Cell membrane

• Also known as plasma membrane. • Function: Maintains homeostasis within the cell

by being selectively permeable (meaning that it will some things into the cell and keep others out)

Cell membrane continued -PHOSPHOLIPID

• Cell membranes are made primarily of a phospholipid bilayer

• Phospholipids have 2 main regions:• Head negative charge, hydrophilic. Heads point toward

the inside and outside of the cell (toward water)• 2 fatty acid Tails nonpolar, hydrophobic. Tails point to the

center of the membrane (away from water)

HYDROPHILIC

HYDROPHOBIC 2

Membranes form spontaneously– This can be demonstrated when a mixture of

phospholipids and water are shaken, the phospholipids organize into bilayers surrounding water-filled bubbles

• This formation of membrane enclosed collections of molecules was a critical step in the evolution of the first cell.

Water-filledbubble made ofphospholipids

Fluid Mosaic model• “mosaic” – surface made of small pieces

• Has diverse protein molecules embedded in a framework of phospholipids.

• “fluid” – most molecules can drift about in the membrane. • The double bonds in the unsaturated fatty acid tails of

many phospholipids produce kinks that prevent them from packing tightly together. (fluid as salad dressing)

• The steroid cholesterol wedged in the bilayer in animal cells helps stabilize the membrane in warm temps., and keeps the membrane fluid at lower temps.

Types of proteins within the cell membrane

• 1) Glycoproteins – involved in cell to cell recognition.

• Carbohydrates outside the surface of the cell membrane function as “id tags”.– Cells in an embryo can sort themselves into

tissue & organs– Immune system to recognize and reject

foreign cells (such as bacteria)– Form junctions between cells.

Types of proteins within the cell membrane

• 2) Enzymes - Many membrane proteins are enzymes that carry out reactions. Remember that enzymes speed up reactions by lowering activation energy.

Enzymes

Types of proteins within the cell membrane

• 3) Receptors – Proteins that receive chemical signals (called ligands) from other cells and cause a reaction to occur in the cell

• Has a shape that fits a specific messenger, such as a hormone. It can either turn on a process directly or initiate a signal transduction pathway (a series of steps to turn on a process).

Messenger molecule (ligand)

Receptor

Activatedmolecule

Types of proteins within the cell membrane

• 4) Transport proteins -• Molecules that are large, polar/ionic (hydrophilic) molecules must use

a transport protein to move across a cell because they cannot mix with the hydrophobic tails of the phospholipid bilayer – Small, nonpolar (hydrophobic) molecules can

move directly across the phospholipid bilayer

because they can interact with the nonpolar

(hydrophobic) tails

– These differences in solute movement is what

allows for selective permeability.

• 3 different types of Transport Proteins (all are specific to what they will move)

1. Channel – move polar/ionic molecules down their concentration gradient

2. Pump – move polar/ionic molecules against their concentration gradient

3. Carrier – bind to one (or a few) specific molecules and move then individually across the membrane; much slower than channel protein

Concentration and concentration gradient

• Concentration = amount of solute / amount of solvent• Concentration gradient = difference in concentration

across the cell membrane (intracellular vs. extracellular)

Extracellular Matrix (ECM) – Present only in animal cells, Helps hold together tissues, protects and supports the plasma membrane

EXTRACELLULAR FLUID

Microfilaments

Collagen fiber

Glycogen of glycoprotein

Glycoprotein

Plasmamembrane

Glycoproteincomplex with longpolysaccharide

CYTOPLASM

11

Passive Transport• Passive transport – cell performs no work to move

molecules into or out of the cell because they are moving DOWN their CONCENTRATION GRADIENT.– Small, nonpolar molecules move directly across the

plasma membrane• Remember that the tails of phospholipids are

nonpolar (hydrophobic) so other nonpolar (hydrophobic) things can move through here

• Example: In our lungs, oxygen enters red blood cells, and carbon dioxide passes out by passive transport.

– Polar molecules can also move by passive transport if they are moving down their concentration gradient, but they must have transport (channel or carrier) proteins to provide a pathway.

Passive Transport• Passive transport will continue until equilibrium is

reached. At this point, the amount of solute moving into and out of the cell will be equal.

• Once equilibrium is reached, there is still movement of particles, but no net change in concentration.

3 Types of Passive Transport• 1. Diffusion – the movement of small, nonpolar

molecules directly across the phospholipid bilayer down their concentration gradient. Diffusion also describes the tendency for particles of any kind to spread out evenly in an available space, moving from highly concentrated areas, to low concentrated areas.– Ex. Ink spreading out in water, Perfume (and other smells)

diffusing across a room

3 Types of Passive Transport - Diffusion continued

• Requires NO work• It is caused by the RANDOM THERMAL MOVEMENT of

molecules. • Molecules are constantly in motion (fast when they are hot

and slow when they are cold). When they are in areas of high concentration, they collide with other molecules and move in the opposite direction.

• Although movement is random, there is a net movement of particles from high to low concentration because in an area of low concentration molecules ARE NOT colliding and bouncing in the other direction as frequently. Once in equilibrium, they collide and move in opposite directions equally.

3 Types of Passive Transport

• 2. Facilitated diffusion – movement of a large, polar, or ionic molecule down its concentration gradient using a transport protein (because it can’t move across the phospholipid bilayer)– Facilitate means to help so facilitated diffusion is just like diffusion

except with the help of a transport protein

– Does NOT require energy because it is moving down its concentration gradient

• Substances that use facilitated diffusion:– Sugars– amino acids– Ions– water

Solutemolecule

Transportprotein

3 Types of Passive Transport• 3. Osmosis – diffusion of WATER across the

membrane using aquaporins (channel proteins for water because water is polar and thus can’t move across phospholipid bilayer).

• The net movement of water down its own concentration gradient which will always be in the opposite direction of diffusion.

Watermolecule

Solute molecule withcluster of water molecules

Net flow of water

3 Types of Passive Transport – Osmosis Continued

Osmosis operates in the opposite direction of diffusion because a high concentration of solute means a low concentration of water. This is because water adhesively bonds to the solute and is thus no longer free to move by itself.

Osmosis – Tonicity

• Solutions of various tonicities (ability of a solution to make a cell gain or lose water) can have three different effects on plant & animal cells.

• Isotonic solutions: – (iso – the same) (tonos – tension)

• The solute concentration in the external environment is equal to that of the cell.– The cell’s volume remains constant. It gains

water at the same rate that it loses water. • Plasma that transports red blood cells.

• Intravenous fluid administered in hospitals.

• Marine animals are isotonic to seawater.

Osmosis – Tonicity continued

• Hypotonic solution:– (hypo – below)

• The solute concentration in the

external environment is below

that of the cell.• The cell gains water, swells,

and may burst (lyse). – The cell’s volume increases. It gains water faster than

it loses water.

Osmosis – Tonicity continued

• Hypertonic solution:– (hyper – above)

• The solute concentration in

the external environment is

above that of the cell.• The cell loses water, shrivels,

and can die from water loss.

This is referred to as plasmolysis. In plants the cell membrane pulls away from the cell wall. – The cell’s volume decreases. It loses water faster that it gains

water.

Osmosis – Tonicity Continued• Water balance differs slightly for plant cells vs. animal cells.

• Animal cells prefer isotonic environments.

• Plant cells prefer hypotonic environments.

– The cell wall of plants exerts pressure on the cell, preventing it from taking in too much water and bursting.

Isotonic solution

(B) Lysed (C) Shriveled

(D) Flaccid (E) Turgid (F) Shriveled

Hypertonic solutionHypotonic solution

Plantcell

Animalcell

(A) Normal

Plasmamembrane

(plasmolyzed)

Active Transport• A cell expends energy to move a solute against

its concentration gradient – that is toward the side where there is more solute.– Transport proteins are used to pump solutes against

their concentration gradient (from low to high). ATP provides the energy to do this.

– This is done to build up the concentration gradient (and potential energy) to be used later. In other words, you put in a little bit of energy now to get back a lot of energy later.

Steps for Active Transport1. Solute on the inside of the cell binds to an

active site on a transport protein.

2. ATP then transfers one of its phosphate groups to the transport protein (this is called phosphorylating the pump).

Steps for active transport continued

3. Causing the protein to change shape, so that the solute is released on the other side of the membrane.

4. Then the phosphate group detaches, and the transport protein returns to its original shape.

Example of Active Transport• Sodium-Potassium pump: a transport protein that helps

generate nerve signals.• Creates a higher concentration of K+ and a lower

concentration of Na+ inside the cell. • The transport protein constantly shuttles the K+ into the

cell, and the Na+ out of the cell.– Because you maintain this large concentration

gradient across your nerve cells, there is lots of potential energy because they want to return to equilibrium. When you send a nerve signal, you open up ion channels that allow NA+ and K+ to rush back through them. This is why you can send nerve signals so quickly.

Na+/K+ pumps

Cytoplasmic Na+ bonds tothe sodium-potassium pump

CYTOPLASMNa+

[Na+] low[K+] high

Na+

Na+

EXTRACELLULARFLUID

[Na+] high[K+] low

Na+

Na+

Na+

ATP

ADP

P

Na+ binding stimulatesphosphorylation by ATP.

Na+

Na+

Na+

K+

Phosphorylation causesthe protein to change itsconformation, expelling Na+

to the outside.

P

Extracellular K+ bindsto the protein, triggeringrelease of the phosphategroup.

PP

Loss of the phosphaterestores the protein’soriginal conformation.

K+ is released and Na+

sites are receptive again;the cycle repeats.

K+

K+

K+

K+

K+

28

Types of Cellular Transport

• PASSIVE

• Does not require

energy.

• Goes with the concentration gradient (high to low).

• Diffusion, Facilitated Diffusion, Osmosis

• ACTIVE

• Requires energy from ATP.

• Goes against the concentration gradient (low to high).

• Active Transport

Diffusion

Requires no energy

Passive transport

Higher solute concentration

Facilitateddiffusion

OsmosisHigher water

concentration

Higher soluteconcentration

Requires energy

Active transport

Solute

Water

Lower soluteconcentration

Lower waterconcentration

Lower soluteconcentration

Vesicular transport (**Add to notes: Also called bulk transport) - Movement

of MACROMOLECULES

Some molecules are too large to move across even with a transport protein. Movement of these macromolecules can be achieved by using vesicles in the following 2 methods:

1.Exocytosis – removal (or exit) of macromolecules from cell

2.Endocytosis – entry of macromolecules into cell

a) Phagocytosis

b) Pinocytosis

c) Receptor-mediated endocytosis

1. Exocytosis• Exocytosis – (exo – outside) export bulky materials

such as proteins or polysaccharides.

A transport vesicle filled with macromolecules buds from the Golgi body.

1. Moves to the cell membrane.

2. Vesicle fuses with the cell membrane.

3. Vesicle contents spill out of the cell.

4. Vesicle membrane becomes part of the cell membrane.

2. Endocytosis

• Endocytosis – (endo – inside) a cell takes in substances.

1. A depression forms in the cell membrane.2. Material outside the cell sits within this

depression.3. The depression pinches in and forms a

vesicle (containing materials).

3 types of endocytosis

• Phagocytosis – “cellular eating” A cell engulfs a particle by wrapping extensions called pseudopodia around it and packaging it within a vacuole.

•Vacuole then fuses with a lysosome, which digests the contents.

3 types of endocytosis

• Pinocytosis – “cellular drinking” A cell gulps droplets of fluid into tiny vesicles.

3 types of endocytosis

• Receptor – mediated endocytosis – 1. Receptor proteins for specific molecules are

embedded in cell membrane.

2. These receptors have picked up particular molecules.

3. Then the cell membrane pinches off to form vesicle containing receptors and their attached molecules.

• Used to take in cholesterol

(LDL) from the blood.