Biology Semester II

Sections:

Introduction |  Section 1 | Section 2 | Section 3

Section Three:

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9 | Part 10

Biology: Plant Hormones, Nutrition, and Transport: Part Ten Phloem, Sugar, and Translocation Phloem consists of several types of cells: sieve tube cells (sometimes referred to as sieve elements), companion cells, and the vascular parenchyma. Sieve cells are tubular cells with perforated end walls, known as sieve plates. Most sieve cells lose their nuclei but remain alive, leaving an empty cell with a functioning plasma membrane. Since the sieve cells lose their nucleus, the adjacent companion cell’s nucleus will control the functioning of the sieve cell. Phloem cells. A source is an area where a material, in this case sugar, is produced. Sugars move from sources (usually leaves) to areas with low sugar concentration, such as a plant’s roots. Companion cells load sugar into the sieve element (sieve elements are connected into sieve tubes). Fluids can move upwards or downwards within the phloem and are transported from one place to another. Sinks are places where sugar is being consumed or stored. What happens to unused sugars? Unused sugars are converted into starch for longer-term storage. Food moves through the phloem by a pressure-flow mechanism. Sugar moves (by an energy-requiring step) from a source (usually leaves) to a sink (usually roots) by osmotic pressure. Movement of sugar into a sieve element (by active transport) causes water to enter that cell. Once the sugar is actively transported into the sieve cell, water will then pass into that cell by osmosis. This increases the pressure of the sugar-water mix (phloem sap). This pressure causes the sap to flow toward an area of lower pressure, which in this case would be the sink (such as in the roots or another non-photosynthetic part of the plant). Once in the sink, the sugar is removed from the phloem by another energy-requiring active transport step. Again, the water follows the sugar in the root. The sugar can either be metabolized to make ATP or converted into the storage polysaccharide starch. Flow of water through xylem and phloem. Practice what you've learned in this interactive. Plant Hormones, Nutrition, and Transport Quiz 25 points Congratulations on completing this section! In this section you learned about: Tropisms Types of Plant Hormones Mycorrhizae, Bacteria, and Mineral Uptake Water Uptake Xylem and Transport of Water and Minerals Guard Cells Phloem, Sugar, and Translocation Now it’s time to take the section quiz. Please make sure to check your understanding of the topics above before proceeding to the quiz. After you have completed the quiz, continue with the unit. Plant Structure Exam 100 points Congratulations on completing this unit!  In this unit, you learned about: Plant Organs, Tissues, and Cells Flowering Plant Reproduction Plant Hormones, Nutrition, and Transport Now it’s time to take the unit exam. Please make sure to check your understanding of the topics above before proceeding to the exam. After you have completed the exam, continue with the course. End of unit.

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