bio U2 L4.pdf

Grade.8 Mr. Ahmed Maher  Plant Processes How do plants obtain and use energy? Plants, like all living things, need energy to survive. But plants don’t exactly “eat” to get energy. Plants get energy from sunlight during the process...

Grade.8 Mr. Ahmed Maher  Plant Processes How do plants obtain and use energy? Plants, like all living things, need energy to survive. But plants don’t exactly “eat” to get energy. Plants get energy from sunlight during the process of photosynthesis.  Plants Capture Light Energy in Chloroplasts Plants use photosynthesis to change light energy to chemical energy in the form of sugar. Unlike animal cells, plant cells have organelles called chloroplasts where photosynthesis takes place. Chloroplasts are made up of two membranes that surround stacks of smaller, circular membranes. These smaller membranes contain chlorophyll, which is a green pigment. Chlorophyll absorbs light energy from the sun. Sunlight is made up of various wavelengths of light. Different wavelengths of visible light are seen as different colors. Chlorophyll absorbs many wavelengths, but it reflects more green light than other colors of light. As a result, most plants look green. Chloroplasts Use Light Energy to Make Sugar The light energy captured in chloroplasts is changed and stored in the bonds of a sugar called glucose. In the same process, oxygen gas is released. Many chemical reactions occur during photosynthesis. The process can be summarized by the equation This equation shows that light energy is used to change six molecules of carbon dioxide and six molecules of water into one molecule of glucose and six molecules of oxygen gas  Mitochondria Release Energy from Sugar In plants, extra glucose is stored as starch or changed to other types of sugar such as fructose or sucrose. Cellular respiration is the process by which cells use oxygen to release the stored energy from the bonds of sugar molecules. This process occurs in mitochondria. Cellular respiration also produces carbon dioxide and water. In cellular respiration, one molecule of glucose and six molecules of oxygen are changed into six molecules of carbon dioxide and six molecules of water. The reaction changes the energy in sugar into energy that can be used to power cell processes.  What are the phases of a plant’s life cycle? All plants complete their life cycles by alternating between two phases, the sporophyte phase and the gametophyte phase. In one phase, plants called sporophytes produce spores by meiosis. Meiosis is a process of cell division in which each daughter cell receives half the chromosomes of the parent cell. The products of meiosis in plant sporophytes are spores. The spores are then released. Under the right conditions, spores grow into plants called gametophytes Female gametophytes make eggs. Male gametophytes make sperm. When a sperm fertilizes an egg, they combine to form an embryo. The embryo develops into a seed, which is released and can grow into a new sporophyte.  How do seedless plants reproduce? The gametophyte generation in plants makes eggs and sperm. In seedless plants, sperm are released in the presence of water. Sperm have whip-like tails. The sperm swim to the eggs and fertilize them. The fertilized eggs then grow into sporophytes. Some seedless plants, such as mosses, have a visible gametophyte phase. The short, dense plant you think of as moss is the gametophyte. Sometimes you can also see the sporophytes of moss if you look closely. They are thin, brown stems topped by a small, brown capsule.  How do seed plants reproduce? The sporophyte is what you see in seed plants. In most seed plants, the sporophyte makes two types of spores, male and female, that grow into microscopic male and female gametophytes. The male gametophyte is pollen, a tiny structure in which sperm form. Pollen may be carried by wind, water, or animals to the female plant reproductive structure. The female gametophyte develops inside an ovule, which is part of the sporophyte. Within the ovule the gametophyte produces eggs. Pollination happens when pollen lands on the female plant reproductive structure and fertilizes the eggs. The fertilized egg develops into an embryo and the ovule becomes the seed.  How do flowering plants reproduce? In flowering plants, sexual reproduction takes place inside the flowers. Flowers are reproductive structures that have specialized leaves called sepals and petals, which often attract animal pollinators such as insects. A stamen is the male reproductive structure of flowers. At the tip of each stamen is an anther, where pollen is produced. A pistil is the female reproductive structure of flowers. When a pollen grain reaches the tip of the pistil, called the stigma, pollination occurs. A pollen tube grows down through the pistil into the ovary. Within the ovary are one or more ovules containing eggs. Sperm travel down the tube, into the ovary, and fertilize the eggs. A fertilized egg develops into an embryo, a tiny, undeveloped plant. The ovule develops into a seed that surrounds and protects the embryo. The ovary becomes a fruit, which protects the seeds and helps seeds to spread. When conditions are right, seeds will sprout and grow into new plants.  How do plants reproduce asexually? Most plants can also reproduce asexually. Asexual reproduction allows a plant to reproduce without seeds or spores. During asexual reproduction, part of a parent plant, such as a stem or root, produces a new plant. Some examples of structures that plants use to reproduce asexually include plantlets, tubers, and runners. * Plantlets are tiny plants that grow along the edges of a plant’s leaves. These plantlets fall off and grow on their own. * Tubers are underground stems that store nutrients and can grow into new plants. A potato is a tuber. Each “eye” can grow into a new plant. * Runners are above-ground stems that can grow into new plants. Strawberries send out lots of runners.  What are some ways plants respond to their environment? Anything that causes a reaction or change in an organism is a stimulus. Plants can respond to internal stimuli, such as water levels in cells. Plants can also respond to external stimuli, such as the amount of light they receive. By Wilting A stoma (plural, stomata) is an opening in the surface of a leaf. Stomata help a plant exchange gases and respond to its water levels. Each stoma is surrounded by two guard cells that open and close the stoma. When stomata are open, carbon dioxide enters the leaf, and oxygen and water vapor exit the leaf. The loss of water from leaves is called transpiration. A plant wilts when it loses more water than it can absorb through its roots. When a plant is wilting, its stomata close, preventing further water loss.  By Growing Plant growth in response to a stimulus is called a tropism. Plant tropisms are controlled by plant hormones. Hormones are chemical messengers that cause changes in cells. A change in the direction of plant growth in response to light is called phototropism. Hormones build up in cells on the shaded side of the stem, causing these cells to lengthen. The lengthening of these cells makes the stem bend in the direction of the light. A change in the direction of plant growth in response to gravity is called gravitropism. Most stems grow upward, away from the pull of Earth’s gravity, and most roots grow downward, toward the pull of gravity.  By Going Dormant The difference between day length and night length is an important stimulus for many plants. Shorter days and longer nights during fall help trigger winter dormancy. Dormant describes the inactive state of a seed or other plant part when conditions are not right for growth. For many plants, it is more energy-efficient to shut down during winter, or during a dry season, than to continue photosynthesis under conditions of reduced sunlight and rain. These plants survive by living off of stored sugars. Many plants and seeds come out of dormancy in the spring. Their growth is triggered by the return of more direct sunlight, longer daylight hours, and increased rain. Each plant species has an ideal temperature at which most of its seeds begin to grow. For many plants, this is about 27 °C (80 °F). But some seeds need extreme conditions, such as forest fires, to break their dormancy.