Introduction to Cell Biology
Membranes
They are smooth sheets that behave as selectively permeable barriers to maintain physicochemical conditions. They have an asymmetrical structure consisting of a lipid bilayer, which constitutes the basic structural component, and a set of proteins distributed on either side of or immersed in the bilayer. These proteins are responsible for specific activities (asymmetric structure). This is referred to as the fluid mosaic model proposed by Singer and Nicholson.
Lipid bilayer: This consists of phospholipids (50%) and a set of proteins (50%).
Properties of Bilayers
Lipids include: phospholipids, glycolipids, and cholesterol. All these components are amphipathic molecules.
Self-Assembly
In aqueous media, the formation of bilayers is spontaneous. The hydrophobic portions of the molecules remain inside, and the hydrophilic portions face outwards.
Self-Sealing
These bilayers tend to close, forming vesicles with hollow hydrophobic ends that could be in contact with water.
Waterproofing
The bilayer acts as a containment barrier, preventing the cell from losing its soluble content and preventing the entry of water-soluble ions and molecules (they are difficult to pass).
Fluency
There is a constant movement of phospholipids, including several types of movements (flexion, rotation, diffusion, lateral motion, and flip-flop).
Cholesterol is a component of the plasma membrane whose function is to increase stiffness and decrease fluidity.
Membrane Proteins
These proteins carry out the possible functions of the membrane, acting as:
- Receptors: for cellular communication
- Transporters: for solute transport
- Electron transporters
- Enzymes: for enzymatic reactions
Types of Membrane Proteins
Integral Proteins
Strongly attached to the membrane. Some integral proteins span the membrane once or multiple times (up to 7 times) and are called multipass proteins.
Peripheral Proteins
These are located on either side of the membrane, weakly bound to the polar heads of lipids or other integral proteins.
Lipids and proteins are capable of movement. However, cells can restrict this movement, resulting in membrane domains. Membrane domains have functions such as nutrient uptake, and reception and transmission of stimuli.
Plasma Membrane
The plasma membrane defines the cell’s boundaries and its relationship with the outside environment. It is similar to other biomembranes. The outer side is called the glycocalyx and is formed by oligosaccharides linked to glycolipids and glycoproteins. The glycocalyx protects against physical and chemical damage and controls the entry and exit of materials (electrical permeability).
Membrane proteins actively participate in specific transport systems (as a result of the selective permeability).
Transport of Small Molecules
Transport can be either passive, when it does not require energy, or active, when it needs an energy source.
Passive Transport
This involves the spontaneous movement of molecules from an area of higher concentration to an area of lower concentration, i.e., down their concentration gradient.
Simple Diffusion
This occurs through the lipid bilayer. Non-polar molecules such as gases and some hormones, such as steroid and thyroid hormones, can cross the cell membrane in this way. Small uncharged polar molecules, such as water, ethanol, glycerol, or urea, can also pass through.
Facilitated Diffusion
This occurs via carrier proteins or permeases, which undergo a conformational change upon binding to specific molecules and carry them across the membrane. It can also occur through ion channels, which form aqueous pores through which ions pass. There are sodium channels, calcium channels, chloride channels, etc., which remain closed until they receive a specific signal.
These signals can be:
- Voltage-dependent (electrical signal)
- Ligand-gated (chemical signal)
Active Transport
Molecules cross the cell membrane against their concentration gradient. This requires energy (ATP). This transport is carried out by carrier proteins called pumps.
An example is the sodium-potassium pump. In this transport model, energy derived from ATP hydrolysis is used to eject three sodium ions from the cell and bring in two potassium ions, both against their concentration gradients. This helps to control intracellular osmotic pressure and membrane potential.
Transport of Macromolecules and Particulate Matter
Endocytosis
During endocytosis, the plasma membrane is internalized. Substances are engulfed by invaginations of the plasma membrane, forming vesicles (membranous sacs) that contain the ingested material.
Types of Endocytosis
Phagocytosis
The ingested material consists of bacteria, cellular debris, or old intact cells. This process destroys invading microorganisms and removes old cells.
Pseudopodia are formed by the membrane surrounding the particles that are being phagocytosed, creating a large vesicle called a phagosome. The phagosome eventually fuses with lysosomes, where the contents are digested.
Clathrin-Dependent Endocytosis
This is used for the entry of macromolecules, which are internalized into vesicles formed in coated pits. Coated pits are small depressions with a roof formed by the protein clathrin. The endocytosed material ends up in lysosomes.
An example of clathrin-dependent endocytosis is the uptake of cholesterol.
Exocytosis
This is the reverse of endocytosis, and it regenerates the plasma membrane. Hormones, neurotransmitters, digestive enzymes, etc., are released from the cell via exocytosis.
These materials are synthesized in the endoplasmic reticulum and then pass through the Golgi apparatus. The products are packaged into secretory vesicles that are directed to the plasma membrane to release their contents outside the cell.
Cellular Junctions
Cellular junctions are regions with a high concentration of proteins that establish connections between two cells or between a cell and the extracellular matrix. According to their shape, there are two types: zonulae and maculae. According to their function, there are three types: occluding, anchoring, and communicating (also including desmosomes and hemidesmosomes).
Functions and Features
- Occluding junctions bind adjacent cells of epithelial sheets to prevent the passage of molecules between them.
- Zonula adherens connect the actin cytoskeletons of adjacent cells.
- Desmosomes connect the intermediate filaments of the cytoskeletons of adjacent cells.
- Communicating junctions connect the cytoplasms of adjacent cells, allowing the passage of soluble ions and small molecules between them.
- Hemidesmosomes anchor intermediate filaments of the cytoskeleton of a cell to the extracellular matrix.
Types of Cell Communication
Based on the distance traveled by the signal molecule:
- Autocrine: Immune system cells use this type of communication, where the signal molecule travels a short distance. e.g., T lymphocytes induce cell proliferation.
- Paracrine: The signal molecule travels a shorter distance than in autocrine signaling. e.g., neurotransmitters.
- Endocrine: The signal molecule, which is a hormone, travels a long distance to reach the target cell.
Proteasomes
Basic Function
Proteasomes eliminate proteins and contribute to protein aging. They are large molecular complexes that degrade defective or short-lived proteins. This process requires ATP (energy). Proteasomes contain two parts: a central hollow cylinder and two protein complexes. Proteins are recognized and destroyed by a system of enzymes called ubiquitin.
Cytoskeleton
Important Functions
The cytoskeleton is composed of filamentous proteins. Its functions include maintaining cell shape, transporting substances and organelles within the cell, and enabling cell movement.
Types of Cytoskeletal Filaments
Actin Microfilaments (Thin Filaments)
Specific Functions:
- Division of the cytoplasm during cell division (contractile ring)
- Muscle contraction
- Strangulation of the cytoplasm (mitosis)
Intermediate Filaments (Fibrous Proteins)
Microtubules
Specific Role:
Microtubules are composed of 13 protofilaments of tubulin. They radiate from the centrosome to all parts of the cell, giving it shape and enabling movement (cilia and flagella).
Properties
Microfilaments and Microtubules
These are polar structures, with their two ends growing at different rates. The fast-growing end is called the plus end, and the other is the minus end.
These structures are labile (not stable) and go through phases of growth and shortening.
Intermediate Filaments
These are stable and non-polar.
Centrosome
Found only in animal cells, the centrosome is the organizing center of cellular microtubules. It has an amorphous matrix.
Structures
Actin Filaments
- Bundles: These have a parallel structure. There are two types: non-contractile bundles, which form microvilli that absorb nutrients, and contractile bundles, which form sarcomeres that allow muscle contraction (actin-myosin interaction).
- Network: This forms a meshwork located beneath the plasma membrane, acting as structural support. It also forms pseudopodia, which allow cell movement and processes such as phagocytosis.
Microtubules: Centrioles
Centrioles are a pair of hollow cylinders found in the centrosome, only in animal cells. Each centriole is composed of nine triplets of microtubules.
The centrioles and centrosome are duplicated during each cell cycle while the DNA is replicated, forming the poles of the mitotic spindle.
Cilia and Flagella
Cilia
Short and numerous. Their movement is a beating motion.
Flagella
Long and few in number. Their movement is a wave-like motion.
Structure of Cilia and Flagella
They consist of an outer portion called the axoneme and an inner portion called the basal body, whose structure is similar to that of centrioles.
The axoneme consists of nine pairs of microtubules (9 + 2 structure).
Movement
The outer doublets of microtubules slide over one another, driven by the motor activity of dynein. In the presence of ATP, the dynein arms that project from the A microtubule (complete) of each doublet “walk” on the B microtubule (incomplete) of the adjacent doublet toward the minus end.