Cell Membrane Structure

Your cell membrane is basically a phospholipid bilayer that's exactly 7.8nm wide, no matter what organism you're looking at. Think of it as a double-layered sandwich where the "bread" loves water but the "filling" hates it.

The phospholipids have hydrophilic $water-loving$ heads pointing outward and hydrophobic $water-hating$ tails pointing inward. This clever arrangement means small, non-polar molecules can slip through easily, but water-soluble substances get blocked.

Membrane proteins are randomly scattered throughout this structure. Extrinsic proteins sit on the surface and help with cell recognition, whilst intrinsic proteins span the entire membrane and act as transport channels. Cholesterol wedges between phospholipids to keep everything stable and rigid.

💡 Remember: The fluid mosaic model describes how proteins of different shapes and sizes are embedded like a mosaic in the fluid phospholipid bilayer.

Membrane Components and Functions

Glycoproteins and glycolipids stick out from the membrane surface like molecular name tags, helping cells recognise each other. Together, they form the glycocalyx - a carbohydrate layer that acts as hormone receptors and cell recognition sites.

The fluid mosaic model explains why membranes work so brilliantly. The phospholipids can move around freely (that's the "fluid" part), whilst proteins of various sizes create a mosaic pattern throughout the membrane.

Channel proteins specifically transport ions across the membrane, acting like selective tunnels. Carrier proteins handle larger, water-soluble substances by changing shape when molecules bind to them - it's like a molecular handshake that opens the door.

💡 Key Point: Different proteins handle different jobs - channels for ions, carriers for larger molecules, and surface proteins for recognition.

Diffusion

Diffusion is simply molecules moving from where there's loads of them to where there's fewer - no energy required, just natural molecular movement. It's like perfume spreading across a room, but it happens because molecules have kinetic energy and bounce around randomly.

The rate of diffusion depends on four key factors. A steeper concentration gradient means faster diffusion - more molecules moving in one direction creates a stronger "flow". Higher temperature speeds things up because molecules have more kinetic energy to move around.

Surface area matters massively - the bigger the area, the more molecules can cross at once. The type of molecule also counts: smaller molecules diffuse faster than larger ones, and non-polar molecules slip through membranes more easily than polar ones.

💡 Formula to remember: Rate of diffusion = (surface area × concentration difference) ÷ diffusion pathway length

Facilitated Diffusion

Some molecules need a helping hand to cross the membrane, and that's where facilitated diffusion comes in. It's still passive (no energy needed), but polar molecules and ions use specific transport proteins as their gateway.

Channel proteins create tunnels for small ions like sodium, whilst carrier proteins handle larger molecules like glucose and amino acids. The carrier protein changes shape when the molecule binds, creating a perfect molecular escort service across the membrane.

Co-transport is particularly clever - it moves two substances simultaneously. For example, glucose and sodium ions bind to the same carrier protein, get transported together, then go their separate ways once inside the cell.

The rate depends entirely on how many appropriate channels are available and whether they're open for business. No channels available? No transport happening.

💡 Remember: Channel proteins = ions, Carrier proteins = larger molecules like glucose and amino acids.

Active Transport

Sometimes cells need to move substances against the concentration gradient - like pushing water uphill. This requires energy in the form of ATP, making it active transport rather than passive diffusion.

The process works like a molecular pump. A molecule binds to a specific carrier protein, ATP provides the energy by transferring a phosphate group, and the protein changes shape to carry the molecule across. Once delivered, everything resets for the next round.

Bulk transport handles really large materials through two methods. Exocytosis packages substances in vesicles that fuse with the cell membrane to release contents outside. Endocytosis does the opposite - the membrane wraps around material to bring it inside.

Phagocytosis engulfs solid materials (think white blood cells eating bacteria), whilst pinocytosis takes in liquid materials. Both are types of endocytosis but handle different states of matter.

💡 Key difference: Active transport requires ATP and can work against concentration gradients, unlike passive transport.

Osmosis Basics

Osmosis is just diffusion of water molecules across a selectively permeable membrane, but we measure it using water potential (Ψ). Think of water potential as water's "eagerness" to move - it always flows from higher potential to lower potential.

Pure water has the highest water potential of 0 kPa. Add any solute (like salt or sugar) and the water potential becomes negative - the more solute, the more negative it gets. This happens because dissolved particles take up space and reduce the number of free water molecules.

Water moves from hypotonic solutions (higher water potential, fewer solutes) to hypertonic solutions (lower water potential, more solutes). When solutions have equal water potential, they're isotonic and no net movement occurs.

You can predict water movement by comparing water potentials between cells. Water always flows toward the more negative value, seeking equilibrium.

💡 Memory trick: HyPOtonic = POsitive outcome for the cell (water moves in), HyPERtonic = cell PERishes (water moves out).

Water Potential in Plant Cells

Plant cells are more complex because they have cell walls, so their water potential depends on both solute potential (Ψs) and pressure potential (Ψp). The formula is: Ψ = Ψs + Ψp.

Solute potential is always negative (solutes reduce water potential), whilst pressure potential can be positive (cell wall pushing back) or zero. In animal cells, pressure potential is always zero because they lack rigid cell walls.

Plasmolysis occurs when plant cells lose so much water that the cell membrane pulls away from the cell wall - the pressure potential becomes zero. Turgid cells are fully inflated with maximum water, whilst flaccid cells have lost water and gone limp.

In isotonic conditions, animal cells maintain their normal shape through homeostasis. Plant cells become flaccid because there's no pressure against the cell wall, so Ψp equals zero.

💡 Plant cell states: Turgid (full of water) → Flaccid (losing water) → Plasmolysed (severely dehydrated).

Osmotic Effects on Cells

Hypotonic conditions cause cells to swell as water rushes in. Animal cells may undergo lysis (bursting) because they can't handle the pressure, but plant cells become turgid and firm as the cell wall provides support.

Hypertonic conditions make cells shrink as water leaves. Animal cells undergo crenation (shrivelling), whilst plant cells become plasmolysed - the cytoplasm shrinks away from the cell wall, causing the plant to wilt.

Isotonic conditions create equilibrium with no net water movement. Animal cells maintain their normal shape naturally, but plant cells become flaccid because there's no pressure potential - they're not firm or crisp.

In plasmolysed cells, the pressure potential (Ψp) equals zero because the cell contents aren't pushing against the cell wall. This means the water potential equals the solute potential: Ψ = Ψs.

💡 Quick check: If a plant looks wilted, its cells are likely plasmolysed from being in hypertonic conditions - they need water!