biology topic 5

8/13/2019 Biology Topic 5

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Biology Topic 5

On the wild side

Photosynthesis

Chloroplasts are the organelles where reactions ofphotosynthesis occur

The light-dependent reactions

In the light-dependent stage, light energy is trapped by thephotosynthetic pigment, chlorophyll.

Chlorophyll molecules do not occur randomly in the grana.Rather, they are grouped together in structures calledphotosystems, held in the thylakoid membranes of the grana.

1. Energy from the light raises 2 electrons in each chlorophyllmolecule to a higher energy level. The chlorophyll moleculesare now in an excited state.


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2. The electrons leave the excited chlorophyll molecules and passalong a series of electron carrier molecules, all of which areembedded in the thylakoid membrane. These moleculesconstitute the electron transport chain.

3. The electrons pass from one carrier to the next in a series ofoxidation and reduction reactions, losing energy in the process.The energy is used in the synthesis of ATP, in a process calledphotophosphorylation.

4. The electrons lost from the chlorophyll must be replaced if the

flow of electrons along the electron transport chain is tocontinue.

5. Within the thylakoid space, an enzyme catalyses the splitting of water (photolysis) to give oxygen gas, hydrogen ions andelectrons. These electrons replace those that were emitted fromthe chlorophyll molecule, so it is no longer positively charged.

The hydrogen ion concentration within the thylakoid space israised as a result of photolysis.

6. The electrons that have passed along the electron transportchain combine with the co-enzyme NADP and hydrogen ionsfrom the water to form reduced NADP. Hydrogen ions are usedin the reduction of NADP.

ATP and reduced NADP do not normally accumulate, however,as they are immediately used in the fixation of carbon dioxidein the surrounding stroma (in the light-independentreactions). Then the ADP and NADP + diffuse back into thegrana for re-use in the light-dependent reactions.


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ATP Adenosine Triphosphate

ATPase catalyses the breakdown of ATP to ADP. The additionof phosphate to ADP is known as phosphorylation.

The Light-independent reactions Calvin cycleIn the light-independent reactions, carbon dioxide is convertedinto carbohydrate. These reactions occur in the stroma of thechloroplasts, using the reduced NADP and ATP from the light-dependent reactions. CO 2 is reduced to carbohydrate andNADP acts as a hydrogen carrier.

1. Carbon dioxide combines with a 5-carbon compound RuBP inthe presence of the catalyst RuBISCO. Carbon dioxide is addedto RuBP in a process known as fixation.

2. The 6-carbon compound formed is unstable and immediately breaks down into two 3-carbon molecules called GP.

3. GP is reduced to form a 3-carbon sugar phosphate calledGALP. The hydrogen for the reduction comes from the reducedNADP from the light-dependent reactions. ATP from the light-dependent reactions provides the energy required for thereaction.

4. 2/12 GALPs formed are involved in the creation of a 6-carbon

sugar which can be converted to other organic compounds forexample amino acids or lipids.

5. 10/12 GALPs formed are involved in the recreation of RuBP.Phosphorylation using ATP forms RuBP.


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Why does Photosynthesis take place inside thechloroplasts?

Thylakoids and the light-dependent reaction

The electron carriers are located in the thylakoid membrane inthe chloroplast. Their positioning within the membrane createsan electron transport chain, allowing electrons to passefficiently from each electron carrier to its neighbour.

The stroma and the light-independent reaction

The compartmentalisation of these reactions within the

chloroplast stroma means that the substrates and enzymes can beat concentrations that allow the reactions to be catalysed quickly.

The productivity of photosynthesis

Only about half the energy emitted by the Sun and reaching theEarths outer atmosphere gets through to soil level and to plantlife the remainder is reflected into space as light or heat energy.

Of the radiation reaching green plants, about 45% is in the visible wavelength range (400 700nm) and can be used by the plant inphotosynthesis. Much of the light energy reaching the plant isreflected from the leaves or transmitted through them, however.Furthermore, of the energy absorbed by the stems and leaves ofplants, much is lost in the evaporation of water.

A small quantity of the energy reaching the green leaf is absorbed by the photosynthetic pigments and used in photosynthesis; onlyone quarter of this light energy ends up as chemical energy inmolecules like glucose. The remainder is lost as heat energy in the various reactions of the light-dependent and light-independentreactions. In green plants, the total amount of light energy fixedthrough photosynthesis in a given period of time is known as thegross primary productivity (GPP).


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The rate at which energy is transferred into the organic moleculethat make up the new plant biomass is called net primaryproductivity (NPP).

NPP = GPP R

The percentage efficiency of photosynthesis is given by dividingthe GPP by the estimated amount of light energy reaching theplant, multiplied by 100. GPP is usually expressed as units ofenergy per unit area per year.

Energy flow through a producer and consumers


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Abiotic Factors

Abiotic factors of a terrestrial habitat are of three types, relatingto:

Climate factors such as solar radiation, temperature, rainfalland wind


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Soil factors such as the parent rock, soil water and soilchemistry, and the mineral nutrients available (edaphic factors)

Topography factors such as slope and aspect of the land, andaltitude.

Biotic Factors

Interactions between organisms, known as biotic factors, are between members of the same species (intraspecific competition)and between members of different species (interspecificcompetition).

Of course, the impact of biotic factors depends upon the numbersof organisms present in relation to resources available, within a

given environment. So we say that biotic factors are densitydependent. Furthermore, we can recognise that biotic factorsmust be highly influential since most species occupy only a smallpart of the environment in which they are equipped to live.

Niche

Niche is a comprehensive ecological term that defines how anorganism feeds, where it lives, and how it behaves in relation toother organisms in its habitat.


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The Carbon Cycle

The Greenhouse Effect

Limiting Factor in Photosynthesis

Photosynthesis is a biochemical process involving a series ofinterconnected reactions. All these reactions contribute to the

overall rate of the process. The rate of photosynthesis will belimited by the slowest of these reactions. In fact, the rate of


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photosynthesis will be limited by the factor that is in shortestsupply (whether it be CO 2 concentration, light intensity or thetemperature). This factor, whichever one it is, is known as thelimiting factor.


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biology topic 5

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