Chapter 4: Cell Physiology — Study Notes
Welcome to Cell Physiology! In this chapter of Unit AS 1 (Molecules and Cells), we explore how substances get into and out of living cells. Understanding transport across cell membranes is fundamental to all of biology—from how plant roots draw up water to how nerve cells send signals. Don't worry if the terminology or the numbers seem tricky at first; we will break down every concept step by step.
---1. Movement Across Cell Membranes
Every living cell is enveloped by a cell surface membrane. To stay alive, cells must absorb raw materials (like oxygen and glucose) and excrete waste products (like carbon dioxide). Depending on the size, charge, and concentration of the substance, cells use different transport mechanisms.
A. Simple Diffusion
Simple diffusion is the passive net movement of solute particles from a region of higher concentration to a region of lower concentration (down a concentration gradient) due to their random kinetic energy.
• Energy requirement: Passive (no metabolic energy or ATP needed).
• Substances transported: Small, uncharged molecules or non-polar (lipid-soluble) molecules, such as \(\text{O}_2\) and \(\text{CO}_2\), which can slip directly between the phospholipid molecules of the membrane.
The rate of diffusion across a membrane is governed by Fick's Law:
\(\text{Rate of Diffusion} \propto \frac{\text{Surface Area} \times \text{Difference in Concentration}}{\text{Diffusion Distance / Membrane Thickness}}\)
Analogy: Imagine a crowded room with an open door leading to an empty corridor. People naturally spread out into the corridor without anyone pushing them—simply moving from where they are crowded (high concentration) to where they have space (low concentration).
B. Facilitated Diffusion
Many essential molecules are polar (e.g., glucose, amino acids) or are charged ions (e.g., \(\text{Na}^+\), \(\text{K}^+\)). Because they are hydrophilic, they are repelled by the hydrophobic fatty acid tails inside the phospholipid bilayer. They require the help of specific transmembrane proteins to cross down their concentration gradient.
• Energy requirement: Passive (down a concentration gradient; no ATP needed).
• Key Transport Proteins:
1. Channel Proteins: Fixed, water-filled hydrophilic pores running through the membrane that allow specific water-soluble ions to diffuse through. Some channel proteins are gated, meaning they can open or close in response to specific signals.
2. Carrier Proteins: These possess a specific binding site for a particular solute (like glucose). When the solute binds, the carrier protein undergoes a conformational (shape) change that carries the molecule across the membrane and releases it on the other side.
C. Active Transport
Active transport is the movement of ions or molecules across a cell membrane against their concentration gradient (from a region of lower concentration to a region of higher concentration) using carrier proteins.
• Energy requirement: Active (requires metabolic energy released from the hydrolysis of ATP).
• Mechanism: Solute binds to a specific carrier protein (often called a pump). ATP provides the energy to change the shape of the carrier protein, pumping the solute across the membrane against its gradient.
• Limiting factors: The rate of active transport is limited by the availability of carrier proteins and the cell's rate of respiration (ATP production).
D. Bulk Transport (Cytosis)
When a cell needs to move very large particles, macromolecules, or large volumes of liquid, it uses bulk transport. These processes do not cross through the phospholipid bilayer directly; instead, they rely on vesicles and require energy from ATP.
• Endocytosis: The cell surface membrane invaginates (folds inward) around external material to form a membrane-bound vesicle inside the cell.
— Phagocytosis: Bulk uptake of solid material (cell eating).
— Pinocytosis: Bulk uptake of liquids and dissolved solutes (cell drinking).
• Exocytosis: Secretory vesicles produced within the cell move to and fuse with the cell surface membrane, releasing macromolecules externally (e.g., secretion of digestive enzymes or hormones).
Key Takeaways: Movement Across Membranes
• Simple diffusion: Passive, non-polar/small molecules, directly through bilayer.
• Facilitated diffusion: Passive, uses channel or carrier proteins.
• Active transport: Active (needs ATP), uses carrier pumps only, moves substances against gradient.
• Bulk transport: Active (needs ATP), uses vesicle formation/fusion (endocytosis & exocytosis).
2. Osmosis and Water Potential
What is Osmosis?
Osmosis is the net movement of water molecules from a region of higher (less negative) water potential to a region of lower (more negative) water potential through a selectively (partially) permeable membrane.
Understanding Water Potential (\(\Psi\))
Water Potential (\(\Psi\)) is a measure of the tendency of water molecules to move by osmosis. It is measured in units of pressure: kilopascals (\(\text{kPa}\)).
• Pure liquid water at standard temperature and pressure has the highest possible water potential, defined as exactly \(0\text{ kPa}\).
• When solutes dissolve in water, water molecules cluster around the solute particles, reducing the freedom of water to move. Therefore, adding solutes always lowers water potential, making \(\Psi\) a negative value (\(\Psi < 0\text{ kPa}\)).
Avoid the "Negative Number Trap"!
Remember how negative numbers work on a number line:
• \(-200\text{ kPa}\) is closer to \(0\text{ kPa}\) than \(-800\text{ kPa}\).
• Therefore, \(-200\text{ kPa}\) is a higher (less negative) water potential than \(-800\text{ kPa}\).
• Water always moves from higher \(\Psi\) \(\to\) lower \(\Psi\) (e.g., from \(-200\text{ kPa} \to -800\text{ kPa}\)).
3. Plant Cell Water Relations
Unlike animal cells, plant cells possess a rigid cellulose cell wall. This allows them to withstand internal pressure when water enters. To calculate water movement in plant cells, we use the Plant Cell Water Relations Equation:
\(\Psi_{\text{cell}} = \Psi_{\text{s}} + \Psi_{\text{p}}\)
Where:
• \(\Psi_{\text{cell}}\) (or \(\Psi\)): Total water potential of the plant cell.
• \(\Psi_{\text{s}}\) (Solute Potential): The osmotic component determined by the concentration of dissolved solutes inside the cytoplasm and vacuole. Solutes lower water potential, so \(\Psi_{\text{s}}\) is always negative or zero.
• \(\Psi_{\text{p}}\) (Pressure Potential): The outward hydrostatic pressure exerted by the cell protoplast against the rigid cell wall. Because the wall pushes back, \(\Psi_{\text{p}}\) is usually positive (ranging from \(0\text{ kPa}\) up to a maximum positive value).
Osmotic States in Plant Cells
1. Turgid (in a hypotonic / dilute / high \(\Psi\) solution):
• Water enters the cell by osmosis.
• The protoplast expands and pushes against the rigid cell wall, causing \(\Psi_{\text{p}}\) to increase.
• Eventually, the outward pressure potential (\(\Psi_{\text{p}}\)) balances the inward pull of the solute potential (\(\Psi_{\text{s}}\)).
• At full turgor, \(\Psi_{\text{p}}\) is equal and opposite to \(\Psi_{\text{s}}\), so: \(\Psi_{\text{cell}} = 0\text{ kPa}\).
• Importance: Turgor pressure provides vital mechanical support to non-woody plant tissues.
2. Incipient Plasmolysis (the transitional state):
• As water leaves the cell, the protoplast shrinks until it just begins to pull away from the cell wall.
• At this precise moment, the cell wall exerts no pressure on the protoplast, so \(\Psi_{\text{p}} = 0\text{ kPa}\).
• Substituting \(\Psi_{\text{p}} = 0\text{ kPa}\) into our equation gives: \(\Psi_{\text{cell}} = \Psi_{\text{s}}\).
• Experimental note: Incipient plasmolysis is defined experimentally as the point where 50% of the cells in a tissue sample show detachment of the plasma membrane from the cell wall.
3. Plasmolysed / Flaccid (in a hypertonic / concentrated / low \(\Psi\) solution):
• Significant water leaves the vacuole by osmosis.
• The protoplast contracts completely and pulls away from the cell wall.
• Because the protoplast is not pushing on the wall, \(\Psi_{\text{p}} = 0\text{ kPa}\), and the cell is plasmolysed.
4. Osmotic Behaviour in Animal Cells
Animal cells do not have a rigid cell wall, meaning they cannot develop pressure potential (\(\Psi_{\text{p}}\)). Their response to external solutions depends entirely on water movement:
• In a hypotonic solution (higher \(\Psi\) than inside cell): Water enters the cell by osmosis. The cell swells and bursts—a process called lysis.
• In a hypertonic solution (lower \(\Psi\) than inside cell): Water leaves the cell by osmosis. The cell loses volume, shrinks, and wrinkles—a process called crenation.
5. Common Pitfalls & CCEA Examiner Advice
• Precision in the definition of Osmosis: Always state: "net movement of water molecules from a higher/less negative water potential to a lower/more negative water potential across a selectively (or partially) permeable membrane." Omitting the membrane or saying "water concentration" loses marks.
• Water Potential Comparisons: Never describe \(-800\text{ kPa}\) as higher than \(-200\text{ kPa}\). \(-200\text{ kPa}\) is higher (closer to zero).
• Incipient Plasmolysis Calculations: Remember that at incipient plasmolysis, \(\Psi_{\text{p}} = 0\text{ kPa}\). Therefore, \(\Psi_{\text{cell}} = \Psi_{\text{s}}\). This allows you to find the solute potential of a plant tissue by identifying the water potential of the surrounding solution that causes 50% plasmolysis.
• Channels vs. Carriers: Channel proteins are only involved in facilitated diffusion (passive transport). Active transport is carried out only by carrier proteins (pumps).
• Bulk Transport Descriptions: In endocytosis and exocytosis, substances do not cross through the phospholipid bilayer; they enter or leave the cell via vesicle formation, invagination, and membrane fusion.