Topic 1.4 - Membrane Transport

active transport

Energy-requiring process that moves material across a cell membrane against a concentration difference

An invagination of the membrane forms a flask-like depression which envelopes the extracellular material. The invagination is then sealed off to form an intracellular vesicle containing the material

Explain Endocytosis

Vesicles (typically derived from the Golgi) fuse with the plasma membrane, expelling their contents into the extracellular environment. The process of exocytosis adds vesicular phospholipids to the cell membrane, replacing those lost when vesicles are formed via endocytosis

Explain Exocytosis

In hypertonic solutions, water will leave the cell causing it to shrivel (crenation). In hypotonic solutions, water will enter the cell causing it to swell and potentially burst (lysis)

Explain how uncontrolled osmosis has negative effects with regards to cell viability

Simple diffusion involves movement of substances that can pass freely through the phospholipid bilayer. For example: Non-polar molecules such as oxygen. If there is a higher concentration of oxygen outside the cell than inside the cell, there will be a net movement of oxygen across the plasma membrane into the cell.

Outline and give an example of simple diffusion.

Endocytosis: The plasma membrane invaginates (bulges in). The vesicle forms inside the cell and contains material from outside the cell. For example: Phagocytosis and pinocytosis. Exocytosis: A vesicle inside the cell fuses with the plasma membrane. Materials within the vesicle are secreted outside the cell. The membrane of the vesicle (and any proteins embedded within it) is now part of the plasma membrane.

Outline endocytosis and exocytosis.

If the membrane is not fluid enough it cannot invaginate to form a vesicle in endocytosis. Cholesterol, an important factor determining membrane fluidity, also helps the membrane curve. The fluidity of the membrane also allows vesicles to fuse with the plasma membrane in exocytosis.

Outline how the fluidity of membranes is required for endocytosis and exocytosis.

To prevent damage to the cells of the organ, an isotonic bathing solution is used. The bathing solution will have the same solute concentration as the cells of the organ. This avoids cells of the organ shrinking or swelling (even bursting) due to the loss or gain of water by osmosis.

Outline the reasons why organs for transplantation need to be bathed in isotonic solutions when outside of a body.

isotonic

Solutions that have the same osmolarity (same solute concentration ⇒ no net water flow)

Hypertonic

Solutions with a relatively higher osmolarity (high solute concentration ⇒ gains water)

hypotonic

Solutions with a relatively lower osmolarity (low solute concentration ⇒ loses water)

Diffusion

The net movement of molecules from a region of high concentration to a region of low concentration

The membrane is principally held together by weak hydrophobic associations between the fatty acid tails of phospholipids. This weak association allows for membrane fluidity and flexibility, as the phospholipids can move around to some extent. This allows for the spontaneous breaking and reforming of the bilayer, allowing larger materials to enter or leave the cell without having to cross the membrane (this is an active process and requires ATP hydrolysis)

What allows for membrane to be fluid and flexible?

selective membranes

membrane proteins may regulate the passage of material that cannot freely cross

semi-permeable

only certain materials may freely cross - large and charged substances are typically blocked

Osmolarity

A measure of solute concentration, as defined by the number of osmoles of a solute per litre of solution (osmol/L)

Stage 1. The pump is open towards the inside of the axon. Three sodium ions bind at specific binding sites. One ATP molecule binds. Stage 2. ATP is hydrolyzed to ADP and inorganic phosphate (Pi). Energy released causes the protein to change shape. The pump is now open towards the outside of the axon. Stage 3. The three sodium ions are released outside the axon. Two potassium ions bind at specific binding sites. Stage 4. Pi is released and the pump returns to its original shape. The potassium ions are released into the axon. The process will occur again if more ATP is present.

Annotate each stage to describe how sodium potassium pumps in the membranes of axons function to prepare for a nerve impulse.

Selectively permeable means that are some molecules are able to pass through or not, this is because there is a size limit for molecules to pass into or out the permeable membrane.

Define selectively permeable in the context of the plasma membrane.

Active transport involves protein pumps that span the phospholipid bilayer. Protein pumps move substances from a region of lower concentration to a region of higher concentration. This is against the concentration gradient. This is an active process, it requires energy in the form of ATP. Protein pumps are specific for a single type of substance. Protein pumps only transport substances in one direction (either out of or into the cell). Different cell types can vary the number and types of protein pumps depending on their function.

Describe active transport.

Particles in liquids and gases are in constant motion. Collisions between particles result in them moving randomly. Diffusion is the spreading out of particles from a region of higher concentration to a region of lower concentration, due to collisions with randomly moving particles. Diffusion will occur if a concentration gradient exists between two regions. Diffusion is a passive process, it requires no energy in the form of ATP.

Explain diffusion.

Carrier proteins will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction. Carrier proteins may move molecules against concentration gradients in the presence of ATP (i.e. are used in active transport). Carrier proteins have a much slower rate of transport than channel proteins (by an order of ~1,000 molecules per second)

Explain how carrier proteins work

Channel proteins are ion-selective and may be gated to regulate the passage of ions in response to certain stimuli. Channel proteins only move molecules along a concentration gradient (i.e. are not used in active transport). Channel proteins have a much faster rate of transport than carrier proteins

Explain how channel proteins work

Integral proteins with a hydrophilic inner pore via which potassium ions may be transported. The channel is comprised of four transmembrane subunits, while the inner pore contains a selectivity filter at its narrowest region that restricts passage of alternative ions. Potassium channels are typically voltage-gated and cycle between an opened and closed conformation depending on the transmembrane voltage

Explain how potassium channels work

The endoplasmic reticulum is a membranous network that is responsible for synthesising secretory materials. Rough ER is embedded with ribosomes and synthesises proteins destined for extracellular use. Smooth ER is involved in lipid synthesis and also plays a role in carbohydrate metabolism. Materials are transported from the ER when the membrane bulges and then buds to create a vesicle surrounding the material

Explain how the endoplasmic reticulum are involved in the movement materials within cells

The vesicle is then transported to the Golgi apparatus and fuses to the internal (cis) face of the complex. Materials move via vesicles from the internal cis face of the Golgi to the externally oriented trans face. While within the Golgi apparatus, materials may be structurally modified (e.g. truncated, glycosylated, etc.). Material sorted within the Golgi apparatus will either be secreted externally or may be transported to the lysosome

Explain how the golgi apparatus are involved in the movement materials within cells

Vesicles containing materials destined for extracellular use will be transported to the plasma membrane. The vesicle will fuse with the cell membrane and its materials will be expelled into the extracellular fluidMaterials sorted by the Golgi apparatus may be either: Released immediately into the extracellular fluid (constitutive secretion). Stored within an intracellular vesicle for a delayed release in response to a cellular signal (regulatory secretion)

Explain how the plasma membrane are involved in the movement materials within cells

The osmolarity of a tissue may be interpolated by bathing the sample in solutions with known osmolarities. The tissue will lose water when placed in hypertonic solutions and gain water when placed in hypotonic solutions. Water loss or gain may be determined by weighing the sample before and after bathing in solution. Tissue osmolarity may be inferred by identifying the concentration of solution at which there is no weight change (i.e. isotonic)

Explain how to estimating osmolarity

Only water can move by osmosis. Osmosis is a passive process, it requires no energy in the form of ATP. Water is a solvent for polar and charged substances. When substances dissolve in water they form bonds with water molecules. Water can move through a partially permeable membrane. The solute cannot move through the membrane. The water molecules that are bonded to the solute, are no longer free to move through the membrane. The net movement of water is from a region of lower solute concentration to a region of higher solute concentration. In a less concentrated solution, there are more 'free' water molecules than in a concentrated solution.

Explain the movement of water across membranes by osmosis.

In plant tissues, the effects of uncontrolled osmosis are moderated by the presence of an inflexible cell wall. In hypertonic solutions, the cytoplasm will shrink (plasmolysis) but the cell wall will maintain a structured shape. In hypotonic solutions, the cytoplasm will expand but be unable to rupture within the constraints of the cell wall (turgor)

How are the effects of uncontrolled osmosis moderated in plant tissues?

Water is considered the universal solvent - it will associate with, and dissolve, polar or charged molecules (solutes). Because solutes cannot cross a cell membrane unaided, water will move to equalise the two solutions. At a higher solute concentration there are less free water molecules in solution as water is associated with the solute. Osmosis is essentially the diffusion of free water molecules and hence occurs from regions of low solute concentration

Explain the process of osmosis

An integral protein that exchanges 3 sodium ions (moves out of cell) with two potassium ions (moves into cell). The process of ion exchange against the gradient is energy-dependent and involves a number of key steps: Three sodium ions bind to intracellular sites on the sodium-potassium pump. A phosphate group is transferred to the pump via the hydrolysis of ATP. The pump undergoes a conformational change, translocating sodium across the membrane. The conformational change exposes two potassium binding sites on the extracellular surface of the pump. The phosphate group is released which causes the pump to return to its original conformation. This translocates the potassium across the membrane, completing the ion exchange

Explain the process of the sodium potassium pump

Active transport involves the use of carrier proteins (called protein pumps due to their use of energy). A specific solute will bind to the protein pump on one side of the membrane. The hydrolysis of ATP (to ADP + Pi) causes a conformational change in the protein pump. The solute molecule is consequently translocated across the membrane (against the gradient) and released

Explain the use of carrier proteins in active transport

By facilitated diffusion. Larger polar molecules, such as glucose, and ions pass through channel proteins spanning the phospholipid bilayer. A channel protein forms a small hydrophilic pore through which hydrophilic substances can pass. Channel proteins are specific for a single type of substance. For example, if there is a higher concentration of Na+ outside the cell than inside the cell, there will be a net movement through a Na+ specific channel protein into the cell. Different cell types can vary the number and types of channel proteins depending on their function.

How do larger polar molecules and ions move passively through the plasma membrane?

Carrier Proteins

Integral glycoproteins which bind a solute and undergo a conformational change to translocate the solute across the membrane

Channel Proteins

Integral lipoproteins which contain a pore via which ions may cross from one side of the membrane to the other

Passive transport

Involves the movement of material along a concentration gradient (high concentration ⇒ low concentration)

Actice Transport

Involves the movement of materials against a concentration gradient (low concentration ⇒ high concentration)

vesicles

Materials destined for secretion are transported around the cell in membranous containers

Osmosis

The net movement of water molecules across a semi-permeable membrane from a region of low solute concentration to a region of high soluteconcentration (until equilibrium is reached)

Facilitated diffusion

The passive movement of molecules across the cell membrane via the aid of a membrane protein

Exocytosis

The process by which large substances (or bulk amounts of small substances) exit the cell without crossing the membrane

Endocytosis

The process by which large substances (or bulk amounts of smaller substances) enter the cell without crossing the membrane

Pinocytosis

The process by which liquids / dissolved substances are ingested (allows faster entry than via protein channels)

Phagocytosis

The process by which solid substances are ingested (usually to be transported to the lysosome)

To move through the phospholipid bilayer, substances pass via gaps between the phospholipids. The two major factors that impact on permeability, are size and charge. The inside of the phospholipid bilayer is hydrophobic/non-polar due to the fatty acid tails. Small non-polar molecules, such as oxygen and carbon dioxide, can pass freely through. Fatty acids and lipid soluble molecules can pass freely through as they are non-polar. Hydrophilic substances include polar molecules and ions. Small polar molecules, such as water or urea, can pass at a low rate. Large polar molecules, such as glucose and amino acids, cannot pass. Ions, such as H+ and Cl−, are charged and cannot pass.

Use your knowledge of the properties of the phospholipid bilayer to explain its partial permeability to different substances.

Temperature (affects kinetic energy of particles in solution) Molecular size (larger particles are subjected to greater resistance within a fluid medium) Steepness of gradient (rate of diffusion will be greater with a higher concentration gradient)

What are the different factors that affect the rate of diffusion?

Simple diffusion - movement of small or lipophilic molecules (e.g. O2, CO2, etc.) Osmosis - movement of water molecules (dependent on solute concentrations) Facilitated diffusion - movement of large or charged molecules via membrane proteins (e.g. ions, sucrose, etc.)

What are the three main types of passive transport?

Primary (direct) active transport - Involves the direct use of metabolic energy (e.g. ATP hydrolysis) to mediate transport Secondary (indirect) active transport - Involves coupling the molecule with another moving along an electrochemical gradient

What are the two types of active transport?

Phospholipid bilayer is permeable to: Water Carbon dioxide Oxygen Fatty acids Urea Phospholipid bilayer is not permeable to: Amino acids Glucose Hydrogen ions (H+) Chloride ions (Cl−)

Which of the following substances will move freely through the phospholipid bilayer: Water Carbon dioxide Amino acids Glucose Oxygen Fatty acids Urea Hydrogen ions (H+ ) Chloride ions (Cl−)

Tissues or organs to be used in medical procedures must be kept in solution to prevent cellular dessication. This solution must share the same osmolarity as the tissue / organ (i.e. isotonic) in order to prevent osmosis from occurring

Why do tissues or organs used in medical procedures must be kept in solution?