3.2 Cells 🦠

Question 1

What do all cells have

Answer 1

Cell-surface membrane

Question 2

Define a Eukaryotic cell

Answer 2

A cell with DNA contained in a nucleus, containing membrane-bound specialised organelles

Question 3

How do Eukaryotic cells replicate

Answer 3

• Mitosis
• Meiosis

Question 4

What do all Eukaryotic cells have

Answer 4

• Cell-surface membrane
• Internal membranes (for membrane-bound organelles)

Question 5

Outline the structure and function of the cell surface membrane

Answer 5

• Structure: ‘Fluid mosaic’ phospholipid bilayer with extrinsic and intrinsic proteins embedded

• Functions:
1. Isolates cytoplasm from the extracellular
environment
2. Selectively permeable to regulate transport of
substances
3. Involved in cell signalling/cell recognition

Question 6

Outline the structure and function of the nucleus

Answer 6

• Structure: Membrane bound structure that contains chromosomes, consisting of protein-bound linear DNA, and one or more nucleoli surrounded by a nuclear envelope

• Functions:
1. Contains DNA
2. Controls cellular processes and activity

Question 7

Outline the structure and function of the nucleolus

Answer 7

• Structure: A round body located inside the nucleus

• Function: Makes ribosomal subunits from proteins and ribosomal RNA (rRNA)

Question 8

Outline the structure and function of the nuclear envelope

Answer 8

• Structure: Phospholipid bilayer that’s perforated with ‘nuclear pores’

• Functions:
1. Helps maintain the shape of the nucleus
2. Assists in regulating the flow of molecules in
and out through the nuclear pores

Question 9

Outline the structure and function of the mitochondria

Answer 9

• Structure: surrounded by a double membrane; the folded inner membrane forms cristae, the site of the electron transport chain. Within this there’s a fluid matrix containing mitochondrial DNA, respiratory enzymes, lipids and proteins

• Function: the site of aerobic respiration to produce ATP

Question 10

Outline the structure and function of the Golgi apparatus

Answer 10

• Structure: planar stack of membrane-bound, flattened cisternae and associated vesicles aligning with the rough endoplasmic reticulum

• Functions:
1. Modifies and packages proteins for export
2. Synthesises glycoproteins

Question 11

Outline the structure and function of the lysosome

Answer 11

• Structure: A type of Golgi vesicle that contains lysozymes (digestive hydrolase enzymes). A glycoprotein coat protects the cell interior
It’s a sphere surrounded by a single membrane embedded H+ pump that maintains acidic conditions.

• Functions
1. Digests contents of a phagosome
2. Exocytosis of digestive enzymes

Question 12

Outline the structure and function of the ribosome

Answer 12

• Structure: Formed of protein and RNA and made up of a large and small subunit, they can be free in the cytoplasm or attached to the endoplasmic reticulum

• Function: Site of protein synthesis via translation, the large subunit joins amino acids and the small subunit contains the mRNA binding site

Question 13

Outline the structure and function of the endoplasmic reticulum

Answer 13

• Structure: Network of tubules and flattened sacs (membranes called cisternae) extends from the cell membrane and connects to the nuclear envelope

• Functions:
1. Smooth ER: Lipid synthesis
2. Rough ER: Many ribosomes attached for protein synthesis and transport

Question 14

Identify the organelles found in animal cells

Answer 14

• Cell-surface membrane
• Nucleus
• Mitochondria
• Golgi apparatus
• Lysosomes
• Ribosomes
• Rough and smooth endoplasmic reticulum

Question 15

Plant cells contain the same organelles as animal cells, identify the extra organelles they have

Answer 15

• Cell wall
• Vacuole
• Chloroplasts

Question 16

What are the four types of Eukaryotic cells

Answer 16

• Animal
• Plant
• Algal
• Fungal

Question 17

Compare and contrast plant and algal cells

Answer 17

• Same organelles and both photosynthesise

• Algae can be multicellular or unicellular, whereas plants can only be multicellular

Question 18

Compare and contrast plant and fungal cells

Answer 18

• Same organelles except fungi don’t have chloroplasts

• Fungal cell wall is made of Chitin while plant cell walls are made of cellulose

Question 19

Outline the structure and function of chloroplasts in plant and algal cells

Answer 19

• Structure:
• Vesicular plastic surrounded by a double membrane envelope, each a phospholipid bilayer
• Filled with a fluid-filled matrix called the stroma
• Contains a series of flattened, fluid-filled sacs called thylakoids (containing photosystems with chlorophyll) that stack to form grana that are connected by membranous channels called stroma lamellae
• Contains small (70s) ribosomes, a loop of DNA and starch grains

• Function: Site of photosynthesis to convert solar energy to chemical energy

Question 20

Outline the structure and function of a cell wall in plant, algal and fungal cells

Answer 20

• Structure: Made of cellulose microfibrils with plasmodesmata channels that allow molecules to pass between cells. Middle lamella act as a boundary between adjacent cell walls

• Functions:
1. Mechanical strength and support
2. Physical barrier against pathogens
3. Part of a apoplast pathway to enable easy diffusion of water (plants)

Question 21

Outline the structure and function of the cell vacuole

Answer 21

• Structure: Surrounded by the tonoplast and contains cell sap that contains mineral ions, water, enzymes and subtle pigments

• Functions:
1. Controls turgor pressure
2. Absorbs and hydrolyses potentially harmful substances to detoxify the cytoplasm

Question 22

What’s the order of organisation

Answer 22

Cell > Tissue > Organ > Organ System > Organism

Question 23

Outline what happens to eukaryotic cells in complex, multicellular organisms

Answer 23

• Cells become specialised for specific functions in multicellular organisms and are organised into tissues, tissues into organs and organs into organ systems.
• Once they’re differentiated to carry out a specific function they can no longer become another type of cell
• They become specialised through genes in the DNA being switched on and off leading the cell to make different proteins

Question 24

Define a Prokaryotic cell

Answer 24

DNA’s free in the cytoplasm and they contain no membrane bound organelles

Question 25

Identify the differences between Eukaryotic and Prokaryotic cells

Answer 25

• Eukaryotes are larger and often multicellular while prokaryotes are smaller and always unicellular

• Eukaryotes have organelles including a nucleus while prokaryotes have a cytoplasm that lacks membrane-bound organelles (no nucleus)

• Eukaryotes have linear DNA associated with histones while prokaryotes have circular DNA (plasmids) not associated with proteins

• Eukaryotes have larger 80s ribosomes while prokaryotes have smaller 70s ribosomes

• Eukaryotes divide my mitosis and meiosis (sexual or asexual reproduction) while prokaryotes divide by binary fission (always asexual)

• Eukaryotes have cellulose (plants) and chitin (fungi) cell walls while prokaryotes have murein, a glycoprotein, cell walls

• Prokaryotes have capsules, sometimes plasmids/ cytoskeleton/flagella while eukaryotes have no capsule, plasmids or flagella and always have a cytoskeleton

Question 26

Identify the similarities between Eukaryotic and Prokaryotic cells

Answer 26

• Contain a cell membrane, cytoplasm and ribosomes (don’t count as an organelle as they aren’t membrane bound)

Question 27

Identify the structure and function of plasmids in prokaryotes

Answer 27

• Structure: Small ring of DNA

• Function: carries non-essential genes and can be exchanged between bacterial cells via conjugation

Question 28

Outline the structure of and function of flagella

Answer 28

• Structure: microscopic hair-like organelle

• Function: Rotating tails that propels the organism

Question 29

Outline the structure and function of a (slime) capsule in prokaryotes

Answer 29

• Structure: Polysaccharide layer

• Function: Prevents desiccation, acts as a food reserve, sticks cells together and provides mechanical protection against phagocytosis and external chemicals

Question 30

Outline viruses

Answer 30

• They’re acellular because they have no cell-surface membrane or cytoplasm

• They’re non-living beacuse they have no metabolism and cannot independently respire/move/replicate/excrete without being inside a host cell

Question 31

Outline the structure of a virus

Answer 31

• Linear genetic material (DNA/RNA) and viral enzymes (e.g. reverse transcriptase) surrounded by a capsid and no cytoplasm

• The capsid’s a protein coat made of capsomeres that protects nucleic acid from degradation by restriction endonucleases.

• Attachment proteins enable the viral particle to bind and enter host/inject their genetic material

• Enveloped viruses are further surrounded by matrix protein and an envelope derived from the cell membrane of the host cell with attachment proteins on the surface. These enable viral particles to bind to complimentary sites on host cells (entry via endosymbiosis)

Question 32

Describe the process of viral replication

Answer 32

• Virus uses attachment proteins on its surface to bind to complementary receptor proteins on the surface of a host cell and injects DNA or RNA into the host cell

• The host-cell uses its nucleic acid and ribosomes to produce new viral particles which are released by:
1. The host cell bursts open, releasing all the new viral particles
2. Viral particles leave individually through the host cell membrane via ‘budding’, taking a section of the membrane with them (forming an enveloped virus)

Question 33

Outline the difference between enveloped (EV) and non-enveloped viruses (NEV)

Answer 33

• NEV are more virulent and cause host cell lysis more often

• EV have a membrane surrounding the capsid, and can use the host-cells membrane to assemble their own membrane, avoiding lysis and escaping the hosts immune system

Question 34

Define endosymbiosis

Answer 34

A symbiotic relationshop where one organism lives inside the other

Question 35

Define symbiotic relationships

Answer 35

A close and long-term biological interaction between two different species, called symbionts that can be mutualistic, commensalistic or parasitic

Question 36

Identify the methods of studying cells

Answer 36

• Microscopes
• Cell fractionisation

Question 37

Outline magnification

Answer 37

• How many times bigger the image of the specimen observed is compared to the actual size of the specimen

• Equation: Size of image
Magnification = ———————-
Actual size

Question 38

Define resolution

Answer 38

The ability to distinguish between two points

Question 39

Define artefact

Answer 39

• Something present in a sample that’s been artificially introduced as a result of processing the sample.
• They’re more likely to be produced from an electron microscope as there’s more steps involved

Question 40

Identify the types of microscope

Answer 40

• Light Microscope
• Transmission Electron Microscope
• Scanning Electron Microscope

Question 41

Explain how an optical microscope works

Answer 41

• Lenses focus rays of light and magnify the view of a thin slice of specimen

• Different structures absorb different amounts and wavelengths of light

• Reflected light is transmitted from the observer via the objective lens and eyepiece

Question 42

Describe how to prepare a sample (tissue) for an optical microscope

Answer 42

• Obtain a thin section of tissue (e.g. using
ultratome or by maceration)
• Place the tissue in a drop of water
• Stain the tissue on a slide to make structures
visible
• Add a code slip using a mounted needle at 45° to
avoid trapping air bubbles

• Place micrometer on stage to calibrate eyepiece
graticule
• Line up the scales on the graticule and
micrometer and count how many graticule
divisions are in 100um on the micrometer
• The length of one eyepiece division = 100um
divided by the number of divisions
• Use calibrated values to calculate actual length
of structures

Question 43

Evaluate optical microscopes

Answer 43

+ Colour image
+ Can show living structures
+ Affordable apparatus

-2D image
-Lower resolution than electron microscopes
because light has a longer wavelength than
electrons so you can’t see ultrastructure

Question 44

Explain how a transmission electron microscope works

Answer 44

• Passes a high beam of electrons through a thin slice of specimen
• More dense structures appear darker as they absorb more electrons
• Focus images onto fluorescent screen or photographic plate using magnetic lenses

Question 45

Evaluate transmission electron microscopes

Answer 45

+ Electrons have a shorter wavelength than light
so it has a higher resolution so ultrastructure is
visible
+ High magnification (X500,000)

-2D image
-Requires a vacuum, so can’t show living
structures
-Extensive preparation may cause artefacts
-No colour image

Question 46

Explain how scanning electron microscopes work

Answer 46

• Focuses a beam of electrons into a specimens surface using electromagnetic lenses

• Reflected electrons hit a collecting device and are amplified to produce an image on a photographic plate

Question 47

Evaluate scanning electron microscopes

Answer 47

+ Electrons have a shorter wavelength than light
so it has a higher resolution so ultrastructure is
visible
+ 3D image

-Requires a vacuum, so can’t show living
structures
-No colour image
-Only shows outer surface

Question 48

Outline Cell Fractionation and Ultracentrifugation

Answer 48

• Cell fractionation is the process of breaking down cells to isolate different organelles that’s used to study the cell’s structure and function

Process:
• Tissues are cut up and placed in a cold, isotonic, buffer solution

• The cell and solution and placed in a homogeniser where tissues are minced and homogenised to break open cells and release organelles, producing a homogenate

• Homogenate is filtered to remove whole cells and cell debris

• Perform differentiation centrifugation in a centrifuge. The homogenate spins in the centrifuge and fragments are separated according to density by centrifugal force

• The most dense organelles form a pellet at the bottom in the order of sedimentation (density): (nucleus, chloroplast, mitochondria, lysosomes, ER, ribosomes)

• Filter off the supernatant and spin again at a higher speed

Question 49

Explain the properties of the solution cells are placed in during Cell Fractionisation

Answer 49

• Cold to denature (protease) enzymes so organelles aren’t broken down

• Isotonic so organelles aren’t damaged from gaining or losing water

• Buffer solution maintains pH to preserve organelles

Question 50

Outline the cell cycle

Answer 50

• Within multicellular organisms, not all cells retain the ability to divide; Eukaryotic cells that do retain the ability to divide show a cell cycle

• DNA replication occurs during 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 identical copies of DNA produced by the parent cell during DNA replication

• Finally, cytokinesis (division of the cytoplasm) usually occurs, producing two new cells

Question 51

Describe interphase during the cell cycle

Answer 51

Split into three stages;
• Gap phase 1: cell growth and synthesis of protein and organelles
• Synthesis phase: DNA synthesis
• Gap phase 2: More growth and synthesis of protein and organelles as well as preparation for cell division

Question 52

Outline mitosis during the cell cycle

Answer 52

• Mitosis is the part of the cell cycle in which a Eukaryotic cell divides to produce two daughter cells, each with identical copies of DNA produced by the parent cell during DNA replication, it’s split into four stages: Prophase, Metaphase, Anaphase and Telophase

• Mitosis has multiple roles:
• The growth of multicellular organisms
• Replacement of cells and repair of tissues
• Asexual reproduction

Question 53

Describe prophase in mitosis

Answer 53

• Chromosomes condense and are visible when stained, consisting of two sister chromatids (containing one DNA molecule each) joined at the centromere

• The two centrosomes (replicated in the G2 phase just before prophase) move towards opposite poles of the nucleus and spindle fibres unravel from them

• The nuclear envelope (nuclear membrane) breaks down into small vesicles

Question 54

Describe metaphase in mitosis

Answer 54

• Chromosomes line up at the equator of the metaphase plate so they’re equidistant to the two centrosome poles

• Spindle fibres (protein microtubules) reach the chromosomes and attach to the centromeres

Question 55

Describe anaphase of mitosis

Answer 55

• The sister chromatids seperate at the centromere

• Spindle fibres begin to shorten and pull the separated sister chromatids (now called chromosomes) to opposite poles of the cell

Question 56

Describe telophase of mitosis

Answer 56

• Chromosomes arrive at opposite poles and begin to decondense

• Nuclear envelopes begin to reform around each set of chromosomes and the spindle fibres break down

• The cell is splitting in two

Question 57

Outline cytokinesis

Answer 57

• The separation of the parent cell into two genetically identical daughter cells once a new nucleus has completely reformed at each like if the parent cell after telophase

• In animals, a ‘cleavage furrow’ forms and separates the daughter cells but in plants a ‘cell plate’ forms at the metaphase plate. Once it reaches the walls of the parent cell, new cell walls are produced, separating the new daughter cells

Question 58

Define and evaluate mitotic index and provide the equation

Answer 58

• The number of cells undergoing mitosis
• Mitotic index is useful for:
1. Determining the speed of growth
2. Determining when a tissues becoming
cancerous
3. Assessing the efficacy of cancer treatment

                        Number of cells with visible  
                                chromosomes  Mitosis index = ——————————————
                           Number of cells (total)

Question 59

How do you calculate the time taken at each stage of mitosis

Answer 59

(Cells in that phase ➗ total cells in mitosis) ✖️ total time taken for 1 cell cycle

Question 60

Outline the formation of cancers

Answer 60

• Mitosis is a controlled process. Uncontrolled cell division can lead to the formation of tumours and cancers

• This can be due to mutations of genes that control cell division; almost half of all people with cancer possess a mutated P53 gene that helps control cell growth

• Proto-oncogenes stimulate cell division and tumour suppressor genes stop cell division. Mutated genes that cause cancer are called oncogenes

Question 61

Identify and explain the two types of tumours

Answer 61

• Benign tumours are slow growing and don’t spread to other parts of the body

• Malignant tumours are fast growing, cancerous tumours that invade neighbouring tissue. When a tumour moves from a primary to a secondary location in the body this is known as metastasis

Question 62

Outline and evaluate cancer treatments

Answer 62

• Many cancer treatments are directed at controlling the rate of cell division by disrupting the cell cycle
-> E.g. Chemotherapy drugs such as Methotrexate
that inhibits synthesis of DNA nucleotides and
Vincristine and Taxol that prevent the formation
of the mitotic spindle

• However these treatments don’t distinguish between normal cells and tumour cells so they’re both affected. Tumour cells for divide more quickly so they’re more likely to be killed.

Question 63

Define carcinogen and provide examples

Answer 63

• An agent that causes cancer, for example: UV light, X-rays and tar in tobacco smoke

Question 64

Define and describe the process of binary fission

Answer 64

• Cell division in prokaryotic cells that involves:

• The replication of the circular DNA and plasmids
• The 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

Question 65

Identify key differences between mitosis and binary fission

Answer 65

There’s no nuclear envelope to breakdown and there are no spindle fibres present

Question 66

Outline the structure and function of the cell surface membrane

Answer 66

Structure:
• Phospholipid bilayer composed of two layers of
phospholipids where the hydrophilic phosphate
head faces outwards and a hydrophobic lipid tail
faces inwards described by the Fluid mosaic
model

Function:
• Creates an enclosed space separating the internal cell environment from the external environment in every cell
• Intracellular membranes help form organelles, control exchange of materials and act as an interface for communication

Question 67

What happens if phospholipids are spread over a surface of water or mixed with water

Answer 67

• If they’re spread over water they form a phospholipid monolayer

• If they’re mixed with water they form spheres with phosphate heads facing outwards called a micelle. Alternatively, two-layered structures may form sheets called phospholipid bilayers

Question 68

Outline the fluid mosaic model

Answer 68

• First outlined in 1972
• Describes cell membranes as fluid because phospholipids and proteins can move around via diffusion. Phospholipids mainly move sideways within their own layer, while proteins move like icebergs in the sea, while some are stationary
• Describes cell membranes as mosaics because the scattered pattern produced by proteins within the phospholipid bilayer viewed from above looks like a mosaic

• Explains how:
1. Biological molecules are arranged to form cell
membranes
2. Passive and active movement between cells and
surroundings
3. Cell-to-cell interactions
4. Cell signalling

Question 69

Outline the structure and function of phospholipids

Answer 69

Structure:
• Phospholipids are made up of a polar,
hydrophilic phosphate head that’s soluble in
water and a non-polar, hydrophobic lipid tail
that’s insoluble in water. They can contain glycolipids, glycoproteins, proteins and cholesterol may also be present

Function:
• Act as a barrier to most water-soluble
substances
• The non-polar fatty acid tails prevent polar
molecules or ions passing across the
membrane ensuring water-soluble molecules
can’t leak out of the cell and unwanted water-
soluble molecules can’t get in

• Can be chemically modified to act as signalling
molecules. This happens by:
• Moving within the bilayer to activate other
molecules (e.g. enzymes)
• Being hydrolysed which releases smaller
water-soluble molecules that bind to specific
receptors in the cytoplasm

• Form compartments that provide the basic
structure of organelles

Question 70

Outline the structure and function of glycoproteins

Answer 70

Structure:
• Proteins with carbohydrate chains attached found in the outer phospholipid monolayer

Function:
• Act as receptor molecules for cell signalling:
1. Signalling receptors for hormones and
neurotransmitters
2. Receptors involved in endocytosis
3. Receptors involved in cell adhesion and
stabilisation as the carbohydrate part can form
hydrogen bonds with water molecules
surrounding the cell

• Act as cell markers of antigens for cell-to-cell recognition
• Binding cells together

Question 71

Outline the structure and function of glycolipids

Answer 71

Structure:
• Lipids with carbohydrate chains attached found in the outer phospholipid monolayer

Function:
• Act as receptor molecules for cell signalling:
1. Signalling receptors for hormones and
neurotransmitters
2. Receptors involved in endocytosis
3. Receptors involved in cell adhesion and
stabilisation as the carbohydrate part can form
hydrogen bonds with water molecules
surrounding the cell

• Act as cell markers of antigens for cell-to-cell recognition

Question 72

Identify the two types of proteins in a phospholipid bilayer

Answer 72

• Intrinsic (integral) proteins, embedded in the membrane and most commonly span the entire membrane (transmembrane proteins E.g. transport proteins)

• Extrinsic (peripheral) proteins are found on the inner or outer surface of the membrane

Question 73

Outline transport proteins

Answer 73

• They’re specific to an ion or molecule and allow the cell to control what enters and leaves

• They create hydrophilic channels to allow ions and polar molecules to travel through the membrane

• Can either be channel or carrier proteins

Question 74

Outline the structure and function of Cholesterol in the cell surface membrane

Answer 74

Structure:
• Steroid molecule made up of hydrophobic tails and hydrophilic heads, similarly to the phospholipid bilayer but are absent in prokaryotes

Function:
• Fits between phospholipid molecules orientated in the same way and regulates the fluidity of the membrane.
It does this by stopping them packing too closely together when temperatures are low, preventing freezing and fracturing. They bind to hydrophobic tails of phospholipids at higher temperatures causing them to pack more closely together, stabilising the membrane, stopping it becoming too fluid

• Contributes to the impermeability of the membrane to ions

• Increases mechanical strength and stability preventing breaking down and bursting

Question 75

Outline the process of the regulation of cell surface membrane fluidity

Answer 75

• Membranes become less fluid because:
- There’s an increased proportion of saturated fatty acid chains as they pack tightly together creating high intermolecular forces between them.
- Temperature decreases as the molecules have less energy and therefore aren’t moving freely, so the structures more closely packed

• Membranes become more fluid because:
- There’s an increased proportion of unsaturated fatty acid chains that are bent so are less tightly packed, decreasing intermolecular forces
- At higher temperatures, molecules have more energy and move more freely increasing membrane fluidity

Question 76

Why and how can substances cross cell surface membranes

Answer 76

Cell surface membranes are partially permeable, so substances can cross them by:
• Simple diffusion
• Facilitated diffusion
• Osmosis
• Active transport
• Co-transport

Question 77

Outline simple diffusion

Answer 77

• The net movement of molecules or ions from a region of high concentration to a region of low concentration down a concentration gradient

• The random movement is caused by the natural kinetic energy of the molecules or ions that tend to reach equilibrium where they’re evenly spread in a given volume of space as a result of diffusion

• Fick’s law states that the rate of diffusion is proportional to:

Concentration gradient x surface area
——————————————————
Thickness of the exchange surface

Question 78

Identify and outline the factors that the rate of diffusion is dependant on

Answer 78

Steepness if the concentration gradient
• A greater difference in concentrations means a
greater difference in the number of molecules
passing from each direction and therefore a
faster rate of diffusion

Temperature
• Molecules and ions have more kinetic energy at
higher temperatures so they move faster,
resulting in a higher rate of diffusion

Surface Area
• The greater the surface area, the more
molecules that can cross at any one moment,
increasing the rate of diffusion
• Surface area of cell membranes can be increased
by folding (e.g. Microvilli in the intestine/cristae in
mitochondria)

Properties of molecules or ions
• Large molecules diffuse more slowly than small
molecules as they require more energy to move
• Uncharged and non-polar molecules diffuse
directly across the phospholipid bilayer. Non-
polar molecules diffuse more quickly than polar
ones because they’re soluble in the non-polar
phospholipid bilayer

Question 79

Outline facilitated diffusion

Answer 79

• The movement of molecules or ions from a region of high concentration to a region of low concentration requiring the aid of certain proteins (there are two main types: channel and carrier proteins)

• Certain substances can only cross the phospholipid bilayer with the help of certain proteins, including large, polar molecules such as glucose and amino acids and ions such as sodium (Na+) and chloride (Cl-)

• Facilitated diffusion is affected by the steepness of the concentration gradient and the number of transport proteins available

Question 80

Outline the proteins involved in facilitated diffusion

Answer 80

Channel proteins
• Used to transport charged molecules
• Fixed shape
• They have water filled pores that allow charged
substances (e.g. ions) to diffuse through the cell
membrane
• Diffusion of these ions doesn’t occur freely, most
channel proteins are gated. This means the
pores can open and close to control the
exchange of ions

Carrier Proteins
• Used to transport large molecules
• Can switch between two shapes. This causes the
binding site of the carrier protein to be open on
one side of the membrane first, and then open to
the other side of the membrane when it switches
shape. The direction of movement of molecules
depends on the concentration on each side of
the membrane and net movement occurs down
a concentration gradient

Question 81

Outline osmosis

Answer 81

• The passive net movement of water from a region of less negative water potential to a region of more negative water potential, through a partially permeable membrane down a concentration gradient

• The partially permeable membrane will allow smaller (e.g. water) but not larger (e.g. solute) molecules through

Question 82

Explain water potential

Answer 82

• The pressure created by free water molecules, measured in Kilopascals (kPa) that describes the tendency of what to move out of solution

• The water potential of pure water at atmospheric pressure is 0kPa a concentrated solution would have a water potential of (e.g.) -500kPa

• A more negative water potential in the cell than the solution means the cell bursts, which is called haemolysis
• A more negative water potential in the cell than the solution means the cell shrinks, which is called crenation
• A normal cell has the same water potential as the solution

• Osmosis is explained in terms of water potential

Question 83

Outline the water potential practical

Answer 83

It’s possible to investigate the effects of immersing plant tissue in solutions of different water potentials and then use the results to estimate water potential of the plant itself

Method:
• Use each cork borer to cut 5 potato cylinders of the same diameter and uses a scalpel and ruler to trim each potato to the same length and blot dry to remove excess moisture

• Measure using a top pan balance accurate to 0.01g the initial mass of each potato cylinder and record in a table of results as well as a table for initial length (mm)

• Measure 10cm3 of each sugar/salt solution (one should be distilled water) and our each into a boiling tube, labelling each one’s clearly including its concentration

• Add one potato cylinder to each tube and leave for a specified amount of time

• Remove the potatoes, blot dry and record the final mass and length of each

Analysis
• The percentage change in mass for each potato cylinder is calculated and put a graph for percentage change in mass (Y) against sugar concentration (X) :

(Final mass - initial mass)
———————————— X100
Initial mass

• A positive change in mass indicates that the potatoes gained water by osmosis so the solution had a higher water potential than the potato. The potato cell is turgid as the water exerts turgor pressure on the cell walls so potatoes feel hard

• A negative change in mass indicates that the solution had a lower water potential than the potato, water moves out of the potato cells by osmosis, making them flaccid and feel floppy

Conclusion
• The point at which the line of best fit crosses the X-axis is the concentration of sugar inside the potato as this is where there would be no change in the mass of the potato

Question 84

Identify what happens if an plant cell is placed in a solution with a higher water potential than the cell

Answer 84

• Water enters the cell through it’s partially permeable membrane by osmosis as the solution has a higher water potential than the plant cell

• Water enters the vacuole so its volume increases and the protoplast (entire cell excluding the cell wall) expands and pushes against the cell wall. The inelastic cell wall prevents the cell from bursting, so the cell becomes turgid.

• Turgid cells fully inflated with water are firm and provide strength and support allowing the plant to stand upright with its leaves out to catch the sunlight, if plants don’t receive enough water they can’t stay turgid and the plant wilts

Question 85

Identify the difference between a plant cell membrane and its cell wall

Answer 85

The cell surface membrane is partially permeable and the cellulose cell wall is freely permeable

Question 86

Identify what happens when a plant cell is placed in a solution with a lower water potential than the cell

Answer 86

• Water leaves the plant cell through its partially permeable cell surface membrane through osmosis

• Water leaves the vacuole so the volume decreases and the protoplast gradually shrinks and pulls away from the cell wall

• This is known as plasmolysis (plant cell is plasmolysed)

Question 87

Identify what happens if an animal cell is placed in a solution with a lower water potential than the cell

Answer 87

• Water leaves the cell through the partially permeable cell surface membrane by osmosis and the cell will shrink and shrivel up.

•. This occurs in a hypertonic environment where the solution outside the cell has a higher solute concentration than the inside of the cell

Question 88

Identify what happens if an animal cell is placed in a solution with a higher water potential than the cell

Answer 88

• Water enters the cell through its partially permeable cell surface membrane by osmosis as the pure water or dilute solution has a higher water potential than the animal cell

• The cell gains water by osmosis until the membranes stretched too far and the cell bursts (cytolysis) because it has no cell wall to withstand the increased pressure

• This occurs when the cell is in a hypotonic environment which means the solution outside the cell has a lower solute concentration than inside the cell

Question 89

Explain what an isotonic environment is

Answer 89

• The solution outside cell has the same solute concentration as inside the cell

• There is no net movement of water molecules and there is no change to the cells

Question 90

Outline active transport

Answer 90

• The movement of molecules or ions into or out of the cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins

Active transport requires:
• Carrier proteins: transmembrane proteins that
undergo conformational change
• Energy: from ATP being hydrolysed to release
energy during respiration which causes carrier
proteins to change shape

Question 91

Identify the uses of active transport in biology

Answer 91

• Reabsorption if useful molecules and ions into the blood after filtration into the kidney tubules

• Absorption of 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 the root hairs

Question 92

Explain the process of active transport

Answer 92

• A molecule or ion bonds to receptor sites on the carrier protein and ATP bonds to the protein (inside the cell) and releases energy when its hydrolysed into ATP + Pi

• The carrier protein changes shape, opening up on the other side of the membrane where the molecule or ion is released

• The inorganic phosphate (Pi) is released from the protein where it combines with ADP to form ATP

• This causes the protein to return to its previous shape

Question 93

Identify differences between facilitated diffusion and active transport

Answer 93

• Facilitated diffusion occurs down a concentration gradient from a high concentration to a low concentration whereas active transport occurs against a concentration gradient from a low concentration to a high concentration

• Facilitated diffusion used channel or carrier proteins whereas active transport uses just carrier proteins

• Facilitated diffusion uses innate kinetic energy whereas active transport uses energy from ATP

Question 94

Outline co-transport

Answer 94

• The coupler movement of substances across a cell surface membrane via a carrier protein, involving a combination of facilitated diffusion and active transport

• Co-transporters are a type of carrier protein that bind to two molecules at the same time. The concentration gradient of one molecule is used to move the other molecule against its own concentration gradient

• This is illustrated by the absolution of sodium ions and glucose by cells lining the mammalian ileum

Question 95

Describe the process of the co-transport of glucose and sodium ions

Answer 95

• To absorb glucose from the lumen of the gut there must be a higher concentration of glucose in the lumen compared to the epithelial cell for facilitated diffusion
• However usually there’s more glucose in the epithelial cells so active transport and co-transport are required. The concentration of glucose in the blood is lower than the epithelial cells because the blood flows and carries away the absorbed glucose

1. Sodium ions are actively transported out of the
   epithelial cells into the blood
2. This reduces the sodium ion concentration in
   the epithelial cell
3. Sodium ions can diffuse from the lumen down
   their concentration gradient into the epithelial
   cell
4. The protein the sodium ions diffuse through is a
   co-transporter protein, so either glucose or
   amino acids also attach and are transported into
   the epithelial cell against their concentration
   gradients
5. Glucose then moves by facilitated diffusion
   from the epithelial cell to the blood
6. Microvilli on the epithelial cell surface increase
   the surface area for co-transporter proteins.
   However, protein channels are finite and can
   become the limiting factor

Question 96

Define immunity and Identify the categories of defence mechanisms against pathogens

Answer 96

• Immunity is the ability of a body to be better prepared for fighting a disease than the last time

Non-Specific immune responses
• Barrier methods
• Inflammation
• Phagocytosis

Specific immune responses
• Cell-mediated response (T-cells)
• Humoral response (B-cells)

Question 97

Explain cell recognition

Answer 97

Each type of cell has specific molecules on its surface that identify it called antigens. These molecules include proteins and enable the immune system to identify:
• Pathogens
• Cells from other organisms of the same species
• Abnormal body cells
• Toxins

Question 98

Outline antigens

Answer 98

• Antigens are specific macromolecules on the cell-surface membrane that stimulate an immune response and allow cell-to-cell recognition

• Antigens produced by the organisms own body cells are known as self antigens and don’t stimulate an immune response
• Foreign antigens not produced by the organisms own body cells are known as non-self antigens and stimulate an immune response

• Every human body cell, microorganism (both pathogenic and non-pathogenic) such as bacteria and viruses have macromolecules called antigens on their cell surface membranes, bacteria cell walls or the surface of viruses. Some glycolipids and glyco-proteins on the outer surface of cell membranes act as antigens

Question 99

Outline antigenic variability

Answer 99

• Antigenic variability refers to the fact that mutations can change the tertiary structure so the immune system can’t detect the pathogen

• Some pathogens exhibit antigenic variability. This is a problem as lymphocytes and memory cells produce a specific immune response complementary in shape to only one antigen. When this shape changes they can no longer bind and there’s no secondary immune response so the host is infected again
• E.g. cold and influenza change year after year

Question 100

Outline non-specific defence mechanisms

Answer 100

• Barrier methods, including physical and chemical defences e.g. skin, mucous membranes, tears (containing lysozyme enzymes) and saliva

• Inflammation, involving the swelling and heating of a region invaded by the pathogen

• Phagocytosis

Question 101

Describe the process of phagocytosis

Answer 101

• The phagocyte is attracted to the pathogen by it’s chemical products and moves towards it along a concentration gradient by chemotaxis

• Receptors in the cell surface membrane of the phagocyte bind to the pathogen. Once attached, the cell surface membrane extends around the pathogen, engulfing it and trapping it within a phagocytic vacuole to form a phagosome. This is called endocytosis.

• Lysosomes within the phagocyte migrate towards the phagosome and release digestive enzymes called lysosymes that hydrolyse the pathogen

• The lysosome fuses with the phagosome membrane to form a phagolysosome

• The hydrolysis products of the bacteria are absorbed by macrophage phagocytes and antigens from the destroyed pathogen can be presented on the surface of the phagocyte due to the major histocompatibility complex.

• An antigen presenting cell is formed that presents foreign antigens on their cell surface membrane to activate other immune cells

Question 102

State the types of white blood cell

Answer 102

• Phagocytes
• Lymphocytes

Question 103

Outline phagocytes

Answer 103

• A type of white blood cell (WBC) that are part of a non-specific immune response (the same for all pathogens) and are produced continuously in the bone marrow.

• They’re responsible for removing dead cells and identifying and eliminating pathogens by engulfing them and digesting their contents through phagocytosis

• There are two types of phagocyte that carry out phagocytosis, Neutrophils and Macrophages

Question 104

Describe the function of neutrophils

Answer 104

• Neutrophils travel throughout the body and leave the blood by squeezing through capillary walls to patrol body tissues. During infection they’re released in large numbers and are short lived.

• Chemicals released from body cells under attack and pathogens attract neutrophils that move towards pathogens (chemotaxis- response to chemical stimuli)
• Antibodies attach to receptor proteins on the neutrophils which stimulates them to attack the pathogen, beginning phagocytosis

Question 105

Describe the function of macrophages

Answer 105

• Long-lived cells that are larger than neutrophils and travel in the blood as monocytes that then develop into macrophages once they leave the blood into organs such as the lungs and lymph nodes

• Macrophages carry out phagocytosis as normal but don’t destroy pathogens completely. They display the antigens on their surface to become an antigen presenting cell through a structure called the major histocompatibility complex.

• These antigen presenting cells are recognised by lymphocytes

Question 106

Outline lymphocytes

Answer 106

There are two types of lymphocyte:
• T-lymphocytes (T-cells) mature in the thymus gland and are associated with cell-mediated immunity (body cells)

• B-lymphocytes (B-cells) mature in bone marrow and are associated with humoral immunity (antibodies in plasma)

Question 107

Outline cell-mediated immunity

Answer 107

• Mediated by T-lymphocytes (T-cells) matured in the thymus that specifically bind to antigens presented on cells. Cell-mediated immunity is more important in eliminating intercellular pathogens

• Helper T-cells have receptors on their surface that bind to antigens on the antigen presenting cell from phagocytosis. This stimulates the rapid differentiation of T-cells by mitosis (cloning):

1. Increase phagocytosis which will make more antigen presenting cells and therefore decrease the spread of bacteria
2. Become a helper T-cell that recognise antigens on antigen presenting cells, stimulate more phagocytosis and activate a B-lymphocyte (B-cell) to divide and secrete antibodies
3. Become a memory T-cell which remain in the blood to respond to future infection by the same pathogen
4. Become a cytotoxic T-cell that produces performing which make holes in cells and fills those cells with water so they lyse (burst)

Question 108

Describe the process of humoral immunity

Answer 108

• Initiated by B-cells matured in the bone marrow that secrete antibodies (unlike T-cells). Humoral immunity is more important to eliminating soluble antigens and extracellular microorganisms

• Helper T-cells stimulate B-cells to rapidly divide by mitosis to form clones of plasma cells (clonal selection) which produce antibodies specific to the antigen. This means the antibodies attach to the antigen and destroy the pathogen

• Some B-cells develop into memory cells that have the specific antibodies attached to their cell surface membranes and remain in the blood and divide by mitosis into more memory cells and plasma cells in response to the same antigen again

• The first contact with pathogen is the primary response and the second contact is the secondary response. During the secondary response, once antibodies have destroyed the pathogen, memory B-cells are formed that more rapidly produce more antibodies at a higher intensity of production on re-exposure

Question 109

Outline the structure and function of antibodies

Answer 109

Structure:
• Proteins that have binding sites complementary to antigens
• Quaternary structure of four polypeptide chains (2 light chains and 2 heavy chains) linked by disulphide bonds
• They have a constant region that’s present in all antibodies and a variable region that makes antibodies specific to an antigen

Function:
• Bind to antigens and destroy pathogens. Antibodies destroy pathogens by agglutination: the antibodies that have bound to antigens to form an antigen-antibody complex all stick together, so pathogens can be easily digested by phagocytosis as the larger clumps are more easily detected by the phagocyte

Question 110

Outline vaccination

Answer 110

• Vaccinations are made up of dead or inactive pathogen that incite a primary response, learning to the formation of memory B-cells so that, upon re infection, a secondary response is stimulated

• This leads to heard immunity where immunising the majority of the population means that unvaccinated people are less likely to come into contact with an infected person.
• Active immunity is immunity that a results from antibody production by the immune system in response to the presence of an antigen e.g. vaccination

• Ethical issues include:
• Whether they should be compulsory (against
people’s will for the good of the population)
• Who should vaccines be tested on (sick/young/
old)
• Are they 100% effective long-term (true for
new vaccines e.g. HPV in girls- LT effects aren’t
yet seen