4.1 Eukaryotic Cell Structure Overview
KEY CONCEPTS
By the end of this section, you will be able to do the following:
- Evaluate the role of membranes in surrounding both the cell and most organelles
- List the major organelles found within eukaryotic cells
- Distinguish between cytosol and cytoplasm
- Identify non-organelles components that are important for eukaryotic structure and function, such as the cytoskeleton
- Briefly describe the importance of extracellular components for different types of eukaryotic cells
Eukaryotic cells have complex structures, which can support a large range of functions. Before we dive into the details of the various components of the eukaryotic cell (Ch. 4.2 – Ch. 4.6), this chapter will give a high-level overview of some core concepts that are important for all eukaryotic cells. These topics include membranes, membrane-bound organelles, the cytoplasm (and how it differs from the cytosol), the cytoskeleton, and extracellular components.
Membranes, Membranes – Everywhere!
As described in Chapter 1, all cells are surrounded by a plasma membrane, which is a phospholipid bilayer with embedded proteins that separates the internal contents of the cell from its surrounding environment (Figure 4.2). The plasma membrane controls the passage of organic molecules, ions, water, and oxygen into and out of the cell. Wastes (such as carbon dioxide and ammonia) also leave the cell by passing through the plasma membrane. See Chapter 5 for more information on the plasma membrane.
Eukaryotes also have membranes within their cells, surrounding various organelles. These membrane-bound organelles are surrounded by one or more phospholipid bilayers that have a similar component (e.g., phospholipids, proteins) as the plasma membrane. The word “organelle” means “little organ,” and, as we already mentioned, organelles have specialized cellular functions, just as your body’s organs have specialized functions. Some of these organelles and other cellular components can be visualized in Figure 4.3 and are listed in Table 4.1. Organelle functions will be explored more thoroughly in Chapter sections 4.2-4.4.
Table 4.1. A summary of different cell structures present in animal, plant and fungi cells.
|
Cell Component |
Function |
Present in…
|
||
|
|
|
Fungal Cells |
Animal Cells |
Plant Cells |
|
Plasma Membrane |
Phospholipid bilayer that separates cell from external environment; controls passage of organic molecules, ions, water, oxygen, and wastes into and out of cell |
Yes |
Yes |
Yes |
|
Cytosol |
Site of many metabolic reactions; medium in which organelles are found |
Yes |
Yes |
Yes |
|
Nucleus |
Membrane-bound organelle that houses DNA and directs synthesis of ribosomes and proteins |
Yes |
Yes |
Yes |
|
Ribosomes |
Organelle that carries out protein synthesis |
Yes |
Yes |
Yes |
|
Mitochondria (singular: mitochondrion) |
Membrane-bound organelles that conducts cellular respiration and produces ATP |
No |
Yes |
Yes |
|
Peroxisomes |
Membrane-bound organelles that oxidize and thus break down fatty acids and amino acids, and detoxify poisons |
Yes |
Yes |
Yes |
|
Vesicles |
Membrane-bound spheres important for transport of molecules within cells |
Yes |
Yes |
Yes |
|
Vacuoles |
Membrane-bound organelles involved in storage and intracellular transport; digestive function in plant cells |
|
|
|
|
Centrosome |
A microtubule organizing centre with a role in animal cell division |
No |
Yes |
No |
|
Lysosomes |
Membrane-bound organelles that carry out digestion of macromolecules; recycling of worn-out organelles |
No |
Yes |
Some |
|
Cell Wall |
Extracellular structure that provides protection, structural support, and maintenance of cell shape |
Yes |
No |
Yes |
|
Chloroplasts |
Membrane-bound organelles that carry out photosynthesis – sugar production using energy from light |
No |
No |
Yes |
|
Endoplasmic Reticulum |
Membrane-bound organelle that modifies proteins and synthesizes lipids |
Yes |
Yes |
Yes |
|
Golgi Apparatus |
Membrane-bound organelle that modifies, sorts, tags, packages, and distributes lipids and proteins |
Yes |
Yes |
Yes |
|
Cytoskeleton |
Network of fibrous proteins that maintains cell shape (animal cells), secures organelles in specific positions, allows cytosol and vesicles to move within cell, and enables single cells to move independently |
Yes |
Yes |
Yes |
Cytoplasm vs. Cytosol
Cytoplasm and cytosol are often (incorrectly) used interchangeably; however they are different! The cytoplasm is the cell’s entire region between the plasma membrane and the nuclear envelope. The cytoplasm is comprised of a diverse group of organelles that are suspended in the gel-like cytosol, the cytoskeleton, and various chemicals (Figure 4.3). Even though the cytosol (fluid surrounding organelles) consists of 70-80% water, it has a semi-solid consistency, which comes from the proteins within it. However, proteins are not the only organic molecules in the cytoplasm. Glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, fatty acids, and derivatives of glycerol are also there. Ions of sodium, potassium, calcium, and many other elements also dissolve in the cytosol. Many metabolic reactions, including protein synthesis, take place in the cytosol.
Cytoskeleton
The cytoskeleton plays a vital role in the structure and function of eukaryotic cells. This is a network of protein fibers that helps maintain the cell’s shape, secures some organelles in specific positions, allows cytoplasm and vesicles to move within the cell, and enables cells to move and change shape. There are three types of fibers within the eukaryotic cytoskeleton: microfilaments, intermediate filaments, and microtubules (Figure 4.4). While the name cytoskeleton suggests something static and structural, we will explore the more diverse roles that the cytoskeleton plays in Chapter 4.5.
Beyond the Plasma Membrane
Most cells release materials into the space outside the plasma membrane, known as the extracellular space. The materials in this extracellular space can carry out extremely important functions for cells. For example, these materials can form a cell wall that supports cell shape and structure, as we see with plant and fungal cells. These cell walls are produced by the movement of materials like cellulose (plants) and chitin (fungi) to the extracellular space.
In multicellular organisms – including animals, most plants, and most fungi – cells must be able to adhere (stick) to each other and organize into tissues and/or organs with other cells. The extracellular materials can help with this. For example, many animal cells can link together into tissues or epithelial layers by attaching to a shared extracellular matrix – a mixture of various materials (proteins, carbohydrates, etc.) in the extracellular space. This extracellular matrix is not as rigid as plant and fungal cell walls but can still provide structure support and facilitate the arrangement of cells into tissues and organs.
In animals, there are also several ways for cells to directly attach to each other via proteins embedded in the plasma membrane. Some of these connections facilitate direct cell-cell communication by allowing components of the cytoplasm to move between neighbouring cells. Plant cells also have a mechanism to do this. We will explore the structure and function of cell walls and extracellular matrices, along with cell-cell connections in Chapter 4.6.
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