Membrane Structure and Transport

List and describe the general functions of the membrane.

Component	Function
Phospholipid bilayer	Main structure — two layers of phospholipids. Hydrophilic heads face outward, hydrophobic tails face inward
Embedded proteins	Transport, communication, enzymes, receptors
Cholesterol	Stabilizes membrane fluidity — keeps it from being too rigid or too fluid
Carbohydrate chains	Cell identification and communication — like a cell's ID tag

Key functions of the membrane:

• Controls what enters and exits (selective permeability)

• Communication with other cells

• Structural support

Differentiate between the different types of membrane proteins.

Type	Function
Transport proteins	Move substances across the membrane
Receptor proteins	Receive signals from outside the cell (like hormones)
Enzymatic proteins	Carry out chemical reactions at the membrane
Cell recognition proteins	Act as ID tags so cells recognize each other
Cell junction proteins	Connect adjacent cells together

Two categories based on location:

• Integral proteins — embedded directly into the membrane, often spanning the whole bilayer

• Peripheral proteins — attached to the surface of the membrane, not embedded

Describe the factors that control membrane fluidity.

1. Temperature

• Higher temperature = molecules move faster = membrane more fluid

• Lower temperature = molecules slow down = membrane more rigid/solid

2. Cholesterol

• Acts as a buffer against temperature extremes

• At high temperatures — cholesterol restrains phospholipid movement = prevents membrane from becoming too fluid

• At low temperatures — cholesterol prevents phospholipids from packing too tightly = prevents membrane from becoming too rigid

• Overall cholesterol stabilizes fluidity

3. Fatty acid composition

• Unsaturated fatty acids (kinked) = more fluid membrane

• Saturated fatty acids (straight) = less fluid, more rigid membrane

Define selective permeability and identify the types of materials that pass the membrane freely or do not pass freely.

• Selective permeability means the membrane only allows certain substances to pass through freely.

Passes freely	Does NOT pass freely
Small nonpolar molecules (O₂, CO₂)	Large molecules (glucose, proteins)
Small uncharged molecules (water)	Charged ions (Na⁺, K⁺, Cl⁻)
Hydrophobic molecules	Hydrophilic molecules

Why? The membrane's core is hydrophobic (the fatty acid tails). So anything hydrophobic passes easily. Anything hydrophilic or charged needs a protein to help it cross.

Differentiate between active and passive transport in terms of the concentration gradient, energy/ATP required, and need for a specialized protein like a pump, pore, or channel.

	Passive Transport	Active Transport
Energy (ATP)	No	Yes
Direction	High to low concentration	Low to high concentration
Proteins needed	Sometimes	Yes always
Examples	Diffusion, osmosis	Sodium/potassium pump

Differentiate between simple and facilitated diffusion.

Both are passive (no ATP needed), both go high to low concentration. The difference is whether a protein is needed:

	Simple Diffusion	Facilitated Diffusion
Protein needed	No	Yes
What crosses	Small nonpolar molecules	Large or charged molecules
Examples	O₂, CO₂, water	Glucose, ions

• Simple diffusion — molecule passes directly through the phospholipid bilayer on its own

• Facilitated diffusion — molecule needs a channel or carrier protein to help it cross but still no ATP required

Define osmosis.

Simple definition:

• Osmosis — the diffusion of water across a selectively permeable membrane from an area of high water concentration to low water concentration

• Water moves to where there is more solute (dissolved stuff)

• No energy required — it's passive

Differentiate between solutions that are hypertonic, hypotonic, and isotonic.

Solution	Solute concentration compared to cell	Water moves	Result
Hypertonic	More solute outside than inside	Water leaves cell	Cell shrinks
Hypotonic	Less solute outside than inside	Water enters cell	Cell swells
Isotonic	Equal solute inside and outside	No net movement	Cell stays same

Explain or diagram what happens when you put a red blood cell (animal) or a plant cell in a solution that is hypertonic. Repeat for these cells in a hypotonic solution and isotonic solution.

Solution	Red Blood Cell (Animal)	Plant Cell
Hypertonic	Crenation — cell shrinks and shrivels	Plasmolysis — membrane pulls away from cell wall
Hypotonic	Lysis — cell swells and bursts	Turgid — cell swells but cell wall prevents bursting
Isotonic	Normal	Normal

Differentiate between active transport proteins – uniporter, symporter, and antiporter.

Type	How it works	Example
Uniporter	Moves ONE substance in ONE direction	Glucose transporter
Symporter	Moves TWO substances in the same direction	Sodium-glucose transporter
Antiporter	Moves TWO substances in opposite directions	Sodium/potassium pump

Diagram the sodium/potassium pump.

1. 3 sodium ions (Na⁺) bind inside the cell to the pump

2. ATP is used to change the pump's shape → 3 Na⁺ pushed outside

3. 2 potassium ions (K⁺) bind outside → pump returns to original shape → 2 K⁺ pulled inside

	Sodium (Na⁺)	Potassium (K⁺)
Direction	Pumped OUT	Pumped IN
Amount	3	2

Differentiate between endocytosis and exocytosis.

	Endocytosis	Exocytosis
Direction	INTO the cell	OUT of the cell
How	Membrane folds inward, forms vesicle	Vesicle fuses with membrane, releases contents
Example	Cell engulfing bacteria	Releasing insulin, neurotransmitters

Differentiate between three types of endocytosis – pinocytosis, phagocytosis, and receptor-mediated endocytosis.

Type	What it takes in	How
Pinocytosis	Fluid and small dissolved molecules	Cell membrane gulps in tiny droplets of extracellular fluid
Phagocytosis	Large solid particles (bacteria, debris)	Cell wraps around and engulfs the particle — forms a large vesicle called a phagosome
Receptor-mediated endocytosis	Specific molecules only	Molecules bind to specific receptors on membrane → membrane folds in → very selective