Nerve Signals

High density of voltage-gated channels.

Extremely active Na⁺/K⁺ pumps.

Specialized for rapid electrical signaling.

Muscle Cells

Calcium channels and pumps dominate.

Control contraction and relaxation.

Must coordinate calcium release and uptake.

Kidney Cells

Incredible variety of transporters.

Filter blood and reclaim valuable molecules.

Fine-tune body's chemical balance.

Channel Proteins

Like gates. Fast. No barrier to speed. Like flood gates. These are like water-filled tunnels that allow polar molecules to move into/out of cells. Always follows the concentration gradients. Large molecules don't fit. Only small molecules fit.

Types

Voltage Gated

Based on electrical charge inside and outside the cell.

Goes with the concentration gradient.

May be open or closed.

Ligand Gated

Binds to a specific molecule to open the gate.

• The molecule it binds to is known as a ligand.

Goes with the concentration gradient.

May be open or closed.

Leak Channels

Always leaky/open.

Does not require ATP energy.

Goes with the concentration gradient.

Potassium leak channels maintain an internal negative voltage and are used for resting membrane potential.

Carrier Proteins

Like a revolving door. Shape shifters. Slow. Like a turn-style gate. Requires the protein to change shape. May be passive transport (through facilitated diffusion), or, more-commonly active transport (uses energy) (when going against the concentration gradient). When it goes against the concentration gradient (uses ATP), it's called pump.

Glucose Transporter.

Sodium-Potassium Pump

Active Transport

Proteins that facilitate the movement of molecules against the concentration gradient are called pumps.

The sodium-potassium pump moves molecules against the concentration gradient to fuel membrane potential.

Action Potentials

Relies on speedy channels for fast communication - or fast water movement for high volume flows.

Nerve Signals Action Potentials

Must change voltage from -70mV to +30mV in 1 millisecond.

Requires millions of sodium ions to enter rapidly.

Only possible with the extreme speed of ion channels.

Muscle Contraction

Calcium must be released instantly for contraction.

Must be pumped back quickly for relaxation.

Channels provide speed, pumps do cleanup.

Heart Rhythm

Precise timing of sodium, potassium, and calcium flow.

Any delay disrupts the rhythm.

Life depends on channel speed.

Sodium-Potassium Pump

Sodium-potassium pump moves 3 Na⁺ from the inside to the outside of the cell. Moves 2 K⁺ ions into the cell from the outside. This creates a voltage difference between the inside and the outside of the cell. More positive ions are pumped to the outside of the cell, and then some potassium leaks out from the inside of the cell through a leaky potassium channel protein.

This is important for volume control of water (sends ions out so water doesn't rush in due to osmosis), creates a voltage difference for nerve and muscle functions. Powers secondary transport.

Sodium-potassium pumps affect all other transport by maintaining crucial cellular energy and gradients.

Steps

Sodium-Potassium pumps use energy to create a loaded spring state with unnatural gradients to store potential energy that can be instantly released by the faster, passive, channels that let ions in and out.

• Na⁺ channels are voltage-gated.
• K⁺ channels are both voltage-gated and leaky.
• Potassium and Sodium ions here are both positive.

Resting

Membrane potential of -70 mV (millivolts)

Cells use 30% of daily energy to maintain or bring back into the resting negative-potential state.

Sodium-Potassium pumps move 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell. This creates a negative internal environment. Some K⁺ ions leak out through a leaky potassium channel that also contributes.

Step 1 Depolarization

When a stimulus triggers a threshold (-55mV), the sodium channels open, allowing sodium cations to flood in.

The sodium channels remain closed until this threshold is met, keeping the positive sodium ions out of the cell.

This reverses the voltage potential to a positive one.

Step 2 Repolarization

K⁺ ions leak out, resetting the voltage potential.

The cell opens voltage-gated potassium channels so that potassium moves out of the cell quickly.

• The cell overshoots and lets out too much, leading to the voltage potential being lower than normal for a bit.

This state has more sodium ions inside the cell and more potassium ions outside the cell.

• The sodium-potassium pump must work overtime to move sodium out of the cell.

Secondary Active Transport

Cells use sodium gradient powered by the sodium-potassium pump. Since the sodium-potassium pump pushes the sodium out to makes more concentration of sodium outside the cell, the sodium want to flow back in. Does not require ATP for secondary transport, but the active transport that facilitates this (the sodium-potassium pump) does require energy.

Sodium-Glucose Cotransporter Does not require ATP

Pulls along water into the cell due to osmosis.

Salt and sugar are absorbed together through sodium-glucose cotransporter.

The sodium-glucose cotransporter moves a sodium and glucose together into the cell in line with the concentration gradient of the sodium.

• Sodium moves down its concentration gradient into the cell.

• The glucose is "dragged along" against its concentration gradient.

Occurs in small intestine and kidney cells to absorb glucose from the intestines when the glucose concentration is low.

Vesicle Transport

For molecules that are too large for active transport.

Endocytosis Requires ATP

Wraps molecules in vesicles to pull into the cell.

• Vesicle is created from the membrane.

Exocytosis Requires ATP

Unwraps molecules from vesicles to cast them out of the cell.

• Vesicle merges with the membrane.

Endocytosis

Cell membrane wraps large molecules in a vesicle to bring them into the cell.

• Very specific - like a lock and key; the cell only takes in what it recognizes.

Phagocytosis

Membrane wraps around specific, large molecules.

Membrane creates internal vesicles that carry the molecules across and into the cell.

Used by immune cells to engulf bacteria.

Pinocyctosis

Less specific, takes in droplets of extracellular fluid.

Membrane creates internal vesicles that carry the droplets across and into the cell.

Exocytosis

Vesicle-wrapped molecules merges with the cell membrane and releases molecules out of the cell.

Neurons releasing neurotransmitters.

Glands secreting hormones.

Cells disposing of waste.