the cell surface. They contain a ring of nine double microtubules plus two central ones. Flagella are larger and usually only one is present, as in a sperm. Cilia are smaller and many are present. Cilia and flagella can be used for locomotion. Cilia can be also be used to create a current in the fluid next to the cell.

Question 18

State the structure and function of chloroplast

Answer 18

A double membrane surrounds the chloroplast. Inside are stacks of thylakoids, which are fattened sacs of membrane. The shape of chloroplasts is variable but is usually spherical or ovoid. They produce glucose and a wide variety of other organic compounds by photosynthesis. Starch grains may be present inside chloroplasts if they have been photosynthesising rapidly.

Question 19

What is resolution?

Answer 19

It is defined as the shortest distance between two points that can be distinguished

Question 20

How do bacterial cells divide?

Answer 20

Most bacteria reproduce asexually by binary fission. The cell’s DNA is replicated and each copy attaches to a different part of the plasma membrane. The cell begins to elongate and the cell wall and cell membrane
begin to grow inwards forming a cross wall. The ingrowing cell walls meet and two identical cells are formed.