Intracellular is a term that refers to the inside of a cell. In biology, an intracellular space is the interior space enclosed by the plasma membrane of a cell. All of the organelles within a cell are located within the intracellular space.
The plasma membrane is a selective barrier that separates the contents of a cell from its surroundings. It regulates what enters and leaves the cell, and also serves as a structural support for the cell. The plasma membrane is composed of two layers of lipids: an inner layer of phospholipids and an outer layer of glycolipids. These lipids are arranged in such a way that they form a bilayer, with their hydrophobic tails pointing towards each other and their hydrophilic heads pointing outwards.
The lipid bilayer is semi-permeable, meaning that it allows some molecules to pass through while others are blocked. This selectivity is achieved by proteins embedded in or attached to the lipid bilayer. These proteins can act as channels or pores that allow specific molecules to pass through, or they can bind to specific molecules and transport them across the membrane.
The main types of cells in animals are epithelial cells, connective tissue cells, muscle cells, and nerve cells. Epithelial cells form the lining of organs and body cavities; connective tissue cells include fibroblasts, adipocytes, macrophages, and leukocytes; muscle cells make up skeletal muscle tissue; and nerve cells constitute nervous tissue. All these cell types have unique functions determined by their structure (morphology) and location within tissues or organs (histology). However, all animal cells share certain common features: they are eukaryotic (have nucleus), lack chloroplasts (are not photosynthetic), have mitochondria (for energy production), express genes encoding for proteins (use DNA as genetic material), secrete extracellular matrix components (such as collagen), possess specialized junctions connecting them to their neighbours (elements of cellular adhesion), display polarity with distinct apical and basal surfaces (oriented toward an external surface or lumen), etcetera.