Department of Kinesiology and Applied Physiology WCR Chapter 3: Cells Overview Plasma membrane: structure Plasma membrane: transport Resting membrane potential

Department of Kinesiology and Applied Physiology WCR

Chapter 3: Cells• Overview• Plasma membrane: structure• Plasma membrane: transport• Resting membrane potential• Cell-environment interactions• Cytoplasm• Nucleus• Cell growth & reproduction• Extracellular materials• Developmental aspects

Department of Kinesiology and Applied Physiology 2

Do exercise scientists need to think about cells?Exercise in a Pill

•“AMPK and PPARδ Agonists Are Exercise Mimetics”•AICAR activates intracellular pathways that are also activated by exercise. Mice taking AICAR look like mice on exercise. Mice on AICAR plus exercise are supermice.

Narkar et al., Cell 2008.

Fibroblasts

Erythrocytes

Epithelial cells

(d) Cell that fights disease

Nerve cell

Fat cell

Sperm

(a) Cells that connect body parts, form linings, or transport gases

(c) Cell that storesnutrients

(b) Cells that move organs and body parts

(e) Cell that gathers information and control body functions

(f) Cell of reproduction

SkeletalMusclecell

Smoothmuscle cells

Macrophage

Figure 3.1

Generalized Cell

• All cells have some common structures and functions

• Human cells have three basic parts:– Plasma membrane—flexible outer boundary– Cytoplasm—intracellular fluid containing

organelles– Nucleus—control center

Copyright © 2010 Pearson Education, Inc. Figure 3.2

Secretion beingreleased from cellby exocytosis

Peroxisome

Ribosomes

Roughendoplasmicreticulum

Nucleus

Nuclear envelopeChromatin

Golgi apparatus

Nucleolus

Smooth endoplasmicreticulum

Cytosol

Lysosome

Mitochondrion

CentriolesCentrosomematrix

Cytoskeletalelements• Microtubule• Intermediate filaments

Plasmamembrane

Plasma Membrane

• Bimolecular layer of lipids and proteins in a constantly changing fluid mosaic

• Plays a dynamic role in cellular activity• Separates intracellular fluid from extracellular

fluid– Interstitial fluid = ECF that surrounds cells


Integralproteins

Extracellular fluid(watery environment)

Cytoplasm(watery environment)

Polar head ofphospholipid molecule

Glycolipid

Cholesterol

Peripheralproteins

Bimolecularlipid layercontainingproteins

Inward-facinglayer ofphospholipids

Outward-facinglayer ofphospholipids

Carbohydrate of glycocalyx

Glycoprotein

Filament of cytoskeleton

Nonpolar tail of phospholipid molecule

Membrane Proteins

• Integral proteins– Firmly inserted into the membrane (most are

transmembrane)– Functions:

• Transport proteins (channels and carriers), enzymes, or receptors

Animation: Transport ProteinsPLAYPLAY

Membrane Proteins• Peripheral proteins

– Loosely attached to integral proteins – Include filaments on intracellular surface and

glycoproteins on extracellular surface– Functions:

• Enzymes, motor proteins, cell-to-cell links, provide support on intracellular surface, and form part of glycocalyx

Animation: Structural ProteinsPLAYPLAY

Animation: Receptor ProteinsPLAYPLAY

Copyright © 2010 Pearson Education, Inc. Figure 3.4a

A protein (left) that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. Some transport proteins (right) hydrolyze ATP as an energy source to actively pump substances across the membrane.

(a) Transport

Copyright © 2010 Pearson Education, Inc. Figure 3.4b

A membrane protein exposed to the outside of the cell may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external signal may cause a change in shape in the protein that initiates a chain of chemical reactions in the cell.

(b) Receptors for signal transductionSignal

Receptor

Copyright © 2010 Pearson Education, Inc. Figure 3.4c

Elements of the cytoskeleton (cell’s internal supports) and the extracellular matrix (fibers and other substances outside the cell) may be anchored to membrane proteins, which help maintain cell shape and fix the location of certain membrane proteins. Others play a role in cell movement or bind adjacent cells together.

(c) Attachment to the cytoskeleton and extracellular matrix (ECM)

Copyright © 2010 Pearson Education, Inc. Figure 3.4d

A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane act as a team that catalyzes sequential steps of a metabolic pathway as indicated (left to right) here.

(d) Enzymatic activity

Enzymes

Copyright © 2010 Pearson Education, Inc. Figure 3.4e

Membrane proteins of adjacent cells may be hooked together in various kinds of intercellular junctions. Some membrane proteins (CAMs) of this group provide temporary binding sites that guide cell migration and other cell-to-cell interactions.

CAMs

(e) Intercellular joining

Copyright © 2010 Pearson Education, Inc. Figure 3.4f

Some glycoproteins (proteins bonded to short chains of sugars) serve as identification tags that are specifically recognized by other cells.

(f) Cell-cell recognition

Glycoprotein

Membrane Junctions

• Three types:– Tight junction – Desmosome – Gap junction


Interlockingjunctional proteins

Intercellularspace

Plasma membranesof adjacent cells

Microvilli

Intercellularspace

Basement membrane

(a) Tight junctions: Impermeable junctions prevent molecules from passing through the intercellular space.


Intercellular space


Microvilli

Intercellularspace

Plaque

Linker glycoproteins(cadherins)

Intermediatefilament (keratin)

(b) Desmosomes: Anchoring junctions bind adjacent cells together and help form an internal tension-reducing network of fibers.

Basement membrane



Microvilli

Intercellularspace

Intercellularspace

Channelbetween cells(connexon)

(c) Gap junctions: Communicating junctions allow ions and small molecules to pass from one cell to the next for intercellular communication.

Basement membrane

Membrane Transport: How things get in and out of cells

Plasma membranes are selectively permeable: some molecules easily pass through the membrane; others do not

Types of Membrane TransportPassive processes

– No cellular energy (ATP) required– Substance moves down its concentration gradient

Active processes– Energy (ATP) required– Occurs only in living cell membranes

Copyright © 2010 Pearson Education, Inc.

Passive Processes

• What determines whether or not a substance can passively permeate a membrane?

1. Lipid solubility of substance

2. Channels of appropriate size

3. Carrier proteins

PLAYPLAY Animation: Membrane Permeability


Passive Processes

• Simple diffusion

• Carrier-mediated facilitated diffusion

• Channel-mediated facilitated diffusion

• Osmosis


Passive Processes: Simple Diffusion

• Nonpolar lipid-soluble (hydrophobic) substances diffuse directly through the phospholipid bilayer

PLAYPLAY Animation: Diffusion


Extracellular fluid

Lipid-solublesolutes

Cytoplasm

(a) Simple diffusion of fat-soluble molecules directly through the phospholipid bilayer


Passive Processes: Facilitated Diffusion

• Certain lipophobic molecules (e.g., glucose, amino acids, and ions) use carrier proteins or channel proteins, both of which:

• Exhibit specificity (selectivity)

• Are saturable; rate is determined by number of carriers or channels

• Can be regulated in terms of activity and quantity


Lipid-insoluble solutes (such as sugars or amino acids)

(b) Carrier-mediated facilitated diffusion via a protein carrier specific for one chemical; binding of substrate causes shape change in transport protein


Small lipid-insoluble solutes

(c) Channel-mediated facilitated diffusion through a channel protein; mostly ions selected on basis of size and charge


Passive Processes: Osmosis

• Movement of solvent (water) across a selectively permeable membrane

•Water diffuses through plasma membranes:

• Through the lipid bilayer

• Through water channels called aquaporins

Copyright © 2010 Pearson Education, Inc. Figure 3.7d

Watermolecules

Lipidbillayer

Aquaporin

(d) Osmosis, diffusion of a solvent such as water through a specific channel protein (aquaporin) or through the lipid bilayer


Importance of Osmosis

•When osmosis occurs, water enters or leaves a cell

• Change in cell volume disrupts cell function

PLAYPLAY Animation: Osmosis


Tonicity

• Tonicity: How much dissolved material there is in a solution. Tonicity determines whether a solution will make cells shrink or swell.

• Isotonic: A solution with the same solute concentration as the inside of a normal cell

• Hypertonic: A solution with a greater solute concentration than than a normal cell

• Hypotonic: A solution with a lesser solute concentration than a normal cell


Summary of Passive Processes

• Also see Table 3.1

Process Energy Source

Example

Simple diffusion

Kinetic energy

Movement of O2 through phospholipid bilayer

Facilitated diffusion

Kinetic energy

Movement of glucose into cells

Osmosis Kinetic energy

Movement of H2O through phospholipid bilayer or AQPs


Membrane Transport: Active Processes

• Two types of active processes:

• Active transport

• Vesicular transport

• Both use ATP to move solutes across a living plasma membrane


Active Transport

• Requires carrier proteins (solute pumps)

• Moves solutes against a concentration gradient

• Types of active transport:

• Primary active transport

• Secondary active transport


Primary Active Transport

• Energy from breakdown of ATP causes shape change in transport protein to “pump” molecules across the membrane

• Example: Sodium-potassium pump (Na+-K+ ATPase)

• Located in all plasma membranes

• Involved in primary and secondary active transport of nutrients and ions

• Maintains “electrochemical gradients” essential for functions of muscle and nerve tissues


Extracellular fluid

K+ is released from the pump proteinand Na+ sites are ready to bind Na+ again.The cycle repeats.

Binding of Na+ promotesphosphorylation of the protein by ATP.

Cytoplasmic Na+ binds to pump protein.

Na+

Na+-K+ pump

K+ released

ATP-binding siteNa+ bound

Cytoplasm

ATPADP

P

K+

K+ binding triggers release of thephosphate. Pump protein returns to itsoriginal conformation.

Phosphorylation causes the protein tochange shape, expelling Na+ to the outside.

Extracellular K+ binds to pump protein.

Na+ released

K+ bound

P

K+

PPi

1

2

3

4

5

6


Secondary Active Transport

• Energy stored in ionic gradients is used indirectly to drive transport of other solutes

• Always involves cotransport – transport of more than one substance at a time

• Two substances transported in same direction (Na+, glucose)

• Two substances transported in opposite directions (Na+, H+)

Mod WCR


The ATP-driven Na+-K+ pump stores energy by creating a steep concentration gradient for Na+ entry into the cell.

As Na+ diffuses back across the membrane through a membrane cotransporter protein, it drives glucose against its concentration gradientinto the cell. (ECF = extracellular fluid)

Na+-glucosesymporttransporterloadingglucose fromECF

Na+-glucosesymport transporterreleasing glucoseinto the cytoplasm

Glucose

Na+-K+

pump

Cytoplasm

Extracellular fluid

1 2


Vesicular Transport

• Transport of large particles, macromolecules, and fluids across plasma membranes

• Requires cellular energy (e.g., ATP)

• Functions:

• Exocytosis—transport out of cell

• Endocytosis—transport into cell (receptor mediated; phago-; pino-)

• Transcytosis—transport into, across, and then out of cell

• Vesicular transport within a cell (see the video)Mod WCR


Phagosome

(a) Endocytosis:PhagocytosisThe cell engulfs a large particle by forming pro-jecting pseudopods (“false feet”) around it and en-closing it within a membrane sac called a phagosome. The phagosome is combined with a lysosome. Undigested contents remain in the vesicle (now called a residual body) or are ejected by exocytosis. Vesicle may or may not be protein-coated but has receptors capable of binding to microorganisms or solid particles.


Vesicle

(b) Endocytosis:PinocytosisThe cell “gulps” drops of extracellular fluid containing solutes into tiny vesicles. No receptors are used, so the process is nonspecific. Most vesicles are protein-coated.


Vesicle

Receptor recycledto plasma membrane

(c) Receptor-mediatedendocytosisExtracellular substances bind to specific receptor proteins in regions of coated pits, enabling the cell to ingest and concentrate specific substances (ligands) in protein-coated vesicles. Ligands may simply be released inside the cell, or combined with a lysosome to digest contents. Receptors are recycled to the plasma membrane in vesicles.


1 Membrane-bound vesicle migrates toplasma membrane.

2 Proteins at vesicle surface (v-SNAREs) bind with t-SNAREs (plasma membrane proteins).

Exocytosis

Extracellularfluid

Plasma membraneSNARE (t-SNARE)

Secretoryvesicle

VesicleSNARE(v-SNARE)

Molecule tobe secretedCytoplasm

Fusedv- and

t-SNAREs

3 Vesicle and plasma membrane fuse and pore opens up.

4 Vesicle contentsreleased to cell exterior.

Fusion pore formed


Summary of Active Processes

Process Energy Source Example

Primary active transport ATP Pumping of ions across membranes

Secondary active transport

Ion gradient Movement of polar or charged solutes across membranes

Exocytosis ATP Secretion of hormones and neurotransmitters

Phagocytosis ATP White blood cell phagocytosis

Pinocytosis ATP Absorption by intestinal cells

Receptor-mediated endocytosis

ATP Hormone and cholesterol uptake

Documents

Department of Kinesiology and Applied Physiology WCR Chapter 3: Cells Overview Plasma membrane: structure Plasma membrane: transport Resting membrane potential