Transport Membranes Notes

SEC 3 BIOLOGY 2011

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CHAPTER 3: TRANSPORT ACROSS MEMBRANES

Learning Outcomes

1. Define diffusion as the movement of molecules from the region of their higher

concentration to a region of their lower concentration, down a concentration gradient.

2. Define osmosis as the passage of water molecules from a region of less negative water

potential to a region of more negative water potential, through a partially permeable

membrane.

3. Describe the importance of water potential gradient in the uptake of water by plants

and the effects of osmosis on animal tissue.

4. Define active transport and discuss its importance as an energy-consuming process by

which substances are transported against a concentration gradient, as in ion uptake by

root hairs and uptake of glucose by cells in the villi

5. Discuss significance of surface area to volume ratio in organisms

6. Define endocytosis and exocytosis

7. Describe how facilitated diffusion is carried out across the cell membrane (details of

mechanism not required)

The learning outcomes dictate what you need to know for the topic. A good way to check if you are

sufficiently prepared for a topic is to you if you can answer all the learning outcomes. If you can’t,

better continue studying!

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3.1 INTRODUCTION

All cells require a wide variety of nutrients to survive, but these nutrients are not always

found or produced by the cell and it has to be absorbed from the surroundings. The

different processes for transport across the cell membrane are thus important for the

uptake of beneficial nutrients (e.g. Glucose, Oxygen) and to excrete toxic substances or

wastes (e.g. Carbon Dioxide).

These processes can be divided into two broad categories: Passive and Active Processes.

Passive Processes Active Processes

Energy Requirement No energy required Energy required, in the form of

ATP

Concentration Gradient

From a region of higher concentration to a region of lower concentration, down a

concentration gradient

From a region of lower concentration to a region of

higher concentration, against a concentration gradient

Examples Diffusion, Facilitated Diffusion,

Osmosis Active Transport, Endocytosis,

Exocytosis

3.2 DIFFUSION

Note: All definitions are to be memorized word-for-word.

Occurs because molecules are in a constant random state of motion (FYI: kinetic

theory of particles)

When a lump of sugar is dissolved in water, sugar molecules will undergo random

collusions with water molecules until there is an even distribution of sugar molecules

in the beaker of water.

Diffusion is the movement of molecules from the region of their higher

concentration to a region of their lower concentration, down a concentration

gradient

definition

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Diffusion is a passive process, energy (ATP) is not needed

For cells, substances transported by diffusion include Oxygen and Carbon Dioxide, as

these molecules are lipid-soluble and can pass through the hydrophobic interior of

the cell membrane.

Can occur in both directions (out of the cell or into the cell) depending on the

concentration gradient until equilibrium is achieved, when the concentration of the

solute is equal on both sides of the membrane

FYI: Dynamic Equilibrium

Diffusion achieves Dynamic Equilibrium, which means that even though the concentration of the

solute is the same on both sides of the membrane, there is still movement of solute molecules

across the membrane. However, the net movement of solute molecules is zero. This means that

the rate of molecules moving from A to B is equal to the rate of molecules moving from B to A.

Factors Affecting Rate of Diffusion

Temperature The higher the temperature, the faster the molecules move and the faster the diffusion process

Concentration Gradient The larger the concentration gradient (or the greater the difference in concentration), the faster the diffusion process

Distance The shorter the distance over which diffusion occurs, the faster the diffusion process. This is why leaves are very thin to increase the rate of gaseous exchange by diffusion.

Surface Area The larger the surface area, the faster the diffusion process.

Nature of the Membrane Increased number of pores in the membrane (a more permeable membrane) will increase the rate of diffusion

Nature of Diffusing Molecule

The smaller the molecule, the faster it passes through the membrane The more lipid-soluble the molecule, the faster it diffuses across the membrane (Remember that cell membranes have a hydrophobic interior due to the hydrophobic carbon chains)

Pressure The greater the pressure, the higher the rate of diffusion

A B

A

A B

A

A B

A

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3.3 FACILITATED DIFFUSION

Remember from the previous topic that cell membranes are a lipid bilayer, and only small

hydrophobic (or lipid soluble) molecules can pass through the membrane. However,

hydrophilic molecules (e.g. Ions, Urea) and larger molecules (e.g. Sucrose, Glucose), cannot

pass through the membrane. These substances are thus transported using a special kind of

diffusion, Facilitated Diffusion.

The process is ‘facilitated’ because it uses transport proteins embedded in the cell

membrane to transport these substances across the cell membrane

Simple diffusion can be compared to people swimming from one side of the river to

the other. Facilitated diffusion would thus be a method to get people who cannot

swim to cross the river, such as using a boat or building a bridge, because these

people cannot swim across the river on their own

No energy is required (ATP is not required)

There are two main ways facilitated diffusion occurs (FYI)

Channel Protein

Channel Proteins have a central water-filled pore that allows for the water soluble substances to travel across the cell membrane, such as Ions.

Imagine a bridge across a river so that people can walk across the river without swimming

Carrier Protein

Substances will bind to one side of the Carrier Protein, after which the protein goes through a structural change and transports the substance across the membrane. Imagine a ferry that fetches people across the

river one group at a time

http://porpax.bio.miami.edu/~cmallery/150/memb/c8.7x15.facilitated.diffusion.jpg

http://porpax.bio.miami.edu/~cmallery/150/memb/c8.7x15.facilitated.diffusion.jpg

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3.3 OSMOSIS

Osmosis is a special type of diffusion, for water molecules

Passive Process – Energy is not required

For osmosis, the concept of water potential is used instead of concentration

Water Potential (Ψ)

Water Potential is defined as the tendency of water molecules to travel form one region to another

Pure Water has the Water Potential of 0, and the more concentrated the solution is, the more negative the Water Potential is.

Since the mathematical values for Water Potential are negative, a more concentrated solution is said to have a more negative water potential and a more dilute solution has a less negative water potential

Important Note: The terms “More/Less Negative” are used instead of “higher/lower” when

discussing water potential

A partially permeable membrane is one that allows only certain molecules to pass

through which preventing others.

Example The membrane is selectively permeable as it only allows water to pass through but the pores are too small for the large solute molecules, hence the solute cannot pass through the membrane. Therefore, water moves from the region of less negative water potential (left) to the area of more negative water potential (right) until equilibrium is reached. That is why the water level on right side of the U-tube is higher than the left

Experiment showing the effects of osmosis

Osmosis is the movement of water molecules from a region of less negative water

potential to a region of more negative water potential, through a partially permeable

membrane

definition

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Significance of Osmosis – Uptake of Water by Plants

Root hair cells uptake water via osmosis, since it has a more negative water potential

than the surrounding soil solution

Water then moves from cell to cell through the root cortex by osmosis along a water

potential gradient; this means that the water potential of each cell is less negative

than the water potential of the next cell

Results in a uni-directional movement of water from the root hair cell to the root

cortex cells and finally into the xylem vessels

Effects of Osmosis on Animal and Plant Cells

When Animal Cells (no cell walls) are placed in different solutions (hypotonic, isotonic

hypertonic), different observations are obtained.

Hypertonic Solution: Water Potential of solution is more negative than the cell Isotonic Solution: Water Potential of solution is equal to that of the cell Hypotonic Solution: Water Potential of solution is less negative than the cell Note: Hyper, Hypo and Iso-tonicity should only be used to describe animal cell systems. For

plant cells, please use the water potential concept.

For Animal Cells

In a Hypertonic Solution, Water Potential in the cell is less negative than the

solution; hence water leaves the cell by osmosis, and the cell becomes shrivelled

In an Isotonic Solution, Water Potential in the cell is equal to the solution, hence

there is no net movement of water and no change in cell volume

In a Hypotonic Solution, Water Potential in the cell is more negative than the

solution; hence water enters the cell by osmosis. The cell swells and lyses (bursts)

since it lacks a cell wall

Water potential gradient

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Effects of putting Red Blood Cells (RBC) in different solutions

For Plant Cells

In a solution of less negative water potential, water potential in the cell is more

negative, hence water enters the cell via osmosis. The cell swells and becomes

turgid, while the cell wall prevents the cell from bursting

In a solution of equal water potential, there is no net movement of water and thus

there is no change in cell volume

In a solution of more negative water potential, water potential in the cell is less

negative, hence water leaves the cell via osmosis.

o The cell becomes flaccid. (FYI: Flaccid refers to the state where the protoplast

does not exert any pressure against the cell wall.

o Further loss of water via osmosis will cause the cell body to shrink and pull

away from the cell wall, and the cell becomes plasmolysed

SUMMARY

RBC Ghosts refer to

Red Blood Cells that

have burst (lysed)

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3.4 ACTIVE TRANSPORT

Active Transport is an Active Process, requiring energy

Occurs for ions and small hydrophilic molecules.

Enables the cell to maintain an optimal internal concentration of molecules that is

either higher or lower than the concentrations of the molecules in the cellular

environment. For example, cells need to maintain a lower concentration of waste

products in the cell than in the environment, while maintaining a higher

concentration of certain mineral salts

FYI: How Active Transport Works

Active Transport utilize carrier proteins embedded in the cell membrane (similar to those in

facilitated diffusion, but requires ATP)

These carrier proteins bind to specific molecules, and using energy in the form of ATP,

undergo a conformational change to transport the molecule across the membrane

An example of such a carrier protein is the sodium-potassium pump, which transports two

potassium ions into the cell while transporting three sodium ions out of the cell

Active Transport is the movement of substances from a region of lower

concentration to a region of higher concentration, against the concentration

gradient. Energy is required in the form of ATP.

definition

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Examples of Active Transport

Example 1: Absorption of Mineral Salts from the soil by Root Hair Cells

Mineral salts exist in the soil solution as ions such as nitrates

These are essential for a plants metabolism and are found in low concentrations in the soil

Therefore, since the concentration of these ions is higher in the root hair cell than the surroundings, the mineral salts cannot be taken in via diffusion

The root hair cells thus use active transport to uptake the minerals from a region of low concentration (soil) to a region of higher concentration (cell), against the concentration gradient

Energy is used in the process

Root Hair Cell

Example 2: Uptake of Glucose by cells in the Villi

The food we eat is digested by the digestive system into amino acids, glucose and fats, which are absorbed in the small intestine by epithelial cells of the villi

Glucose cannot be taken in via diffusion due to the high concentration of glucose already within the epithelial cells

Therefore, the cells use active transport to transport the glucose from an area of lower concentration (small intestines) to an area of higher concentration (epithelial cells) against the concentration gradient

Energy is used in the process

Small Intestine, Villi

Questions to Ponder

Since active transport is an energy-consuming process, can you figure out which cellular organelle is

highly abundant in the two cells discussed above?

How are the two cells discussed above adapted to maximize their function?

Hint: The concept of surface area to volume ratio is involved

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3.5 ENDOCYTOSIS & EXOCYTOSIS

Endocytosis Exocytosis

Endocytosis is a process where the cell takes in

solid particles or fluid through the formation of

vesicles.

FYI: These vesicles are formed by the

invagination of the cell membrane

Exocytosis is a process where the cell secretes

substances via the fusion of vesicles with the cell

membrane

This process is used by cells for the uptake of

extracellular substances or molecules

This process is often used by secretory cells to

export products out of the cell or to remove

waste materials

Note: Both are energy-consuming processes as the formation of the vesicles as well as the

movement of the vesicles requires energy. Hence it is an Active Process

FYI: Endocytosis: Phagocytosis, Pinocytosis, Receptor-Mediated Endocytosis

Phagocytosis involves the extension of pseudopodia outwards to engulf large solid particles

Pinocytosis involves the cell incorporation small droplets of extracellular liquid into small

vesicles.

Receptor-mediated endocytosis is triggered when the molecules bind to specific protein

receptors on the cell membrane, which stimulates the invagination of the membrane to

form vesicles. This is a selective form of endocytosis and allows the uptake of specific

substances from the extracellular fluid.

Transport Membranes Notes

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