Photosynthesis at the Cellular Level

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Photosynthesis Recap

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 stages in the process of photosynthesis. The first called the light reactions (the photo stage) use the energy of light to produce the energy of the chemical bonds in energy carriers (ATP and NADP). Water is split and oxygen produced.

The second stage is called the Calvin cycle (the synthesis stage). Carbon dioxide is used with the hydrogen from water to form the sugar glucose. This glucose is turned into insoluble starch and stored. If this did not happen, the osmotic balance of the cell would be upset.

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.

More on the Photosynthesis Reactions

📕 article 📸 image 🔎 interactive 🎦 video

Glossary

Key terms and definitions.

Leaf Structures that Maximise the Rate of Photosynthesis

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.

The carbon dioxide moves into the plant through the stomata (stoma = singular, stomata = plural) which are tiny holes mostly on the underside of leaves. Each stoma has 2 guard cells that can change shape to make the hole open or closed, as seen in these microscope photos.

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. Stomata are mostly on the underside of leaves to reduce water loss from the leaves, as they are protected from direct sunlight.

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.

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