Download Cells

Introduction to Cells
&
The Cell Membrane
Chapter 4
Overview
• Cell structure & function
• Cell membrane structure & function
• How cells interact
Cell Theory
1. Organisms consist of 1 or more cells.
2. Cell = smallest unit of life
3. Continuity of life comes from growth &
division of cells
What is a Cell?
Made of C, H, O, N + trace elements
Has all the properties of life:
– Metabolism
– Responsiveness
– Growth
– Reproduction
Cells differ in size, structure, & function
Cell Function
Performs all vital physiological functions
Maintains homeostasis via processes
happening within
Building blocks of all organisms
Generalized Cell Structure
Plasma membrane
Region of DNA
Cytoplasm
Plasma Membrane
Lipid bilayer
– Separates internal & external environments
Semi-permeable
– Controls passage of substances in/out of cell
Region of DNA
Cellular control centre
Holds genetic information
Eukaryotic cells:
– Inside membrane-bound
nucleus
Prokaryotic cells:
– Free-floating in cytoplasm
Cytoplasm
“Guts” of cell
Semi-fluid matrix
– Colloidal properties
Contains all structural
components
(organelles, etc.)
Why Are Cells So Small?
Surface area:volume ratio
Volume increases with cube of diameter
V = 4/3πr3
SA increases with square of diameter
SA = 4πr2
Diameter
(cm)
0.5
1.0
1.5
SA (cm2)
0.79
3.14
7.07
Volume
(cm3)
0.06
0.52
1.77
SA:volume
ratio
13:1
6:1
4:1
If cells were big:
– Plasma membrane would have to work very hard
to service all of cytoplasm
– Materials would have a harder time moving
through the cytoplasm
– Movement of substances across membrane
would not be fast enough to maintain cell activity
Cells that aren’t tiny are usually long & thin or
have folds to increase SA
Cell (Plasma) Membrane
Semi-permeable barrier between interior of
cell & external environment
Fluid-mosaic model:
Lipid bilayer
(prevents movement of water-soluble substances)
Dynamic pattern of proteins
(some able to move through membrane)
(involved in membrane function)
Lipid Bilayer
Consists of:
– Phospholipids
– Glycolipids
– Cholesterol
– Lipid Rafts
Membrane Lipids: Phospholipids
Hydrophilic (polar) head
Hydrophobic (non-polar) fatty acid tails
(unsaturated = kinks in tails ↑ membrane fluidity)
Biological membranes naturally form
closed, spherical structures that can
reseal quickly if torn
Membrane Lipids: Glycolipids
5% of membrane lipids
Phospholipids with sugar groups
Found only on outer membrane surface
(cell signalling & recognition)
Membrane Lipids: Cholesterol
20% of membrane lipids
Wedges between phospholipid tails
Has both hydrophobic & hydrophilic
regions
Maintains integrity of membrane by
keeping it firm & impermeable to
some water-soluble molecules
Increases fluidity of membrane by
ensuring that fatty acid chains don’t
crystallize
Membrane Lipids: Lipid Rafts
20% of membrane lipids
Found only on outer membrane surface
Variety of tightly packed saturated lipids
(= stable, less fluid)
Used as concentrating platforms for cell signalling
molecules
Overview of Membrane Proteins
50% of membrane mass
Carry out most membrane functions
• Integral proteins
• Peripheral proteins
Overview: Integral Proteins
Most are transmembranal
Have both hydrophobic &
hydrophilic regions
Used mainly for transport
(channels, carriers, receptors)
Overview: Peripheral Proteins
Usually located at one membrane surface
Attach loosely to integral proteins or membrane
lipids
Functions include: structural support for cell,
enzymatic action, joining cells, changing cell
shape
Major Membrane Proteins
Receptor proteins
Recognition proteins
Adhesion proteins
Communication proteins
Transport proteins
(passive & active transporters)
1. Receptor Proteins
Binding sites for hormones, etc.
Allow changes in cell activities
(protein synthesis, cell division, etc.)
Different cells have different
receptor proteins
2. Recognition Proteins
In multicellular organisms
Identify cells as foreign or self
Used in tissue defense, cell adhesion, etc.
3. Adhesion Proteins
In multicellular organisms
Allow cells of same type to find & stick to
each other or to other substances
(e.g. proteins in EC matrix)
4. Communication Proteins
Multicellular organisms
Form channels between
cytoplasm of 2 cells
Allow chemical & electrical
signals to flow between
cells
5. Transport Proteins
Have interior channels
Solute enters channel & binds weakly to
protein
Protein changes shape
Channel closes behind solute & opens in front
of solute
Solute is released on other side
Protein regains normal shape
5a. Passive Transport Proteins
Move solutes & water across
membrane down concentration
gradients
No energy input required
One-way or bi-directional
Some are ion-selective channels:
(have gates that open/close depending
on molecular, chemical, etc. signal)
5b. Active Transport Proteins
Pump solutes across membrane against
concentration gradients
Require energy input
One-way or bi-directional
Some are co-transporters:
(allow passive transport of some solutes while pumping
others in the opposite direction)
So What is a Concentration Gradient?
Different in concentration of ions or
molecules between 2 adjacent areas
With no energy input, molecules move down
gradient from [high] to [low]
Diffusion
Net movement of ions/molecules down
concentration gradient
Each ion/molecule has its own gradient
Factors Affecting Diffusion Rate
1. Steepness of concentration gradient
2. Temperature
3. Size of ions/molecules
4. Electric gradients
5. Pressure gradients
So … the cell membrane is semi-permeable
= allows passage of some substances but not
of others
What controls what, how much, & when
substances cross the membrane?
Membrane is mostly non-polar
= allows small, non-polar molecules to
cross (e.g. O2, CO2, etc.)
= impermeable to ions & large, polar
molecules (e.g. glucose, Na+, K+, etc.)
Water (although polar) can slip through
gaps caused by kinks in tails or can use
aquaporin transporters
Movement Mechanisms
Passive transport
– Simple diffusion
– Facilitated diffusion
Active transport
Exocytosis
Endocytosis
1. Passive Transport
Net movement down concentration gradient
No ATP energy required
1a. Simple Diffusion
Direct diffusion through membrane
Non-polar, lipid-soluble, & small molecules
e.g. O2, CO2, fat-soluble vitamins
e.g. [O2] in blood is higher than in cell, so
continuously diffuses in
1b. Facilitated Diffusion
Substances transported passively via
channels/carriers (proteins)
Some channels are always open; others open &
close on cue
Transport limited by # & activity of
channels/carriers
2. Active Transport
Pumps solutes against concentration gradient
ATP energy required
Substrate-specific transporters
(activated by phosphate group from ATP)
e.g. sodium-potassium pump
• Uses carrier enzyme
Na+K+ATPase
• [K+] ↑ inside cell, [Na+] ↑
outside cell
• Both seep through
leakage channels down
concentration gradients
• Na+K+ pump drives Na+
out & pumps K+ in
Now we know how ions & molecules cross
the selectively permeable plasma
membrane.
What about water?
Osmoregulation
= control of water balance
Prevents excessive uptake / loss of water
e.g. fish have gills & kidneys
Osmosis
Diffusion of water across semi-permeable
membrane from [high] to [low]
How can water have a concentration?
[H2O] ↑ as [dissolved solute ] ↓
In other words, the more dilute a solution is,
the more concentrated the water is
Tonicity
Relative solute concentrations of two fluids
e.g. on opposite sides of a membrane
Determines the direction & how much water
movement will occur across a membrane
When comparing two fluids:
Hypotonic solution:
= the one with fewer solutes
Hypertonic solution:
= the one with more solutes
Isotonic solutions:
= have the same concentrations
Water diffuses from hypotonic fluids to
hypertonic fluids
a. Hypotonic Animal Cell
b. Hypertonic Animal Cell
c. Isotonic Animal Cell
So Why Don’t Animal Cells Burst Easily?
As H2O enters cell via osmosis, solutes are
transported out
= osmoregulation
Hydrostatic pressure against the membrane
also controls osmosis
Note: there is a point where cells will lyse
(burst)
Because of their rigid cell walls, plant cells
face a different scenario
Plant Cells & Osmoregulation
Isotonic solution
= becomes flaccid & wilts
Hypertonic solution
= shrivels
= plasmolysis
(plasma membrane
separates from cell wall)
Plant cells are happiest in hypotonic
environments
Become turgid
Net inflow of water
Cell wall expands but does not
burst
Other Transport Mechanisms
Large particles & other substances can be
moved between the external environment,
the plasma membrane, & the interior of the
cell via:
• Exocytosis
• Endocytosis
Remember:
Membranes are self-sealing
Hydrophobic interactions between
phospholipid tails & water molecules
creates spherical structures
1. Exocytosis
Vesicle w/i cytoplasm moves to cell membrane
Fuses with membrane
Releases contents to exterior of cell
2. Endocytosis
Outer membrane inpouches around particle
outside of cell
Contents released to cell interior
(can then be moved to or stored in organelles)
3 types:
– Receptor-mediated endocytosis
– Phagocytosis
– Bulk-phase endocytosis
2a. Receptor-Mediated Endocytosis
Substance (hormone, vitamin, mineral, etc.)
binds to receptors on membrane
Pit forms beneath receptor
Pit sinks into cytoplasm, forming vesicle
2b. Phagocytosis
“Cell eating”
e.g. amoebas, macrophages, etc.
Pseudopods extend around substance, forming
vesicle
Vesicle moves into cytoplasm & fuses with
lysosome, which digests contents of vesicle
2c. Bulk-Phase Endocytosis
Non-selective process
Vesicle forms around ECF & carries it into
cytoplasm
Doesn’t the continuous formation of vesicles
drastically change the surface area of the
cell membrane?
Endocytosis & exocytosis occur at rates that
maintain the total SA of the plasma
membrane
Losses via endocytosis ≈ replacements via exocytosis
Cell Junctions
Some cells are free-floating
Most knit together
Molecular structures allow communication b/w cells:
Plant cells: plasmodesmata
Animal cells: tight junctions, desmosomes, gap junctions
a. Plasmodesmata
Channels
Connect cytoplasms of 2
adjacent cells
Allow rapid exchange of
materials
b. Tight Junction
Integral proteins of adjacent cells fuse
Impermeable to leakage between cells
Join cells of most body tissues
e.g. stomach lining
c. Desmosomes
a.k.a. adhering junction
Zipper-like seal
Binds cells together internally &
externally by protein filaments
Distributes tension evenly to ↓
risk of tearing
e.g. skin, heart muscle, neck of
uterus
d. Gap Junctions
Channels that connect
cytoplasms of 2 adjacent cells
Allow rapid exchange of
materials
Occur in electrically-excitable
tissues
e.g. heart muscle, smooth
muscle
(allows synchronicity of electrical
activity & contraction)
Cell-Environment Interactions
Membrane receptors
Integral proteins & glycoproteins used as
binding sites
– Contact signalling
– Chemical signalling
a. Contact Signalling
Physical contact between cells
Glycocalyx
= glycoproteins with branching
sugar sidechains
= unique to each cell type
Aids in cell recognition
b. Chemical Signalling
Ligands
= signalling chemicals that bind to membrane
receptors
(include neurotransmitters, hormones, etc.)
Receptors sense molecules outside cell &
activate signal transduction pathways that
lead to cellular responses
Many receptors involved in diseases but also
used as targets for drugs
e.g. G-protein linked receptors
• Ligand (1st messenger) binds to
receptor
• Receptor activates G-protein
• G-protein stimulates effector
protein (enzyme)
• Effector protein produces 2nd
messenger inside cell
• 2nd messenger activates kinase
enzymes
• Kinases activate other enzymes
→ various cellular responses