What is Transport Across Cell Membrane?
- The cell membrane controls movement of substances into and out of the cell.
- This is called membrane transport.
- Transport is essential for nutrient intake, waste removal, ion balance, etc.
TYPES of Transport Across Cell Membrane
There are 2 main types:
- Passive Transport (No energy/ATP needed)
- Active Transport (Requires energy/ATP)
1. Passive Transport
- Substances move from high to low concentration.
- No energy needed (natural movement – like sliding downhill).
Types of Passive Transport:
A. Simple Diffusion
- Movement of small, lipid-soluble substances directly across membrane.
- Example: O₂, CO₂
Mnemonic: "O2CO2 slide"
B. Facilitated Diffusion
- Movement with help of carrier or channel proteins.
- For larger or polar molecules.
- Example: Glucose via GLUT transporters
Mnemonic: "Facilitated = Friendly carrier"
C. Osmosis
- Movement of water molecules across a semi-permeable membrane.
- From low solute → high solute concentration.
- Important for fluid balance.
Mnemonic: "Water follows salt!"
Mnemonic for Passive Transport Types:
"SFO" = Simple, Facilitated, Osmosis
2. Active Transport
- Substances move against concentration gradient (low to high).
- Needs energy (ATP) and carrier proteins.
Types of Active Transport:
A. Primary Active Transport
- Direct use of ATP.
- Example: Sodium-Potassium Pump (Na⁺/K⁺-ATPase)
o Pumps 3 Na⁺ out and 2 K⁺ in
Mnemonic: "Pump 3 Na⁺ out, 2 K⁺ in – powered by ATP"
B. Secondary Active Transport
- Uses energy indirectly (from another ion's gradient).
- Example: Na⁺-glucose co-transport in intestine.
Types:
- Symport – both substances move in same direction
- Antiport – substances move in opposite directions
Mnemonic:
"Symport = Same way"
"Antiport = Alternate way"
3. Bulk Transport (Vesicular Transport)
- Transport of large molecules or particles via vesicles.
- Requires energy.
A. Endocytosis – Into the cell
- Phagocytosis – cell eating (e.g., WBC engulfing bacteria)
- Pinocytosis – cell drinking (fluids)
B. Exocytosis – Out of the cell
- E.g., release of neurotransmitters, hormones
Mnemonic:
"Endo = Enter, Exo = Exit"
Comparison Table
|
Type |
Energy Used |
Direction |
Example |
|
Simple Diffusion |
No |
High → Low |
O₂, CO₂ |
|
Facilitated Diff. |
No |
High → Low |
Glucose (GLUT) |
|
Osmosis |
No |
Water only |
Water transport |
|
Active Transport |
Yes |
Low → High |
Na⁺/K⁺ pump |
|
Endocytosis |
Yes |
Into cell |
Phagocytosis, Pinocytosis |
|
Exocytosis |
Yes |
Out of cell |
Hormone release |
Final Mnemonic Recap:
Mnemonic: "SFO-PAE"
- S – Simple Diffusion
- F – Facilitated Diffusion
- O – Osmosis
- P – Primary Active Transport
- A – Antiport/Symport (Secondary)
- E – Endo/Exocytosis
FAQs for Exams
Q1. What is the difference between passive and active transport?
Ans:
- Passive = No energy, high to low concentration
- Active = Requires ATP, low to high concentration
Q2. What is the sodium-potassium pump?
Ans:
- It is a primary active transport system that uses ATP to pump 3 Na⁺ out and 2 K⁺ in the cell.
Q3. What is facilitated diffusion?
Ans:
- It is passive transport that uses carrier proteins to move substances like glucose across the membrane.
Q4. What is the difference between phagocytosis and pinocytosis?
Ans:
- Phagocytosis = cell "eating" solids
- Pinocytosis = cell "drinking" fluids
Q5. What is osmosis?
Ans:
- Osmosis is the movement of water across a membrane from low solute to high solute concentration.
Tags
PHYSIOLOGY