MATSEC AM Biology flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Water potential

The tendency of water to move from one region to another, measured in kPa; pure water at atmospheric pressure is 0 kPa, and solutes lower it to negative values, driving osmosis into cells.

Glycosidic bond

A covalent bond formed by condensation between the hydroxyl groups of two monosaccharides, releasing water; it can be hydrolysed by enzymes such as amylase to regenerate the sugars.

Cellulose structure

A polymer of β-glucose joined by β-1,4 glycosidic bonds; alternate glucose units are inverted, producing straight chains held by hydrogen bonds into strong microfibrils that resist tension in plant cell walls.

Quaternary structure

The arrangement of two or more folded polypeptide subunits into a single functional protein, held by the same bonds as tertiary structure; haemoglobin with four subunits is a classic example.

Induced fit model

The explanation of enzyme specificity in which the active site changes shape slightly as the substrate binds, moulding around it to strain bonds and lower activation energy more effectively than a rigid lock-and-key site.

Competitive inhibition

An inhibitor resembling the substrate binds reversibly to the active site, so its effect is reduced by raising substrate concentration; it increases the apparent Michaelis constant but leaves maximum rate unchanged.

Non-competitive inhibition

An inhibitor binds a site other than the active site, altering the enzyme's shape so catalysis slows; raising substrate concentration does not overcome it, and maximum rate falls while the Michaelis constant is unchanged.

Magnification formula

Magnification equals image size divided by actual size; rearranged, actual size equals image size divided by magnification. Units must be converted consistently, usually to micrometres for cells.

Prokaryotic ribosome

A 70S ribosome found in bacteria and archaea, made of 50S and 30S subunits, smaller than the eukaryotic 80S ribosome; this difference is exploited by antibiotics such as tetracycline.

Fluid mosaic model

The description of membranes as a phospholipid bilayer with proteins, cholesterol and glycoproteins able to move laterally within it, accounting for selective permeability, flexibility and cell recognition.

Osmosis

The net movement of water across a partially permeable membrane from a region of higher water potential to lower water potential, driven by solute concentration differences and requiring no metabolic energy.

Active transport

Movement of ions or molecules against their concentration gradient through carrier proteins, requiring energy from ATP hydrolysis; examples include the sodium-potassium pump and glucose uptake in the ileum.

Crossing over

Exchange of equivalent chromosome segments between non-sister chromatids of a homologous pair during prophase I of meiosis, producing recombinant chromatids and increasing genetic variation in gametes.

Independent assortment

The random orientation of homologous pairs at metaphase I, so maternal and paternal chromosomes of different pairs are distributed to gametes independently, generating 2ⁿ possible combinations.

Bohr shift

A rightward shift of the haemoglobin dissociation curve caused by raised carbon dioxide, lower pH or higher temperature, reducing affinity so oxygen is released more readily in respiring tissues.

Cohesion-tension theory

Explains water movement up xylem: evaporation from leaves creates tension, and cohesion between water molecules plus adhesion to walls transmits the pull upward through continuous water columns.

Pressure flow hypothesis

Explains phloem translocation: sucrose loaded at a source lowers water potential, water enters and raises pressure, pushing sap to a sink where sucrose is unloaded and water leaves.

Glycolysis

The anaerobic first stage of respiration in the cytoplasm, where glucose is phosphorylated and split into two pyruvate molecules, yielding a net gain of two ATP and two reduced NAD.

Chemiosmosis

Synthesis of ATP as protons flow down their electrochemical gradient through ATP synthase in the inner mitochondrial membrane or thylakoid membrane, following electron transport.

Light-dependent reactions

Thylakoid reactions in which light excites photosystems, water is split to release oxygen and electrons, and the resulting proton gradient drives ATP synthesis while NADP is reduced.

Calvin cycle

Light-independent reactions in the stroma where rubisco fixes carbon dioxide onto ribulose bisphosphate; ATP and NADPH reduce the product to triose phosphate, regenerating the acceptor.

Limiting factor

The factor in shortest supply that restricts the rate of a process such as photosynthesis; increasing it raises the rate until another factor becomes limiting.

Negative feedback

A control mechanism in which a deviation from a set point triggers responses that reduce the deviation, restoring stability; it underlies thermoregulation and blood glucose control.

Insulin action

Insulin, secreted by pancreatic beta cells when blood glucose rises, increases glucose uptake by cells, stimulates glycogenesis and fat synthesis, and lowers blood glucose concentration.

ADH

Antidiuretic hormone, released from the posterior pituitary when blood water potential falls; it increases collecting duct permeability to water, so more water is reabsorbed and urine becomes concentrated.

Test cross

A cross between an individual showing a dominant phenotype and a homozygous recessive individual; offspring ratios reveal whether the tested individual is homozygous dominant or heterozygous.

Semi-conservative replication

DNA replication in which each new double helix contains one original parental strand and one newly synthesised strand, confirmed by the Meselson-Stahl density gradient experiment.

Degenerate genetic code

The property that most amino acids are encoded by more than one codon, so several different base triplets specify the same amino acid, reducing the effect of some point mutations.

Frameshift mutation

Insertion or deletion of bases not in multiples of three, shifting the reading frame so all subsequent codons change, usually producing a non-functional protein.

Ecological efficiency

The percentage of energy at one trophic level transferred to the next, typically about 10 %; the rest is lost as heat through respiration, in excretion and in uneaten material.

Carrying capacity

The maximum population size an environment can sustain indefinitely given available resources; logistic growth levels off at this value, shown as the plateau of an S-shaped curve.

Nitrogen fixation

Conversion of atmospheric nitrogen gas into ammonia by bacteria such as Rhizobium in root nodules or free-living Azotobacter, making nitrogen available for amino acid and nucleic acid synthesis.