Reading: Chloroplasts and Vacuoles

Chloroplasts

This illustration shows a chloroplast, which has an outer membrane and an inner membrane. The space between the outer and inner membranes is called the intermembrane space. Inside the inner membrane are flat, pancake-like structures called thylakoids. The thylakoids form stacks called grana. The liquid inside the inner membrane is called the stroma, and the space inside the thylakoids is called the thylakoid space.

Figure 1. The chloroplast has an outer membrane, an inner membrane, and membrane structures called thylakoids that are stacked into grana. The space inside the thylakoid membranes is called the thylakoid space. The light harvesting reactions take place in the thylakoid membranes, and the synthesis of sugar takes place in the fluid inside the inner membrane, which is called the stroma. Chloroplasts also have their own genome, which is contained on a single circular chromosome.

Like the mitochondria, chloroplasts have their own DNA and ribosomes (we’ll talk about these later!), but chloroplasts have an entirely different function.Chloroplasts are plant cell organelles that carry out photosynthesis. Photosynthesis is the series of reactions that use carbon dioxide, water, and light energy to make glucose and oxygen. This is a major difference between plants and animals; plants (autotrophs) are able to make their own food, like sugars, while animals (heterotrophs) must ingest their food.

Like mitochondria, chloroplasts have outer and inner membranes, but within the space enclosed by a chloroplast’s inner membrane is a set of interconnected and stacked fluid-filled membrane sacs called thylakoids (Figure 1). Each stack of thylakoids is called a granum (plural = grana). The fluid enclosed by the inner membrane that surrounds the grana is called the stroma.

The chloroplasts contain a green pigment called chlorophyll, which captures the light energy that drives the reactions of photosynthesis. Like plant cells, photosynthetic protists also have chloroplasts. Some bacteria perform photosynthesis, but their chlorophyll is not relegated to an organelle.

Vacuoles

Vacuoles are membrane-bound sacs that function in storage and transport. The membrane of a vacuole does not fuse with the membranes of other cellular components. Additionally, some agents such as enzymes within plant vacuoles break down macromolecules.

The Central Vacuole

Previously, we mentioned vacuoles as essential components of plant cells. If you look at Figure 2b, you will see that plant cells each have a large central vacuole that occupies most of the area of the cell. The central vacuole plays a key role in regulating the cell’s concentration of water in changing environmental conditions. Have you ever noticed that if you forget to water a plant for a few days, it wilts? That’s because as the water concentration in the soil becomes lower than the water concentration in the plant, water moves out of the central vacuoles and cytoplasm. As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the cell walls of plant cells results in the wilted appearance of the plant.

The central vacuole also supports the expansion of the cell. When the central vacuole holds more water, the cell gets larger without having to invest a lot of energy in synthesizing new cytoplasm. You can rescue wilted celery in your refrigerator using this process. Simply cut the end off the stalks and place them in a cup of water. Soon the celery will be stiff and crunchy again.

Part a: This illustration shows a typical eukaryotic animal cell, which is egg shaped. The fluid inside the cell is called the cytoplasm, and the cell is surrounded by a cell membrane. The nucleus takes up about one-half the width of the cell. Inside the nucleus is the chromatin, which is composed of DNA and associated proteins. A region of the chromatin is condensed into the nucleolus, a structure where ribosomes are synthesized. The nucleus is encased in a nuclear envelope, which is perforated by protein-lined pores that allow entry of material into the nucleus. The nucleus is surrounded by the rough and smooth endoplasmic reticulum, or ER. The smooth ER is the site of lipid synthesis. The rough ER has embedded ribosomes that give it a bumpy appearance. It synthesizes membrane and secretory proteins. In addition to the ER, many other organelles float inside the cytoplasm. These include the Golgi apparatus, which modifies proteins and lipids synthesized in the ER. The Golgi apparatus is made of layers of flat membranes. Mitochondria, which produce food for the cell, have an outer membrane and a highly folded inner membrane. Other, smaller organelles include peroxisomes that metabolize waste, lysosomes that digest food, and vacuoles. Ribosomes, responsible for protein synthesis, also float freely in the cytoplasm and are depicted as small dots. The last cellular component shown is the cytoskeleton, which has four different types of components: microfilaments, intermediate filaments, microtubules, and centrosomes. Microfilaments are fibrous proteins that line the cell membrane and make up the cellular cortex. Intermediate filaments are fibrous proteins that hold organelles in place. Microtubules form the mitotic spindle and maintain cell shape. Centrosomes are made of two tubular structures at right angles to one another. They form the microtubule-organizing center. Part b: This illustration depicts a typical eukaryotic plant cell. The nucleus of a plant cell contains chromatin and a nucleolus, the same as an animal cell. Other structures that the plant cell has in common with the animal cell include rough and smooth endoplasmic reticulum, the Golgi apparatus, mitochondria, peroxisomes, and ribosomes. The fluid inside the plant cell is called the cytoplasm, just as it is in an animal cell. The plant cell has three of the four cytoskeletal components found in animal cells: microtubules, intermediate filaments, and microfilaments. Plant cells do not have centrosomes. Plant cells have four structures not found in animals cells: chloroplasts, plastids, a central vacuole, and a cell wall. Chloroplasts are responsible for photosynthesis; they have an outer membrane, an inner membrane, and stack of membranes inside the inner membrane. The central vacuole is a very large, fluid-filled structure that maintains pressure against the cell wall. Plastids store pigments. The cell wall is outside the cell membrane.

Figure 2. These figures show the major organelles and other cell components of (a) a typical animal cell and (b) a typical eukaryotic plant cell. The plant cell has a cell wall, chloroplasts, plastids, and a central vacuole—structures not found in animal cells. Plant cells do not have lysosomes or centrosomes.