Cells: The Living Units

Introduction

Life in general occurs in an aqueous environment

All chemical processes essential to life occur within the aqueous
environment of the cell and surrounding fluids contained in the
body

To survive humans must maintain a normal volume and
composition in ICF and ECF

TBW = 50 to 60% BW (Approx. 40L)

ICF ~ 25L (Cytoplasm)

ECF ~ 15L (Interstitial fluid, CSF , Blood Plasma & Lymph)
Plasma Membrane: Structure (Fig. 3.3)
 Structure that forms the boundary between the cell and its
surroundings
 Cellular
membranes surround cell organelles
 Made from lipids that form a lipid bilayer
 Materials enclosed within the plasma membrane may be loosely
referred to as cytoplasm
Plasma Membrane: Structure

Singer and Nicolson (1972) published the fluid mosaic model of the
cell membrane

Primary structure of the cell membrane is the lipid bilayer


Phospholipids:

Major constituent of lipid bilayer

Move freely within the membrane
Cholesterol:

About 20% of lipids

Stabilizes neighboring phospholipids and decrease flexibility of
cell membrane
Plasma Membrane: Structure
 Proteins:
 Make
up approx. half the mass of the cell membrane
 Associated
with the cell membrane either by insertion or by
association with one surface
 Types
of proteins
 Integral
Proteins (Transmembrane Proteins)
 Peripheral
proteins
Plasma Membrane: Structure

Integral Proteins

Tightly associated with the membrane

Part extends into lipid bilayer

Extended portion has hydrophobic amino acids held by
hydrophobic interactions

Transmembrane proteins extend all the way through the
membrane

Peripheral Proteins

Bound to the membrane surface

Held in place by weak, non-covalent bonds
Plasma Membrane: Structure

Glycoproteins

Coat outer surface of the membrane

Forms sticky layer called glycocalyx

Different cell types within the body have different CHO in the
glycocalyx thus cells are able to recognize each other

Flexibility of membranes allow changes in shape of cells and
organelles

Fluidity allows membranes to fuse and break apart

Most body cells are attached to other cells (RBCs are exceptions)
Plasma Membrane: Structure (Fig. 3.5)
 Specialized “Membrane Junctions”
 Tight
 An
Junctions
impermeable junction formed by the fusion of a series of
integral protein molecules in the plasma membrane of
adjacent cells
 Desmosomes
 Mechanical
(Anchoring junctions)
couplings scattered like rivets along the sides of
abutting cells that prevent their separation
 Gap
Junctions (Nexus)
 Communicating
junction that allows chemical substances to
pass between adjacent cells
Plasma Membrane: Functions

Diffusion

Tendency of molecules or ions to disperse and be evenly
distributed in an environment


Recall that all molecules possess kinetic energy
Net movement occurs along a concentration gradient, from
higher concentration to lower concentration

Cellular membranes restrict and regulate the movement of
particles within the body

Membranes that allow movement across are described as
semi-permeable membranes
Plasma Membrane: Functions
 Simple (Passive) Diffusion
 Direct
movement of particles through the lipid bilayer along a
concentration gradient
 No
energy is provided by the cell (passive)
 Such
molecules are usually small and nonpolar (02, CO2, CH4)
 Some
larger nonpolar molecules can also cross in this manner
(steroid hormones)
Factors That Affect The Rate of Diffusion

Particle size

Charge

Non-polar molecules move easily

Polar molecules do not move easily

Charged particles cannot move across
(all in reference to the lipid bilayer)

Temperature

Concentration gradient

Number of carrier proteins or channels
Plasma Membrane: Functions

Facilitated Diffusion

‘Assisted’ movement of particles across cellular membranes
along a concentration gradient

‘Assist’ is provided by carrier (transport) proteins, or channels
(both transmembrane proteins)

Carriers attach to the given particle and shuttles it across the
membrane

Channels are made up of proteins that form a hole through
the lipid bilayer

Open Channels

Gated channels
Plasma membrane: Functions (Fig. 3.9)
 Osmosis
 Passive
movement of solvent molecules along a concentration
gradient
 Recall

solute, solvent, and solution
Two solutions are isotonic if they have the same
concentration

Hypertonic solution:Higher concentration (more solute; less
solvent; high osmotic pressure)

Hypotonic solution: Lower concentration (less solute, more
solvent, low osmotic pressure)
Plasma Membrane: Functions
 Filtration
A
process that forces water and solutes through a membrane or
capillary wall by fluid or hydrostatic pressure
 Passive
process, not selective and involves a pressure gradient
 Examples
 Fluid
movement out of capillaries
 Fluid
excreted by kidneys as urine
Plasma Membrane: Functions
 Active Transport
 Movement
 Energy
of particles against a concentration gradient
(generally in the form of ATP) is required and is provided
by the cell
 Example:
 Active
pumping of two K+ and three Na+ when Na+/K+ ATPase
hydrolyses a molecule of ATP
 Active
transport is NOT diffusion
Plasma Membrane: Functions
 Vesicular Transport
 Two
main categories
 Endocytosis
 Exocytosis
 Molecules
(bringing substances into the cell)
(expelling substances from within the cell)
are usually too large to cross membrane even with the
help of a carrier
Plasma Membrane: Functions
 Bulk-Phase endocytosis
 Plasma
membrane folds inward trapping substances from the
surrounding
 Invaginated
region of membrane pinches off from plasma
membrane and enters cell
A
vesicle (phagosome), containing the trapped substances, is
formed
Plasma Membrane: Functions
 Phagocytosis
 Describes
a situation where one cell engulfs another cell
 Receptor-mediated endocytosis (Fig. 3.12)
 Small
particles attach to receptor on the external surface of cell
membrane
 Receptor
and particle aggregates at one region of cell
membrane
 Region
of cell membrane invaginates to form a vesicle
Active or Passive?
 Osmosis
 Exocytosis
 Filtration
 Simple diffusion
 Endocytosis
 Facilitated diffusion
Match the following
A. Osmosis
B. Exocytosis
C. Filtration
D. Simple diffusion
E. Endocytosis
F. Facilitated diffusion
1. White blood cell engulfing bacteria
2. Neurotransmitter release at neuron terminal
3. Gas exchange at lung
4. Exchange at capillaries due to pressure gradient
5. Calcium movement through voltage-gated channels
6. Movement of a solvent along its concentration gradient