CEL - Subcellular Components [LESSON]

Subcellular Components

As we discussed in the last lesson, prokaryotic and eukaryotic cells have multiple structures that work together to enable the cell to work efficiently.  First, know that ALL cells have the following three structures.

  1. Cell membrane – this structure determines what goes into and out of the cell. We will learn more about this in a later lesson.
  2. DNA – all cells have DNA since DNA holds the instructions for building proteins. You can return to Module 1 to review the structure of DNA if necessary.
  3. Ribosomes – all cells have ribosomes in order to build proteins. Remember, the ribosomes put amino acids together in the correct order according to the DNA code.

All cells also have cytoplasm (or cytosol) to surround everything in an aqueous solution.

Cells have structures, which are not true organelles since they do not meet the criteria for an organelle (must be bound by a membrane).  For example, within the cytoplasm, there is a network of protein fibers known as the cytoskeleton. This structure is responsible for both cell movement and stability. The major components of the cytoskeleton are microtubules, intermediate filaments, and microfilaments. The cytoskeleton serves as a road for other proteins to “walk” on and transport things across the cell.

Eukaryotes also have some combination of the following organelles. Remember that organelles are bound by a membrane. Read about each one and create a chart to organize the information. 

First, watch the Organelles in Eukaryotic Cells video below and then read about each organelle. Know that this is not a comprehensive list, but just some of the most common organelles.

Nucleus

Though part of the function of the nucleus is to separate and protect the DNA from the rest of the cell, molecules must still be able to move in and out (e.g., RNA). Protein channels known as nuclear pores form holes in the nuclear envelope. The nucleus itself is filled with liquid (called nucleoplasm) and is similar in structure and function to the cytoplasm. It is here within the nucleoplasm where chromosomes (tightly packed strands of DNA) are found.

Rough Endoplasmic Reticulum

The surface of the rough endoplasmic reticulum surface is studded with ribosomes, the molecules in charge of protein production. When a ribosome finds a specific RNA segment, that segment may tell the ribosome to travel to the rough endoplasmic reticulum and embed itself. The protein created from this segment will find itself inside the lumen of the rough endoplasmic reticulum, where it folds and is tagged with a (usually carbohydrate) molecule in a process known as glycosylation that marks the protein for transport to the Golgi apparatus. The rough endoplasmic reticulum is continuous with the nuclear envelope. Proteins made in the rough endoplasmic reticulum as destined to either be a part of a membrane or to be secreted from the cell membrane out of the cell.

Smooth Endoplasmic Reticulum

The smooth endoplasmic reticulum makes lipids and steroids, instead of being involved in protein synthesis. These are fat-based molecules that are important in energy storage, membrane structure, and communication (steroids can act as hormones). The smooth endoplasmic reticulum is also responsible for detoxifying the cell.

Golgi Apparatus

 It is responsible for packing proteins from the rough endoplasmic reticulum into membrane-bound vesicles (tiny compartments of lipid bilayer that store molecules) which then translocate to the cell membrane. At the cell membrane, the vesicles can fuse with the larger lipid bilayer, causing the vesicle contents to either become part of the cell membrane or be released to the outside. Different molecules actually have different fates upon entering the Golgi. This determination is done by tagging the proteins with special sugar molecules that act as a shipping label for the protein.

Lysosome

The lysosome is the cell’s recycling center. These organelles are spheres full of enzymes ready to hydrolyze (chop up the chemical bonds of) whatever substance crosses the membrane, so the cell can reuse the raw material. These disposal enzymes only function properly in environments with a pH of 5, two orders of magnitude more acidic than the cell’s internal pH of 7.

Peroxisome

The peroxisome is another spherical organelle responsible for destroying its contents. Unlike the lysosome, which mostly degrades proteins, the peroxisome is the site of fatty acid breakdown. It also protects the cell from reactive oxygen species (ROS) molecules which could seriously damage the cell. ROSs are molecules like oxygen ions or peroxides that are created as a byproduct of normal cellular metabolism, but also by radiation, tobacco, and drugs. They cause what is known as oxidative stress in the cell by reacting with and damaging DNA and lipid-based molecules like cell membranes.

Mitochondrion

ATP (adenosine triphosphate) is the energy currency of the cell and is produced in a process known as cellular respiration. Though the process begins in the cytoplasm, the bulk of the energy produced comes from later steps that take place in the mitochondria. There are two membranes in the mitochondria and the center is called the mitochondrial matrix.

Chloroplast

 The chloroplast also has a double membrane, as discussed in the previous lesson. Within the inner membrane are stacks of thylakoid membrane that contain the pigments that can capture light energy. This is the site of photosynthesis, which is the process by which autotrophs make their own sugar. In photosynthesis, light energy is collected and used to build sugars from carbon dioxide. The sugars produced in photosynthesis may be used by the plant cell or may be consumed by animals that eat the plant, such as humans. We will learn about this process in detail in the later unit. 

Vacuole

Plant cells are also unique because they have a lysosome-like organelle called the vacuole. The large central vacuole stores water and waste, isolate hazardous materials, and contains enzymes that can break down macromolecules and cellular components, like those of a lysosome.

Cell wall

The cell wall is a rigid covering that surrounds the cell, protecting it and giving it support and shape. The major organic molecule of the plant cell wall is cellulose, a polysaccharide composed of glucose units that you learned in Module 1.

Plant vs Animal Cells

Watch the Overview of Animal and Plant Cells video looking at the differences between these two types of EUKARYOTIC cells. Remember, there are other types of eukaryotic cells as well, such as fungi and protists.

Now, can you sort the organelles below into plant, animal, or both based on your knowledge from this lesson?

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