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AL Biology Chapter 13 Photosynthesis

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Blanca Peris

on 16 January 2019

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Transcript of AL Biology Chapter 13 Photosynthesis

Chapter 13

Outline of photosynthesis
Photosynthesis is the fixation of carbon dioxide and its subsequent reduction to carbohydrate, using hydrogen from water
The absorbed light energy excites electrons in the pigment molecules and they transfer this energy to drive the process of photosynthesis.
The photosynthetic pigments are arranged in clusters called PHOTOSYSTEMS: PSI (p700) and PSII (p680)
Light-dependent reactions (or light reactions): light energy is needed. Light is absorbed by pigments.

Inner membrane called the thylakoid membrane.

Thickened regions called thylakoids. A stack of thylakoids is called a granum. (Plural – grana)

Stroma is a liquid surrounding the thylakoids.
Chloroplast Structure
Most photosynthesis occurs in the palisade layer.
Gas exchange of CO2 and O2 occurs at openings called stomata surrounded by guard cells on the lower leaf surface.
Leaf Structure
An absorption spectrum is a graph of the absorbance of different wavelengths of light by a pigment.
Absorption spectrum
A photon of violet light packs nearly twice as much energy as photon of red light.
A membrane system or
runs through a ground substance called

Spongy mesophyll is mainly adapted as a surface for exchange of CO2 and O2.
Contains chloroplast, but in smaller amount than in palisade cells.
High light intensities allows photosynthesis to happen
Irregular packing with large air space provides large surface area of moist cell wall for gaseous exchange.
Long cylinders arranged at right angles to the upper epidermis, reducing the number of light-absorbing cross walls.
Its cells have several adaptions for light absorption:
Plants, protists (like phytoplankton), and cyanobacteria can do what no other organisms on Earth can do: photosynthesize, or make sugar out of water and carbon dioxide.
These photons are transferred to the surroundings as light of longer (less energetic) wavelength (seen as red fluorescence) and some waste heat energy.
In living plants, these energy are passed to another chlorophyll pigment or to an electron acceptor.
It is this energy that drives the process of photosynthesis.
Absorbed light energy excites electrons in the pigment molecules
If you illuminate a solution of chlorophyll with UV light you will see a red fluorescence when it is observed in the dark
After absorbing photons (light energy), the electrons jumps to a state of higher energy
When excited electrons return to their original lower energy state, a photon is given off
He used aerobic bacteria which concentrate near an oxygen source to determine which segments of the alga were releasing the most O2.
Bacteria congregated in greatest number around the parts of the alga illuminated with red or blue light.
An action spectrum is a graph of the rate of photosynthesis at different wavelengths of light.
Different pigments absorb light of different wavelengths.

Excess carbohydrates from photosynthesis is sometimes stored as starch grains.
In eukaryotes, photosynthesis takes place in chloroplasts.
Africa's desert Tree Tumbo plant only has few broad leaves and can live for almost 1,000 years with exposure to little rainfall.
Some have been estimated to be over 2,000 years old! It is also regarded a living fossil, so it really seems to be doing alright for itself!
Sea-slug (Kleptoplasty)  ingests a chloroplast-containing prey (often algae) and retains only the plastids, while it digests the rest. The predator can thenceforth photosynthesise to produce its own fuel
PURE genius!
palisade cells are the mains site of photosynthesis
There are more chloroplasts per palisade mesophyll than in the spongy mesophyll cells.
Structure and function of palisade cell in photosynthesis
Ways in which the structure of the leaf contributes to its successful functioning are:
Two sets of reactions are involved:
Light is a form of energy known as electromagnetic energy
Electromagnetic energy travels in waves. This entire range of radiation is known as the electromagnetic spectrum. The segment most important to life is the visible light
13.4 Chloroplast structure and function
Pine trees and other conifers have evolved to grow in a triangle shape because of photosynthesis. The tree's shape exposes most of its needles to the sun, especially the ones near the top of the tree, enabling it to produce enough energy to grow taller.
Long, narrow air spaces between them
- To give large surface area of contact between cell and air
Thin cell walls
- So that the gases can diffuse through them more easily
Palisade Cells’ adaptation for gaseous exchange
How are the palisade cells able to absorb light?
Leaves have usually many stomata in the lower epidermis.
In flowering plants, the major photosynthetic organ is the leaf.
Engelmann illuminated a filamentous alga on a slide with light that had been passed through a prism exposing different wavelengths.
Engelmann’s Experiment
Trapping Light Energy
Action spectrum
Chromatography can be used to separate the different pigments in a mixture of them extracted from a leave.
Essay question
They contain photosynthetic pigments that absorb light.
Section of a leaf
If the guard cells gain water, the pore is open, and viceversa.
Palisade mesophyll
Main site of photosynthesis.
Large central vacuole - Restricts chloroplasts to a layer near the outside of the cell where they can be reached by light more easily.
Cells are densely packed together giving a large surface area to air
Its cells have also adaptions for gaseous exchange:
Cuticle: waxy and transparent secreted by epidermis.
Spongy mesophyll
PSI absorbs electrons from PSII and the PSII receives electrons from the photolysis of water.
PSII has an enzyme that catalyses the breakdown of water:
The hydrogen ions and electrons combine with NADP to reduce it to be used in the next reactions
2 H + 2e + NADP ==> Reduced NADP
3) Each photoexcited electrons passes through electron transport chain.
1) When PSII absorbs light, an electron is excited to a higher energy level in the reaction centre P680 and captured by the electron acceptor.
4) When an electron reaches the “bottom” of the electron transport chain, it fills an electron “hole” in P700, the chlorophyll a molecule in the reaction centre of PSI.
Light intensity and wavelength.
At low light intensities,
as light intensity increases
, the rate of photosynthesis
increases proportionately
The availability of water is
considered to be limiting as there is more water available than CO2. Water supply can affect the rate
because a plant that is short of water will
close its stomata
preventing CO2 from diffusing into the leaf.
An increase in the CO2 concentration increases the rate until it is limited by another factor.
As it is present in the atmosphere at very low concentration (0.04%), it is usually the limiting factor.
Describe how the structure of a dicotyledonous leaf is related to its functions in photosynthesis. [7]
1. thin / flat to give large surface area to volume ratio ;
2. held at right angles to sun to allow max. light absorption ;
3. ref. to arrangement of cells in palisade mesophyll ;
4. ref. to spongy mesophyll large surface area for CO2 uptake / gaseous exchange;
5. ref. to stomata / guard cells and entry of CO2 ;
6. ref. to moist surfaces ;
7. ref. to xylem and supply of water / mineral ions ; and support ;
8. ref. to phloem and translocation of products of photosynthesis ;
9. ref. to cuticle on upper surface ;
Describe the transfer of energy to produce ATP during photosynthesis [7]
1. light absorbed by chlorophyll / AW ;
2. ref. photosystems ;
3. ref. harvesting clusters / accessory pigments ;
4. reaction centre / P680 / P700 ;
5. excitation of electrons / AW ;
6. ETC ;
7. idea of different energy levels ;
8. ADP + Pi → ATP ;
9. cyclic / non-cyclic, photophosphorylation ;
10. chemiosmosis / ATP synthase / description
The top band of pigments in the separation are carotenoids called carotenes, most likely beta-carotene, and appear yellowish-orange. The second type of carotenoid separated in the experiment are xanthophylls, which appear bright yellowish .
The "loading line" is the location of the original pigment line painted on the paper. (point of origin)

3) The solvent level must be below the pencil line.
1) Fresh spinach leaves are grind with solvent (acetone or propanone). The leaf extract contains a mixture of pigments.
: the photosynthetic pigments from spinach leaves will be extracted and separated using the technique of paper chromatography.
Separating Photosynthetic Pigments
Paper Chromatography is a technique used to separate the components of a mixture.
Separating Photosynthetic Pigments
6)The Rf value can be calculated.
This sugar gives energy to them and to all other organisms on Earth.
The nitrifying bacteria. (e.g.: Nitrosomonas, Nitrobacter): the conversion of ammonia to nitrite, and nitrite to nitrate provides energy.
Green plants, photosynthetic prokaryotes and algae.
Almost all the energy transferred to all the ATP molecules in all living organisms is derived from light energy used in photosynthesis by autotrophs.
A few autotrophs do not depend on light energy but use chemical energy sources.
Requires sunlight, water, and carbon dioxide. It occurs in chloroplasts
The overall equation can be written as:
Light-independent reactions (dark reactions or Calvin cycle): trapped light energy is converted to the chemical energy of sugars. Light energy not needed.
Red-blue violet
The photosynthetic pigments of higher plants form two groups: chlorophylls and carotenoids.
Different pigments absorb different wavelengths of light.
Light behaves like discrete particles (containing fixed quantity of energy) called photons.
The shorter the wavelength, the greater the energy it contains.
Substances that absorb visible light are called pigments.
As light meets matter, it may be reflected, transmitted or absorbed.
The ability of a pigment to absorb various wavelengths of light can be measured with a spectrophotometer.
The shorter the wavelength, the greater the energy it contains.
It shows the effectiveness of the different wavelengths relating their absorption and their energy content.
The pigments absorb the light energy and convert it into chemical energy.
 The functions of a leaf are best achieved:
and have a water and solute supply/transport route.
absorbing carbon dioxide (and disposing of oxygen)
by containing chlorophyll,
Leaves are very thin allowing a quick gaseous exchange.
Blade held at right angles to incident light
Arrangement of leaves (leaf mosaic) helps the plant to absorb as much light as possible.
Large surface area of the lamina: for maximum exposure to light and efficient gas exchange.
They have water supply and they can export the carbohydrates they produce.
Upper epidermis: made of thin, flat, transparent cells that allow light through them
Each stomata is bounded by two guard cells, and changes in their turgidity cause them to change shape so that they open and close the pore.
They are pores through which gaseous exchange occurs.
Cells have many more chloroplasts than spongy mesophyll.
Chloroplasts can be moved by cytoplasmic proteins to absorb more light or protect them.
Thin cell walls - to allow gases to more easily diffuse through them.
Removes products of photosynthesis and take them to the rest of the plant (phloem)
Irregular packing with large air space provides large surface area of moist cell wall for gaseous exchange.
Cells have less chloroplasts than palisade cells.
It is adapted mainly to gas exchange, it has a lot of air spaces.
Acts as supporting skeleton together with lignified tissues.
Vascular System
Supplies water and mineral salts (xylem)
Surrounded by two phospholipid membranes:
Diameter 3 – 10 µm, they are visible with a light microscope.
They contain chlorophyll and other photosynthetic pigments located on a system of membranes
The thylakoids are the site of the light-dependent reactions.
The stroma is gel-like containing soluble enzymes for the Calvin cycle and other chemicals such as sugars and organic acids.
They also have ATP synthase so chemiosmosis also takes place.
The membranes are covered with chlorophyll and other pigments, enzymes and electron carriers.
Some thylakoids are joined together forming the
They are many flattened, fluid-filled sacs.
Chloroplasts contain 70S ribosomes, circular DNA coding for some proteins and lipid droplets.
1) PHOTOLYSIS OF WATER: water is split to give hydrogen ions (protons) and oxygen.
It can be cyclic (PSI) or non-cyclic (PSII-PSI).
Oxygen is given out as waste product.
The hydrogen ions are combined with NADP producing reduced NADP.
During this process enough energy is released to make ATP from ADP and Pi in chemiosmosis
Chlorophyll a is photoactivated (one electron is excited).
The electron passes through a chain of electron carriers until it is passed back to the chlorophyll a.
PSI and PSII are excited and their electrons pass along the chain of electron carriers leaving the PS positively charged.
ATP is synthesized as the electrons lose energy while passing along the carrier chain
Oxygen is the waste product.
2) An enzyme attracts electrons from water and supplies them to P680, replacing the lost electrons when it absorbed light energy.
The oxidized chlorophyll is now a very strong oxidizing agent.
This is the water-splitting step of photosynthesis that releases O2.
This reaction splits a water molecule into 2 hydrogen ions and an oxygen atom, which immediately combines with another oxygen atom to form O2.
As electrons are passed through the chain, their fall to a lower energy level is used to produce ATP.
This ATP synthesis is called photophosphorylation because it is driven by light energy.
5) The electron acceptor passes the photoexcited electrons to a second electron transport chain.
This hole is created when light energy drives an electron from P700 to the electron acceptor.
The electrons are transferred to NADP. Reduced NADP will provide reducing power for the synthesis of sugar in the Calvin cycle.
TRIOSE PHOSPHATE is the first carbohydrate produced.
ATP and NADPH molecules created during the light reactions power the Calvin cycle.
The GP in the presence of ATP and NADPH is converted into TRIOSE PHOSPHATE (TP) (3C sugar).
Dark reactions occur in the stroma.
The fixation of carbon dioxide:
it is combined with RIBULOSE BISPHOSPHATE (5C sugar) (RuBP) to give two molecules of 3 carbons (GLYCERATE 3- PHOSPHATE, GP or PGA).
From it the plant can make glucose, sucrose, starch, cellulose or amino acids and lipids.
Most is used to
the RuBP.
) catalyses the combination of CO and RuBP
Main external factors affecting the rate of photosynthesis:
Carbon dioxide concentration.
The factor that has the greatest effect in reducing the rate is said to be the limiting factor.
As light intensity is increased further the rate of photosynthesis is eventually
limited by some other factor
Light intensity and rate of photosynthesis at a constant temperature:
At high light intensities the rate of photosynthesis reaches a plateau.
Chlorophyll a is used in both photosystems. The wavelength of light is also important.
So the rate plateaus.
At very high light intensity, chlorophyll may be damaged and the rate drops steeply.
Light with a higher proportion of energy concentrated in these wavelengths will produce a higher rate of photosynthesis.
PSI absorbs energy most efficiently at 700 nm and PSII at 680 nm.
Carbon dioxide and rate of photosynthesis
Temperature and rate of photosynthesis at constant light intensities
No books!!!
The separation is achieved by distribution of components between a stationary phase and a mobile phase.
Separating Photosynthetic Pigments
2) Prepare the chromatography paper: a pencil line is drawn at the bottom;
the extract is placed on the line;
the paper is placed vertically in a jar with the solvent.
4) The solvent rises up the paper by capillary action, past the sample, toward the end of the paper.
5) Each pigment travels at different speed so are separated as they ascend.
7) Two dimensional chromatography achieves a better separation of pigments.
The Rf values can be used to identify the different solutes.
Photosynthesis also gives us the oxygen that we breathe. 
13.6 C4 PLANTS
C3 plants
: first sugar molecule produced in the light-independent reaction is a 3-carbon molecule.
The first carbon compound that maize and most tropical grasses produce has 4 carbons.
Rubisco catalyses the reaction of CO2 with the RuBP but it also catalyses the reaction of oxygen with RuBP, called PHOTORESPIRATION.
• C4 plants have a group of cells called
bundle sheath cells
around the vascular bundles.
• The ring of mesophyll cells are in contact with air spaces, contain the enzyme PEP carboxylase.
They contain RuBP and Rubisco, but have no direct contact with the air so they are not exposed to high concentrations of oxygen.
• Oxaloacetate is then converted to malate
It is passed on to the bundle sheath cells where carbon dioxide is removed from the malate and combined with RuBP.
The Calvin cycle then proceeds as normal.
They are arranged in light-harvesting clusters = PHOTOSYSTEMS.
Two categories: primary or accessory pigments
I or II
Light is absorbed by PSI and is passed to the primary pigment.
The ATP passes then to the light independent reactions.
Reduced NADP passes to the light independent reactions.
The photolysis of water can be demonstrated by the Hill reaction.
Robert Hill showed (1939) that isolated chloroplasts had reducing power and released oxygen from water in the presence of an oxidising agent.
The reducing power was demonstrated with a redox agent that changed colour on reduction.
Paper 5
This technique can be used to investigate the effect of light intensity or wavelength on the rate of photosynthesis of a chloroplast suspension.
DCPIP is usually used, it becomes colourless when reduced.
The rate increases as the temperature is increased over a limited range
(Blackman's experiments)
(Blackman's experiments)
These experiments illustrate the concept of limiting factors
If a process is affected by several factors, the rate will be limited by the one which is nearest its lowest factor.
At higher concentrations the rate is limited by the temperature or the light intensity.
AND BOX 13.2 P. 293
Glasshouses are used to increase the yield of a certain crop.
Sensors monitor the light intensity, humidity and concentration of CO2
Pests and fungal diseases are more easily controlled.
This happens in high temperatures light intensity as result less photosynthesis takes place
It catalyses the combination of CO2 with phosphoenolpruvate or PEP forming oxaloacetate.
Enzymes in C4 plants usually have higher optimum temperatures as an adaptation to hot climates.
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