This document discusses cellular homeostasis and the role of the cell membrane in maintaining homeostasis. It begins by defining homeostasis as the ability to maintain stable internal conditions despite external fluctuations. It then discusses how homeostasis operates at the whole body, tissue, and cellular levels. The document focuses on the cellular level and the cell membrane's role in regulating transport using channel, carrier, and active transport proteins. It describes different types of cellular transport including passive diffusion, facilitated diffusion, osmosis, and active transport processes like endocytosis and exocytosis. The document emphasizes how these transport mechanisms help cells regulate conditions to survive in varying external environments.

1. Presented by:
Manju Chhetri
Kathmandu University
(2015)

2. HOMEOSTASIS ???
 Homeostasis is the capability of a biological system to
maintain a relatively stable internal milieu in a
fluctuating external environment.
 Homeostatic control operates at all levels of biological
organizations.
1. Whole‐body level
2. Tissue/organ level
3. Cellular level
Adaptive cellular stress response is the process by which a
homeostatic mechanism is activated to restore the internal
state of the cell after initial perturbation.

4. Contd….
 cellular homeostasis, which we define here as the
study of the mechanisms by which cells maintain a
constant intracellular environment.

5. CELL MEMBRANE FUNCTION and
STRUCTURE
The CELL MEMBRANE is chiefly responsible for
maintaining homeostasis inside a living cell using
different methods to transport molecules in and out of
the cell.
 Too much water can burst the cell
 Too many wastes can poison the cell
 The cell cannot tolerate any great variations in ion
conditions.

6. Cell Membrane : Important for cell
homeostasis
1. Isolate the cytoplasm from the external
environment
2. Regulate the exchange of substances (gases and
ions)
3. Communicate with other cells
4. Identification (proteins and carbohydrates on its
surface)

7. Proteins Embedded in Membrane Serve
Different Functions
Transport Proteins : regulate movement of substance
Channel Proteins
 form small openings for molecules to diffuse through
like water
Carrier Proteins
 binding site on protein surface "grabs" certain molecules
and pulls them into the cell

8. Channel Protein

9. Carrier Protein

10. Gate Protein

11. Why transport protein?
 Enable the cell to maintain and regain needed
chemical components for metabolism and cell
function.
 Help cell to maintain concentrations of chemicals and
water
 To get rid of excess materials without interfering with
continual cell processes.

12. Gated Channels
similar to carrier proteins, not always
"open"—eg. Bind and pull in calcium ions
when needed. This requires cell energy—
active transport.

13. Receptor Proteins
molecular triggers that set off cell
responses (such as release of hormones or
opening of channel proteins)
e.g. The junction between
nerve cells requires the
transmission of
neurotransmitters between
synaptic gaps—these
chemicals bind onto
receptor proteins.

14. Types of Cellular Transport
Passive Transport
cell doesn’t use energy
1. Diffusion
2. Facilitated Diffusion
3. Osmosis
Active Transport
cell does use energy
1. Protein Pumps
2. Endocytosis
3. Exocytosis
high
low
Weeee!!!
high
low
This is
gonna be
hard
work!!

15. 1. Passive Transport
Simple Diffusion - water, oxygen and other
molecules move from areas of high concentration to
areas of low concentration, down a concentration
gradient.
Note – Osmosis is the diffusion of water
Facilitation Diffusion - diffusion that is enabled by
proteins (channel or carrier proteins) which bind
onto required molecules so that they flow into the cell.

16. Cellular Homeostasis and water
concentrations
 Three basic types of cellular environments;
1. Isotonic solutions: these solutions have the same or
similar amounts of dissolved materials as the cytoplasm of
a cell
2. Hypotonic solutions: have a greater concentration of
water than what is present in the cell
3. Hypertonic solutions: have greater amounts of dissolved
materials than what is present inside the cell.
 Each of the solutions describe have an effect on the
cell’s ability to survive.

17. Cellular Homeostasis and water
concentrations
 Isotonic Solutions: do not have an effect on a cell, as they
mimic the cell’s inner solution. Ie. saline solution for eyes.
 Hypotonic Solutions: will cause a cell to enlarge or break.
Since the concentration of water is greater outside the cell,
the water will flow into the cell until equilibrium is found.
 Hypertonic Solutions: will cause a cell to shrink or
collapse. Since the environment has a greater
concentration of dissolved materials, water from the cell
will move to the environment until the environment has
the same concentration of water as the cell.

20. Factors Affecting Rate of
Diffusion

21. 1. Size
 small molecules can slip through phospholipids
bilayer easier than large molecules
 very large molecules may not be able to diffuse at all
2. Concentration
the greater the concentration gradient (bigger range)
the quicker a material diffuses (makes the molecules
want to move faster) – think of a crowded room

22. 3. Temperature
In general as temperature increases – molecules move
faster which translates into faster diffusion
4. Polarity of molecules
Water-soluble (polar) molecules will not easily move
through the membrane because they are stopped by
the middle water-insoluble (nonpolar) layer

23. 5. Surface Area
As a cell’s size increases its volume increases much
quicker than it’s surface area.
 If you double individual lengths (1 cm to 2 cm) the
surface areas increases 4 times, and the volume
increases 8 times.
If cell size is doubled, it would require 8 times
more nutrients and have 8 times s much waste. SA
only increases by a factor of 4 – not enough surface
area through which nutrients and wastes could
move.
 Cell would either starve or be poisoned (waste products)

24. ACTIVE TRANSPORT

25. Active Transport
Involves moving molecules "uphill" against the
concentration gradient, which requires energy.
 Uses carrier protein molecules as receptors.
 One may transport calcium ions , another glucose
molecules.
 There are hundreds of these types of protein molecules.
*Each one changes shape to accommodate a specific
molecule.

26. CONTD…..
Their activity can be stopped from transporting
molecules with inhibitors (unfortunately, these are
usually poisons) which:
 either destroy the membrane protein
 or just plug it up
(e.g. for neurons – tetanus &
botulinum-B secrete a poison that
suppress the Na/K pump)

27. Sodium-Potassium Pump
Pumps out 3 sodium atoms for ever 2 potassium atoms
taken in against gradient in the cell.

28. The H+/K+ ATPase
The parietal cells of stomach (lining) use this
pump to secrete gastric juice.
These cells transport hydrogen ions (H+) from a
concentration of about 4 x 10-8 M within the cell to
a concentration of about 0.15 M in the gastric juice
(giving it a pH close to 2).
 Recall: pH – power of the H+ ion

29. The H+/K+ ATPase (Proton Pump)

30. Exocytosis
Moves large, complex molecules such as proteins out
of the cell membrane.
Large molecules, food, or fluid droplets are packaged
in membrane-bound sacs called vesicles.

31. Endocytosis
Endocytosis moves large particles (huge molecules or
molecular conglomerates) into a cell.

32. Phagocytosis
Phagocytosis is another type of endocytosis
used for massive transport.
Cell membrane extends out forming pseudopods
(fingerlike projections) that surround the particle.
Membrane pouch encloses the material & pinches
off inside the cell making a vesicle.
Vesicle can fuse with lysosomes(digestive
organelles) or release their contents in the
cytoplasm

33. Used by ameba to feed & white blood cells to kill
bacteria.
Known as “killer cells"

34.  Pinocytosis is another type of endocytosis
 Cell membrane surrounds fluid droplets
 Fluids taken into membrane-bound vesicle
 Known as “cell drinking”

35. •Exocytosis is used to remove
large products from the cell
such as wastes, mucus, & cell
products such as hormones
and antibodies.
•Exocytosis is the process
used by our memory cells
(white blood cells that
produce antibodies to fight
infection).
•It is also used by our gland
cells to secrete hormones
when needed.

36. Transport Flowchart
Transport of Materials
Across a Membrane
Active
PhagocytosisPinocytosis
Ion
Pump
Facilitated
Diffusion
Osmosis
Simple
Diffusion
Passive
Endocytosis Exocytosis

37. Active Transport Proteins
 These proteins, called pumps, move molecules and
ions against concentration gradients.
 They move materials from areas of low concentration
to high concentration.
 To do this they require ATP.
 The Na-K pump (sodium potassium pump) is an
example of an active transport protein.

38. Active Transport Pump

39. Thank you!

Any queries please write to me on comment below or
email me at manjuchhetri@gmail.com