Cells

Function [2] and structure [2] of the CELL-SURFACE MEMBRANE:

• Its selectively permeable, controling the movement of substances in and out of the cell.

• Phospholipids & proteins.

Function [2] and structure [5] of MITOCHONDRIA:

• Site of aerobic respiration, and produces ATP.

• Matrix (enzyme rich liquid), double membrane, cristae, 70S ribosomes and circular DNA.

Function [6] and structure of GOLGI APPARATUS:

• Packages, modifies and transports lipids & proteins. Produces lysosomes.

• Cisternae.

Function [2] and structure [2] of LYSOZOMES:

• Hydrolyses pathogens and breaks waste & debris.

• Hydrolytic enzymes & membrane.

Function and structure [2] of RIBOSOMES:

• Site of proteinsynthesis.

• Proteins & RNA.

Function [3] and structure [2] of the RER:

RER:

• Synthesises, packages and transports proteins.

• Cisternae & ribosomes.

Function [2] and structure of the SER:

SER:

• Synthesises and transports lipids.

• Cisternae.

What are the 3 ORGANELLES only found in PLANTS, and their function [2 & 1 & 2] and structure [2 & 7 & 2]?

Cell wall:

• Provides strength and prevents the cell from bursting or shrinking.

• Cellulose & plasmodesmata.

Chloroplasts:

• Site of photosynthesis.

• Thylakoids, stroma, starch, grana, chlorophyll, DNA, ribosomes.

Vacuole:

• Maintains tugor pressure (to prevent plant from wilting). Stores cell sap.

• Tonoplast membrane, cell sap.

How is a BACTERIAL and FUNGI cell wall different to a plant cells?

Bacteria cells wall is made of Murein.

Fungi cell walls are made of Chitin.

What 3 ‘extra' organelles that are ‘sometimes present' in a BACTERIA cell? And what is their function and structure?

Capsule:

• Protects the cell from antibiotics/WBCs.

• Polysaccharides.

Pili:

• Helps it attach to surfaces.

• Is short and hair-like.

Plasmids:

• Extra genes.

Flagellum:

• Rotates and moves the cell.

• Is long and hair-like.

What is the general structure [3] of a VIRUS have? And what is its function and structure?

Envelope:

• Helps the virus enter host cells.

• Phospholipid membrane.

Capsid:

• Protects the genetic material.

Glycoproteins:

• Binds to the receptors on the host cell.

• Protein & carbohydrate chains.

Enzymes:

• Converts RNA to DNA.

What happens in a VIRUSES LYTIC CYCLE [5]?

• Virus attaches to the host cell / fuses to the hosts membrane and injects its DNA in the host.

• The DNA uses the hosts machinery to make viral proteins.

• Viruses assemble in the host.

• Host lyses (bursts).

• The viruses then go to infect other cells.

What happens in the VIRUSES LYSOGENIC CYCLE [4]?

When the virus injects its DNA in the host…

• The DNA becomes incorporated with the hosts genome - causing the cell to become a provirus.

• The host divides and copies the viral DNA.

• The virus remains inactive until the viral DNA is triggered into the lytic cycle.

How does a LIGHT MICROSCOPE work [3]?

• Visible light passes through the specimen.

• Then the visible light passes through the glass lens.

• This magnifies the image when you look through the eyepiece lens.

The advantages [4] & disadvantages [3] of the LIGHT MICROSCOPE:

• Advantages: used on living cells, portable, cheaper, coloured images.

• Disadvantages: low resolution and magnification, can't see organelles.

How does a TRANSMITTION ELECTRON MICROSCOPE work [3]?

• A beam of electrons is controlled by electromagnets and passes through a thin specimen.

• The denser parts of the specimen absorbs more electrons so the image appears darker.

• The less dense areas appear lighter.

The advantages [3] and disadvantages [6] of the TEM:

• Advantages: highest resolution and magnification, can be used for studying subcellular organelles.

• Disadvantages: has to be used in a vacuum, sample must be dead and thin - this may cause the creation of artefacts, sample preparation is complex, produces 2D black and white images.

How does a SCANNING ELECTRON MICROSCOPE work [2]?

• A beam of electrons is fired at the surface of the sample.

• The beam reflects off the surface and is detected by a computer to form a 3D image.

This provides the surface structure rather than internal.

What are the advantages [4] & disadvantages [4] of the SEM:

• Advantages: high magnification, high resolution, produces 3D images, can be used on thicker samples.

• Disadvantages: must be used in a vacuum, sample must be dead, complex sample prep so artefacts could be produced, produces black and white images.

Why do electron microscopes have a higher resolution & magnification than light microscopes [2]?

Electron beams have much shorter wavelengths creating a higher resolution (the ability to distinguish two points close together as seperate).

Electron microscopes use electromagnets to control the beam, which achieves more control.

What's the method of CELL FRACTIONATION / Ultracentrifugation [6]?

1. The tissue is placed in a cold, isotonic, buffered solution.

2. The cells are homogenised by a homogeniser to create homogenate.

3. The homogenate is filtered to remove large debris and unbroken cells.

4. The homogenate is placed in a centrifuge and spun at a low speed for a short period of time until a pellet is formed with a supernatant ontop.

5. The supernatant is placed in a centrifuge and spun at a faster speed for a longer period of time.

6. This process is repeated until you get your desired organelle as a pellet.

The organelles are seperated from heaviest to lightest: nucleus, chloroplast, mitochondria, lysozome, ribosome.

Why is the homogenate solution: cold, isotonic & buffered?

• Cold: to prevent enzyme activity and the digestion of organelles.

• Isotonic / same water potential: to prevent osmosis and the shrinkage or bursting of cells.

• Buffered: to maintain pH so that the proteins do not denature.

What happens in the 1st part of the CELL CYCLE (Interphase) and its phases [4]?

This is when DNA replication occurs to ensure that there are enough organelles for division (but the chromosomes aren't visible yet).

The phases of Interphase:

• Gap 1: the cell grows and produces new organelles.

• Synthesis: DNA replication occurs.

• Gap 2: further growth.

• G0: happens immediately after cytokinesis, this is the resting phase where the cell doesn't divide.

What happens in the 2nd part of the Cell cycle (Mitosis) and its stages [4]?

Stages of Mitosis:

• Prophase: the chromosomes condense into 2 sister chromatids, the nuclear envelop breaks down, the centroiles move to the opposite poles of the cell and begin to form spindle fibres.

• Metaphase: the spindle fibres attach to the centromeres of the chromosomes and aligns them in the cells equator.

• Anaphase: the spindle fibres contract, the sister chromatids seperate and are pulled to the opposite poles of the cell.

• Telophase: the chromosomes decondense into chromatin, nuclear envelop reforms around each set of the chromosomes.

What are the stages of BINARY FISSION [4]?

• Replication of DNA: its genetic loop and any plasmids are replicated.

• Cell growth: the cell increases in size and its organelles replicate.

• Seperation of DNA: the copies of DNA move to the opposite poles of the cell and the plasmids are split between the 2 poles.

• Cytokinesis: the cell membrane pinches inwards forming a new cell wall in the middle of the cell, producing 2 daughter cells.

What are the components [6] of the FLUID-MOSAIC MODEL?

• Partially permeable phospholipid bilayer.

• Intrinsic (channel or carrier) proteins.

• Extrinsic proteins (provides structural support).

• Glycoproteins (receptors made of protein & carbohydrate chains).

• Glycolipids (lipids & carbohydrate chains).

• Cholestrol (maintains membrane fluidity).

What are the 2 types of DIFFUSION? and how does it work?

• Simple diffusion: the movement of small non-polar molecules from an area of high to low concentration.

• Facilitated diffusion: the movement of larger charged molecules via channel or carrier proteins.

What is the function of CHANNEL & CARRIER PROTEINS?

• Channel proteins: creates hydrophillic pores.

• Carrier proteins: transports larger specifically shaped molecules by, the molecule attaching to its binding site causing the protein to change shape and release the molecule on the other side.

What is the definition of OSMOSIS?

The net movement of water molecules through a partially permeable membrane, moving down the concentration gradient. From a region of high to low water potential.

What are the 2 types of ACTIVE TRANSPORT? And how does it work?

• Active transport: the movement of molecules across a membrane against their concentration gradient, requiring energy.

• Co-transport: the movement of 2 molecules across a membrane using the same carrier protein. One molecule moves down the concentration gradient while the other moves against its concentration gradient.

Why can't large molecules diffuse through the membrane [3]?

• The bilayer is tightly packed so large molecules can't pass through.

• If there is a high concentration of cholestrol in the membrane it loses fluidity so diffusion could occur slowly.

• But if there are more unsaturated fatty acids the fluidity increases so the rate of diffusion also increases, due to their C=C that create ‘kinks' so that they can't pack closely together.

Why can only non-polar molecules pass through the membrane?

The hydrophobic phospholipid tails repel water-soluble and polar substances. (so they require proteins to cross)

How do you increase the rate of transport [7]?

• Increase SA.

• Increase the number of channel & carrier proteins.

• More mitochondria.

• Thin exchange surfaces.

• Increase in unsaturated fatty acids.

• Decrease in cholestrol.

• Increase in temperature.

What is an antigen?

A protein that is recognised as foreign by the immune system and stimulates an immune response.

What is ANTIGEN VARIABILITY and its effect?

Antigen variability is when pathogens change their surface protein over time so the immune system no longer recognises the pathogen. This makes it so that antibodies no longer bind to the new antigens - causing people to get sick from the same pathogen again, making vaccines less effective.

What is an ANTIBODY? And what is Agglutination, Opsonisation & Neutralisation?

A Y-shaped quaternary structured globular protein that is produced by plasma cells in response to a specific antigen. They form antigen-antibody complexes.

• Agglutination: antibodies binds and forms clumps of pathogens to reduce spread and be engulfed easier.

• Opsonisation: antibodies coat bacteria in a protein layer so that they can be detected and destroyed easier.

• Neutralisation: antibodies bind and neutralises toxins.

What are the differences in the PRIMARY & SECONDARY IMMUNE RESPONSE?

• Primary response: produces plasma cells (antibody levels rise slowly, due to clonal selection & expansion), produces memory cells, you experience symptoms.

• Secondary response: plasma cells produce antibodies at a faster rate, memory cells rapidly divide into plasma cells (no need for clonal selection to occur), they pathogen is destroyed before you experience symptoms.

What is the difference between ACTIVE & PASSIVE IMMUNITY?

• Active: your own immune system produces antibodies (naturally or vaccination), memory cells are produced, slow but long-term protection.

• Passive: antibodies are given by an external source, T & B cells aren't activated, no memory cells produced, immediate but short-term protection.

What type of immune response is PHAGOCYTOSIS? What is its effect?

• Non-specific immunity.

• Neutrophils & Macrophages (WBCs) send out pseudopodias to engulf the pathogen and create a phagosome. The cells lysozomes bind to the phagosome and release hydrolytic enzymes to digest the pathogen. The phagolysozomes left over are moved to the cells surface to create an Antigen Presenting Cell, to alert the immune system.

What type of immune response is the CELLULAR RESPONSE? And what effect does it have [4]?

Specific immunity.

• After phagocytosis T-helper-cells are activated and bind to the APC.

• They release cytokines.

• Undergo clonal selection and expansion.

• Divide by mitosis, then differentiate into T active & memory cells.

What type of immune response is the HUMOURAL IMMUNITY? And what effect does it have [3]?

Specific immunity.

• When a B cell comes into contact with an antigen, it becomes an APC.

• A T-active-cell binds to the APC and releases cytokines.

• This stimulates the B-cell to undergo clonal selection & expansion to produce: a B-memory-cell and a plasma cell.

What type of immune response is CELLULAR IMMUNITY? And what effect does it have [5]?

Specific immunity.

• Abnormal cells act as an APC.

• A cytotoxic-T-cell binds to the APC and releases cytokines.

• To produce cytotoxic memory & active cells.

• The active cell then binds to other APCs. Then releases perforin.

• Perforin makes cells more porous, allowing granzymes to enter and trigger apoptosis.

What is the aim & method [5] for the ELISA test?

AIM: This detects the presence & quantity of a specific antigen or antibody in a sample.

1. Monoclonal antibodies are bound to a plate full of wells.

2. Sample is exposed to the plate and creates an antigen-antibody complex.

3. An enzyme is added to the wells and binds to the bound antigen-antibody complex.

4. The wells are rinsed - to prevent false positives.

5. A substrate is added. The enzyme breaks down the substrate creating a colour change (which is proportional to the amount of antigen present in the sample).

What is a VACCINATION? What is Herd immunity?

A weakened or inactive version of the pathogen that stimulates a primary immune response so B-cells with complementary receptors can bind to the antigen and create plasma cells to secrete antibodies and memory cells.

Herd immunity = when the majority of the population is vaccinated so when they encounter the antigen, their immune system rapidly destroys it before it can spread. This makes non-vaccinated individuals protected against the pathogen, due to the pathogen having fewer hosts.

What are the ETHICAL ISSUES [5] associated with Vaccines & Monoclonal antibodies?

• Tested on animals.

• Some are compulsory - protection vs choice. [vaccine]

• Expensive - unfair on poorer individuals.

• Can cause side effects, you must weight the risks & benefits.

• The long-term effects are unknown. [monoclonal antibodies]

How does HIV replicate in T-helper-cells [5]?

• HIV binds to the cells receptors, with its attachment protein.

• Then fuses to its membrane and injects its RNA & enzymes (reverse transcriptase) into the cell.

• The reverse transcriptase converts its viral RNA into DNA, which is then inserted into the hosts genome.

• Once the viral DNA is activated, it uses the hosts machinery to produce viral proteins & RNA.

• These then assemble into viruses, which bud of the host and form a lipid envelop for itself, destroying the T-cell.

Why does HIV cause AIDS? And why are antibiotics ineffective on viruses?

HIV targets T-cells (which are necesary for the activation of B-cells & plasma cells). Fewer Cytotoxic-T-cells are made so more abnormal cells remain. This causes for a weak immune system, making an individual vulnerable to infections - AIDS.

Antibiotics don't work due to viruses lacking sub-cellular structures that antibodies target. And to destroy the virus in the cell the antibiotic would have to destroy the host too.