OCR A-Level Biology: Understanding the Cell Membrane

Cell Membrane Functions and Structure

Your cells are basically tiny factories that need proper security systems to function properly. The plasma membrane acts as a selective barrier, keeping your cell's contents safe whilst allowing essential molecules to pass through when needed.

Inside your cells, internal membranes create separate compartments around organelles like mitochondria and the nucleus. Think of these like different departments in a factory - each area can focus on specific jobs without interference from other processes.

The fluid mosaic model explains how membranes are built. Picture a double layer of phospholipids (like a sandwich) with various proteins floating around like icebergs. Cholesterol molecules squeeze between the phospholipids to keep everything stable and control how fluid the membrane stays.

Key Point: The membrane isn't rigid like a wall - it's more like a flexible boundary that can adapt whilst maintaining control over what passes through.

Membrane Components and Cell Recognition

Glycoproteins and glycolipids are basically the cell's ID cards - they stick out from the membrane surface with unique carbohydrate chains that help other cells recognise whether you're friend or foe. This is why your immune system can tell the difference between your own cells and invading bacteria.

These recognition molecules also work as binding sites for hormones and drugs. When you take medicine like salbutamol for asthma, it specifically targets receptors on your airway muscles, causing them to relax and helping you breathe easier.

Membrane proteins are the real workhorses, forming channels and pumps that move substances across the membrane. Some create pores for molecules that can't dissolve in the fatty phospholipid layer, whilst others actively transport materials against concentration gradients.

Temperature and solvents like alcohol can seriously mess with membrane structure. As temperature rises, membranes become leakier, and eventually proteins can denature, causing the whole system to break down.

Key Point: Cell signalling depends entirely on the right shapes fitting together - like biological lock and key systems that coordinate your entire body's activities.

Transport Across Membranes

Moving stuff across membranes happens in two main ways, and understanding this difference will save you marks in exams. Passive transport doesn't need any cellular energy - molecules simply follow their natural tendency to spread out from crowded areas to less crowded ones.

Diffusion works for small or fat-soluble molecules that can slip through the phospholipid bilayer directly. Facilitated diffusion uses protein channels or carriers to help larger molecules cross, whilst osmosis specifically refers to water movement down its concentration gradient.

Several factors speed up diffusion rates: higher temperatures give molecules more energy to move faster, steeper concentration gradients create stronger driving forces, and larger surface areas provide more space for crossing.

Active transport is completely different - it burns ATP energy to pump molecules uphill against their concentration gradients. This is like pushing water uphill instead of letting it flow downhill naturally, requiring special pump proteins that change shape to move molecules across.

Key Point: Remember that passive transport is like rolling downhill (no energy needed), whilst active transport is like cycling uphill (requires energy input).