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What is photosynthesis?

All life on Earth depends on photosynthesis. The process is described by the following word and symbol equations:

These equations are summaries of a complex two-step process that takes place in the chloroplasts of green plants. The end products are not just glucose, but complex organic molecules such as carbohydrates, amino acids, lipids and nucleic acids.

carbondioxide + water oxygen + glucose

light energy

6CO2 + 6H2O 6O2 + C6H12O6

light energy


Why do plants photosynthesize?

Photosynthesis is an essential biological process. This is because it produces:

energy for processes in the organism

complex organic molecules needed for growth

oxygen, which is then used for respiration.

The oxygen produced is released into the atmosphere and is available for other organisms.

If the plant is eaten, the organic molecules are used to provide energy to organisms higher up the food chain.

There are not just advantages for the plant itself:


Leaves and photosynthesis


Structure of the chloroplast


Plant photosynthetic tissues


Plant anatomy


Functions of photosynthetic structures



In plants there are two types of pigments: chlorophylls and carotenoids. They are coloured because they absorb particular wavelengths of light and reflect others.

A photosynthetic pigment is a coloured biological compound that is present in chloroplasts and photosynthetic bacteria, and which captures light energy for photosynthesis.

Chlorophyll is the pigment that gives plants their green colour by reflecting green light. Carotenoids reflect red, orange or yellow light.

Photosynthetic pigments






Adenosine triphosphate

Adenosine triphosphate, or ATP, is an important molecule found in all living cells. It readily diffuses around the cell and provides energy for cellular processes.

ATP releases energy in the light-independent reaction when a bond between inorganic phosphate groups is broken, producing ADP and an inorganic phosphate group.

adenine

ribose

3 phosphate groups

ATP is made in the light-dependent reaction in photosynthesis from adenosine diphosphate (ADP) and an inorganic phosphate group (Pi). This requires energy.


Adding a phosphate molecule is phosphorylation. In this case, light is used as an energy source so the process is called photophosphorylation.

In the light-dependent reaction:

+ ®+ energy

In the light-independent reaction:

The use of water makes this a hydrolysis reaction.

Photophosphorylation and hydrolysis

ADP ATP

+ energy+®H2O ADPATP


NADP and NADPH

Nicotinamide adenine dinucleotide phosphate (NADP) is a coenzyme involved in the photosynthesis reactions.

The compound is a dinucleotide, containing an adenine base and a nicotinamide base. The nucleotides are joined through their phosphate groups. In addition there is an extra phosphate on the ribose of the adenine-containing nucleotide.

NADP can accept electrons to be reduced to reduced NADP, often called NADPH. NADPH can be oxidized back to NADP, releasing electrons.

nicotinamide

adenine

ribose


Which molecule?



Location of the reaction

The light-dependent reaction takes place in the thylakoid membranes of the chloroplasts.

Situated in these membranes are light-harvesting systems called photosystems. There are two types, photosystem I (PSI) and photosystem II (PSII). Both have chlorophyll at their centres.

chlorophyll

lightphotosystem


Light-dependent reaction


Light-dependent reaction: summaryCyclic photophosphorylation

Non-cyclic photophosphorylation

photolysis


Products of the light-dependent reaction

Products of the light-dependent reaction that pass into the light-independent reaction:

Products of the light-dependent reaction that leave the plant:

Products of the light-dependent reaction that are re-used in another part of the light-dependent reaction:

reduced NADP

ATP

oxygen

H+ ions

electrons


How much do you know?



Location of the reaction

The light-independent reaction can also be called the “dark reactions”, or the Calvin Cycle.

The light-independent reaction takes place in the stroma of the chloroplasts.

The reaction consists of a cycle of enzyme-controlled processes making complex molecules such as carbohydrates.

stroma


The light-independent reaction


Products of the Calvin cycle

Products of the light-independent reaction that pass back into the light-dependent reaction:

Products of the light-independent reaction that are used in other processes:

NADP

ADP

triose phosphate – used to build complex carbohydrates, amino acids and lipids.

inorganic phosphate


Making complex molecules

Pairs of TP molecules combine to form hexose sugars, such as glucose, some of which may isomerize to form fructose.

Triose phosphate is a three-carbon sugar that can be used to make a variety of complex biological molecules.

TP can be converted to glycerol and this may be combined with fatty acids to make lipids.

These monosaccharides can combine to form disaccharides such as sucrose, and polysaccharides such as cellulose and starch.


How much can you remember?


Summary of photosynthesis

light- dependent reactions

light- independent

reactions

light

carbohydrates, other complex molecules


Photosynthesis: true or false?



Glossary


What’s the keyword?


Inputs and outputs to the reactions


Multiple-choice quiz

Documents

Photosynthesis: The Reaction - Groby Bio Page - Homegrobybio.weebly.com/.../2/6/2/3/26235017/… · PPT file · Web view · 2017-06-05light energy. Boardworks A2 Biology . ... The