Cell Membranes

What is the Fluid mosaic model ?

• Because the membrane contains molecules of different substances it is called a mosaic

• The lipid bilayer gives the membranes its fluid characteristics because the phospholipids and proteins move within the membrane

What is a Glycolipid ?

• Lipids with a carbohydrate attached by a glycosidic bond

• Their role is to serve as markers for cellular recognition and also to provide energy

What is Cholesterol ?

Regulates the fluidity of the plasma membrane, acting as a buffer

What are Glycoproteins ?

Cell recognition and contribute to membrane stability

What are enzymes ?

Catalyse reactions in the cytoplasm

What are channel proteins ?

• Allows large, charged (polar) molecules to pass through

• Allows movement with concentration gradients

• Can open/close

What are carrier proteins ?

• Allow large, polar molecules to pass through

• Require energy to change shape

• Can have movement against a concentration gradient

What are receptors ?

Proteins have a very specific shape, making them ideal as receptor molecules for chemical signalling between cells

Cell recognition

Antigens :

• Act as cell identity markers

• Often glycoproteins

What is cell adhesion ?

Cells needing to stick together

What is diffusion ?

• The movement of particles from an area of high concentration to an area of low concentration down a concentration gradient

• Passive process

• Diffusion continues until equilibrium is reached

• The particles are spread evenly in the available space

• Random movement of particles still occurs but there is no net movement in any particular direction

What is lipid (simple) diffusion ?

• A few substances can diffuse directly through the lipid bilayer

• The only substances that can do this are lipid-soluble molecules

• Since this is a passive process, no energy is involved and substances can only move down their concentration gradient

• Cannot be controlled by the cell

Factors affecting diffusion - SURFACE AREA OF MEMBRANE

• Increased surface area = increased rate

• More room for molecules to pass through

Factors affecting diffusion - CONCENTRATION GRADIENT

Increased concentration gradient = increased rate

Factors affecting diffusion - DIFFUSION DISTANCE

Shorter diffusion distance = increased rate

Factors affecting diffusion - TEMPERATURE

Increased temperature = increased movement of particles = increased rate

Factors affecting diffusion - SIZE OF MOLECULES

Larger molecule = decreased rate

Factors affecting diffusion - CHEMICAL NATURE OF MOLECULES (POLARITY)

• Non polar = increased rate

• Polar = decreased rate

Factors affecting diffusion - DIAMETER AND NUMBER OF TRANSPORT PROTEINS

• Increased number of transport proteins = increased rate

• Wider diameter = increased rate

What is facilitated diffusion (passive transport) ?

• Involves transmembrane proteins

• The diffusion of substances across a membrane through a trans-membrane protein molecule

• Transport proteins tend to be specific for one molecule

• No energy is involved and substances can only move down their concentration gradient

What is a channel protein ?

Have pores which can open and close (gated)

What is a carrier protein ?

• High to low concentration

• Have specific binding/receptor sites (not active sites)

• Doesn’t require energy as energy created from binding is enough to use to change shape

Protein carriers are specific

Molecules will bind to the binding site of the carrier protein and to do this it needs to be specific in shape to let specific molecules through

It is a passive process

Doesn’t require energy

Some ions move through gated proteins

Not all as some may not bind to carrier proteins binding site of the (ions are charged)

Facilitated diffusion is similar to diffusion but also different

• Similar in allowing molecules to pass through the membrane

• More molecules and more types of molecules pass through in facilitated diffusion

What is active transport ?

The movement of molecules from an area of low concentration to an area of high concentration against the concentration gradient, this requires energy and a specific carrier protein

What does active transport do ?

Enables essential substances to be transported in/out of cells even when the external concentration is low

What is co-transport ?

• Glucose and other polar molecules are absorbed by a combination of facilitated diffusion and active transport along with a second substance

• For example, special carrier proteins in the cell surface membranes of epithelial cells only transport glucose in the presence of sodium ions

What is Osmosis ?

• The movement of water molecules from a high water potential to a lower water potential across a partially permeable membrane, through the phospholipids

• Water moves to a more negative water potential

What is water potential ?

• Water molecules are in constant motion as they randomly hit the cell membrane, exerting a pressure

• This pressure is known as water potential

• Water potential is simply the effective concentration of free water

• It is measured in units of pressure (Pa/kPa)

How do solutes affect water potential ?

• Solutes lower the water potential of the cytoplasm because they attract water molecules, which form a ‘shell’ around them

• The water molecules collide less often with the cell membrane

• Water always moves from a high to a lower water potential

Osmosis in animal cells - SURROUNDING SOLUTION HAS A HIGH WATER POTENTIAL (E.G. FRESHWATER)

Net diffusion of water into cell, so cell swells and bursts (lysis)

Osmosis in animal cells - SURROUNDING SOLUTION HAS EQUAL WATER POTENTIAL

Cell is normal size (isotonic)

Osmosis in animal cells - SURROUNDING SOLUTION HAS LOW WATER POTENTIAL (E.G. SEA WATER)

Net diffusion of water out of cell, so cell shrinks and crenates

Osmosis in plant cells - SURROUNDING SOLUTION HAS HIGH WATER POTENTIAL (E.G. FRESHWATER)

Net diffusion of water into cell, so cell swells a bit and becomes turgid

Osmosis in plant cells - SURROUNDING SOLUTION HAS EQUAL WATER POTENTIAL

No net diffusion of water, so cell is normal size

Osmosis in plant cells - SURROUNDING SOLUTION HAS LOW WATER POTENTIAL (E.G. SEAWATER)

Net diffusion of water out of cell, so cytoplasm shrinks from cell wall and the cell plasmolyses

What are antigens ?

Usually large molecules - proteins, polysaccharides or glycoproteins on the outer surface of the cell - they trigger an immune response

What are foreign antigens ?

• Antigens that aren’t normally found in the body

• It’s these antigens that the immune system usually responds to

What do antigens do ?

Allow the immune system to identify pathogens, cells from other organisms of the same species, abnormal body cells and toxins

What are Phagocytes ?

• Form the non-specific immune system, this is a non specific method of killing microbes that have passed the first line of defence and entered the body

• Phagocytes are large, irregularly shaped leukocyte cells that remove bacteria, viruses, cellular debris and dust particles

• Phagocytes are constantly changing shape, and they flow over microbes, surrounding and ingesting them through the process of phagocytosis

Stages of phagocytosis

• Phagocytes are attracted to the pathogen due to the chemicals they release

• The pathogen is engulfed and a phagosome is formed around it

• Lysosomes move towards and fuse with the phagosome, releasing lysosome (enzyme)

• The pathogen is hydrolysed by the lysosome

• Useful products are moved out the cell and recycled by the body

• Antigens are displayed on the surface of the phagocyte and it is now referred to as an Antigen Presenting Cell (APC)

T-cells

• Made in bone marrow

• Move to the thymus to mature

What happens to cloned T-cells ?

• Develop into memory cells that enable a rapid response to future infections by the same pathogen

• Stimulate phagocytes to engulf pathogens by phagocytosis

• Stimulate B-cells to divide and secrete antibodies

• Activate cytotoxic T-cells

CYTOTOXIC (KILLER) T-CELL

• Bind to APC (pathogen, tumour)

• Release perforin which forms holes in the membrane of the APC

• Water moves into the APC by osmosis and causes lysis

HELPER T-CELL

• Make cytokines which help trigger B-cells and upregulate all aspects of the immune response

MEMORY T-CELL

• Remain in the body after an immune response

• They can respond quickly upon reinfection

B-Cells

• Clone by mitosis and differentiate into plasma and memory B-cells

• Antibodies are produced by B plasma cells in response to a non-self antigen

What is the constant region of an antibody ?

The same for every antibody

What is variable region of an antibody ?

Where the antigens bind to form a highly specific antigen-antibody complex

What are disulfide bridges ?

Hold the polypeptide bridges together

What is the antigen binding site ?

Complementary to a specific antigen

What are the roles of antibodies ?

• Bind to toxins and viruses to prevent them entering our cells

• Immobilise bacteria by attaching to flagella

• Agglutinate bacteria by sticking them together into clumps

• Coat bacteria, identifying them for phagocytosis

What is the primary response carried out by plasma cells ?

• Plasma cells secrete antibodies which destroy the pathogen and any toxins

• Only survive for a few days but make 2000 antibodies every second

• Memory B-Cells are also produced

• Can live for decades circulating in the blood and tissue fluid

What is the secondary response carried out by Memory cells ?

• When a second infection by the same pathogen occurs

• A smaller amount of antigen will induce an immune response because memory cells are already present

• The result is much more rapid and many more antibodies are produced

• The pathogen may be destroyed before infection takes hold

• Memory cells provide long term immunity

What is antigenic variability ?

• Changing the tertiary structure of antigens

• Secondary immune response doesn’t work with some diseases because the microbes have constantly changing antigens

• Caused by mutations in DNA or RNA replication due to poor polymerase enzymes

• Each infection causes a new primary response

What is immunisation ?

• Inject with an antigen that will promote the primary immune response, but has been modified to be non-pathogenic

• The immune system is forced to make memory cells and so if the person is ever infected with the real pathogen, a more powerful secondary response is triggered

What is active immunity ?

• When the body responds to antigens and makes its own antibodies

• Infection

• Vaccination

What is passive immunity ?

• When the body is provided with antibodies instead of making them itself

• Monoclonal antibodies

• Maternal antibodies - placenta, breastmilk

What is herd immunity ?

When a high percentage of the population is vaccinated, it is difficult for infectious diseases, that are also contagious, to spread because there are not many people who can be infected

What is a monoclonal antibody ?

Antibodies with the same tertiary structure formed from a single plasma B cell

How are monoclonal antibodies used in research ?

• If we want to locate a specific protein within a cell, we add monoclonal antibodies complementary to that protein

• These monoclonal antibodies are labelled with a (fluorescent) dye

How are monoclonal antibodies used in diagnosis ? - PROSTATE CANCER

• Add monoclonal antibody to the patient sample for the antigen of interest which has a dye attached

• Add sample to the test plate

• The antibody and dye will bind if the antigen is present

• PSA = Prostate Specific Antigen

How are monoclonal antibodies used in diagnosis ? - PREGNANCY TESTS

• Testing for the presence of hCG

• Add patients urine

• hCG will bind to mobile monoclonal antibodies for hCG with a dye attached if pregnant

• At the test window immobile hCG antibodies will bind to the hCG/antibody complex if pregnant

• At the control window immobile antibodies that are complementary to the mobile antibodies will bind to the antibodies if pregnant

How are monoclonal antibodies used in cancer treatment ?

• Monoclonal antibodies can be used to carry drugs to specific tissues

• This is because they bind to the antigens in a tumour, for example, they can be used to take drugs or radioactive substances directly to the cancer cells

What is an indirect ELISA test used for ?

Used to test to see if the patient has ever made any antibodies against it

What are the pros of vaccines ?

• Herd immunity

• Protect from disease

What are the cons of vaccines ?

• Rare to vaccinate everyone in a population

• Patients who are ill may not have the vaccine

• People refuse to be vaccinated

• Allergies

• Drug trials may be harmful

What are the pros of monoclonal antibodies ?

• Can diagnose disease

• Used for pregnancy tests

• Used in research

What are the cons of monoclonal antibodies ?

• Production involves mice

• New technology

• May be harmful during drug trials