cellular transport sc.912.l.14.2 relate structure to function for the components of plant and animal cells. explain the role of cell membranes as a highly selective barrier (passive and active transport).
1. what is facilitated diffusion? is it active or passive transport?

2. list and describe 3 types of active transport.

3. what is meant by the term \concentration gradient?\

4. how does the concentration gradient affect movement across the membrane?

5. what is the role of carrier proteins in active transport?

Question

cellular transport sc.912.l.14.2 relate structure to function for the components of plant and animal cells. explain the role of cell membranes as a highly selective barrier (passive and active transport).
1. what is facilitated diffusion? is it active or passive transport?

2. list and describe 3 types of active transport.

3. what is meant by the term \concentration gradient?\

4. how does the concentration gradient affect movement across the membrane?

5. what is the role of carrier proteins in active transport?

Accepted Answer

### Question 1
# Brief Explanations:
Facilitated diffusion is the process where molecules (like polar or large ones) cross the cell membrane with the help of transport proteins (channel or carrier proteins). It is passive transport because it moves substances down their concentration gradient (from high to low concentration) and does not require energy (ATP).
# Answer:
Facilitated diffusion is the movement of molecules across a cell membrane via specific transport proteins (channel or carrier proteins) from an area of higher concentration to an area of lower concentration. It is passive transport (as it does not require energy and moves down the concentration gradient).

### Question 2
# Brief Explanations:
1. **Primary Active Transport**: Uses energy from ATP hydrolysis to directly move molecules against their concentration gradient. For example, the sodium - potassium pump ($Na^+/K^+$ pump) in animal cells. It moves 3 $Na^+$ ions out of the cell and 2 $K^+$ ions into the cell, maintaining the electrochemical gradient.
2. **Secondary Active Transport (Cotransport)**: Uses the energy stored in an electrochemical gradient (created by primary active transport) to move molecules against their concentration gradient. There are two types:
    - **Symport**: Both the molecule being transported and the ion (e.g., $Na^+$) move in the same direction. For example, in the small intestine, glucose and $Na^+$ are transported into the cell together. The $Na^+$ moves down its concentration gradient (established by the $Na^+/K^+$ pump), and this energy is used to move glucose against its gradient.
    - **Antiport**: The molecule being transported and the ion move in opposite directions. For example, the sodium - calcium exchanger ($Na^+/Ca^{2+}$ exchanger) in some cells. $Na^+$ moves into the cell (down its gradient), and $Ca^{2+}$ moves out (against its gradient).
3. **Bulk Transport (Endocytosis and Exocytosis)**:
    - **Endocytosis**: The cell takes in large molecules or particles by engulfing them. There are three types:
        - **Phagocytosis**: “Cell eating” - the cell engulfs large solid particles (e.g., bacteria by immune cells). The cell membrane extends around the particle, forming a vesicle (phagosome) that enters the cell.
        - **Pinocytosis**: “Cell drinking” - the cell takes in small droplets of extracellular fluid along with dissolved solutes. Small vesicles are formed.
        - **Receptor - Mediated Endocytosis**: Specific molecules (ligands) bind to receptors on the cell membrane. The membrane then invaginates to form a vesicle, bringing the ligand - receptor complex into the cell (e.g., uptake of cholesterol - bound to LDL - by cells).
    - **Exocytosis**: The cell releases large molecules (e.g., hormones, neurotransmitters) or waste products. Vesicles containing the substances fuse with the cell membrane, releasing their contents outside the cell.
# Answer:
1. **Primary Active Transport**: Directly uses ATP energy to move molecules against their concentration gradient (e.g., $Na^+/K^+$ pump).
2. **Secondary Active Transport (Cotransport)**: Uses the energy from an electrochemical gradient (created by primary active transport) to move molecules against their gradient. Symport (e.g., glucose - $Na^+$ cotransport) moves substances in the same direction; antiport (e.g., $Na^+/Ca^{2+}$ exchanger) moves them in opposite directions.
3. **Bulk Transport**:
    - **Endocytosis**: Cell takes in large materials. Phagocytosis (engulfs solids, e.g., bacteria), pinocytosis (takes in fluid), receptor - mediated endocytosis (specific ligand - receptor uptake, e.g., cholesterol).
    - **Exocytosis**: Cell releases large materials (e.g., hormones) via vesicle - membrane fusion.

### Question 3
# Brief Explanations:
A concentration gradient is the difference in the concentration of a substance between two regions (e.g., inside and outside a cell). For example, if there are more glucose molecules outside the cell than inside, there is a glucose concentration gradient from outside (high) to inside (low) the cell.
# Answer:
A concentration gradient is the difference in the concentration of a substance between two areas (e.g., across a cell membrane, between two regions in a solution). It represents the driving force for passive transport (substances move from high - concentration to low - concentration regions).

### Question 4
# Brief Explanations:
In passive transport (e.g., simple diffusion, facilitated diffusion), substances move down the concentration gradient (from high to low concentration). The steeper the concentration gradient (the larger the difference in concentration between the two regions), the faster the rate of movement. In active transport, substances move against the concentration gradient (from low to high concentration), and the concentration gradient opposes this movement, so energy (ATP) is required to overcome it.
# Answer:
In passive transport (e.g., diffusion, facilitated diffusion), substances move **down** the concentration gradient (from high to low concentration); a steeper gradient increases the rate of movement. In active transport, substances move **against** the concentration gradient (from low to high concentration), and the gradient resists this movement (energy is needed to overcome it).

### Question 5
# Brief Explanations:
In active transport, carrier proteins bind to specific molecules (or ions) and change their shape (conformational change) to transport these substances across the cell membrane **against** their concentration gradient. This process requires energy (usually from ATP hydrolysis for primary active transport or from an electrochemical gradient for secondary active transport), and the carrier proteins are specific to the substances they transport.
# Answer:
Carrier proteins in active transport bind to specific substances (molecules/ions), undergo a conformational change, and transport these substances **against** their concentration gradient across the cell membrane. They require energy (e.g., from ATP or an electrochemical gradient) to perform this transport and are specific to the substances they carry.

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