Tissues are composed of cells, extracellular matrix (ECM), and extracellular substances. Cells are the fundamental structural and functional units of tissues and are composed of two principal components:
- [[Nucleus]]
- Cytoplasm
The cell is separated from the extracellular environment by the plasma membrane (plasmalemma), which serves as a selective barrier and mediates interactions with the ECM. Specialized membrane proteins, such as integrins, link intracellular cytoskeletal elements to extracellular matrix components, thereby contributing to structural integrity and signal transduction.
The cytoplasm consists of a fluid component and various structural and functional elements:
The cytosol is the aqueous component of the cytoplasm containing numerous enzymes involved in cellular metabolism, including pathways such as glycolysis. It serves as the medium in which cellular organelles are suspended.
Organelles are metabolically active structures within the cytoplasm and are classified into:
- Membranous organelles (e.g., mitochondria, endoplasmic reticulum)
- Non-membranous organelles (e.g., ribosomes)
The cytoskeleton is composed of protein filaments that:
- Maintain cell shape
- Facilitate cell motility
- Enable intracellular transport
Cytoplasmic inclusions are non-living substances such as:
- Glycogen (carbohydrate storage)
- Lipid droplets
- Pigments
The plasma membrane is a dynamic, selectively permeable structure that regulates the movement of substances into and out of the cell.
- Thickness: approximately 7.5–10 nm
- Visible only under an electron microscope
- Composed primarily of a phospholipid bilayer with embedded proteins
The membrane consists of amphipathic phospholipids:
- Hydrophilic (polar) heads face the aqueous extracellular and intracellular environments
- Hydrophobic (nonpolar) fatty acid tails face inward, forming the lipid core
Phospholipids exhibit dynamic movement:
- Lateral movement: frequent and rapid
- Flip-flop movement: rare
Cholesterol molecules are interspersed among phospholipids in animal cell membranes and play a key role in regulating membrane fluidity and permeability:
- Restrict excessive movement of phospholipids
- Decrease permeability to small molecules
- Stabilize membrane structure
Glycolipids are lipids with attached carbohydrate chains located on the extracellular surface. Together with glycoproteins, they form the glycocalyx, which is involved in:
- Cell recognition
- Protection
- Cell adhesion
Membrane proteins constitute approximately 50% of the membrane by weight and are classified as:
- Embedded within the lipid bilayer
- Often span the membrane (transmembrane proteins)
- Loosely attached to the membrane surface
- Proteins with carbohydrate moieties projecting extracellularly
- Contribute to the glycocalyx
Membrane transport is mediated by specialized proteins known as permeases, which include:
- Form hydrophilic pores or channels
- Allow passage of ions or small polar molecules
- Transport occurs passively down the concentration gradient
- Examples include ion channels and aquaporins
Ion channels contribute to the establishment of a membrane potential, defined as the electrical charge difference across the membrane due to unequal ion distribution.
- Bind specific molecules
- Undergo conformational changes to transport substances across the membrane
- Can mediate: Passive transport (facilitated diffusion) Active transport (against the concentration gradient, requiring energy)
Certain membrane proteins function as enzymes, with their active sites exposed at the membrane surface, catalyzing specific biochemical reactions.
Membrane proteins act as receptors for extracellular signaling molecules. For example, insulin binds to its specific receptor on the cell membrane without entering the cell, triggering intracellular signaling pathways.
Mediated primarily by glycoproteins, which serve as identification markers allowing cells to recognize one another.
Membrane proteins facilitate adhesion between adjacent cells, forming intercellular junctions essential for tissue integrity.
Membrane proteins, particularly integrins, provide anchorage by linking:
- The intracellular cytoskeleton
- The extracellular matrix (in animal cells)
This connection is essential for maintaining cell shape, mechanical stability, and signal transduction.
The plasma membrane is a highly organized and dynamic structure composed of lipids, proteins, and carbohydrates. It plays critical roles in selective permeability, cellular communication, structural support, and interaction with the extracellular environment. Transport proteins, including channel and carrier proteins, are essential for maintaining cellular homeostasis and enabling communication between the cell and its surroundings.