Without leaves, there would not be life on Earth. Leaf size can vary from the tiniest leaf of the common water fern (Azolla filiculoides) that are just one mm in length, to the largest leaves of the raffia palm (Raphia regalis) measuring 25 meters in length. No matter the size, most leaves are adapted for photosynthesis. This is a very important process where plants convert light energy into sugars and oxygen. To learn more about photosynthesis, see Light and Plants.
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Leaves are complex organs consisting of many different cell types (see Figure 1) including the epidermis, palisade mesophyll layer, spongy mesophyll layer, and vascular bundles.
Figure 1: Cross-section of a dicot leaf showing its various tissues and anatomy.
Source: Let"s Talk ScienceEpidermis is the “skin” of the leaves. Leaves have an upper epidermis that is located on the upper part of the leaf. A cuticle can also sometimes be present on the outside of the epidermis. This waxy layer helps prevent water loss, especially in dry regions. The palisade mesophyll layer is made up of closely-packed, elongated cells located just below the upper epidermis. They contain chloroplasts and carry out most of the photosynthesis. Vascular bundles are made up of xylem and phloem cells. These are the cells that carry water and nutrients throughout the plant and are visible as the veins in leaves. The spongy mesophyll layer is located directly below the palisade mesophyll layer. It consists of irregularly-shaped cells that are loosely packed with air spaces in between. Cells in the spongy layer usually contain few chloroplasts (especially in dicot plants) and are the storage place for the products of photosynthesis. The air spaces are all interconnected and lead to the outside of the leaf through stomata. The lower epidermis is located on the underside of leaves. Stomata are usually present on the lower epidermis. In order to minimize transpiration that occurs with gas exchange, most dicot plants have their stomata on the lower epidermis. On the other hand, monocot plants such as corn can have their stomata on both the top and bottom sides of leaves. This is because corn leaves grow upright rather than parallel to the ground and therefore both upper and lower surfaces of leaves experience transpiration.
Chloroplasts and Photosynthesis
Inside chloroplasts, there are membrane bound structures called thylakoids that are surrounded by the stroma (see Figure 2).
Figure 2: Diagram of a chloroplast and its structure.
Source: Let"s Talk Science
Thylakoid discs are often arranged into stacks called grana. Grana are connected together by stromal thylakoids, also called lamellae. The thylakoid membrane contains chlorophyll which acts to trap energy from the sun as well as the other protein complexes required for photosynthesis. Chlorophyll molecules reflect green light so that leaves appear green to us. To learn more about chlorophyll, see Role of Pigments in Plants.
Stomata and Gas Exchange
Stomata or pores in the leaf surface are surrounded by specialized leaf cells called guard cells (see Figure 3). Guard cells regulate the opening and closing of stomata. Stomata allow oxygen and carbon dioxide to either enter or leave the plant. Water vapour leaves the plant through stomata as well; by a process known as transpiration.
Figure 3: Diagrams of open (left) and closed (right) stomata.
Source: Let"s Talk Science
How is the opening and closing of stomata regulated?
Each guard cell contains a large vacuole as well as nucleus, chloroplasts, and other typical plant cell components. Generally, when the plant senses favourable conditions, such as high light or high humidity, stomata open. Channels in the guard cell walls are opened to release protons out of the cells while other channels allow for potassium ions to enter. This causes diffusion of water by osmosis into the guard cells, causing them to swell and open the pore. The reverse process occurs when stomata close due to unfavourable conditions.
Spaces between spongy mesophyll cells where gas exchange takes place.Carbon dioxide:
A gas used by plants for photosynthesis; a gas produced as waste by animals during cellular respiration. Plants also make carbon dioxide through cellular respiration, but they use more during photosynthesis than they make during cellular respiration.Chlorophyll:
A class of pigments produced in plants that give plants their green colour. These include chlorophyll a and b.Chloroplast:
An organelle found in plants and some algae where photosynthesis takes place.Cuticle:
A waxy layer usually present on the outside of the epidermis in plants.Dicot:
A group of flowering plants. The seeds in this group of plants contain two seed leaves.Epidermis:
A single layer of cells that covers all parts of a plant. Plants leaves contain an upper epidermis which is located on the upper side of the leaf and a lower epidermis located on the underside of the leaf.Grana (singular granum):
A stack of thylakoid discs that resembles a stack of coins or pancakes.Guard cells:
Specialized cells surrounding stomata that also control stomatal opening and closing.Lamellae:
Connect grana stacks together.Monocot:
A group of flowering plants. The seeds in this group of plants contain one seed leaf.Nucleus:
An organelle that stores the hereditary information in the cell and coordinates the cell’s activities.Osmosis:
The movement of molecules through a semi-permeable membrane from a region of higher concentration to lower in order to equalize the concentration on both sides of the membrane.Oxygen:
A gas used by animals during respiration; a gas produced by plants during photosynthesis.Palisade mesophyll layer:
Tightly packed layer of elongated cells located directly below the upper epidermis. These cells contain most of the chloroplasts in a leaf.Phloem:
Specialized cells within vascular bundles that transport nutrients throughout the plant.Photosynthesis:
The process used by plants to change light energy into biochemical energy (sugar). Light energy is used to change carbon dioxide and water chemically into oxygen and sugar.Potassium:
A major ion present inside cells.Protons:
Positively-charged particles that are found in the nucleus of every atom.Respiration:
The process used by plants and animals to get energy from sugar molecules. Respiration changes oxygen and sugar chemically into carbon dioxide and water and heat.Spongy mesophyll layer:
A loosely packed layer of irregularly-shaped cells. Air spaces that surround this cell layer allow gas exchange to take place.Stomata (singular stoma):
Small pores (holes) located on leaves. They are usually present on the underside of leaves but can also be found on the upper side as well.Stroma:
The fluid surrounding the grana within chloroplasts.Thylakoids:
A membrane bound structure within the chloroplast. Thylakoids consist of a thylakoid membrane surrounding a thylakoid space or lumen. Thylakoids contain chlorophyll and are where photosynthesis takes place.Transpiration:
The process of water movement through plants and eventual evaporation from small pores, or stomata, in leaves.Vacuole:
An organelle that stores food, nutrients or waste for a cell.Vascular bundles:
Strands of vascular tissues connecting all of plant parts in order to transport nutrients and water through phloem and xylem.Water vapour:
Water molecules in the form of gas.
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Specialized cells within vascular bundles that transport water throughout the plant.