Ch 5 Membrane Structure and Cell Transport

Surface Area:Volume Ratio

• Cells must be small to maintain a large surface area to volume ratio (SA:V)

• Large S.A. allows ↑ rates of chemical exchange between cell and environment

Surface Area Example (Animal)

Small Intestine: highly folded surface to increase absorption of nutrients

• Villi: finger-like projections on SI wall

• Microvilli: projections on each cell

Ex. Surface Area & Maintaining Body Temperature

• Mouse: Body shape is as close as it can be to that of a sphere → minimum amount of surface area

  • Covered with fur to maintain heat

• Elephant: skin is covered with wrinkles, large ears → increases surface area

  • No fur because it’s trying to lose excess heat

Surface Area Example (Plant):

Root hairs: extensions of root epidermal cells; increase surface area for absorbing water and minerals

Cell Membrane

1. Plasma membrane is selectively permeable

   • Allows some substances to cross more easily than others

Fluid Mosaic Model

• Fluid: membrane held together by weak interactions

• Mosaic: phospholipids, proteins, carbs, cholesterol

Membrane Fluidity- Low temps

• phospholipids w/unsaturated tails (kinks prevent close packing)

• animal in cold= more fats

Membrane Fluidity- Cholesterol resists changes by:

• limit fluidity at high temps

• hinder close packing at low temps

• right fluidity, right viscosity

Membrane Fluidity- Adaptations

1. bacteria in hot springs have unusual lipids

2. winter wheat have → unsaturated phospholipids (to keep from freezing)

Membrane Protein- Integral Proteins

• Embedded in membrane (transmembrane)

• Transmembrane with hydrophilic heads/tails and hydrophobic middles

Membrane Protein- Peripheral Proteins

• Extracellular or cytoplasmic sides of membrane

• NOT embedded

• Held in place by the cytoskeleton or ECM

• Provides stronger framework

Carbohydrates

• Function → cell-cell recognition

• Glycolipids, glycoproteins

  • Ex. blood transfusions are type-specific (A, B and RH)

  • Used for immunity purposes

Passive Transport

• NO ENERGY (ATP) needed!

• Diffusion down concentration gradient ([high] → [low])

  • Ex. small hydrocarbons, CH₄, CO₂, O₂

Osmosis

diffusion of H2O

Hypotonic Solutions

• There is less solute outside the cell

• Is there more or less H₂O inside the cell?

• Water will move into the cell

  • Cell will swell 

  • Animal cell in distilled (pure) H₂O would burst

  • Cell walls prevent this

Hypertonic Solutions

• More solute outside the cell

• Water flows out & cell will shrivel

  • Ex. Plants wilt when in a hypertonic solution

Turgor pressure

• pressure inside a plant cell from H₂O pushing against cell wall

Plasmolysis

• when turgor pressure is lost – plants wilt

Water Potential (ψ)

• free energy of water

• H₂O moves from high ψ →  low ψ potential

• Water potential equation: 

  ψ = ψ_S + ψ_P

Pure water (open beaker):

• ψ_P = 0 Mpa (0 bars)

Plant cells

• ψ_P = 1 Mpa (10 bars)

Solute potential

• solute concentration (osmotic potential)

• ψ_S = -iCRT

• The addition of solute to water lowers the solute potential (more negative) and therefore decreases the water potential.

  
  

Facilitated Diffusion

(no ATP)

Transport proteins (channel or carrier proteins) help hydrophilic substances cross

two ways facilitated diffusion

• Two ways:

  1. Provide hydrophilic channel

  2. Loosely bind/carry molecule across

• Ex. ions, polar molecules   (H₂O, glucose, amino acids)

Aquaporin

channel protein that allows passage of H2O

glucose transport protein

carrier protein

• need insulin to open

Active Transport

• Requires ENERGY (ATP)

• Proteins transport substances against concentration gradient ([low] →  [high])

  • Ex. Na⁺/K⁺ pump or proton (H+) pump

Electrogenic Pumps

generate voltage across membrane, which has PE. like a water dam.

• Na+-/K+- Pump

• Proton Pump

Na+-/K+- Pump

• Pump Na⁺ out, K⁺ into cell

• Nerve transmission

Proton Pump

• Push protons (H⁺) across membrane

• Ex. mitochondria (ATP production)

Cotransport

membrane protein enables “downhill” diffusion of one solute to drive “uphill” transport of other

Ex. sucrose-H⁺ cotransporter (sugar-loading in plants)

Passive transport

• no Energy

• [High] →  [low] 

• DOWN the [gradient]

• Ex. diffusion, osmosis, facilitated diffusion (w/transport protein)

passive tranports

• simple diffusion (non-polar mlcs)

• facilitated diffusion

• transport protein

• osmosis

Active transport

• Requires Energy (ATP)

• [Low] → [high]

• AGAINST the [gradient]

• Ex. pumps, exo/endocytosis

Osmoregulation

• Control solute & water balance

• Contractile vacuole →  “bilge pump” forces out fresh water as it enters by osmosis

• Ex. Paramecium caudatum – freshwater protist

Bulk Transport

Transport of proteins, polysaccharides, large molecules

• Endocytosis: take in macromolecules, form new vesicles

• Exocytosis: vesicles fuse with cell membrane, expel contents

• (active)

types of endocytosis

• active

• Phagocytosis: “cellular eating”- solids

• Pinocytosis: “cellular drinking”- fluids