Eukaryotic Cells: Cell Membrane and Cytoskeletal Structures (A-level Biology)

Eukaryotic Cells: Cell Membrane and Cytoskeletal Structures

Cell Surface Membrane

The cell surface membrane is also commonly called the cell membrane, or the plasma membrane.

• Provides structure to the cell. The cell surface membrane is made up primarily of a phospholipid bilayer, with proteins regularly incorporated into it throughout its length. The cell surface membrane also has additional lipids and carbohydrates attached to it, each of which serve their own specialised functions.
• Regulates the movement of substances into and out of cells. The cell surface membrane is specially designed to act as a barrier to protect the cell from the external environment. It tightly regulates what gets to come in and out. Everything that comes inside of a cell or goes outside the cell (including water, ions, and large molecules) has to cross the cell membrane.
• Plays a role in cell to cell communication. All cells communicate with each other and their environment. The cell surface membrane has special proteins called receptors (glycoproteins and glycolipids) which receive signals, such as chemical hormones, from neighbouring cells or from the environment, and then relay these chemical messages to the inside of the cell to initiate specific responses related to these messages.

Cytoskeletal Structures

• The cytoskeleton is a network of protein filaments. The cytoskeleton is made up of protein filaments which form a mesh-like network in the cytoplasm of the cell. They are present in all cells, including bacteria. Cytoskeletal proteins in eukaryotes are present in the form of microtubules (cylindrical proteins) and microfilaments (protein strands)
• The cytoskeleton perform a range of functions. Cytoskeletal structures are heavily involved in a variety of cellular activities:
  1. Strengthening and maintaining the structure of the cell
  2. Providing support to cellular organelles and maintaining their position
  3. Involved in movements of cellular material e.g. moving chromosomes during cell division
  4. Mobilising the cell
• There are specialised cytoskeletal structures. Rather than just acting as protein filaments in the cytoplasm, some cytoskeletal structures have become specialised to adapt to the needs of the cells. Examples include cilia and flagella.

Centrioles

• Centrioles are made up of microtubules. Centrioles consist of small, hollow cylindrical proteins called microtubules.
• Centrioles are mainly present in animal cells. They are found in most animal cells but are only present in certain plants.
• Centrioles help chromosomes separate during cell division. During cell division, it is vital that chromosomes are equally separated so that each of the daughter cells will have the same number of chromosomes. Centrioles ensure this happens by pulling the chromosomes apart equally to each end of the dividing cell.

Cilia

• Cilia are hair-like structures on surface membranes. Some animal cells have cilia, which are tiny hair-like structures that protrude from the surface membrane. 
• There are motile and non-motile cilia. Motile cilia can move due to the activity of the microtubules inside it. This type of cilia help substances to move along the surface of cell membranes e.g. respiratory cilia help waft mucus along the respiratory tract. Non-motile cilia do not move and are typically found in sensory organs e.g. some photoreceptor cells in the eye have non-motile cilia as a part of its structure.

Flagella

• Flagella are long protrusions from the cell surface. Flagella are similar to cilia; however, they are usually much longer and thicker. They protrude from the cell surface and is covered in a layer of plasma membrane.
• Flagella help the cell move. Flagella also have microtubules inside of it like cilia, which contract to help the flagella move. This type of movement propels the cell and allow it to be motile. An example of a flagellum would be the tail on a sperm cell, which contracts to help it “swim” through the vaginal canal of a female and into the uterus.