(2.4) Biology - Transport Across Cell Membranes

Why is it called the ‘fluid mosaic model’

• fluid → phospholipid molecules can move around layers via diffusion

• mosaic → the membrane has protein molecules and their arrangement varies

label ts

• A → intrinsic protein

• B → fatty acid tail

• C → glycoprotein

• D → phospholipid head

• E → peripheral protein (support)

• F → phospholipid bilayer

• G → glycolipid

• H → carbohydrate

• I → cholesterol

function of membranes?

• provide support for cell

• controls what enters and exits

function of phospholipids ?

• allow lipid soluble substances to enter and exit cell

• prevent water soluble molecules from entering and exiting cell

• make membrane flexible

function of glycoprotein and glycolipids

• act as recognition sites

• help cells attach to eachother to form tissues

function of cholesterol

• decreases fluidity at high temps which decreases water loss

function of channel protein

• allows specific water soluble ions and water molecules to diffuse through

• used for facilitated diffusion

function of carrier protein

• changes shape when molecules or ions bind to active site and transports them across membrane

• used for active transport

what is the difference between simple and facilitated diffusion

• diffusion is the net movement of particles from high concentration to low concentration

• simple diffusion → allows direct transport of non-polar molecules across cell membrane

• facilitated diffusion → hydrophilic substances diffuse across membrane via channel proteins / carrier proteins

describe the process of active transport

• movement of molecules from low conc. to high conc. using ATP and carrier proteins

  1. Molecule binds to receptor on carrier protein

  2. ATP binds to carrier protein and is hydrolysed into ADP + Pi

  3. Carrier protein changes shape and releases molecule on other side

  4. Phosphate ion is released and carrier protein returns to original shape

  5. ADP and Pi reform ATP during respiration

describe the process of cotransport

1. Sodium ions enter bloodstream from the epithelial cell via active transport

2. This creates a concentration gradient for facilitated diffusion of Na⁺ to enter epithelial cell from ileum

3. Glucose enters epithelial cell from ileum via cotransport with sodium

4. (Glucose exits epithelial cell into bloodstream via facilitated diffusion)

what is an advantage of cell membrane fluidity

membrane proteins can diffuse to where they are needed

explain 3 factors that affect the fluidity of the cell membrane

• higher temperature → increases fluidity because there is more kinetic energy

• higher conc. of saturated acids decrease fluidity as there are more intermolecular forces

• cholesterol decreases fluidity at high temperatures