Photosynthesis (DP IB Biology)

Carotene produces an R_f value of 0.95.

The two types of chromatography are paper chromatography and thin-layer chromatography.

A pencil is used as the ink in pens will separate into pigments within the experiment and obscure the results

An absorption spectrum is a graph that shows the absorbance of different wavelengths of light by a particular pigment in the chlorophyll.

Plants absorb only some wavelengths of light because each pigment absorbs only certain wavelengths (colours) of visible light. Pigments reflect the colour of the wavelengths that they cannot absorb. For example, chlorophyll a, is seen as green by humans because green light is reflected by chlorophyll a.

Carotenoids absorb wavelengths of light mainly in the blue-violet region of the spectrum

Light energy results in the excitation of electrons within a pigment molecule (chlorophyll) which triggers the transfer of electrons leading to a series of reactions which make up the process of photosynthesis.

During photosynthesis, light energy is transformed into chemical energy when glucose is formed.

Chlorophylls mainly aborbs light between 400 and 500 nm (blue) and around 650 to 680 nm (red).

An action spectrum is a graph that shows the rate of photosynthesis at different wavelengths of light.

Both absorbance spectra and action spectra have:

• two main peaks – at the blue-violet region and the red region of the light spectrum

• a trough in the green-yellow region of the light spectrum

The rate of photosynthesis is highest at the blue-violet and red regions of the light spectrum. This is because these are the wavelengths of light that photosynthetic pigments can absorb.

The rate of photosynthesis can be determined by measuring the volume of oxygen produced or the carbon dioxide consumption at different wavelengths of light.

A control variable is that the light source should be kept the same distance from the plant.

Aquatic plants are often used to determine the rate of photosynthesis because the volume of oxygen produced can be easily measured using a gas syringe or bubbles of oxygen can be counted within the water.

The independent variable when determining the rate of photosynthesis for varying wavelengths of light are different wavelengths (colours) of light.

The data required to plot an action spectrum are:

• Wavelength of light (nm) on the x-axis

• Rate of photosynthesis on the y-axis

True.

The rate of photosynthesis can be measured by counting oxygen bubbles from a cutting of Elodea.

A hypothesis is a proposed explanation for an idea which may be true or false. A hypothesis can be proposed before experimentation or based on evidence from an experiment already carried out.

The independent variable is the factor that is deliberately changed throughout the experiment. For example, when investigating the effect of light intensity on the rate of photosynthesis, the independent variable would be the distance of the plant.

The dependent variable is the factor that is measured to see if it is affected by the independent variable. For example, when investigating the effect of temperature on the rate of photosynthesis, the dependent variable would be the rate of photosynthesis, measured (usually) by the volume of oxygen produced.

As light intensity increases, the rate of photosynthesis also increases, up to a certain point where it levels off. After this point, another factor is limiting the rate of photosynthesis.

True.

Temperature must be controlled during photosynthesis experiments to prevent it from affecting the results. This can be done using a heat shield in front of a light source (which generates heat) or a water bath.

The CO₂ concentration can be controlled by boiling and re-cooling water and then adding a set mass of sodium hydrogen carbonate.

The rate of photosynthesis decreases because enzymes begin to denature and cannot catalyse the reaction.

A limiting factor is a condition or resource that limits the rate of photosynthesis when it is in short supply. Limiting factors that affect the rate of photosynthesis are the concentration of carbon dioxide, light intensity or temperature.

A gas syringe can be used to measure the volume of gas produced in a photosynthesis experiment. Alternatively, a less accurate method is to use an inverted measuring cylinder placed over an aquatic plant.

Light intensity can be varied by using a light source and placing it at regular distances from a plant, e.g. 5 cm, 10 cm, 15 cm, 20 cm, 25 cm.

Suggested hypotheses for the effect of limiting the concentration of carbon dioxide on the rate of photosynthesis include:

• Carbon dioxide concentration will affect the rate of photosynthesis

• As carbon dioxide concentration increases the rate of photosynthesis will increase

Carbon dioxide enrichment experiments aim to study the effects of increased CO₂ levels on plant growth and photosynthesis to predict future impacts.

Free air carbon dioxide enrichment (FACE) experiments are conducted in natural ecosystems where CO₂ is pumped in to study its effects on larger plants and trees.

False.

In FACE experiments, variables cannot be controlled because they are carried out in natural ecosystems, but variables can be monitored and taken into account when analysing results.

True.

Enclosed greenhouse experiments allow for the control and manipulation of variables. They should be controlled to ensure that the effect of only one variable is being considered at any one time.

Controlled variables are any factors that may affect the experiment's results and need to be controlled or monitored. This may include temperature, light intensity, light wavelength, carbon dioxide concentration, and water availability. The control variables will depend on the variable that is being manipulated (the independent variable).

Enclosed greenhouse experiments are used to control and manipulate variables like light, CO₂, and temperature in photosynthesis studies.

The types of plants used in enclosed greenhouse experiments are small species that can be contained in a greenhouse can be studied using enclosed greenhouse experiments.

Controlling variables is essential to ensure valid results by isolating the effect of only the independent variable on the dependent variables (rate of photosynthesis).

A photosystem is an array of chlorophyll and accessory pigments with a special chlorophyll reaction centre that absorbs light and emits an excited electron.

False.

Photosystems are always located in membranes within cyanobacteria and photosynthetic eukaryotes.

False.

Photosystems are present in both cyanobacteria and the chloroplasts of photosynthetic eukaryotes.

The reaction centre in a photosystem gains energy that has been harvested by the accessory pigments and then emits excited electrons.

False.

A single molecule of chlorophyll cannot perform photosynthesis alone; this requires a structured array of pigments within a photosystem.

Having a range of different accessory pigments expand the range of light wavelengths that can be absorbed, allowing photosynthesis occur at a higher rate.

Examples of photosynthetic pigments include:

• chlorophyll, e.g. chlorophyll a, chlorophyll b

• carotenoids, e.g. carotene, xanthophyll

The light-dependent reactions of photosynthesis occur on the thylakoid membrane, including:

• photolysis of water

• synthesis of ATP by chemiosmosis

• reduction of NADP

The two useful products of the light-dependent reactions are:

• NADPH

• ATP

Photolysis is the process of splitting water using light energy, producing protons, electrons, and oxygen.

False.

The oxygen generated by photolysis is a waste product in photosynthesis.

The specific location of photolysis is photosystem II in the thylakoid membrane of chloroplasts.

Photolysis is represented by the equation 2H_₂O → O_₂ + 4H^⁺ + 4e^⁻.

In photolysis, two water molecules are split to release 4 hydrogen protons, 4 electrons and 2 molecules of oxygen.

Chemiosmosis is the movement of protons across a membrane, driving ATP synthesis.

The proton gradient results in the movement of protons across the thylakoid membrane. This powers ATP synthase to produce ATP.

ATP synthase is an enzyme that synthesises ATP from ADP and inorganic phosphate, using energy from the proton gradient.

Cyclic photophosphorylation is the process during which electrons from photosystem I are passed down the electron transport chain and back to PSI. This produces ATP only, without producing NADPH.

True.

Thylakoids facilitate ATP synthesis via chemiosmosis and the reduction of NADP in light-dependent reactions.

Photophosphorylation is the process by which light energy is used to phosphorylate ADP to produce ATP. NADP is reduced during this process.

The photosystems involved in photophosphorylation are:

• cyclic phosphorylation = photosystem I only

• non-cyclic photophosphorylation = photosystems I and II

The process of NADP reduction that occurs during the light-dependent reactions of photosynthesis involves:

• NADP is reduced by accepting two electrons

• the electrons come from photosystem I

• NADP also accepts a hydrogen ion from the stroma

Rubisco is the enzyme responsible for carbon fixation in the Calvin cycle, attaching CO₂ to RuBP.

The substrates are RuBP and CO₂, and the product is glycerate-3-phosphate (GP).

True.

Rubisco is the most abundant enzyme on Earth.

NADPH provides the reducing power (electrons) needed to convert glycerate-3-phosphate (GP) into triose phosphate (TP).

RuBP is ribulose bisphosphate, a five-carbon molecule that reacts with CO₂ in the Calvin cycle (the light-independent stage of photosynthesis).

Triose phosphate (TP) is a three-carbon molecule produced in the Calvin cycle, derived from GP using ATP and NADPH.

RuBP is regenerated in the Calvin cycle when five molecules of triose phosphate are converted into three molecules of RuBP. This regeneration allows the cycle to continue.

True.

Five-sixths of all triose phosphate produced is converted back to RuBP for the cycle's continuation.

Carbon fixation is the process by which CO₂ is attached to an organic molecule (RuBP) in photosynthesis, catalysed by Rubisco.

Carbohydrates, amino acids, and other carbon compounds are synthesised from Calvin cycle intermediates using mineral nutrients.

False.

Light-independent reactions rely on CO₂, and a lack of it prevents the Calvin cycle from functioning. If the Calvin cycle was to stop, then NADP and ADP + P_i would not be returned to the light-dependent reaction which inhibits the light dependent stage of photosynthesis.

Light-dependent reactions are the part of photosynthesis that requires light to produce ATP, NADPH, and O₂ in the thylakoids.

Light-independent reactions stop without light because they need ATP and NADPH produced by light-dependent reactions.

ATP provides energy, and NADPH provides reducing power to drive the conversion of CO₂ into carbohydrates.

The Calvin cycle is the light-independent reaction in photosynthesis that fixes CO₂ into organic molecules.

False.

A lack of CO₂ prevents the Calvin cycle, which in turn affects the entire photosynthesis process as the products (NADP and ADP + P_i) are not recycled back to the light-dependent reactions. This means that the light-dependent reaction is affected, including Photosystem II.

Photosynthesis stops without light because the light-dependent reactions cannot produce ATP and NADPH. In turn this means that the Calvin cycle cannot continue and organic molecules will not be synthesised.

The light-dependent reactions provide the energy (ATP) and reducing power (NADPH) for the Calvin cycle.

The Calvin cycle regenerates ADP and NADP⁺, which are essential for the light-dependent reactions to continue.

Compound M is CO₂. Carbon dioxide enters the Calvin cycle and is used to fix 5C RuBP (compound O).

Compound P represents glucose, a 6C compound and a product of the Calvin cycle.