Biology For Dummies
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Plants must get food into their systems in order to acquire energy and continue living, similar to animals. Plants create energy for animals to use, so they must replenish their nutrients. And plants breathe, in a way. They take in the carbon dioxide that all the animals give off, and they give off oxygen for all the animals to use. Pretty cool design, isn’t it?

Making energy from the ultimate energy source

Photosynthesis is the process by which plants convert energy from the sun. It is the process that allows plants to create organic molecules that they use as fuel. Here is how it works.
  • The molecules of chlorophyll contained in the chloroplasts absorb energy in the form of light from the sun. Some plants need more sunlight than others, but all need at least a little.

  • Instead of taking in oxygen and breathing out carbon dioxide like animals do, plants take in carbon dioxide from the atmosphere. Plants absorb water from the ground up through their roots.

    During photosynthesis, the energy from the sun splits the water molecules into hydrogen and oxygen. The oxygen molecules are given off by the plant and emitted into the atmosphere. Molecules of ATP are created within the plant cell. These reactions are called photochemical or light reactions because they require light to occur.

  • Enzymes within the plant then catalyze the combination of hydrogen and carbon dioxide to create a carbon compound that is called an intermediate. An intermediate is a compound used to continue a process to create a different compound.

    In plants, the intermediate is called phosphoglyceraldehyde (PGAL). PGAL goes on in the process to produce glucose, which the plant uses as fuel to survive. These reactions are called carbon-fixation reactions (or dark reactions to differentiate them from the light reactions above) because atoms of carbon are “fixed”; that is, they are put into stable compounds that can be used purposefully instead of just floating around the cell aimlessly.

  • When the plant has created more glucose than it needs to sustain life, it combines glucose molecules into larger carbohydrate molecules called starch. The starch molecules are stored within the large vacuoles in the plant cells. When necessary, the plant can break the starch molecules down to retrieve glucose for energy or to create other compounds, such as proteins, nucleic acids, or fats.

Flowin’ through the xylem and phloem

Plants undergo photosynthesis to produce energy for themselves (and ultimately humans). Light and water are needed to perform this process. But, how do the plants get the water and light into their cells?

Tissues called the xylem and the phloem usually are found together in what are called vascular bundles. Both types of tissue conduct substances up through the root and stem of a plant. The xylem conducts water and minerals from the soil; the phloem “flows” sugar molecules.

All plant cells have a cell wall, but cells in the xylem have an additional cell wall to give them extra strength (helps to avoid a blowout of water through the stem). Vessel elements are specialized cells in the xylem that form columns called vessels. Water passes through holes at the ends of each vessel element, and continues up through the entire vessel column.

Phloem tissue contains cells called sieve-tube elements, which connect in columns called sieve tubes. Each sieve-tube element has a pore on the end of it, through which the cytoplasm from one sieve-tube element can “touch” the cytoplasm of the next sieve-tube element. This structure allows the fuel that the plant makes in the leaves to pass through and nourish the rest of the plant. This process is called translocation.

Transporting water from cell to cell

Plants have two ways of moving water from outside the root toward the inside of the root to the xylem and phloem tissue. Water can flow between the cell walls of adjacent cells. Think of this area as a hallway. Or water can flow between cells through tubes connecting the cytoplasm of each cell, much like people can walk through doors of adjoining rooms.

The inspiration for transpiration

Transpiration is the technical term for the evaporation of water from plants. As water evaporates from the leaves (or any part of the plant exposed to air), it creates a tension in the leaves and tissues of the xylem. Because plants lose water through openings in the leaves called stomata, they must regain water. Therefore, the inspiration for transpiration is the loss of water. The loss of the medium that carries necessary minerals inspires the plant to pull more water in from the ground.

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