Describe the structure of a phospholipid.
A phospholipid consists of a hydrophilic phosphate head and two hydrophobic fatty acid tails.
Describe the fluid mosaic model of the membrane.
The membrane is a phospholipid bilayer with proteins embedded within it; the phospholipids can move laterally, giving the membrane fluidity, and proteins form a mosaic pattern.
Why do phospholipids form a bilayer?
Hydrophilic heads face the aqueous environments inside and outside the cell, while hydrophobic tails face inward, avoiding water.
Role of cholesterol in membranes?
holesterol restricts phospholipid movement, increasing membrane stability and reducing permeability.
State two functions of membrane proteins.
Transport of substances and cell recognition (also acceptable: receptors, enzymes, structural support).
Which substances can diffuse directly through the phospholipid bilayer?
A: Small, non-polar molecules such as oxygen and carbon dioxide.
Why can ions not diffuse directly through the bilayer?
They are charged and therefore hydrophilic, so cannot pass through the hydrophobic core.
Define diffusion.
The net movement of particles from a region of higher concentration to lower concentration down a concentration gradient
Two factors affecting rate of diffusion across membranes?
: Concentration gradient and surface area (also acceptable: thickness of membrane, temperature).
Define facilitated diffusion.
The movement of molecules down a concentration gradient via carrier or channel proteins, without the use of ATP.
Why is facilitated diffusion selective?
Carrier proteins have specific binding sites complementary to particular molecules.
Difference between diffusion and facilitated diffusion?
Facilitated diffusion requires membrane proteins, whereas diffusion does not.
Define osmosis.
The diffusion of water across a partially permeable membrane from a region of higher water potential to lower water potential
What is water potential?
measure of the tendency of water molecules to move; pure water has the highest water potential.
Effect of placing an animal cell in a hypotonic solution?
Water enters by osmosis, causing the cell to swell and burst (lysis).
Effect of placing a plant cell in a hypotonic solution?
Cell becomes turgid as water enters and presses the cytoplasm against the cell wall.
Define active transport
The movement of substances against their concentration gradient using ATP and carrier protein
Why is ATP required for active transport?
To change the shape of the carrier protein and move substances against the gradient.
One example of active transport in organisms?
Uptake of mineral ions by root hair cells.
Define endocytosis.
The bulk transport of materials into a cell via the formation of vesicles from the cell membrane.
Q: Define exocytosis
The bulk transport of materials out of a cell via vesicles that fuse with the cell membrane.
Role of glycoproteins in membranes?
Act as receptors for cell signalling and cell recognition.
Why is cell recognition important?
Enables cells to identify each other, important in immune responses and tissue formation.
Compare facilitated diffusion and active transport (2 marks).
Both use carrier proteins; facilitated diffusion occurs down a concentration gradient without ATP, while active transport moves substances against the gradient using ATP.
