Cell Membrane n Transport

Cell membrane (plasma membrane)

Phospholipid bilayer + proteins + cholesterol + carbohydrates

Phospholipid

Amphipathic molecule with hydrophilic head + hydrophobic fatty acid tails

Phospholipid bilayer arrangement

Hydrophilic heads face ECF/ICF, hydrophobic tails face inward

Why is the membrane semipermeable?

Hydrophobic core blocks polar/charged molecules

Fluid mosaic model

Membrane is fluid phospholipid bilayer with proteins embedded and moving laterally

What molecules diffuse easily through lipid bilayer?

Small nonpolar lipid-soluble molecules (O₂, CO₂, steroids)

Why do ions not cross the lipid bilayer easily?

They are charged and repelled by hydrophobic membrane core

Why does glucose require a transporter?

Large and polar → cannot pass hydrophobic core

Cholesterol function

Regulates membrane fluidity and stability

Cholesterol at high temperature

Decreases membrane fluidity by stabilizing bilayer

Cholesterol at low temperature

Increases fluidity by preventing phospholipid packing

Integral membrane protein

Protein embedded in lipid bilayer (strongly bound)

Peripheral membrane protein

Protein loosely attached to membrane surface (weak interactions)

Transmembrane protein

Integral protein spanning entire lipid bilayer

Channel protein

Transmembrane protein forming pore for specific molecules/ions

Carrier protein (transporter)

Binds solute and changes shape to move it across membrane

Glycocalyx

Carbohydrate coat on extracellular membrane surface (glycoproteins + glycolipids)

Glycocalyx main functions

Protection + prevents dehydration + cell recognition + immune identification

Blood group antigens are found where?

Glycocalyx (surface carbohydrates)

Passive transport

Movement down concentration gradient without ATP

Simple diffusion

Direct movement through lipid bilayer down gradient

Examples of simple diffusion

O₂ and CO₂

Osmosis

Movement of water across semipermeable membrane due to solute concentration difference

Facilitated diffusion

Movement down gradient using transport proteins (no ATP)

Example of facilitated diffusion

GLUT4 glucose transporter

Aquaporin function

Facilitated diffusion of water through channel

Ion channels transport type

Facilitated diffusion (passive)

Voltage-gated channel

Opens in response to membrane potential change

Ligand-gated channel

Opens when ligand binds

Mechanically-gated channel

Opens when membrane is stretched or pressured

PIEZO receptor

Mechanically-gated ion channel

Active transport

Movement against gradient requiring energy

Primary active transport

Direct ATP hydrolysis powers transport

Example of primary active transport

Na⁺/K⁺ ATPase

Na⁺/K⁺ ATPase function

3 Na⁺ pumped out, 2 K⁺ pumped in using ATP

Secondary active transport

Uses energy stored in ion gradient (no ATP directly)

Symporter

Moves 2 substances in same direction

Antiporter

Moves 2 substances in opposite directions

SGLT (Na⁺-glucose cotransporter)

Secondary active transport symporter

How does SGLT work?

Na⁺ moves down gradient providing energy to move glucose against gradient

Electrochemical gradient

Combination of concentration gradient + electrical gradient

CFTR protein function

Chloride ion channel regulating salt/water movement

Cystic fibrosis cause

Mutation in CFTR → defective chloride transport

CFTR protein type

Transmembrane protein

Endocytosis

Cell takes substances in by vesicle formation (ATP required)

Phagocytosis

Endocytosis of large particles (“cell eating”)

Pinocytosis

Endocytosis of fluid/small solutes (“cell drinking”)

Receptor-mediated endocytosis

Selective uptake using receptor-ligand binding

Why is receptor-mediated endocytosis selective?

Only specific ligands bind receptors and are internalized

Exocytosis

Vesicle fuses with membrane to release contents outside cell

Golgi vesicle budding function

Packages and delivers proteins to organelles or secretion outside cell

0.9% NaCl effect on cells

Isotonic → no net water movement → cell normal

0.45% NaCl effect on cells

Hypotonic → water enters → cell swells

1.5% NaCl effect on cells

Hypertonic → water leaves → cell shrinks

3.0% NaCl effect on cells

Strongly hypertonic → severe cell shrinkage

Hypotonic solution definition

Lower solute concentration than cell → water enters cell

Hypertonic solution definition

Higher solute concentration than cell → water leaves cell

Isotonic solution definition

Same solute concentration as cell → no net water movement

Facilitated diffusion key feature

Saturable (rate reaches maximum due to limited carriers)

Simple diffusion key feature

Not saturable (rate rises proportionally with gradient)

If transport reaches Vmax, what mechanism is likely?

Facilitated diffusion or carrier-mediated transport

If transport is linear with gradient, what mechanism is likely?

Simple diffusion