Unit 12: Diseases and Infections
A: Investigate different types of diseases and infections that can affect humans
Infectious and non-infectious diseases
My local hospital is offering clinical insight work experience placements to Key Stage 5 students who are planning to
pursue a career in pharmacy, physiotherapy or nursing. I have been asked to conduct research into different types of
infectious agents, the causes of non-infectious diseases and the relationship between disease progression and the
effects of this on human health.
Communicable diseases
A communicable disease is a disease which can be inherited from another individual. Communicable diseases can
mainly be passed from person to person through five pathogens: bacteria, fungi, viruses, protozoa and parasites.
Bacteria
Bacteria are prokaryotic and unicellular organisms cells [1]. A typical bacterial cell is 2 micrometres long and 0.5
micrometres in diameter [2]. Bacteria can be found in hot springs, polar ice and glaciers, soil, water, plants, animals,
radioactive waste and deep inside the earth's crust [3]. They can also be found in the stratosphere, between 6 and
30 miles up in the atmosphere as well as in the deepest parts of the ocean, 10,000 metres or 32,800 feet below the
surface [3]. Bacteria cause disease by secreting toxins that are produced when bacteria decompose, or by
developing sensitivity to their antigenic characteristics [4].
Figure 1 - Diagram of bacterial cell [5]
Structure and function of organelles in bacterial cells
The main functions of capsules are to prevent the bacteria from drying out and to shield it from being phagocytosed
by larger germs [6]. Escherichia coli and Streptococcus pneumoniae, two of the main disease-causing bacteria, both
have a capsule [6].
The cell envelope is made up of two to three layers: the interior cytoplasmic membrane, the cell wall and in some
species of bacteria, an outer capsule [6]. They serve as the initial line of defence against biotic and abiotic threats
from the outside [7].
A protein-sugar (polysaccharide) compound known as peptidoglycan makes up the cell wall [6]. It shapes the cell
and envelops the cytoplasmic membrane, shielding it from the exterior of the cell [6]. It also helps to anchor
,Unit 12: Diseases and Infections
A: Investigate different types of diseases and infections that can affect humans
appendages like the pili and flagella, which originate in the cytoplasm membrane and protrude through the wall to
the outside [6]. When there are significant osmotic pressure differences between the cytoplasm and the
environment, the cell is prevented from bursting by the strength of the cell wall [6].
Cell development, metabolism and replication take place in the cytoplasm of bacterial cells [6]. Water, enzymes,
nutrients, waste products and gases make up its gel-like matrix which also contains ribosomes, chromosomes and
plasmids [6].
The cytoplasmic membrane controls the interior of the bacteria, regulating the flow of materials into and out of the
cell [6]. This structural feature, which enables all living cells to interact with their environment in a selective manner,
is shared by all living cells [6].
Flagella are structures that resemble hairs and allow for mobility [6]. They can be found at any point on a
bacterium's surface as well as at either or both of its ends [6]. The flagella, which beat in a propeller-like manner,
help the bacterium travel in one of three directions: toward nutrition, away from dangerous substances, or in the
case of photosynthetic cyanobacteria, toward the light [6].
The nucleoid, a region of the cytoplasm, contains chromosomal DNA [6]. Most bacteria have a single, circular
replication-controlling chromosome, while some species do have two or more [6].
Pili are protruding structures on the surface of the cell that are present in a variety of bacterial species [6]. These
protrusions enable the bacteria to attach to a variety of tissues and construct elements, such as teeth, intestines and
rocks [6]. Due to their inability to adhere to host tissue, many disease-causing bacteria lose their ability to infect [6].
Through specialised pili, two bacteria exchange plasmid DNA fragments during conjugation [6].
Nucleic acids, which are the building blocks of proteins, are converted into amino acids by ribosomes in order to
carry out genetic functions [6].
Mode of reproduction in bacteria
Bacteria reproduce through binary fission, which is an asexual process [8]. DNA replication of the bacteria is the first
step that takes place [9]. At a specific spot near the origin, where DNA replication starts, the circular DNA
chromosome is joined to the cell membrane [9]. The DNA replicates in both directions from the point of replication
until the two replicating strands come together and DNA replication is finished [9].
The bacterial cell expands at the same time that the DNA is being replicated [9]. As the cell multiplies, the
chromosome stays bound to the plasma membrane [9]. The replicating DNA chromosomes will therefore start to
segregate to opposite ends of the cell as the cell expands [9].
As the bacteria cell develops and the DNA chromosomes duplicate, genome segregation continues [9]. Cytokinesis
starts once the chromosome has finished replicating and has moved past the cell's midpoint [9]. Cytokinesis begins
with the formation of a FtsZ protein ring [9]. FtsZ helps in the recruitment of additional proteins and these proteins
start the synthesis of new plasma membranes and cell walls [9]. A structure known as a septum develops when the
components for the cell wall and plasma membrane build up [9]. Similar to the cell plate in plant cells during
cytokinesis, this septum serves a similar purpose [9]. The daughter cells will ultimately be separated by the septum,
which will fully develop into a new cell wall and plasma membrane, concluding cell division through binary fission in
bacteria [9].
Bacterial diseases
Some bacterial diseases include meningitis (caused by Meningococcal bacteria [12]), pneumonia (caused by
Streptococcus pneumoniae [13]) and tetanus (caused by clostridium tetani [12]).
,Unit 12: Diseases and Infections
A: Investigate different types of diseases and infections that can affect humans
Pneumonia is an illness that causes the air sacs in one or both lungs to become inflamed or clogged with fluid or pus
(purulent material). This can result in phlegm or pus cough, fever, chills and trouble breathing [13]. An overwhelming
infection or excessive inflammation in reaction to pneumonia can cause serious lung injury, resulting in acute
respiratory distress syndrome (ARDS) [14]. ARDS causes shortness of breath which is generally accompanied by fast,
shallow breathing [14]. People suffering from ARDS typically require mechanical ventilation for an extended period
of time [14].
Common pneumonia symptoms include a cough (which may be dry or produce thick yellow, green, brown, or
blood-stained mucus), difficulty breathing (breathing may be rapid and shallow, a rapid heartbeat, fever, feeling
generally unwell, sweating and shivering [15].
Life cycle of streptococcus pneumoniae
Streptococcus pneumoniae colonises the upper respiratory tract (URT) mucosa [16]. This carriage is required for
both transmission to other people and the development of invasive disease in the carrier [16]. Carriers can shed
streptococcus pneumoniae in nasal secretions, allowing the pathogen to spread [16]. Invasive illnesses such as otitis
media, community-acquired pneumonia, sepsis and meningitis can result from spreading beyond its niche along the
nasal epithelium by aspiration, bacteraemia or local dissemination [16]. Because all of these diseases are 'dead ends'
in the organism's life cycle, the bacterial elements that cause them must also be adaptable for colonisation and/or
transmission [16].
Figure 2 - Diagram showing the life cycle of Streptococcus pneumoniae [16]
Fungi
Fungi are eukaryotic cells [17]. They can be single-celled or multicellular organisms [18]. An example of a
single-celled fungi is yeast [19] and an example of a multicellular fungi is mould [19]. They are usually 2–3 µm to
, Unit 12: Diseases and Infections
A: Investigate different types of diseases and infections that can affect humans
20–50 µm in length and 1–10 µm in width [21]. Fungi may grow in soil, on plants outside, in the air, indoors on
surfaces and within people's bodies [22]. They can cause disease by inadvertently breaching host barriers or when
immunologic abnormalities or other debilitating circumstances favour fungal entry and proliferation [23].
Figure 3 - Diagram of fungal cell [24]
Structure and function of organelles in fungal cells
Cell rigidity and shape are provided by the fungal cell wall, as are metabolism, ion exchange and interactions with
host defence mechanisms [25].
The cell membrane's primary role is to act as a barrier between the cell and its surroundings [26].
Mitochondria use cellular respiration to generate energy in the form of adenosine triphosphate (ATP) [27]. They also
play an important role in fungal virulence and pathogenesis, (the ability of fungi to cause disease and the severity of
the infection) [27].
Cytoplasm allows for long-distance transportation of resources without the need for a separate circulatory system
[28].
The nucleus stores the DNA of fungal cells [17].
Ribosomes convert messenger RNA into proteins [29]. They are produced at the nucleolus and then spread
throughout the cytoplasm [29].
Mode of reproduction in fungi
Fungi can reproduce both sexually and asexually [30]. Imperfect fungi only reproduce asexually (through mitosis),
whereas perfect fungi reproduce both sexually and asexually [30]. Fungi develop spores in both sexual and asexual
reproduction, which disperse from the parent organism by either drifting on the wind or hitching a ride on an animal
[30]. Plant seeds are larger and heavier than fungal spores [30]. The gigantic puffball mushroom explodes, releasing
millions of spores [30]. The massive quantity of spores expelled improves the likelihood of landing in a favourable
environment for growth [30]. Fungi reproduce asexually through fragmentation, budding, or spore production [30].
Hyphae fragments can form new colonies [30]. Mycelial fragmentation happens when a fungal mycelium splits into
pieces, each of which grows into a distinct mycelium [30]. During budding (a type of cytokinesis), a bulge grows on
the cell's side, the nucleus divides mitotically and the bud eventually detaches itself from the mother cell [30]. The
most prevalent process of asexual reproduction is the formation of asexual spores, which are produced by one
