Cell Membrane Structure and the Fluid-Mosaic Model

Think of a cell membrane as a flexible barrier that's only 7-8 nanometres thick - so tiny you need an electron microscope to see it! The membrane uses a clever phospholipid bilayer structure where each phospholipid has a water-loving head and water-hating tails.

The fluid-mosaic model explains why this structure works so well. It's "fluid" because the phospholipids can move around like people in a crowded room, and "mosaic" because proteins of different shapes and sizes are scattered throughout like tiles in a mosaic pattern.

Extrinsic proteins sit on the surface providing structural support, whilst intrinsic proteins span the entire membrane creating channels and transport routes. The membrane also contains cholesterol (making it more rigid) and glycoproteins that act like ID cards for cell recognition.

Key Insight: Lipid-soluble substances (like oxygen) can slip right through the phospholipid bilayer, but water-soluble substances (like glucose) need special protein channels to get across.

Simple Diffusion and Factors Affecting Membrane Permeability

Simple diffusion is nature's way of evening things out - particles naturally move from areas of high concentration to low concentration until they're evenly spread. It's completely passive transport, meaning no energy required from the cell.

Several factors speed up or slow down diffusion rates. A steeper concentration gradient means faster diffusion, whilst thicker membranes slow things down. Larger surface areas give more space for particles to cross, and smaller molecules zip across faster than bulky ones.

Temperature, pH, and ethanol can seriously mess with membrane permeability. Heat and extreme pH levels denature membrane proteins, whilst ethanol literally dissolves the lipid components, creating holes in the membrane.

Fick's Law gives you the maths: Rate of diffusion = (Surface area × Concentration difference) ÷ Length of diffusion path. This formula often appears in exam calculations, so get familiar with it!

Exam Tip: Remember that lipid-soluble molecules diffuse much faster than water-soluble ones because the phospholipid bilayer is naturally hydrophobic.

Facilitated Diffusion and Carrier Proteins

When water-soluble substances can't squeeze through the phospholipid bilayer, they need help from facilitated diffusion. This process uses specific transport proteins but still doesn't require any energy from the cell.

Channel proteins work like selective doorways with hydrophilic pores that open and close as needed. Ions love these channels because they're water-friendly inside. Carrier proteins are more like personal escorts - they bind to larger molecules like sugars, change shape, and release them on the other side.

The key difference from simple diffusion is that facilitated diffusion can become saturated. Once all the carrier proteins are busy, adding more substrate won't increase the transport rate - you'll see this as a plateau on graphs.

Graph Analysis: In exam questions, look for uptake curves that level off - this indicates facilitated diffusion rather than simple diffusion, which would show a straight line.

Active Transport and Co-transport

Sometimes cells need to move substances against the concentration gradient - like pushing water uphill. Active transport does exactly this using energy from ATP breakdown, making it the cell's energy-hungry transport option.

The process involves specific carrier proteins that change shape when ATP adds a phosphate group. The molecule gets carried across, the phosphate is released, and the carrier returns to its original shape. It's like a molecular conveyor belt powered by cellular respiration.

Co-transport is a clever variation where two substances hitch a ride together on the same protein. The classic example is sodium-glucose co-transport in your intestines and kidneys, where glucose gets a free ride with sodium ions.

Anything that affects respiration (like the poison cyanide) will reduce active transport rates because less ATP is available. This is why exam questions often mention respiratory inhibitors when testing your understanding.

Memory Aid: Active transport always works against the concentration gradient and needs ATP - think "active" means it takes effort and energy, just like active exercise!

Water Potential and Osmosis Fundamentals

Water potential (ψ) measures how badly water molecules want to move somewhere else - it's like measuring the "pressure" of water to escape. Pure water has a water potential of zero, and adding any solute makes it negative.

Water always moves from higher water potential (less negative) to lower water potential (more negative). Think of it like water rolling downhill, but instead of height, it's following the water potential gradient.

Solute potential (ψs) measures how tightly water molecules are held by dissolved substances. More solutes mean more negative solute potential, which pulls the overall water potential down.

Osmosis is simply water diffusion across a selectively permeable membrane, following water potential gradients. It's passive transport that doesn't need energy, just concentration differences.

Formula Focus: Water potential = Pressure potential + Solute potential $ψ = ψp + ψs$. This equation is essential for calculations and understanding cell behaviour.

Plant Cells and Water Relations

Plant cells are brilliant at handling water changes because of their cell walls. When water enters by osmosis, it creates pressure potential (ψp) as the cell contents push against the rigid wall.

A turgid cell is fully inflated with water - the cell wall prevents further expansion, creating internal pressure that stops more water entering. This turgidity gives plants their structural support and keeps leaves upright.

Hypotonic solutions (higher water potential than the cell) cause water to flow in, whilst hypertonic solutions (lower water potential) cause water to flow out. Isotonic solutions create no net water movement.

When plant cells lose too much water, plasmolysis occurs - the cell membrane pulls away from the cell wall as the vacuole shrinks. Incipient plasmolysis is the point where this just begins to happen, and pressure potential becomes zero.

Practical Application: Understanding plant water relations explains why plants wilt in salty soil and why adding fertiliser can sometimes damage plants through osmotic stress.

Animal Cells and Bulk Transport

Animal cells have no protective cell wall, so they're more vulnerable to osmotic changes. In distilled water, red blood cells swell and burst (haemolysis), whilst in concentrated solutions they shrink and become crenated.

This is why medical saline solutions must be isotonic with blood cells - getting the concentration wrong could be fatal. Animal cells rely on the body's regulatory systems to maintain proper osmotic balance.

Endocytosis and exocytosis are active transport processes for moving large particles or volumes of fluid. Phagocytosis engulfs solid particles (like white blood cells eating bacteria), whilst pinocytosis takes in liquid droplets.

Exocytosis works in reverse - vesicles fuse with the cell membrane to release substances like digestive enzymes or hormones. Both processes require ATP because the membrane must actively change shape.

Real-world Connection: These bulk transport processes are essential for immune responses, hormone secretion, and neurotransmitter release at synapses.