## 2 Microplastic and Nanoplastic Uptake
### MP and NP Internalization at the Molecular Level  
The internalization mechanisms vary across different cell types and are influenced by factors such as the particle size, morphology, and chemical composition, all of which can affect particle uptake and toxicity [100; 101; 102; 103; 104]. There are two types of internalization pathways--energy-dependent and non-energy-dependent transport. Energy-dependent transport can be further divided into phagocytosis and pinocytosis. Phagocytosis is primarily employed by specialized cells, including macrophages, neutrophils, and monocytes [105; 106]. Nanoparticles can be absorbed through phagocytosis and pinocytic mechanisms, including macropinocytosis, caveolin- and clathrin-mediated endocytosis, and dynamin-independent endocytosis [107]. The actin cytoskeleton and Rho family GTPases play a role in the processes of macropinocytosis and phagocytosis. Large vacuoles known as phagosomes are produced by phagocytosis, whereas macropinosomes are produced by macropinocytosis. These processes are distinguished by the fact that macropinocytosis generates macropinosomes by fusing endocytic vesicles with the plasma membrane, whereas phagocytosis generates phagosomes [108]. These processes are facilitated by receptors and dependent on dynamin. They rely on invagination into the plasma membrane and the formation of clathrin-coated vesicles and caveolar vesicles, which are then transported within the cell [105; 107; 109]; see Figure 3.  
Figure 3: Main pathways of nanoplastic particle internalization at the cellular level: phagocytosis, macropinocytosis, clathrin-dependent pathway, and caveolin-dependent pathway. Understanding the uptake pathways of nanoparticles is important in mediating both their intracellular fates and the biological response. Within some cell types, clathrin-mediated endocytosis is the main pathway of internalization, while caveolin-mediated endocytosis occurs mainly in endothelial cells [109]. In previous research, clathrin-mediated endocytosis was found to play a role in nanoparticle absorption. NPs enter the cell through similar internalization mechanisms. In addition, renal epithelial cells (293T), bone marrow-derived macrophages (BMDM), and fibroblasts (129) can be accessed by NPs through macropinocytosis and phagocytosis [107]. According to some research, the uptake of nanoparticles in particular cell types may not be dependent on energy or receptors. This has led researchers to conclude that particle uptake can occur through epithelial barriers such as tight junctions between cells [100,110]. The process of particle internalization takes their size into account, but there is debate regarding the minimum size required for absorption. Some researchers suggest that clathrin-mediated endocytosis is only possible for clathrin-coated pit sizes of about 120 nm. However, others claim that typical clathrin-coated pit sizes range from 60 to 200 nm [101,106,111]. Caveolae-mediated particle uptake can occur for particles ranging in size from 200 nm to less than 1 \(\upmu\)m [101,112]. On the other hand, macropinocytosis-derived particles should have a size greater than 150 nm [113]. In the case of PS particles, the optimal size for phagocytosis is 88 nm to 2.68 \(\upmu\)m. However, smaller particles accumulate more rapidly and are more efficient. At tight junctions, nanoparticles can be internalized through paracellular transport if their size is less than 10 nm [100,114]. Internalization is also conditioned by the change in the surface area of the particles. Another non-endocytic internalization pathway is interaction with the lipid bilayers of plasma membranes and the higher uptake of positively charged PS NPs relative to negatively charged particles [113]. Positively charged PS NPs are absorbed through clathrin-mediated endocytosis, as described by [PERSON] et al., whereas experiments with single PS NPs showed internalization via micropinocytosis [115,116]. In a study conducted by [PERSON] et al.