Welcome to Cell Physiology!
Hello and welcome to one of the most exciting and fundamental chapters in your CCEA AS 1 Biology journey: Cell Physiology. If you have ever wondered how your cells take in vital nutrients, get rid of toxic waste, or prevent themselves from bursting like overfilled water balloons, you are in the right place!
Don't worry if cell transport seems a bit heavy on technical terms at first. We will break every concept down step-by-step using everyday analogies, simple calculations, and clear revision checkpoints. Let's dive in!
1. The Gateway: Cell Membrane Structure & Permeability
Before looking at how things move in and out of cells, let's remind ourselves of the boundary: the plasma membrane.
The membrane is described by the Fluid Mosaic Model:
• Fluid: The individual phospholipid molecules and proteins can move laterally (side-to-side) within their layer, giving the membrane flexibility.
• Mosaic: Proteins of different shapes and sizes are scattered throughout the phospholipid bilayer, looking like tiles in a mosaic pattern.
Membrane Components and Their Roles in Physiology:
• Phospholipid Bilayer: Has hydrophilic (water-loving) phosphate heads pointing outwards and hydrophobic (water-hating) fatty acid tails pointing inwards. It acts as a barrier to water-soluble, polar, and charged particles, while allowing small, non-polar molecules (like \(O_2\) and \(CO_2\)) to pass through freely.
• Intrinsic (Integral) Proteins: Span the entire bilayer. These include channel proteins (water-filled pores for specific ions) and carrier proteins (which change shape to transport specific molecules).
• Extrinsic (Peripheral) Proteins: Found on one surface only; often act as receptors or enzymes.
• Cholesterol: Regulates membrane fluidity and provides mechanical stability.
• Glycoproteins & Glycolipids: Carbohydrate chains attached to proteins or lipids that act in cell recognition, stability, and as receptor sites.
Did you know? The plasma membrane is selectively permeable (or partially permeable). It does not simply let everything through; it controls cell entry strictly based on size, charge, and lipid solubility.
Key Takeaway: Small, non-polar, lipid-soluble molecules glide directly through the phospholipid bilayer, while polar or charged molecules require specialised protein helpers.
2. Passive Transport: Simple and Facilitated Diffusion
Passive transport processes do not require metabolic energy in the form of \(ATP\) (adenosine triphosphate). Substances move entirely down their natural concentration gradient using their own kinetic energy.
A. Simple Diffusion
Definition: The net movement of particles from a region of their higher concentration to a region of their lower concentration down a concentration gradient until dynamic equilibrium is reached.
• What moves this way? Small, non-polar molecules (e.g., Oxygen \(O_2\), Carbon Dioxide \(CO_2\)), and small lipid-soluble molecules (e.g., steroid hormones, glycerol).
Factors Affecting the Rate of Diffusion (Fick's Law)
In CCEA Biology, we quantify the rate of diffusion using Fick's Law:
\(\text{Rate of Diffusion} \propto \frac{\text{Surface Area} \times \text{Difference in Concentration}}{\text{Thickness of Exchange Surface}}\)
To maximise the rate of diffusion, biological systems have evolved:
1. Large surface area (e.g., microvilli on epithelial cells).
2. Steep concentration gradient (maintained by blood flow or rapid metabolism of the substance).
3. Thin exchange pathway (short diffusion distance, e.g., single-cell-thick capillaries).
B. Facilitated Diffusion
Some molecules, such as glucose, amino acids, and inorganic ions (\(Na^+\), \(K^+\), \(Cl^-\)), are polar or charged. They are repelled by the hydrophobic fatty acid tails of the bilayer and cannot cross by simple diffusion. Instead, they use transport proteins.
Two Types of Transport Proteins:
1. Channel Proteins: Water-filled hydrophilic pores that allow specific charged ions to pass straight through. Some are gated, meaning they open or close in response to specific stimuli (such as voltage changes or hormones).
2. Carrier Proteins: Possess a specific binding site for a particular solute (e.g., glucose). When the solute binds, the protein changes its tertiary shape, releasing the molecule on the opposite side of the membrane.
Analogy: Think of simple diffusion like walking through an open field. Facilitated diffusion with a channel protein is like walking through an open turnstile, while a carrier protein is like a revolving door that fits only one person at a time.
Important Graph Concept: Unlike simple diffusion (where the rate increases linearly without limit as concentration increases), the rate of facilitated diffusion eventually plateaus at high concentrations. Why? Because all the carrier/channel proteins become fully occupied (saturated). The number of transport proteins becomes the limiting factor.
Key Takeaway: Diffusion is passive (no \(ATP\)). Simple diffusion goes straight through the bilayer; facilitated diffusion requires channel or carrier proteins down the concentration gradient.
3. Osmosis and Water Potential (\(\Psi\))
Osmosis is a special case of diffusion that deals strictly with water molecules.
CCEA Definition: Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential across a selectively permeable membrane.
Understanding Water Potential (\(\Psi\))
• Water Potential (\(\Psi\)) is a measure of the free kinetic energy of water molecules in a system, measured in units of pressure: kilopascals (\(\text{kPa}\)).
• Pure water at standard atmospheric pressure and temperature has the maximum possible water potential: \(\Psi = 0\text{ kPa}\).
• Adding solute binds water molecules, reducing their free kinetic energy. Therefore, all solutions have a negative water potential (e.g., \(-200\text{ kPa}\), \(-500\text{ kPa}\)).
• Water always moves spontaneously from a less negative (higher) \(\Psi\) to a more negative (lower) \(\Psi\).
The Water Potential Equation for Plant Cells
For plant cells, the total water potential is governed by two forces:
\(\Psi_{cell} = \Psi_s + \Psi_p\)
Where:
• \(\Psi_{cell}\) = Overall Water Potential of the plant cell.
• \(\Psi_s\) = Solute Potential (or Osmotic Potential). This represents the effect of dissolved solutes in the vacuole and cytoplasm. Solutes always lower water potential, so \(\Psi_s\) is always negative.
• \(\Psi_p\) = Pressure Potential (or Turgor Pressure). As water enters the cell, the expanding protoplast pushes against the rigid cellulose cell wall. The wall pushes back with an equal and opposite force. This is an inward mechanical pressure, so \(\Psi_p\) is almost always positive (it is \(0\text{ kPa}\) when the membrane ceases pushing against the wall).
Effects of Osmosis on Animal Cells vs. Plant Cells
1. In Pure Water / Dilute Solution (Hypotonic Environment - High \(\Psi\)):
• Animal Cells (e.g., Red Blood Cells): Water enters by osmosis down the water potential gradient. Animal cells lack a cell wall; the delicate cell membrane stretches and eventually bursts. This bursting is called lysis (or haemolysis in red blood cells).
• Plant Cells: Water enters by osmosis into the cytoplasm and vacuole. The protoplast swells and presses firmly against the cell wall. The cell becomes fully swollen and firm—a state known as turgid. Turgor pressure (\(\Psi_p\)) rises until \(\Psi_{cell} = 0\text{ kPa}\), stopping any further net entry of water. Turgidity is crucial because it provides mechanical support to non-woody plant stems and leaves!
2. In Equal Concentration Solution (Isotonic Environment - Same \(\Psi\)):
• Dynamic equilibrium is reached. Water enters and leaves at equal rates (\(\text{net movement} = 0\)). Cells retain their normal shape and volume.
3. In Concentrated Solute Solution (Hypertonic Environment - Low \(\Psi\)):
• Animal Cells: Water leaves the cell rapidly by osmosis. The cell shrinks, shrivels, and wrinkles. This process is called crenation.
• Plant Cells: Water leaves the vacuole and cytoplasm by osmosis. The volume of the protoplast decreases. The cell membrane pulls away from the cell wall. This state is called plasmolysis, and the cell is said to be plasmolysed.
Special Concept: Incipient Plasmolysis
Incipient plasmolysis is the precise point at which the protoplast has just lost contact with the cell wall. At this exact threshold:
• The protoplast exerts zero pressure on the wall: \(\Psi_p = 0\text{ kPa}\).
• Using our equation \(\Psi = \Psi_s + \Psi_p\), substituting \(\Psi_p = 0\) gives: \(\Psi_{cell} = \Psi_s\).
• In practical experiments, incipient plasmolysis is defined as the condition when exactly \(50\%\) of the cells in a tissue sample show signs of plasmolysis.
Common Mistake to Avoid: Never say "the cell shrinks" for a plasmolysed plant cell! The outer cellulose cell wall remains largely rigid; it is the protoplast (vacuole + cytoplasm + plasma membrane) that shrinks away from the wall. The space between the wall and the shrunken membrane becomes filled with the external hypertonic solution!
Key Takeaway: \(\Psi_{cell} = \Psi_s + \Psi_p\). Water always flows from higher (less negative) \(\Psi\) to lower (more negative) \(\Psi\). Plant cell walls prevent bursting and generate turgor pressure.
4. Active Transport
Sometimes cells must absorb nutrients or pump out wastes against nature's flow—from an area of low concentration to an area of high concentration.
CCEA Definition: Active transport is the movement of ions or molecules across a cellular membrane against a concentration gradient (from lower to higher concentration), requiring metabolic energy in the form of \(ATP\) and specific carrier proteins (often called protein pumps).
How Active Transport Works (Step-by-Step):
1. The target molecule or ion binds to a specific recognition site on the carrier protein on the low-concentration side of the membrane.
2. \(ATP\) binds to the carrier protein and is hydrolysed into \(ADP\) and an inorganic phosphate group (\(P_i\)):
\(ATP \xrightarrow{\text{hydrolysis}} ADP + P_i + \text{Energy}\)
3. The released energy causes the carrier protein to undergo a conformational change (a change in tertiary shape).
4. The molecule is transported across the membrane and released on the high-concentration side.
5. The phosphate group is released from the protein, allowing it to return to its original shape, ready to repeat the cycle.
Factors Affecting Active Transport:
Because active transport depends on \(ATP\) generated by aerobic cellular respiration in mitochondria, any factor that slows respiration will directly reduce or halt active transport:
• Oxygen Concentration: Less \(O_2 \implies\) less aerobic respiration \(\implies\) less \(ATP \implies\) slower active transport.
• Temperature: Low temperature reduces enzyme kinetic energy; high temperatures denature respiratory enzymes.
• Respiratory Poisons (e.g., Cyanide): Inhibit key respiratory enzymes (like cytochrome c oxidase), preventing \(ATP\) synthesis and stopping active transport entirely.
Real-World Examples:
• Absorption of mineral ions (e.g., nitrates, magnesium) by plant root hair cells from dilute soil solutions.
• Reabsorption of all glucose from kidney filtrate back into the blood in the proximal convoluted tubule.
• Maintenance of resting potential in nerve axons via the sodium-potassium pump (\(Na^+/K^+\) ATPase).
Key Takeaway: Active transport moves substances uphill (against concentration gradients), requiring carrier proteins and metabolic energy from \(ATP\) hydrolysis.
5. Bulk Transport: Endocytosis and Exocytosis
What happens when particles are far too large for carrier proteins (like whole bacteria, large polypeptide hormones, or massive enzyme secretions)? The cell uses bulk transport. This is an active process that requires \(ATP\) to reshape the cytoskeleton and move vesicles.
A. Endocytosis (Transport INTO the Cell)
The plasma membrane invaginates (folds inward) around the external material, pinching off to form an intracellular membrane-bound vesicle or vacuole.
There are two primary forms of endocytosis:
1. Phagocytosis ("Cell Eating"): Uptake of solid, particulate matter. For example, a white blood cell (neutrophil or macrophage) engulfing a foreign bacterium to form a phagocytic vacuole, which then fuses with a lysosome for destruction.
2. Pinocytosis ("Cell Drinking"): Uptake of extracellular fluid and small dissolved solutes via very small vesicles.
B. Exocytosis (Transport OUT of the Cell)
Substances produced inside the cell (e.g., digestive enzymes, hormones, antibodies, structural proteins) are packaged by the Golgi apparatus into secretory vesicles. These vesicles travel along microtubules to the plasma membrane, fuse with it, and discharge their contents to the cell exterior.
• Membrane Recycling: Notice how exocytosis adds vesicle membrane back to the plasma membrane, perfectly balancing the membrane lost during endocytosis!
Key Takeaway: Bulk transport moves large quantities of materials or oversized molecules via vesicle formation and fusion. Both endocytosis and exocytosis require \(ATP\).
6. Summary Comparison of Transport Mechanisms
Let's lock in everything you have learned with this master summary checklist:
• Simple Diffusion: Passive (No \(ATP\)) | Down gradient (\(\text{High} \to \text{Low}\)) | Direct through phospholipid bilayer | Non-polar, small molecules (e.g., \(O_2, CO_2\)).
• Facilitated Diffusion: Passive (No \(ATP\)) | Down gradient (\(\text{High} \to \text{Low}\)) | Uses Channel or Carrier proteins | Polar/charged molecules (e.g., Glucose, \(Na^+\)).
• Osmosis: Passive (No \(ATP\)) | From High \(\Psi\) to Low \(\Psi\) | Across selectively permeable membrane | Water molecules only.
• Active Transport: Active (Requires \(ATP\)) | Against gradient (\(\text{Low} \to \text{High}\)) | Uses specific Carrier proteins (pumps) | Ions, minerals, glucose (e.g., \(NO_3^-\) in roots).
• Bulk Transport (Endo/Exocytosis): Active (Requires \(ATP\)) | Independent of concentration gradient | Uses Vesicles & Membrane fusion | Large macromolecules, bacteria, bulk fluids.
7. Quick Review & Self-Check Questions
Test your understanding with these classic CCEA exam-style review questions:
1. Why does the rate of facilitated diffusion reach a maximum plateau as concentration increases, whereas simple diffusion does not?
Answer: In facilitated diffusion, carrier or channel proteins are required. At high solute concentrations, all protein binding sites become saturated (occupied), making the number of transport proteins the limiting factor.
2. A plant cell has a solute potential (\(\Psi_s\)) of \(-750\text{ kPa}\) and a pressure potential (\(\Psi_p\)) of \(+250\text{ kPa}\). Calculate its overall water potential (\(\Psi_{cell}\)).
Answer: \(\Psi_{cell} = \Psi_s + \Psi_p = -750\text{ kPa} + 250\text{ kPa} = -500\text{ kPa}\).
3. If this cell is placed in a solution with \(\Psi = -200\text{ kPa}\), which direction will water move?
Answer: Water moves from higher water potential (\(-200\text{ kPa}\)) to lower water potential (\(-500\text{ kPa}\)), so water will enter the cell by osmosis.
4. Why do cyanide-treated cells fail to carry out active transport while still performing simple diffusion?
Answer: Cyanide inhibits aerobic respiration in mitochondria, halting \(ATP\) production. Active transport strictly requires \(ATP\), whereas simple diffusion relies solely on the intrinsic kinetic energy of molecules and does not need metabolic energy.
Congratulations on completing these study notes on Cell Physiology! Review these terms and equations regularly, and you'll be well-prepared for your AS 1 examination.