Above Image: Diagram showing the special types of cells present in leavesWithout 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. Show
Leaf StructureLeaves are complex organs consisting of many different cell types (see Figure 1) including the epidermis, palisade mesophyll layer, spongy mesophyll layer, and vascular bundles.
Chloroplasts and PhotosynthesisInside chloroplasts, there are membrane bound structures called thylakoids that are surrounded by the stroma (see Figure 2). 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 ExchangeStomata 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. 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. GlossaryAir spaces: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. Xylem:Specialized cells within vascular bundles that transport water throughout the plant. References
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What is one of the main functions of the specialized cells on the leaf?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.
What are the 3 main functions of a leaf?These are:. Photosynthesis.. Transpiration.. Photosynthesis.. What are the 4 main functions of a leaf?CONTENTS. Photosynthesis.. Transpiration.. Guttation.. Storage.. Defense.. What are the specialized cells in plants?Specialised plant cells include root hair cells, palisade cells, xylem cells and phloem cells.
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