Plant Biology

Introduction to plant biology

Plant biologists have many roles. Some study the chemical signals plants use to defend themselves, others may study the way the climate affects a plant's ability to reproduce.

Plant biology is a wide-reaching subject, but some categories receive more attention based on their importance to us and our interests. See below for more about the different categories of plant biology:

1. Evolution: We study the adaptations that plants have acquired which have helped them to diversify and occupy new environments.

2. Life history: We study the length of a plant's life and the style of reproduction.

3. Plant structure: From the chloroplast to the whole plant, plant structure can tell us about what makes a plant unique and how it survives in its environment.

4. Function: We study plant function to understand how plants work.

The definition of a plant in biology

Plants are multicellular, eukaryotic organisms that have chloroplasts for photosynthesis. Scientists believe that the beginning of plants occurred when a eukaryote engulfed a cyanobacterium. Cyanobacteria are bacteria that can photosynthesize, and so it is believed this led to the development of the chloroplast in plants. This theory is called primary endosymbiosis.

For the purpose of AP biology, we will be concerned with the land plants, which are a major group within the "green plants" that developed a number of adaptations to survive on land. These plants are typically split into nonvascular plants (mosses, hornworts, and liverworts) and vascular plants (trees, ferns, horsetails, and many more).

Plant cell: diagram and its biology

The plant cell, because it is eukaryotic, contains a membrane-bound nucleus and a number of organelles. Below, important parts of the plant cell are highlighted. You may notice that many of the organelles are also shared with animal cells.

Figure 1. This diagram shows the basic parts of a plant cell.

Plant specific organelles

• Chloroplast: Chloroplasts are the site of photosynthesis. They help plant cells capture the sun's energy to use to make sugars. The plant can then use those sugars as food.
• Vacuole: Vacuoles are storage sites in cells for water, nutrients, and some byproduct chemicals from cell processes. In a plant cell, the big vacuoles at the center of the cell are filled with water and help provide stability to the plant cells.
• Cell wall: Unlike animal cells, plant cells have a cell wall, fortified by cellulose, which provides support for the structure of the plant. Plants do not have skeletons like some animals do, so having cell walls provides some rigidity for plants.
• Plasmodesmata: The plasmodesmata are small junctions in plant cell walls that connect the cytoplasm of adjacent cells.

Organelles shared with animal cells

• Peroxisomes: These organelles help to break down chemicals that are toxic, known as peroxides. They are more common in plants, but also are found in animal cells.

• Ribosomes:Ribosomes are points of protein synthesis, cells have both free ribosomes in the cytoplasm, and ribosomes attached to the rough endoplasmic reticulum.

• Rough and smooth endoplasmic reticulum (ER): The rough ER has ribosomes where proteins are made. The smooth ER has a number of functions, including making products that the cell needs (i.e. lipids).

• Golgi apparatus:Ribosomes help to make proteins for the cell, while the Golgi apparatus packages and processes these proteins for use inside or outside of the cell.
• Mitochondria: Mitochondria help to produce energy from the sugars that the plant makes in its chloroplasts.
• Nucleus and nucleolus: The nucleus is the site for DNA storage as chromatin. In the nucleus, the nucleolus is the site where ribosomes begin construction, before being finished in the cytoplasm.

Plant biology: life cycles and reproduction

One quality of all land plants (nonvascular and vascular plants) is the life cycle mechanism, known as the alternation of generations. In the alternation of generations life cycle, a plant goes through both a multicellular diploid stage and a multicellular haploid stage.

Below, the basic steps of alternation of generations are highlighted, starting with the diploid plant:

1. The diploid multicellular plant, called a sporophyte, will produce spores via meiosis.

2. The spores will grow into a haploid plant, called a gametophyte.

3. The gametophyte will produce gametes via mitosis (NOT meiosis).

4. Two gametes when brought together, or fertilized, form a diploid zygote which will begin the process again.

Figure 2. This diagram illustrates the alternation of generations in plants.

Note that plants produce haploid spores via meiosis, but gametes (sex cells) are produced via mitosis. Because plants have a haploid gametophyte generation, gametes must be produced by mitosis, unlike in other organisms (like humans), who produce gametes by meiosis.

Plant biology: structure, function, and adaptations

Land plants are of particular interest to us and biologists because they had to evolve to be able to survive on land, meaning they have many adaptations to new problems faced on land, like desiccation (drying out), retaining structure, and getting water and nutrients from the soil.

Evolution of land plants: adaptations and their importance

Land plants can be split into two major groups: nonvascular plants and vascular plants. The main adaptation connecting all land plants is that their embryos have protection from drying out from maternal tissue. This adaptation allowed plants the ability to reproduce on land and was, therefore, an important first step for terrestrial plant species.

Figure 3. Diagram of land plant evolution, from bryophytes to angiosperms, with important adaptations highlighted.

Nonvascular plants do not have vascular systems and therefore grow only a few inches tall. Vascular plants all have vascular systems, which function to transport water and nutrients throughout the plants.

Plant structure and function

The structure and function of plants are two closely related categories of plant biology. Biologists who study plant structure and function may say they study "plant physiology", a formal term for these branches of plant biology. The structure of plants is often split into the root system and the shoot system. The root system includes the roots and the shoot system includes the stem and the leaves.

Roots

Roots have evolved to help to anchor a plant and absorb water and nutrients. They vary greatly from plant to plant, but they can grow extensive lengths to help keep plants anchored in the soil. Roots also have special extensions of their cells called root hairs. Root hairs help increase the surface area of the root so that more water and nutrients can be absorbed more efficiently.

Roots also have vascular tissue in them (the xylem and phloem) which allows them to transport water and mineral directly from the soil to the plant. Additionally, the root tip is a place of mitosis- meaning the root system of plants can grow as plants get larger.

Stems

The stems of the plant are the long, central plant body. For example, the trunk of the tree is the centralized plant body.

Stems are composed of vascular tissue (xylem and phloem) and other types of tissues, some of which are strengthened by proteins that provide support for the plant against outside forces (wind, rain, etc.).

Leaves

Leaves have vascular tissue (veins) and photosynthetic tissue (mesophyll). The mesophyll tissue of leaves contain chloroplasts, which makes it the site of photosynthesis, or sugar production. Additionally, leaves are often thin and broad, allowing for a large surface area in which a plant can capture light energy from the sun (in the form of photons).

Leaves also have stomata, which are typically located on their undersides. Stomata are cell openings that are controlled by guard cells. Guard cells function by opening to let CO₂ into the plant for photosynthesis and closing back up to protect from too much water getting out.