Membranes

Membranes are cell boundaries. They control what enters and exits a cell or organelle.

Plasma Membrane

The outer boundary of cells.

Nuclear Membrane

Around DNA.

Mitochondria Membrane

Around Mitochondria.

All organelles have membranes.

Membranes have selective permeability.

• They keeps toxins out.
• They control salt/electrolyte levels.
• They help maintain pH levels.
• They keeps glucose in.

Parts of a Membrane

Phospholipid Bilayer

The foundation of the membrane.

Proteins

For cellular processes.

For transport of large/polar molecules through the membrane.

Cholesterol

Provides structure and support.

Fills in gaps to complete the circle.

Phospholipid Bilayer

Has two opposing sides of phospholipids. Since the membranes are surrounded by water, the water-loving (glycerol) head will face the water and the water-phobic (fatty-acid) tail will face each other.

Phospholipids Amphipathic

Glycerol Hydrophilic

Water-Loving Head

Fatty Acids Hydrophobic

Water-Hating Tail

Phosphate Group

Attached to the head used to bond with other molecules to form the membrane.

⇑⇑⇑⇑⇑🛢⇑⇑⇑⦚⇑⇑⦚⇑⦚⇑⇑⇑⇑ഄ⇑⇑
⇓⇓⇓⇓⇓🛢⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓

⇑ = phospholipid, ⇓ = phospholipid, ⦚ = cholesterol, 🛢 = transmembrane protein, ഄ = surface protein

Proteins

These transport molecules, receive signals, and recognize other cells.

Surface Proteins

Sits on top of the membrane.

Transmembrane Proteins

Go all the way through the membrane.

Special Transport Proteins

A type of transmembrane protein. These are highly selective. Using these makes it facilitated diffusion.

Channel Proteins Like Tunnels

Generally facilitates diffusion through passive transport mechanisms.

• Generally does not require ATP energy.

Sodium channels can only move sodium.

Potassium channels can only move potassium.

Carrier Proteins Like Turnstiles

Can either facilitate diffusion through passive or active transport mechanisms.

• May or may not require ATP energy.

Glucose transporters can only transport glucose.

Transport

Passive Transport No Energy

Does not require ATP energy.

May or may not involve proteins.

Molecules follow the concentration gradient.

Active Transport Uses Energy

This uses proteins along with ATP energy to move molecules against the concentration gradient.

Osmosis

Water molecules and their hydrogen bonds move through membranes towards the side w/ more dissolved particles. Doesn't necessarily follow simple diffusion rules. Water prefers to form stronger covalent bonds over its own.

Water wants to form stronger covalent bonds with other ions (sa. salt) over its own hydrogen bonds.

Water wants to diffuse and even out the concentration of ions, not the amount of water.

Water doesn't diffuse when there are uneven amounts of water w/ equal ion concentration.

Water does diffuse when there are equal water levels with uneven ion concentrations.

After osmosis, does diffusion occur? Depends on the molecules.

Diffusion

Natural spreading of molecules. No energy required. Molecules diffuse from high concentrations to low concentrations across cells.

Concentration Gradient

The gradual change in the concentration of solutes within a solution, typically between adjacent areas.

Simple Diffusion No Proteins

Molecules move from high concentrations to low concentrations.

Does not require transport proteins to move into and out of cells.

Never requires energy. Follows the concentration gradient.

Has issues. Certain molecules need special transporter proteins to facilitate diffusion.

• Glucose (sugar) is too polar.

• Amino acids are too large and polar.

• Sodium and potassium are too charged.

Small molecules

Uncharged particles

Non-polar molecules

Facilitated Diffusion Uses Proteins

May or may not require ATP energy whether the protein needs to move the molecule against the concentration gradient.

Requires transmembrane/transport proteins to move molecules across the cell membrane.

Fatty-acid tail blocks charged molecules/ions because the hydrophobic nature of it blocks water from passing though and the charged molecules tend to associate with water.

Large molecules

Charged particles (ions)

Polar molecules