Making things with Light Photosynthesis


Table of Contents

Photosynthesis

Glucose

Glucose is probably the most important molecule in biology

It is made of just 3 types of atoms = Carbon, Oxygen and Hydrogen

It is made from Carbon and Water

It is a Carbo-hydrate

The Water component comes from .... water

The Carbon part comes from Carbon Dioxide

The Dioxide part of Carbon Dioxide floats away while the glucose is being made. It floats away as O2

Glucose is important for us as it is the main fuel for respiration

Cellular Respiration turns the Glucose back into Carbon Dioxide and Water, by re-combining the Carbon with O2, thus making CO2. The remaining atoms are then Hydrogen, Oxygen and Hydrogen. These make H2O, or water


The Process to make glucose needs energy.

The energy comes from light, the light comes from the sun

The process is Photosynthesis

Photosynthesis is the synthesis of a molecules using energy from light, photons (photo)

To make glucose, you need 24 atoms:

  • 6 Carbon atoms

  • 6 Oxygen atoms

  • 12 Hydrogen atoms

To get these, you could use the REACTANTS

  • 6 Carbon Dioxide molecules

  • 6 Water molecules

After the reaction, you end up with these PRODUCTS

  • 1 Glucose

  • 6 Oxygen molecules 6O2

To break apart the Carbon dioxide and the water, you need Light energy and a catalyst (something that is going to conduct the reaction)

But what is the Catalyst for photosynthesis?

This process needs a little help. this is where Chlorophyll comes in.

Chlorophyll is the amazing molecule that uses light energy to break apart the carbon dioxide and the water molecules that are then used to make glucose (and molecular oxygen)

Once glucose has been synthesized through photosynthesis, it can then be used as the fuel for cellular respiration

Or it can be combined with other glucoses to make starch

Or it can be combined to make cellulose

Or it can be disassembled, combined with nitrogen and restructured into an amino acid, or even turned into an oil

The Chlorophyll is found in the Chloroplast of Plants

Chlorophyll = molecule = Catalyst for the photosynthetic reaction

Chloroplast = organelle


Photosynthesis and Cellular Respiration

Photosynthesis and Cellular Respiration are mirror images of each other

Photosynthesis uses Carbon dioxide and Water to make Glucose and Oxygen

Cellular respiration uses Glucose and Oxygen to make Water and Carbon Dioxide

So, what is the point if it just goes round and round?

Follow the energy....

Energy enters Biological systems, that we are part of, via the sun. That energy then becomes chemical energy stored in glucose. We eat something. That glucose gets into our cells and is then 'burnt' via cellular respiration back into water and carbon dioxide (this is the reason we breath in oxygen and breath out carbon dioxide). But, then we use the energy released via cellular respiration to do stuff... to do everything that we do. To go for a run, groom the dog, play PS5 and read this. :-)

The purpose of Photosynthesis

Food for plants = starting point of the planets food web

Photosynthesis is the N in MRSGREN = Nutrition

Super detailed and complex overview of photosynthesis (how plants make their food)

Plants are autotrophs, this means they make their own food

In terms of life processes, photosynthesis is the N in MRSGREN

Photosynthesis is how plants get their Nutrition

The glucose made by photosynthesis is 'burnt' in cellular respiration to obtain the chemical potential energy that is then used throughout the cell (and thus throughout the plant) to power everything that happens in the cell.

The glucose made by photosynthesis is then used by the plants as a source of atoms for the building of all of the structures within the plant cells and thus the plant

this includes the Starch granules in the cells. The Starch in the Cotyledon and Endosperm in the seed.

This also includes the cellulose of the cell wall, the cellulose that we can't digest and refer to as dietary fiber

If we eat the glucose from a plant, such as eating oranges, so of the glucose is 'burnt' via cellular respiration for us to get energy to do stuff - hence if we eat glucose we can get a 'sugar high'

Also, if we eat the glucose from a plant, such as eating oranges, some of the atoms will stay as part of our body - in fact if we eat lots of glucose then the atoms will be stored in our body as fat.

Likewise, glucose is the source of energy and of atoms in the plant. So photosynthesis is how plants make their food. Thus, photosynthesis is the source of a plants Nutrition

Easy-as overview of photosynthesis

Factors affecting the rate of photosynthesis

Light Intensity

Light provides the energy for photosynthesis - this is the primary limiting factor for photosynthesis - and this is why photosynthesis does not occur at night

The more light there is, the more energy there is, so the faster the rate of photosynthesis.

As a light dependent reaction, the reaction rate increases proportionally with light intensity

This increases to a point, it then plateaus, with other factors acting as the limiters.

Carbon Dioxide

As one of the two substrates for photosynthesis, the more carbon dioxide present, the more carbon that is available for construction into glucose.

As carbon dioxide is present at very low concentrations (0.04%) in the atmosphere, it is a key limiting factor. An increase in carbon dioxide results in an immediate increase in the rate of photosynthesis.

Temperature

The hotter the temperature, the faster carbon dioxide and water can move within the leaf and the faster the enzymes can move around and operate. So, the faster the reaction.

Notice very carefully, the dramatic drop in the graph below. Once the temperature gets too hot, the enzymes start to change their shape, they denature, thus the active sites of the enzymes no longer fit the substrates and the reaction stops.

Chlorophyll

If we give the plant plenty of Carbon Dioxide, Light and Warmth, then the limiting factor of photosynthesis finally becomes the quantity of Chlorophyll in the leaf

Light Intensity Data from Y11 Biology

Leaf Structure

Why are leaves green?

Leaves are green because the chlorophyll reflects green light

Chlorophyll absorbs Blue and Red light (Infra-red is also reflected)

Chlorophyll is kept small organelles called chloroplasts

There are more chloroplasts, and thus chlorophyll, at the top of a leave than the bottom, this is why the top is greener.

The absorbed blue and red light energy is used by the chloroplasts to split water and carbon dioxide molecules up, they then rejoin to make oxygen and glucose

What is light?

Light is pure energy

Light has properties of both a particle (photon) but it travels in waves

This is known as the "Wave Particle Duality".

Its a hard one and physicists are still trying to work it out.

The best way for us to think of it is as a particle of pure energy traveling in a wave pattern

The speed that the particle is traveling in is known as the speed of light

Roughly 1 Billion Kilometers per hour

This speed is the same for all wave sizes

The size of the up and down of the wave correlates with the distance between the two 'tops' of two wave

The distance between the two tops or crests is known as the wave length

The smaller the distance between the two tops, the shorter the wavelength and the more energy the photon has

Lets look at the size of these wavelengths

Waves with very large wavelengths are like radio-waves. They get smaller, down to microwaves like our microwave oven. They keep getting smaller, down to infra-red then finally visible light

Red has the largest wavelengths, then Orange, Y, G, B, Indigo then Violet.

As we get smaller the photons get more energy, Violet becomes the invisible and dangerous Ultra Violet - that gives us a tan, and skin cancer

Smaller still are the dangerous X Rays and finally extremely dangerous and very high energy Gamma Rays

Leaves absorb the energy from the ROY and BIV parts of the rainbow, using this energy to break water and carbon dioxide and make sugar and molecular oxygen.

Leaves do not use the G part of the rainbow. So this is 'reflected

and so you see this reflected light and to you the leaf 'looks' green.

Virtual Photosynthesis simulator

Have a play with the Virtual Photosynthesis simulator below

  1. Use the clock on the wall of the simulator to monitor time at 1 minute

  2. Keep the bulb brightness (wattage) and distance the same

  3. Change the colour of the filter - Clear, Red, Green, Blue

  4. Have a look at the number of bubbles formed for each filter

  5. What do you notice?

Palisade Mesophyll Cells - Why so upright?

There are 2 types of mesophyll cells - Palisade and Spongy

The Palisade Mesophyll Cells are were most Photosynthesis occurs

They are the top layer of cells after the epidermis

They are at the top of the leaf to get the most light

They are upright to pack as much chloroplast into the space as possible

They are also upright to maximize the surface area for photosynthesis by absorbing most of the light, and reflecting a small amount

In the diagram you can see the advantage to being upright. Any unabsorbed light can bounce (reflect) to the neighbouring cell so it can then be absorbed. A small amount might not be, it will the bounce off to the next cell to be absorbed by the chlorophyll. In the diagram below you will see the change in colour for each reflection - this is to show that only a small amount is reflected with most of the energy being absorbed

Any light that gets past the Palisade Mesophyll Cells will be absorbed by the Spongy Mesophyll Cells which also photosynthesis (just not as much as the Palisade Mesophyll Cells)

Spongy Mesophyll Cells - Where's the Air?

The haybales are like the mesophyll cells, the space to walk and crawl is like the 'air spaces' in the spongy mesophyll

Leaves are green because the chlorophyll in the chloroplasts absorb Blue and Red light, but green is reflected

This blue and red light energy is used to split water and carbon dioxide molecules up, they then rejoin to make oxygen and glucose

To get the carbon dioxide molecules in to the leave they move through stomata.

To get carbon dioxide from the stomata to the photosynthesizing palisade cells, the carbon dioxide must move through air spaces in the leaf. The area that contains the air spaces is called the spongy mesophyll.

Oxygen created by photosynthesis will also move through the spongy mesophyll and out through the stomata.

With oxygen and carbon dioxide moving through the stomata this is the same, but if reverse to our own breathing. Thus, the stomata is like the nostrils of the plant.

The air spaces are kept open by the network of cells, acting like haybales in a haybale maze - these cells are called the Spongy mesophyll cells

With the leaf getting warm due to the sun, water could evaporate from it, to minimize this, water can only leave through the stomata. Evaporation of the water from the stomata, causes water to then be sucked into the leaf through the straw like xylem, from the roots. This process is called transpiration (like the transport of water through breathing)

To prevent water from leaving through the leave, from any other area, the top of the leaf, where there is the most heat energy, has a waxy layer that water can't get through. This is called the waxy cuticle.

Just like our skin, the outside layer of cells is called the Epidermis

Water gets to the leaf through the xylem.

Sugary water leaves the leaf like fanta, through the phloem.

Leaves are green

  • Leaves are green because chlorophyll reflects green light

  • Chlorophyll absorbs blue and red light and uses this energy for photosynthesis

  • Infra-red is also reflected

  • Chloropyll is kept inside of organelles (little organs) inside of the cells called Chloroplasts


Structure of the leaf from the top down

Cuticle - waxy to prevent water getting out

Epidermis - outside most layer of cells

Mesophyll cells - these are the cells of the leaf

All mesophyll cells contain chloroplasts

Palisade mesophyll - these are at the top of the leaf to capture as much of the sun light as possible, and are the reason that the top of the leaf is more green than the bottom of the leaf. These cells do the photosynthesizing

Spongy mesophyll - these cells do some photosynthesizing. However, their main role is to provide structure to the leaf to allow water and air to move within the leave

Air channels - the spongy mesophyll cells act like the walls of a channel that allows carbon dioxide to diffuse from the air to the palisade mesophyll for photosynthesis

Stomata - these are the nostrils of the leaf. They are underneath the leaf. They are holes. They are made up of 2 Guard cells. The guard cells start to close the stomata if there is not enough water - so they regulate water loss from the plant

Vascular Bundle - contains the Vascular Tissues - Xylem and Phloem




In the Vascular Bundle - Vascular Tissue

Like our own Veins and Arteries. The plant has Liquid filled Vessels.

The vessels taking water from the roots up to the leaves are called Xylem

  • Xylem = zup to the leaves

The vessels taking sugar from the leaves to the rest of the plant are called Phloem

  • Phloem = flow away

  • Fanta = sugary drink

  • Phloem = sugary drink

  • Fanta = Flow'em = Phloem

Transpiration

Two Vessels: Xylem and Phloem

Xylem = Just water = water xylophone

Phloem = Sugar = Fanta = Flow'em

Transpiration process

Transpiration its like a giant straw

  1. Water evaporates from the leaves, through the stomata

  2. This causes water to flow from the inner leave closer to the stomata to replace the lost water

  3. This causes water to flow from the Xylem into the leaf

  4. This causes water to flow up the Xylem

  5. This causes water to move from the roots into the Xylem

  6. This causes water to move from the ground into the roots

Transpiration - like combining perspiration and respiration - its the transport of WATER

Translocation

the opposite

Translocation is how sugar moves from the leaves, through the Phloem, and out to the rest of the plant, to its very tip, and down to its smallest root hair cells

  1. Leaf

  2. Phloem

  3. Roots

My Drawing of how the structure of the leaf facilitates photosynthesis

Another version of the drawing