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Soap molecules are similar in structure to phospholipids and interact with the cell membrane. They can clearly cause damage to the cell membrane.
If this is the case, why then, does soap not break down your skin every time you use it? The skin cells, keratinocytes still have a membrane, so wouldn't soap attack our skin in the same way it does to plant cells and cause deterioration?
The outer most layer of the mammalian epidermis (cornified layer or stratum corneum) is composed of 15-20 layers of dead cells called corneocytes, which are basically dead keratinocytes filled with keratin intermediate filament cross-linked other proteins as well as some lipids. As keratinocytes differentiate into corneocytes (a process called cornification), the plasma membrane is replaced by what is called a conified envelope which is composed of cross-linked structural proteins and some lipids (more details in here).
Surfactants in soap do interact with components of the cornfield layer such as keratin but not in the same way they would with the lipid bilayer of a typical plasma membrane. Outcome of such interaction depends on the kind of soap as well as amount and duration of application among other factors, but even a normal soap, upon normal use on a normal skin, may cause some keratin denaturation. But the cornified layer is actually continually shed (process called desquamation) and replaced by keratinocytes that proliferate in the inner most layer of the epidermis.
You can find more details on effect of detergents on skin here and here. You can also find useful info on skin structure here and here.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2674689/
"The SC [stratum corneum] consists of corneocytes, keratinocytes that have undergone terminal differentiation, surrounded by a neutral lipid-enriched, extracellular matrix composed primarily of ceramides, cholesterol, and free fatty acids. The hydrophobic extracellular lipid matrix provides the principal barrier to the transcutaneous movement of water and electrolytes."
The keratinocytes have an extracellular ground substance that basically bar things like soap and water from diffusing/otherwise entering the body; this is even a part of our innate immune system. The answer then is the keratinized stratified squamous epithelial extracellular matrix.
