Selective permeability, modelling and bulk transport

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The membrane lets **some substances cross but blocks others**, mainly depending on their **size** and whether they are **polar**.

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**Small, non-polar** molecules such as **oxygen** and **carbon dioxide** (water crosses too, helped by aquaporins).

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**Large** molecules such as **starch** and **proteins** — they are too big to pass through.

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As a **model** of a partially permeable membrane: its pores let **small** molecules through but hold back **large** ones.

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**Glucose passes out** through the pores (it is small); **starch stays inside** (it is too large).

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Its molecules are **too large** to fit through the pores of the partially permeable tubing.

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Moving **large amounts of material**, or particles too big to cross the bilayer, **in vesicles** — it **uses ATP**.

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Bulk transport that brings material **INTO** the cell: the membrane **folds inwards** and pinches off a vesicle around the material.

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Bulk transport that releases material **OUT** of the cell: a vesicle **fuses** with the plasma membrane and empties its contents.

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**Yes** — both endocytosis and exocytosis **use ATP**, so bulk transport is **active**.

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Taking in **large food particles** or engulfing a **pathogen** (e.g. a white blood cell engulfing a bacterium).

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**Secreting** proteins, **enzymes** or **hormones** (e.g. a gland cell releasing a digestive enzyme).

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**Endo** = **into** the cell ('enter'); **exo** = **exit** the cell.