Lecture 7

BIOL 118 Exam 1

extracellular fluid (ECF)

fluid outside cells (blood plasma, interstitial fluid)

intracellular fluid (ICF)

fluid inside cells (cytoplasm)

What separates ECF and ICF?

cell membrane

phospholipid

a special type of lipid that is the main building block of cell membranes

Which region of this molecule is hydrophilic?

head (polar)

Which region of this molecule is hydrophobic?

fatty acid tails (nonpolar)

How are phospholipids arranged in a membrane?

• Heads face water

• Tails face inward

→ forms a phospholipid bilayer

Why is the membrane a “fluid mosaic”?

• Fluid → lipids and proteins move sideways

• Mosaic → many different proteins embedded

general functions of cell membranes

• Physical barrier

• Control what enters/leaves

• Communication

• Cell identity

• Transport

• Homeostasis

How does membrane flexibility alter if phospholipids contain more saturated fatty acids than normal?

it becomes less flexible

How does membrane flexibility alter if phospholipids contain more unsaturated fatty acids than normal?

it becomes more flexible

What is the role of cholesterol in the cell membrane?

• Stabilizes membrane

• Prevents it from being too stiff or too fluid

• Maintains proper permeability

passive transport

• No ATP

• Uses kinetic energy

• Moves down concentration gradient

active transport

• Requires ATP

• Moves against gradient

examples of passive transport

• Simple diffusion

• Channel-mediated diffusion

• Carrier-mediated (facilitated) diffusion

• Osmosis

examples of active transport

• Pumps

• Endocytosis

• Exocytosis

Which transport method uses ATP?

active transport

Which transport method uses kinetic energy of the molecule?

passive transport

Which transport method goes “down” a concentration gradient?

passive transport

vesicular transport

a way cells move large materials into or out of the cell using membrane-bound sacs called vesicles (requires ATP and uses the cytoskeleton)

What are the two main types of vesicular transport?

endocytosis and exocytosis

endocytosis

the cell membrane engulfs material and pinches off a vesicle (into the cell)

exocytosis

a vesicle fuses with the cell membrane and releases its contents outside (out of the cell)

How does vesicular transport use ATP?

ATP is required for:

• Moving vesicles along the cytoskeleton

• Membrane reshaping and fusion

• Actively controlling transport

Why is vesicular transport important?

It allows cells to move:

• Very large molecules

• Many molecules at once

• Things that cannot cross membranes by diffusion or carriers

channel proteins

• Form pores

• Select by size/charge

• Fast

• No binding

• Passive

carrier proteins

• Bind molecule

• Change shape

• Can be passive (facilitated diffusion) or active

Do channels proteins require ATP?

no (always passive transport)

simple diffusion

the passive movement of molecules from an area of higher concentration to lower concentration (through lipid bilayer)

What drives simple diffusion?

kinetic energy (without ATP or transport proteins)

How does temperature (kinetic energy) affect the rate of diffusion?

Temperature ↑ → rate ↑

How does concentration gradient affect the rate of diffusion?

Concentration gradient ↑ → rate ↑

How does molecular/ion mass affect the rate of diffusion?

Molecular mass ↑ → rate ↓

channel-meditated diffusion

• Protein forms a pore (tunnel) through the membrane

• Molecules move directly through

• Selected by size and charge

• Can be gated (open/close)

carrier-mediated (facilitated) diffusion

• Molecule binds to a carrier protein

• Protein changes shape

• Molecule released on the other side

Active transport (using carrier proteins)

• Molecule binds to a carrier protein

• Protein changes shape

• Uses ATP

How do facilitated diffusion differ from active transport?

active transport uses ATP and moves against the concentration gradient

What happens after GLUT2 brings glucose into beta cells?

• Glucose enters via GLUT2

• ATP increases

• K⁺ channels close

• Cell depolarizes

• Ca²⁺ enters

• Insulin released

Osmosis

movement of water across a membrane due to solute concentration differences

What causes water to move?

water moves toward higher solute concentration

Is osmosis active or passive?

passive

Isotonic

equal solutes (no net movement)

Hypotonic

lower solutes outside → cell swells

Hypertonic

higher solutes outside → cell shrinks

How is osmosis related to cellulose?

Cellulose is indigestible and remains in the intestines, increasing the solute concentration of the intestinal contents. This causes water to move into the colon by osmosis. The added water softens and bulks the feces, making it easier to move through the digestive tract and easier to pass.

cellulose

• Indigestible fiber

• Stays in intestines

• Increases solute concentration

• Pulls water in by osmosis

• Makes stool softer and easier to pass