Cell Transport

Plasma Membrane

  • Composed of phospholipid bi-layer

    • hydrophobic tails facing inward and hydrophilic heads facing outward

  • Fluid - Mosaic Model

    • Lots of different molecules which move around

  • Two types of proteins

    • Integral Proteins - embedded/goes through entire membrane

    • Peripheral Proteins - through one side, doesn’t poke through other

      • includes:

        • Glycolipids

        • Glycoproteins - Celluar ID

          • MHC - Major Histocompatibility Complex

        • Cholesterol - adds fluidity and flexibility


Functions of Membrane structure

  1. Channel Proteins - small, uncharged tunnels for substances to move

  2. Receptor Proteins - Binds to substances in enviornment and triggers cell responses

  3. Carrier Proteins - Transport specific substances. Ex - glucose protein carrier

    1. used in facilitated transport

  4. Enyzmes - Breaks down molecules and glues molecules together

    1. involved in ATP metabolism



  • Carbohydrate Chains - forms a “sugar coat”

    • - glycocalyx - layer of cell ID

  • diversity of these chains makes “fingerprints”

    • chains aid in

      • recognitition of self vs non-self

      • immune responses

      • protect from outer surface



Plasma Membrane Permeability

  • Membrane is Selectively Permeable - not all molecules can cross membrane

  • Concentration - the amount of a specific substance dissolved in a given volume of fluid

  • High Concentration - lots of particles

  • Gradient - area available with low concentration and high concentration


  • Macromolecules and charged molecules - NEED PERMISSION to move within cell

  • Noncharged molecules and water - are lipid soluble and can move by themselves


Passive Transport - molecules move from HIGH to LOW

  • HIGH —→ LOW CONCENTRATION

  • Moving down the gradient

  • Kinetic Energy(energy of motion) - drives passive mechanisms

  • Associated with diffusion, facilitated diffusion(carrier mediated)

  • Facilitated transport is a type of passive transport monitered by a carrier protein


Diffusion - molecules move from HIGH to LOW

  • Driven by random kinetic energy

  • any molecule but H2O

  • lipid soluble - the ability to dissolve through the lipid (fat) portion of a membrane

  • lipid solid molecules, gases, and some small water soluble molecules

    • - can diffuse freely across cell membrane


  • The goal of diffusion is to reach equilibrium

  • Equilibrium - concentration of a substance is equal on both sides of a semipermeable membrane


Solute - the minor component in a solution, dissolved in the solvent

/ - Example: sugar

Solvent - a substance that dissolves a solute, resulting in a solution

/ - Example: water, tea


3 types of solutions:

  • Hypertonic - more solute

    • comparative

  • Hypotonic - less solute

    • comparative

  • Isotonic - equal amount of solute

    • no net change in cell size

    • water still moving (one molecule goes in —→ one comes out)


  • water always moves toward the hypertonic solution


  • Osmosis - form on diffusion focused on water across membrane

    • Osmotic Pressure - force that causes water to move in a direction

      • occurs due to number of non diffusable particles in a solution


  • Isotonic: no change

  • Hypertonic: if water leaves cell - cell shrivels/crenates

  • Hypotonic: water joins - cell swells/bursts

    • known as Turgor Pressure - plant cell gains influx of water but cannot burst due to cell wall

    • Plasmolysis - plant cell becomes dehydrated

Active Transport - molecules move from LOW to HIGH concentration

  • LOW ——> HIGH CONCENTRATION

    • AGAINST CELL GRADIENT

  • 3 Requirements:

    1. Availible area of concentration gradient

    2. ATP

    3. “pump” / carrier protein


Exocytosis - transports cell products and waste OUT of cell

  • - when a cell releases its contents by fusing a vesicle with the cell's outer membrane, and then the vesicle membrane becomes part of the outer layer

  • Vesicle mediated


Endocytosis - transports substances INTO of cell

  • - when the cell membrane folds in to surround a substance, then pinches off to form a vesicle that brings the substance into the cell

  • Vesicle Mediated

  • 3 types:


    • Phagocytosis - material taken in is large

      • Example - bacteria or cell debris

      • “cell eating”

      • entire cells possible

    • Pinocytosis - involves small molecules

      • Example - cell taking in glucose

      • “cell drinking”

        • Receptor Mediated - specific type of pinocytosis which occurs in response to receptor stimulation

    • ONLY difference between pinocytosis and phagocytosis is size of particle