Biology P530: BY NAKAPANKA JUDE MAYANJA 0704716641. Biology department SMACON 2017
MOVEMENT IN AND OUT OF CELLS
The plasma membrane isolates the inside of the cell protoplasm from its extracellular environment. Materials are exchanged between
the protoplasm and the extracellular environment across the plasma membrane. The plasma membrane is selectively permeable and
allows transport of materials across it.
Substances move in and out of cells by the following processes:
a. Simple diffusion e. Endocytosis
b. Facilitated diffusion i. Phagocytosis
c. Osmosis ii. Pinocytosis
d. Active transport iii. Receptor mediated endocytosis
f. Exocytosis
The transport of substances is important to;
a) Supply cells with oxygen for respiration and raw materials for anabolism (synthesis of biological molecules)
b) Regulate the pH and solute concentration for maintaining a stable internal environment for enzymes to function
optimally
c) Excrete toxic waste substances
d) Secrete useful substances for cell activities
Note: the transport of substances across the cell membrane takes place by two major fundamental processes.
SIMPLE DIFFUSION
Diffusion is the random movement of ions or molecules from a region where they are at higher concentration to a region of
lower concentration. That is, to move down a concentration gradient until equilibrium is reached.
The phospholipid bilayer is permeable to very small and
uncharged molecules like oxygen and carbon dioxide. These
molecules diffuse freely in an out of the cell through the
phospholipid bilayer. Hydrophobic substances (lipid-soluble)
e.g. steroids, can also diffuse through. These non-polar
molecules do not require the aid of membrane proteins (channel
or carrier) to move across the cell membrane.
Even though water is an extremely small, its polar therefore it
does not move across the cell membrane by simple diffusion.
A charged molecule or atom and its surrounding shell of water, find the hydrophobic layer (non-polar) of the membrane more
difficult to penetrate thus the lipid bilayer partly accounts for the membrane’s selective permeability by preventing very large
molecules and small polar molecules of ions to move across it.
The rate of diffusion depends upon;
The concentration gradient
This refers to the relative concentration on either side of the membrane or between two points. The greater the difference
between the points, the faster the rate of diffusion and if the difference is less, the slower the diffusion rate. Therefore a
reduced concentration gradient causes a reduced rate of diffusion and vice versa.
Temperature
When increased, temperature causes an increased rate of diffusion because the particles acquire increased kinetic energy
which causes increased speed of movement hence increased rate of diffusion.
At low temperatures, the kinetic energy is very low and the speed of movement by particles is equally very low.
Surface area
The larger the surface area over which the molecules are exposed, the faster the rate of diffusion.
Distance over which diffusion takes place
Page 1 of 82
, BY NAKAPANKA JUDE MAYANJA 0704716641 2017.
This is the distance over which the molecules are to travel i.e. the surface thickness across which the molecules move. The
greater the distance the lower the rate of diffusion
Size and nature of diffusing molecules
The smaller the size of the diffusing particles, the faster they diffuse i.e. smaller particles move very fats while the large
ones will move slowly.
Permeability
The more porous a surface is, the greater the number of particles that diffuse through it hence the greater the rate of Page | 2
diffusion
Significance of diffusion In order maximize the rate of diffusion, tissues where diffusion occurs
attained special adaptations. These include;
a) It’s a means by gaseous exchange occurs in plants
and animals e.g. in plants diffusion of gases occur 1. The lungs are ventilated by the respiratory tract (trachea,
through the stomata and in animals, in gills of fish, , bronchus, bronchioles) which maintain a steep concentration
the skin and buccal cavity of amphibians alveoli of gradient between the lung alveoli and blood in the capillaries.
reptiles, mammals and birds. 2. Respiratory surfaces like the lung alveoli and intestine epithelial
b) Absorption of certain digested food materials e.g. lining possess a rich supply of blood vessels which transport
glucose in the ileum. away the diffusing materials hence maintaining a steep gradient
c) A means of exchange of materials between blood in which sustains the fast diffusion
capillaries and the tissues 3. Diffusion surfaces e.g. lung alveoli and intestines (ileum) are
d) During formation of the nerve impulse, sodium ions covered by a thin epithelium lining which reduces the distance
diffuse into the nerve cells facilitating generation of over which diffusion takes place.
nerve impulses and ensures transmission of nerve 4. The epithelial lining covering the alveoli and rumen of the ileum
impulses from one neurone to another i.e. diffusion is very permeable to allow molecules to travel across them
facilitates synaptic transmission 5. In lungs there are numerous alveoli and in the ileum infoldings
e) It ensures excretion of waste products e.g. ammonia known as villi and microvilli which is coupled with a very long
in fresh water fishes ileum also increases the surface area along which particles move
f) It’s the main means of transportation of materials into cells hence increase the rate of diffusion.
within the cell’s cytoplasm e.g. in unicellular 6. Flattened body e.g. platyhelminthes (flatworms) which increases
organisms the surface area for movement of materials by diffusion
g) Absorption of mineral salts by plants from the soil is 7. Some organisms are of small size e.g. unicellular organisms
effected by diffusion as one of the mechanisms which increases the surface area to volume ratio of the surface
that permits increased rate of diffusion
FACILITATED DIFFUSION
This refers to the transport of molecules and ions across a membrane by specific transport proteins, carrier and channel
proteins, found within the membrane in the direction of lower concentration of the ions or molecules i.e. in favour of the
concentration gradient (difference) of ions.
Facilitated diffusion is a faster form of movement than simple diffusion and it involves transport of large polar molecules
and ions that cannot be transported by simple diffusion.
Trans-membrane proteins form channels or act as transport proteins to facilitate and increase the rate of diffusion across the
semi permeable membrane. The transport protein molecules involved in facilitated diffusion include channel and carrier
proteins.
Facilitated diffusion by carrier proteins Facilitated diffusion by protein channels:
Some small hydrophobic organic molecules e.g. amino acids and These trans-membrane proteins form water-filled functional
glucose pass through the cell membrane by facilitated diffusion pores in the membrane. This allows charged substances,
using carrier proteins. These proteins are specific for one usually ions, and polar molecules to diffuse across the cell
molecule, so substances can only cross a membrane if it contains membrane. Most channels can be gated (opened or closed),
the appropriate proteins. allowing the cell to control the entry and exit of the ions,
The transport of glucose across the plasma membrane of fat cells, these include the ligand-gated and voltage gated channels.
skeletal muscle fibres, the microvilli of the ileum mucosa and The proteins form specific water filled hydrophilic channels
across proximal convoluted tubule cells of vertebrate kidneys is that permit the diffusion of various ions such as K+, Na+,
brought about by a change in the shape of the carrier protein once Ca2+,Cl-,HCO-3.
the glucose molecule bonds to it.
Carrier proteins alter the conformation of the carriers moving the There are also specialized channels for water known as
solute aquaporins found in both plant and animal cells. The
P 530across the membrane as the shape of the carrier changes.
SMACON aquaporins speed up the rate of diffusion of water molecules
The solute molecule is released on the other side of the
membrane, down its concentration gradient. down its water potential gradient.
,BY NAKAPANKA JUDE MAYANJA 0704716641 2017.
Page | 3
Comparison between simple and facilitated diffusion
Differences
Simple Facilitated
The rate of diffusion depends on the The rate of diffusion does not depend
concentration gradient on the concentration gradient Similarities
Diffusion can occur in either direction Diffusion occurs in only one direction
Similar molecules diffuse at the same Specific molecules diffuse faster than Both move molecules from a region of
rate others high concentration to a region of low
does not require special transport Occurs via special channels or carrier concentration through a partially
proteins proteins permeable membrane.
ACTIVE TRANSPORT
It is the movement of molecules or ions across a cell membrane against their concentration gradient aided by the protein
pump with specific binding sites, involving the expenditure of energy.
Cells which carry out active transport have a high respiratory rate and a large number of mitochondria to generate a high
concentration of Adenosine Tri Phosphate (ATP). The energy from ATP can be directly or indirectly used in active transport.
Active transport can be slowed or inhibited by respiratory poisons (inhibitors) e.g. cyanide or lack of oxygen.
Mechanism of active transport
This can be direct active transport if the energy from ATP is used directly to transport the substances, ions or molecules, or it
can be indirect active transport if the energy is not directly used to transport a substance across a membrane.
a) Direct active transport (e.g. Na+- K+ pump)
ATP is hydrolysed and the binding of the
phosphate group to the protein pump changes the
protein conformation. The protein pump actively
transports three sodium ions (3 Na+) out of the cell
for every two potassium ions (2K+) pumped
against their concentration gradient into the cell.
This generates a difference in ionic charge on the
two sides of the membrane which is important for
the transmission of nerve impulses. The Na+
gradient is also used in the coupled uptake of
solutes such as glucose into the cells against its
concentration gradient.
P 530 SMACON
, BY NAKAPANKA JUDE MAYANJA 0704716641 2017.
Indirect active transport mechanism (secondary
active transport)
This is also known as co-transport e.g. the coupled
uptake of glucose into cells lining the ileum in
mammals where glucose and Na+ ions are absorbed Page | 4
into the cells. Sodium ions down a concentration
gradient while the glucose molecules against the
concentration gradient.
In co-transport of Na+ and glucose, ATP is used by
the protein pump to pump Na+ out of the cell
creating a Na+ concentration gradient.
The Na+ and glucose molecules then bind to trans-
membrane protein (carrier protein), also called co-
transport proteins/coupled transport proteins. They
are then moved by the proteins inside the cells i.e.
the Na+ moves down its concentration gradient
while the glucose molecules moves down against
its concentration gradient.
Types of membrane proteins involved in active transport
Three main types of membrane proteins exist;
a) Uniport carriers. They carry (transport) a single ion
or molecule in a single direction.
b) Simport carriers. They carry two substances in the
same direction.
c) Antiport carriers. They carry two substances in
opposite directions.
P 530 SMACON
MOVEMENT IN AND OUT OF CELLS
The plasma membrane isolates the inside of the cell protoplasm from its extracellular environment. Materials are exchanged between
the protoplasm and the extracellular environment across the plasma membrane. The plasma membrane is selectively permeable and
allows transport of materials across it.
Substances move in and out of cells by the following processes:
a. Simple diffusion e. Endocytosis
b. Facilitated diffusion i. Phagocytosis
c. Osmosis ii. Pinocytosis
d. Active transport iii. Receptor mediated endocytosis
f. Exocytosis
The transport of substances is important to;
a) Supply cells with oxygen for respiration and raw materials for anabolism (synthesis of biological molecules)
b) Regulate the pH and solute concentration for maintaining a stable internal environment for enzymes to function
optimally
c) Excrete toxic waste substances
d) Secrete useful substances for cell activities
Note: the transport of substances across the cell membrane takes place by two major fundamental processes.
SIMPLE DIFFUSION
Diffusion is the random movement of ions or molecules from a region where they are at higher concentration to a region of
lower concentration. That is, to move down a concentration gradient until equilibrium is reached.
The phospholipid bilayer is permeable to very small and
uncharged molecules like oxygen and carbon dioxide. These
molecules diffuse freely in an out of the cell through the
phospholipid bilayer. Hydrophobic substances (lipid-soluble)
e.g. steroids, can also diffuse through. These non-polar
molecules do not require the aid of membrane proteins (channel
or carrier) to move across the cell membrane.
Even though water is an extremely small, its polar therefore it
does not move across the cell membrane by simple diffusion.
A charged molecule or atom and its surrounding shell of water, find the hydrophobic layer (non-polar) of the membrane more
difficult to penetrate thus the lipid bilayer partly accounts for the membrane’s selective permeability by preventing very large
molecules and small polar molecules of ions to move across it.
The rate of diffusion depends upon;
The concentration gradient
This refers to the relative concentration on either side of the membrane or between two points. The greater the difference
between the points, the faster the rate of diffusion and if the difference is less, the slower the diffusion rate. Therefore a
reduced concentration gradient causes a reduced rate of diffusion and vice versa.
Temperature
When increased, temperature causes an increased rate of diffusion because the particles acquire increased kinetic energy
which causes increased speed of movement hence increased rate of diffusion.
At low temperatures, the kinetic energy is very low and the speed of movement by particles is equally very low.
Surface area
The larger the surface area over which the molecules are exposed, the faster the rate of diffusion.
Distance over which diffusion takes place
Page 1 of 82
, BY NAKAPANKA JUDE MAYANJA 0704716641 2017.
This is the distance over which the molecules are to travel i.e. the surface thickness across which the molecules move. The
greater the distance the lower the rate of diffusion
Size and nature of diffusing molecules
The smaller the size of the diffusing particles, the faster they diffuse i.e. smaller particles move very fats while the large
ones will move slowly.
Permeability
The more porous a surface is, the greater the number of particles that diffuse through it hence the greater the rate of Page | 2
diffusion
Significance of diffusion In order maximize the rate of diffusion, tissues where diffusion occurs
attained special adaptations. These include;
a) It’s a means by gaseous exchange occurs in plants
and animals e.g. in plants diffusion of gases occur 1. The lungs are ventilated by the respiratory tract (trachea,
through the stomata and in animals, in gills of fish, , bronchus, bronchioles) which maintain a steep concentration
the skin and buccal cavity of amphibians alveoli of gradient between the lung alveoli and blood in the capillaries.
reptiles, mammals and birds. 2. Respiratory surfaces like the lung alveoli and intestine epithelial
b) Absorption of certain digested food materials e.g. lining possess a rich supply of blood vessels which transport
glucose in the ileum. away the diffusing materials hence maintaining a steep gradient
c) A means of exchange of materials between blood in which sustains the fast diffusion
capillaries and the tissues 3. Diffusion surfaces e.g. lung alveoli and intestines (ileum) are
d) During formation of the nerve impulse, sodium ions covered by a thin epithelium lining which reduces the distance
diffuse into the nerve cells facilitating generation of over which diffusion takes place.
nerve impulses and ensures transmission of nerve 4. The epithelial lining covering the alveoli and rumen of the ileum
impulses from one neurone to another i.e. diffusion is very permeable to allow molecules to travel across them
facilitates synaptic transmission 5. In lungs there are numerous alveoli and in the ileum infoldings
e) It ensures excretion of waste products e.g. ammonia known as villi and microvilli which is coupled with a very long
in fresh water fishes ileum also increases the surface area along which particles move
f) It’s the main means of transportation of materials into cells hence increase the rate of diffusion.
within the cell’s cytoplasm e.g. in unicellular 6. Flattened body e.g. platyhelminthes (flatworms) which increases
organisms the surface area for movement of materials by diffusion
g) Absorption of mineral salts by plants from the soil is 7. Some organisms are of small size e.g. unicellular organisms
effected by diffusion as one of the mechanisms which increases the surface area to volume ratio of the surface
that permits increased rate of diffusion
FACILITATED DIFFUSION
This refers to the transport of molecules and ions across a membrane by specific transport proteins, carrier and channel
proteins, found within the membrane in the direction of lower concentration of the ions or molecules i.e. in favour of the
concentration gradient (difference) of ions.
Facilitated diffusion is a faster form of movement than simple diffusion and it involves transport of large polar molecules
and ions that cannot be transported by simple diffusion.
Trans-membrane proteins form channels or act as transport proteins to facilitate and increase the rate of diffusion across the
semi permeable membrane. The transport protein molecules involved in facilitated diffusion include channel and carrier
proteins.
Facilitated diffusion by carrier proteins Facilitated diffusion by protein channels:
Some small hydrophobic organic molecules e.g. amino acids and These trans-membrane proteins form water-filled functional
glucose pass through the cell membrane by facilitated diffusion pores in the membrane. This allows charged substances,
using carrier proteins. These proteins are specific for one usually ions, and polar molecules to diffuse across the cell
molecule, so substances can only cross a membrane if it contains membrane. Most channels can be gated (opened or closed),
the appropriate proteins. allowing the cell to control the entry and exit of the ions,
The transport of glucose across the plasma membrane of fat cells, these include the ligand-gated and voltage gated channels.
skeletal muscle fibres, the microvilli of the ileum mucosa and The proteins form specific water filled hydrophilic channels
across proximal convoluted tubule cells of vertebrate kidneys is that permit the diffusion of various ions such as K+, Na+,
brought about by a change in the shape of the carrier protein once Ca2+,Cl-,HCO-3.
the glucose molecule bonds to it.
Carrier proteins alter the conformation of the carriers moving the There are also specialized channels for water known as
solute aquaporins found in both plant and animal cells. The
P 530across the membrane as the shape of the carrier changes.
SMACON aquaporins speed up the rate of diffusion of water molecules
The solute molecule is released on the other side of the
membrane, down its concentration gradient. down its water potential gradient.
,BY NAKAPANKA JUDE MAYANJA 0704716641 2017.
Page | 3
Comparison between simple and facilitated diffusion
Differences
Simple Facilitated
The rate of diffusion depends on the The rate of diffusion does not depend
concentration gradient on the concentration gradient Similarities
Diffusion can occur in either direction Diffusion occurs in only one direction
Similar molecules diffuse at the same Specific molecules diffuse faster than Both move molecules from a region of
rate others high concentration to a region of low
does not require special transport Occurs via special channels or carrier concentration through a partially
proteins proteins permeable membrane.
ACTIVE TRANSPORT
It is the movement of molecules or ions across a cell membrane against their concentration gradient aided by the protein
pump with specific binding sites, involving the expenditure of energy.
Cells which carry out active transport have a high respiratory rate and a large number of mitochondria to generate a high
concentration of Adenosine Tri Phosphate (ATP). The energy from ATP can be directly or indirectly used in active transport.
Active transport can be slowed or inhibited by respiratory poisons (inhibitors) e.g. cyanide or lack of oxygen.
Mechanism of active transport
This can be direct active transport if the energy from ATP is used directly to transport the substances, ions or molecules, or it
can be indirect active transport if the energy is not directly used to transport a substance across a membrane.
a) Direct active transport (e.g. Na+- K+ pump)
ATP is hydrolysed and the binding of the
phosphate group to the protein pump changes the
protein conformation. The protein pump actively
transports three sodium ions (3 Na+) out of the cell
for every two potassium ions (2K+) pumped
against their concentration gradient into the cell.
This generates a difference in ionic charge on the
two sides of the membrane which is important for
the transmission of nerve impulses. The Na+
gradient is also used in the coupled uptake of
solutes such as glucose into the cells against its
concentration gradient.
P 530 SMACON
, BY NAKAPANKA JUDE MAYANJA 0704716641 2017.
Indirect active transport mechanism (secondary
active transport)
This is also known as co-transport e.g. the coupled
uptake of glucose into cells lining the ileum in
mammals where glucose and Na+ ions are absorbed Page | 4
into the cells. Sodium ions down a concentration
gradient while the glucose molecules against the
concentration gradient.
In co-transport of Na+ and glucose, ATP is used by
the protein pump to pump Na+ out of the cell
creating a Na+ concentration gradient.
The Na+ and glucose molecules then bind to trans-
membrane protein (carrier protein), also called co-
transport proteins/coupled transport proteins. They
are then moved by the proteins inside the cells i.e.
the Na+ moves down its concentration gradient
while the glucose molecules moves down against
its concentration gradient.
Types of membrane proteins involved in active transport
Three main types of membrane proteins exist;
a) Uniport carriers. They carry (transport) a single ion
or molecule in a single direction.
b) Simport carriers. They carry two substances in the
same direction.
c) Antiport carriers. They carry two substances in
opposite directions.
P 530 SMACON
