Exam Revision Notes: Infectious Diseases and Multicellular Organisms

Disease

A disease is a condition that makes an organism unwell and prevents it from functioning properly in its environment.
A disease upsets the steady state between an organism and its environment, resulting in temporary or permanent changes to physiological functions.
Diseases can be infectious or non-infectious.

Non-Infectious Disease

A disease that cannot be transferred from one person to another, also known as a non-communicable disease.
Caused by a wide range of factors.

Infectious Disease

A disease that can be transferred from person to person, also known as a communicable disease.
Caused by agents called pathogens.

Pathogens

Disease-causing microbes that exploit the host's warm, nutrient-rich, and moist environment.
- Cross protective barriers.
- Multiply rapidly.
- Avoid the immune system.

Types of Pathogens

Bacteria
- Description: Single-celled organisms without a nucleus.
- Example: Escherichia coli.
- Human Diseases: Strep throat, staph infections, tuberculosis, food poisoning, tetanus, pneumonia, syphilis.
Viruses
- Description: Non-living particles that reproduce by taking over living cells.
- Example: Herpes simplex.
- Human Diseases: Common cold, flu, genital herpes, cold sores, measles, AIDS, genital warts, chicken pox, small pox.
Fungi
- Description: Simple organisms, including mushrooms and yeasts, that grow as single cells or thread-like filaments.
- Example: Death cap mushroom.
- Human Diseases: Ringworm, athlete's foot, tineas, candidiasis, histoplasmosis, mushroom poisoning.
Protozoa
- Description: Single-celled organism with a nucleus.
- Example: Giardia lamblia.
- Human Diseases: Malaria, "traveler's diarrhea", giardiasis, trypanosomiasis ("sleeping sickness").

Bacteria

Unicellular, prokaryotic organisms (no membrane-bound organelles).
DNA organized as a single circular chromosome.
The most abundant lifeform on Earth, first evolved around 3500 million years ago.
Important for decomposition of dead organisms for recycling.
Classified based on their shape.

Lifecycle of Bacteria

Prokaryotic parent cell initiates replication.
A copy of the cell's DNA is created.
Cell elongates and cross wall forms.
Cross wall forms completely, and daughter cells separate.

Viruses

Viruses do not display all processes of living things (MRSGREN).
Extremely small compared to cells, even bacteria.
Nucleic acid (RNA/DNA) packaged in a protein coat = virion (single virus particle).

Lifecycle of Virus (example: influenza virus)

Attachment: Influenza virus becomes attached to a target epithelial cell.
Penetration: The cell engulfs the virus by endocytosis.
Uncoating: Viral contents are released.
Biosynthesis: Viral RNA enters the nucleus, where it is replicated by the viral RNA polymerase.
Assembly: New phage particles are assembled.
Release: New viral particles are made and released into the extracellular fluid. The cell, which is not killed in the process, continues to make new virus.

Fungi

Heterotrophs - do not make their own food.
Feed on organic matter.
Secrete enzymes that break down organic matter.
Like warm, damp places.

Chain of Infection

Causative Agent:
- Bacteria
- Fungi
- Viruses
- Protozoa
- Helminths (worms)
Reservoir:
- Humans
- Animals
- Soil
- Water
Portal of Entry:
- Gastrointestinal tract
- Urogenital tract
- Upper respiratory tract
- Blood
- Broken Skin
- Mucous membrane
Portal of Exit:
- Gastrointestinal tract
- Urogenital tract
- Upper respiratory tract
- Blood
- Broken Skin
- Mucous membrane
Method of Transmission:
- Indirect/direct contact
- Airborne
- Food
- Fomites
- Water
- Body Fluids
Host:
- A susceptible host can be any person: patient, client, or health worker

Disease Transmission

Pathogens are transmitted from host → host or vector → host through a variety of ways.
Host: An organism that holds a pathogen, providing a nutrient-rich environment (e.g., humans).
Vector: An intermediate host involved with the transmission of the pathogen (e.g., tick or mosquito).

Modes of Infectious Disease Transmission

General Transmission
- Abiotic environmental factors:
  - Wind: Inhalation of spores.
  - Water: Entry into skin.
- Animal vectors:
  - Mosquitoes (malaria, dengue).
  - Fleas (bubonic plague).
Human to Human Transmission
- Direct Contact:
  - Pathogen survives best inside the body.
  - Eg: HIV, Herpesviruses, Ebola
- Indirect Contact:
  - Pathogen survives harsh environment
  - Pick up pathogen from surface or air
  - Eg Influenza, norovirus
- Droplets:
  - Pathogens are in droplets, but do not survive long this way
  - Eg: Ebola, Bordetella pertussis
- Airborne:
  - Pathogens aerosolized and stay infective
  - Eg: Influenza, Tuberculosis
- Fecal-Oral:
  - Through contaminated water or food
  - Eg: Cholera, Norovirus, Shigella

Disease Control

Four main tactics:
- Control / Kill the vectors that cause spread.
- Kill pathogens.
- Quarantine.
- Immune system (we will look at this later).

First Line of Defence: Physical Barriers

Skin: Made up of different layers of tissues and is the largest organ of the body.
- Upper layer of epidermis (stratum corneum): consists of dead cells which contain keratin.
- Lipid molecules are water repellent.
- The skin secretes these from sebaceous glands which provide a waterproof coat.
Cough/Sneeze Reflex
- Reflex responses brought about by mechanical (touch) or chemical (pathogen) stimuli.
- Helps to expel the pathogen
- Stimulus → receptor → message → effector → response.
- Reflex = involuntary action
Mechanical Protection
- Blinking is a reflex action - irritation by particles including pathogens.
- Cornea provides a physical barrier to make it harder for pathogens to enter.
- Tears are produced to wash away potential harmful particles.
Chemical Protection
- Cells in the eye secrete different chemicals that possess antimicrobial properties.
- Lysozyme enzyme in tears can destroy bacterial cell walls.
Ear wax
- Secreted by the ear, wax is made of a range of chemicals that are antimicrobial and prevent entry.
Mucus
- Secretion found on the surface (epithelium) of several body organs.
  - Respiratory system: nasal passages, bronchi and bronchioles.
  - Digestive system: esophagus, stomach and intestines.
  - Urogenital system: urethra, vagina.
- Produced by epithelial cells called goblet cells.
Epithelial cells contain hair like structures called cilia, that sway back and forth.
- Direct mucus and foreign matter towards the nose and mouth to be expelled or swallowed.
Stomach Acid
- Cells lining the stomach secrete acid which makes the stomach highly acidic (low pH).
- Acid kills many pathogens, but good bacteria thrive here.

Second Line of Defence: Innate Immune System

Immune Cells:
- Dendritic cell
- Macrophage
- Neutrophil
- Mast cell
- Natural killer cell
- Monocyte
Soluble Proteins:
- Complement proteins
- Cytokines
Inflammation

Phagocytosis

When pathogens first enter the body they encounter white blood cells called phagocytes.
The phagocytes (neutrophils in the blood and macrophages in body tissue) recognise and bind to the surface antigens on pathogens.
Engulf the pathogen into the cell, break it down and expel the contents by exocytosis.

Natural Killer Cells

Once pathogens have entered the host cell they cannot be killed by phagocytosis.
Natural killer cells:
- Recognise protein markers on invading cells that need to be destroyed.
- Release cytokines which signal other responses.
- Punch holes through the cells they need to destroy.

Soluble Proteins

Complement: group of 30 proteins that are secreted by a range of cells
- Inactive until they bind to surface antigens on pathogens.
Interferons: chemicals secreted by cells which are infected with a virus.
- Aimed at limiting the spread and multiplication of viruses.

The Inflammatory Response

Major component of the innate immune system.
- Swelling
- Redness
- Pain
- Heat

Third Line of Defence: Adaptive Immune System

Characterised by:
- Specific antigens and specific pathogen recognition.
- Highly selective and able to detect differences between pathogens.
- Retains memory of the pathogen in event of subsequent infections.
- Second exposure to the same antigen is bigger than the first response.

Major-Histocompatibility Complex

Set of protein markers found on the surface of cells that are unique for every individual.
Each cell has two different types of these markers:
- ‘Self’ proteins that are unique to the individual.
- Short sections of pathogen antigens, which are called ‘non-self’. These help WBC to recognise and bind to the cells.

Cells of the Adaptive Immune System

All blood cells come from bone marrow.
- B cells remain in the bone marrow to mature.
- T cells mature in the thymus gland.

B Cells

These cells have specific receptors that are designed to recognise ONE specific pathogenic antigen.
They release a specific antibody which binds to its complementary antigen.
Antibodies signal other cells (like phagocytes) that the cell is infected.

B Cells (cont.)

When an antigen binds to a B cell, it divides to create:
- Plasma cells which secrete more antibodies.
- Memory B cells which remain in the blood in case of a second exposure to the pathogen.

B Cells (Memory)

Memory B cells allow for much quicker response to a ‘second exposure’ of the pathogen.
More antibodies are produced.

T Cells

Each person has around 10 million different T cells with their own specific antigen binding capacity.
T cells possess receptors on the surface of their membranes the can bind to one specific type of antigen.
T cells bind to antigen fragments bound to the MHC which initiates the action of the T cells.

T Cells (Killer)

Killer T cells (also known as cytotoxic T cells): identify infected cells and secrete chemicals that destroy them.

T Cells (Helper)

Helper T cells:
- Activate killer T cells.
- Provide memory T cells.
- Activate specific B cells which secrete antibodies.

T Cells (Memory)

Memory T cells: increase in number as a result of initial infection.
Responsible for quicker response on second exposure.

Active Immunity

Adaptive immune response activated, memory cells are produced and stored in the spleen and lymph nodes.
Active immunity can be brought about in two ways:
- Exposure to the actual pathogen and hence antigen (get the disease).
- Exposure to a vaccine that consists of harmless antigens derived from the pathogen (vaccination).

Passive Immunity

Brought about by the acquisition of antibodies.
Not long term.
- Naturally for unborn babies or new born babies whose immune systems have not fully developed yet (breast milk or placenta).
- By injection to provide protection for extremely virulent pathogens where immediate action is required to prevent death (snake bite).

Cell Differentiation

Cells in a multicellular organism develop differently to become specialised.
For example, sperm cells need a flagellum for mobility – therefore they produce strands of proteins called microtubules
Cells will produce different proteins in order to become specialised

Cell Differentiation (cont.)

How do cells determine which proteins are produced?
Genes can be switched on/off through
- Activators – which ‘opens up’ DNA and allows the DNA to be ‘read’
- Repressor – make a region of DNA tightly coil and unable to be ‘read’

Protein Synthesis

Transcription

DNA from a gene is copied to produce an RNA transcript called messenger RNA (mRNA)
Carried out by an enzyme called RNA polymerase which uses available bases from the nucleus of the cell to form the mRNA chain
RNA is similar in chemical structure to DNA but only contains a single strand of bases
RNA uses a base called uracil (U) instead of thymine (T)

Translation

The premature mRNA is spliced to remove introns (non-coding regions) and joins together the exons (coding regions). This becomes mature mRNA.
mRNA moves out of the nucleus to the ribosomes (protein making factories) for translation to occur.

Translation (cont.)

The message carried by the mRNA is read by the carrier molecule called transfer RNA (tRNA)
The mRNA is read three letters at a time (called a codon)
Each codon specifies a particular amino acid.
As there are only 20 amino acids but 64 possible codon combinations, more than one codon can code for the same amino acid.

Translation (cont.2)

Each amino acid is attached specifically to its own tRNA molecule
When the mRNA is sequence is read, each tRNA molecule delivers it's amino acid to the ribosome and binds temporarily to the corresponding codon on the mRNA molecule
The tRNA releases is amino acid and they all join together to form a polypeptide.

Organisation of Multicellular Organisms

Cells → tissues → organs → organ systems → organism

Tissues

Lining of organs (all hollow organs eg stomach)
Skin
Brain
Spinal cord
nerves
Cardiac muscle
Smooth muscle
Skeletal muscle
Fat
Bone
Tendon and ligament
Blood

Exchange Surfaces

Exchange surfaces allow for exchange of vital substances with the environment
In the human body:
- Alveoli in the lung
- Villi in the small intestine
- Nephrons in the kidney
Exchange surfaces require four specific characteristics to be effective

Exchange Surfaces Characteristics

Thin
- Reduces the distance that materials need to move during exchange
- Increases the speed of exchange
- Often 1-2 cells thick
Moist
- Assists the transport of materials across the exchange surface
- Eg oxygen is dissolved in water to allow it to be transported across the membrane
Large Surface Area
- Provides more surface for exchange to take place
- Many exchange surfaces will have folds to increase the surface area
- Eg villi and microvilli in the small intestine
Blood Supply
- Good, constant blood supply maintains a large diffusion gradient and ensure the exchanged substances are constantly moving to the area needed
- Exchange surfaces are always surrounded by capillaries (one cell thick)

Transport Processes

Active transport
Passive transport

Transport Processes (cont.)

Active = requires energy for substances to be transported across cell membrane
Passive = no energy required for substances to be transported across cell membrane

Simple Diffusion

The movement of small uncharged molecules with the concentration gradient across the cell membrane
High concentration to low concentration
Eg. Oxygen, carbon dioxide, ethanol and small lipids

Facilitated Diffusion

The movement of molecules with a concentration gradient
Requires a channel protein
Eg. Glucose

Active Transport

The movement of molecules against a concentration gradient
Requires a transport protein and ATP (energy)
Eg. Glucose, ions and amino acids

Circulatory System

Blood

Blood is made up of four components:
- Plasma
- Red blood cells
- White blood cells
- Platelets

Red Blood Cells

Flattened, biconcave disc shape: ensures maximum surface area to volume ratio for gas exchange
Red: Large amount of haemoglobin which transports oxygen
Diameter larger than the capillary ( $6-8$ micrometres): slows blood flow to enable diffusion
No nucleus or organelles: maximises space for haemoglobin so more oxygen can be transported

Blood Vessels

Arteries

Carry blood away from the heart
Generally carry oxygenated blood
Are thick and elastic
Have thick muscles for contraction (vasoconstriction) and relaxing (vasodilation)
Carry blood under high pressure
Branch into arterioles
Have a relatively small lumen (space in the middle)
No valves

Veins

Carry blood towards the heart
Generally carry deoxygenated blood (pulmonary vein exception)
Are thinner and less elastic and muscular than arteries
Have valves to force blood in one direction
Require skeletal contractions to move the blood
Carry blood under low pressure
Branch into venules
Have a large lumen (space in the middle)

Capillaries

Link arterioles and venules
Only site of exchange of materials
Thin walls (one cell thickness)
Close proximity to cells to assist diffusion
Provide large surface area
Not muscular or elastic
No valves
Relatively large lumen
Blood pressure begins high and finishes low

The Heart

The human heart is a combination of two pumps that work together
The right side of the heart receives deoxygenated blood
The left side of the heart receives oxygenated blood

Respiratory System

Gas Exchange at the Alveolus

Air moves in and out of alveolus
Gases dissolve in moist mucus lining
Oxygen diffuses into blood
Oxygen is transported around body by red blood cells
$CO_2$ diffuses from blood to be exhaled

Digestive System

Physical vs Chemical Digestion

Physical: Breakdown of food by chewing and grinding in the mouth before swallowing, churning of food in the stomach
Chemical: Enzymes break down foods into the various building blocks of the food consumed. Macromolecules found in food are polysaccharides/carbohydrates, lipids, protein and nucleic acids

Factors Affecting Enzymes

Concentration of enzyme: increased enzyme, increased rate of reaction
Concentration of substrate: increased substrate, increased rate of reaction - until saturation
Temperature: increased temperature, increased molecular collisions, increased rate of reaction
- At a certain temperature: denaturation of enzyme
pH: optimal pH maintains enzyme shape
- At a certain pH: denaturation of enzyme
Salt concentration
- Above the optimum salt level: denaturation of enzyme

The Villus

The villi project into the intestinal lumen
Each villus is one cell think, lined with epithelial cells and contains a capillary and a lacteal (lymph vessel)
Each villus is also lined with microvilli

Absorption of Nutrients

Absorption of Carbohydrates

Carbohydrates are broken down into glucose
Absorbed into the capillary by diffusion or active transport (depending on concentration gradient)

Absorption of Lipids

Lipids (fats) are broken down into fatty acids and glycerol
Absorbed into the lymph vessels (lacteals) by diffusion or active transport (depending on concentration gradient)

Absorption of Proteins

Proteins are broken down into amino acids
Absorbed into the capillary by diffusion or active transport (depending on concentration gradient)

Absorption of Nucleic Acids

Nucleic acids are broken down into nucleotides
Absorbed into the capillary by diffusion.