Topic 2: sub 1 Flashcards
exchange surfaces:
-multicellular organisms require gas exchange systems in order to obtain sufficient oxygen for respiring cells, and to expel the carbon dioxide created by these cells
-in order to maximise the rate of exchange of substances, gas exchange surface are adapted to have a:
-large surface area:volume ratio- the larger the ratio, the greater the surfacearea for the organism to carry out exchange, so a faster transfer of substances across the surface
-short diffusion pathway- a short distance for substance to move across means they move faster
-steep concentration gradient- a large difference in concentrations between 2 areas means diffusion of particles from an area high concentration to an area of lower concentration occurs faster
Fick’s Law:
-the rate of diffusion is also dependent on the main features
-this is shown by Fick’s law that states that the rate of diffusion is proportional to the surface area multiplied by difference in concentration, divided by the length of diffusion pathway
Rate of diffusion (a) surface area x concentration difference/distance
the mammalian lung:
-when we breathe, air enters the mouth, passes into the trachea which splits into 2 bronchi; one on the left side and one on the right side of the body
-each broncus branches out into smaller bronchioles which end in tiny air sacs called alveoli, where gas exchange takes place
-deoxygenated blood flows into the alveoli, where carbon dioxide diffuses out of the capillary, through the alveolar membrame and into the surrounding air, down a concentration gradient
-oxygen moves in the opposite direction from the surrounding air and into the blood steam, making the blood oxygenated
the mammalian lung: alveoli adaptations
-the extensive branching of vessels in the lungs means there are many alveoli-over 300 million in an average adult- this gives them a very large surface area which increases the rate of diffusion of oxygen into the blood, and carbon dioxide out of the blood
-the alveoli are adapted further, they have a rich blood supply from surrounding capillaries which mantains a steep concentration gradient between the blood in the capillaries and the air entering the lungs, which again increases the rate of diffusion
-alveoli also have a moist outer lining, allowing gases to dissolve and move across their membrame faster
-finally, alveoli have a wall of only one cell thick and pores in the endothelium, this creates a short distance for the gases to travel over so they can diffuse quickly
cell membrames: structure
-all cells and organelles are surrounded by a partially permeable membrame composed of a sea of phospholipids with protein molecules between the phospholipid molecules
-the main function of the membrame is controlling the movement of substances in and out if the cell/organelle
-however, it also contains receptors for other molecules, such as hormones, and enables adjacent cells to stick together
-the main structure of a membrame is the phospholipid bilayer-2 rows of phospholipids (lipids made from 1 molecule of glycerol, 1 phosphate group and 2 fatty acid chains)
-the phosphate groups are hydrophilic (water loving), so form the outside of the bilayer
-whereas fatty acid chains are hydrophobic (water hating), so lie in between the 2 rows of phosphate heads
-the fatty acid chains ae non-polar, allowing non-polar molecules like carbon dioxide to pass straight through the phospholipid bilayer
-while polar substance, like water, have to move though channel proteins, since they aren’t soluble in the fatty acid tails
Fluid Mosaic Model:
-the fluid mosaic model is the name given to the model suggested for the structure of the cell membranes
-it is described as ‘fluid’ due to the fluidity of the phospholipid bilayer which allows all molecules to movel freely within it
-the cell membrame also contains a ‘mosaic’ of transport proteins, receptor proteins, enzymes, structural and recognition proteins of varying shapes and sizes
-cholesterol molecules are also found in the bilayer, these give the membrame stability and reduce its fluidity
-scientific models such as this one are based on data and results of investigations by scientists
-models like this may be update over time, the way the model of the atom was updated numerously, as new data and discoveries are made
transport of substances:
-the movement of molecules through cell membrane depends on the properties of the molecule (for instance its size and whether its polar or nonpolar) as well as the requirements of the cell
-movement can be passive (require no energy) or active (requires energy released from respiration)
transport of substances: diffusion
-> is the passive movement of molecules down a concentration gradient, from an area of high concentration to an area of lower concentration through a partially permeable membrame
-these are specific types of diffusion- simple diffusion, facilitated diffusion and osmosis
transport of substances: simple diffusion
-> is the passive movement of small, non-polar lipid soluble molecules, such as carbon dioxide and oxygen, from an area of high concentration to an area of low concentration
-the molecules move directly through the phospholipid bilayer
transport of substances: facilitated diffusion
-> requires a membrame protein to transport polar molecules, charged and water soluble molecules across the membrame
-since these molecules cannot pass through the non-polar inside of the bilayer
transport of substances: active transport
-> can transport all types of molecules through carrier proteins from an area of low concentration to an area of high concentration
-this process moves particles against the concentration, and so requires energy in the form of ATP
transport of substances: exocytosis + endocytosis
-> transport large particles
-the particles are enclosed in vesicles made from the cell surface membrane and transported into the cell/ organelle in endocytosis
-in exocytosis, vesicles containing large particles are fused with the membrane and leave the cell/organelle
osmosis: a special type of diffusion
-> is the net diffusion of water molecules from an area of higher water potential to an area of lower water potential, through a partially permeable membrane
-in the case of osmosis, ‘water potential’ is used to describe the relative concentration of water molecules
-water potential is the tendency of a solution to gain or lose water
-a high water potential means there is a low concentration of solute, in other words a high concentration of water
-pure water has the highest water potential possible
-water potential is measured in pascals, pure water has a value of 0 pascals and all other values are negative, as water potential becomes lower
osmosis: isotonic, hypetonic and hypotonic
isotonic: solutions have equal osmotic pressures, so there is not net water movement, and the size of the cell remains the same
-hypotonic: solutions have a lower osmotic pressure, so water will enter the cell, and the cell will swell
-hypertonic: solutions have a high osmotic pressure, so water will leave the cell, and the cell will shrink
membrame proteins:
-carrier proteins: can move particles through the membrame by both active transport and facilitated diffusion
-channel proteins: forms pores in the membrame for polar particles to move through by facilitated diffusion
-extrinsic proteins: a membrame protein that goes through only 1 layer of the bilayer
-intrinsic proteins: a membrame protein that goes through both layers of the bilayer
Enzymes:
- Enzymes are biological catalysts that increase the rate of reaction by lowering the activation energy (the energy needed for a reaction to
occur) of the reactions they catalyse, including both intracellular (within cells) and extracellular (outside the cells) reactions
-part of the enzyme is known as the active site, this is where the reaction with the substrate takes place
-since the enzymes are proteins, they have a very specific 3D shape due to the bonding in their tertiary structure
-this means enzymes have a specific and complementary shape to the substrate they bind to, meaning that only one type of substrate fits into the active site of the enzyme
-when the enzyme and substrate form a complex (an enzyme-substrate complex), the tertiary structure of the enzyme is altered so that the active site of the enzyme fits around the substrate
-this is called the induced fit model
-once an enzyme-substrate complex is formed and there is sufficient energy, the reaction can take place, often breaking down the substrate or combining 2 to build up larger molecules
Nucleotides:
-both deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are polymers of nucleotides which consist of:
1. A Penrose sugar - a 5 carbon sugar
2. A nitrogen contIning organic base
3. A phosphate group
-nucleotides join together by phosphodiester bonds formed in condensation reactions
-the phosphodiester bonds are formed between the phosphate groups and a carbon on the pentose sugar of adjacent nucleotides
Nucleotides: DNA
-the components of a DNA nucleotide are a pentose sugar called deoxyribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or thymine
-adenine and guanine both have double ring structure and are classified as purine bases
Nucleotides: RNA
-the component of an RNA nucleotide are a pentose sugar called ribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or uracil
-thymine, uracil and cytosine all have single ring structure and are classified as pyramidine bases
Complementary base pairing:
-a DNA molecule is a double helix composed of 2 polynucleotides chains running antiparallel to each other, joined together by hydrogen bonds between the 2 strands
-2 hydrogen bonds form between the complementary bases adenine and thymine, and 3 hydrogen bonds form between the complementary bases cytosine and guanine
-this bonding is essential for DNA to maintain a stable structure, as it prevents the chemical bases being corrupted by other outside chemicals or forces
-the bonding is also what twists the strand into the double helix shape
-RNA is a single-stranded polynucleotide chain, so does not have hydrogen bonding between the bases within it, however its bases are still complementary to each other- cytosine and guanine, and adenine and uracil are complementary
DNA replication: and the process
-the semi-conservative replication of DNA ensures genetic continuity between generations of cells, meaning that genetic information is passed on from one generation from the next
-the process is described as ‘semi-conservative’ because each strand formed contains 1 new strand, and 1 original strand
-the steps of semiconservative replication of DNA are as follows:
1. The double helix unwinds and the enzyme DNA Helicase breaks the hydrogen bonds between the complementary bases, separating the two strands of DNA
2. One of the strand is used as the template and complementary base pairing occurs between the template strand and free nucleotides that attach to the exposed strands
3. The enzyme DNA polymerase moves along the strands, joining the adjacent nucleotides by forming phosphodiester bonds in condensation reactions
Meselson and Stahl’s experiment:
2 scientists names Matthew Meselson and Franklin Stahl did an experiment to prove that DNA is replicated semi-conservatively, this is what they did:
1. They began by growing bacteria in a broth containing only one isotope of nitrogen- Nitrogen-15, a heavier isotope than the more abundant Nitrogen-14
-after allowing the bacteria to grow and take the heavy nitrogen into their DNA they isolated the DNA and centrifuged it; the DNA settles at a point equal to it’s density
2. The bacteria containing only ^15N DNA were transferred to a broth now only containing a ^14N medium
-since DNA replicates semi-conservatively, after the first replication of DNA, each double helix contained 1 ^14N strand, and 1 ^15 strand, so settled at a different point when centrifuged
-the scientists observed this was between the settling points of ^15N only DNA, and ^14N only DNA, so concluded the DNA must contain one strand of each
3. They then carried out a second replication of the DNA, still in the ^14N medium, the results of the centrifuge conformed the semi-conservative hypothesis still, with half of the DNA settling out at the ^14N only point, and half of the DNA settling with a combing ^14N and ^15N point
The genetic code:
-the order of nucleotide
bases on DNA makes up the genetic code which consist of triplets of bases; each triplet of bases codes for a particular amino acid and is known as a codon
-for instance, the codon GCA codes for the amino acid alanine
-in ribosomes during protein synthesis the amino acids are joined together by peptide bonds and form a polypeptide chain
-therefore, a gene is a sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide chain
-however, not all of the genome codes for proteins-the non-coding sections of DNA are called introns and the coding regions are called exons
Features of the genetic code:
-the genetic code is-overlapping meaning that each triplet is only read once and triplets don’t share any bases
-genetic code is also degenerate meaning that more than one triplet codes for the same amino acid, this reduces the number of mutations which are mistakes in the base sequence such as base deletion, insertion or substitution
-for instance, GCA, GCC, GCG, and GCU all code for the amino acid alanine
-the genetic code contains start and stop codons which either start or stop protein synthesis
