B1 - CELL LEVEL SYSTEMS (9-1 SPECIFICATION)

Eucaryotic Cells - clearly defined nucleus where chromosomes are located

Prokaryotic Cells - no membrane-bound organelles including a nucleus

Organelle Functions:

• Nucleus -> contains DNA as chromosomes, controls cell activity
• Cytoplasm -> where chemical reactions occur
• Mitochondria -> cellular respiration, contain enzymes
• Cell membrane -> holds cell, controls what substances leave / enter
• Rigid cell wall -> cellulose, gives cell support
• Chloroplasts -> photosynthesis, contain chlorophyll
• Chromosomal DNA -> one long circular chromosome, floats freely
• Plasmids -> small loops of extra DNA, passed between bacteria

PLANT CELLS ONLY

PROKARYOTIC CELLS ONLY

Microscopes:

• They are ued to magnify an image.

• They increase an image's resolution (how well two close points can be distinguished).

• Light microscopes allow us to see larger organelles e.g. nuclei, chloroplasts.

• Electron microscopes allow us to see smaller things in greater detail e.g. plasmids, viruses.

• Trandmission electron microscopes have higher resolution and magnification but are not portable, are expensive, and are complicated to use.

total magnification = eypiece lens magnification x objective lens magnification

image size = magnification x real size

DNA:

• Contains an organism's genetic material

• Chromosomes are long coils of DNA which is divided into shorter genes.

• DNA is a double helix where each DNA strand is made up of lots of nucleotides which join together to make a long polymer.

• Each nucleotide contains a base: A,T,G or C.

• A+T is complementary and G+C is complementary.

Nucleotides -> contain a sugar, phosphate and base

Polymer -> larger, complex molecules made from chains of smaller monomers (DNA is made from nucleotide monomers)

Proteins:

• Genes code for proteins.
• Proteins are made from chains of amino acids. Each proteins has a different number / order of amino acids so each protein has a different shape and function.
• Order of bases in a gene determines order of amino acids in a protein.
• Each amino acid is coded for by a triplet code of bases.

Protein Synthesis:

1. DNA contains gene coding for proteins.

2. Transcription -> In nucleus, DNA strands unzip around gene. DNA is used as a template for mRNA and base pairing ensures it's complementary.

3. mRNA moves out of nucleus and into cytoplasm.

4. Translation -> Amino acids matching triplet codes on mRNA are joined in correct order. The protein has been coded for by the gene.

Enzymes:

• Enzymes act as biological catalysts which speed up reactions without being used up themselves.
• Reduce the need for really high temperatures to speed up useful chemical reactions in the body.
• Every reaction has a specific enzyme and each enzyme is a protein coded for by a different gene so has its own unique shape.

The Lock and Key Hypothesis:

• Enzymes only work with one substrate.
• If the substrate's shape doesn't exactly match the enzyme's active site then the reaction won't be catalysed.

Temperature:

• Higher temp. = more movement = more likely for substrate and actiive site to collide.
• However when conditions are too hot, the enzyme's active site denatures so the substrate can no longer fit in.
• Optimum temp. is when the reaction goes fastest and is about 37 degrees in humans.

pH:

• If pH is too low / high the active site may denature enzymes.
• Optimum pH is often neutral 7 

Enzyme and Substrate Concentration:

• The more enzymes there are, the more likely one will meet with a substrate but eventually there will be more than enough enzyme molecules to deal with all available substrate.
• The higher the substrate concentration, the more likely the enzyme will will meet with a substrate but eventually all active sites will be full so adding more makes no difference.

Respiration -> the process of transferring energy from the breakdown of glucose

• Energy transferred can't be directly used by cells, makes ATP which stores it.
• Controlles by enzymes
• Exothermic reaction

Aerobic respiration:

• Occurs when there's plenty of oxygen available
• Most efficient way to transfer energy form glucose

glucose + oxygen -> carbon dioxide + water

Anaerobic Respiration:

• Transfers much less energy per glucose molecule than aerobic respiration.
• Occurs when you do vigorous exercise and your body can't supply enough oxygen to muscles.
• Lactic acid builds up in muscles which is painful and causes muscle fatigue.
• When you stop exercising you'll have an oxygen debt where you need extra oxygen to break down all of the built-up lactic acid.

glucose -> lactic acid

Anaerobic Respiration in Plants:

• In waterlogged soil where there is little / no oxygen
• Some fungi, e.g. yeast, respire anaerobically 

glucose -> ethanol + carbon dioxide

Biological structures can be broken down to fuel respiration.

Carbohydrates:

• Contain carbon, hydrogen, oxygen
• Monomers are simple sugars 
• Broken down by enxymes in mouth and small intestine

Proteins:

• Monomers are amino acids
• Amino acids contain carbon, nitrogen, hydrogen, oxygen
• Broken down by enzymes in stomach and small intestine

Lipids:

• Made from glycerol and fatty acids, not polymers
• Lipids contain carbon, hydrogen, oxygen
• Broken down by enzymes in small intestine

Photosynthesis:

• Energy from the sun is used to make glucose.
• Occurs inside chloroplasts where the chlorophyll absorbs light.
• Endothermic reaction.
• Light dependent stage - energy transferred by light is used to split water into oxygen gas an hydrogen ions.
• Light independent stage - carbon dioxide gas combines with hydrogen ions to make glucose.

carbon dioxide + water -> glucose + oxygen

Light:

• Light tranfers energy needed for photosynthesis so rate increases when light does but only up to a point.

Carbon Dioxide:

• Concentration of CO2 only increases rate up to a point.

Temperature:

• If temp. is the limiting factor it's usually because it's too low so enzymes are moving too slowly.
• If the plant gets too hot then enzymes denature so rate rapidly decreases. This normally occurs at about 45 degrees.