Cell Structure

All objects need to define where they end and the outside world begins. The region of the object that interacts with the outside is often critical for its function and behavior. All cells in our body are surrounded by a membrane composed of lipids, proteins and sugars that we’ll call the cell membrane. The cell membrane defines the outer border of cells and provides a functional interface with the surrounding environment.

Cell Membrane

The most obvious function of the cell membrane is to prevent the loss of cellular material and restrict the passage of material from the external environment into a cell. Proteins and other cellular material undergo rapid diffusion due to thermal energy. Without a cell membrane, all of those precious molecules would simply diffuse away into the surrounding environment. Likewise, the surrounding environment contains a myriad of molecules and chemicals, some useful and some harmful. The cell membrane provides routes of selective entry of material from the surrounding environment.

Membranes restrict passage of material based on size and charge. Large molecules, such as DNA, RNA, protein and carbohydrates, can’t diffuse freely across membranes. In contrast, gases freely diffuse which is critical for the exchange of oxygen and CO2. NO is a key signaling molecule. Some larger molecules such as steroids can diffuse across membranes. Steroids are large but hydrophobic which allows them to diffuse the membrane. Steroids are important in cell communication and signaling.

Cells regulate the passage of many molecules and ions. Amino acids, nucleic acids and carbohydrates require special portals to pass across the cell membrane due to their size and/or charge. Even single atoms, such as sodium, potassium and chloride, can’t diffuse across the membrane without a special passageway because of the strong charge of the ionized versions of the atoms.

The ability of membranes to restrict the diffusion of ions allows cells to create ion gradients across their cell membrane. For most cells, the concentration of sodium is significantly higher outside the cell than inside. For potassium, the reverse is true: the concentration inside the cell is higher than outside the cell. The distribution of other ions also differs between inside and outside. One consequence of this asymmetric distribution of ions is that the overall charge inside the cell is negative compared to the outside. An electrical potential difference exists across cell membranes which affects the diffusion of ions into and out of cells.

Cells use these ion gradients in a variety of ways. Because sodium readily flows into the cell, cells couple the movement of sodium with the uptake of key nutrients such as glucose. This electrical potential across the membrane is critical for cell communication. For example, neurons generate action potentials by reversing the electrical potential across their membranes.

What makes membranes an effective barrier to prevent diffusion of proteins and ions? Membranes consist of a bilayer of phospholipids. The layers of phospholipids are arranged into an outer leaflet that faces the external environment and an inner leaflet that faces the inside of the cell. Membranes have two key chemical properties. The outer surfaces is charged and is soluble in water. All cells exist in aqueous environments and must interact readily with water. The central core of membranes, in contrast, is hydrophobic or repels water. The hydrophobic core is what inhibits the diffusion of charge molecules and ions. The ability of phospholipids to pack together tightly is what restricts the diffusion of larger molecules.

One key feature of phospholipids is that in water, their most stable conformation is to form a vesicle that encloses water. This resembles a cell.

The diagram illustrates the general structure of a phospholipid that composes membranes. There are two important chemical and structural features of phospholipids. The polar head group makes phospholipids soluble in water, where as the long hydrophobic tails allows phospholipids to self-assemble into bilayers to form membranes.

The hydrophobic tails are composed of long chains of hydrocarbons. If the carbons are all linked by single bonds, then the tail is called saturated and tends to be more straight, allowing saturated lipids to pack more closely together. A double-bond introduces a kink in the tail and prevents lipids from packing as closely together as in saturated lipids. Lipids with a double bond are called unsaturated. Biological membranes contain a mix of saturated and unsaturated lipids as a membrane with all saturated lipids would be a solid at physiological temperature.

One intriguing feature of phospholipids is that they can have a variety of chemically distinct head groups, a few of which are shown here. If the cell membrane merely forms a diffusion barrier, why would phospholipids need different head groups? As we’ll see the cell membrane does more than restrict the diffusion of material. It also plays a critical role in cell signaling reactions. To perform these additional functions it needs proteins. Some proteins can differentiate between these head groups and cell membrane can recruit specific proteins by have more or less of certain types of phospholipids.

Sphingolipids are another class of lipid found in cell membranes. Similar to phospholipids, sphingolipids have long chains of hydrocarbon, but the structure of the polar head group is different and can be quite elaborate as shown below. Although less common than phospholipids, sphingolipids have a variety of different cellular activities in addition to creating a membrane barrier. These activities include cell growth, cell adhesion and inflammation.

In a typical membrane, there will be a variety of different types of lipids and the composition of lipids in the cell membrane will differ between cells. In addition, the lipid composition will differ between the two leaflets. Because the outer leaflet faces the external environment, it will contain lipids that help cells interact with and respond to changes in the surrounding environment. In contrast, the inner leaflet will contain lipids that interact with proteins in the cytoplasm or participate in signaling pathways.

Membrane Proteins

The lipid bilayer in cell membranes is impermeable to most molecules and ions, but cells need theses molecules, such as amino acids, sugars, nucleotides, to grow and survive. How do cells take up this material from the external environment while preventing the diffusion of others?

Cell membrane contains many different types of proteins that impart different functions on membranes. One class of proteins is channels that span both bilayers and contain a pore that allows that passage of specific molecule or ion. The opening of some of these these pores is tightly regulated and occasionally requires energy.

Cells must also be able to sense their surrounding environment. Single-celled organisms sense the amount of nutrients in the environment to know when is a good time to grow and divide. The cells in our body must communicate with each other. That communication is mediated by small molecules that are released to the environment and then must be detected by cells. Phospholipids cannot differentiate between all of these small molecules, so cells express proteins in membranes which bind to specific molecules with high affinity.

A second type of protein found in the cell membrane is receptors that allow cells to sense and respond to the external environment. Receptors interact with specific molecules and chemicals outside the cell and relay their binding state across the membrane to activate or inactivate specific cellular events.

Lastly, as multicellular organisms, our cells must adhere to each other to form functional units. Those cells must stick to each other or tissues and organs would fall apart. Phospholipids in neighboring cells cannot interact to tether cells together. Cells express proteins in their membranes that interact with proteins in the membranes of adjacent cells. The interactions between these proteins is strong enough to hold cells together.

All of the above functions performed by the cell membrane are mediated by proteins associated with cell membranes. There are different ways that proteins can associate with membranes . One class is integral membrane proteins. Theses proteins span the membrane at least once and can cross the membrane multiple times. Because the proteins span the membrane, they have a domain that faces the external environment and a domain that faces the interior of the cell. These proteins are permanently embedded in the membrane and cells must expend considerable energy to extract theses proteins from membranes.

A second class is peripheral membrane proteins that associate with phospholipids or integral membrane proteins but don’t span the membrane. These proteins form transient interactions with the membrane as changes in the structure of the proteins or the phospholipid composition of the membrane can disrupt the interaction.

Another important concept to remember is that cell membranes are fluid. Due to thermal energy, the lipids and proteins rapidly diffuse within membranes. Consequently, proteins and lipids will eventually randomly distribute throughout the cell membrane unless their diffusion is restricted. This will be important when we consider cell adhesion.

Cytoskeleton

Lipid membranes are very flexible and extensible. This property is critical for cell growth and changes in cell shape and morphology. However, the compliance of the membranes make them prone to damage after repeated, extensive deformation. Cells in skeletal muscle and skin are subject to strong physical forces that can easily damage cell membranes.

The cytoskeleton provides mechanical support to the cell membrane. Actin filaments are the primary cytoskeletal filaments that support the cell membrane. Actin filaments can be arranged in a variety of configurations to create different shapes of the cell membrane. If the interaction between the cytoskeleton and cell membrane is disrupted, then the cell membrane is weakened and prone to damage.

Actin

Actin filaments are a polymer of a single globular protein of about 43 kD. Actin monomers bind and hydrolyze ATP and have an asymmetric structure as one side of the protein contains the pocket to bind ATP. Actin monomers polymerize into filaments in a helical fashion. Actin filaments appear to be of two single filaments wrapped around each other. The lateral interactions between monomers allows filaments to grow to greater lengths than if the filament were a single, straight chain of monomers. Individual actin filaments are shorter and less rigid than microtubules.

Microtubules

Microtubules are another cytoskeletal filament important for cell structure and function. Microtubules are composed of a heterodimer of alpha and beta tubulin. Both bind the nucleotide GTP. Heterodimers assemble into filaments end on end to form protofilaments. 13 protofilaments interact to make a single microtubule. The arrangement of the protofilament creates a long, hollow tube similar to a PVC pipe. The extensive lateral contacts between subunits in neighboring protofilaments gives microtubules greater strength and allows them to grow longer than actin filaments. Microtubules are also polarized with one end, the minus end, containing an exposed alpha subunit and the other end, the plus end, having an exposed beta subunit. Microtubules grow in length from their plus ends, but their minus ends are unstable and are usually associated with proteins that prevents subunits from depolymerizing from the filament.

The video below shows the behavior of microtubules in vitro. Note that the minus ends of the microtubules emanate from a common center called the microtubule organizing center. Microtubules grow from their plus ends as more dimers are added to the filament. Occasionally, microtubule will stop growing and then shrink as dimers fall off plus end. Inside cells, the plus ends of microtubules are capped or stabilized so they don’t shrink but for certain cellular events, such as cell division, the dynamic nature of microtubule growth is important.

Integrated cytoskeleton

Although structurally and functionally distinct, actin and microtubules form an integrated network to provide structural support and as we’ll see, the means for moving material throughout the cytoplasm. This image is stained for actin filaments in red and microtubules in green. Note the different distribution of the two filaments. Actin filaments usually localize near the cell membrane or at sites where cells attach to another cell or external surface. Actin filaments provide structural support to the cell membrane and as we will discuss in another lecture, allow cells to generate tension on the cell membrane which can lead to cell contraction.

Microtubules are longer than actin filaments and span the entire length of the cell. This makes them effective for transporting material across cells.

Intracellular transport along filaments

Microtubules also mediate the transport of organelles, proteins and nucleic acids within the cytoplasm of cells. Microtubules-based motor proteins, kinesins and dynein, use microtubules to move cellular material within cells. Kinesins comprise a large family of motor proteins most of which move toward the plus ends of microtubules. In contrast, dynein moves toward the minus ends. All motor proteins contain a domain that binds microtubules and hydrolyzes ATP. The energy released through ATP hydrolysis propels the motor protein along a microtubule. Motor proteins that transport cellular material also contain a domain that interacts specifically with an organelle, protein or nucleic acid. By controlling the type of motor protein is attached to an organelle, cells control the location of that organelle.

Similar to microtubules, actin filaments also support the transport of organelles, proteins and nucleic acids. Instead of kinesins and dynein, actin filaments use myosins to transport intracellular organelles and other cellular material. These myosins resemble the structure of kinesin in that contain motor domains that bind actin filaments and hydrolyze ATP and a domain at their C-terminus that links the motor to different cargo. The motor domains of myosin generate force along actin filaments to move themselves within the cytoplasm.

Because actin filaments are shorter than microtubules, myosins usually transport organelles over shorter distances compared to kinesins and dynein. Myosin-based transport is often used near the cell membrane, an area rich in actin filaments. One model of how kinesins and myosins work together is that kinesins transport organelles from the center of the cell towards the periphery, where myosins take over moving organelles near the cell membrane. For example, a secretory vesicle might be transported from the trans-Golgi network towards the cell membrane along microtubules. When the secretory vesicle reached the actin filaments underneath the cell membrane, myosins would complete the transport of the vesicle to the cell membrane.

Actin and myosin change cell shape

In addition to transporting organelles, actin and myosin filaments are used to change the shape of cells. Recall that actin filaments localize near the cell membrane. Cells express a type of myosin that assembles into filaments (this myosin is similar to the one found in your muscle cells). These large myosin filaments are capable of two different actin filaments. The myosin filament is organized so that its motors pull on both actin filaments. If both actin filaments are attached to the cell membrane, the the myosin filament will pull those attachment points closer together. The net result is a small contraction of the cell.

Intermediate filaments

Intermediate filaments are the third component of the cytoskeleton but less well studied than actin and microtubules. Intermediate filaments primarily provide mechanical support and are structurally and functionally different than microtubules and actin filaments.

In the image below, intermediate filaments are stained in green and the cell membrane is stained blue. Intermediate filaments extend from the nucleus (dark, round structure) to the cell membrane. At the cell membrane they interact with proteins that bind proteins in the cell membranes of neighbor cells. The proteins in the neighbor cells are also linked to intermediate filaments. In this way, intermediate filaments are integrated into large network that spans a group of cells, increasing the mechanical strength of the cells and tissue and protecting it against external stress. For this reason intermediate filaments are found predominantly in cells that face significant mechanical stress, such as skin.

One of the unique properties of intermediate filaments is that they respond differently to low and high forces. At low forces intermediate filaments will stretch but as the force increases, intermediate filaments become less flexible and start resisting the force. This property makes intermediate filaments more mechanically robust than either microtubules or actin filaments. The graph shows the two phases of intermediate filaments under an external force by relating the amount of external force with the increase in length in the intermediate filament. At low forces, intermediate filaments stretch and increase in length but as the force increases, the intermediate filament stiffens and its length changes less as the force increases. Intermediate filaments are more mechanically robust than microtubules or actin filaments.

Similar to microtubules and and actin filaments, intermediate filaments are polymers that from from smaller subunits. However, the structure of the base subunit of intermediate filaments is very different from actin or tubulin. First, it is not a globular protein but contain a long helical region or coiled coil. The helical region mediates interaction with another intermediate filament protein. The dimer contains globular domains on either end. Two dimers associate in head to tail fashion with slight offset. The tetramer is considered the base subunit of intermediate filaments. Note that the base subunit lacks the inherent polarity of actin or alpha-beta tubulin. Consequently, intermediate filaments are not polarized as are actin filaments and microtubules. Soluble base subunit of IFs.

Tetramers polymerize end on end to form protofilaments. 8 protofilaments twist together to form an intermediate filament. The extensive lateral interactions are in part what gives intermediate filaments their tremendous strength.

One other difference between intermediate filaments and actin filaments and microtubules is that intermediate filaments do not support transport of material by motor proteins.

Intermediate filaments comprise large family of proteins. Keratin is the largest class with ~50 members and found prominently in skin and hair. Neurofilaments are found in axons and lamins localize to the inner nuclear membrane.