Transport Across Cell Membranes

CELL MEMBRANESI. Cell membranes

A. Function – barrier that separates inside of cell from the external environment

B. Cell membranes made mainly of:1. Phospholipids – form bilayer

a. Hydrophilic headsb. Hydrophobic fatty acid tails

2. Proteins a. Some have sugar groups bound to them – glycoproteins

3. Cholesterol (animal cells only)

C. Consistency similar to olive oil

MEMBRANE STRUCTURE

Membrane Permeability

D. Permeability - what is allowed to cross a barrier

1. Impermeable – nothing gets through

2. Permeable – anything gets through

3. Cell membranes are semipermeable -

a. Some molecules cross freely

b. Some must be transported across

Membrane Permeability

Membrane Permeability

a. Molecules that pass freely:1. Small uncharged molecules like glycerol, ethanol

2. Small hydrophobic molecules like oxygen, carbon dioxide

b. Molecules that must be transported:1. Water

2. Macromolecules (proteins, sugars, etc..) too large

3. Ions like H+, Na+, Cl-, Ca++

MEMBRANES• For cells to survive they must be able transport

materials (water, oxygen, food, waste products, ions) into and out of the cells.

II. Transport mechanisms used by cells include (handout):A. Passive mechanisms (require no energy expenditure)

1. Diffusion2. Osmosis3. Facilitated diffusion

B. Active transport (requires energy output)1. Active transport2. Endocytosis and exocytosis

DiffusionA. Passive mechanisms – fueled by

concentration gradient

- Difference in concentration across a given space

Diffusion1. Diffusion - movement of a substance from a place of

higher concentration to a place of lower concentrationa. Remember, all molecules in constant, random motion unless

at absolute zerob. If concentration gradient exists, there will be net movement

of substance in greater concentration until equilibrium (equal concentration ) is reached.

Diffusion

• Animation http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ce/m3/s2/index.htm

Diffusionc. Example – American pioneers

d. Rate of diffusion is related to kinetic energy and the size of the gradient

1) Increased kinetic energy…

2) Large concentration gradient…

e. Demonstration

Diffusion

f. Diffusion is one way materials are moved in and out of cells.

- Cytoplasm is mostly water containing dissolved solutes (salts)

Diffusion

Gas exchange in the lungs:1) Oxygen follows its

concentration gradient into the capillary

2) Carbon dioxide (waste) follows its concentration gradient into the lung, is exhaled

Diffusion

g. Diffusion works very well over short distances1) As size of cell increases, volume increases faster than

surface area

2) Cells remain relatively small because center of large cells would not get adequate gas exchange, would die.

3) Multicellular organisms made up of many small cells because they can be efficiently supplied by circulatory system, even if deep inside organism.

Osmosis

2. Osmosis - diffusion (movement) of water (only) across a semipermeable membrane

a. Water moves along its concentration gradient from an area of high water concentration (less solute) to an area of lower water concentration (more solute)

• Potato demonstration

Osmosis

a. Animation: http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ce/m3/s3/index.htm

b. Can generate significant osmotic pressure-Turgor pressure in plants

Osmosis

c. 3 possibilities for direction of water movement in living cells:

1) Hypotonic environment: greater concentration of water outside the cell than inside. Water enters, cell swells.

a) Animal cell bursts (lysis, rupture)

b) Plant cell – water pushes against cell wall, creating turgor pressure.

- Helps plants stand up against gravity

- Cell wall protects from bursting

Osmosis

2) Hypertonic environment: greater concentration of water inside cell. Water moves out, cell shrinks

a) Animal cell – shrivels up (crenation)

b) Plant cell – plasmolysis (low turgor pressure). Cell membrane draws away from cell wall, plant wilts.

Osmosis

3) Isotonic: solute and water concentration is the same inside and outside cell

a) At equilibrium, molecules continue to move across membranes evenly (but there is no net movement)

b) Cell size and volume do not change

Osmosis

http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ce/m3/s3/index.htm

Osmosis

4) How do organisms that live in water deal with osmosis?

a) Freshwater animals…?

b) Marine (saltwater) organisms…?

c) Plants…?

FACILITATED DIFFUSION

3. Facilitated diffusion - passive transport of specific substances down their concentration gradient by a carrier protein

a. Examples: water, glucose

ACTIVE TRANSPORT

B. Active transport - uses energy (ATP) and a membrane protein to move materials against their concentration gradient from an area of lower to higher concentration

1. Used to move ions (Na+, Ca++, Cl-, K+), amino acids, nucleotides across the cell membrane

Active Transport

• Animation - http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ce/m3/s5/index.htm

Membrane-Assisted TransportC. Membrane-assisted transport

uses energy to move large, complex molecules across the cell membrane

1. Large molecules like proteins, food, or fluid droplets are packaged in vesicles, then sent into or out of the cell

2. Exocytosis – large products removed from cell

a) Vesicle from inside cell fuses with cell membrane to secrete its contents

b) Examples: insulin, mucus

Membrane-Assisted Transport3. Endocytosis moves large particles into a cell

a) Phagocytosis – particles

1) Cell membrane extends out, surrounds the material & pinches off inside the cell making a vesicle2) Used by amoeba to feed & white blood cells to kill bacteria

b) Pinocytosis - Cell membrane surrounds fluid droplets

Membrane-Assisted Transport

Animation -http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ce/m3/s6/index.htm

Transport review

Membrane Transport

III. Real world examples:A. Fresh produce

B. Wrinkled fingers and toes when swimming

C. Lungs

D. Destruction of bacteria, viruses by immune system

Membrane Transport

Membrane Transport

E. Cystic fibrosis – remember symptoms? Cause?a. Genetic mutation changes membrane protein controlling

Cl- ion exchange

b. Cl- ions retained in cell, along with water

Membrane Transport

c. Mucus too thick, clogs lungs (infections), pancreas (trouble with digestion, poor growth), usually fatal by 20-40 years old

d. Treatment – antibiotics, physical therapy (vest), diet, enzymes, transplants

• Membrane-assisted transport activity

Homeostasis

• Homeostasis - maintaining a stable internal environment even when external conditions change – Homes – furnace, a/c – Cells – membrane transport – Organisms – have systems that help maintain

relatively constant conditions inside the organism

• Examples?

Homeostasis– Temperature (sweat when hot, shiver when cold)– pH (buffers)– Nutrients (maintain level blood sugar)– Blood pressure

– O2/CO2 levels (gas exchange, respiratory system)

– Concentration of waste products (urinary system)

Homeostasis

– Maintaining homeostasis is critical to survival of organisms!

– Look at 2 organ systems used by body to maintain homeostasis

• Respiratory system (gas exchange)• Excretory system (waste removal)

Respiration

I. IntroductionA. All organisms need constant supply of energy for…

B. Energy from food released through cellular respiration (in the form of ???)

1. Requires O2

2. Generates CO2.

C. Organisms need a steady supply of O2 and a way to get rid of CO2.

This is called respiration = breathing = gas exchange

RespirationII. Water-dwelling organisms

A. Breathing (movement of O2 and CO2) happens by diffusion across a membrane

B. Some O2 dissolved in water, but concentration low (<1%), so organisms need very efficient breathing mechanism

- Large surface area to volume ratio required for efficient diffusion of O2

RespirationC. Gills have very large surface area

1. Made of fine, threadlike filaments

2. Allow high rate of water flow and close contact between water and gill surface

Respiration

3. Constant flow of water over gills + huge surface area allows O2 to diffuse into extensive blood supply, while CO2 diffuses out along concentration gradients.

4. Blood then flows via circulatory system to all cells in organism

RespirationII. Land (terrestrial)

organismsA. Differences/challenges

1. Air has 25-50 times higher O2 concentration than water

2. But gases (O2 and CO2) must be dissolved in water for diffusion to occur

3. Therefore, gas exchange can occur only on moist surfaces

4. Land organisms must battle water loss by evaporation from moist respiratory surfaces.

RespirationB. Human respiratory system

1. To minimize water loss, many land animals internalize respiratory surface (lungs)

- Found in thoracic cavity with heart

- Air is filtered, warmed and humidified before entering lungs

Respiration

• www.youtube.com/watch?v=DoSTehS7iq8

Respiration

2. Structures and functionsa. Trachea (windpipe) –

tube reinforced with cartilage that carries air from pharynx to bronchi.

b. Bronchi – branching tubes that carry air into (and out of) the lungs

Respiration

c. Alveoli (folds or pockets) that maximize respiratory surface area (thus diffusion)

- Each lung has hundreds of million alveoli

- Total surface area of all alveoli in 1 person’s lungs would cover a tennis court!

- Respiratory surfaces very thin with many capillaries to maximize diffusion of O2 and CO2

Respirationd. Diaphragm contracts, rib cage expands to create larger volume and thus negative pressure in thoracic cavity. Air rushes in to fill space. As muscles relax, exhalation occurs.

Respiration

3. Respiratory system disorder – asthma

a. Airway walls become irritated, swollen and very sensitive

b. Exposure to triggers (allergens, smoke, perfume, exercise or illness) causes airways to react and become narrower.

c. Lungs get less air, causing wheezing, coughing, chest tightness and difficulty breathing

Respiration

d. Treatments (often inhaled medicines)

- Fast acting medicines that dilate the airways

- Longer acting medicines like steroids to stabilize airways, make them less reactive

Respiration

C. Plants1. Leaves covered with waxy cuticle to reduce water

loss from evaporation

2. Gases enter leaf through stomates on leaf surface

Respiration

- Stomates controlled by guard cells that are open when water abundant, closed when water level low.

Waste RemovalI. Introduction

A. Nitrogen wastes produced from protein and nucleic acid metabolism

1. Amino group (NH2) removed prior to complete catabolism

2. NH2 combines with a hydrogen ion (H+) to form ammonia (NH3). This is a toxic substance!

3. Many organisms convert ammonia to urea, which is less toxic

Waste Removal

B. Methods of waste removal1. Diffusion: Simple aquatic organisms (sponge)

excrete wastes through external surfaces

2. Excretory system: Larger, more complex organisms have specialized organs that:

a. Remove and concentrate wastes from body fluids

b. Return other substances to body fluids as necessary for homeostasis

c. Eliminate excretory products from the body

Waste Removal

C. Human excretory system consists of:

1. Kidneys – filter waste from blood and collect it as urine.

- Entire blood supply of human body (about 5.5 L) filtered once every 5 minutes!

2. Ureters – transport urine from kidney to bladder

3. Bladder – stores urine4. Urethra – conducts

urine out of body

Waste Removal

D. Kidney1. Nephron is kidney's functional unit

a. 1 million per kidney

b. Components

1) Glomerulus

2) Tubules

3) Loop of Henle

Waste Removal

2. Functionsa. Filtration – fluid part

of blood forced into nephron by pressure

b. Reabsorption -nephron cells process filtered material, return valuable substances (salt, glucose, amino acids, water) to blood, send wastes for excretion

• Kidney animation:

http://www.biologymad.com/resources/kidney.swf

RespirationD. Countercurrent

exchange – a. Oxygenated water and

blood flow in opposite directions

b. Allows over 80% of the initial concentration difference to be transferred

C. Maximizes efficiency of gas exchange