OCR A level Biology

Describe the nucleus

-surrounded by nuclear envelope

• contains chromatin
  -instructs for protein synthesis

Describe the nucleolus

Inside the nucleus
No membrane
Contains RNA
Where ribosomes are made

Describe the rough endoplasmic reticulum (RER)

-Membranes continuous with nuclear envelope
-fluid-filled cavities called cisternae
-Ribosomes attached
-Produce proteins
-A transport system

Describe the smooth endoplasmic reticulum (SER)

-membranes not continuous with nuclear membrane
-continuous with RER
-cisternae
-lipid enzymes
-absorption, synthesis and transport of lipids
-no ribosomes

Describe the Golgi Apparatus

-stack of membrane-bound flattened sacs
-modify proteins (e.g. add sugar to make glycoproteins)
-proteins packaged into vesicles

Describe lysosomes

-single membrane
-contain powerful hydrolytic enzymes
-Abundant in phagocytic cells
-can engulf old cell organelles
-keep enzymes separate from cell

Describe mitochondria

-double membrane
-inner membrane folded into cristae
-fluid-filled matrix
-self-replicating
-ATP production
-abundant in cells with lots of metabolic activity (muscle, liver etc.)

Describe chloroplasts

-double membrane
-only in plants
-thylakoids containing chlorophyll
-each thylakoid stack called a granum
-Stroma - fluid-filled matrix
-loops of DNA and starch grains
-site of photosynthesis
-Abundant in palisade mesophyll layer

Describe vacuole

-surrounded by tonoplast membrane
-contains water and solutes
-maintains stability (turgid when full)
-only in plants

What type of cell wall do plants have?

cellulose

What type of cell wall do bacteria have?

peptidoglycan

What type of cell wall do fungi have?

chitin

Describe the cellulose cell wall

-outside plasma membrane
-bundles of cellulose fibres
-prevents cell from bursting when turgid

• strength and support
• maintains cell shape
  -permeable
  -only in plants

Describe cilia and undulipodia

-protrusions surrounded by cell membrane
-contains microtubules
-formed from centrioles
-nearly all cells have cilia containing receptors

What type of cell has cilia and why?

Epithelial cells have cilia to move mucus

What type of cell has undulipodium and why?

A sperm cell has a undulipodium to move

Describe ribosomes . What is the difference between the functions of free ribosomes and RER ribosomes?

-no membrane
-made of ribosomal RNA

• made in nucleolus in two parts
  -those bound to RER, synthesise proteins for use outside cell
• those free in cytoplasm, assemble proteins for use inside the cell

Describe the process of protein synthesis

mRNA made in nucleus (transcription)
mRNA leaves nucleus through pore and attaches to ribosome.
Ribosome assembles protein (translation)
Protein travels in vesicle which fuses with Golgi apparatus
Golgi apparatus modifies and packages protein.
Protein travels to and fuses with plasma membrane in a vesicle.
Plasma membrane opens up to release protein (exocytosis)

What are the three features of the cytoskeleton and what are their functions?

Microfilaments -mechanical strength

• support and shape
  Microtubules - provide tracks for movement
• act as motors using ATP
  -made of tubulin
  Intermediate filaments - anchor nucleus within cytoplasm
  -extend between cells enabling cell to cell signalling
  All - work together to help cell move

Describe centrioles

Two bundles of microtubules at right angles to eachother. They are arranged to form a cyclinder.
Forms the spindle before the cell divides
Also involved in formation of cilia and undulipodium
Usually only in animals

Describe the difference between features of prokaryotes and eukaryotes.

Prokaryotes (e.g. bacteria) - no nucleus

• 70s ribosomes not 80s (smaller)
  -no centrioles
  -no membrane-bound organelles
• different cell wall (peptidoglycan or chitin)
  -naked DNA (in cytoplasm)
  -plasmids
  -waxy capsule
  -flagella
  -pili

What are the advantages of an optical microscope?

-relatively cheap
-easy to use
-portable
-can study whole living specimens

What are the disadvantages of an optical microscope?

Resolution is limited (0.2μ)
Only up to x1500 magnification
Cannot see ribosomes

What is magnification?

size of image/actual size

What is resolution?

The degree to which it is possible to distinguish two points that are close together

Describe a laser scanning microscope

Image displayed on computer
high resolution
series of optical sections
Microscopes have depth selectivity
Can study whole living organisms

What are the two types of electron microscope?

Transmission electron microscope and Scanning electron microscope

Describe a transmission electron microscope

resolution 0.1nm, high magnification
Specimen needs to be dehydrated and thinly sliced. It is then stained with metal salts
Black and white electron micrograph

Describe a scanning electron microscope

Resolution 10nm, high magnification
Gives 3D image of surface of whole specimen
Electron gun fired and secondary and back-scattered electrons used to generate image.
Specimens must be dried and coated (metal film)

What are the disadvantages of electron microscopes?

They are large
Expensive
Need skill and training to use
Specimens must be dead

What is methylene blue used for?

All purpose stain

What is differential staining?

Stains bind to specific cell structures staining each one differently

What does acetic orcein bind to and its colour?

Binds to DNA, staining chromosomes dark red

What does eosin stain and its colour?

Cytoplasm (pink)

What does sudan stain and its colour?

Lipids (red)

What does iodine in KI solution stain and what colour?

Cellulose (yellow)
Starch (blue/black)

What is the general formula for carbohydrates?

Cx(H2O)y

What are the properties of monosaccharides?

Sweet, soluble and crystalline
Reducing sugars

Draw alpha glucose

Draw beta glucose

What are the properties of disaccharides?

Sweet, soluble, crystals

Which disaccharides are reducing sugars?

maltose and lactose

Which disaccharide is a non-reducing sugar?

sucrose

How is a dissacharide formed?

Two monosaccharides join in condensation reaction forming a 1-4 glycosidic bond

How is maltose made?

α glucose + α glucose

How is lactose made?

α glucose and galactose

How is sucrose made?

α glucose + fructose

How is cellobiose made?

β glucose and β glucose

What are the properties of polysaccharides?

Insoluble, not sweet, not crystalline
Compact and dense granules (branched chains are more compact)
1-4 glycosidic bonds in chain, 1-6 glycosidic bonds to attach branches

Describe glycogen

Lots of branches (more than amylopectin)
Very compact
Granules
Soluble in water
Energy store in liver and muscles
Made up of α glucose.
Branches formed between c-1 and c-6

Describe amylose and amylopectin

Make up starch in plants
Amylopectin has branches but amylose doesn't
Form a spiral with h-bonds
Insoluble in cold water or alcohol
White tasteless powder
Energy store

Describe cellulose

Alternate B-glucose molecules upside-down. Unable to coil or form branches. 1,2 bonds. Cellulose molecules joined by H-bonds to form microfibrils, then macrofibrils, then fibres.
Insoluble in water
White powder
High tensile strength
Makes up cell wall (permeable, strong, flexible)

What are proteins and what elements do they contain?

Large polymers of amino acids (around 20)
contain C,H,O,N (and sometimes S)

What is the general structure of an amino acid?

Draw cysteine

Draw glycine

What is the primary structure of a protein?

the sequence of amino acids in the polypeptide chain, held together by peptide bonds

What is the secondary structure of a protein?

The folding of the polypeptide chain.
Stabilised by hydrogen bonds
Alpha helix or beta pleated sheet

What is the tertiary structure of a protein?

Folding of regions of secondary structure to give 3D shape.
Stabilised by bonds between R-groups
-Disulphide bridges
-Ionic bonds
-hydrogen bonds
-hydrophobic interactions

What is the quaternary structure of a protein?

How multiple polypeptide chains are arranged (e.g. in haemoglobin)

What are the properties of globular proteins?

Roughly spherical shape
Soluble in water

What are the functions of globular proteins?

messengers
enzymes (catalyse reactions)
structural support

Give three examples of globular proteins

Haemoglobin
Insulin
DNA polymerase

What are the properties of fibrous proteins

long and narrow
insoluble in water

What are the types of proteins?

fibrous and globular (and conjugated)

What is the function of fibrous proteins?

Offer structural support

Give three examples of fibrous proteins

Collagen-ligaments, tendons, muscle, cartilage
Keratin-horns, hooves, nails
Elastin -in connective tissue

What are three types of lipids?

triglycerides, phospholipids, steroids (e.g. cholesterol)

What is a triglyceride made of?

1 glycerol and 3 fatty acids joined with an ester bond in a condensation reaction

What are the functions of triglycerides?

-Energy source (ester bonds hydrolysed)
-Energy store (insoluble so doesn't affect water potential)
-Insulation (heat + electricity)
-Buoyancy (less dense than water)
-Protection (insoluble + shock absorber)

What are the difference between fats and oils?

Fats- saturated, solid at room temp
Oils- unsaturated, liquid at room temp

What is a phospholipid?

glycerol + 2 fatty acids + phosphate group

Describe the phospholipid bilayer

Phospholipid head is hydrophilic and the tail is hydrophobic. They form a double layer.
It forms membranes around cells and organelles
Selectively permeable - only small non-polar molecules can move through

What is cholesterol and what is its function?

Four carbon-based rings
Small and hydrophobic so sits in membrane regulating fluidity.
Some hormones are made from cholesterol

What is a wax?

A very long hydrocarbon chain. They waterproof

How do you test for a reducing sugar?

Add benedict's reagent and heat (and centrifuge). If reducing, it will go (blue to) brick red.
(It will be oxidised)

How do you test for a non-reducing sugar?

Add acid (to hydrolyse glycosidic bond) and heat
Then neutralise with sodium bicarbonate
Add benedict's reagent and heat (and centrifuge). It will go (blue to) brick red

How do you test for starch?

Iodine turns blue/black in the presence of starch

How do you test for a lipid?

Make an emulsion - dissolve in ethanol
-shake and leave (2-3 mins)
-Pour into water
-If a milky layer is formed, a lipid is present

How do you test for a protein?

Add to water and biuret
Shake and leave (5mins)
Blue to violet if protein present (not for amino acids)

What is a DNA nucleotide made of?

a phosphate group (negative charge), a deoxyribose sugar, and a nitrogen-containing base

Draw deoxyribose

What is a RNA nucleotide made of?

A phosphate group, a ribose sugar and a nitrogen-containing base

Draw ribose

How are DNA nucleotides joined together?

Phosphodiester bonds in condensation reactions

What are the DNA bases?

Adenine, Thymine, Cytosine, Guanine

What are the purines?

A double-ring base
Adenine and Guanine

What are pyrimidines?

Single ring bases
Cytosine, Thymine (and uracil)

What are the RNA bases?

Adenine, Uracil, Cytosine, Guanine

Which bases pair together?

Adenine and thymine (two hydrogen bonds)
Guanine and Cytosine (three hydrogen bonds)

DNA strands are…

Antiparallel with complementary bases.
They form a double helix

What are the two ends of a DNA strand called?

3 prime (3') and 5 prime (5')

What are the three forms of RNA?

messenger RNA, transfer RNA, ribosomal RNA

How many strands do DNA and RNA have?

DNA - 2 strands
RNA - 1 strand

Describe DNA replication

-DNA helicase splits the two strands by breaking hydrogen bonds
-DNA polymerase adds nucleotide bases from 5' to 3'
-Both strands act as a template
-On lagging strand, built in Okazaki fragments joined by DNA ligase
-occurs in both directions

What does semi-conservative mean?

one old strand and one new strand

What are four features of the genetic code?

Triplet code - three bases for one amino acid
Degenerate - more than one combination for the same amino acid
Universal - codons code for the same amino acid for most organisms
Non-overlapping - each codon is read discretely

Describe RNA transcription

-RNA polymerase attaches at start codon (3')
-breaks H-bonds to expose bases
-Nucleotides in cytoplasm activated (adding two phosphoryl groups)
-Complementary RNA base pairs with base on template strand
-RNA polymerase moves along the strand

• As the two RNA bases come close, the phosphoryl groups break off releasing energy to form a phosphodiester bond
  -The enzyme rewinds the DNA as it goes along
• reaches a stop codon and detaches

Describe RNA translation

-AUG is the start codon on the mRNA strand
-First tRNA with corresponding anticodons will hydrogen bond
-Ribosome binds and two codons can fit (moves 5' to 3')
-As the tRNA is released, its amino acid forms a peptide bond with the next one
-Continues until it reaches a stop codon (no complementary tRNA)

What is a polysome?

Several ribosomes using a mRNA molecule to synthesize protein at the same time