Chapter 5:Genetic Control & Protein Synthesis

(Week 10 & 11)

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PowerPoint® Slides

by Ms Lina Tang

Learning Objective (s)

• Understanding photosystem

• Understanding the light dependent and light independent

reaction of photosynthesis

• Understanding the structure of a dicotyledonous leaf

• Understanding limiting factors in photosynthesis

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Learning Outcome (s)

• Explain the cyclic and non-cyclic photophosphorylation

• Outline the processes in Calvin cycle

• Describe the structure of a dicotyledonous leaf and

relate the structure to its role in photosynthesis

• Discuss limiting factors in photosynthesis

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Figure 1: The diagram shows how energy enters an ecosystem as light which is captured and converted into the chemical energy of food

• During photosynthesis, the chlorophyll traps energy from

sunlight.

• This energy is used to combine water and carbon

dioxide to produce glucose and oxygen.

• The captured energy in plant material becomes the

ultimate source of food, because animals either eat

plants or other animals.

• Energy, unlike matter, is not recycled and does not

remain in an ecosystem.

• Some of the energy is used to drive the chemical

reactions of the body that keep the organism alive

• For example, during life processes such as respiration,

some energy is used. However, most energy is

converted into heat, which is released

• In this way, most of the energy that enters an ecosystem

as light leaves the ecosystem as heat.

• A one way flow of energy occurs in all ecosystems as

energy is transferred from one organism to another as

shown

Light energy > Autotrophs > Heterotrophs > Heat energy

• Autotrophs – organisms that can use an inorganic

carbon source in the form of carbon dioxide ▫ Organic molecule = compound including carbon and hydrogen

• Heterotrophs – organisms that need a ready-made

organic supply of carbon

Photosynthesis

• Photosynthesis transfers light energy into chemical

potential energy of organic molecules

• Photoautotrophs – green plants, the photosynthetic

prokaryotes and both single-celled and many-celled

protoctists (including green, red and brown algae)

• Chemoautotroph

▫ Autotrophs that do not depend in light energy, but use chemical

energy sources

▫ Include the nitrifying bacteria in the nitrogen cycle

▫ They obtain their energy from oxidising ammonia to nitrite, or

nitrite to nitrate

• Photosynthesis – trapping (fixation) of carbon dioxide and its

subsequent reduction to carbohydrate, using hydrogen from

water

• Overall equation:

light energy in the presence of chlorophyll

▫ nCO2 + nH2O --------> (CH2O)n + nO2

carbon dioxide water carbohydrate oxygen

• Hexose sugars (glucose) and starch are commonly formed: light energy in the presence of chlorophyll

▫ 6CO2 + 6H2O --------> C12H2O6+ 6O2

• In photosynthesis, two sets of reactions are involved: ▫ Light-dependent reactions

Only take place in the presence of suitable pigments which absorb

certain wavelengths of light

Light energy – splitting of water into hydrogen and oxygen (waste

products)

Light energy also needed to provide chemical energy (ATP) for the

reduction of carbon dioxide to carbohydrate in the light-independent

reactions

▫ Light-independent reactions

1. Trapping the light energy

Pigments Colour

Chlorophylls Chlorophyll a Yellow-green

Chlorophyll b Blue-green

Carotenoids carotene Orange

Xanthophyll Yellow

Photosynthetic pigments Absorbs different wavelengths of light

Trapping the light energy

• Light energy captured is then converted to chemical

energy

• Absorbed light energy excites electrons in the pigment

molecules

• Ultraviolet light absorbed (high in energy), causing

electrons to be excited

Photosynthesis pigments

• There are two kinds:

▫ Primary pigments, eg chlorophyll a

Chlorophyll a – 2 forms of it with slight difference in the absorption

peaks

▫ Accessory pigments, eg other forms of chlorophyll a, chlorophyll

b, caretenoids

• The photosynthesis pigments are arranged in light

harvesting clusters, known as photosystems

• Several hundreds of accessory pigment molecules

surround a primary pigment molecule and the energy of

the light absorbed by the different pigments is passed to

the primary pigment ▫ Primary pigment acts as a reaction centre

Photosystem

• Photosystem I ▫ Arranged around a molecule of chlorophyll a with a peak

absorption at 700nm

▫ Reaction centre therefore is called P700

• Photosystem II ▫ Based on a molecule of chlorophyll a with a peak absorption of

680nm

▫ Reaction centre called P680

Light-dependent reactions

• Synthesis of ATP in photo-phosphorylation and the

splitting of water by photolysis to give hydrogen ions

• Hydrogen ion combines with NADP to make reduced

NADP

• ATP and reduced NADP are passed from the light-

dependent to light-independent reaction

• Phosphorylation of ADP to ATP can be

Cyclic

OR

Non-Cyclic

• depending on the pattern of electron flow in one or both

photosystems

Cyclic phosphorylation

• Only in Photosystem I

• Light is absorbed by PS I, passed to chlorophyll a (P___)

• An electron in the chlorophyll a molecule is excited to a

higher energy level and emitted from the chlorophyll

molecule

• Instead of falling back into the photosystem and losing

its energy a fluorescence, it is captured by an electron

acceptor and passed back to chlorophyll a molecule via

a chain of electron chains

• During this process, enough energy is released to

synthesize ATP from ADP and Pi

• ATP then passes to the light-independent reactions

Non-cyclic phosphorylation

• Light is absorbed by both photosystems and excited

electrons are emitted from primary pigments of both

reaction centres (P680 and P700)

• These electrons are absorbed by electron acceptors and

pass along chains of electron carriers leaving the

photosystem positively charged

• The P700 of PS I absorbs electrons from PS II

• P680 receives replacement electrons from splitting

(photolysis) of water

• ATP synthesized as the electron loss energy whilst

passing along the carrier chain

Light-independent reactions

• Main process – fixation of carbon dioxide,

• Carbon dioxide combines with a 5-carbon sugar,

ribulose bisphosphate (RuBP) to give two molecules of a

3-carbon compound, glycerate-3-phosphate (G3P)

• G3P (in presence of ATP and reduced NADP) is reduced to triose phosphate (3-Carbon sugar)

• This is the point where carbohydrate is produced in photosynthesis

• Some of the triose phosphates are condensed to form hexose phosphates, sucrose, starch and cellulose or even converted to acetylcoenzyme A to make amino acids and lipids

• Other triose phosphate regenerate ribulose

bisphosphate (RuBP)

• This cycle – worked by Calvin, Benson and Bassham in

between 1946 – 1953, usually called Calvin cycle

• Most common enzyme in the world – ribulose

bisphosphate carboxylase (rubisco) catalyses the

combination of CO2 and RuBP

Chloroplast

Leaf structure and function

• Leaf – main photosynthetic organ in dicot

• Broad, thin lamina, a midrib and a network of veins

• May also have a leaf stalk (petiole)

• To act as a photosynthetic organ, the leaf must:

▫ Contain chlorophyll and other photosynthetic pigments arranged in such a way that they can absorb light

▫ Absorb carbon dioxide and dispose of the waste product oxygen

▫ Have a water supply and be able to export manufactured carbohydrate to the rest of the plant

• Large surface area of lamina – absorbance of light

• Its thinness – minimises the diffusion pathway for

gaseous exchange

• Arrangement of leaves on the plant (the leaf mosaic) –

absorb as much light as possible

• Upper epidermis – made of thin, flat, transparent cells

which allow light through the cells of the mesophyll

below

• Waxy transparent cuticle – secreted by the epidermal

cells, provide watertight layer

• Cuticle + epidermis protective layer

Palisade cell

• Palisade cell – main site for photosynthesis (more

chloroplast per cell compared to spongy mesophyll)

• These cells had being adapted for maximum light

absorption

• Long cylinders arranged at the right-angles to the upper epidermis

• Large vacuole with a thin peripheral layer of cytoplasm

• Chloroplasts can be moved by the proteins in the cytoplasm, to absorb more light or to protect them against excessive light

• The cylindrical cells pack together with long, narrow air spaces between them – large surface area between cell and air

• Thin cell walls – gases diffuse through them easily

Spongy mesophyll

• Adapted mainly as exchange surface for carbon dioxide

and oxygen

• Smaller amount of chloroplasts – photosynthesis occurs

here, only at high light intensities

• Irregular packing and large air spaces – large surface

area of moist cell wall for gaseous exchange

Lower epidermis

• Lower epidermis has about the same structure as upper

epidermis, except that most mesophytes have many

stomata in the lower epidermis (some have few stomata

in the upper epidermis too)

• Stomata – pores in the epidermis through which

diffusion of gases occurs

• Each stoma is bounded by two sausage-shaped guard

cells

• Changes in the turgidity of the guard cells cause them to change shape so that they open and close the pore

• When the guard cells gain water, the pore opens; as they lose water, it closes

• Guard cells have unevenly thickened cell walls

• The wall adjacent to the pore is very thick, whilst the wall furthest from the pore is thin

• Guard cells gain and lose water by osmosis

• A decrease in water potential is needed before water

can enter the cells by osmosis

• This is achieved by the active removal of hydrogen ions,

using energy from ATP and hence, the intake of

potassium ions

How a stoma is opened

1. ATP-powered proton pump actively transports H+ out of

the guard cell

2. The low H+ concentration and negative charge inside

the cell causes the K+ channels to open. K+ diffuses

into the cell down a electrochemical gradient.

3. The high concentration of K+ inside the guard cell

lowers the water potential.

4. Water moves in by osmosis, down a water potential

gradient.

5. The entry of the water increases the volume of the guard

cells, so they expand. The thin outer layer wall expands

most, so the cells curve apart, and the stoma is

opened.

Chloroplast

• Chloroplast – photosynthetic organelle

• Each chloroplast – surrounded by an envelope of two

phospholipid membranes

• System of membranes also runs through the ground

substance, stroma

• Thylakoids – series of flattened fluid-filled sacs, forming

stacks, called grana

• Membrane of grana provides large surface area which

holds the pigments, enzymes and electron carriers

needed for the light-dependent reactions

• The arrangement is to enables the absorption of

maximum amount of light

• Stroma – the site for light-independent reactions

• It contains enzymes for the Calvin cycle, sugars and some organic acids

• There are also small (70s) ribosomes, a loop of DNA, lipid droplets and starch grains in te stroma

• The loop of DNA codes for some of the chloroplast proteins, made by chloroplast’s ribosomes, but other chloroplast proteins are coded for by the DNA in the plant cell nucleus

Factors affecting rate of photosynthesis

1. Light intensity

2. CO2

3. Temperature

References:

Jones, M. 2007, AS/A Level Biology (Cambridge

International Examinations), 2nd edn, Cambridge

University Press.

Lee, S.C. & Liew, S.L. (2011). Success in Biology For

STPM (vol 1). Fajar Bakti.

Lee, S.C., Liew, S.L. & Choong, N.M. (2011). Success in

Biology For STPM (vol 2). Fajar Bakti.

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Key terms:

Term Definition

Photophospho-

rylation

The synthesis of ATP from ADP and Pi using light

energy in photosynthesis