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Cell membranesStef Elorriaga

4/11/2016

BIO102

Announcements

• Lab report 2 is due now

• Quiz 2 is on Wednesday on cells, part of the cells, plasma membrane, and enzymes

Outline of the day

• Activity on the parts of the cells• Lab write-ups are graded for lab 1

• Lecture on the plasma membrane

• Activity on osmosis

• Lecture on reactions and enzymes

Learned so far

• Introduction

• Matter

• Macromolecules

• Cells

• Parts of the cells

Plasma membrane is a phospholipid bilayer embedded

with other components

Alpha-helix

protein

Plasma membrane is fluid and dynamic – Fluid mosaic model• Model was introduced in 1972 by S.J. Singer

and Garth L. Nicolson, and still stands

Alpha-helix

protein

Cell membrane functions

1. Isolation

2. Regulation

3. Sensitivity

4. Attachment

Cell membrane functions

1. Physical isolation• Separates inside of cell from extracellular fluid

Cell membrane functions

2. Regulates intracellular-extracellular exchange• Controls ions, nutrients, waste, and secretory

product exchange

Cell membrane functions

3. Sensitivity to the extracellular environment• Receptors allow cell recognition/response to

molecules in the environment

Cell membrane functions

4. Attachment (within cell)• Cytoskeleton

• Microfilaments (actin)

• Microtubules (tubulin)

• Intermediate filaments

Cytoskeleton

intermediatefilaments

Light micrograph showing the cytoskeleton

microtubules

microfilaments

microtubules (red)

microfilaments (blue)

nucleus

Cell membrane functions

4. Attachment (outside cell)• Cells don’t live floating in fluid – must attach to

surface and to each other

• Provide structural and biochemical support, stiffness, and elasticity

• Examples: fibronectin, cadherins

Cell membrane structure

• Membrane separates inside/outside• Inside cell = aqueous

• Outside cell = aqueous

• Solutes are mostly polar

• How do we keep molecules where they need to be?

Cell membrane structure

• Solution: cell membrane must be fundamentally non-polar, but able to exist in aqueous environment

• Molecule that makes this possible = phospholipid

Cell membrane structure

• Polar heads face both outside and inside

• Hydrophilic heads face aqueous environments

• Non-polar tails protected in between

phospholipid

hydrophilicheads

hydrophobictails

hydrophilicheads

extracellular fluid(watery environment)

cytoplasm(watery environment)

Plasma membrane

Cell membrane structure

• Membrane must be very fluid

• Fluidity adjusted by changing saturation of fatty acid tails• More saturated = less fluid

phospholipid

hydrophilicheads

hydrophobictails

hydrophilicheads

extracellular fluid(watery environment)

cytoplasm(watery environment)

Plasma membrane

Cell membrane structure

• Problem: system is too effective; cell

membranes isolate the cell from the outside

• Like a room with no doors and no windows

• How does anything get in or out?

• Selective permeability

Cell membrane structure

• Solution: membranes not 100%

phospholipid

• Cell membranes have:

• Phospholipid

• Cholesterol

• Carbohydrates

• Proteins

Cholesterol

• 50% dry weight of membrane

• Adds stiffness

• Straightens phospholipid tails

• Prevents small polar molecules from passing

through membrane

Membrane carbohydrates

• Carbohydrates attach to other molecules in

the membrane

• Attach to protein = glycoprotein

• Replace phosphate in phospholipid =

glycolipid

Membrane carbohydrates

• Carbohydrate functions• Multiple functions

• Allow cell-to-cell interactions, ex. Platelet aggregation

• Cell recognition - “fingerprint”

Membrane proteins

• Proteins = major functional component

Connection proteins

Types of membrane proteins

• Two configurations• Integral

• Span entire width of membrane

• Part of membrane structure

• Peripheral• Bind to inner or outer

surfaces

• Distinct from membrane

Integral membrane proteins

• Hydrophilic surface region

• Hydrophobic transmembrane segments made of alpha-helices or beta-sheets

How do the cells get nutrients?

• Diffusion allows molecules dissolved in liquids to move from a highly concentrated region to a lesser concentrated region

• The interior of the cell must be close to the external environment

Cells are small!

• Most cells range in size from about 1 to 100 micrometers in diameter• Because cells need to exchange nutrients and wastes

with the environment through the plasma membrane

Why are cells so small?

• Reactants needed for metabolism are present in low concentrations

• Low concentration means reactants don’t collide often

• This makes chemical reactions slow

Cell size and reactant concentration

• Concentration gets lower as cells get bigger

• What happens to chemical reaction rate in cells as cells get bigger?

Reaction rate and cell size

• Concentration gets lower as cells get bigger

• What happens to chemical reaction rate in cells as cells get bigger?

How are eukaryotic cells larger than prokaryotic ones?

• Eukaryotic cells are 10 to 100X larger than prokaryotic ones

• Eukaryotic cells have found a way around this: membrane-bound organelles• Serve to concentrate reactants in appropriate

compartments

• Improves cell efficiency

Cell size difference

• This means eukaryotic cells can be larger than prokaryotic cells

Cell size exercise

• Still, being small has some advantages

• Solutes taken into cells through membrane

• Consider 2 cubes (even though most cells are spherical)…

2 m1 m

Cell size exercise

• Consider the following calculations:

2 m1 m

Cell 1 Cell 2

Surface Area: length x width x 6

Volume: length x width x height

Surface Area/Volume

Cell size exercise

• Which cell has the greater surface area?

• Which cell has the greater volume?

• Which cell has the greater ratio of surface area to volume?

Cell 1 Cell 2

Surface Area: length x width x 6

Volume: length x width x height

Surface Area/Volume

6 m2 24 m2

1 m3 8 m3

6 3

6 m2

1 m3

6

24 m2

8 m3

3

Cell size exercise

• Volume increases faster than surface area (x3 vs x2)

• So as cells get bigger, the proportion of surface area decreases

• Keeps cells small

• Cells need surface area to absorb solutes

• Less surface area = fewer reactions

Diffusion Leads to the Even Distribution of Molecules

A drop of dye isplaced in water

Dye moleculesdiffuse into the water;water molecules diffuseinto the dye

Both dye moleculesand water molecules areevenly dispersed

drop of dye

water molecule

2

13

Types of passive transport (diffusion) across membranes

water glucose

carrierprotein

aquaporinchannelprotein

phospho-lipidbilayer

(cytoplasm)

(extracellularfluid)

Cl–

O2

(a) Simple diffusion through

the phospholipid bilayer

(b) Facilitated diffusion

through channel proteins

(c) Osmosis through

aquaporins or the

phospholipid bilayer

(d) Facilitated diffusion through

carrier proteins

Facilitated diffusion or facilitated transport allows for the transport of specific molecules

Osmosis and Cells

• Osmosis is the diffusion of water across selectively permeable membranes• Water diffuses from a region of high water

concentration to one of low water concentration across a membrane

• Dissolved substances (solutes) reduce the concentration of free water molecules (solvent)

Water moves in or out of cells depending on the relative tonicity of the

solution• Isotonic

• No net movement of water across the membrane

• Hypertonic

• Water moves across a membrane toward the hypertonic solution

• Hypotonic

• Water moves across a membrane away from the hypotonic solution

Osmosis and cells

hypertonicisotonic hypotonic

10% salt90% water

10% salt90% water

10% salt90% water

10% salt90% water

0% salt100% water

30% salt60% water

Osmosis and plants cells

Plasmolysis Turgor pressure

Active Transport: Using Energy to Move Against the Gradient

The transportprotein binds bothATP and Ca2

Energy from ATPchanges the shape of thetransport protein and movesthe ion across the membrane

The protein releases the ion andthe remnants of ATP(ADP and P) and closes

ATPbindingsite

recognitionsite

ATP

P

ADP

Ca2

(extracellular fluid)

(cytoplasm)

ATP

1 2 3

Cells use active transport to concentrate molecules in places they are needed

Electrochemical gradient across plasma membrane

Types of transporters

• Examples: Na+-K+ ATPase, H+-K+ ATPase, Ca2+ ATPase, and H+ ATPase

Primary active transport

Secondary active transport

Bulk transport for movement of large molecules

• Endocytosis• Phagocytosis

• Pinocytosis

• Potocytosis

• Receptor-mediated

Bulk transport for movement of large molecules

• Exocytosis• Transcytosis