C4 : cell membranes and transport

function of cell surface membrane

• barrier between internal and external environments

• form compartments within cell

• control exchange of substances

what happens when phospholipids form a bilayer in water

spontaneously form bilayers or vesicles (spheres)

why do phospholipids form a bilayer in water

driven by hydrophobic and hydrophilic interactions

• hydrophilic phosphate head face water

• hydrophobic fatty acid tails face away from water

intrinsic (integral) membrane proteins

Proteins embedded within the phospholipid bilayer, with hydrophobic regions interacting with fatty acid tails and hydrophilic regions exposed to water.

extrinsic (peripheral) membrane proteins

Proteins found on the inner or outer surface of the membrane and do not span the bilayer

Why is the membrane described as “fluid” “mosaic”

• fluid : phospholipids and proteins move laterally by diffusion , allowing flexibility and shape change

• mosaic : proteins are scattered irregularly throughout the phospholipid bilayer

where is cholesterol found in membranes

• fits between phospholipid molecules
• Oriented with hydroxyl group near phosphate heads
• Present in eukaryotes only (absent in prokaryotes)

where glycolipids located

• Lipids with carbohydrate chains

• Carbohydrate chains project outwards from membrane surface

glycoprotein

• Proteins with carbohydrate chains
• Carbohydrate chains project outwards from membrane surface

three roles of phospholipids in membranes

• Form bilayer structure
• Act as barrier to most water-soluble substances
• Provide membrane fluidity via lateral movement

How do phospholipids contribute to cell signalling

They can be chemically modified or hydrolysed, releasing water-soluble signalling molecules

How does cholesterol affect membrane fluidity

• Prevents phospholipids packing too closely

• Regulates membrane fluidity

Effect of cholesterol at low temperatures

Prevents close packing → prevents membrane freezing → increases fluidity

How does cholesterol affect permeability and stability

• Reduces permeability
• Increases mechanical strength and stability

State the roles of glycolipids

• Act as antigens for cell recognition

• Cell-to-cell adhesion

• Maintain membrane stability

State the roles of glycoproteins

• Receptors for hormones and neurotransmitters
• Cell-to-cell adhesion
• Formation of tissues
• Cell recognition

Differences between channel and carrier proteins

• Channels have hydrophilic pores, carriers do not
• Channels do not change shape, carriers undergo conformational change
• Channels only do facilitated diffusion , carriers can do facilitated diffusion and active transport

how do membrane proteins act as receptors

they have specific complementary binding sites for signalling molecules ; binding triggers a response inside the cell

factors affecting membrane fluidity

• cholesterol content

• temperature

• fatty acid tail length

• degree of saturation of fatty acid

• phospholipid composition

effect of unsaturated fatty acid tails on fluidity

• contain kinks → prevents close packing → increase membrane fluidity

effect of fatty acid chain length on fluidity

• shorter chain → weaker hydrophobic interactions

• less close packing → increased fluidity

what is cell signalling

the process by which cells interact with their environment and with other cells

stages of cell signalling

1) Synthesis and secretion of specific chemicals (ligands) from cells

2) transport of ligands to target cells

3) binding of ligands to specific cell surfcae receptors on target cell

ligands

molecules that bind to specific biological molecules, often proteins or glycoproteins

how ligands transported in animals

via blood circulatory system → tissue fluid surrounding target cells

How are ligands transported in plants?

Via phloem sap, plasmodesmata, or through cell walls

What happens if cells have different surface antigens

they are recognised as foreign and destroyed in an immune response

role of cell surface antigens

allow immune system to recognise self cells and distinguish them from foreign cells

molecules are involved in cell recognition

glycoproteins and glycolipids acting as cell surface antigens

define diffusion

the net movement of particles from a region of higher concentration to lower concentration, due to the random motion of molecules or ions

effect of concentration gradient on diffusion rate

steeper gradient → greater difference in number of particles moving → faster diffusion

effect of surface area to volume ratio on diffusion

lower SA:V ratio → slower diffusion

molecules diffuse directly through the phospholipid bilayer

mall, uncharged, non-polar, lipid-soluble molecules (e.g. O₂, CO₂)

define facilitated diffusion

The diffusion of particles down a concentration gradient through channel or carrier proteins.

channel protein

• Fixed shape
• Hydrophilic, water-filled pores
• Allow ions and water-soluble molecules
• Most are gated

carrier protein

• Binding site for specific molecule
• Undergo conformational change
• Binding site alternately opens to either side of membrane

simple diffusion vs facilitated diffusion (molecules transported)

simple: small, non-polar, lipid-soluble
facilitated: ions, polar, large molecules

simple vs facilitated diffusion (saturation)

simple: no saturation, linear increase
facilitated: saturates, plateaus when proteins occupied

define osmosis

the net movement of water molecules from a region of higher water potential to lower water potential, across a partially permeable membrane.

define water potential (ψ)

pressure created by water molecules, measured in kPa

effect of solute concentration on water potential.

more solute → more negative water potential

define active transport

movement of substances against a concentration gradient, using ATP and carrier proteins

describe the mechanism of active transport

• Molecule binds to carrier protein
• ATP hydrolysed to ADP + Pi
• Carrier protein changes shape
• Molecule transported across membrane
• Phosphate released, carrier returns to original shape

active transport example

• Glucose & amino acid uptake in ileum
• Mineral ion uptake by plant roots
• Na⁺/K⁺ pump in neurons
• H⁺ excretion in kidneys

facilitated vs active transport (gradient)

facilitated: down gradient
active: against gradient

what is bulk transport

movement of very large substances across membranes using vesicles, requiring ATP

define endocytosis

transport of substances into the cell by membrane infolding and vesicle formation

define exocytosis

transport of substances out of the cell by vesicles fusing with the membrane

desc exocytosis of proteins

• Proteins packaged in Golgi vesicles
• Vesicles move to membrane
• Fuse with membrane
• Contents released outside cel4

shagocytosis vs pinocytosis

Phagocytosis: uptake of solids
Pinocytosis: uptake of liquids

what is a co-transporter

a carrier protein that binds two substances simultaneously, using the gradient of one to move the other

example of co-transport

glucose absorption in ileum using sodium ions

why is diffusion alone insufficient for glucose absorption

glucose concentration in epithelial cells becomes higher than lumen, so diffusion cannot continue

why use % change in mass instead of raw mass?

removes variation in starting mass and allows comparison

direction of water movement in osmosis

from less negative → more negative water potential

hypotonic solution on plant cells

water enters → cell becomes turgid

hypertonic solution on plant cells

water enters → cell swells → cytolysis

identifying linear graph and curve that plateous

• linear : simple diffusion (no saturation)

• plateau : facilitated diffusion (limited protein)