5A - photosynthesis

Photosynthesis is the biological process where photoautotrophs capture light energy from the sun and convert it into chemical energy.

6 CO2 + 12 H2O

sunlight

C6H12O6 glucose

Sugar

+ 6 O2 + oxygen

6 H2O water

carbon dioxide

water

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Theory details
Overview of photosynthesis

Plants are photoautotrophs, meaning that they do not consume the food they need to survive as we humans and other animals do. Instead, they create their own energy via photosynthesis. Photosynthesis is the process in which light energy is harnessed to produce glucose – the energy source of plants. Algae and photosynthetic cyanobacteria are also photoautotrophs capable of undertaking photosynthesis.

In essence, photosynthesis uses two inputs – carbon dioxide and water – to produce the outputs – glucose, oxygen, and water (Figure 1). For this process to occur, sunlight is also required to energise the reaction. Glucose is the primary product of photosynthesis. It is either used immediately as a source of energy for cellular respiration, stored as starch, or used to form more complex molecules such as cellulose. Photosynthesis is not as simple as Figure 1 makes it look. Instead, it is a complex series of biochemical reactions (and their regulatory enzymes) that can be broken into two stages, which we will explore later in this lesson.

photoautotroph an organism capable of undertaking photosynthesis

photosynthesis the process of capturing light energy to power the production of glucose and oxygen from carbon dioxide and water

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Figure 1 The chemical and word equations of photosynthesis. Note the inclusion of sunlight (above the arrow) to represent the need in the process.

6 CO2 + 6 H2O sunlight C6H12O6 + 6 O2

Figure 2 The simplified equation for photosynthesis. As water is both an input and output of the process, we can simplify the equation by subtracting 6 H2O from each side.

Plant structures involved in photosynthesis

Leaves are the main site of photosynthesis in plants and typically have a large surface area to maximise the amount of light hitting the surface. Leaves contain many different cell types that perform varying functions, from structural support to channelling water and nutrients. The main cells in leaves that photosynthesise are called mesophyll cells. Inside mesophyll cells are large populations of chloroplasts, the organelle that is the
site of both stages of photosynthesis. To zoom in even further, within chloroplasts is the photosynthetic pigment known as chlorophyll, which is directly responsible for initiating photosynthesis by capturing and being energised by light energy.

Tiny pores on the surface of leaves known as stomata open to allow carbon dioxide in the atmosphere to diffuse into the leaf. Stomata can also close to prevent water loss from the leaf in dry conditions. Water is absorbed by the root hair cells of plants from the soil and transported through the xylem to photosynthesising cells.

mesophyll cell a plant cell type found in leaves that contain large amounts of chloroplasts

chloroplast a membrane-bound organelle only found in plant and photoautotroph cells that is the site of photosynthesis

chlorophyll a chemical found in the thylakoids of chloroplasts. It is responsible for absorbing light energy in photosynthesis

stoma (pl. stomata) a small pore on the leaf’s surface that opens and closes to regulate gas exchange

xylem vascular tissue in plants responsible for transporting water and minerals from the roots to the leaves

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light

carbon dioxide

water

oxygen

water

mesophyll cells

wax cuticle

wax cuticle

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Figure 3 The inputs and outputs of photosynthesis and the structure of a leaf
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stomata

In the next lesson, lesson 5B, we will consider the difference between three types of plants – C3, C4, and CAM plants. These plants have evolved differences

in their photosynthetic pathways. For the purposes of this lesson, however,
we will focus solely
on the process of C3 photosynthesis.

242 ChapTer 5: phOTOSYNTheSIS

The light-dependent stage 3.2.4.1 Overview

In the first stage of photosynthesis, plants are dependent on light to split water into oxygen and hydrogen. This light-dependent stage occurs on the thylakoid membranes of chloroplasts.

Theory details

As the name suggests, the light-dependent stage of photosynthesis only occurs
when light is present. The light-dependent reactions occur on the chlorophyll-filled thylakoid membranes which make up the grana inside a chloroplast (Figure 4). The reactions in the pathway are catalysed by various enzymes and the purpose of this first stage is to generate the high energy coenzymes NADPH and ATP to power the second stage of photosynthesis.

light-dependent stage the first stage of photosynthesis, where light energy splits water molecules into oxygen and hydrogen inside the thylakoid membranes. Also known as the light-dependent reactions

thylakoid a flattened sac-like structure housed inside the chloroplast. Each thylakoid is made up of a chlorophyll-containing membrane enclosing a lumen. Thylakoids are the location of

the light-dependent stage of photosynthesis

granum (pl. grana) a stack of thylakoids

NADPH a coenzyme that is a proton (H+) and electron carrier in photosynthesis

ATP adenosine triphosphate, a high energy molecule that, when broken down, provides energy for cellular processes

thylakoid lumen

thylakoid membrane

granum

Figure 5 An electromicrograph of thylakoids in a chloroplast

photolysis the process in which molecules are broken down by the action of light

inner membrane

granum

outer membrane stroma

thylakoid

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Figure 4 The key structures of a chloroplast. The light-dependent stage of photosynthesis occurs on the thylakoid membranes of the grana, whereas the light-independent stage occurs in the stroma.

The inputs of the light-dependent stage are:

  • 12 water (H2O) molecules

  • 12 NADP+

  • 18 ADP + Pi.
    The outputs of the light-dependent stage are:

  • 6 oxygen (O2) molecules

  • 12 NADPH

  • 18 ATP.

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water

NADP+

light-dependent reactions

ADP +Pi

NADPH oxygen

ATP

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Figure 6 Summary of the light-dependent stage of photosynthesis

The steps in the light-dependent stage are:

  1. 1  Inside the thylakoid, light energy excites electrons in chlorophyll. The excited electrons (e-) move along proteins in the thylakoid membrane. As they move, the energy in the electrons powers the pumping of H+ into the thylakoid lumen. Water donates electrons to chlorophyll to replace the electrons that leave, which causes water to split into oxygen and two H+. This process is known as photolysis.

  2. 2  The oxygen is released from the chloroplast. It will either diffuse out of stomata and into the environment or be used as an input for aerobic cellular respiration (you will learn more about this in chapter 6).

  3. 3  The H+ ions from water molecules are used to generate the high energy coenzyme NADPH (NADP+ + H+  NADPH). The movement of H+ down its concentration gradient (maintained by energy from excited electrons in step 1) generates the high energy coenzyme ATP (ADP + Pi  ATP). (Figure 7).

4 ATP and NADPH coenzymes then move on to the light-independent stage.
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chloroplast thylakoid

thylakoid membrane

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thylakoid lumen

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2

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light

H2O 1

chlorophyll

H+

e– e–

O2 H+ H+ H+ H+

H+ H+ 3 H+ e–

H+

ATP synthase stroma

4

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membrane H+ proteins

NADP+ + H+ H+ +

H

NADPH
ADP + Pi ATP

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1 2 3 4

Light energy energises chlorophyll which pumps H+ and splits water Oxygen released
H+ and e– generate NADPH and ATP
NADPH and ATP are inputs for the light-independent stage

Figure 7 An overview of the light-dependent stage of photosynthesis. You do not need to know all the mechanisms, but an understanding can help you remember the inputs and outputs.

Ultimately, these steps demonstrate how the thylakoid turns: (1) 12 H2O, (2) 12 NADP+, and (3) 18 ADP + Pi.
into:
(1) 6 O2, (2) 12 NADPH, and (3) 18 ATP.

In summary, during the light-dependent stage of photosynthesis:

  • Sunlight excites an electron within chlorophyll.

  • Water absorbed by a plant’s root hairs is split into O2 and H+ as it donates one electron to the chlorophyll.

  • The excited electron and H+ ion from water lead to the production of the coenzymes NADPH and ATP.

  • The oxygen is released out of the chloroplast, and the coenzymes are ready for the second stage of photosynthesis.

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The VCAA does not require you to know the details of the biochemical pathways in the light-dependent and independent stages of photosynthesis (i.e. Figure 7 is not directly examinable). However, the VCAA specifically states you must know the inputs, outputs, and locations of the stages of photosynthesis.

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In the VCE Biology 2015 Exam Report, Question 3b, the VCAA stated the following:

  • Light is not an input molecule of photosynthesis.

  • NADP is an acceptable notation for NADP+, however, NAD is not acceptable.

  • ADP and Pi both can be acceptable inputs of the light-dependent stage (or outputs of the light- independent stage) as both are required to produce ATP. Although, listing both ADP + Pi as inputs will not negatively impact your answer.

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244 ChapTer 5: phOTOSYNTheSIS Enzymes and coenzymes in photosynthesis

Enzymes catalyse most of the reactions in photosynthesis. For example, in the light- dependent stage, the enzyme ATP synthase catalyses the reaction ADP + Pi  ATP using energy from the flow of H+ down its concentration gradient (Figure 7). Having enzymes regulate each step in photosynthesis ensures reactions are sped up and controlled, so plants can metabolise efficiently. We will focus on the function of another key enzyme in photosynthesis, called Rubisco, in the next lesson.

The coenzymes NADPH and ATP cycle through both stages of photosynthesis. In the light-dependent stage, the NADP+ and ADP + Pi inputs are turned into high-energy NADPH and ATP outputs. These two high-energy molecules then go on to be inputs for the light-independent stage. As you will soon see, the coenzymes donate their energy

in the light-independent reactions, forming the ‘unloaded’ NADP+ and ADP + Pi. The unloaded coenzymes then return back to the light-dependent reactions and the cycling continues (Figure 8).

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In the VCE Biology 2019 Exam Report, Question 2b, the VCAA summarised the role of both coenzymes in photosynthesis: NADPH transfers hydrogen ions while ATP transfers energy.

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light-dependent reactions

NADPH

NADP+ ATP

ADP + Pi

light-independent reactions

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Figure 8 NADPH and ATP cycle between the stages of photosynthesis. This cycling of energy is necessary to turn carbon dioxide and water into glucose.

The light-independent stage 3.2.4.2 Overview

During the second stage of photosynthesis, glucose is produced from carbon dioxide, NADPH, and ATP through a cycle of reactions occurring in the stroma of chloroplasts.

Theory details

Unlike the light-dependent stage, the light-independent stage of photosynthesis does
not require light to occur. Instead, the reactions are energised by the ATP and NADPH coenzymes produced in the light-dependent reactions. The light-independent stage occurs in the stroma, is facilitated by enzymes, and cycles through multiple reactions (which is why the stage is also referred to as the Calvin cycle).

The inputs of the light-independent stage are:

  • 6 carbon dioxide (CO2) molecules

  • 12 NADPH

  • 18 ATP.
    The outputs of the light-independent stage are:

  • glucose (C6H12O6)

  • 6 water (H2O) molecules

  • 12 NADP+

  • 18 ADP + Pi.

light-independent stage

the second stage of photosynthesis where carbon dioxide is used to form glucose in the stroma of a chloroplast. Also known as the Calvin cycle, the dark stage, or

the light-independent reactions

stroma the fluid substance that makes up the interior of chloroplasts. It is the site of the light-independent stage of photosynthesis

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Cycle back to lesson 3B if
you need to brush up on
your coenzyme knowledge. For now, it is enough to remember that enzymes catalyse (speed up) reactions and coenzymes assist enzyme functioning.

NADPH NADP+ carbon dioxide light-independent reactions

ATP ADP + Pi

Figure 9 Summary of the light-independent stage of photosynthesis

The steps in the light-independent stage are:

glucose + water

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  1. 1  Carbon dioxide molecules enter the Calvin cycle and undergo initial reactions. During these changes, the carbon from CO2 combines with a five-carbon molecule, then splits into 2 x three-carbon molecules, which continue along the cycle.

  2. 2  NADPH molecules formed in the light-dependent reactions donate their hydrogen ions and electrons, and ATP molecules break into ADP and Pi to release energy to facilitate further changes to the carbon molecules.

  3. 3  Carbon molecules continue to change and rearrange as they move around the cycle. Eventually, one specific three-carbon molecule is created and leaves the cycle, going on to contribute to the formation of glucose. Overall, six CO2 molecules must enter the cycle to produce glucose (carbon dioxide = one carbon; glucose = six carbons).

  4. 4  Some of the oxygen molecules leftover from the breaking of CO2 at the beginning of the cycle combine with hydrogen ions from NADPH to create the output water.

CO2

intermediate reaction glucose

NADPH and ATP NADP+ and ADP + Pi

Figure 10 An overview of the light-independent stage of photosynthesis. You do not need to know all the mechanisms, but an understanding can help you remember the inputs and outputs.

Overall, within the stroma the reactions turn:

(1) 6 CO2, (2) 12 NADPH, and (3) 18 ATP

into:

(1) C6H12O6, (2) 12 NADP+, (3) 18 ADP + Pi, and (4) 6 H2O.

To summarise, in the light-independent stage of photosynthesis:

  • CO2 collected from the stomata in leaves enters a cyclic reaction.

  • The carbon, from CO2, undergoes reactions powered by ATP and NADPH to produce a series of carbon-based molecules.

  • Eventually, a specific carbon molecule is reached that goes on to contribute to the formation of glucose, with water also being produced in this stage.

    The production of glucose is the main outcome of photosynthesis. The plant, through a series of reactions over the two stages, has now converted sunlight energy into chemical energy stored within the bonds of a glucose molecule. The glucose is transported out of the chloroplast for cellular respiration or conversion into complex carbohydrates.

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changes to CO2

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In the next lesson, we will revisit Figure 10 and add a lot more detail to it.