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 = 4/3 πr 3
SA increases with square of diameter
SA = 4 πr 2

Diameter
(cm)
SA (cm 2 )
Volume
(cm 3 )
SA:volume ratio
0.5
0.79
0.06
13:1
1.0
3.14
0.52
6:1
1.5
7.07
1.77
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. O
2
, CO
2
, 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. O
2
, CO
2
, fat-soluble vitamins e.g. [O
2
] 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?
[H
2
O] ↑ 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 H
2
O 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 (1 st messenger) binds to receptor
• Receptor activates G-protein
• G-protein stimulates effector protein (enzyme)
• Effector protein produces 2 nd messenger inside cell
• 2 nd messenger activates kinase enzymes
• Kinases activate other enzymes
→ various cellular responses