A Level Biology Edexcel B

Features and function of the Nucleus

Present in all eukaryotic cells
Surrounded by double membrane (nuclear envelope)
Membrane contains nuclear pores to allow movement of mRNA and ribosomes out of nucleus, and enzymes into nucleus
Stores genetic material as chromatin
nucleolus (site of ribosome production)

Where is the site of ribosome production?

Nucleolus

Features and Function of Rough Endoplasmic Reticulum

Folds of membrane with it's surface covered in ribosomes
Folds and processes proteins made on the ribosomes

Features and Function of Smooth Endoplasmic Reticulum

Folds of membrane (no ribosomes)
Production, processing and storage of lipids, carbohydrates and steroids

How to distinguish between smooth or rough endoplasmic reticulum?

Observe under electron microscope

If ribosomes present, it is RER
If ribosomes not present, it is SER

Features and function of Golgi Apparatus

Flattened Fluid filled curved sacs called cisternae
Sorts and modifies proteins and lipids before packaging them into vesicles to be transported.

Features and function of mitochondria

Site of aerobic respiration
Surrounded by a double membrane (envelope)
Inner membrane folded to form cristae
Matrix contains enzymes needed for aerobic respiration Also contains ribosomes and small pieces of DNA needed for replication of mitochondria before cell division

Features and function of centrioles

Not found in plant or fungi cells
Made out of hollow fibres called microtubules
Microtubules are filaments of a protein that is used to move substances inside the cell and supports shape of cell
Two centrioles at a right angle to each other form a centrosome
Organises spindle fibres during cell division

Features and function of 80s Ribsosomes

Found in eukaryotic cells only

Site of protein synthesis and translation

Made up of large 60s subunit, and small 40s subunit

Features and function of lysosomes

Bound by single membrane

Form of vesicle containing hydrolytic enzymes
Used by cells of immune system for programmed cell death

Features and function of vacuole

Large and central in plants (animal cells may have one, but they are often small and temporary)

Fluid filled cavity surrounded by a membrane (tonoplast)

Helps maintain hydrostatic pressure and store cell sap

Features and function of plasmids

Small loops of DNA sometimes found in bacterial cells

Contain genes that can be passed between prokaryotes (antibiotic resistance)

Features and function of slime capsule

Help keep bacteria moist and prevent them from drying out

Help protect bacterial cells from attack of host organisms immune system cells

Features and function of Nucleoid

Area in which circular DNA is found

Genetic material in prokaryotic cells often found as single circular strands

Features and function of Pilus

Thread like structures found on cell surface membrane

Enables bacteria to attach to other cells and surfaces
Allows plasmids to be passed from cell to cell

Features and function of 70s Ribosomes

Large 50s subunit

Small 30s subunit
Site of protein synthesis

Features and function of cell wall (prokaryotic)

Made of peptidoglycan

Provides strength and support to the cell

Difference in structure of Gram negative and Gram positive bactera

Gram positive:

Thick layer of peptidoglycan
Inner plasma membrane

Gram negative:
Thin layer of peptidoglycan
Outer lipopolysaccharide membrane

Gram Staining Test Procedure + Results

1. Crystal violet solution for 1 minute, then wash with water

2. Iodine solution for 1 minute, wash with water.

3. Alcohol added for 1 minute (lipopolysaccharide soluble in alcohols so outer membrane dissolves)

4. Red safranin for 1 minute, then dried
   
   Gram positive: Purple
   Gran negative: Pink/Red

Resolution

Minimum distance between two objects where they can still be seen as two separate objects

Features of transmission electron microscope

Beams of electrons transmitted through the object, producing a 2D image.

Resolution 2000x better than light microscope

Features of Scanning electron microscope

Beam of electrons scans back and forth over the surface of the sample producing a 3D image

What type of staining is required for electron and light microscopes?

Electron microscopes - heavy metals (reflect electrons)

Light microscopes - methylene blue

Advantages of electron microscopy

Higher resolution and magnification

Disadvantages of electron microscopy

Sample must be placed in a vacuum, therefore not living

Expensive and not portable
Produces only black and white images

What is the protective coat of Nucleic acid in viruses called?

Capsid

Describe the lytic cycle of a virus

1. Attachment - virus attaches to the surface of the host

2. Penetration - viral DNA/RNA is inserted into the cytoplasm of host call

3. Biosynthesis - viral DNA replicated and viral proteins are made

4. Maturation - new viruses assembled

5. Lysis - Lysis of cell releases newly made phage (independent of host cell's genome)

Describe the lysogenic cycle of a virus

1. Virus attaches to surface of host cell and inserts DNA/RNA in the form of a provirus

2. Viral DNA integrates with host cell’s DNA

3. Enables viral DNA to be replicated through cell division of host cell.

4. Provirus can remain dormant if virus produces repressor proteins, inhibiting the transcription of the provirus.

Ethical implications to using untested drugs

Difficult to obtain informed consent

Unknown side effects
May not ending up being more effective than other existing alternatives/treatment

What are the 5 stages of mitosis?

interphase, prophase, metaphase, anaphase, telophase

Describe interphase

G1, S, G2 - DNA replication and preparation for cell division - chromosomes and some organelles are replicated

Describe prophase

Chromosomes condense (2 sister chromatids joined by a centromere
Nuclear envelope breaks down
Centrioles move to opposite poles forming spindle network

Describe Metaphase

Chromosomes align along equator of the cell

Spindle fibres attach to chromosomes by centromeres

Describe Anaphase

Spindle fibres contract and pull chromatids to opposite poles of the cell

Centromere divides

Describe Telophase

Chromosomes uncoil and become chromatin

Nuclear envelope forms around two sets of chromosomes
Two nuclei that form are identical

Describe cytokinesis

Cytoplasm divides

Produces 2 daughter cells

What is the main purpose of meiosis?

Production of haploid gametes

Maintenance of chromosome number

How is genetic variation acheieved?

Crossing over - exchange of sections of DNA between homologous chromosomes

Independent assortment - various combinations of maternal and parental chromosome arrangement

What is a translocation mutation?

Where 1 part of a chromosome breaks off and attaches to a completely different chromosome

What is a non-disjunction mutation

Homologous chromosomes or sister chromatids fail to separate

What is polysomy? examples!

More than 2 chromosomes in a cell

Down's syndrome

What is monosomy? examples!

Less than 2 chromosomes in a cell

Turner's syndrome

Describe the process of Oogenesis in steps

1. Primordial germ cell divides several times by mitosis to form oogonia

2. Oogonia matures and continues to grow to form primary oocyte.

3. Primary oocyte undergoes mitosis to form 1 secondary oocyte and 1 polar body.

4. Secondary oocyte undergoes meiosis to form 1 haploid ootid and one more polar body.

5. First polar body produced undergoes meiosis to produce 2 polar bodies

6. Polar bodies die as ootid develops

Describe the process of spermatogenesis in steps

1. Primordial germ cell divides several times by mitosis to form spermatogonia.

2. Spermatogonia continues to grow the form primary spermatocytes

3. Primary spermatocytes undergoes meiosis to form secondary spermatocytes

4. Secondary spermatocytes undergo meiosis to form spermatids

5. Spermatids differentiate and grow to form spermatozoa

Features of the ovum

Zona pellucida (protective coating to prevent polysomy (fertilisation by more than 1 sperm)

Cortical granules - release substances causing zona pellucida to harden
Follicle cells - protective coating

Features of spermatozoa

Many mitochondria to provide more energy for movement (rotation of flagellum)

Acrosome contains digestive enzymes to break through zona pellucida

Describe sexual reproduction in mammals in steps

1. Sperm head contacts zona pellucida and acrosome reaction occurs (acrosome fuses with cell membrane of sperm to release enzymes).

2. Sperm head fuses with cell membrane of egg to allow sperm nucleus to enter egg cell.

3. Cortical reaction occurs.

4. Nuclei fuse - full set of chromosomes is restored, forming diploid zygote.

How does a zygote form a blastocyst?

Zygote undergoes mitosis to form morula.

Morula continues to divide and undergo differentiation to form blastocyst

Describe pollen formation

1. Diploid microspore mother cells undergo meiosis to form 4 haploid microspores.

2. Haploid microspores undergo mitosis to form pollen grains

What two types of nuclei do pollen grains consist of?

Generative nucleus and pollen tube nucleus

Describe ovum formation

1. Diploid megaspore mother cells undergo mitosis to form an ootid and 3 polar bodies.

2. Ootid undergoes mitosis 3 times to form an embryo sac

Describe the process of fertilisation in plants

1. Pollen grain sticks to stigma and later germinates.

2. Pollen tube grows down style via secretion of digestive enzymes (digest surrounding tissue as a source of nutrients).

3. Pollen tube grows through micropyle into embryo sac.

4. Generative nucleus divides by mitosis to form 2 sperm cells which enter embryo sac.

5. Double fertilisation occurs:
   one sperm cell fuses with female nucleus to form diploid zygote.
   one sperm cell fuses with 2 polar body nuclei to form a triploid endosperm nucleus (served as nutrients for embryo).

Describe aseptic culture technique

1. Decide on microorganism you want to culture and obtain the culture

2. Provide microorganism with appropriate nutrients in sterile nutrient medium: either broth or agar (microorgnaisms that need very specific combination of nutrients can be grown in selective medium

3. Inoculate the culture

• If broth used, swirl inoculating loop in culture

• If agar used, make streak or spread plate\

Describe methods of aseptic technique

All equipment should be sterile by flaming equipment with bunsen flame

Lids replaced as quickly as possible

Describe growth curve of microorganism in closed culture

1. Lag phase

• Microorganisms slowly adapting to environment.

• Reproduction rate increases slowly

2. Log Phase

• Microorganisms grow at maximum rate as long as ther eis sufficient nutrients

3. Stationary phase

• Death rate = reproduction rate

• Buildup of waste products and lack of nutrients

4. Death phase

• Death exceeds new cell population as conditions continue to deteriorate

Ways in which bacterial growth can be measured?

Cell count, Turbidimetry and Dilution plating

Describe process, and pros and cons of cell count

Haemocytometer

• Sample broth stained with trypan blue

• Useful because includes only counting living cells and is accurate

Cons:

• Slow

• Expensive equipment

Describe process, and pros and cons of turbudimetry

Form of colorimetry

As turbidity increases, transmission decreases

Use calibration graph to obtain cell count

Pros:

• Can be conducted in field & quick

Cons:

-Equipment is expensive

• Values affected by other variables

• Counts non-viable cells as well

• Calibration curve required

• Assumes density of cells is equal across the culture

Describe process, and pros and cons of dilution plating

Original culture serially diluted

Multiplied by dilution factor to obtain cell count

Pros:

• Doesn't require complex or expensive equipment

• Obtains direct cell count

Cons:

• Slow (incubation period)

• Dilutions required

What are endotoxins?

Lipopolysaccharides in outer lipid membrane of gram negative bacteria eg. Salmonella

Effects local to site of infection

Effects local to site of infection

What are exotoxins?

Soluble proteins produced and released by bacteria as they metabolise and reproduce eg. Staphylococcus

Spread around the body through blood and bodily fluids.

Spread around the body through blood and bodily fluids.

Host tissue invasion

Tuberculosis (Mycobacterium tuberculosis)