Transport Across Cell Membranes

Cell Membrane Structure• The cell membrane functions to: • form a barrier between the internal and external environment of

the cell.• Regulate what is allowed to enter and exit the cells internal

environment. • Provides protection and support.

Fluid mosaic model• The fluid mosaic model was developed to explain how the cell

membrane is structured. • Phospholipid bilayer with proteins floating in and on it.

Proteins in the cell membrane• Water is able to travel through pores in proteins found in the

cell membrane• Proteins have polar and non-polar regions, this way they can

span the membrane• There are both peripheral (on the outside) proteins and

integral (part of the membrane) proteins

• Some proteins within the membrane have carbohydrates attached. These are called glycoproteins• Lipids in the bilayer can

also have carbohydrates attached. These are called glycolipids.• Carbohydrate chains

only occur on the outside of cells and function to identify the cell

• Proteins perform several functions in the cell membrane:• Channel proteins: allow a specific molecule to cross the

membrane freely• Carrier protein: combine with a specific molecule to move it

across the membrane• Recognition proteins: allow the cell to be recognized by other

cells (glycoproteins) • Ex. Tissue rejection in organ donation

• Receptor protein: have a special shape to be able to bind with other molecules

• Enzymatic proteins: carry out metabolic reactions

Selective Permeability • Permeable- the ability to pass through a membrane• The cell membrane is a selectively permeable membrane.• This means that it allows some things through and prevents

others from entering or exiting.

• http://www.youtube.com/watch?v=Pt4Ch-YW-xs

• Transport across a membrane is classified as either passive or active

• Why would cells be selectively permeable?

• Solution- a liquid with one or more substances dissolved in it• Solvent- the liquid that the solute is dissolved in• Solute- the substance dissolved in a solution• Concentration- how strong it is… the

solute/volume (percentage)

Jigsaw Activity

• Form groups of 3 • Each member of the group find information on

one type of passive transport (p.71-74)• 1. Diffusion• 2. Osmosis (ignore “osmosis in cells” section)• 3. Facilitated transport

Transport through the Membrane

• Simple Diffusion

• Facilitated Diffusion

• Active Transport

Diffusion• Particles moving from an area of greater concentration toward

an area of lesser concentration until it is equally distributed• Diffusion refers to the process by which the molecules of two

different substances intermingle (mix) as a result of their kinetic energy of random motion.

• Does not use a transport protein• Moves with the concentration gradient (high to low)• Does not require additional cell energy• Passive• Examples:• Water• Carbon dioxide• oxygen

• The rate of diffusion can be increased by:• Increased temperature• Increased surface area• Change in shape of molecules• Change in concentration

• (Why for each?)

Osmosis

• The net movement of water molecules from the area of greater concentration of water to the area of lesser concentration of water until it is evenly distributed• Must be across a selectively permeable membrane• Water passes through pores in membrane proteins• Osmotic pressure can work against physical pressure

Review• https://www.youtube.com/watch?v=JShwXBWGMyY

Facilitated Transport• An assisted form of diffusion• Moves molecules that would not be able to cross the

membrane alone• Moves toward the concentration gradient• Requires carrier proteins• No energy is needed

Tonicity

• Water molecules travel freely through protein channels to try to maintain an environmental solute concentration.

• Osmotic pressure is a measure of the number of collisions water molecules make with the cell membrane surface.

• If there is a high concentration difference between the inside and outside of the cell there will be a high osmotic pressure… more collisions as water tries to get across the membrane

• Turgor pressure is the force the cytoplasm exerts against the inside of the cell membrane.

• Large and prolonged changes in turgidity can have effects on cell function.

• What the solution concentration outside the cell looks like has effect on the cells turgor pressure.

Isotonic• If the cell is placed in a solution that

has equal solute concentration to inside the cell the solution is called isotonic

• The cell will not shrink or swell because the osmotic pressure on each side of the cell membrane is equal

• The concentrations of solution on both sides of the membrane are equal

Hypertonic• When a cell is exposed to an environment with a

greater solute concentration, water will leave the cell to try to bring the inside and the outside solutions into equilibrium.

• The cell will shrivel as water leaves and the solute inside the cell will concentrate so that it is more similar to the concentration outside the cell.

• The outside solution is called hypertonic (hyper=high). It has a higher concentration of solute than the cytoplasm.

• The shrinking of a cell in a hypertonic solution is called plasmolysis. If it is a red blood cell it is called crenation.

Hypotonic• When a cell is placed in a solution that has a

lower concentration of solute compared to the cell cytoplasm it is called hypotonic (hypo=low)

• Water moves from outside the cell to inside the cell to try to lower the solute concentration inside the cell

• The cell swells and may even burst if the osmotic pressure is great enough.

• When a red blood cell bursts it is called hemolysis.

Practice Question• Which set of conditions would cause the fluid levels to change

as shown after 10 minutes?

Solution in Side A Solution in Side B

Distilled water 5% salt

2% glucose 5% glucose

5% protein 2% protein

2% salt 2% salt

Practice Question• Is the solution hypertonic, isotonic, or hypotonic?• Which way will water move?• What will happen to the cell?

Active Transport

• Ions or molecules move across the membrane and collect either inside or outside the cell

• works in the opposite direction as diffusion from low concentration to high concentration.

• Requires carrier proteins and energy to move molecules across the membrane

• Active transport uses carrier proteins like one form of facilitated diffusion.

• Energy (in the form of ATP) is required for the protein to bind with the molecule to move it across the membrane.

• Active transport is used by cells of the thyroid gland to collect iodine from the body. Iodine moves against the concentration gradient toward the more concentrated area inside the cell.

• Up to 40% of a cells energy supply can be used up to perform active transport. Cells that primarily gather certain molecules by active transport have lots of mitochondria close to their cell membrane to produce energy.

• Proteins involved in active transport are often called pumps

Sodium-Potassium pump• All animal cells, especially nervous cells, contain sodium-

potassium pumps.

• This protein moves sodium to the outside of the cell and potassium into the cell.

• Both molecules are moved by the same protein: the sodium-potassium pump.

• https://www.youtube.com/watch?v=GTHWig1vOnY

• The attachment and detachment of a phosphate group from ATP causes a shape change in the protein that allows it to bind sodium

NaCl• Salt is very important in the functions of cells.

• Sodium is pumped across the membrane by carrier proteins

• Chloride ions flow across the membrane through protein channels because they are attracted by the positively charged sodium.

Exocytosis and Endocytosis• The cell needs a way to get very large or macromolecules

across the cell membrane and these do not fit through cell membrane proteins.

• These large molecules are transported by vesicle formation.

• Vesicle formation requires energy, so this is a form of Active Transport.

Exocytosis• Is a process of the cell to get molecules out of the cell

(exo=exit)

• Vesicles are like small packages of molecules that need to be released encased in cell membrane

• The contents are released to outside the cell when the vesicle membrane fuses with the cell membrane.

• Vesicles are often formed by the Golgi apparatus and contain proteins that the cell needs in it’s membrane or is getting out of the cell.

• Vesicles are used by the cell as a way to get membrane bound proteins from Golgi apparatus membrane to the cell membrane surface where they adhere to the surface and become part of the membrane.

• Some cells are specialized to produce molecules and excrete them out of the cell

• Ex. Pancreatic cells excrete digestive enzymes.

• These cells wait for a signal from outside the cell before they exocytosis takes place.

Endocytosis• Endocytosis is the process cells use to get macromolecules

from outside the cell membrane to inside the cell membrane

• The cell membrane Pushes in toward the cellIn one are

• The macromolecule is Surrounded by the cell Membrane and the Membrane pinches off Forming a vesicle inside the cell.

Phagocytosis• When endocytosis is used to take

in a large object like a food particle or cell the process is called phagocytosis and the vesicle formed is called a vacuole.

• Single cell organisms like amoebas often perform phagocytosis to obtain food.

• Some human white blood cells also do phagocytosis to engulf objects in the body that they see as foreign

Pinocytosis • Pinocytosis is when vesicles form around a liquid or very small particles.

• Blood cells, kidney cells, and intestinal wall cells perform pinocytosis constantly

• To balance out the cell membrane lost by endocytosis, there is exocytosis also happening.

Receptor-Mediated Endocytosis• This is a form of very specific pinocytosis. • The cell wall is lined with receptors in one location called a

coated pit, that bind a specific molecule. When that molecule is bound the vesicle forms and is moved into the cell.