Category: 1. Plant Cells

Plant cells are also notorious for having huge vacuoles. Up to 90% of the volume of a mature cell may be taken up by a single large vacuole or several vacuoles. The vacuole is bound by a special membrane, called the tonoplast, and contains cell sap—which is composed of dissolved substances and may include pigments.

The cell cycle contains the process in which cells are either dividing or in between divisions. Cells that are not actively dividing are said to be in interphase, which has three distinct periods of intense activity that precedes the division of the nucleus, or mitosis. The division of the rest of the cell occurs as an end result of mitosis and this process occurs in regions of active cell division, called meristems. Meristems will be looked at in the plant tissue tutorial.

Mitosis is a process within the cell cycle that is divided into four phases which we will sum up here:

1. Prophase—the chromosomes and their usual two-stranded nature becomes apparent, the nuclear envelope breaks down.
2. Metaphase—the chromosomes become aligned at the equator of the cell. A spindle composed of spindle fibers is developed and some attach to the chromosomes at their centromere.
3. Anaphase—the sister chromatids of each chromosome, that is now called the daughter chromosomes, separate lengthwise and each group of daughter chromosomes migrates to the opposite ends of the cell.
4. Telophase—the groups of daughter chromosomes are grouped within a developing nuclear envelope which makes them separate nuclei. A wall forms between the two sets of daughter chromosomes thus creating two daughter cells.

In plants, as the cell wall is developing, droplets or vesicles of pectin merge forming a cell plate that eventually will become the middle lamella of the new cell wall.

It is necessary to note a bit about the form of chloroplasts, as you will encounter them throughout this tutorial. Inside a chloroplast is a matrix called the stroma. Enzymes are found in the stroma as well as grana—stacks of coin-shaped discs, called thylakoids. It is within the thylakoids that photosynthesis takes place. Note that chloroplasts, like mitochondria, contain their own DNA. They do rely on proteins from the nucleus and are considered semi-autonomous organelles.

• The cytoskeleton is a filamentous network of proteins that are associated with the processes that maintain and change cell shape and produce cell movements in animal and bacteria cells. In plants, it is responsible for maintaining structures within the plant cell, rather than whole-cell movement. The cytoskeleton also forms tracks along which cell organelles move propelled by contractile proteins attached to their various surfaces. Like a little highway infrastructure inside the cell. Three types of filaments make up the cytoskeleton.
  • Microfilaments are the thinnest and most abundant of the cytoskeleton proteins. They are composed of actin, a contractile protein, and can be assembled and disassembled quickly according to the needs of the cell or organelle structure.
  • Intermediate filaments are slightly larger in diameter and are found most extensively in regions of cells that are going to be subjected to stress. Once these filaments are assembled they are not capable of rapid disassembly.
  • Microtubules are hollow tubes composed of a protein called tubulin. They are the thickest and most rigid of the filaments. Microtubules are present in the axons and long dendrite projections of nerve cells. They are capable of rapid assembly and disassembly according to need. Microtubules are structured around a cell region called the centrosome, which surrounds two centrioles composed of 9 sets of fused microtubules. These are important in cell division when the centrosome generates the microtubular spindle fibers necessary for chromosome separation.

Peroxisomes are also bound by a single membrane. They consume oxygen and work to drive reactions that remove hydrogen from various molecules in the form of hydrogen peroxide. They are important in maintaining the chemical balances within the cell.

Lysosomes are bound by a single membrane and contain highly acidic fluid. The fluid acts as digesting enzymes for breaking down bacteria and cell debris. They play an important role in the cells of the immune system.

Mitochondria are some of the most important structures in the cell. They are the site of various chemical processes involved in the synthesis of energy packets called ATP (adenosine triphosphate). Each mitochondrion is surrounded by two membranes. The outer membrane is smooth, while the inner one is folded into tubule structures called cristae. Mitochondria are unique in that they contain small amounts of DNA containing the genes for the synthesis of some mitochondrial proteins. The DNA is inherited solely from the mother. Cells with greater activity have more mitochondria, while those that are less active have less need for energy-producing mitochondria.

Endosomes are membrane-bound tubular and vesicular structures located between the plasma membrane and the Golgi apparatus. They serve to sort and direct vesicular traffic by pinching off vesicles or fusing with them.

The Golgi apparatus is a membranous sac that serves to modify and sort proteins into secretory/transport vesicles. The vesicles are then delivered to other cell organelles and the plasma membrane. Most cells have at least one Golgi apparatus, although some may have multiple. The apparatus is usually located near the nucleus.

The endoplasmic reticulum (ER) is collectively a network of membranes enclosing a singular continuous space. As mentioned earlier, the granular endoplasmic reticulum is associated with ribosomes (giving the exterior surface a rough, or granular appearance). Sometimes granular endoplasmic reticulum is referred to as rough ER. The granular ER is involved in packaging proteins for the Golgi apparatus. The agranular, or smooth, ER lacks ribosomes and is the site of lipid synthesis. In addition, the agranular ER stores and releases calcium ions (Ca²⁺).

Ribosomes are the sites where protein molecules are synthesized from amino acids. They are composed of proteins and RNA. Some ribosomes are found bound to the granular endoplasmic reticulum, while others are free in the cytoplasm. The proteins synthesized on ribosomes bound to granular endoplasmic reticulum are transferred from the lumen (open space inside endoplasmic reticulum) to the Golgi apparatus for secretion outside the cell or distribution to other organelles. The proteins that are synthesized of free ribosomes are released into the cytosol.