Cell physiology

Habari Mwananchi wa Tiba! Welcome to Medical Physiology!

Karibu sana to your first year! Forget everything you thought you knew about the cell being just a simple circle with a dot in the middle from your KCSE Biology. We are about to dive deep into the city that is the cell. Think of the human body as the entire nation of Kenya. Each organ is a county, like Nairobi or Mombasa. But what makes up these counties? The people, the homesteads, the bomas! That's our cell. It's a bustling, living unit with its own government, its own border control, and its own economy. As future doctors, understanding the physiology of this fundamental unit is the key to understanding health and disease. Twende kazi!

The Cell Membrane: The Boma's Gatekeeper and National Border

Every homestead needs a fence, and every country needs a border. The cell membrane, also known as the plasma membrane, is this border. But it's not just a static wall; it's a dynamic, intelligent gatekeeper. We call its structure the Fluid Mosaic Model.

• Fluid: The components, like the lipids and proteins, can move around. Think of it like the bustling Gikomba market – everything is in motion, not fixed in one spot.
• Mosaic: It's made of many different parts – phospholipids, cholesterol, proteins, and carbohydrates – fitted together like the beautiful mosaic tiles at the National Archives.

Key Components of the Border Post:

• Phospholipid Bilayer: This is the main fence. It has hydrophilic (water-loving) heads facing outwards and inwards (towards the water) and hydrophobic (water-fearing) tails hiding in the middle. Imagine people in Kisumu who love being near the lake (hydrophilic) versus someone in Lodwar trying to find shade from the sun (hydrophobic).
• Membrane Proteins: These are the askaris (guards) and customs officials at the border. They have specific jobs:
  • Channels: Tunnels that allow specific ions like Na+ or K+ to pass through.
  • Carriers/Transporters: They bind to a substance (like glucose) and change shape to carry it across.
  • Pumps: Use energy to actively move substances, often against their will!
  • Receptors: Act like communication aerials, receiving signals from hormones or neurotransmitters.
• Glycocalyx (Carbohydrates): These are like the ID cards or the national flag on the outside of the cell. They help cells recognise each other ("Hey, you're a liver cell, you belong here!") and are crucial for our immune system.

    ---EXTRACELLULAR FLUID (Outside the 'Boma')---
       /   \
  {O}   {O}   {O}  {O}  {O} <-- Glycocalyx (Cell ID)
   |     |     |    |    |
 [==P==] [==P==] [=CH=] [==P==] <-- Proteins (Askaris) & Channels
  / \   / \   / \  / \   / \
 ooooo ooooo ooooo ooooo ooooo  <-- Hydrophilic Heads
  |||   |||   |||   |||   |||
  |||   |||   |||   |||   |||   <-- Hydrophobic Tails (The Fatty Core)
  |||   |||   |||   |||   |||
 ooooo ooooo ooooo ooooo ooooo  <-- Hydrophilic Heads
  / \   / \   / \  / \   / \
 [==P==] [==P==] [=====] [==P==] <-- More Proteins
 
    ---CYTOPLASM (Inside the 'Boma')---

Image Suggestion: A vibrant, detailed 3D digital illustration of the fluid mosaic model. The phospholipid heads are bright blue, and the tails are yellow. Embedded proteins are depicted in various functional shapes, some as channels (tunnels) and some as carriers. Chains of carbohydrates (glycocalyx) are attached to the outer proteins, glowing slightly to signify their role in cell identification. The entire structure looks fluid and dynamic, not static.

Transport Across the Membrane: Vitu Zinaingiaje na Zinatokaje?

Just like at the Namanga border, not everything can cross freely. There are rules! We can group transport into two main categories.

1. Passive Transport (No Energy Required)

This is like rolling down a hill on a bicycle from Upper Hill towards Uhuru Park – it requires no effort. Substances move down their concentration gradient (from an area of high concentration to low concentration).

• Simple Diffusion: Small, uncharged molecules like Oxygen (O2) and Carbon Dioxide (CO2) just slip through the phospholipid bilayer.

  Real-Life Example: The delicious smell of nyama choma being roasted at a roadside joint. The aroma particles are in high concentration at the grill and diffuse through the air to your nose, where they are in low concentration. Mmmh!

• Facilitated Diffusion: For slightly larger molecules like glucose or ions that can't easily cross the fatty middle. They need help from a channel or carrier protein. It's still passive because it's moving down the gradient.

  Analogy: Trying to cross the busy Thika Superhighway. You can't just run across (simple diffusion is impossible!). You need to use a footbridge (the carrier protein) to get to the other side safely.

• Osmosis: The special diffusion of water across a selectively permeable membrane. Water moves from an area of high water concentration (low solute) to an area of low water concentration (high solute).

  Kenyan Kitchen Science: Ever wondered why your mother soaks wilted sukuma wiki in water before cooking? The vegetable cells have lost water. By placing them in pure water, water moves via osmosis back into the cells, making them firm and crisp again! This is due to turgor pressure.

2. Active Transport (Energy is a MUST!)

This is like pushing a stalled matatu up the hill towards the KNH bus stop – it requires a lot of energy! This process moves substances against their concentration gradient (from low to high concentration). The energy currency for this is ATP (Adenosine Triphosphate).

• The Sodium-Potassium (Na+/K+) Pump: This is the superstar of active transport. It's found in almost all our cells and is critical for nerve function and muscle contraction. It tirelessly pumps 3 Sodium ions (Na+) OUT of the cell and 2 Potassium ions (K+) IN. This uses up a lot of our body's energy!

        ************ PASSIVE TRANSPORT ************          ************ ACTIVE TRANSPORT ************
        (High Concentration) --> (Low Concentration)         (Low Concentration) --> (High Concentration)
        
        [High O2]   ||   [Low O2]                          [Low Na+]   ||   [High Na+]
          O2 -->    ||     O2                              Na+ -->    ||     <--PUMP-- Na+
                    ||                                                ||          (ATP Used!)
         No Energy Needed!                                          Energy Needed!

Image Suggestion: A split-panel cartoon. On the left, titled 'Passive Transport', a person on a bicycle is effortlessly coasting downhill. On the right, titled 'Active Transport', a group of people are sweating and straining to push a colorful Kenyan matatu uphill. The downhill slope is labeled 'High to Low Concentration', and the uphill slope is labeled 'Low to High Concentration'. The matatu has 'ATP' written on its license plate.

The Resting Membrane Potential: A Charged Battery

Because of the Na+/K+ pump and leaky channels, the inside of our cells is slightly more negative than the outside. This difference in charge is called the Resting Membrane Potential (RMP). Think of it like a fully charged phone battery, holding potential energy, ready to make a call (which would be an 'action potential' like a nerve impulse).

Three main reasons for this negative charge inside:

1. The Na+/K+ Pump is electrogenic: it pumps out 3 positive charges (Na+) for every 2 positive charges (K+) it brings in, resulting in a net loss of positive charge from the inside.
2. The membrane is more "leaky" to Potassium (K+) than Sodium (Na+). So, more positive K+ ions leak out of the cell than Na+ ions leak in, leaving the inside more negative.
3. There are large, negatively charged proteins and anions trapped inside the cell that cannot leave.

Calculating the Equilibrium Potential: The Nernst Equation

How much electrical force is needed to perfectly balance the chemical force pushing an ion across the membrane? We use the Nernst Equation to find this for a single ion. It helps us predict the membrane potential if the membrane were only permeable to that one ion.

    E_ion = (RT / zF) * ln([ion]_out / [ion]_in)

    Where:
    E_ion = Equilibrium potential for the ion (in Volts)
    R = Gas constant
    T = Absolute temperature (in Kelvin)
    z = Valence (charge) of the ion (+1 for K+, +2 for Ca2+)
    F = Faraday's constant
    ln = Natural logarithm
    [ion]_out = Concentration of the ion outside the cell
    [ion]_in = Concentration of the ion inside the cell

    Let's simplify for a typical mammalian cell at 37°C:
    E_ion (in mV) = (61.5 / z) * log10([ion]_out / [ion]_in)

    Step-by-step example for Potassium (K+):
    - Typical [K+]_out = 4 mM
    - Typical [K+]_in = 140 mM
    - z = +1

    1. E_K = (61.5 / 1) * log10(4 / 140)
    2. E_K = 61.5 * log10(0.0286)
    3. E_K = 61.5 * (-1.544)
    4. E_K = -94.9 mV

    This means that for K+ to be in equilibrium, the inside of the cell would need to be about -95mV. Since the actual RMP is around -70mV, it tells us that K+ is not the *only* ion at play!

Clinical Correlation: From the Lecture Hall to the Ward

A Case of Cholera and Oral Rehydration Solution (ORS)

Imagine you are at a rural clinic in an area with a cholera outbreak. A mother brings in her child who is very weak, with severe watery diarrhea ("rice-water stool"). The cholera toxin has caused the chloride channels in the intestinal cells to stay open, leading to a massive outflow of Cl- ions, followed by Na+ and a huge amount of water into the gut. The child is severely dehydrated.

You don't have IV fluids, but you have packets of Oral Rehydration Solution (ORS) – a simple mixture of water, salt (NaCl), and glucose. Why is this a lifesaver? It's pure cell physiology in action!

In the small intestine, there is a transporter protein called the Sodium-Glucose Co-transporter 1 (SGLT1). This transporter is a type of secondary active transport; it grabs one molecule of glucose and one sodium ion from the gut and pulls them *both* into the intestinal cell. This transporter is NOT affected by the cholera toxin! By giving the child sugar and salt, you are using this SGLT1 pathway to actively pull sodium back into the body's cells. And as we learned from osmosis, where salt goes, water follows! You are literally tricking the body into rehydrating itself. This simple, life-saving solution is based entirely on the principles of membrane transport we just discussed.

See? Understanding the cell is not just for exams. It's for saving lives. Master these fundamentals, and you will build a powerful foundation for your entire medical career. Keep up the great work!