An Introduction to Photosynthesis – 6.1, 6.2

My name is Dr. Matt Ivory.

I'm Electra in the School of Pharmacy and Pharmaceutical sciences.

And in this series of talks, we're gonna look at plants.

We'll look at how they make their food and the various

factors that can affect that.

And we'll look at the structure of plants both on a cellular

level and a larger organism level and look at how their

structure is adapted to the different environments that they live in.

So in this first talk, we're gonna look at photosynthesis.

So, plants are really important

because they fix energy. So all organisms

on the earth need a source of energy to survive,

so food basically.

And

it could be that organisms all ate one another to get food. So

obviously, we don't produce our own food.

We can't photosynthesize.

And so we need to eat plants and animals and other organisms

in order to get their energy.

So,

if this was the case for every single organism on the earth,

then there'd be energy lost every time one organism ate

another. So if you haven't seen the food chain talk, it's

explained in more detail there.

But a good way to think about it is,

if you think about Newton's cradle,

So the series of ball bearings where they transfer energy to

one another. So if you pick up that first ball and let it go,

it transfers most of its energy to the other balls in the system.

And then the one at the end moves up. But eventually,

because it's not perfect energy transfer,

it runs out of energy and stops.

So if all of the organisms on earth relied on each other for

food, eventually,

all the energy would be lost and all of the organisms would

die out. So we need some source of energy that can fix

outside energy and bring it into the ecosystems of the earth.

So, This is where photosynthetic organisms come in. So,

this is any organism that can use photosynthesis to make its own food.

So green plants and algae are two of the main examples of that.

And so they've evolved a way of trapping the energy that hits

the earth in the sun's light.

And turning it into food and they do so through a process

called photosynthesis.

And that's why they're called photosynthetic organisms.

So these organisms are called producers. So in terms of

trophic levels, they are always the lowest trophic level.

Because they make their food from the sun.

So they don't consume other organisms usually.

And so they get their energy from the sun's light. And so this provides

a renewable source of energy.

You could say that the sun obviously constantly shining in the day.

Lesser in the UK. But in other countries, lots of sun.

And so lots of energy.

So you don't have to worry about it running out and not

least for many, many years from now.

So this injects energy into these trophic levels and allows

other organisms that eat producers or organisms that eat

the organisms that eat producers to get energy and to

build biomass and to survive.

So photosynthesis itself is a chemical reaction.

And it occurs in cellular structures called chloroplasts.

So in the talk that discuss the different structures of

different cell types. So plant cells contain chloroplasts.

And within the chloroplasts, there's a pigment,

a green pigment called chlorophyll.

And so this is what they use to absorb the energy from the sun

and that helps to drive the chemical reaction that builds

glucose molecules.

We'll talk a little bit later about how it does that.

What it uses is its substrates.

So it takes in carbon dioxide from the atmosphere as well.

So it needs the sunlight.

It needs carbon dioxide and it needs water as well.

So it draws water through its root system from the soil.

So carbon dioxide from the air,

water from the soil and the sun's light from the sun. And

so when it performs this chemical reaction,

their byproducts are glucose,

so that's the main thing that it wants to make,

and it also releases oxygen. So This is really important for

life on earth as well.

So not only do plants provide food chains with energy but it

releases oxygen into the atmosphere.

And this oxygen is used by any organism that relies on aerobic respiration

to get its energy to drive cellular processes and to survive.

So photosynthesis as a chemical reaction is described as being

endothermic and that's because the products at the end of the

reaction have more energy than the reactants that are used at

the start. So overall,

the reaction takes in energy as you move from left to right.

If you compare this to respiration, the opposite is

true. So, respiration,

you're releasing that energy from the molecules,

so it can be used in cells. And that's an exothermic reaction.

So, the chemical equation for photosynthesis

So you take carbon dioxide and water and then in the presence

of light and chlorophyll.

You produce glucose and oxygen.

And in terms of the number of molecules,

an easy way to remember it is that it's six of one and a half

a dozen of the other for the reaction.

So six molecules of carbon dioxide, six molecules of water.

And then you get one molecule of glucose.

So that's the chemical formula c six h twelve o six,

and six molecules of oxygen. So three sixes and one molecule of

glucose between the two sides of that equation.

So once the plant cell has made glucose,

then it will be joined together.

So if they don't do anything with it,

it would just build up in the cell.

And if you've watched or talked about osmosis,

you'll know that glucose dissolved in water will raise

the osmotic pressure of a cell, which can cause damage.

And so they link together all these different molecules of

glucose. And form starch,

which is an insoluble polymer of glucose molecules.

And so this allows the plant cell to store the glucose that

it's produced without affecting its osmotic potential.

The cell might then break down the starch to release some

glucose, and it might use that glucose for respiration.

It might be that it uses it for

producing other bio molecules, so things like cellulose,

or amino acids or lipids,

or it could be that it transports it around the plant.

So you get photosynthetic

cells tend to occur in the leaves of plants.

And you've obviously got lots of other tissues and cells in

the plants that will rely on glucose for aerobic respiration.

And so, it'll transport sugars in the form of sucrose

to the various other tissues of the plant to make sure that all

of the cells in the plant have enough energy to perform their tasks.

Some plants as well will have storage organs, so things like

potatoes, parsnips, will store excess sugar

in their root system. So that's a potato is essentially a big

store of all the energy that the plants made.

And so, yeah, it's important that these cells transport

their produce to all the other cells within the plant so that

everything has enough energy.

So I mentioned briefly about synthesizing other bio

molecules, so things like amino acids and lipids and from the

glucose that's produced by photosynthesis.

And that's really important because that allows the plant

to build biomass. So biomass is the mass of an organism.

And so by building its biomass, it's able to

undergo cell division.

Within the plant and that allows it to grow.

Cell elongation can occur as well and we'll talk more about that later.

And this builds,

basically the plant can grow and become a bigger organism.

So

This biomass is essential in food chains. So the bigger the

biomass that's available,

the more biomass can be supported on the next level,

the more organisms can be supported as well.

So photosynthetic producers are essential in supporting food chains.

And so,

a change in the amount of biomass produced can affect

trophic levels further down as well.