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Membrane Structure
and Transport

Explore how the plasma membrane controls what enters and exits cells through diffusion, osmosis, and active transport.

Anatomy & Physiology

What Is the Plasma Membrane?

Every living cell is surrounded by a thin, flexible barrier called the plasma membrane. This structure is not a solid wall: it is a dynamic, selectively permeable boundary that allows certain substances to pass while blocking others.

The Fluid Mosaic Model

Channel
Peripheral
Cholesterol
Extracellular Fluid
Cytoplasm
Hydrophilic head
Hydrophobic
tails

Key Membrane Components

Phospholipids
Form the bilayer: hydrophilic heads face water, hydrophobic tails face inward
Proteins
Integral (span membrane) and peripheral (surface-bound) proteins enable transport and signaling
Cholesterol
Stabilizes fluidity: prevents too rigid in cold, too fluid in heat
Carbohydrates
Glycoproteins and glycolipids on the outer surface aid in cell recognition

Two Categories: Passive vs Active

Passive Transport
No energy (ATP) required. Substances move down their concentration gradient: from high to low concentration.
Active Transport
Requires energy (ATP). Substances move against their concentration gradient: from low to high concentration.

Diffusion: Movement Down the Gradient

High Concentration
Low Concentration
Membrane

Diffusion is the net movement of particles from a region of high concentration to a region of low concentration. It requires no energy and continues until equilibrium is reached.

Osmosis: Water Follows Solute

Low Solute (Hypotonic)
High Solute (Hypertonic)
Water moves toward solute →
Water
Solute

Tonicity: How Solutions Compare

Isotonic
Equal solute on both sides. No net water movement. Cell stays normal.
Hypotonic
Less solute outside. Water moves into the cell. Cell swells (may lyse).
Hypertonic
More solute outside. Water flows out of the cell. Cell shrivels (crenation).

Facilitated Diffusion

Channel
Carrier
Extracellular
Intracellular
Ions (e.g. \( \text{Na}^+ \))
Glucose
No ATP

Facilitated diffusion uses membrane proteins to help large or charged molecules cross the membrane. Channel proteins form pores for ions, while carrier proteins change shape to shuttle molecules like glucose. Still passive: no ATP needed.

Active Transport: Against the Gradient

Pump
ATP →
Extracellular
Intracellular
\( 3 \text{Na}^+ \) out
\( 2 \text{K}^+ \) in

Diffusion vs Osmosis vs Active Transport

Feature Diffusion Osmosis Active Transport
What moves? Solute particles Water Solutes (ions, molecules)
Direction High → Low High water → Low water Low → High
Energy (ATP) No No Yes
Proteins needed? Sometimes Aquaporins help Always (pumps)

Bulk Transport: Endo and Exocytosis

Endocytosis
Cell membrane folds inward to engulf large particles into vesicles. Includes phagocytosis (solids), pinocytosis (liquids), and receptor-mediated endocytosis.
Exocytosis
Vesicles fuse with the membrane to release contents outside the cell. Used for secretion of hormones, neurotransmitters, and waste.

Identifying Transport Type

1
Identify the substance. Is it water, a small nonpolar molecule, an ion, or a large particle?
2
Determine the gradient direction. Is the substance moving from high to low concentration, or low to high?
3
Check for energy. If moving down the gradient: passive. If against: active (ATP required).
4
Look for helpers. Channels and carriers = facilitated diffusion. Pumps = active transport. No protein = simple diffusion.
5
If water is the substance, it is osmosis: water moving toward higher solute concentration.

Example 1: Oxygen in the Lungs

Alveolus
High \( \text{O}_2 \)
Capillary
Low \( \text{O}_2 \)
\( \text{O}_2 \)
Simple Diffusion: No ATP, No Protein

Example 2: Glucose Entering a Cell

Blood (high glucose)
Cell (low glucose)
GLUT
Facilitated Diffusion via GLUT transporter

Example 3: The \( \text{Na}^+/\text{K}^+ \) Pump in Neurons

Neurons must maintain a resting membrane potential of about \( -70 \, \text{mV} \). The \( \text{Na}^+/\text{K}^+ \) pump keeps \( \text{Na}^+ \) concentrated outside and \( \text{K}^+ \) concentrated inside.

1
\( 3 \text{Na}^+ \) ions bind inside the cell to the pump protein.
2
ATP is hydrolyzed (\( \text{ATP} \to \text{ADP} + \text{P}_i \)), and the pump changes shape.
3
\( 3 \text{Na}^+ \) released outside, then \( 2 \text{K}^+ \) bind from outside.
4
Pump reverts shape, releasing \( 2 \text{K}^+ \) inside the cell. Cycle repeats.

Common Mistakes

✕ Confusing Osmosis Direction
Water moves toward more solute, not toward "more water." A hypertonic solution pulls water out of a cell.
✕ Thinking All Diffusion Needs Proteins
Simple diffusion of small nonpolar molecules (\( \text{O}_2 \), \( \text{CO}_2 \)) crosses the bilayer directly. Only polar or large molecules need facilitation.
✕ Mixing Up Active and Passive
If it moves down the gradient: passive (no ATP). If against: active (ATP). Facilitated diffusion is still passive.

Transport Summary

Type Energy? Protein? Example
Simple Diffusion No No \( \text{O}_2 \), \( \text{CO}_2 \)
Facilitated Diffusion No Yes Glucose, ions
Osmosis No Aquaporins help Water
Active Transport Yes (ATP) Yes \( \text{Na}^+/\text{K}^+ \) pump
Endocytosis Yes (ATP) Vesicles White blood cells
Exocytosis Yes (ATP) Vesicles Neurotransmitters

Question 1 of 5

A red blood cell is placed in a hypertonic solution. What will happen to the cell?

Question 2 of 5

Which of the following is an example of facilitated diffusion?

Question 3 of 5

The \( \text{Na}^+/\text{K}^+ \) pump moves \( 3 \text{Na}^+ \) out and \( 2 \text{K}^+ \) in per cycle. What type of transport is this?

Question 4 of 5

Which membrane component is primarily responsible for stabilizing membrane fluidity across temperature changes?

Question 5 of 5

During osmosis, water moves from a region of ______ water concentration to a region of ______ water concentration.