3.2 Cells

Question 1

Describe the structure of the cell surface membrane

Answer 1

Structure:
-formed from a phospholipid bilayer with a diameter of around 10nm
-partially permeable so only allows some molecules through
-its hydrophilic heads form the inner and outer surface of the membrane
-its hydrophobic tails form the inside of the membrane, so the membrane’s surface can interact with the water inside and outside of the cell, but water-soluble substances cannot diffuse through the hydrophobic core
-cell membrane’s structure is called the fluid mosaic model, as it is made up of many structures that are constantly moving within the bilayer (hence they are fluid)
-cholesterol molecules are embedded between phospholipids to prevent too much movement
-channel proteins and carrier proteins are found within the bilayer, allowing large molecules/ions to be transported across the membrane
-receptor proteins, glycoproteins and glycolipids are scattered throughout the membrane (glycoproteins are proteins with a carbohydrate attached to them, and glycolipids are lipids with a carbohydrate attached to them)

Question 2

Describe the function of the cell surface membrane

Answer 2

-physical barrier; controls the exchange of materials between internal cell environment and external environment
-substances can be transported across the cell membrane through diffusion, osmosis and active transport
glycoproteins/glycolipids:
-respond to insulin in liver cells, resulting in the cell absorbing glucose from the bloodstream
-establish blood type
-immune responses
-respond to neurotransmitters involved in nervous responses

Question 3

Describe the structure and function of the cell wall

Answer 3

Structure:
-made of cellulose in plant and algae
-made of chitin in fungi
-peptidoglycan in most bacterial cells
-rigid
-surrounds cell membrane

Function:
-helps the cell maintain its shape
-Narrow threads of cytoplasm (surrounded by a cell membrane) called plasmodesmata connect the cytoplasm of neighbouring plant cells
-provides cell with protection against invading pathogens

Question 4

Describe the structure and function of the nucleus and its components

Answer 4

Structure:
-encased within a double membrane called the nuclear envelope, which has spaces within it called nuclear pores that allow substances like RNA to move between the cell’s nucleus and cytoplasm
-nuclear pores are also responsible for allowing enzymes (e.g. DNA polymerases) and signalling molecules to travel in
-DNA is linear and associates with proteins called histones, which coil tightly to form chromosomes which are found in the nucleolus
-DNA is too large to fit through nuclear pores, and so it cannot leave the nucleus
-some cells have more than 1 nucleolus

Function:
-controls the cell’s functions by controlling its DNA transcription
-control gene expression, protein synthesis and DNA storage
-protein synthesis and ribosome production occur in the nucleolus
-chromatin, a substance made up of DNA and protein is dispersed throughout it
-the nucleolus consists of DNA, RNA and proteins

Question 5

Describe the structure and function of chloroplasts (in plants and algae)

Answer 5

Structure:
-have a double membrane that surrounds the gel-like stroma, which has many membrane-bound, fluid-filled sacs called thylakoids
-thylakoids contain chlorophyll for photosynthesis, and stack to form structures called grana
-grana are joined together by lamellae (thin and flat thylakoid membranes)
-contain DNA and ribosomes for synthesise proteins needed in chloroplast replication and photosynthesis

Function:
site of photosynthesis:
-The light-dependent stage takes place in the thylakoids
-The light-independent stage (Calvin Cycle) takes place in the stroma

Question 6

Describe the structure and function of the Golgi body

Answer 6

Structure:
-made up of the Golgi apparatus, flattened sacs of membrane similar to the smooth endoplasmic reticulum, as well as Golgi vesicles
-the vesicles are detached, fluid-filled pockets found at the edges of the complex

Function:
-modifies proteins and lipids before packaging them into Golgi vesicles
-vesicles then transport the proteins and lipids to their required destination
-produces lysosomes
-Proteins that go through the Golgi apparatus are usually exported (e.g. hormones such as insulin), put into lysosomes (such as hydrolytic enzymes) or delivered to membrane-bound organelles

Question 7

Describe the structure and function of the lysosomes

Answer 7

Structure:
-Specialist forms of vesicles
-membrane-bound
-no obvious internal structure but has hydrolytic enzymes including digestive enzymes called lysozymes
-the pH inside lysozymes is acidic compared to the alkaline pH of the cytoplasm

Function:
-digest invading cells, complex biomolecules and waste materials such as worn-out organelles
-the membrane ensures that the lysozymes are kept separate from the cell’s cytoplasm, which prevents self-digestion

Question 8

Describe the structure and function of ribosomes

Answer 8

Structure:
-very small, consisting of a large subunit and a small subunit
-composed of almost equal amounts of ribosomal RNA (rRNA) and proteins
-not surrounded by a membrane

Function:
-formed in the nucleolus
-often associated with the rough endoplasmic reticulum, but otherwise found floating freely within the cytoplasm
-site of proteinsynthesis

Question 9

Describe the structure and function of the smooth endoplasmic reticulum

Answer 9

SER:
Structure:
-similar to RER, but does not have ribosomes attached
-typically attached to RER and linked to nuclear membrane
-large surface area = increased rate of synthesis of lipids and other molecules

Function:
-stores, synthesises and processes lipids, steroids and cholesterol
-within skeletal muscle cells, the SER stores other substances (e.g. calcium ions)
-within some endocrine glands, the SER has enzymes to detoxify harmful substances (e.g. breakdown of carcinogens in the liver cells)

Question 10

Describe the structure and function of the rough endoplasmic reticulum

Answer 10

RER:
Structure:
-network of channel-like structures filled with fluid
-ribosomes attached along outer surface
-large surface area = increased rate of photosynthesis
-formed from continuous folds of membrane continuous with the nuclear envelope

Function:
-works in conjunction with the attached ribosomes to process 3D protein structures
-site of glycoprotein synthesis
-cells that make a lot of protein tend to have a lot of RER

Question 11

Describe the structure and function of the cell vacuole (in plants).

Answer 11

Structure:
-permanent packets of cell sap (a solution of salts, sugar and water)
-surrounded by a selectively permeable membrane called the tonoplast

Function:
-maintains osmotic pressure inside cells, ensuring it remains turgid to stop plant wilting
-important for storing unwanted chemicals that are discarded by the cell

Question 12

Describe the structure and function of mitochondria

Answer 12

Structure:
-oval-shaped
-surrounded by a double membrane, with the inner membrane folded to form cristae (finger-like structures that increase the surface area available for chemical reactions to happen)
-small circular pieces of (mitochondrial DNA) and ribosomes are also found in the matrix (needed for replication)

Function:
-site of aerobic respiration within eukaryotic cells, which produces adenosine triphosphate (ATP), a molecule essential for cellular activity
-cells needing large amounts of energy tend to have a lot of mitochondria
-the matrix formed by the cristae contains enzymes needed for aerobic respiration, producing ATP

Question 13

What are centrioles?

Answer 13

-hollow fibres made of microtubules
-two centrioles at right angles to each other form a centrosome, which organises the spindle fibres during cell division
Not found in fungi and flowering plants

Question 14

How are phloem vessel cells adapted to their function?

Answer 14

Function: transport of dissolved sugars and amino acids

Adaptations:
-made of living cells, which are supported by companion cells

-cells also have very few subcellular structures to aid the flow of materials

-cells are joined end-to-end

-contain holes in the end cell walls (sieve plates) forming tubes which allow sugars and amino acids to flow easily (by translocation)

Question 15

How are xylem vessel cells adapted to their function?

Answer 15

Function: transport tissue for water and dissolved ions

Adaptations:
-no top and bottom walls between cells to form continuous hollow tubes through which water is drawn upwards towards the leaves by transpiration

-cells are dead, without organelles or cytoplasm, to allow free movement of water

-outer walls are thickened with a substance called lignin, strengthening the tubes, which helps support the plant

Question 16

How are red blood cells adapted to their function?

Answer 16

-biconcave
-do not contain a nucleus, to make more space inside the cell so that they can transport as much oxygen as possible

Question 17

How are root hair cells adapted to their function?

Answer 17

Function: to absorb water and mineral ions from soil

Adaptations:
-root hair to increase surface area (SA) so the rate of water uptake by osmosis is greater

-thinner walls, so shorter diffusion distance, so water can move through easily

-permanent vacuole contains cell sap which is more concentrated than soil water, maintaining a water potential gradient

-mitochondria for active transport of mineral ions

Question 18

How are nerve cells (neurones) adapted to their function?

Answer 18

Function: to conduct nerve impulses

Adaptations:
-has a cell body where most of the cellular structures are located and most protein synthesis occurs

-extensions of the cytoplasm from the cell body form dendrites (which receive signals) and axons (which transmit signals), allowing the neurone to communicate with other nerve cells, muscles and glands

-axon is covered with a fatty myelin sheath, which speeds up nerve impulses

-axons are long, so can enable fast communication over long distances

Question 19

How are muscle cells adapted to their function?

Answer 19

Function: Contraction for movement

Adaptations:
-all muscle cells have layers of protein filaments in them, which can slide over each other causing muscle contraction

-have a high density of mitochondria to provide sufficient energy (via respiration) for muscle contraction

-skeletal muscle cells fuse together during development to form multinucleated cells that contract in unison

Question 20

How are sperm cells adapted to their function?

Answer 20

Function: Reproduction - to fuse with an egg, initiate the development of an embryo and pass on fathers genes

Adaptations:
-head contains a nucleus that contains half the normal number of chromosomes (haploid, no chromosome pairs)

-acrosome in the head contains digestive enzymes to break down the outer layer of an egg cell so that the haploid nucleus can enter to fuse with the egg’s nucleus

-mid-piece is packed with mitochondria to release energy (via respiration) for the tail movement

-tail rotates, propelling the sperm cell forwards and allowing it to move towards the egg

Question 21

How may a cell that makes a large amount of protein be adapted

Answer 21

contains more ribosomes

Question 22

How do prokaryotic cells differ from eukaryotic cells?

Answer 22

Prokaryotic cells have:
-no membrane-bound organelles
-smaller (70S) ribosomes
-no nucleus; instead they have a single circular DNA molecule that is free in the cytoplasm and is not associated with proteins
-a cell wall that contains murein, a glycoprotein.

In addition, prokaryotic cells have:
-one or more plasmids (circular pieces of DNA)
-a capsule surrounding the cell (protective slimy layer which helps the cell to retain moisture and adhere to surfaces)
-one or more flagella (tail-like structure that rotates to allow cell movement)
-pili (Hair-like structures which attach to other bacterial cells)

Question 23

Explain what viruses are

Answer 23

-non-cellular, infectious particles that are non-living
-much smaller than prokaryotic cells (with diameters between 20 and 300 nm)
-all viruses are parasitic as they can only reproduce by infecting living cells and using their ribosomes to produce new viral particles

Structurally they have:
- a nucleic acid core (their genomes are either DNA or RNA, and can be single or double-stranded)
-a protein coat called a ‘capsid’
-some viruses have an outer layer called an envelope formed usually from the membrane-phospholipids of a cell they were made in

Question 24

Compare optical microscropes, TEM and SEM

Answer 24

Optical (light) microscopes
-use light to form an image
-can be used to observe eukaryotic cells, their nuclei and possibly mitochondria and chloroplasts, but not to observe smaller organelles e.g. ribosomes, endoplasmic reticulum or lysosomes
-maximum useful magnification of optical microscopes is about ×1500

Advantages of optical:
-cheaper
-portable
-species is alive

Disadvantages of optical:
-limited resolution and magnification

Transmission electron microscopes (TEMs)
-TEMs use electromagnets to focus a beam of electrons that are transmitted through the specimen
-Denser parts of the specimen absorb more electrons, which makes these denser parts appear darker on the final image produced (produces contrast between different parts of the object being observed)

Advantages of TEMs:
-give high-resolution images (more detail), which allows the internal structures within cells (or even within organelles) to be seen

Disadvantages of TEMs:
-can only be used with very thin specimens
-cannot be used to observe live specimens (as there is a vacuum inside a TEM, all the water must be removed from the specimen and so living cells cannot be observed, meaning that specimens must be dead, unlike optical microscopes that can be used to observe live specimens)
-lengthy treatment required to prepare specimens means that artefacts can be introduced (artefacts look like real structures but are actually the results of preserving and staining)
-do not produce a colour image (unlike optical microscopes that produce a colour image)

Scanning electron microscopes (SEMs)
scan a beam of electrons across the specimen
bounces off the surface of the specimen and the electrons are detected, forming an image
This means SEMs can produce three-dimensional images that show the surface of specimens

Advantages of SEMs:
can be used on thick or 3-D specimens
allow the external, 3-D structure of specimens to be observed

Disadvantages of SEMs:
-lower resolution images (less detail) than TEMs
-cannot be used to observe live specimens (unlike optical microscopes that can be used to observe live specimens)
- do not produce a colour image (unlike optical microscopes that produce a colour image)

Question 25

How do electron microscopes work

Answer 25

Electron microscopes use electrons to form an image

increases the resolution of electron microscopes compared to optical microscopes, giving a more detailed image

A beam of electrons has a much smaller wavelength than light, so an electron microscope can resolve (distinguish between) two objects that are extremely close together

maximum resolution of around 0.0002 µm or 0.2 nm (i.e. around 1000 times greater than that of optical microscopes)

can be used to observe small organelles such as ribosomes, the endoplasmic reticulum or lysosomes

maximum useful magnification of electron microscopes is about ×1,500,000

There are two types of electron microscopes:
Transmission electron microscopes (TEMs)
Scanning electron microscopes (SEMs)

Question 26

Explain the difference between magnification and resolution

Answer 26

Magnification tells you how many times bigger the image produced by the microscope is than the real-life object you are viewing

Resolution is the ability to distinguish between objects that are close together (i.e. the ability to see two structures that are very close together as two separate structures)

Question 27

Practical skills:

Answer 27

Many biological structures are too small to be seen by the naked eye
Optical microscopes are an invaluable tool for scientists as they allow for tissues, cells and organelles to be seen and studied
For example, the movement of chromosomes during mitosis can be observed using a microscope
When using an optical microscope always start with the low power objective lens:
It is easier to find what you are looking for in the field of view
This helps to prevent damage to the lens or coverslip incase the stage has been raised too high
A graticule must be used to take measurements of cells:
A graticule is a small disc that has an engraved scale. It can be placed into the eyepiece of a microscope to act as a ruler in the field of view
As a graticule has no fixed units it must be calibrated for the objective lens that is in use. This is done by using a scale engraved on a microscope slide (a stage micrometer)
By using the two scales together the number of micrometers each graticule unit is worth can be worked out
After this is known the graticule can be used as a ruler in the field of view

Electron microscopes can produce highly detailed images of animal and plant cells
The key cellular structures within animal and plant cells are visible within the electron micrographs below

Drawing Cells
To record the observations seen under the microscope (or from photomicrographs taken) a labelled biological drawing is often made
Biological drawings are line pictures which show specific features that have been observed when the specimen was viewed
There are a number of rules/conventions that are followed when making a biological drawing
The conventions are:
The drawing must have a title
The magnification under which the observations shown by the drawing are made must be recorded
A sharp HB pencil should be used (and a good eraser!)
Drawings should be on plain white paper
Lines should be clear, single lines (no thick shading)No shading
The drawing should take up as much of the space on the page as possible
Well-defined structures should be drawn
The drawing should be made with proper proportions
Label lines should not cross or have arrowheads and should connect directly to the part of the drawing being labelled
Label lines should be kept to one side of the drawing (in parallel to the top of the page) and drawn with a ruler

Question 28

Principles of cell fractionation and ultracentrifugation as used to separate cell components.

Answer 28

split into three stages:

1) Homogenisation
-breaking up of cells
-sample of tissue must first be placed in a cold, isotonic buffer solution
-solution must be:
-Ice-cold to reduce the activity of enzymes that break down organelles
-Isotonic (same WP as cells being broken up) to prevent water from moving into organelles via osmosis, which would cause them to expand and eventually damage them
-Buffered to prevent organelle proteins/enzymes, from becoming denatured
-the tissue-containing solution is then homogenised using a homogeniser, which grinds the cells up
-homogeniser breaks the plasma membrane of the cells and releases the organelles into a solution called the homogenate

2) Filtration
-homogenate is then filtered through a gauze, to separate out any large cell debris or tissue debris that were not broken up
-the organelles are all much smaller than the debris and are not filtered out (they pass through the gauze)
-the filtrate is the mixture of organelles remaining

3) Ultracentrifugation
-filtrate is placed into a tube, which is placed in a centrifuge to separate materials by spinning
-filtrate is first spun at a low speed
-causes the largest, heaviest organelles (e.g. the nuclei) to settle at the bottom of the tube, where they form a thick sediment known as a pellet
-rest of the organelles stay suspended in the solution above the pellet (solution is known as the supernatant)
-supernatant is drained off and placed into another tube, which is spun at a higher speed
-Once again, this causes the heavier organelles (e.g. mitochondria) to settle at the bottom of the tube, forming a new pellet and leaving a new supernatant
-new supernatant is drained off and placed into another tube, which is spun at an even higher speed
-process is repeated at increasing speeds until the desired organelle is separated out
-each new pellet formed contains a lighter organelle than the previous pellet

-order of mass of these organelles (from heaviest to lightest) is usually:
Nuclei
Chloroplasts (if carrying out cell fractionation of plant tissue)
Mitochondria
Lysosomes
Endoplasmic reticulum
Ribosomes

Question 29

Give the formula that links magnification, image size and actual size

Answer 29

M = I/A
A = I/M
I = AM

Question 30

All cells arise from other cells (check spec)

Answer 30

DNA replication occurs during the interphase of the cell cycle.
Mitosis is the part of the cell cycle in which a eukaryotic cell divides to produce two daughter cells, each with the identical copies of DNA produced by the parent cell during DNA replication.
The behaviour of chromosomes during interphase, prophase, metaphase, anaphase and telophase of mitosis. The role of spindle fibres attached to centromeres in the separation of chromatids.

Division of the cytoplasm (cytokinesis) usually occurs, producing two new cells.

Mitosis is a controlled process. Uncontrolled cell division can lead to the formation of tumours and of cancers. Many cancer treatments are directed at controlling the rate of cell division.

Binary fission in prokaryotic cells involves:

replication of the circular DNA and of plasmids
division of the cytoplasm to produce two daughter cells, each with a single copy of the circular DNA and a variable number of copies of plasmids.
Being non-living, viruses do not undergo cell division. Following injection of their nucleic acid, the infected host cell replicates the virus particles.

Question 31

interphase

Answer 31

-movement from one phase to another is triggered by chemical signals called cyclins
-during Interphase the cell increases in mass and size and carries out its normal cellular functions (eg. synthesising proteins and replicating its DNA ready for mitosis)
-consists of three phases:
G1 phase
gap between the previous cell division and the S phase
-a signal is received telling the cell to divide again
-Cells make the RNA, enzymes and other proteins required for growth during the G1 phase

S phase
-DNA in the nucleus replicates (resulting in each chromosome consisting of two identical sister chromatids)
-the S phase is relatively short

G2 phase
-Between the S phase and the next cell division event
-cell continues to grow
-new DNA that has been synthesised is checked and any errors are usually repaired
-Other preparations for cell division are made (eg. the production of tubulin protein, which is used to make microtubules for the mitotic spindle)

Question 32

why is mitosis important

Answer 32

Replacement of cells & repair of tissues:
-cells constantly die
-damaged tissues can be repaired by mitosis followed by cell division

Asexual reproduction
-production of new individuals of a species by a single parent organism
-the offspring are genetically identical to the parent

Growth of multicellular organisms:
-two daughter cells produced are genetically identical to one another (clones)
-have the same number of chromosomes as the parent cell
-enables unicellular zygotes (as the zygote divides by mitosis) to grow into multicellular organisms

Question 33

Mitosis and cytokinesis

Answer 33

Mitosis:
- nuclear division by which two genetically identical daughter cells are produced
- Also genetically identical to parent cell
- Due to same number of chromosomes in nucleus

Prophase:
- Chromosomes condense and now visible when stained
- Two centrosomes move towards opposite poles
- Spindle fibres begin to emerge from them
- Nuclear membrane breaks down into small vesicles

Metaphase:
- Centrosomes reach opposite poles
- Chromosomes line up at the equator
- Spindle fibres continue to emerge and attach to the centromere
- Each sister chromatid is attached to a spindle fibre originating from opposite poles

Anaphase:
- Sister chromatids separate at the centromere
- Centromere divides into two
- Chromosomes are now pulled to poles by spindle fibres
- Spindle fibres begin to shorten

Telophase:
- Chromosomes arrive at opposite poles and start to decondense
- Nuclear membrane begins to reform around each set of chromosomes
- Spindle fibres break down

Cytokinesis:
- Cytoplasm divides
- Two genetically identical cells are produced

Question 34

recognising stages of mitosis

Answer 34

Prophase
Chromosomes are visible
The nuclear envelope is breaking down

Metaphase
Chromosomes are lined up along the middle of the cell

Anaphase
Chromosomes are moving away from the middle of the cell, towards opposite poles

Telophase
Chromosomes have arrived at opposite poles of the cell
Chromosomes begin to decondense
The nuclear envelope is reforming

Cytokinesis
Animal cells: a cleavage furrow forms and separates the daughter cells
Plant cells: a cell plate forms at the site of the metaphase plate and expands towards the cell wall of the parent cell, separating the daughter cells

Question 35

cancer

Answer 35

-arise due to uncontrolled mitosis
-Cancerous cells divide repeatedly and uncontrollably, forming a tumour (an irregular mass of cells)
-start when changes occur in the genes that control cell division
-a change in any gene is known as a mutation
-if the mutated gene is one that causes cancer it is referred to as an oncogene
-Mutations are common events and don’t lead to cancer most of the time
-Most mutations either result in early cell death or result in the cell being destroyed by the body’s immune system
-As most cells can be easily replaced, these events usually have no harmful effect on the body
-mutations that result in the generation of cancerous cells do not result in early cell death or in the cell being destroyed by the body’s immune system
-means that the harmful mutation occurring in the original cell can be passed on to all that cell’s descendants

-Carcinogens are any agents that may cause cancer (eg. UV light, tar in tobacco smoke and X-rays)
-Some tumours (such as warts) do not spread from their original site – these are known as benign tumours and do not cause cancer
-Some tumours spread through the body, invading and destroying other tissues – these are known as malignant tumours and cause cancer
-Malignant tumours interfere with the normal functioning of the organ / tissue in which they have started to grow (eg. they may block the intestines, lungs or blood vessels
-Malignant tumour cells can break off the tumour and travel through the blood and / or lymphatic system to form secondary growths in other parts of the body
-spreading of cancers in this way is known as metastasis, and it is very dangerous as it can be very difficult to detect, locate and remove secondary cancers

Question 36

binary fission

Answer 36

-process of cell division in prokaryotic cells
-prokaryotic cells do not possess a nucleus, chromosomes, membrane-bound organelles or spindle fibres
-have a single, circular DNA molecule and plasmids
-plasmids are smaller, circular DNA molecules that are also replicated and inherited

The process of binary fission:
-different from mitosis as there is no nuclear envelope to breakdown and no spindle fibres present

-the single, circular DNA molecule undergoes DNA replication

-any plasmids present undergo DNA replication

-parent cell divides into two cells, with the cytoplasm roughly halved between the two daughter cells

-two daughter cells each contain a single copy of the circular DNA molecule and a variable number of plasmids

-there are mechanisms to ensure that all daughter cells inherit a copy of the single, circular DNA molecule along with some plasmids

-If a daughter cell does not receive the single circular DNA molecule or at least one copy of a plasmid they die

Question 37

simple diffusion (involving limitations imposed by the nature of the phospholipid bilayer)

Answer 37

The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration (down a concentration gradient)
The random movement is caused by the natural kinetic energy of the molecules or ions

Question 38

factors affecting simple diffusion

Answer 38

-the larger the surface area of the exchange surface, the higher the rate of simple diffusion across that surface

-the greater the difference in concentration on either side of the surface, the higher the concentration gradient across the exchange surface, hence the rate of simple diffusion is also higher
-as diffusion occurs, concentration gradient decreases until an equilibrium is reached

-the thinner the exchange surface, the higher the rate of simple diffusion
-due to molecules having to travel shorter distances

Question 39

explain how surface area, number of channel or carrier proteins and differences in gradients of concentration/water potential affect the rate of movement across cell membranes.

Answer 39

the higher the concentration gradient, the faster the rate of diffusion (up to a point, depending on the number of channel and carrier proteins available)

once all the carrier proteins and channel proteins are in use, the rate of diffusion of a particular molecule can no longer increase
-therefore, the more channel and carrier proteins available, the faster the rate of diffusion

Question 40

facilitated diffusion (involving the roles of carrier proteins and channel proteins)

Answer 40

Facilitated diffusion:
-some substances cannot diffuse through the phospholipid bilayer of cell membranes, such as large polar molecules (e.g. glucose and amino acids) and ions like sodium ions (Na+) and chloride ions (Cl-)
-these substances can only cross the phospholipid bilayer with the help of certain proteins
-two types of proteins that enable facilitated diffusion:
-channel proteins
-carrier proteins
-both are highly specific (they only allow one type of molecule or ion to pass through)

Channel proteins
-water-filled pores that allow charged substances (eg. ions) to diffuse through the cell membrane
-diffusion of these ions does not occur freely, most channel proteins are ‘gated’, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore
-this allows the channel protein to control the exchange of ions

Carrier proteins
-unlike channel proteins which have a fixed shape, carrier proteins can switch between two shapes
-causes the binding site of the carrier protein to be open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape
-direction of movement of molecules diffusing across the membrane depends on their relative concentration on each side of the membrane
-net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient (from an area containing many of that specific molecule to an area containing less of that molecule)

Question 41

osmosis (explained in terms of water potential)

Answer 41

-net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane
-water potential = tendency of water to move out of a solution, and is used to avoid confusion between water concentration and concentration of a solution
-dilute solution = high water potential
-concentrated solution = low water potential
-water potential of pure water (without any solutes) at atmospheric pressure is 0 kPa, therefore any solution that has solutes will have a water potential lower than 0 kPa (it will be a negative value)

in plants:
If a plant cell is placed in pure water or a dilute solution, water will enter the plant cell through its partially permeable cell surface membrane by osmosis, as the pure water or dilute solution has a higher water potential than the plant cell
As water enters the vacuole of the plant cell, the volume of the plant cell increases
The expanding protoplast (living part of the cell inside the cell wall) pushes against the cell wall and pressure builds up inside the cell – the inelastic cell wall prevents the cell from bursting
The pressure created by the cell wall also stops too much water entering and this also helps to prevent the cell from bursting
When a plant cell is fully inflated with water and has become rigid and firm, it is described as fully turgid
This turgidity is important for plants as the effect of all the cells in a plant being firm is to provide support and strength for the plant – making the plant stand upright with its leaves held out to catch sunlight
If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant wilts
If a plant cell is placed in a solution with a lower water potential than the plant cell (such as a concentrated sucrose solution), water will leave the plant cell through its partially permeable cell surface membrane by osmosis
As water leaves the vacuole of the plant cell, the volume of the plant cell decreases
The protoplast gradually shrinks and no longer exerts pressure on the cell wall
As the protoplast continues to shrink, it begins to pull away from the cell wall
This process is known as plasmolysis – the plant cell is plasmolysed

in animals:
Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane
Like plant cells, animal cells can also lose and gain water as a result of osmosis
As animal cells do not have a supporting cell wall (unlike plant cells), the results of this loss or gain of water on the cell are more severe
For example, if an animal cell is placed in a solution with a lower water potential than the cell (such as a concentrated sucrose solution), water will leave the cell through its partially permeable cell surface membrane by osmosis and the cell will shrink and shrivel up
This occurs when the cell is in a hypertonic environment (the solution outside of the cell has a higher solute concentration than the inside of the cell)
Conversely, if an animal cell is placed in pure water or a dilute solution, water will enter the cell through its partially permeable cell surface membrane by osmosis, as the pure water or dilute solution has a higher water potential. The cell will continue to gain water by osmosis until the cell membrane is stretched too far and the cell bursts (cytolysis), as it has no cell wall to withstand the increased pressure created
This occurs when the cell is in a hypotonic environment (the solution outside of the cell has a lower solute concentration than the inside of the cell)
This is why a constant water potential must be maintained inside the bodies of animals
If an animal cell is in an isotonic environment (the solution outside of the cell has the same solute concentration as the inside of the cell), the movement of water molecules into and out of the cell occurs at the same rate (no net movement of water) and there is no change to the cells

Question 42

Compare osmosis in plant and animal cells

Answer 42

In a solution of lower water potential:
-in both plant/animal cells, water leaves cell by osmosis through the partially permeable cell surface membrane
-volume decreases in both plant/animal cells
-animal cells shrink/shrivel up
-in plants cell, the protoplast shrinks and pulls away from cell wall
-plant cell is plasmolysed

In a solution of higher water potential
-in both plant/animal cells, water leaves cell by osmosis through the partially permeable cell surface membrane
-volume increases in both plant/animal cells
-in animal cells, there is no cell wall to withstand increased pressure created : the cell membrane is eventually stretched too far and cell bursts
-in plant cells, cell wall withstands increased pressure created
-pressure increases until cell is rigid and turgid
-cell is fully inflated with water and no more can enter

Question 43

active transport (involving the role of carrier proteins and the importance of the hydrolysis of ATP)

Answer 43

the movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration
Active transport requires carrier proteins (each carrier protein being specific for a particular type of molecule or ion)
Although facilitated diffusion also uses carrier protein, active transport is different as it requires energy
The energy is required to make the carrier protein change shape, allowing it to transfer the molecules or ions across the cell membrane
The energy required is provided by ATP (adenosine triphosphate) produced during respiration. The ATP is hydrolysed to release energy
important in:
Reabsorption of useful molecules and ions into the blood after filtration into the kidney tubules
Absorption of some products of digestion from the digestive tract
Loading sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant
Loading inorganic ions from the soil into root hairs

Question 44

co-transport (illustrated by the absorption of sodium ions and glucose by cells lining the mammalian ileum).

Answer 44

the coupled movement of substances across a cell membrane via a carrier protein
It involves a combination of facilitated diffusion and active transport
A well-known example of a co-transporter protein can be found on the cell surface membrane of the epithelial cells lining the mammalian ileum
This specific co-transport protein is involved in the absorption of glucose
Sodium ions and glucose molecules are transported into the epithelial cells via facilitated diffusion
The facilitated diffusion can only continue if a concentration gradient is maintained
The active transport of sodium ions out of the cell into the blood helps to maintain this gradient
The glucose molecules exit the epithelial cell and enter the blood via facilitated diffusion

Question 45

How are epithelial cells adapted to their function?

Answer 45

Epithelial cells are cells that line the surfaces of organs.
Some epithelial cells have cilia which are hair-like structures on their surface.

-Ciliated epithelial cells’ main role is to move substances in one direction
-structures move together to waft substances
-E.g. in the airways, ciliated epithelial cells help move mucus (that traps unwanted inhaled substances) up towards the throat.
-This is then swallowed and doesn’t reach the lungs thus, protecting the lungs

Question 46

Explain the difference between intracellular and extracellular

Answer 46

Intracellular = inside cell
Extracellular = outside cell

Question 47

Explain what intrinsic and extrinsic proteins are

Answer 47

Intrinsic proteins:
-span both bilayers of the plasma membrane
-act as channels or carrier proteins to transport water-soluble molecules.

Extrinsic proteins:
-found on the surface of the plasma membrane
-usually function as enzymes and catalyse chemical reactions inside the cell.

Question 48

what defence mechanisms does our body have against pathogens

Answer 48

Preventing the entry of pathogens by a variety of physical and chemical defences, such as the skin, mucous membranes, tears (containing the enzyme lysozyme, which destroys bacteria) and saliva

Inflammation (swelling and heating) of the region invaded by the pathogen, a process known as a non-specific inflammatory response

Recognising ‘foreign’ cells and targeting any pathogenic cells, a process known as a specific immune response

Question 49

example of the importance of antigens in defending against pathogens:

Answer 49

-white blood cells known as phagocytes have surface proteins that act as receptors and bind to the proteins (antigens) on the surface of pathogens

-enables pathogens to be engulfed and digested

-antigens that were found on the pathogen can then be presented on the surface of the phagocyte (now an antigen-presenting cell)

-this is then used to recruit other cells of the immune system, leading to a specific immune response

Question 50

phagocytosis

Answer 50

-phagocytes are white blood cells that are produced continuously in the bone marrow
-stored in the bone marrow before being distributed around the body in the blood
-responsible for removing dead cells and invasive microorganisms
-carry out what is known as a non-specific immune response
There are two main types of phagocyte, each with a specific mode of action
-two types are: Neutrophils and Macrophages
-as both are phagocytes, both carry out phagocytosis (the process of recognising and engulfing a pathogen) but the process is slightly different for each type of phagocyte

Neutrophils
-travel throughout the body and often leave the blood by squeezing through capillary walls to ‘patrol’ the body tissues
-during an infection, they are released in large numbers from their stores
-however, they are short-lived cells
-chemicals released by pathogens, as well as chemicals released by the body cells under attack (eg. histamine), attract neutrophils to the site where the pathogens are located (this response to chemical stimuli is known as chemotaxis)
-neutrophils move towards pathogens (which may be covered in antibodies)
-antibodies are another trigger to stimulate neutrophils to attack the pathogens (neutrophils have receptor proteins on their surfaces that recognise antibody molecules and attach to them)
-once attached to a pathogen, the cell surface membrane of a neutrophil extends out and around the pathogen, engulfing it and trapping the pathogen within a phagocytic vacuole
-this part of the process is known as endocytosis

Lysosomes
-contain digestive enzymes called lysozymes, whichdigest unwanted material present in cells
-phagocytic vacuole formed around a pathogen once it has been engulfed by a neutrophil is called a phagosome
-a lysosome fuses with the membrane of the phagosome (to form a phagolysosome) and releases lysozymes (digestive enzymes) to digest the pathogen
-these digestive enzymes destroy the pathogen
-after killing and digesting the pathogens, the neutrophils die
-pus is a sign of dead neutrophils

Macrophages
-larger than neutrophils and are long-lived cells
-instead of remaining in the blood, they move into organs including the lungs, liver, spleen, kidney and lymph nodes
-after being produced in the bone marrow, macrophages travel in the blood as monocytes, which then develop into macrophages once they leave the blood to settle in the various organs listed above
-important role in initiating an immune response
-although they still carry out phagocytosis in a similar way to neutrophils, they do not destroy pathogens completely
-they cut the pathogens up so that they can display the antigens of the pathogens on their surface (through a structure called the major histocompatibility complex)
-these displayed antigens (the cell is now called an antigen-presenting cell) can then be recognised by lymphocytes (another type of white blood cell)

Question 51

the T-lymphocyte response

Answer 51

Lymphocytes are another type of white blood cell
They play an important part in the specific immune response
They are smaller than phagocytes
They have a large nucleus that fills most of the cell
They are produced in the bone marrow before birth
There are two types of lymphocytes (with different modes of action). The two types of lymphocytes are:
T-lymphocytes (T cells)
B-lymphocytes (B cells)
T-lymphocytes and the cellular immune response
Immature T-lymphocytes leave the bone marrow to mature in the thymus
Mature T-lymphocytes have specific cell surface receptors called T cell receptors
These receptors have a similar structure to antibodies and are each specific to one antigen

The maturation of T-lymphocytes – some become helper T cells and others become killer T cells

T-lymphocytes are activated when they encounter (and bind to) their specific antigen that is being presented by one of the host’s cells (host cells being the human’s own cells)
This antigen-presenting host cell might be a macrophage or a body cell that has been invaded by a pathogen and is displaying the antigen on its cell surface membrane
These activated T-lymphocytes (those that have receptors specific to the antigen) divide by mitosis to increase in number (similar to the clonal selection and clonal expansion of B-lymphocytes)
These T-lymphocytes differentiate into two main types of T cell:
helper T cells
cytotoxic T cells(also known as killer T cells)

Question 52

role of helper T-cells

Answer 52

The Role of Helper T cells
Activated T-lymphocytes (those that have receptors specific to an antigen) divide by mitosis to increase in number (similar to the clonal selection and clonal expansion of B-lymphocytes)
These T-lymphocytes differentiate into two main types of T cell:
helper T cells
cytotoxic (killer) T cells
Helper T cells assist other white blood cells in the immune response
They release cytokines (hormone-like signals) which stimulate:
The maturation of B-lymphocytes into antibody-secreting plasma cells
The production of memory B cells
The activation of cytotoxic T cells, which destroy virus infected cells and tumour cells
An increased rate of phagocytosis

Question 53

b-lymphocyte response

Answer 53

B-lymphocytes and the humoral immune response
B-lymphocytes (B cells) remain in the bone marrow until they are mature and then spread through the body, concentrating in lymph nodes and the spleen
Millions of types of B-lymphocyte cells are produced within us because as they mature the genes coding for antibodies are changed to code for different antibodies
Once mature, each type of B-lymphocyte cell can make one type of antibody molecule
At this stage, the antibody molecules do not leave the B-lymphocyte cell but remain in the cell surface membrane
Part of each antibody molecule forms a glycoprotein receptor that can combine specifically with one type of antigen
If that antigen enters the body, B-lymphocyte cells with the correct cell surface receptors will be able to recognise it and bind to it (clonal selection)
These specific B-lymphocytes divide repeatedly by mitosis (clonal expansion) and differentiate into two main types of cell:
Plasma cells
Memory cells
These two cell types each have a specific function

Primary immune response
When an antigen enters the body for the first time, the small numbers of B-lymphocytes with receptors complementary to that antigen are stimulated to divide by mitosis
This is known as clonal selection
As these clones divide repeatedly by mitosis (the clonal expansion stage) the result is large numbers of identical B-lymphocytes being produced over a few weeks
Some of these B-lymphocytes become plasma cells that secrete lots of antibody molecules (specific to the antigen) into the blood, lymph or linings of the lungs and the gut
These plasma cells are short-lived (their numbers drop off after several weeks) but the antibodies they have secreted stay in the blood for a longer time
The other B-lymphocytes become memory cells that remain circulating in the blood for a long time
This response to a newly encountered pathogen is relatively slow

Question 54

antibodies

Answer 54

Structure:
-quaternary globular glycoproteins called immunoglobulins
-represented as Y-shaped, with two ‘heavy’ (long) polypeptide chains bonded by disulfide bonds to two ‘light’ (short) polypeptide chains
-each polypeptide chain has a constant region and variable region
-constant region determines the mechanism used to destroy antigens
-amino acid sequence in the variable regions of the antibodies (the tips of the “Y”) are different for each antibody
-variable region = where the antibody attaches to antigen to form antigen-antibody complex
-at the end of the variable region is a site called the antigen-binding site
-antigen-binding sites vary greatly giving the antibody its specificity for binding to antigens
-if pathogen or virus presents multiple antigens, different antibodies need to be produced
-the ‘hinge’ region (where the disulfide bonds join the heavy chains) gives flexibility to the antibody molecule
-allows the antigen-binding site to be placed at different angles when binding to antigens

Question 55

antigen-antibody complex

Answer 55

An antigen and its complementary antibody have complementary molecular shapes
This means that their molecular structures fit into each other
When an antibody collides (randomly) with a foreign cell that possesses non-self antigens with a complementary shape, it binds with one of the antigens
When this occurs, the two molecules combine to form an antigen-antibody complex
antibodies have at least two antigen-binding sites
This means they can bind to more than one bacterium or virus at the same time
This cause groups of the same pathogens to become clumped together
This process is known as agglutination
The binding of antibodies to the antigens either neutralises the pathogen or acts like a marker to attract phagocytes to engulf and destroy the pathogens
Due to agglutination, phagocytes can often phagocytose many pathogens at the same time, as they are all clumped together

Question 56

plasma and memory cells

Answer 56

During an immune response, B-lymphocytes form two types of cell: plasma cells and memory cells
Memory cells form the basis of immunological memory – the cells can last for many years and often a lifetime
There are two types of immune response:
Primary immune response (responding to a newly encountered antigen)
Secondary immune response (responding to a previously encountered antigen)
Primary immune response
After clonal selection and expansion, the B-lymphocytes that have become plasma cells secrete lots of antibody molecules (specific to the antigen) into the blood, lymph or linings of the lungs and the gut
These plasma cells are short-lived (their numbers drop off after several weeks) but the antibodies they have secreted stay in the blood for a longer time
The other B-lymphocytes become memory cells that remain circulating in the blood for a long time
This response to a newly encountered pathogen is relatively slow
Secondary immune response
If the same antigen is found in the body a second time, the memory cells recognise the antigen, divide very quickly and differentiate into plasma cells (to produce antibodies) and more memory cells
This response is very quick, meaning that the infection can be destroyed and removed before the pathogen population increases too much and symptoms of the disease develop
This response to a previously encountered pathogen is, relative to the primary immune response, extremely fast

Question 57

Explain what antigens are in detail

Answer 57

-macromolecules which are markers that allow cell-to-cell recognition
-found on cell surface membranes, bacterial cell walls, or the surfaces of viruses
-some glycolipids and glycoproteins on the outer surface of cell surface membranes act as antigens
-can be either self antigens or non-self antigens
-self antigens do not stimulate an immune response
-non-self antigens stimulate an immune response

Question 58

Ethical issues associated with the use of vaccines and monoclonal antibodies

Question 59

Spec coverage begins here:

Question 60

Each type of cell has specific molecules on its surface that identify it, such as proteins. What do these enable the immune system to identify?

Answer 60

• pathogens
• cells from other organisms of the same species
• abnormal body cells
• toxins

Question 61

Effect of antigen variability on disease and disease prevention

Answer 61

-pathogen’s DNA can mutate frequently
-if mutation occurs in the gene which codes for the antigen, then the shape of antigen will change
-memory cells in blood will have a memory of old antigen shape
-no longer complementary
-cannot recognise pathogen, hence any previous immunity to this pathogen is no longer effective
-host gets infected and suffers from the disease again

Question 62

Phagocytosis of pathogens

Answer 62

-Phagocyte = macrophage (type of WBC) found in blood and tissues that carries out phagocytosis
-non-specific response
-any non-self cell that is detected will trigger the same response every time

Stages:
-pathogens/abnormal cells release chemicals/debris
-phagocytes attracted to chemicals so move towards pathogen
-surface of phagocytes has receptor binding points that can be used to attach to chemicals/antigens on the pathogen
-phagocyte changes shape to engulf pathogen
-pathogen is contained with a phagosome vesicle
-lysosome from the phagocyte fuses with membrane of phagosome
-forms phagolysosome
-releases lysozymes into phagosome to digest pathogen
-pathogen is hydrolysed

Question 63

How do lymphocytes recognise cells?

Answer 63

-made when you are foetus
-foetuses sre unlikely to be exposed to any other cells than self cells
-lymphocytes complementary to the antigens on self-cells will die/production is suppressed
-prevents lymphocytes from attacking own cells
-remaining lymphocytes are complementary to pathogenic and non-self cells
-same process occurs after birth in bone marrow
-new lymphocytes made in bone marrow are complementary in shape to antigens on self-cells will be destroyed
-if this process goes wrong —> symptoms of autoimmune disease

all lymphocytes are made in the bone marrow but t cells mature in the thymus

Question 64

What do T-lymphocytes do?

Answer 64

T Cells (T lymphocytes)can be…T helper cells:
-Stimulate B cell maturation-Promote phagocytosis
-secrete cytokines, which activate or control the activities of other lymphocytes.
-Most helper T cells die out once a pathogen has been cleared from the body, but a few remain as memory cells.
-These memory cells are ready to produce large numbers of antigen-specific helper T cells like themselves if they are exposed to the same antigen in the future.
T killer cells:
-Identify and kill infected host cells-Especially important during viral infections

Question 65

Key points

Answer 65

Key point 1:
In the non-specific defence system, macrophages engulf and digest pathogens in phagocytosis. They process the antigens from the surface of the pathogen to form antigen-presenting cells (ADCs).

Key point 2:
The receptors on some of the T helper cells fit the antigens. These T helper cells become activated and produce interleukins, which stimulate more T cells to divide rapidly by mitosis. They form clones of identical activated T helper cells that all carry the right antigen to bind to a particular pathogen.

Key point 3:
The cloned T cells may:
-develop into T memory cells, which give a rapid response if this pathogen invades the body again
produce interleukins that stimulate phagocytosis
produce interleukins that stimulate B cells to divide
-stimulate the development of a clone of T killer cells that are specific for the presented antigen and then destroy infected cells.

Question 66

How do cytotoxic T cells destroy cells infected with viruses

Answer 66

-Once activated, a cytotoxic T cell divides rapidly
-releases a protein called perforin that forms pores in the membrane of the infected cell
-any substance can enter and exit
-causes the cell to burst, destroying both the cell and the viruses inside it.
-After an infection has been brought under control, most cytotoxic T cells die off.
-However, a few remain as memory cells.

Question 67

Role of B cells in immunity (textbook)

Answer 67

-surface antigens of an invading pathogen are taken up by a B-cell
-B cell processes the antigens and presents them on its surface
-helper T cells attach to the processed antigens on the B cell thereby activating the B cell
-B cell is now activated to divide by mitosis to give a clone of plasma cells
-the cloned plasma cells produce and secrete the specific antibody that fits the antigen on the pathogen’s surface exactly
-the antibody attaches to antigens on the pathogen and destroys them
-some B cells develop into memory cells that can respond to future infections by the same pathogen by rapid division and developing into plasma cells that produce antibodies (known as secondary immune response)

Question 68

Differences between humoral and cell mediated immunity

Answer 68

Humoral immunity:
-B lymphocytes
-produced and mature in bone marrow
-involves production of antibodies
-pathogens identified via antigens floating in blood
-pathogens killed by antibodies
-once stimulated, cells divide into either plasma cells or memory cells

Cell mediated immunity:
-T lymphocytes
-produced in bone marrow, but mature in thymus gland
-does not involve production of antibodies
-pathogens are identified via antigens on the surface of infected cells
-pathogens killed by specialised killer T cells
-once stimulated, cells divide into different types of specialist T cells

Question 69

Humoral response

Question 70

Define antibody and describe the structure
Define antigen
Define APC

Answer 70

Antibody:
-quaternary structure globular protein (4 polypeptide chains)
-has a variable region that binds to the antigen (known as antigen-binding site) and the constant region
-has a heavy chain and light chain

Antigen:
protein on the surface of a pathogen that stimulates an immune response

Antigen-Presenting Cell:
-any cell that presents a non-self antigen on their surface
E.g. infected body cells, macrophage that has engulfed and destroyed a pathogen, cells of a transplanted organs, cancer cells

Question 71

The formation of an antigen-antibody complex, leading to the destruction of the antigen, limited to agglutination and phagocytosis of bacterial cells.

Answer 71

-antibodies are flexible so can bind to multiple antigens to clump them together known as agglutination
-antigen-antibody complex can be formed
-makes it easier for phagocyte to locate and destroy pathogens

Question 72

The roles of plasma cells and of memory cells in producing primary and secondary immune responses.

Question 73

differences between active and passive immunity.

Answer 73

Passive:
-antibodies are introduced to the body
-pathogen doesn’t enter body so plasma cells and memory cells are not made
-no long-term immunity
-e.g. antibodies passed to foetus through placenta or breastmilk to a baby

Active:
-immunity created by own immune system following exposure to pathogen or its antigen
-natural: following infection and creation of body’s own antibodies and memory cells
-artificial: following introduction of weakened version of pathogen or antigens via a vaccine

Question 74

The use of vaccines to provide protection for individuals and populations against disease
Concept of herd immunity

Answer 74

-small amounts of weakened/dead pathogen, or antigens are introduced to body
-exposure to antigens activates B cell to go through clonal expansion and differentiation (clonal selection)
-B cells undergo mitosis to make large number of cells that differentiate into plasma cells or memory cells
-plasma cells make antibodies
-B memory cells can collide with antigen and divide rapidly into plasma cells when reinfected, so large number of antibodies made quickly

Herd immunity:
-if enough of the population are vaccinated, the pathogen cannot spread easily amongst the population
-protects those who are not vaccinated

Question 75

Structure of the human immunodeficiency virus (HIV)

Answer 75

Core = RNA and reverse transcriptase enzyme, which are needed for viral replication
Capsid = outer protein wall
Envelope = extra outer layer, made out of membrane taken from the host cell’s membrane
Attachment proteins = on the exterior of the envelope to enable the virus to attach to the host’s helper T cell

Question 76

HIV replication in T helper cells

Answer 76

-HIV is transported around in the blood until it attaches to a CD4 protein on the helper T cells

-HIV protein capsule fuses with helper T cell membrane, enabling the RNA and enzymes to enter

-reverse transcriptase copies viral RNA into a DNA copy and moves to the helper T cell nucleus, hence why it is called a retrovirus

-here, mRNA is transcribed and the helper T cell starts to make viral proteins to make new viral particles

-new HIV proteins and RNA move to the surface of the cell and assemble

-they bud, forming new HIV particles as they burst out of the host cell, killing it in the process

Question 77

How HIV causes symptoms of AIDS

Answer 77

AIDS is when the replicating viruses in the helper T cells completely weaken the immune system

Question 78

The use of monoclonal antibodies in:
• targeting medication to specific cell types by attaching a therapeutic drug to an antibody
• medical diagnosis

Question 79

Why are antibiotics ineffective against viruses

Answer 79

They do not have a cell wall that can be attacked

Question 80

Cell-mediated response (final)

Answer 80

-“cell-mediated” are specific as T cells only respond to antigens which are presented on cells (APC),
-no response to antigens detached from cells and within body fluids (e.g. blood)

Stages:
-once phagocyte has engulfed/destroyed pathogen, the antigens are positioned on the cell surface (hence it is an APC)
-helper T cells have receptors on their surface, which can bind to complementary antigens on APC
-when attached, activates helper T cells to divide by mitosis, making large numbers of clones

-cloned helper T cells differentiate into different cells:
-can become memory cells to enable a rapid response to future infections by the same pathogen
-can stimulate phagocytes (i.e. macrophages) to engulf pathogens by phagocytosis
-can remain as helper T cells and stimulate B cells to divide and secrete their antibody
-can become and activate cytotoxic T cells (killer T cells)

Question 81

What is an ELISA test

Answer 81

-stands for enzyme-linked immunosorbent assay
-can be used to see if a patient has any antibodies to a certain antigen (or any antigens to a certain antibody)
-e.g. they can be used to test for infections by pathogens or for allergies

In an ELISA test:
-An enzyme is attached to antibodies
-When this enzyme reacts with a certain substrate, a coloured product is formed, causing the solution in the reaction vessel to change colour
-If a colour change occurs, this shows that the antigen or antibody of interest is present in the sample being tested (e.g. blood plasma)

There are different types of ELISA test:
-Direct ELISA tests use a single antibody that is complementary to the antigen being tested for
-Indirect ELISA tests use two different antibodies (known as primary and secondary antibodies)

Question 82

Example of an indirect ELISA test: testing for antibodies (e.g. for HIV)

Answer 82

-first, HIV antigens bind to the bottom of the reaction well

-blood plasma sample is then taken from the patient and added to well

-any HIV-specific (primary) antibodies present in the blood plasma now bind to the HIV antigens

-any other antibodies that are present in the blood plasma are washed out

-secondary antibody with an enzyme attached to it is added to the reaction well

-these secondary antibodies bind to the primary antibodies

-the well is washed out again to remove any unbound secondary antibodies

-helps avoid false-positive test results

-solution with substrate for enzyme attached to the secondary antibody is added

-if any secondary antibodies present, a coloured product is formed, causing the solution in the well to change colour

-indicates that the patient has HIV-specific antibodies in their blood (and therefore they are infected with HIV)

Question 83

Example of an indirect ELISA test: testing for antigens

Answer 83

-prostate cancer is a cancer of the prostate gland
-the blood plasma of a patient can be tested for the presence of prostate-specific antigens (PSAs)
-high PSA concentration of blood plasma suggests that patient has prostate cancer
-so further diagnostic tests will be carried out

-antibodies to PSA are bound to the bottom of the reaction vessel (instead of antigens, as in the HIV example)

Question 84

Humoral response (final)

Answer 84

1. Helper T cell/TH cell binds to the antigen (on the antigen-presenting cell/phagocyte);
2. This helper T/TH cell stimulates a specific B cell;
3. B cell clones
   OR
   B cell divides by mitosis;
4. (Forms) plasma cells that release antibodies;

Question 85

Function of DNA ligase

Answer 85

Joins DNA fragments together by facilitating the formation of a phosphodiester bond between two DNA monomers at a time

Question 86

Function of integrase

Answer 86

Catalyses integration of viral DNA into host genome