Components Extracellular & Connections
Extracellular Components
Plasma membrane as cell boundary
Most cells synthesize & secrete materials that function outside of the cell membrane
Cell Walls of Plants
The cell wall is an extracellular structure of plant cells that distinguishes them from animal cells Functions:
Cell Walls of Plants
Structure:
Cellulose microfibrils embedded in a polysaccharide & protein matrix Primary cell wall Middle lamella Secondary cell wall
Plant Cell Walls
Central vacuole of cell Plasma membrane Secondary cell wall Primary cell wall Middle lamella
Central vacuole of cell 1 m
Central vacuole
Cytosol Plasma membrane
Plant cell walls
Figure 6.28
Plant Cell Wall
Secondary cell wall
Primary cell wall
The Extracellular Matrix (ECM)
Animal cells lack cell walls They are covered by an elaborate matrix, the ECM
ECM Structure
Glycoproteins
Proteoglycans
Attach to proteins in plasma membrane
ECM Structure
EXTRACELLULAR FLUID Collagen A proteoglycan complex Polysaccharide molecule Carbohydrates Core protein Fibronectin
Plasma membrane
Integrins
Proteoglycan molecule
Integrin
Microfilaments
CYTOPLASM
Figure 6.29
ECM Function
Support Adhesion Movement Regulation
Intercellular Junctions
Help coordinate cellular activities
Direct contact between cells
Numerous types
Intercellular Junctions: Plant Cells
Plasmodesmata Channels through cell walls lined by plasma membrane
Plasmodesmata
Cell walls
Interior of cell
Interior of cell
Figure 6.30
0.5 m
Plasmodesmata
Plasma membranes
Intercellular Junctions: Animal Cells
Three types of intercellular junctions
Tight junctions Desmosomes Gap junctions
TIGHT JUNCTIONS
Tight junctions prevent fluid from moving across a layer of cells Tight junction
0.5 m
DESMOSOMES Tight junctions Intermediate filaments Desmosome Gap junctions
1 m
GAP JUNCTIONS
Space between cells
Extracellular matrix Plasma membranes of adjacent cells Gap junction
Figure 6.31
0.1 m
Tight Junctions
Where the membranes of neighboring cells are very tightly pressed against each other
Tight Junctions
Desmosomes
Anchoring Junctions Function like rivets, fastening cells together
Desmosomes
Gap Junctions
Communicating junctions Cytoplasmic channels between adjacent cells
Gap Junctions
Membrane Structure & Function
Figure 7.1
The Plasma Membrane
The boundary that separates the living cell from its nonliving surroundings Exhibits selective permeability
Allows some substances to cross it more easily than others
Fluid Mosaic Model
The membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids
Hydrophobic region of protein
Phospholipid bilayer
Figure 7.3
Hydrophobic region of protein
The Fluidity of Membranes
Lateral movement (~107 times per second)
Flip-flop (~ once per month)
(a) Movement of phospholipids
Figure 7.5 A
The Fluidity of Membranes
Fluid Viscous
Unsaturated hydrocarbon tails with kinks
Saturated hydroCarbon tails
(b) Membrane fluidity
Figure 7.5 B
The Fluidity of Membranes
Cholesterol (c) Cholesterol within the animal cell membrane
The Membrane as a Mosaic
A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer
ECM Glycoprotein Carbohydrate
GlycolipidEXTRACELLULAR SIDE OF MEMBRANE
Microfilaments Cholesterol Peripheral of cytoskeleton protein
Integral CYTOPLASMIC SIDE proteinOF MEMBRANE
Membrane Proteins
Proteins determine the specific functions of a membrane Two types of membrane proteins
Integral membrane proteins Peripheral membrane proteins
Integral Membrane Proteins
Penetrate the hydrophobic core of the lipid bilayer Transmembrane proteins
N-terminus
EXTRACELLULAR SIDE
C-terminus Helix CYTOPLASMIC SIDE
Peripheral Membrane Proteins
Appendages loosely bound to the surface of the membrane
Membrane Protein Function
1. Transportfunction to move substances from one side of membrane to other (channel protein shown-- left, carrier protein shown--right)
ATP
Enzymes
2. Enzymatic Activity- some receptor
proteins have this
3. Signal Transductionspecific binding site at top. Receives signal, sends to nucleus/rna to make another protein
Signal
Receptor
Membrane Protein Function
4. Intercellular Recognition- green structure is
antibody that attacks glyco-protein
Glycoprotein
5. Intercellular Attachment- transmembrane
proteins b/w 2 cells
6. Attachment to cytoskeleton and/or ECM
Membrane Carbohydrates
Important role in cell-cell recognition
Usually short, branched chains of sugars Glycolipids short oligosaccharides attached to membrane
lipids
Glycoproteins short oligosaccharides attached to
membrane proteins
Synthesis & Sidedness of Membranes
Membranes have distinct inside and outside faces
2 lipid layers may differ in composition
This affects the movement of proteins synthesized in the endomembrane system
Where do you build a glycoprotein that you want on the outside of a cell??? In the endoplasmic reticulum
Synthesis & Sidedness of Membranes
ER Transmembrane glycoproteins Secretory protein Glycolipid 1
Golgi 2 apparatus Vesicle
3
4 Secreted protein Plasma membrane: Cytoplasmic face Extracellular face Transmembrane glycoprotein
Figure 7.10
Membrane glycolipid
Selective Permeability
Membrane structure results in selective permeability A cell must exchange materials with its surroundings, a process controlled by the plasma membrane
The Permeability of the Lipid Bilayer
Hydrophobic molecules
Lipid solubale and can pass through the membrane rapidly
Hydrophilic molecules
Polar molecules Do not cross the membrane rapidly
***Ions have the hardest time moving across the plasma membrane b/c they are surrounded by a sphere of water molecules
Transport Proteins
Membrane-spanning proteins
Channel proteins Carrier proteins Highly specific
Allow passage of hydrophilic substances across the membrane
Selective Permeability
Overall traffic across membrane depends the on lipid bilayer and transport proteins Two major modes of transport across membranes
Passive Transport Active Transport
Passive Transport
Diffusion of a substance across a membrane with no energy investment Diffusion
The tendency for molecules of any substance to spread out evenly in the available space
Diffusion
Substances diffuse down their concentration gradient
The difference in concentration of a substance form one area to another Net movement from more concentrated to less concentrated
Diffusion
Molecules of dye
Membrane (cross section)
Net diffusion
Net diffusion
Equilibrium
Diffusion
Net diffusion Net diffusion
Net diffusion Net diffusion
Equilibrium Equilibrium
Osmosis
The movement of water across a semipermeable membrane Water diffuses across the membrane from the region of lower solute concentration to that of higher solute concentration
Is affected by the concentration gradient of dissolved substances
Osmosis
Figure 7.12
Water Balance: In Cells
Solute concentration and membrane permeability are important Tonicity
The ability of a solution to cause a cell to gain or lose water Depends on concentration of nonpenetrating solutes Has a great impact on cells without walls
Tonicity
The tonicity of a solution is in relation to the concentration of solute in the cell
Isotonic solutions
The concentration of solutes is the same as it is inside the cell There will be no net movement of water
Hypertonic solutions
The concentration of solutes is greater that it is inside the cell The cell will lose water Causes shriveling
Hypotonic solutions
The concentration of solutes is less than it is inside the cell The cell will gain water
Water Balance: Cells Without Walls
Hypotonic solution Isotonic solution Hypertonic solution
H2 O
H2 O
H2 O
H2 O
Lysed
Normal
Shriveled
Water Balance: Cells Without Walls
Osmoregulation is the control of water balanc
Important in animals and other organisms without rigid cell walls living in hypertonic or hypotonic environments
Water Balance: Cells with Walls
Plants, prokaryotes, fungi, protists
Cell walls help maintain water balance
Water Balance: Cells with Walls
Turgid cells
Hypotonic solutions Very firm, a healthy state in most plants
Flaccid cells
Plasmolysis
Hypertonic solution Cell shrivels, etc
Water balance: Cells with Walls
H2 O
H2 O
H2 O
H2 O
Turgid (normal)
Flaccid
Plasmolyzed
hypotonic hypertonic
isotonic
Facilitated Diffusion
Passive transport aided by proteins
Transport proteins speed the movement of molecules across the plasma membrane
Channel proteins Carrier proteins
Channel Proteins
Hydrophilic passageways
Aquaporins Ion channels Gated channels
Channel Proteins
EXTRACELLULAR FLUID
Channel protein
Solute
CYTOPLASM
Figure 7.15
Carrier Proteins
Undergo a subtle change in shape that translocates the solute-binding site across the membrane
Carrier Proteins
Carrier protein
Solute
Figure 7.15
Active Transport
Uses energy to move solutes against their gradients Allows a cell to maintain concentrations of molecules that differ from the external environment Only carrier proteins involved
The Sodium-Potassium Pump
1 Cytoplasmic Na+ binds to the sodium-potassium pump.
Na+ Na+
[Na+] high [K+] low
Na+ Na+ Na+ Na+ CYTOPLASM [Na+] low [K+] high P ADP ATP
2 Na+ binding stimulates phosphorylation by ATP.
Notice: Na out =3... K in =1. Means net change of 1
Na+ Na+ Na+
3 K+ is released and Na+
K+ K+ P
4 Phosphorylation causes the
sites are receptive again; the cycle repeats.
protein to change its conformation, expelling Na+ to the outside.
K+
K+
P K+
Pi
5 Loss of the phosphate
K+
Figure 7.16 restores the protein's original conformation.
6 Extracellular K+ binds to the
protein, triggering release of the Phosphate group.
Passive vs. Active Transport
ATP
Figure 7.17
Ion Transport
Membrane potential
The voltage difference across a membrane Cytoplasm negative compared to outside
Electrochemical gradient
Concentration gradient Membrane potential
+
EXTRACELLULAR FLUID H+
ATP
+
H+ Proton pump
H+
+ H+ + H+
CYTOPLASM
Figure 7.18 + + H+
Co-transport
Coupled transport by a membrane protein Occurs when active transport of a specific solute indirectly drives the active transport of another solute
Co-transport
Active transport driven by a concentration gradient +
ATP +
H+
H+ H+
H+
Proton pump
+
H+
+
Sucrose-H+ cotransporter
H+ Diffusion of H+
H+
+
+
Figure 7.19
Sucrose
Bulk Transport Across Membranes
Large proteins cross the membrane by different mechanisms Exocytosis: move something big out of the cell Endocytosis: move something big inside of the cell.
Exocytosis
Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents Secretion
Endocytosis
The cell takes in macromolecules by forming new vesicles from the plasma membrane
3 types of endocytosis:
Phagocytosis (cell eating) Pinocytosis (cell drinking) Receptor-mediated endocytosis (take in specific large molecule)
Phagocytosis
PHAGOCYTOSIS
EXTRACELLULAR CYTOPLASM FLUID Pseudopodium 1 m
Pseudopodium of amoeba Food or other particle
Bacterium
Food vacuole
Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM).
PINOCYTOSIS
Plasma membrane 0.5 m Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM).
Vesicle
EXTRACELLULAR CYTOPLASM FLUID Pseudopodium
1 m
Pinocytosis
Food or other particle
Pseudopodium of amoeba
PHAGOCYTOSIS
Bacterium Food vacuole Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM).
PINOCYTOSIS
Plasma membrane 0.5 m Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM).
Vesicle
ReceptorMediated Endocytosis
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein Receptor Coated vesicle
Ligand
Coated pit
Coat protein
A coated pit and a coated vesicle formed during receptormediated endocytosis (TEMs).
Plasma membrane 0.25 m