Topic 5A p4 Flashcards
(19 cards)
Semi-Quantitative
A semi-quantitative method provides an approximate measurement of the amount of a substance but not an exact value. It offers a relative comparison between samples rather than a precise numerical result. For example, comparing the intensity of colour in test strips can suggest more or less of a substance without stating the exact concentration.
Semi-Qualitative
A semi-qualitative method focuses on identifying whether a substance is present and may suggest its relative level or category, but it is not strictly numerical. It gives more information than a purely qualitative test, which simply shows presence or absence, but less than a full quantitative analysis. For instance, using chromatography to compare how strongly substances interact with the solvent and stationary phase is semi-qualitative.
Silica Gel
Silica gel is a porous, granular form of silicon dioxide used as the stationary phase in thin-layer chromatography (TLC). It helps separate compounds based on their polarity and solubility. Non-polar substances move further up the plate, while polar ones stick to the silica gel more strongly. It is also commonly used as a drying agent due to its high absorbency.
Rf Values
Rf (retention factor) value is a ratio used in chromatography to indicate how far a substance moves relative to the solvent front. It is calculated as:
Rf = distance moved by substance ÷ distance moved by solvent.
Rf values are used to identify compounds by comparing them with known standards under the same conditions.
Photosystem
A photosystem is a protein–pigment complex in the thylakoid membrane that captures light energy for photosynthesis. It includes a reaction centre chlorophyll molecule and an antenna complex of pigments that gather light. There are two main types: Photosystem I (PSI) and Photosystem II (PSII), each playing a specific role in the light-dependent reactions.
Photosystem I (PSI)
PSI is the second photosystem in the light-dependent stage of photosynthesis, though it was discovered first. It absorbs light best at 700 nm and is involved in producing reduced NADP (NADPH). It receives electrons from PSII and re-energises them using light energy before transferring them to NADP⁺.
Photosystem II (PSII)
PSII is the first photosystem to function in the light-dependent reactions. It absorbs light best at 680 nm and is responsible for splitting water molecules (photolysis) to release oxygen, protons, and electrons. These electrons are passed through the electron transport chain to PSI.
Intergranal Lamellae
Intergranal lamellae are membranous extensions of the thylakoid membrane that connect grana (stacks of thylakoids) to each other. They help distribute energy and materials between grana and contain some of the photosynthetic pigment-protein complexes. This structure helps maintain the organisation and efficiency of the chloroplast.
Light-Dependent Reactions
These are the first stage of photosynthesis, occurring in the thylakoid membranes. They require light to excite electrons in chlorophyll, leading to the production of ATP and reduced NADP (NADPH). Oxygen is also released as a by-product from the photolysis of water. The energy products fuel the next stage.
Light-Independent Reaction
Also called the Calvin Cycle, this stage occurs in the stroma of the chloroplast. It uses ATP and NADPH from the light-dependent reactions to fix carbon dioxide into organic molecules like glucose. It does not directly require light, but depends on the products of the light-dependent phase.
Cyclic Photophosphorylation
In cyclic photophosphorylation, only Photosystem I is involved. Electrons are excited by light and passed through an electron transport chain back to PSI, generating ATP but no NADPH or oxygen. It is used when the cell needs more ATP than NADPH.
Non-Cyclic Photophosphorylation
This is the main pathway of light-dependent photosynthesis involving both PSI and PSII. Electrons move from water (via PSII) through the transport chain to NADP⁺, producing ATP, NADPH, and oxygen. It is a linear, one-way flow of electrons.
NADP (Nicotinamide Adenine Dinucleotide Phosphate)
NADP⁺ is a hydrogen carrier that becomes reduced to NADPH by accepting electrons and hydrogen ions during the light-dependent stage. NADPH then supplies reducing power to the Calvin cycle in the stroma. It is essential for the synthesis of carbohydrates from CO₂.
Photochemical Reaction
A photochemical reaction is a chemical reaction initiated by light energy. In photosynthesis, light is absorbed by chlorophyll, causing electrons to become excited and leave the molecule. This triggers a chain of reactions including electron transport and ATP formation.
Temperature-Sensitive Reactions
These are reactions that depend on enzyme activity, and are therefore affected by temperature. In photosynthesis, the light-independent reactions are temperature-sensitive because enzymes like RuBisCO are involved. Low temperatures slow the rate, and high temperatures may denature the enzymes.
Excite an Electron
To excite an electron means to supply it with energy, causing it to move to a higher energy level or orbital within an atom or molecule. In photosynthesis, this is done using light energy absorbed by chlorophyll. Excited electrons can then be transferred through the electron transport chain.
How Can You Excite an Electron?
Electrons can be excited by absorbing light energy (photons) or by receiving energy from nearby excited molecules. In photosynthesis, chlorophyll absorbs photons, and the energy raises electrons to a higher energy level. This is the initial step in converting solar energy to chemical energy.
Why Do Electrons Get Excited?
Electrons get excited when they absorb sufficient energy (often from sunlight), which boosts them to higher orbitals. This occurs in photosynthesis to allow the electron to leave the chlorophyll molecule and enter an electron transport chain. This energy is later used to produce ATP and NADPH.
RuBP (Ribulose-1,5-bisphosphate)
RuBP is a 5-carbon sugar that acts as the carbon dioxide acceptor in the Calvin cycle. It reacts with CO₂ in a reaction catalysed by the enzyme RuBisCO. The resulting 6-carbon compound quickly splits into two molecules of GP (glycerate-3-phosphate). RuBP must be continually regenerated using ATP.
