Basically the title says it all. What factors determine whether a molecule can enter a cell or not?
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The main factor is whether or not the molecule is hydrophobic.
An ion or polar molecule is hydrophilic, and hydrophilic substances cannot diffuse through a membrane because they can't cross the hydrophobic interior.
Another factor comes into play to a lesser degree: size. While as a rule hydrophilic molecules cannot diffuse through a membrane, if they are small enough - such as water - they can.
The above covered simple diffusion. But that's not the only way substances can enter a cell. Polar molecules and even ions can enter a cell if there is a channel or carrier protein in the membrane for that ion or molecule, for example. Then there is endocytosis, which can, in some organisms, bring almost anything (including an entire bacterium) inside a cell.
An ion or polar molecule is hydrophilic, and hydrophilic substances cannot diffuse through a membrane because they can't cross the hydrophobic interior.
Another factor comes into play to a lesser degree: size. While as a rule hydrophilic molecules cannot diffuse through a membrane, if they are small enough - such as water - they can.
The above covered simple diffusion. But that's not the only way substances can enter a cell. Polar molecules and even ions can enter a cell if there is a channel or carrier protein in the membrane for that ion or molecule, for example. Then there is endocytosis, which can, in some organisms, bring almost anything (including an entire bacterium) inside a cell.
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Mitochondria !!!!