OCR A level Biology Module 4

• What are are pathogens?

  Microorganisms that cause communicable diseases.
• What are the 4 types of pathogens?

  1. Bacteria
  2. Fungi
  3. Viruses
  4. Protoctista
• What diseases are caused by bacteria?

  Tuberculosis (lungs), bacterial meningitis (brain) and ring rot (kills tomatoes and potatoes)
• What diseases are caused by Viruses?

  Tobacco mosaic virus (plants leaves), Influenza (animals and humans) and HIV -causes AIDS (affects immune system).
• What diseases are caused by Fungi?

  Athletes foot (foot skin), cattle ringworm (cattles skin) and black Sigatoka (kills banana plants)
• What diseases are caused by protoctista?

  Malaria (red blood cells) and late blight (tomatoes and potatoes to decay)
• In order for a population of pathogens to survive they need too be transferred from an infected host to an uninfected host.
• This transmission is either direct or indirect.
• In humans what are the 2 methods of direct transmission?

  Direct contact (kissing/handshaking) or droplets (when an infected person coughs or sneezes they expel pathogens in droplets of fluid and and uninfected person can breath them in)
• In humans what are the 2 methods of indirect transmission?

  Ingestion of contaminated foods and water and vectors - organisms that transfer pathogen from and infected individual while not being harmed by pathogen themselves e.g. mosquitos, water and air. Spores - small reproductive structures that can be highly infectious and can travel through water and air causing disease when inhaled.
• Factors affecting transmission in humans?

  1. Dense population
  2. Few trained healthcare workers
  3. Insufficient health education
  4. Climate change - Warmer more pathogens and vectors are able to survive
• What is a method of direct transmission in plants?

  Direct contact
• What is are the methods of indirect transmission in plants?

  Contaminated soil and vectors including humans.
• What factors affect transmission in plants?

  1. Growing susceptible crops to disease
  2. Soil is overcrowded
  3. Crops grown in damp, warm environments
• What are the physical plant defences?

  1. Barriers preventing pathogens from entering the plant including waxy cuticles in leaves, thick cellulose cell wall in palisade cells and ability to close stomata preventing pathogens from entering
  2. If passes through barriers then they can prevent infection spreading e.g. stimulates the plant to produce a polysaccharide callose. Callose deposited between cell surface membrane and the cell wall reinforcing it making harder for pathogen to enter. Also, plants can detach an infected leaf through a process called abscission.
• What are the plants chemical defences?

  Specific chemicals the plant produces. Repel, kill or prevent the growth of pathogens e.g. release of hydrolytic enzymes to break down the cell wall of invading pathogens.
• What are the primary non-specific defences (prevent the entry of pathogens) in humans?

  • Skin
  • Mucus membranes - lie in airway, gut and reproductive systems. Contain mucus secreting goblet cells traps pathogens and contains enzymes and white blood cells that kill bacteria.
  • Expulsive reflexes - coughs and sneezes
• What does skin do?

  If the body is wounded than the body responds to close this wound. Blood clotting - blood near the wound clots this forms a scab that seals the wound preventing pathogens from entering. Beneath the scab cells divide by mitosis to heal the wound. If pathogens manage to enter through a wound then the site of the wound becomes inflamed (inflammation). This occurs when blood vessels around the site dilate and their permeability increases this causes fluid to enter the tissue which creates swelling and allows white blood cells to enter the tissue and kill pathogens.
• What is a lymphocyte?


  A type of white blood cell that identifies non-self cells e.g. pathogens, toxins, cells from other humans and abnormal body cells.
• What is an antigen?

  A protein in the cell surface membrane that triggers an immune response only found in the non-self cell.
• How do Lymphocytes recognise antigens?

  Lymphocytes are able to recognise antigens because they have complementary receptors that bind to antigens. Each lymphocyte has receptors that are capable of recognising one specific antigen so trigger a specific immune response.
• What is another type of white blood cell?

  Phagocytes have receptors that are complementary to molecular patterns found on all pathogens. Phagocytes form part of the non-specific immune response, because phagocytes are able to recognise all pathogens.
• Phagocytes destroy pathogens by carrying out a process called phagocytosis. Phagocyte engulfs the pathogen , Phagocyte breaks down the pathogen.
• What happens during phagocytosis? (Stages 1-3)

  A phagocyte engulfs and breaks down a pathogen. When a pathogen invades the body  the pathogen releases chemicals that attract the phagocyte. A phagocyte moves towards a pathogen from an area with a low concentration of chemicals to an area with a high concentration of chemicals. After moving towards the pathogen along a concentration gradient, the phagocyte it  binds to the pathogen via its receptors. The phagocyte engulfs the pathogen, encasing it in a vesicle (sac-like structure). It is known as a phagosome.
• What happens during phagocytosis? (Stages 4-5)

  The final two steps of phagocytosis are as follows:

  • Lysosomes in the phagocyte move towards the phagosome and fuse

  with it, forming a phagolysosome.

  •  They release lysoszymes which breaks down the pathogen through 

  hydrolysis reactions.
• Explain how a pathogen can be destroyed by phagocytosis.

  1. The phagocyte is attracted to chemicals released by the pathogen and moves along a concentration gradient.
  2. The phagocyte binds to the pathogen and recognises it as non-self. 
  3. The phagocyte engulfs the pathogen to form a phagosome.
  4. Lysosomes in the phagocyte fuse with the phagosome, forming a phagolysosome. 
  5. The lysosomes release lysozymes which hydrolyse the pathogen.
• What are cytokines?

  Cytokines are produced by phagocytes that have engulfed a pathogen. They attract other phagocytes to the site of the infection. The phagocyte can then engulf and destroy the pathogen.
• What are opsonins?

  Opsonins bind to a pathogen’s antigens and prevent the pathogen from repelling phagocytes. Phagocytes have receptors complementary to opsonins, so the two bind together. This makes it easier for phagocytes to engulf and destroy pathogens.
• How do you tell the difference between neutrophils and lymphocytes?

  Neutrophils can be identified by their lobed nucleus while lymphocytes have a round nucleus.
• Explain why it is important to stain blood smears with a differential stain.

  Without a stain, white blood cells are transparent. Differential stains allow the identification of different cells and different organelles within cells, by providing a contrast between the colours.
• What are the two types of lymphocytes involved in the specific immune response?

  B cells and T cells

  In humans, B​ cells mature in the  bone marrow and T ​ cells mature in the thymus
• What is the role of B and T cells in specific immune response?

  During the specific immune response, a naive B cell’s receptors bind to 

  complementary antigens on a pathogen. The B cell internalises the pathogen and presents the antigens on its surface. The processed antigens bind to complementary receptors on a T helper cell. This cell releases interlukins that stimulate the B cell to clone itself. Finally, the B cells differentiate into plasma cells or memory B cells.
• What is the role of plasma cells?

  A typical plasma cell survives for a few days and produces antibodies.
• What are antibodies?

  Proteins produced by B cells in response to a specific antigen.
• Label the structure of an antibody. A)Antigen-binding site B) Disulfide bridge C) Heavy chain D) Constant region

  
• What type of antibodies do plasma cells secrete?

  Monoclonal antibodies are antibodies that have the same tertiary structure, produced from cloned B cells. When an antibody binds to a complementary antigen, they form a/an antigen-antibody complex .

  Antibodies can bind to 2 antigens at one time. 
• What is agglutination?


  Antibodies bind to antigens on pathogens, causing them too clump together.

  Antibodies that carry out agglutination are called agglutinins.
• Antibodies

  • Function as Opsonins - Bring pathogens to phagocytes to allow them to be destroyed
  • Function as Anti-toxins - Bind to pathogen toxins and prevent them from damaging cells
• Antibodies as Opsonins

  1. Phagocytes have receptors complementary to some antibodies

  2. Antibodies bring pathogens to phagocytes

  3. Phagocytes can then destroy the pathogens
• Antibodies as Anti-toxins

  1. Pathogens produce toxins that can damage cells

  2. Antibodies bind to the toxins

  3. Prevents the toxins from functioning

  4. Keeps cells healthy