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Chapter 7��Membrane Structure and Function

Cell Size

Review

Chapter 7��Membrane Structure and Function

Cell Size

Review

https://www.youtube.com/watch?v=5OetGMX9Lus

Chapter 7��Membrane Structure and Function

Cell Size

Review

Chapter 7��Membrane Structure and Function

Cell Size

Review

Chapter 7��Membrane Structure and Function

Cell Size

Review

https://www.youtube.com/watch?v=AdpLDyb0G_I&t=23s

Chapter 7��Membrane Structure and Function

Cell Size

Review

Chapter 7��Membrane Structure and Function

Cell Size

Review

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A spherical bacterial cell has a radius of  3μm. The human egg cell has a radius of 100μm.

Which statement correctly indicates the cell that is able to more efficiently exchange materials with the external environment and provides a correct explanation?

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�A) The egg cell, because it has the smallest surface-to-volume ratio.��B) The egg cell, because it has the largest surface-to-volume ratio.��C) The bacterial cell, because it has the smallest surface-to-volume ratio��D) The bacterial cell, because it has the largest surface-to-volume ratio.��

Chapter 7��Membrane Structure and Function

Cell Size

Review

Of the two cells represented in the figure, which would likely be more efficient at exchanging substances with the surrounding environment?��

Of the two cells represented in the figure, which would likely be more efficient at exchanging substances with the surrounding environment?��A) Cell A, because it has the larger surface-area-to-volume ratio.��B) Cell A, because it has the smaller surface-area-to-volume ratio.��C) Cell B, because it has the larger surface-area-to-volume ratio.�

D) Cell B, because it has the smaller surface-area-to-volume ratio.��

Chapter 7��Membrane Structure and Function

Cell Size

Review

Simple cuboidal epithelial cells line the ducts of certain human exocrine glands. Various materials are transported into or out of the cells by diffusion. (The formula for the surface area of a cube is 6xS^2, and the formula for the volume of a cube is S^3, where S = the length of a side of the cube.)

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Which of the following cube-shaped cells would be most efficient in removing waste by diffusion?

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Chapter 7��Membrane Structure and Function

Plasma Membranes and

Membrane Permeability

Chapter 7��Membrane Structure and Function

Life at the Edge

• The plasma membrane is _______________________
• The plasma membrane exhibits selective permeability, ________________________________________

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Chapter 7��Membrane Structure and Function

Selective permeability— allowing “some” substances to cross more easily than others

Chapter 7��Membrane Structure and Function

Potassium ion

Potassium ion channel protein

Chapter 7��Membrane Structure and Function

Phospholipids

​

​

Amphipathic?

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Chapter 7��Membrane Structure and Function

Other Embedded Proteins

​

​

Amphipathic? Hydrophilic? Hydrophobic?

Chapter 7��Membrane Structure and Function

N-terminus

EXTRACELLULAR

SIDE

α helix

C-terminus

CYTOPLASMIC

SIDE

Describe the chemical properties required of this embedded protein.

Chapter 7��Membrane Structure and Function

Fluid Mosaic Model

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​

• In the fluid mosaic model, ____________________________________________________________________

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Chapter 7��Membrane Structure and Function

Fluid Mosaic Model

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• In the fluid mosaic model, ____________________________________________________________________

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Chapter 7��Membrane Structure and Function

The Fluidity of Membranes

• Membranes are held together mainly by weak hydrophobic interactions
• Most of the lipids and some proteins can move sideways within the membrane
• Rarely, a lipid may flip-flop across the membrane, from one phospholipid layer to the other

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Chapter 7��Membrane Structure and Function

The Fluidity of Membranes

• Membranes are held together mainly by weak hydrophobic interactions
• Most of the lipids and some proteins can move sideways within the membrane
• Rarely, a lipid may flip-flop across the membrane, from one phospholipid layer to the other

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Chapter 7��Membrane Structure and Function

Membranes must be fluid to work properly; membranes are usually about as fluid as salad oil!

Chapter 7��Membrane Structure and Function

• As temperatures cool, membranes switch from a fluid state to a solid state
• The temperature at which a membrane solidifies depends on the types of lipids (sat/unsat)

Chapter 7��Membrane Structure and Function

How do membranes with more saturated fatty acids compare to membranes with more unsaturated fatty acids?

Chapter 7��Membrane Structure and Function

• The steroid cholesterol has different effects on the membrane fluidity of animal cells at different temperatures
• At warm temperatures (such as 37ºC), cholesterol restrains movement of phospholipids
• At cool temperatures, it maintains fluidity by preventing tight packing
• Though cholesterol is present in plants, they use related steroid lipids to buffer membrane fluidity

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Steroids in the Membrane

Chapter 7��Membrane Structure and Function

Evolution of Differences in Membrane Lipid Composition

• Variations in lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditions
• Ability to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary

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Describe the cell membranes of each organism shown and how that benefits the organism in their environment.

Chapter 7��Membrane Structure and Function

Chapter 7��Membrane Structure and Function

Chapter 7��Membrane Structure and Function

What are the pieces of the fluid mosaic??

Chapter 7��Membrane Structure and Function

• Peripheral proteins are bound to the surface of the membrane
• Integral proteins penetrate the hydrophobic core
• Integral proteins that span the membrane are called transmembrane proteins
• The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into α helices

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Chapter 7��Membrane Structure and Function

Enzymes

Signaling

molecule

Receptor

ATP

Signal Sent

(a)

(b)

(c)�

Glyco-

protein

(d)

(e)�

(f)

Membrane proteins can carry out several functions:

Chapter 7��Membrane Structure and Function

Chapter 7��Membrane Structure and Function

The Role of Membrane Carbohydrates

in Cell-Cell Recognition

• Cells recognize each other by binding to molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane
• Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or, more commonly, to proteins (forming glycoproteins)
• Carbohydrates on the extracellular side of the plasma membrane vary among species, individuals, and even cell types in an individual

Chapter 7��Membrane Structure and Function

Explain the chemical differences between each blood type.

Chapter 7��Membrane Structure and Function

The Permeability of the Lipid Bilayer

• ___________________ molecules
• Pass through the membrane rapidly
• _______________________________ molecules
• Do not cross the membrane easily
• Proteins built into the membrane play key roles in regulating transport

What types of molecules can and cannot easily cross the membrane?

Chapter 7��Membrane Structure and Function

Membrane Transport

and

Diffusion

Chapter 7��Membrane Structure and Function

Chapter 7��Membrane Structure and Function

Passive transport is diffusion of a substance

across a membrane with no energy investment

• Diffusion is the tendency for molecules to spread out evenly into the available space
• Although each molecule moves randomly, diffusion of a population of molecules is directional
• At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other

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Chapter 7��Membrane Structure and Function

Chapter 7��Membrane Structure and Function

Passive transport is diffusion of a substance

across a membrane with no energy investment

• Substances diffuse down their concentration gradient, the region along which the density of a chemical substance increases or decreases
• No work must be done to move substances down the concentration gradient
• No energy is expended by the cell to make it happen

Chapter 7��Membrane Structure and Function

Figure 7.10

Molecules of dye

Membrane (cross section)

WATER

(a) Diffusion of one solute

Net diffusion

Equilibrium

Net diffusion

Net diffusion

Equilibrium

Equilibrium

(b) Diffusion of two solutes

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Chapter 7��Membrane Structure and Function

Figure 7.10

Molecules of dye

Membrane (cross section)

WATER

Net diffusion

(a) Diffusion of one solute

Net diffusion

Equilibrium

Net diffusion

Net diffusion

Net diffusion

Net diffusion

Equilibrium

Equilibrium

(b) Diffusion of two solutes

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Chapter 7��Membrane Structure and Function

Facilitated Diffusion: Just Passive Transport That’s Aided by Proteins!

• In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane, with no energy investment

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Chapter 7��Membrane Structure and Function

Facilitated Diffusion by Transport Proteins

• Transport proteins allow passage of __________________ across the membrane

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Chapter 7��Membrane Structure and Function

Facilitated Diffusion by Transport Proteins

• Transport proteins allow passage of hydrophilic substances across the membrane
• A transport protein is specific for the substance it allows to move

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Chapter 7��Membrane Structure and Function

Facilitated Diffusion by Transport Proteins

• Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane
• Aquaporins for water in larger amounts
• Ion channels facilitate the transport of ions
• Some ion channels, called gated channels, open or close in response to a stimulus
• For example, in nerve cells, ion channels open in response to electrical stimulus

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Chapter 7��Membrane Structure and Function

• Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane
• Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane
• This change in shape can be triggered by the binding and release of the transported molecule

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Facilitated Diffusion by Transport Proteins

Chapter 7��Membrane Structure and Function

Draw and Summarize the 2 types of transport proteins.

Chapter 7��Membrane Structure and Function

(a) A channel�protein

EXTRACELLULAR

FLUID

Channel protein

CYTOPLASM

Solute

Carrier protein

Solute

(b) A carrier protein

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• Channel proteins provide hydrophilic passageways for water and ionic molecules to go down their concentration gradient
• Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane

Chapter 7��Membrane Structure and Function

• Facilitated diffusion is still passive because the solute moves down its concentration gradient, and the transport requires no energy
• What type of transport requires an input of energy?

Is facilitated diffusion passive transport?

Chapter 7��Membrane Structure and Function

Active transport uses energy to move solutes against their gradients

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Chapter 7��Membrane Structure and Function

Active transport uses energy to move solutes against their gradients

• Active transport proteins can move solutes against their concentration gradients

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Chapter 7��Membrane Structure and Function

The Need for Energy in Active Transport

• Active transport requires energy, usually in the form of ATP hydrolysis, to move substances against their concentration gradients
• All proteins involved in active transport are carrier proteins

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Chapter 7��Membrane Structure and Function

Chapter 7��Membrane Structure and Function

Organize the terms!!!

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Passive

Active

Simple diffusion

Channel protein

Transport protein

Carrier protein

Facilitated diffusion

Chapter 7��Membrane Structure and Function

Figure 7.16

________ transport

________ transport

___________

__________________

____

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___________

___________

Chapter 7��Membrane Structure and Function

Maintaining Membrane Potential

• All cells have voltages across their membranes! (Membrane potential)
• Voltage= separation of opposite charges
• The cytoplasmic side of the membrane is negative in charge relative to the extracellular side, because of an unequal distribution of of anions and cations

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Chapter 7��Membrane Structure and Function

Maintaining Membrane Potential

• Membrane potential favors the passive transport of cations into the cell and anions out of the cell
• Two forces drive the diffusion of ions across a membrane
• 1. Chemical force (ion’s concentration gradient)
• 2. Electrical force (effect of membrane potential on ion movement)

Electrochemical gradient!!!!!!!

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Chapter 7��Membrane Structure and Function

Maintaining Membrane Potential

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Chapter 7��Membrane Structure and Function

How Ion Pumps Maintain Membrane Potential

• How to redefine passive transport for ions?

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Chapter 7��Membrane Structure and Function

How Ion Pumps Maintain Membrane Potential

• How to redefine passive transport for ions?
• Ion diffuses simply down its electrochemical gradient!!
• Ex: [Na+] inside is lower than outside cell. Cell is stimulated, gated channels open that facilitate Na+ diffusion into cell.
• Both electrical and chem contributions act in same direction in this ex!
• This is not always the case….

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Chapter 7��Membrane Structure and Function

How Ion Pumps Maintain Membrane Potential

• An electrogenic pump is a transport protein that actively generates voltage across a membrane
• The sodium-potassium pump is the major electrogenic pump of animal cells
• The main electrogenic pump of plants, fungi, and bacteria is a proton pump, which actively transports hydrogen ions (H+) out of the cell
• Electrogenic pumps help store energy that can be used for cellular work

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Chapter 7��Membrane Structure and Function

The Sodium-potassium Pump: a Specific Case of Active Transport

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Chapter 7��Membrane Structure and Function

https://youtu.be/7NY6XdPBhxo

Chapter 7��Membrane Structure and Function

The Sodium-potassium Pump: A Specific Case of Active Transport (Part 1: Na⁺ Binding)

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Chapter 7��Membrane Structure and Function

Figure 7.15b The Sodium-potassium Pump: A Specific Case of Active Transport (Part 2: K⁺ Binding)

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Chapter 7��Membrane Structure and Function

Figure 7.15c The Sodium-potassium Pump: A Specific Case of Active Transport (Part 3: K⁺ Release)

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Chapter 7��Membrane Structure and Function

One more type of membrane protein transport ….

Chapter 7��Membrane Structure and Function

Cotransport: Coupled Transport by a Membrane Protein

• Cotransport occurs when active transport of a solute indirectly drives transport of other substances
• The diffusion of an actively transported solute down its concentration gradient is coupled with the transport of a second substance against its own concentration gradient

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Chapter 7��Membrane Structure and Function

Figure 7.16

Passive transport

Active transport

Diffusion

Facilitated diffusion

ATP

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Chapter 7��Membrane Structure and Function

Bulk

Transport

Chapter 7��Membrane Structure and Function

Bulk transport across the plasma membrane occurs by exocytosis and endocytosis

• Small molecules and water enter or leave the cell through the lipid bilayer or via transport proteins
• Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles
• Does bulk transport require ATP?

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Chapter 7��Membrane Structure and Function

Exocytosis

• In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell
• Many secretory cells use exocytosis to export�their products

Examples?

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Chapter 7��Membrane Structure and Function

Exocytosis

• In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell
• Many secretory cells use exocytosis to export�their products

Examples?

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Chapter 7��Membrane Structure and Function

Endocytosis

• In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane
• Endocytosis is a reversal of exocytosis, involving different proteins
• There are three types of endocytosis
• Phagocytosis (“cellular eating”)
• Pinocytosis (“cellular drinking”)
• Receptor-mediated endocytosis

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Chapter 7��Membrane Structure and Function

Endocytosis

• In phagocytosis, a cell engulfs a particle in a vacuole
• The vesicle fuses with a lysosome to digest the particle

Examples?

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Chapter 7��Membrane Structure and Function

Endocytosis

• In pinocytosis, molecules dissolved in droplets are taken up when extracellular fluid is “gulped” into tiny vesicles

Examples?

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Chapter 7��Membrane Structure and Function

Endocytosis

• In receptor-mediated endocytosis, binding of specific solutes to receptors triggers vesicle formation
• Receptor proteins, receptors, and other molecules from the extracellular fluid are transported in the vesicles
• Emptied receptors are recycled to the plasma membrane

Examples?

https://youtu.be/GbptpDSHQEM

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Chapter 7��Membrane Structure and Function

Exploring Endocytosis in Animal Cells

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Chapter 7��Membrane Structure and Function

BioFlix: Membrane Transport

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Chapter 7��Membrane Structure and Function

Chapter 7��Membrane Structure and Function