B1 Cell-level systems

mm to µm

1mm = 1000µm

Eukaryotic cells

• contain genetic material in a nucleus

• complex and relatively large

• sizes between 10-100µm

• plant & animal cells

Prokaryotic cells

• do not contain a nucleus, genetic material floats in cytoplasm

• simple and relatively small

• sizes between 1-10µm

• bacterial cells

Differences between plant and animal cells

Nucleus

controls the activities of the cell, and contains the organism’s genetic material (eukaryotic cells).

Cell membrane

a selective barrier that controls the movement of substances into and out of the cell.

Cytoplasm

a ‘jellylike’ substance; the chemical reactions that keep the cell alive occur here.

Mitochondria

where respiration happens; enzymes enable glucose and oxygen to react together to release and transfer energy in the form of ATP.

Cell wall

made of a fibre called cellulose (which makes the wall rigid and supports the cell).

Vacuole

full of cell sap, which also helps to keep the cell rigid and supports it.

Chloroplasts

contain chlorophyll, which transfer energy from the Sun to the plant, which is used in photosynthesis to make glucose. (eukaryotic)

Flagella

thin ‘tail-like’ structures that allow the cell to move through liquids. (prokaryotic)

Pili

tiny ‘hair-like’ structures that allow cells to attach to structures. Also used to transfer genetic material between bacteria. (prokaryotic)

Slime wall

protects cell from drying out and from poisonous substances. Also helps bacteria stick to smooth surfaces. (prokaryotic)

Plasmids

circular piece of DNA used to store extra genes. This where antibiotic resistance genes are normally found. (prokaryotic)

Process of applying stain to a colourless cell

1. place cells on a glass slide

2. add one drop of the stain

3. place a coverslip on top

4. tap the coverslip gently with a pencil to remove bubbles

Resolution

the smallest distance between two points that can be seen as separate entities.

Differences between light & electron microscopes

DNA

deoxyribonucleic acid - the molecule that contains genetic material in all organisms. It is a polymer of nucleotides.

Chromosome

a long molecule of DNA; all cells in the human body have 46 chromosomes (diploid) apart from gametes, which have 23 (haploid).

Gene

a small section of DNA on a chromosome; each gene codes for a specific sequence of amino acids to make a specific protein.

Genome

the entire genetic material of an organism.

The shape of DNA

double-helix structure/shape.

Composition of a DNA nucleotide

phosphate + deoxyribose + base (A,T,C,G)

The rule for base pairing

Complementary base pairing:

A — T

C — G

Transcription

the process by which a section of DNA (one gene at a time) is copied onto mRNA.

Formation of mRNA (stage 1 of transcription)

• RNA polymerase binds to the DNA just before the gene starts.

• the DNA around the gene unzips, separating the strands and exposing the bases.

• one DNA strand acts as the template strand.

• RNA polymerase lines up free RNA nucleotides complementary to the template strand to form the mRNA strand.

Stage 2 of transcription

• once the mRNA strand is formed, it detaches itself from the DNA template and the DNA zips back up.

• the mRNA strand exits the nucleus via a nuclear pore and travels to a ribosome for translation.

The base which replaces T in mRNA

Uracil

Why mRNA is important

DNA molecules are too large to leave the nucleus, which is why mRNA is required.

Translation

the process by which proteins are made at ribosomes.

Translation stage 1

• mRNA attaches to a ribosome.

• the ribosome ‘reads’ the nucleotides on the strand in groups of 3 (codon).

• each codon codes for a specific amino acid.

• the ribosome continues to read the codons, adding more and more amino acids to a chain.

• this chain is a protein.

The importance of the sequence of amino acids in a protein

determines how the protein will fold; each type of protein has a specific shape which is important for protein function.

Transcription & translation

Enzymes

biological catalysts (speed up the rate of a reaction) that do not get used up in a reaction.

Two types of enzyme-controlled reactions

• Anabolic - building larger molecules from smaller ones

• Catabolic - breaks larger molecules into smaller ones

Do enzymes bind to many different types of substrate molecules?

nah - they’re highly specific, and only bind to one type of substrate molecule.

Lock and key hypothesis

Factors affecting enzymes

• temperature

• pH level

• enzyme concentration

• substrate concentration

Effect of temperature on enzyme-controlled reactions

at higher temperatures the enzymes have more kinetic energy, increasing the frequency of successful collisions. the rate of reaction is faster.

at very high temperatures the enzymes’ active sites change shape, preventing them from binding to substrate molecules. this is denaturation.

Effect of pH level on enzyme-controlled reactions

every enzyme works best at a certain pH level (optimum pH).

if the pH level varies too much, the bonds holding the amino acid chain together may be broken or disrupted, changing the active sites’ shapes, preventing them from binding to substrate molecules. this is denaturation.

Effect of enzyme concentration on enzyme-controlled reactions

an increase in enzyme conc. increases the rate of reaction, because more active sites are available to bind to substrate molecules (increased frequency of collisions).

after a certain point, the rate of reaction doesn’t increase, as the enzyme conc. is no longer a limiting factor.

Effect of substrate concentration on enzyme-controlled reactions

an increase in substrate conc. increases the rate of reaction, because more substrate molecules are available to bind to active sites (increased frequency of collisions).

after a certain point, the rate of reaction doesn’t increase, as all the active sites are saturated. the only way to increase rate of reaction is to increase enzyme conc.

Cellular respiration

the process of transferring energy from the breakdown of glucose. it is a universal chemical process continually occurring in all living cells.

How is energy from cellular respiration stored?

in the form of ATP.

Why do organisms require energy?

• chemical reactions (to synthesise larger molecules from smaller ones)

• movement (ATP enables muscles to contract)

• warmth (constant temp. for enzyme activity)

Aerobic respiration

• requires oxygen

• completely breaks down glucose molecules to release a relatively large quantity of energy.

Anaerobic respiration

• does NOT require oxygen

• the incomplete break down of glucose molecules releases significantly less energy compared to aerobic respiration.

Why does anaerobic respiration occur?

• during strenuous exercise, muscles need to release more energy than normal

• our heart rate & breathing rate cannot increase fast enough to supply muscle cells with enough oxygen

• glucose is not completely oxidised, releasing less energy quicker.

Why do we normally respire aerobically?

• aerobic respiration produces more ATP molecules per glucose molecule than anaerobic respiration.

• lactic acid from anaerobic respiration can cause cramps. the build up of lactic acid in muscle cells causes pain, and the muscles stop contracting - fatigue.

Oxygen debt

the quantity of extra oxygen required to break down the lactic acid which has built up.

Anaerobic respiration in microorganisms and plant cells

• fermentation

• occurs in anerobic conditions ✌😅 (e.g. in the roots of plants in waterlogged soils)

Metabolic rate

the rate at which all chemical reactions in the body occur.

Different groups of carbohydrates

• monosaccharides - simple sugars consisting of a single monomer unit (e.g. fructose, glucose)

• disaccharides - consist of two monosaccharides joined together (e.g. maltose is formed from two glucose molecules)

• polysaccharides - lots of monomer units join together in a long chain to form a polymer (e.g. starch is formed when lots of glucose molecules join together).

Enzyme digesting carbohydrates

carbohydrase; however, amylase is used to break down starch

Proteins

• synthesised from different sequences of amino acids.

  • there’s 20 different types of amino acids

Enzyme digesting proteins

protease

Lipids

• synthesised from 3 fatty acid molecules & 1 glycerol molecule

Enzyme digesting lipids

lipase

Test for sugars

• Benedict’s test

• positive test = dark red/orange

• negative test = remains blue

Test for starch

• Iodine test

• positive test = blue-black

• negative test = orange-brown

Test for protein

• Biuret test

• positive test = purple / violet

• negative test = blue

Test for lipids

• Emulsion test

• postive test = cloudy emulsion ☁

• negative test = no change (colourless)

Photosynthesis

the process by which autotrophs use light energy, CO₂ & H₂O to produce glucose.

Photosynthesis equation

Site of photosynthesis

chloroplasts in leaves (which contain chlorophyll). the pigment absorbs sunlight, where the energy is used to react CO₂ & H₂O to produce C₆H₁₂O₆.

Stage 1 of photosynthesis

energy transferred from light splits water molecules into oxygen gas & hydrogen ions.

Stage 2 of photosynthesis

carbon dioxide combines with the hydrogen ions to make glucose.

Uses of glucose produced in photosynthesis

• converted to starch to provide an energy store for respiration at night

• converted into cellulose to form cell walls

Limiting factors of photosynthesis

• carbon dioxide

• temperature

• light intensity

Effect of carbon dioxide concentration on rate of photosynthesis

• the more CO₂, the faster the rate of photosynthesis until a certain point

• once that point is reached, some other factor prevents the rate of photosynthesis increasing further

• CO₂ is no longer the limiting factor

Effect of temperature on rate of photosynthesis

• increasing temperature = increasing kinetic energy of reactant particles

• frequency of successful collisions increases

• at very high temperatures, enzymes controlling photosynthesis denature

Effect of light intensity on rate of photosynthesis

• the more light a plant receives, the faster the rate of photosynthesis until a certain point

• once that point is reached, some other factor prevents the rate of photosynthesis from increasing further

• light intensity is no longer a limiting factor

The inverse square law