Plant Biology: Structure, Nutrition, Transport and Reproduction

Plants and Their Importance

Plants play a vital role in maintaining life on the planet, and they also have great commercial value. They are multicellular eukaryotes and autotrophs. They contain chlorophyll, release oxygen into the atmosphere, and produce wood, charcoal and paper.

Plant Cell Structure

A plant cell consists of a cell wall, vacuoles, plastids, a nucleus and a membrane.

• Chloroplast: contains chlorophyll structures.
• Leucoplast: structures that store starch.
• Chromoplasts: pigment structures that give yellow or red (orange) colors.

Plant Tissues

• Growth tissue (meristem): responsible for plant growth. It is found at the tips of roots and stems.
• Support tissue: gives firm organization to the roots, stems and leaves, providing support and protection.
• Coating tissue (epidermis): protects the surface of the plant, prevents water loss and covers the plant.
• Conducting tissue: transports substances throughout the plant. The xylem carries water and mineral salts from the roots to all parts of the plant, while the phloem carries the elaborated sap (organic substances) produced in the leaves to the rest of the plant.

Plant Nutrition

Plant nutrition is made up of absorption, conduction, photosynthesis and respiration.

Absorption

Absorption is the responsibility of the roots, which absorb water and mineral salts from the soil. This raw material is called raw sap. Absorbent hairs are small, thin extensions of the root epidermis that capture water and nutrients from the substrate. Some plants, such as bryophytes, have no true roots.

Conduction

Sap conduction in larger plants is carried out by vascular tissue, made of woody xylem vessels. Two main mechanisms take part in conduction:

• Capillarity: the property by which liquid rises through very fine tubes.
• Transpiration: the elimination of excess water vapor into the air. It occurs through small structures on the leaves called stomata.

Photosynthesis

Most plants are capable of producing their own food through photosynthesis, in the presence of light energy. The sugar produced is called glucose. Photosynthesis takes place inside the leaves, in the chloroplasts, which contain chlorophyll that gives plants their green color. Plants that perform photosynthesis need water, carbon dioxide and light energy. Water is absorbed by the root hairs, while carbon dioxide enters the leaves through the stomata. Chlorophyll captures light energy from the Sun, which drives the formation of elaborated sap.

Elaborated sap is conducted through the phloem, whose conducting vessels are called sieve tubes.

Respiration

In respiration, the plant breaks down organic matter using oxygen, producing energy and releasing water and carbon dioxide.

Plant Groups

Pteridophytes: are plants of small size that live in humid environments. They have conducting vessels but do not produce flowers, fruits or seeds. Example: ferns.

Bryophytes: are small plants that live in humid and shaded places. They have no flowers and no conducting vessels. Example: mosses and liverworts.

Angiosperms: have varied sizes and a wide geographic distribution. They have conducting vessels, flowers and fruits. Examples: beans and guava trees.

Gymnosperms: are plants that can reach great heights. They live mainly in temperate regions, have conducting vessels adapted to cold weather, and produce naked seeds, that is, seeds without fruits. Examples: pines and sequoias.

Reproduction

Reproduction of Bryophytes

Bryophytes have a reproductive cycle with two phases: the sexual phase, with the production of gametophytes, and the asexual phase, in which sporophytes are generally produced. Bryophytes are dioecious, meaning there are male and female individuals. Male plants produce male gametes called antherozoids, while female plants produce female gametes called oospheres. Bryophytes depend on water for reproduction.

Reproduction of Pteridophytes

Pteridophytes also have two phases. The sporophyte phase is the dominant one in pteridophytes and produces asexual spores. Taking the fern as an example, observe the small brown spots on the lower surface of its leaves. These spots are called sori, and within each sorus there are countless spores. The spores can mature when they fall on soil and germinate in moisture, starting the sexual phase of the reproductive cycle.

Reproduction of Gymnosperms

Gymnosperms may be individuals that are unisexual, with separate male and female plants, or they may be hermaphrodite, when the male and female reproductive systems are on the same individual plant. Gymnosperms developed reproductive structures called strobili. Male strobili produce pollen grains. On contact with the female strobilus, a pollen tube forms from the pollen grain. During the formation of the pollen tube, the cells of the pollen grain produce male gametes, which travel through the tube to the ovary of the female gametophyte.

Reproduction of Angiosperms

Angiosperm reproduction involves four important stages: pollination, fertilization, seed germination and formation of fruits and seeds.

Angiosperms

Example: pea pods or wheat.