AP Biology Cell Membranes and Transport

Cell Theory

1. All organisms are composed of cells

2. Cell are the basic units of life

3. Cells arise only from pre-existing cells

Prokaryote vs Eukaryote Cell Size

• Prokaryote: 1-10μm diameter

• Eukaryote: 10-100μm diameter

Cell Size Limitation

• Most cells are relatively small due to reliance on diffusion of substances in and out of cells

Diffusion Rates

• Affected by:

  • SA/V ratio

  • Temperature

  • Concentration Gradient

  • Distance

Surface Area/Volume Ratio

• Good for it to be big

• As cell size increases, volume grows faster than surface area, making exchange of materials less efficient. High SA:V allows faster diffusion of nutrients, gases, and wastes

Small Cell Size

• Most cells are small

• Skeletal muscle cells are large but have multiple nuclei

• Some cells are long and skinny to transmit signals faster like nerve cells

Cell Membrane Functions

• Holds in contents

• Regulates what goes in and out of the cell

Cell Membrane Structure

• Made of phospholipids

  • Phosphate head (hydrophilic)

  • Fatty acid tail (hydrophobic)

• Forms a bilayer where only certain molecules can get through

Plasma Membrane

• The boundary that separates a cell from its surroundings

• Exhibits selective permeability and allows some substances to cross it more easily than others

Amphipathic Molecules

• Contain both hydrophobic and hydrophilic parts

Ex. Phospholipids

Fluid Mosaic Model

States that a membrane is a fluid structure (phospholipids and unanchored proteins can move around) with a mosaic of various proteins embedded in it

Cholesterol

• A steroid

• Keeps the phospholipids from locking together/affects membrane fluidity

  • At warm temps (37°C-body temp), cholesterol restricts movement of phospholipids

  • At cool temps, it protects fluidity by preventing tight packing

Proteins

• Help move things across the membrane

Transmembrane/Integral Protein

• Moves things across

Peripheral Protein

Attached to the outside of inside

Carbohydrates

• Helps cells identify each other

Glycolipid

Carb + lipid

Glycoprotein

Carb + protein (look like a Y)

6 Major Functions of Membrane Proteins

1. Transport

2. Enzyme

3. Cell-surface receptors

4. Cell-surface identity markers

5. Cell-to-cell adhesion proteins

6. Attachments to cytoskeleton

Diffusion

• Random movement of particles from high to low concentration

• Only occurs if there is a concentration gradient (uneven distribution of solute)

• Will continue until equilibrium is reached (even distribution)

• A passive process; does not take any added energy to happen

Simple Diffusion

Small, hydrophobic molecules (like H2 and O2) can go right through the phospholipid bilayer

Facilitated Diffusion

Bigger, hydrophilic molecules (like water and sugar) need to use a protein in the membrane to go through

Osmosis

• Diffusion of water through a selectively permeable membrane

  • The membrane will only let water through

• Affected by pressure, temp, and solute concentration

Isotonic Solution

• Same concentration of solute inside and outside the cell

• No net water movement across the plasma membrane

  • Animal cells keep their shape

  • Plant cells wilt/become flaccid

Hypertonic Solution

• Higher concentration of solute outside cell than inside

• The cell will shrink because water goes out to equalize the concentration

  • Animal cells shrink and shrivel; still okay

  • Plant cells tear away from cell walls/become plasmolyzed; not okay

Hypotonic Solution

• Lower concentration of solute outside cell than inside

• Cell will grow because water goes in to equalize concentration

  • Animal cells swell and can burst/become lysed; bad

  • Plant cells are turgid and are happy and normal as the cell wall is tough enough

U-Tube and Urea

1. Urea is polar and water is polar, forming hydration shells as water is attracted to the larger concentration of urea

2. Equilibrating both sides

Active Transport

• Goes against the concentration gradient (unlike simple and facilitated diffusion)

• Needs energy (ATP) to pump molecules across the cell membrane

• Uses membrane proteins (like facilitated diffusion)

3 Carrier Proteins used in Active Transport

1. Uniporters

2. Symporters

3. Antiporters

Uniporters

Move one molecule at a time

Symporters

Move two molecules in the same direction

Antiporters

Move two molecules in opposite directions

Sodium-Potassium Pump

• Uses ATP for active transport

• Uses an antiporter to move 3 NA+ out of the cell and 2 K+ into the cell AGAINST their concentration gradient

• ATP energy is used to change the conformation of the carrier protein

• Affinity of the carrier protein for either NA+ or K+ changes so the ions can be carried across the membrane and then released

Bulk Transport

Large molecules cross the membrane by vesicles

Endocytosis

• Movements of substances into the cell

• Phagocytosis: Where the cell takes in particles and may fuse with a lysosome for digestion

Exocytosis

• Movement of materials out of the cell (vesicle fuses with membrane)

• Used in plants to export cell wall material

• Used in animals to secrete hormones, neurotransmitters, and digestive enzymes

Water Potential (ψw or psi)

• A measurement that combines the effects of solute concentration and pressure

  • Measured in units of pressure like bars or megapascals

• Determines the direction of movement of water during osmosis

• Water flows from regions of higher water potential to regions of lower water potential

  • Areas with low water potential (more negative #) tend to pull water and areas with high water potential tend to push water

• ψw = 0 for pure water at sea level and room temp

• Both pressure and solute concentration affect it

Pressure Potential (ψp)

• The physical pressure on a solution

  • If the cell is an animal cell (no cell wall) and in an open container, ψp = 0

  • Higher pressure potential from pressure against cell wall = higher pressure potential

Solute Potential (ψs)

• Proportional to the number of types of ions created

• Higher solute potential = More negative solute potential

  • The higher concentration, the more water will be pulled to it

i = Ionization constant

• For molecules that do ot break apart or do not ionize, i = 1

Ex. NaCl: i = 2, CaCl2: i = 3

C = concentration (will be given)

R = constant (0.0831 liter bars/mole K)

T = temp in Kelvin (degrees C + 273

• The solute potential will always be zero or a negative number