Chapter 5- Membrane Structure and

Function

Cell (plasma) membrane● Cells need an inside & an

outside…○ separate cell from its environment○ ability to discriminate chemical

exchanges

Phospholipid Bilayer

A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer (fluid mosaic model)

● Cholesterol maintains membrane fluidity

● The carbohydrates are not inserted into the membrane○ too hydrophilic ○ attached to

embedded proteins -- glycoproteins.

Membrane Carbohydrates

● Play a key role in cell-cell recognition○ ability of a cell to distinguish

neighboring cells from another○ important in organ & tissue

development○ basis for rejection of foreign

cells by immune system

Cholesterol

● Provides stability in animal cells

● “temperature buffer” quality for membrane

● Replaced with sterols in plant cells

Selectively permeable membrane● Nonpolar, hydrophobic molecules

dissolve in the membrane and cross with ease

● Polar, hydrophilic molecules that are small enough to pass between lipids (H20, ethanol) move with ease

● Larger molecules (glucose) and ions (Na+) have difficulty passing hydrophobic layer○ transport proteins

■ hydrophilic tunnel■ specific for the substance

they move

Membrane Proteins● Proteins determine most of membrane’s

specific functions○ cell membrane & organelle membranes

each have unique collections of proteins

● Membrane proteins:○ peripheral proteins = loosely bound to

surface of membrane○ integral proteins = penetrate into lipid

bilayer, often completely spanning the membrane = transmembrane proteins

Membrane Protein Types● Channel proteins – wide open passage● Ion channels – gated● Aquaporins – water only, kidney and

plant root only● Carrier proteins – change shape● Transport proteins – require ATP● Recognition proteins - glycoproteins● Adhesion proteins – anchors● Receptor proteins - hormones

Bell RingersYour table has a glucose-starch solution in front of you:

● what macromolecule is glucose? starch?○ which is larger?

● you also have a glucose testing strip in front of you. This is used to test the glucose concentration in urine. ○ what color does the strip turn when dipped in the

solution (may take a minute or two).○ What condition would result in excessive

amounts of glucose in urine?● Record the color of the water in the beaker on your

lab handout. The Lugol’s agent has been added

Getting through the cell membrane● Passive transport

○ no energy needed○ movement down

concentration gradient

● Active transport○ movement against

concentration gradient■ low to high

○ requires ATP

Diffusion● 2nd Law of Thermodynamics

○ universe tends towards disorder○ energy disperses or spreads out

● Diffusion○ movement from high to low concentration○ result of intrinsic kinetic energy ○ result from random molecular motion○ rate regulated by the permeability of the

membrane

Diffusion of 2 solutes● Each substance diffuses down its own

concentration gradient, independent of concentration gradients of other substances○ not influenced by other substances

When is diffusion needed?● Respiratory and Circulatory systems

○ Oxygen and carbon dioxide transport into and out of bloodstream■ skin, gills, aveoli, capillaries

● Excretory systems○ Movement of waste into or out of blood

■ skin, nephridia, nephrons, gills

Countercurrent exchange systemWater carrying gas flows in one direction, blood flows in opposite direction

How does countercurrent exchange work?

Blood and water flow in opposite directionsmaintains diffusion gradient over whole length of gill capillarymaximing oxygen transfer from water to blood

Gas Exchange on Land● Advantages

○ higher concentration of O2○ O2 and CO2 diffuse much faster through air

■ respiratory surfaces exposed to air do not have to be ventilated as thoroughly as gills

○ air is much lighter than water and therefore easier to pump■ less energy moving air in and out

● Disadvantages○ keeping larger respiratory surface moist causes high

water loss

Facilitated diffusion● Move from high to low concentration

through an integral protein channel○ passive transport○ no energy needed○ “facilitated”=with help

● Share some properties with enzymes○ specific for the solute they transport○ help diffusion of ions that are impeded

● Two models○ conformational change when solute

binds○ selective channels for specific solutes

Active Transport● Cells may need molecules to move against concentrations○ need to pump against

concentration○ requires energy (ATP)○ oscillates between two

conformations● Plants have nitrate and

phosphate pumps in their roots- why?○ why do they need N or

P?

What about large molecules?● Moving large molecules into & out

of cell requires ATP(energy)!○ through vesicles & vacuoles○ endocytosis

■ phagocytosis = “cellular eating”

■ pinocytosis = “cellular drinking”

■ receptor-mediated endocytosis

○ exocytosis■ export macromolecules

Osmosis is diffusion of water● Diffusion of water from high

concentration of water to low concentration of water○ across a semi-permeable

membrane that may not allow for diffusion of molecules

● Direction of osmosis is determined by comparing total solute concentrations○ Hypertonic - more solute, less water than compared to inside the

cell○ Hypotonic - less solute, more water than compared to inside the cell○ Isotonic - equal solute, equal water

Water Potential● Water moves from a place of greater water

potential to a place of lesser water potential (net).○ which area has the greatest potential

to move?

● As the concentration of a solute increases in a solution, the water potential will decrease accordingly.○ Which has the greater water potential:

■ 0.2M or 0.8M? ○ Which has the greater water potential:

■ 20% or 80% water?

Water Potential

● Solute potential decreases with increasing solute concentration○ Results in decrease in overall

water potential

Water Potential

i= 2 i= 1

Animal systems evolved to support multicellular life

● Diffusion is too slow for clustered cells○ what reduces when cells are clumped

together?○ Distance for diffusion is reduced

because the circulatory system connects cells with organs specific for exchange

● Developed exchange systems for:○ distributing nutrients

■ circulatory system○ removing waste

■ excretory system

Intracellular Waste● Animals poison themselves from the inside by

digesting proteins● What do we digest our food into?

○ Carbohydrate= CHO -> CO2 +H20○ Lipids= CHO -> CO2 + H20○ Proteins= CHON -> CO2 + H20+ N○ Nucleic acids = CHONP -> CO2+H20+P+N

Water Soluble

Nitrogen Waste● Aquatic organisms

○ can afford to lose water○ ammonia

■ most toxic● Terrestrial

○ need to conserve water○ can’t excrete ammonia fast enough○ urea

■ less toxic (100,000x less)● Terrestrial egg laying

○ need to conserve water○ need to protect embryo in egg○ uric acid

■ least toxic■ released with almost no water= paste-like

Land Animals● Nitrogen waste disposal on land

○ need to conserve water○ must process ammonia so less toxic○ Liver combines ammonia with CO2

■ urea= large molecule= less soluble= less toxic● cost energy to synthesize with CO2

■ can be stored safely which reduces the water necessary for release

○ Kidney■ filter solutes out of blood■ reabsorb H20■ excrete waste

● urine= urea +salts+excess sugar + H20

urea= ammonia +CO2

Kidneys● Kidneys generate urine by filtering

waste from blood○ Nephrons- basic unit of kidney

■ water will move towards urine as it becomes hypoteronic via aquaporins

■ anti-diuretic hormone increase water movement back to blood

■ Blood pressure highly regulated by kidney● no pressure = no

filtration = toxic blood

Osmotic Control in Nephron

● How is all this re-absorption achieved?○ tight osmotic control to

reduce the energy cost of excretion

○ use diffusion instead of active transport wherever possible

○ countercurrent exchange

Osmoregulation- battling osmosis

● Water balance vs habitat○ freshwater

■ hypotonic to body fluids■ water flows into cells and salt

loss○ saltwater

■ hypertonic to body fluids■ water loss from cells

○ terrestrial■ dry environment■ need to conserve water