Photosynthesis & Respiration

Concept 5: Photosynthesis

Success Criteria & Vocabulary

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  • I can explain the purpose of photosynthesis in words and equations.

  • I can describe where and how photosynthesis happens in chloroplasts, in terms of light-dependent and light-independent reactions.

  • I can discuss how the internal structures of a leaf are adapted for photosynthesis.

Click this drop-down menu to see the list of Vocabulary.

Chlorophyll: Green pigment in plants responsible for capturing light energy. This light energy is used to split water molecules during photosynthesis.

Chloroplast: Organelle found only in plants, that is the site of photosynthesis.

Glucose: Simple sugar that stores chemical energy.

Grana: Stack of thylakoids.

Light-dependent reaction: Set of reactions in photosynthesis that use energy from light to split water into oxygen and hydrogen atoms. Occurs in the thylakoids/grana.

Light-independent reaction: Set of reactions in photosynthesis that combines hydrogen atoms with carbon dioxide to make glucose. Occurs in the stroma.

NADPH: Carrier molecule that transfers hydrogen from thylakoid (light-dependent reactions) to stroma (light-independent reactions).

Palisade layer: Layer of tightly packed cells that have many chloroplasts; part of the leaf where most photosynthesis occurs.

Spongy layer: Layer of loosely packed cells that have few chloroplasts. Large amount of space between cells so that carbon dioxide and oxygen easily diffuse.

Stomata: Small openings on the underside of a leaf through which oxygen and carbon dioxide can diffuse.

Stroma: Colourless fluid surrounding grana/thylakoids.

Thylakoid: Flattened membranes that contain chlorophyll.


Watch my teaching video on Photosynthesis.

Complete Education Perfect:

Task called '2.4 Concept 5'.

  • Photosynthesis

  • Chloroplasts and photosynthesis.

Complete sciPad:

  • A Closer look at Organelles - Chloroplasts (pg 26)

  • Leaf Cross-Section (pg 27)

  • Photosynthesis (pg 28)

  • The Chemistry of Photosynthesis (pg 29)

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Concept 5: Support Notes

What is Photosynthesis?

Photosynthesis is the process in which plants use light energy , water, and carbon dioxide to produce glucose and oxygen. Glucose is the sugar molecule plants use to make the energy in the form of ATP they need to survive.

For photosynthesis to occur, plants need water, carbon dioxide, and light.

  • Water is absorbed from the soil via osmosis through the roots.

  • Carbon dioxide is taken up by diffusion through very small pores on the undersides of its leaves called stomata

  • Light energy is captured by chlorophyll in chloroplasts.

Two Important Set of Reactions during Photosynthesis

There are two chemical pathways in the process of photosynthesis.

Light-Dependent Reactions

The first set of chemical reactions are called the light-dependent reactions. It’s called light dependent, because for it to happen it needs light. This takes place on the thylakoid membranes of the chloroplast.

Here the light energy is absorbed by the pigment found within the thylakoid membranes called chlorophyll. This light energy is used to split water into hydrogen and oxygen, and this light energy produces a little bit of ATP. After water has been split, the oxygen is released through the stomata and becomes part of the air we breathe, and the hydrogen is then taken to the second chemical pathway by a carrier molecule called NADPH.

The ATP and NADPH produced from the light-dependent phase are then used to provide the chemical energy/hydrogen ions to make glucose in the next phase of photosynthesis.

Light-Independent Reactions

The second set of chemical reactions are called the light-independent phase or the Calvin cycle. It is called light-independent, because it doesn’t need light to happen.

The light-independent phase occurs in the stroma of the chloroplast.

Here, carbon dioxide from the air and hydrogen (carried to the stroma by NADPH) are joined together through a series of enzyme controlled reactions to produce the final product, glucose. Oxygen is produced as a by-product (waste product) and is eliminated by diffusion through the stomata.

The end product, glucose, is used in cell respiration (aerobic and anaerobic) to create energy in the form of ATP.

You must remember that all of the chemical pathways involved in photosynthesis are controlled by enzymes.

Structure of a Leaf

The cells that make up a plant’s leaves are structured in such a way that maximises the rate of photosynthesis.

1) Waxy cuticle prevents water loss.

Covering the outside of a leaf is a waxy, shiny layer called a waxy cuticle. This waxy cuticle prevents water loss from the upper surface of the leaf, which is important for stopping the plant from drying out. Underneath the waxy cuticle is a single layer of cells called the upper epidermis. These cells are transparent, allowing light to pass through it into the cells below.

2) Role of palisade cells.

Beneath the upper epidermis is the palisade layer. The palisade cells are packed very tightly together and have a rectangular shape. They also have many chloroplasts and are the main site of photosynthesis.

The upright positioning of the palisade cells allows these chloroplasts to receive as much of the sunlight that is entering the cell as possible. The chloroplasts are also found close to the palisade cell membrane (because there is a large, central vacuole that pushes chloroplasts to the periphery). This reduces the diffusion distance for carbon dioxide and water to move into the chloroplast and for oxygen to move out. These two factors combine to maximise the rate of photosynthesis.

3) Loosely packed spongy cells allow gases to diffuse easily.

Under the palisade layer is the spongy cell layer. The cells within this layer are more round than palisade cells, have very few chloroplasts, and are loosely packed. This layer resembles a sponge, hence its name spongy layer. This means there is a large amount of space around these cells, which provides some space for carbon dioxide and oxygen to easily diffuse into and out of the plant. This means carbon dioxide can quickly reach the palisade cells where it is needed for the process of photosynthesis, maximising its rate.

4) Stomata can open and close to regulate diffusion of molecules into and out of the leaf.

Underneath the spongy cell layer is the lower epidermis. This is the same as the upper epidermis, except it contains many stomata that allow the diffusion of molecules into and out of the leaf.

5) Chloroplasts can re-distribute within a cell to get more or less light.

Chloroplasts can move within a cell to get more light. This allows the chlorophyll molecules to absorb more light energy and makes sure that they all have equal access to the materials they need to carry out photosynthesis.

6) Many thylakoid membranes on top of each other maximises surface area.

The thylakoid membrane within chloroplasts are stacked on top of each other. This increases the surface area available for the absorption of light.

7) Cell membrane and stroma are transparent.

The stroma surrounding the structures within chloroplasts is transparent, meaning it doesn’t block out light. It also means that if the sunlight coming into the chloroplast doesn’t hit a thylakoid it can easily pass through to the next chloroplast!

This combination of leaf cell structure and the structure of chloroplasts helps plants maximise their rate of photosynthesis.

Video: Photosynthesis (Amoeba Sisters) + WORKSHEET Fantastic summary of photosynthesis, chloroplast, and light dependent/independent reactions.
Video: Plant Structure and Adaptations (Amoeba Sisters). More information on plant adaptation, that indirectly optimises the rate of photosynthesis.

Concept 6: Respiration

Success Criteria & Vocabulary

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  • I can explain the purpose of cellular respiration in words and equations.

  • I can describe where and how aerobic and anaerobic respiration happens in terms of glycolysis, Krebs cycle, and electron transport chain.

  • I can emphasise the difference between aerobic and anaerobic respiration.

  • I can compare the number and structure of mitochondria in specialised cells.

Click this drop-down menu to see the list of Vocabulary.

Aerobic respiration: Enzyme controlled process which requires oxygen to produce 34-38 ATP from the breakdown of glucose.

Anaerobic respiration: Enzyme controlled process which occurs in the absence of oxygen to produce 2 ATP from the breakdown of glucose.

ATP: Energy 'currency' of the cell, used to perform many cell processes.

Cristae: Infoldings of the inner mitochondrial membrane that is the site of the electron transport chain.

Cytoplasm: Jelly-like fluid organelle inside the cell in which the organelles are suspended.

Electron transport chain: Enzyme controlled process that uses a hydrogen ion concentration gradient to produce a large number of ATP. Occurs across the mitochondrial inner membrane/cristae.

FADH2: Carrier molecule that transfers hydrogen from the Krebs cycle to the electron transport chain.

Glucose: Simple sugar that stores chemical energy.

Glycolysis: Enzyme controlled process that breaks down glucose into 2 x pyruvate, ATP, and NADH. Occurs in the cytoplasm.

Krebs cycle: Enzyme controlled process that breaks down pyruvate derivative (acetyl CoA) into CO2, NADH, and FADH2. Occurs in the mitochondrial matrix.

Lactic acid: Toxic chemical produced during anaerobic respiration.

Matrix: Fluid contained within the inner membrane of mitochondria, that is the site of the Krebs cycle.

Mitochondria: Organelle found in both plants and animals that converts the chemical energy stored in glucose into ATP.

NADH: Carrier molecule that transfers hydrogen from glycolysis and the Krebs cycle to the electron transport chain.

Pyruvate: Three-carbon compound that forms as an end product of glycolysis.

Surface area: Total area occupied by the surface of an object.


Watch my teaching video on Respiration.

Complete Education Perfect:

Task called '2.4 Concept 6'.

  • Mitochondria and Respiration

  • Aerobic and Anaerobic Respiration

  • Respiration Specialisation

Complete sciPad:

  • Page 16 - A Closer look at Organelles - Mitochondria

  • Page 17 - Mitochondria and Aerobic Respiration

  • Page 18 - The Cytoplasm and Anaerobic Respiration

  • Page 19 - Cellular Respiration in Intertidal Mussels

  • Page 21 - Different Cells - Different Energy Requirements

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Concept 6: Support Notes

Plant AND Animal Cells Need ATP

ATP (adenosine triphosphate) is the “energy molecule” or “energy currency” of cells. This means it stores the ATP required to fuel many life processes. This includes growth and repair, mitosis, reproduction, photosynthesis, and active transport.

Without ATP, plant and animal cells can't do anything. So it will continue to occur as long as a cell is alive. This means that even when you are asleep, your cells are still active and working to constantly make ATP, to keep your cells functioning.

Cellular respiration is the process of converting GLUCOSE (from food in animals or photosynthesis in plants) into ATP.

There are TWO Types of Cellular Respiration

The two types of respiration are: AEROBIC and ANAEROBIC RESPIRATION.

  1. Aerobic respiration requires oxygen and is very efficient (i.e. it can produce a high number of ATP molecules per GLUCOSE molecule).

  2. Anaerobic respiration does not require oxygen, but is only about 5% as efficient as aerobic respiration (i.e. it produces a low number of ATP molecules per glucose molecule).

On top of that, there are three chemical pathways in the process of respiration. These are:

  1. GLYCOLYSIS in the CYTOPLASM, glucose is broken down into 2 x PYRUVATE molecules

  2. KREBS CYCLE in the MITOCHONDRIA, 2 x pyruvate molecules used to produce hydrogen and CO2 (by-product)

  3. ELECTRON TRANSPORT CHAIN in the mitochondria, where hydrogen is used to produce 38 ATP.

Aerobic respiration uses all three of the chemical pathways of respiration, but anaerobic respiration only uses the first pathway, glycolysis. This is because both the Krebs cycle and the electron transport chain require oxygen.

Structure of the Mitochondria

A MITOCHONDRION'S outer membrane is smooth. This membrane acts as a selective barrier, which is responsible for transporting material into and out of the organelle.

The inner membrane of a mitochondrion is highly folded into structures called CRISTAE. This membrane is the site of the ELECTRON TRANSPORT CHAIN, the process which makes the majority of ATP molecules produced by respiration.

The presence of cristae gives this membrane a high SURFACE AREA so the electron transport chain can maximise ATP production.

In case you're interested: Basically, during cellular respiration, each glucose molecule is GRADUALLY (i.e. step-by-step) broken down through a complex series of reactions (listed above). These three reactions can only occur because they are catalysed by enzymes. Consequently, any changes in enzyme activity can affect the rate of respiration.
Glycolysis is the first pathway of respiration and the only one that does not occur within mitochondria. Glycolysis takes place within the cytoplasm. The products of this reaction are then transported to the matrix of a mitochondrion where the second pathway, the Krebs cycle, occurs. The last pathway is the electron transport chain. This process produces ATP by moving charged ions across the inner membrane.

Anaerobic Respiration in the Cytoplasm



  • 1 x GLUCOSE only creates 2 x ATP = inefficient

  • Not many ATP produced, therefore the ATP made is used up quickly.

  • LACTIC ACID is produced as a by-product → lowers cell pH → toxic to enzymes.

  • Less reactions involved than AEROBIC RESPIRATION = fastest way to make ATP.

Basically, during GLYCOLYSIS, a glucose (6-carbon) molecule is broken down into 2 x PYRUVATE molecules (3-carbon). During this process, 2 ATP molecules and 2 NADH molecules (hydrogen carrier molecules) are produced. The NADH molecules are each carrying one hydrogen ion.

What’s important is what happens to the pyruvate molecules. In the presence of oxygen, the pyruvate molecules enter the MITOCHONDRIA to be further metabolised in the KREBS CYCLE. In the absence of oxygen, the pyruvate is converted into LACTIC ACID in the CYTOPLASM (through a series of ENZYME-controlled reactions).

Why do cells make ATP via Anaerobic Respiration?

ANAEROBIC RESPIRATION occurs in the ABSENCE of oxygen, in the CYTOPLASM of an animal or plant cell.

You can think of anaerobic respiration as incomplete respiration. Anaerobic respiration only uses GLYCOLYSIS so far fewer reactions need to occur. The reactions can also occur anywhere within the cytoplasm, so there are not many limitations on the SPEED of these reactions. This makes anaerobic respiration a very fast way to produce ATP.

Anaerobic respiration also produces LACTIC ACID as a by-product. When lactic acid builds up, it lowers the pH of a cell. This can affect the structure and function of enzymes that are needed for cells to function properly. Additionally, lactic acid is a large molecule that cannot be easily removed from a cell until it has been broken down. Lactic acid is what causes muscle cramps!

Cells only use anaerobic respiration when they cannot get enough oxygen, or when they need ATP more quickly than aerobic respiration can produce it. If the cells use up what little oxygen they have stored, and they can’t get more oxygen fast enough, anaerobic respiration allows them to continue to produce ATP.

Aerobic Respiration in the Cytoplasm


In AEROBIC RESPIRATION, all pathways (GLYCOLYSIS, KREBS CYCLE, and the ELECTRON TRANSPORT CHAIN (ETC)) are involved. Oxygen is needed for Krebs cycle and ETC in the mitochondria.

  • 1 x glucose creates 38 ATP molecules = efficient

  • No LACTIC ACID produced. CO2 is a waste product that can easily DIFFUSE out of the cell.

  • More reactions involved = a longer process to make (a lot of) ATP.

When cells have access to oxygen, they will usually use all three chemical pathways to produce ATP. This is aerobic respiration. The two chemical pathways that require oxygen, the Krebs cycle and electron transport chain, both occur within the mitochondria.

During aerobic respiration, glucose is broken down in the presence of oxygen to produce carbon dioxide (by-product), water (by-product), and ATP. Carbon dioxide and water molecules are very small so they can be transported out of a cell through simple diffusion.

For each molecule of glucose, aerobic respiration can produce 38 ATP molecules, creating 95% more ATP from each molecule glucose than anaerobic respiration. As long as cells are able to access sufficient oxygen, they will produce ATP through aerobic respiration, so this occurs the vast majority of the time.

As covered already, we know that glucose is converted into 2 x PYRUVATE molecules during GLYCOLYSIS. In the presence of oxygen, these 2 pyruvate molecules enter the mitochondrion MATRIX to undergo the KREBS CYCLE.

The Krebs cycle is a series of ENZYME-controlled reactions that uses pyruvate molecules to produce a small number of ATP molecules, hydrogen-carrying molecules (NADH and FADH2), and carbon dioxide (by-product). The carbon dioxide diffuses out of the cell and is eliminated from the body via the lungs. The important products of the Krebs cycle are the hydrogen-carrying molecules (NADH and FADH2) - these molecules are used in the ELECTRON TRANSPORT CHAIN to produce a very large number of ATP molecules.

The electron transport chain is a series of enzyme-controlled reactions that happen across the inner mitochondrial membrane. (Basically, the NADH and FADH2 hydrogen carrier molecules create a huge accumulation of H+ ions in the intermembrane space. This huge H+ gradient across the inner mitochondrial membrane drives an enzyme called ATP synthase to produce 38 ATP molecules).

Comparing Aerobic and Anaerobic Respiration

Comparing Aerobic and Anaerobic Respiration

Plants and animals have many different types of cells that are all specialised to perform different functions. Due to this specialisation, different types of cells contain different compositions of organelles. For example, mammalian red blood cells have no organelles at all. This is so they can pack in as much haemoglobin molecules inside the cell as possible.

Cell specialisation has a large effect on the number of MITOCHONDRIA different types of cells contain. The tasks that cells are specialised for may require a great deal of energy or only a small amount. The amount of energy required by different cell types is strongly correlated to the number of mitochondria those cells contain.

The structure of mitochondria within different types of cells can also vary. For example, mitochondria within cells with high energy requirements such as muscles, can have more CRISTAE. This increases the surface area of the inner membrane, which allows for a greater production of ATP.

Some cellular processes that require energy are very obvious to us, such as the muscles helping us move. However, there are many other cellular processes that require energy that are not as visible. This includes actions such as:

  • Metabolising nutrients

  • Building proteins

  • Transporting molecules across membranes

  • Breaking down waste and toxins

  • Growth and replication

Almost all cellular processes require ATP and without it, organisms would quickly die.

Example: Cardiac Muscle Cells

Cardiac (heart) muscle cells have extremely high numbers of mitochondria. Cardiac muscles are always working to pump blood around the body of an animal to keep it alive. To achieve this, cardiac muscle cells contain many thousands of mitochondria to produce enough ATP.

Example: Liver Cells

Liver cells also contain a high number of mitochondria, usually 1 or 2 thousand. You might not realise how active your liver cells are because they are working out of sight. Liver cells process and store nutrients, toxins, and drugs, filter the blood, produce proteins and bile, and have to transport a huge number of molecules. All of these processes require ATP!

Example: Red Blood Cells

The animal cells with the fewest mitochondria are the red blood cells. These cells have no mitochondria at all! Red blood cells transport oxygen around the body, but because oxygen is a small, uncharged molecule, it can move into and out of cells passively without the need of ATP.

Mammalian red blood cells break down and remove all of their organelles when they mature. This allows them to be very small so they can move through capillaries easily. Without organelles, there are very few processes that need to be fueled. What little ATP they do require can be produced anaerobically (via glycolysis) in the cytoplasm.

Example: Skin Cells

Skin cells provide an important protective barrier for organisms; however, they are relatively inactive cells so they have few mitochondria (usually just a few hundred). The ATP produced by skin cells is mostly used for DNA replication. This is because skin cells are constantly being shed, so they need to be replaced by new cells.

Concept 7: Comparing Photosynthesis & Respiration

Success Criteria & Vocabulary

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  • I can discuss the factors that affect the rate of photosynthesis.

  • I can discuss the factors that affect the rate of respiration.

  • I can describe the similarities and differences between photosynthesis and respiration.

Click this drop-down menu to see the list of Vocabulary.


Watch my teaching video:

Complete Education Perfect:

Task called '2.4 Concept 7'.

  • Factors Affecting the Rate of Photosynthesis.

  • Maximising the Rate of Photosynthesis.

  • Factors Affecting Respiration Rate

  • Photosynthesis vs. Respiration

Complete sciPad:

  • Page 30-32 - Factors Limiting the Rate of Photosynthesis

  • Page 37 - Cell Structures Chapter Review Question 5

  • Page 39 - Cell Structures Review Question 8 and 9

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Concept 7: Support Notes

Limiting Factors of Photosynthesis

A limiting factor is an essential resource that is in least supply or availability.

The rate of any reaction or process will always correspond to the factor which is in least supply.

The rate of photosynthesis will always correspond to the factor which is in least supply:

  • CO2 concentration

  • Water availaility

  • Light availability

  • Chloroplast number/distribution

  • Time of day

  • Nutrient availability

  • Other factors that affect enzyme function (e.g. temperature, pH, enzyme concentration, inhibitors, co-factors/co-enzymes, substrate concentration).

Carbon dioxide concentration

The carbon and oxygen atoms from CO2 are needed to make glucose.

↓ CO2 conc → ↓ photosynthesis rate

↑ CO2 conc → ↑ photosynthesis rate (upward slope in graph)

UP UNTIL other factors become limiting factors (plateau in graph).

Water availability

Hydrogen atoms from water are needed to make glucose.

↓ H2O conc → ↓ photosynthesis rate

↑ H2O conc → ↑ photosynthesis rate

UP UNTIL other factors become limiting factors.

*Extended explanation for carbon dioxide concentration*

Carbon dioxide is one of the raw materials needed for the process of photosynthesis because it provides the carbon and oxygen atoms needed to make glucose, making it an important limiting factor that can affect the rate of photosynthesis.

If the concentration of carbon dioxide in the atmosphere is low, the rate of photosynthesis will also be low. It doesn’t matter how much sunlight or water is available, if carbon dioxide concentration is low, the rate of photosynthesis will be low because carbon dioxide is essential for photosynthesis. There won’t be enough carbon and oxygen atoms to make glucose. If concentration of carbon dioxide in the atmosphere increases, there will be more carbon dioxide available to diffuse into the leaves of a plant. This increases the rate of photosynthesis up to a maximum point.

If carbon dioxide continues to increase past this maximum point, the rate of photosynthesis will level off. This is because other factors, such as the availability of sunlight, become limiting factors. No matter how much carbon dioxide is available for a plant to absorb, the other factors involved in photosynthesis also need to increase to bring about a further rise in the reaction rate.

*Extended explanation for water concentration*

Water is also one of the raw materials needed for photosynthesis, because it provides the hydrogen molecules needed to make glucose.

If there is a lack of rain, or plants are not watered regularly, then the soil can dry out. This means water can no longer diffuse from the soil into the roots by osmosis, and the rate of photosynthesis will decrease. This is because there will be less hydrogen, so fewer glucose molecules will be formed. That’s why the leaf is coated on the outer side with a waxy cuticle prevents or reduces transpiration also known as water loss which would decrease the rate of photosynthesis. Specialised guard cells that open and close stomata on the underside of leaves also function to control the movement of gases and the rate of transpiration.

As the amount of water increases, the rate of photosynthesis will increase to a certain point after which a further increase in water will no longer increase the rate of photosynthesis any further. That’s because other factors will become limiting.

Light availability and wavelength

Different intensities of light provide different amounts of energy.

Different wavelengths of light provide different amounts of energy.

↑ intensity → ↑ photosynthesis rate

UP UNTIL other factors become limiting factors


Too high → bonds break → enzymes denature → ↓ rate of photosynthesis.

Too low → molecules move more slowly → ↓ enzyme/substrate collisions → ↓ rate of reaction.

*Extended explanation for light availability and wavelength*

Light energy is important for the process of photosynthesis. It is absorbed by chlorophyll and provides the energy that is needed to split water into hydrogen and oxygen. Similar to carbon dioxide, increasing the intensity (or brightness) of the light will increase the rate of photosynthesis to a maximum point.

Above this maximum point, any further increases in the light intensity will have no effect on the rate of photosynthesis. The rate of photosynthesis will eventually level off as other factors such as temperature or the concentration of chlorophyll molecules become the limiting factors. It’s also important to note that different wavelengths of light provide different amounts of energy.

*Extended explanation for temperature*

Each type of enzyme that is associated with photosynthesis has a unique range of temperatures in which they can function. Within this range, there will be a specific temperature where the rate of photosynthesis will be the fastest. This is called the optimum temperature.

(Normally, an increase in light intensity leads to an increase in temperature). If the temperature increases slightly, it increases the kinetic energy of the enzymes and substrates and causes them to move faster. This causes them to collide more frequently, increasing the rate at which the enzymes and substrates bind together at the active site. This will increase the rate of photosynthesis.

Generally, warmer temperatures are better than cooler temperatures for photosynthesis. This means enzymes can collide more with substrates (because they have more kinetic energy) and more chemical reactions can occur.

But, if the temperature gets too high, the rate of photosynthesis may decrease or stop completely. This is because the enzyme may lose its shape/denature, resulting in the active site changing shape (no longer fitting the substrate). As a result, the chemical reactions would stop, and rate of photosynthesis will decrease or stop completely.

If the temperature gets too low, the enzymes and substrates will have less kinetic energy. This means they move much slower and the frequency of collisions between the enzymes and their substrates will decrease. This will also cause the rate of photosynthesis to decrease.

Time of Day/Season

Greater rate of photosynthesis at midday than early morning/early evening. Greater rate of photosynthesis in summer than winter.

Because of temperature and light availability.

Nutrient Availability

Nutrients are absorbed from the soil via active transport.

  • Potassium → co-factor for enzymes

  • Nitrogen → used to synthesise some AAs.

  • Magnesium → used to make chlorophyll.

  • Amino acids → used to synthesise enzymes.

*Extended explanation for time of day/season*

The time of day, and the season (winter, or summer) affects the rate of photosynthesis because time of day and season determines light intensity and temperature

The rate of photosynthesis at is greater at midday than early morning or evening, and there is little to no photosynthesis at night.

The rate of photosynthesis is greater in the summer months compared to the winter months.

*Extended explanation of nutrient availability*

Soil is a major source of nutrients for plants. They need to absorb essential nutrients such as potassium, nitrogen, and magnesium to support healthy cellular processes and growth. Some soils naturally have very low nutrient concentrations, so farmers and gardeners use fertilisers to supply their plants with the nutrients they need for healthy and fast growth.

Nutrients must be dissolved in water for plants to use them. They are taken up by the root hair cells found on the roots of plants. Unlike water and carbon dioxide, the concentration of nutrients in the soil is much lower than inside the plant. Therefore, they cannot be taken up by osmosis or diffusion.

Instead, active transport has to be used to move nutrients into a plant. This requires energy, as the nutrients are being moved against the concentration gradient.

Chloroplast structures

There are several structural features of chloroplasts that maximises the rate of photosynthesis.

  1. The cell membrane and stroma is clear and transparent. This means the stroma does not block the light (i.e. and so light can reach the chlorophyll).

  2. The increased numbers of flattened thylakoids (containing chlorophyll) increases the surface area for absorbing light.

  3. The chloroplasts themselves are can move within the cell (they can distribute themselves) in response to light availability.

  4. In low light, chloroplasts move in order to get more light. This also gives chloroplasts equal access to light. As a result, chloroplasts/thylakoids absorb more total light energy that splits water molecules (into hydrogen and oxygen) and increases the rate of photosynthesis.

  5. In extreme light conditions, the high light intensity can actually damage chloroplasts, so they move to adjust/reduce their light exposure.

Limiting Factors of Respiration

Factors that can affect the rate of respiration fall into 3 categories - the state of the cell, limiting molecules, and factors that affect enzyme activity.


Cells that are highly active have a high rate of respiration.

This means that whenever the rate of chemical reactions in a cell increases, the rate of respiration will also increase to produce more ATP to carry out the chemical reactions. For example, liver cells use ATP to break down and remove toxic substances from the blood. A common toxic substance the liver needs to break down is alcohol. When people consume alcohol, the work their liver cells need to do is increased, and therefore cellular respiration is increased in the liver.

Growth during childhood and puberty also increases the rate of respiration

And growth occurs due to a process of cell division called mitosis. Mitosis is a process that uses ATP, so during growth, cells need more ATP, which increases the rate of cellular respiration.

Growth is a particularly important factor affecting the rate of cellular respiration in plants. For example, growing young leaves use more ATP than old leaves; growing stem tips use more ATP than older branches; and use fruit a lot of ATP as they grow and ripen. The areas where the plant is growing have higher respiration rates because more ATP is being used to fuel reactions.

Repair/healing increases the rate of respiration

Respiration rates can also be increased when cells are healing from damage or infection. In both animals and plants, healing processes require ATP. The more ATP being used to heal, the higher the rate of respiration. If you have any cuts or scratches on your skin, these areas will be respiring more than the skin around them.



Glucose is required for both aerobic and anaerobic respiration. When cells have limited access to glucose, their rates of respiration will decrease.


Aerobic respiration requires oxygen. When oxygen levels are low, the reactions of the Krebs cycle and electron transport chain will be restricted.

Lactic acid

Lactic acid reduces the rate of respiration because it lowers the pH of the cell, and can cause enzymes to denature. When enzymes are denatured, they cannot catalyse the reactions of respiration. Until the lactic acid can be broken down and the pH of the cell restored, the rate of respiration will be limited.



Low temperatures inhibit the action of enzymes by slowing down the movement of molecules. When molecules are moving less, successful enzyme-substrate collisions will occur less frequently which will slow down, or even stop enzyme activity.

High temperatures decrease the rate of enzyme activity by denaturing enzymes. When the temperature gets too hot, the bonds that hold an enzyme in its specific shape can be disrupted and the enzyme will unfold and lose its shape. When an enzyme has lost its shape, it can no longer able to bind to a substrate, and will not be able to catalyse any reactions.


Changes to the pH of a cell can also affect the rate of enzyme activity. This is because a pH that is too high or too low can also disrupt the bonds that hold the specific shape of an enzyme. As mentioned earlier, one factor that can reduce the pH of a cell is the build-up of lactic acid but there are many other molecules that can affect the pH of a cell.


Enzyme activity can also be lowered by the presence of inhibitors. The common inhibitors are heavy metals. These bind to enzymes and prevent them from binding with their specific substrates. For example, cadium can combine with the active site/enzyme, blocking it or changing its shape, and prevents substrates from attaching and a product being formed. This prevents the enzyme functioning correctly and is irreversible.

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