The Cell Membrane

How permeable is the membrane?

Semi-permeable

• Allows some substances to cross more easily than others

Fluid Mosaic Model

Fluid: membrane held together by weak interactions

Mosaic: phospholipids, proteins, carbs

What do phospholipids make up?

The bilayer of the cell membrane

Amphipathic

• In phospholipids

Hydrophilic head, hydrophobic tail

What does the hydrophobic barrier do?

Keeps hydrophilic molecules out

What are the types of membrane proteins?

Integral and Peripheral Proteins

Integral Proteins

Embedded in membrane

• Determined by freeze fracture

• Transmembrane with hydrophilic heads/tails and hydrophobic middles

Peripheral Proteins

Extracellular or cytoplasmic sides of membrane

• NOT embedded

• Held in place by the cytoskeleton or ECM

• Provides stronger framework

Carbohydrates (function in cell membrane)

• Example?

Cell-cell recognition, developing organisms

• Glycolipids, glycoproteins

  • eg. blood transfusions are type-specific

What does cholesterol do in the cell membrane?

Keeps membranes fluid and stable

What types of molecules can pass through passive transport?

Small, nonpolar molecules

Passive Transport

No energy (ATP) needed

• Diffusion by concentration gradient (high to low)

What are the types of passive transport?

Simple Diffusion, Facilitated Diffusion

Simple Diffusion

• Examples

Molecules diffuse right across the phospholipid bilayer

• CO₂, O₂, N₂

Facilitated Diffusion

• Examples

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

• Provide hydrophilic channel

• Loosely bind/carry molecule across

Eg. ions, polar molecules (H2O, glucose)

What does water use to get across the membrane?

Osmosis and Aquaporins (proteins)

Osmosis

Diffusion of H₂O across a semi-permeable membrane

Tonicity

The TOTAL dissolved solutes in a solution relative to another solution across a nearby membrane

Hypertonic solution

HIGHER solute concentration

• relative to another solution across nearby membrane

Hypotonic Solution

LOWER solute concentration

• relative to another solution across nearby membrane

Isotonic Solution

Same dissolved solute concentration

• relative to another solution across nearby membrane

Why are ions and large polar molecules restricted from passing the membrane?

• What do they do instead?

Their hydrophobic cores prevent movement

• Use embedded channels and transport proteins

Can any protein channel be used for any molecule?

No, they are specific to certain molecules

What is needed by the cell?

• Cellular Respiration

Energy (glucose → ATP)

Oxygen

What is released and rid of by the cell

• Cellular Respiration

Carbon dioxide

When a solution is at equilibrium, do they molecules move?

• How?

They do move, back and forth

• At an equal rate

How to phospholipids move in the cell membrane?

Laterally

• Always moving

What do collagen fibers make up?

The extracellular matrix

What is the purpose of glycoproteins?

Identification

What is the journey of glycoproteins and glycolipids?

They are synthesized in the ER, modified and packaged in the Golgi, and sent out in vesicles

• They then attach to the cell membrane where they do their job of identification (receptor)

What happens to a plant cell in a hypotonic solution?

• What is this called?

Water rushes in, making the cell swell/expand

• Turgid (normal)

What happens to an animal cell in a hypotonic solution?

• What is this called?

Water rushes in, making the cell swell/expand, eventually bursting

• Lysed

What happens to a plant cell in a hypertonic solution?

• What is this called?

Water rushes out, shrinking the cell and making the membrane fall away from the cell wall

• Plasmolyzed

What happens to an animal cell in a hypertonic solution?

• What is this called?

Water rushes out, shrinking the cell

• Shriveled

What happens to a plant cell in an isotonic solution?

• What is this called?

The water flowing in and out occurs at a constant rate; equilibrium

• Flaccid

What happens to an animal cell in an isotonic solution?

• What is this called?

The water flowing in and out occurs at a constant rate; equillibirum

• Normal

Why does water flow across membranes so fast?

Aquaporin proteins allow for the transfer of water extremely quickly, leading to the back-and-forth transfer of water across membranes

Active Transport

Requires ENERGY (ATP)

• Proteins transport substances against concentration gradient (low to high conc.)

Electrogenic Pumps

Generate voltage across membrane

What are the types of electrogenic pumps?

Na+/K+ Pump (sodium potassium) and Proton Pump

Na+/K+ Pump

• Where is it used?

Pump Na⁺ out, K⁺ into cell

• Used specifically in nerve cells for nerve transmission

Proton Pump

• Example?

Push protons (H⁺) across membrane

• Eg. mitochondria (ATP production)

How does a Na+/K+ Pump work?

The outside of the nerve cell is slightly more positive while the inside is slightly more negative.

• When a stimulus is picked up, that concentration of sodium ions flips, sending a signal.

  • pos become neg and vice-versa

  • Sodium rushes into cell

  • Also changes the electrical charge

What is the connection and transfer of sodium and potassium in a Na+/K+ Pump

• Sodium (3) bonds to the protein from inside cell

• ATP is added (phosphorylation) → ATP becomes ADP

• Protein changes shape and pushes sodium out of cell

• New protein shape can bind with potassium

• The potassium (2) bond results in the release of phosphate group (dephosphorylation); protein becomes original shape

• Potassium is released into the cell

• REPEAT

How is it determined whether sodium or potassium leaves or enters the cell in the Na+/K+ Pump

Sodium or potassium can be higher on inside or outside; depending on scenario

• Higher concentration inside = moves outside

  • Vice-versa

  • Against concentration gradient

    • Low to High concentration

Cotransport

• Example?

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

• Sucrose-H⁺ cotransporter (sugar-loading in plants)

What does the production of a concentration gradient by the cell through active transport create?

An ELECTROCHEMICAL GRADIENT (both electrical charge difference AND chemical concentration difference) across the membrane for the potential to do work!

What are the charges on sodium and potassium?

positive

What is electrical potential of a cell?

The difference between the electrical voltage inside and outside the cell

Is the resting potential of a cell more positive or negative?

negative

Osmoregulation

• What does it do?

• Example?

Control solute & water balance

• Eg. paramecium caudatum – freshwater protist

Contractile vacuole

“bilge pump” forces out fresh water as it enters by osmosis

• Used for osmoregulation

Bulk Transport

• Types?

Transport of proteins, polysaccharides, large molecules

• Endocytosis, Exocytosis

Endocytosis

• Types

Takes in macromolecules and particulate matter, forms new vesicles from plasma membrane

• Phagocytosis, Pinocytosis, Receptor-Mediated Endocytosis

Exocytosis

Vesicles fuse with plasma membrane, secrete contents out of cells

Phagocytosis

“cellular eating”

• Solids

Pinocytosis

“cellular drinking”

• Fluids

Receptor-Mediated Endocytosis

• Examples?

Ligands bind to specific receptors on cell surface

• Ex: Growth hormones, LDL’s attached to cholesterols

What does hypertonic mean in terms of solute?

Higher solute concentraion

Water Potential

Free energy of water

• Potential of water to do work

What is tonicity always in terms of?

Solute, hypertonic

• Never in terms of water

How does water flow?

From a higher concentration or water potential, to a lower concentration/water potential

Water potential formula

• What does each part represent?

ψ = ψ₋ + ψₙ

₋ = s = solute potential

ₙ = p = pressure potential

Solute Potential

Solute concentration, always negative

• More solute, more negative

Why does a plant cell not burst in a hypotonic solution?

The cell wall exerts pressure that counteracts the water, keeping it from bursting.

What is the relationship between solute addition in solute potential and water potential

The addition of solute both lowers the solute potential and the water potential

• And vice-versa

Pressure Potential

Physical pressure

• Turgor pressure in plants

• The force acting upon the water

Tank A = 5% solute

Tank B = 30% solute

• Which tank has a lower water potential?

Tank B

Tank A = 5% solute

Tank B = 30% solute

• Which tank has a lower solute potential?

Tank B

Tank A = 5% solute

Tank B = 30% solute

• In which direction will osmosis occur?

  • The tanks are connected.

A → B

Tank A = 5% solute

Tank B = 30% solute

• If tank 1 has a water potential of -2000 kPa, and tank 2 has -1000 kPa, which tank has the higher water potential?

Tank 2

What happens to water potential during transpiration?

Moving up the roots of a tree to their air (transpiration occurs here), the water potential decreases

• Allows water to move up the tree