1

Membranes: Structure & Function

Biology 11A. Allen

Properties of the plasma membrane• Cells live in a fluid environment (i.e. blood, water, extra-

cellular fluid). Cells exchange materials with this fluid. • Each cell is a separate unit of life (remember cell theory?) It

therefore has a boundary – the plasma membrane (AKA cell membrane).

...Properties of the plasma membrane• The plasma membrane is the cell’s “gate keeper” It is

selectively permeable. It allows the passage of some materials in and out of the cell but not others. Ex a tea bag allows passage of H2O but not the leaves.

• The plasma membrane brings in the ‘good’ things and keeps them in, and it exports the ‘bad’ things and keeps them out.

...Properties of the plasma membraneWhat will/will not pass through the lipid bilayer?• Small particles pass through easily.• Lipids (and particles that are soluble in lipids) pass through

with least difficulty. The plasma membrane tends not to be permeable to polar molecules unless they are small.

• lipid:– an organic compound with Carbon, Hydrogen and Oxygen.– The # of hydrogen atoms are MUCH greater than the # of oxygen

atoms.– ex: fats, oils, waxes.

• Ions (atoms/molecules that have a charge) will NOT pass through easily.

• magnification of ~200 000X reveals the plasma membrane as having two layers.

• each of these layers are made of phospholipids, hence the term phospholipid bilayer.

Structure of the plasma membrane...Structure of the plasma membrane

• a phospholipid molecule is made of two fatty acid tails, glycerol and a phosphate group.

2

• Think Critically: Suppose you added some phospholipids to water. If you could see the individual molecules, how do you suppose they would be arranged? Draw a sketch.

...Structure of the plasma membrane

• phospholipid animation

• phosphate groups are polar and are therefore hydrophilic. They will point to the watery environments inside and outside the cell.

• the fatty acid tails are hydrophobic. They avoid water and seek ‘each other’

• The properties of fatty acid molecules are the reason for the plasma membrane’s structure. It forms spontaneously. No energy is required of the cell to maintain the integrity of the plasma membrane.

• The plasma membrane is often referred to as a “phospholipid bilayer”

...Structure of the plasma membrane Why do the phospholipids assume this orientation?

...Structure of the plasma membrane• polar molecules have charged ends (+ and -)

and are therefore soluble in water (another polar molecule) They are hydrophilic.

• non- polar molecules are insoluble in water. They are hydrophobic (avoid water).

• The plasma membrane is fluid-like. The phospholipids can move and slide past each other. (pool-full-of-balls analogy). In some cells, cholesterol molecules bind to the phospholipids to help restrict their movement.

...Structure of the plasma membrane

3

Think Critically!• An amoeba relies on

phagocytosis to ingest food. Infer what levels of cholesterol might be in it’s plasma membrane.

...Structure of the plasma membrane

Membrane Proteins• the plasma membrane is embedded with proteins. They

have polar and non-polar ends to determine their position in the bilayer. They can move sideways throughout the plasma membrane. The roles of these proteins include: regulation of the movement of particles across membrane; involvement in chemical reactions; act as ‘marker’ on the cell.

• Biologists now use the fluid mosaic model to describe the plasma membrane. Why is this term appropriate?

THINK Critically!

• Draw two sketches of a plasma membranes with embedded membrane proteins. Label the parts of the proteins that are hydrophillic & hydrophobic– Drawing # 1 : protein goes through entire membrane

– Drawing # 2 : protein does not pass through to inside of membrane

Brownian Motionof a microscopic particle

start

end

http://www.doc.ic.ac.uk/~nd/surprise_95/journal/vol4/ykl/report.html

• Brownian motion- The rapid and chaotic motion of particles suspended in a fluid at rest as a consequence of fluctuations in the rate at which fluid molecules collide with the particles.

• The diagram traces the movement of a microscopic particle. Account for the motion.

4

Think!• Over time, the sugar molecules will spread

throughout the water. What effect will heat have on the rate of diffusion?

• diffusion animation

Diffusion through a Plasma Membrane

• Some substances can move freely through a plasma membrane (the membrane is permeable to these substances). i.e. small, non-polar, non-ionic, lipid soluble.

• Diffusion always happens with the concentration gradient (high to low concentration)

Osmosis• Osmosis diffusion of water across a semi--permeable

membrane.– occurs from a region of higher concentration of water to a region

of low concentration of water.– Water molecules are small and can diffuse through cell

membranes. The direction of the diffusion depends only on the difference in concentration of water on each side of the membrane.

EFFECTS OF OSMOSIS• Movement of water into or out of the cell depends on the

concentration gradient. Cells will react differently to different concentrations of water. A cell’s environment is either a isotonic, hypotonic, or hypertonic solution depending on the relative amounts of solute inside and outside the cell….

How osmosis_works

Types of SolutionsIsotonic solution: ‘iso’ means ‘same’. • this solution has the same concentration of dissolved

substances (solute) in the water as a living cell placed in it. Therefore there is no concentration gradientand the cell does not have a net gain, or loss of water.

Hypotonic solution: ‘hypo’ means ‘less’• this solution contains a lower concentration of dissolved

substances (solute) than that of a living cell. Therefore the concentration of water molecules outside the cell is higher than that inside the cell, creating a concentration gradient. This results in a movement of water into the cell (osmosis).

…Types of Solutions• Hypertonic solution: ‘hyper’ means ‘more’• this solution contains a higher concentration of

dissolved substances than that of the living cell. Therefore the concentration of water molecules inside the cell is higher than that outside the cell, creating a concentration gradient. This results in a movement of water out of the cell.

…Types of Solutions

5

THINK!

• Suppose you place animal cells in isotonic, hypotonic and hypertonicsolutions. Describe what might happen to the cells.

• Predict what would happen to plant cellsin isotonic, hypotonic and hypertonic solutions.

• animal cell placed in hypotonic solution swells and bursts due to osmotic pressure.

• A plant cell placed a hypotonic solution the large vacuole (which stores water) swells to fill the interior of the cell, pushing the cell contents against the cell wall. The resulting pressure in the plant cell is known as turgor pressure.

• An animal cell placed in a hypertonic solution will shrink.

• In a plant cell placed in a hypertonic solution, the vacuole collapses, causing the cytoplasm to shrink. This shrinking of cytoplasm by osmosis is called plasmolysis.

…Types of Solutions

Think!Identify which cells are in hyper/hypo/iso tonic solutions

Osmotic Pressure• Osmotic Pressure is the increased pressure resulting

from osmosis. The rate of diffusion of water depends on concentration and pressure.

• Water will diffuse from an area of high pressure to an area of low pressure. As the pressure increases inside the membrane, the water will diffuse back across the membrane at a faster rate than before.

• Equilibrium is reached when:– water is still passing into the membrane because of the

concentration level but…– water also passes out at the same rate because of the

pressure gradient in the opposite direction.

THINK!

• Suppose an animal cell was placed in a hypotonic solution:– What will happen if the maximum force the

plasma membrane can withhold is less than osmotic pressure?

– What will happen if the maximum force the plasma membrane can withhold is greater than osmotic pressure?

Osmotic Pressure

Suppose the membrane is impermeable to the solute.

What will happen to the water levels over a period of time?

osmosis animation

6

Passive TransportPassive transport is the movement of materials across the cell membrane without requiring energy from the cell. Materials move in the direction of the concentration gradient (high concentration to low concentration)

Types of Passive transport: diffusion, facilitated diffusion, osmosis

FACILITATED DIFFUSION• The cell membrane contains proteins which aid in the passage of certain

particles across the membrane. These proteins are known as transport proteins.

• Certain particles that a cell needs are either too large or are not soluble in lipids to diffuse easily across the cell membrane.

• The transport proteins aid or facilitate these particles as they move from an area of high concentration to an area of low concentration. Transport proteins are selective because each one has a distinctive shape and charge to allow only one kind of particle to enter or leave the cell.

TYPES OF TRANSPORT PROTEINS USED IN FACILLITAED DIFFUSION:

1. CHANNEL PROTEINS: Also called pores. Water-filled passages through which small dissolved particles, such as ions, can diffuse. Ions pass through different pores depending on size and charge.

Animation: channel protein

2. CARRIER PROTEINS: Change shape to allow different molecules to cross the plasma membrane.

animation: facilitated diffusion (carrier protein)

3. GATE PROTEINS : Certain chemicals combine with the transport protein to signal it to “open”. Glucose diffuses into a cell this way.

animation: ligand-gated ion channel

…Passive Transport

ligand

• Scanning electron micrograph of hair cells from the bullfrog inner ear, which contain the mechanically-gated ION CHANNEL TRPA1.

• Sound waves bending the cilia-like projections on the hair cells of the inner ear open up ion channels leading to the creation of nerve impulses that the brain interprets as sound

ACTIVE TRANSPORT• In active transport, cellular energy

is used to change the shape of a carrier protein so that required particles move from an area of low concentration to an area of high concentration. Unlike diffusion, particles go against the concentration gradient. It is analogous to rowing a boat upstream against the current (concentration gradient).

Molecular Active Transport

• The cell uses its own energy to move materials from low to high concentration across a membrane (against the concentration gradient)

animation-proton pump

• Examples– Plants move minerals (ions) into their roots by active transport – Cells in the vertebrate kidney reabsorb sodium ions from urine– The thyroid gland cells bring in iodine for use in producing

hormones

EndocytosisEndocytosis is the process by which the plasma membrane of the cell surrounds the material, engulfs it and takes in the materials from the environment.

THERE ARE THREE TYPES OF ENDOCYTOSIS:

1. PHAGOCYTOSIS– The transport of large particles, whole cells, or solids. – common in unicellular organisms

2. PINOCYTOSIS– The transport of solutes or fluids.– ingesting liquid droplets

3. RECEPTOR-AIDED ENDOCYTOSIS– protein receptors (in membrane) hook up with a specific

molecule. Indentation in membrane forms and pinches off inside cell.

Animation: Receptor-mediated endocytosis

Animation: Phagocytosis

Endo and exocytosis

7

Receptor-Mediated Endocytosis:

• A faulty gene for the LDL receptor results in LDL not binding to the cells. The Cholesterol remains in the blood and becomes deposited on arteries. Reduced blood flow in arteries that supply the heart causes heart attacks in patients as early as 6years.

Exocytosis• Exocytosis• a reverse process of endocytosis to

get rid of wastes (indigestible substances) or secrete hormones.

• the substance is enclosed in a vesicle

• (surrounded by a membrane)• -fuses with cell membrane• -the fused membranes open• to let material outside cell.

Animation: Exocytosis