25 marker

Describe mass flow hypothesis from the mechanism of translocation

-sucrose is co-transported from source cells to sieve tube element with H+ ions

-ATP is produced by companion cells, which is hydrolysed to release energy for this active transport

-movement of sucrose into the sieve tube element decreases the water potential gradient in comparison to the xylem tissue, water moves down the gradient into sieve tube element by osmosis, increases the hydrostatic pressure

-sucrose moves down the hydrostatic pressure gradient by mass flow to respiring cells

-sucrose unloaded by active transport into the sink cell, sink cell stores sucrose as starch or uses it in respiration, sucrose composed of alpha glucose and fructose

-alpha glucose is a monomer of starch

semi conservative replication

-DNA helicase breaks hydrogen bonds between complementary bases in the polynucleotide strands

-causing the double helix to unwind

-separated parental strands of DNA act as a template

-free floating DNA nucleotides within the nucleus attracted to complementary base pairs on template strands

-adjacent nucleotides joined in condensation reaction, forming phosphodiester bonds

-catalysed by DNA polymerase

-2 sets of daughter DNA, 1 parental strand and 1 newly synthesized

describe the stages of co-transport

• Glucose cannot diffuse simply across the ileum epithelial cell membrane as it is a large polar molecule, so must be transported into the cell by co-transport with sodium ions. There is a lower concentration of Na+ inside the ileum epithelial cell compared to the bloodstream, so 3Na+ are actively transported out of the ileum epithelial cell through the Na+/K+ pump (in exchange for 2K+ ions moving into the cell). This step establishes a higher concentration of sodium ions in the ileum epithelial cell compared to the lumen. Consequently, Na+, coupled with the active uptake of glucose, is transported by facilitated diffusion through SGLT1 (a symporter) which is embedded in the ileum epithelial cell membrane. This establishes a higher concentration of glucose in the epithelial cell than in the blood, so glucose is able to be transported by facilitated diffusion down the concentration gradient - through GLUT2 and be absorbed into the bloodstream. 

• Vital for the absorption of glucose - an important respiratory substrate in the body, used in aerobic respiration to produce ATP that can be hydrolysed to release energy. This energy is vital for processes such as muscle contraction 

ATP

-immediate source of energy in organisms

-ATP contains phosphoanhydride bonds between the 3 phosphates, allows rapid release of energy during ATP hydrolysis

-phosphoanhydride bonds are high energy bonds, releasing large amounts of energy when broken, why ATP is immediate energy source and not stored

-allows organisms using ATP to respond to sudden changes in energy demand, such as exercising, increasing demand for ATP to fuel muscle contraction

-muscle cells therefore have a high concentration of mitochondria to produce this

-inner mitochondrial membrane folded to form the cristae, increase SA for proteins and enzymes to attach such as ATP synthase

-increased conc of synthase increases efficiency of ATP production in the condensation reaction ADP+Pi, this increase in ATP production allows for more muscle contraction to occur

-beneficial in organisms for completing exercise, for maintaining healthy bodily function

importance of phosphodiester bonds

-strong covalent bonds, formed by the condensation of nucleotides and consist of a phosphate group and 2 ester bonds

-form between the phosphate group of one nucleotide

-and the pentose sugar of another

-chain of alternating phosphate groups and pentose sugars forms the sugar-phosphate backbone

-backbones shield the genetic information, held by weak hydrogen bonds and protects it from being damaged, backbone is hydrophilic as phosphates negatively charged while nitrogenous bases are hydrophobic so are protected from reacting with water

-damage to the bases can lead to potential mutations so this is prevented

Antibodies

Antibodies are globular proteins produced by plasma cells in the humoral response, consisting of amino acid monomers joined by peptide bonds.

They have a quaternary structure made up of four polypeptide chains – 2 heavy chains and 2 light chains that are held together by disulphide bridges, thus allowing the antibody to maintain its specific antigen binding site. The amino acid sequence in the primary structure determines the tertiary structure of the antibody’s variable regions by dictating whether ionic bonds, hydrogen bonds and disulphide bridges can form - thus producing an antigen binding site with a complementary shape to a specific antigen. this allows antigens to bind to the antibodies and form an antigen-antibody complex.

As each antibody possesses two antigen-binding sites, it can bind to two antigens simultaneously, causing the agglutination of pathogens. This makes it easier for the pathogens to be located by phagocytes, thereby allowing the pathogen to be more effectively engulfed and destroyed through phagocytosis

Respiration (oxidative phosphorylation)

• The reduced NADH from previous respiratory stages (e.g. glycolysis, as just discussed) is oxidised – releasing a hydrogen atom that splits into an electron and an H+ ion. This hydrogen ion is released into the mitochondrial matrix, and is pumped across the inner mitochondrial membrane into the intermembrane space (using energy released by electrons moving down an energy level in the electron transport chain). The movement of these hydrogen ions is important in establishing an electrochemical gradient, with a higher concentration of H+ ions in the intermembrane space than the mitochondrial matrix. Therefore, hydrogen ions move by facilitated diffusion back across the membrane through ATP synthase. This movement provides energy for the phosphorylation of ADP to ATP, where phosphate ions are used to synthesise new ATP molecules. After the electrons have left the electron transport chain, they are accepted by oxygen as the final electron acceptor in the matrix. The electrons combine with the hydrogen ions to form a hydrogen atom, which then combines with oxygen to form water. 

• The importance of the hydrogen ions is that (when combined with electrons and oxygen) it allows the formation of water, where oxygen acts the final acceptor of electrons. If oxygen didn’t accept electrons, then no more electrons would be able to move down the chain of protein complexes. This would release no energy for the active transport of hydrogen ions across the membrane, so therefore no electrochemical gradient would be produced, and consequently no ATP made. (no NADH and FADH oxidised, therefore no regenerated coenzymes to return to Krebs - Krebs stops)

• Importance of Krebs - it produces hydrogen atoms that are carried by NAD to the electrons transport chain and provide energy for oxidative phosphorylation. This leads to the production of ATP that provides metabolic energy for the cell 

• Importance of respiration is the production of ATP, which is an immediate energy source for cells. For example, root hair cells require the hydrolysis of ATP to provide energy for the active uptake of mineral ions against the concentration gradient - name ion (importance of ATP also in K+ pumps in opening stomata)

viral replication 

• Gp120 attachment proteins embedded in the lipid envelope of the HIV have a complementary shape to the tertiary structure of the binding site on CD4 receptors embedded into the cell surface membrane of helper T-cells. This allows them to bind, which causes the HIV lipid envelope to fuse with the cell surface membrane of the helper T cell. The capsid releases viral RNA, reverse transcriptase and integrase into the helper T cell, which enables the synthesis of new DNA that can be used to transcribe viral particles. Once the viral particles have assembled, the virus buds off of the helper T cell, using part of its cell surface membrane to make the lipid envelope 

• This ls is vital for the reproduction of the HIV, as viruses are acellular and unable to reproduce on their own without a host cell. Do not have the subcellular structures to carry out their own metabolic reactions

synaptic transmission

1. The arrival of an action potential depolarises the pre-synaptic membrane. This causes voltage-gated calcium channels in the pre-synaptic membrane to open, and as a result, Ca2+ enter the cells by diffusion 

2. Calcium ions cause synaptic vesicles to fuse with the presynaptic membrane. This results in the exocytosis of the neurotransmitter acetylcholine 

3. The acetylcholine neurotransmitter diffuse across the synaptic cleft and bind to receptor molecules in the post-synaptic membrane 

7. Binding of neurotransmitters causes voltage-gated sodium ion channels to open, allowing sodium ions to diffuse into the post-synaptic neurone by facilitated diffusion, depolarising the membrane

8. If the depolarisation exceeds the threshold, an action potential is generated

8. Effects are short-lived because the neurotransmitter is quickly hydrolysed by enzymes. This causes ion channels to close, and the synaptic response is terminated. Acetylcholinesterase hydrolyses acetylcholine --> acetate + choline. Choline is actively reabsorbed into the presynaptic neurone where it reforms acetylcholine