Physiology - Reband Ahmed

208 Pages·2010·10.46 MB·English

by  Dr. Reband

Masaryk University Faculty of Medicine Physiology Exam
Review of
Physiology

st
1 edition
By: REBAND AHMED

REBAND AHMED Spring Semester 2010
Masaryk University Faculty of Medicine Physiology Exam

REBAND AHMED Spring Semester 2010
Masaryk University Faculty of Medicine Physiology Exam
Part A
1. Structure and function of cell membranes
Structure
Lipid bilayer
The cell membrane consists primarily of a thin layer of amphipathic
phospholipids which are arranged so that the hydrophobic "tail"
regions are shielded from the surrounding polar fluid, causing the
more hydrophilic "head" regions to associate with the cytosolic and
extracellular faces of the resulting bilayer. This forms a continuous,
spherical lipid bilayer.
The arrangement of hydrophilic heads and hydrophobic tails of the lipid bilayer prevent polar solutes
(e.g. amino acids, nucleic acids, carbohydrates, proteins, and ions) from diffusing across the
membrane, but generally allows for the passive diffusion of hydrophobic molecules.
Integral membrane proteins
The cell membrane contains many integral membrane proteins. These structures, which can be
visualized by electron microscopy or fluorescence microscopy, can be found on the inside of the
membrane, the outside, or membrane spanning. These may include integrins, cadherins,
desmosomes, clathrin-coated pits, caveolaes, and different structures involved in cell adhesion.

Function
The cell membrane surrounds the protoplasm of a cell and, in animal cells, physically separates the
intracellular components from the extracellular space, thereby serving a function similar to that of
skin. In fungi, some bacteria, and plants, an additional cell wall forms the outermost boundary;
however, the cell wall plays mostly a mechanical support role rather than a role as a selective
boundary. The cell membrane also plays a role in anchoring the cytoskeleton to provide shape to the
cell, and in attaching to the extracellular matrix and other cells to help group cells together to form
tissues. The barrier is differentially permeable and able to regulate what enters and exits the cell,
thus facilitating the transport of materials needed for survival. The movement of substances across
the membrane can be either passive, occurring without the input of cellular energy, or active,
requiring the cell to expend energy in moving it. The membrane also maintains the cell potential.
Specific proteins embedded in the cell membrane can act as molecular signals that allow cells to
communicate with each other. Protein receptors are found ubiquitously and function to receive
signals from both the environment and other cells

REBAND AHMED Spring Semester 2010
Masaryk University Faculty of Medicine Physiology Exam
2. Structure and functions of cell organelles

Chromosomes

- Usually in the form of chromatin
- Contains genetic information
- Composed of DNA
- Set number per species (i.e. 23 pairs for human)

Nuclear membrane

- Surrounds nucleus
- Composed of two layers
- Numerous openings for nuclear traffic

Nucleolus

- Spherical shape
- Visible when cell is not dividing
- Contains RNA for protein manufacture

Centrioles

- Paired cylindrical organelles near nucleus
- Composed of nine tubes, each with three tubules
- Involved in cellular division
- Lie at right angles to each other

Chloroplasts

- A plastid usually found in plant cells
- Contain green chlorophyll where photosynthesis takes place

Cytoskeleton

- Composed of microtubules
- Supports cell and provides shape
- Aids movement of materials in and out of cells

REBAND AHMED Spring Semester 2010
Masaryk University Faculty of Medicine Physiology Exam
Endoplasmic reticulum

- Tubular network fused to nuclear membrane
- Goes through cytoplasm onto cell membrane
- Stores, separates, and serves as cell's transport system
- Smooth type: lacks ribosomes
- Rough type (pictured): ribosomes embedded in surface
Golgi apparatus

- Protein 'packaging plant'
- A membrane structure found near nucleus
- Composed of numerous layers forming a sac

Lysosome

- Digestive 'plant' for proteins, lipids, and carbohydrates
- Transports undigested material to cell membrane for removal
- Vary in shape depending on process being carried out
- Cell breaks down if lysosome explodes

Mitochondria

- Second largest organelle with unique genetic structure
- Double-layered outer membrane with inner folds called cristae
- Energy-producing chemical reactions take place on cristae
- Controls level of water and other materials in cell
- Recycles and decomposes proteins, fats, and carbohydrates, and forms urea
Ribosomes

- Each cell contains thousands
- Miniature 'protein factories'
- Composes 25% of cell's mass
- Stationary type: embedded in rough endoplasmic reticulum
- Mobile type: injects proteins directly into cytoplasm

Vacuoles
- Membrane-bound sacs for storage, digestion, and waste removal
- Contains water solution
- Contractile vacuoles for water removal (in unicellular organisms)

Cell wall

- Most commonly found in plant cells
- Controls turgity
- Extracellular structure surrounding plasma membrane
- Primary cell wall: extremely elastic
- Secondary cell wall: forms around primary cell wall after growth is complete

Plasma membrane

- Outer membrane of cell that controls cellular traffic
- Contains proteins (left, gray) that span through the membrane and allow passage of materials
- Proteins are surrounded by a phospholipid bi-layer.

REBAND AHMED Spring Semester 2010
Masaryk University Faculty of Medicine Physiology Exam
3. Passive transport across membranes. Cotransport.
Passive transport means moving biochemicals and atomic or molecular substances across the cell
membrane. Unlike active transport, this process does not involve chemical energy.
The four main kinds of passive transport are diffusion, facilitated diffusion, filtration and osmosis.
I. Diffusion is a process by which there is a net flow of matter from a region of high
concentration to one of low concentration. It is driven by a conc. gradient. It can be
measured using the following equation: J = -PA (C – C )
1 2
2
where: J = flux (flow) [mmol/sec], P = permeability (cm/sec), A = area (cm ), C = conc.
1 1
(mmol/L), C2 = conc.2 (mmol/L)

II. Facilitated diffusion is the spontaneous
passage of molecules or ions across a
biological membrane passing through
specific transmembrane transport proteins.
It occurs down an electrochemical gradient
(“downhill”), similar to simple diffusion.
Glucose transport in muscle an adipose cells is
“downhill”, is carrier-mediated, and is inhibited by
sugars as galactose; therefore, it is categorized as facilitated diffusion.

III. Filtration is a mechanical or physical operation which is
used for the separation of solids from fluids (liquids or
gases) by interposing a medium through which only the
fluid can pass. Oversize solids in the fluid are retained,
but the separation is not complete; solids will be
contaminated with some fluid and filtrate will contain
fine particles (depending on the pore size and filter
thickness).

IV. Osmosis is the diffusion of water molecules across a
selectively permeable membrane. The net movement of water
molecules through a partially permeable membrane from a
solution of high water potential to an area of low water
potential.

Co-transport
Also known as coupled transport, refers to the simultaneous or sequential passive transfer of
molecules or ions across biological membranes in a fixed ratio. Cotransporters can be classified as
symporters and antiporters depending on whether the substances move in the same or opposite
directions.
REBAND AHMED Spring Semester 2010
Masaryk University Faculty of Medicine Physiology Exam
4. Compartmentalization of body fluids
The total body fluid is distributed mainly between two compartments: the extracellular fluid
and the intracellular fluid. The extracellular fluid is divided into the interstitial fluid and the
blood plasma.
There is another small compartment of fluid that is referred to as transcellular fluid. This
.
compartment includes fluid in the synovial, peritoneal, pericardial, and intraocular spaces, as
well as the cerebrospinal fluid; it is usually considered to be a specialized type of extracellular
fluid, although in some cases, its composition may differ markedly from that of the plasma
or interstitial fluid. All the transcellular fluids together constitute about 1 to 2 liters.
In the average 70-kilogram adult human, the total body water is about 60 per cent of the
body weight, or about 42 liters. This percentage can change, depending on age, gender, and
degree of obesity. As a person grows older, the percentage of total body weight that is fluid
gradually decreases. This is due in part to the fact that aging is usually associated with an
increased percentage of the body weight being fat, which decreases the percentage of water
in the body. Because women normally have more body fat than men, they contain slightly
less water than men in proportion to their body weight.

Intracellular Fluid Compartment
About 28 of the 42 liters of fluid in the body are inside the 75 trillion cells and are collectively
called the intracellular fluid. Thus, the intracellular fluid constitutes about 40 per cent of the
total body weight in an “average” person.

Extracellular Fluid Compartment
All the fluids outside the cells are collectively called the extracellular fluid. Together these
fluids account for about 20 per cent of the body weight, or about 14 liters in a normal 70-
kilogram adult. The two largest compartments of the extracellular fluid are the interstitial
fluid, which makes up more than three fourths of the extracellular fluid, and the plasma,
which makes up almost one fourth of the extracellular fluid, or about 3 liters. The plasma is
the noncellular part of the blood; it exchanges substances continuously with the interstitial
fluid through the pores of the capillary membranes. These pores are highly permeable to
almost all solutes in the extracellular fluid except the proteins. Therefore, the extracellular
fluids are constantly mixing, so that the plasma and interstitial fluids have about the same
composition except for proteins, which have a higher concentration in the plasma.

Measurement
The volume of a fluid compartment in the body can be measured by placing an indicator
substance in the compartment, allowing it to disperse evenly throughout the compartment’s
fluid, and then analyzing the extent to which the substance becomes diluted.

REBAND AHMED Spring Semester 2010
Masaryk University Faculty of Medicine Physiology Exam
5. Differences between intra- and extracellular fluids

Ionic Composition of Plasma and
Interstitial Fluid Is Similar
Because the plasma and interstitial fluid are separated
only by highly permeable capillary membranes, their
ionic composition is similar. The most important difference
between these two compartments is the higher
concentration of protein in the plasma; because the
capillaries have a low permeability to the plasma proteins,
only small amounts of proteins are leaked into
the interstitial spaces in most tissues.
Because of the Donnan effect, the concentration of
positively charged ions (cations) is slightly greater
(about 2 per cent) in the plasma than in the interstitial fluid.

Important Constituents of the
Intracellular Fluid
The intracellular fluid is separated from the extracellular fluid by a cell membrane that is highly
permeable to water but not to most of the electrolytes in the body.
In contrast to the extracellular fluid, the intracellular fluid contains only small quantities of sodium
and chloride ions and almost no calcium ions. Instead, it contains large amounts of potassium and
phosphate ions plus moderate quantities of magnesium and sulfate ions, all of which have low
concentrations in the extracellular fluid. Also, cells contain large amounts of protein, almost four
times as much as in the plasma.

REBAND AHMED Spring Semester 2010
Masaryk University Faculty of Medicine Physiology Exam

6. Production and resorption of interstitial fluid (Starling forces)
Interstitial fluid is a solution that bathes and surrounds the cells of
multicellular animals. It is the main component of the extracellular
fluid, which also includes plasma and transcellular fluid.
The interstitial fluid is found in the interstitial spaces, also known as
the tissue spaces.
On average, a person has about 11 litres (2.4 imperial gallons) of
.
interstitial fluid, providing the cells of the body with nutrients and a
means of waste removal.
Formation of interstitial fluid
Hydrostatic pressure is generated by the systolic force of the heart. It pushes water out of the
capillaries (Starling forces).
The water potential is created due to the ability of small solutes to pass through the walls of
capillaries. This buildup of solutes induces osmosis. The water passes from a high concentration (of
water) outside of the vessels to a low concentration inside of the vessels, in an attempt to reach an
equilibrium. The osmotic pressure drives water back into the vessels. Because the blood in the
capillaries is constantly flowing, equilibrium is always reached.
The balance between the two forces differs at different points on the capillaries. At the arterial end
of a vessel, the hydrostatic pressure is greater than the osmotic pressure, so the net movement
favors water and other solutes being passed into the tissue fluid. At the venous end, the osmotic
pressure is greater, so the net movement favors substances being passed back into the capillary. This
difference is created by the direction of the flow of blood and the imbalance in solutes created by the
net movement of water favoring the interstitial fluid.
Removal of interstitial fluid
To prevent a build-up of interstitial fluid surrounding the cells in the tissue, the lymphatic system
plays a part in the transport of interstitial fluid. Interstitial can pass into the surrounding lymph
vessels, and eventually ends up rejoining the blood.
Sometimes the removal of tissue fluid does not function correctly, and there is a build-up. This
causes swelling, and can often be seen around the feet and ankles, for example Elephantiasis. The
position of swelling is due to the effects of gravity.

REBAND AHMED Spring Semester 2010
Masaryk University Faculty of Medicine Physiology Exam
7. Ion channels
Ion channels are integral proteins that span the membrane and, when open, permit the passage of
certain ions.
1. Ion channels are selective; they permit the passage of some ions, but not others. Selectively
is based on the size of the channel and the distribution of charges that line it.
2. Ions channels may be opened or closed. When the channel is open, the ion(s) for which it is
selective can flow through it. When the channel is closed, ions cannot flow through them.
3. The conductance of a channel depends on the probability that the channel is open. The
higher the probability that a channel is open, the higher the conductance, or permeability.
Opening and closing of channels are controlled by gates.
a. Voltage-gated channels are open or closed by changes in membrane potential.
 The activation gate of the Na+ channel in nerve is opened by depolarization;
when open, the nerve membrane is permeable to Na+ (e.g., during the
upstroke of the nerve action potential).
 The inactivation gate of the Na+ channel in nerve is closed by
depolarization; when closed, the nerve membrane is impermeable to Na+
(e.g., during the repolarization phase of the nerve action potential).
b. Ligand-gated channels are opened or closed by hormones, second messengers, or
neurotransmitters.
 For example, the nicotinic receptor for acetylcholine (ACh) at the motor end
plate is an ion channel that opens when Ach binds to it. When open, it is
permeable to Na+ and K+, causing the motor end plate to depolarize.

REBAND AHMED Spring Semester 2010