1) explain why the cell membrane is considered amphipathic.
2) how does the amphipathic nature of the cell membrane dictate its selective permeability?
3) which molecule types are able to permeate the membrane freely (without assistance)?
4) which molecule types are unable to permeate the membrane freely (needs assistance)?
5) define passive transport.
6) list the types of passive transport.
7) define active transport.
8) list the types of active transport.
9) differentiate between diffusion & facilitated diffusion.

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Accepted Answer

### Question 1
# Brief Explanations:
The cell membrane's main component is the phospholipid bilayer. A phospholipid has a hydrophilic (water - attracting) phosphate head and two hydrophobic (water - repelling) fatty acid tails. Since the membrane has both hydrophilic and hydrophobic regions, it is amphipathic.
# Answer:
The cell membrane is amphipathic because it is composed of phospholipids. Each phospholipid has a hydrophilic phosphate head and two hydrophobic fatty acid tails, giving the membrane both water - attracting and water - repelling regions.

### Question 2
# Brief Explanations:
The amphipathic nature (hydrophilic heads facing aqueous environments, hydrophobic tails in the middle) creates a hydrophobic core. Small, non - polar molecules can dissolve in the hydrophobic core and pass through. Polar or charged molecules, due to the hydrophobic core, have difficulty passing without assistance, thus dictating selective permeability.
# Answer:
The amphipathic cell membrane has a hydrophobic core (from fatty acid tails) and hydrophilic outer/inner surfaces (from phosphate heads). Small, non - polar molecules can pass through the hydrophobic core. Polar, charged, or large molecules are blocked by the hydrophobic core and need assistance (like transport proteins) to cross, leading to selective permeability.

### Question 3
# Brief Explanations:
Small, non - polar molecules can permeate freely. Examples include oxygen ($O_2$), carbon dioxide ($CO_2$), and small lipid - soluble molecules. They can dissolve in the hydrophobic core of the membrane and move across.
# Answer:
Small, non - polar molecules (e.g., $O_2$, $CO_2$, small lipid - soluble molecules) can permeate the membrane freely as they can dissolve in the hydrophobic core of the amphipathic cell membrane.

### Question 4
# Brief Explanations:
Polar molecules (e.g., glucose, water to some extent without aquaporins), charged molecules (e.g., ions like $Na^+$, $Cl^-$), and large molecules (e.g., proteins) are unable to permeate freely. The hydrophobic core of the membrane repels polar/charged substances, and large molecules are too big to pass through the membrane's structure without help.
# Answer:
Polar molecules (e.g., glucose), charged molecules (e.g., $Na^+$, $Cl^-$), and large molecules (e.g., proteins) are unable to permeate the membrane freely. The hydrophobic core of the amphipathic membrane repels polar/charged substances, and large molecules lack the ability to pass through the membrane's structure unassisted.

### Question 5
# Brief Explanations:
Passive transport is the movement of substances across a cell membrane without the input of cellular energy (ATP). It occurs down a concentration gradient (from an area of higher concentration to an area of lower concentration).
# Answer:
Passive transport is the movement of substances across a cell membrane that does not require cellular energy (ATP) and occurs along a concentration gradient (from high to low concentration).

### Question 6
# Brief Explanations:
The types of passive transport are simple diffusion (movement of small, non - polar molecules down their concentration gradient), facilitated diffusion (movement of polar/charged molecules down their concentration gradient with the help of transport proteins), and osmosis (diffusion of water across a selectively permeable membrane down its water potential gradient).
# Answer:
The types of passive transport are:
1. Simple diffusion (e.g., for $O_2$, $CO_2$)
2. Facilitated diffusion (e.g., for glucose with carrier proteins, for ions with channel proteins)
3. Osmosis (diffusion of water)

### Question 7
# Brief Explanations:
Active transport is the movement of substances across a cell membrane that requires cellular energy (usually ATP). It moves substances against their concentration gradient (from an area of lower concentration to an area of higher concentration).
# Answer:
Active transport is the movement of substances across a cell membrane that requires cellular energy (ATP) and moves substances against their concentration gradient (from low to high concentration).

### Question 8
# Brief Explanations:
The main types of active transport are primary active transport (uses ATP directly, e.g., the sodium - potassium pump which uses ATP to move $Na^+$ out and $K^+$ into the cell) and secondary active transport (uses the energy from an electrochemical gradient, often created by primary active transport, e.g., cotransport of glucose and $Na^+$ in intestinal cells).
# Answer:
The types of active transport are:
1. Primary active transport (e.g., sodium - potassium pump, uses ATP directly)
2. Secondary active transport (e.g., cotransport of glucose and $Na^+$, uses energy from an electrochemical gradient)

### Question 9
# Brief Explanations:
- **Diffusion (Simple Diffusion)**: Involves the movement of small, non - polar molecules (e.g., $O_2$, $CO_2$) down their concentration gradient directly through the lipid bilayer. No transport proteins are required.
- **Facilitated Diffusion**: Involves the movement of polar/charged molecules (e.g., glucose, ions) down their concentration gradient, but requires transport proteins (carrier proteins or channel proteins) to cross the membrane.
# Answer:
- **Diffusion (Simple Diffusion)**: Moves small, non - polar molecules (e.g., $O_2$, $CO_2$) down the concentration gradient through the lipid bilayer; no transport proteins needed.
- **Facilitated Diffusion**: Moves polar/charged molecules (e.g., glucose, ions) down the concentration gradient; requires transport proteins (carrier/channel proteins) to cross the membrane.

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