CIE As Level Biology: Cell Surface membrane and transport

What are cell membranes

The cell surface membrane creates an enclosed space separating the internal cell environment from the external environment, and intracellular membranes form compartments within the cell such as the nucleus, mitochondria and RER
Membranes do not only separate different areas but also control the exchange of material across them, as well as acting as an interface for communication
Membranes are partially permeable

How can substance be transported across the cell surface membrane

Substances can cross membranes by diffusion, osmosis and active transport

What does the fluid mosaic model help to explain

Passive and active movement between cells and their surroundings
Cell-to-cell interactions
Cell signalling

Structure of phospholipids

Phospholipids structurally contain two distinct regions: a polar head and two nonpolar tails. The phosphate head of a phospholipid is polar (hydrophilic) and therefore soluble in water. The lipid tail is non-polar (hydrophobic) and insoluble in water
If phospholipids are spread over the surface of water they form a single layer with the hydrophilic phosphate heads in the water and the hydrophobic fatty acid tails sticking up away from the water, This is called a phospholipid monolayer
If phospholipids are mixed/shaken with water they form spheres with the hydrophilic phosphate heads facing out towards the water and the hydrophobic fatty acid tails facing in towards each other, This is called a micelle
Alternatively, two-layered structures may form in sheets. These are called phospholipid bilayers - this is the basic structure of the cell membrane

What can phospholipids form

Phospholipid bilayers can form compartments - the bilayer forming the cell surface membrane establishing the boundary of each cell. Internally, membrane-bound compartments formed from phospholipid bilayers provide the basic structure of organelles, allowing for specialisation of processes within the cell

Are lysosomes membrane bound

An example of a membrane-bound organelle is the lysosome (found in animal cells), each containing many hydrolytic enzymes that can break down many different kinds of biomolecule. These enzymes need to be kept compartmentalised otherwise they would breakdown most of the cellular components

Strucure of the membrane

The phospholipid bilayers that make up cell membranes also contain proteins. The proteins can either be intrinsic (or integral) or extrinsic (peripheral)
Intrinsic proteins are embedded in the membrane with their arrangement determined by their hydrophilic and hydrophobic regions
Extrinsic proteins are found on the outer or inner surface of the membrane

Why does the fluid mosaic model describe the cell membrane as 'fluid'

The fluid mosaic model describes cell membranes as 'fluid' because:
The phospholipids and proteins can move around via diffusion
The phospholipids mainly move sideways, within their own layers
The many different types of proteins interspersed throughout the bilayer move about within it (a bit like icebergs in the sea) although some may be fixed in position

Why does the fluid mosaic model describe the cell membrane as a 'mosaic'

The fluid mosaic model describes cell membranes as 'mosaics' because:
The scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above

What lipids do cell membranes contain

Phospholipids:Form a bilayer (two layers of phospholipid molecules)Hydrophobic tails (fatty acid chains) point in towards the membrane interior
Hydrophilic heads (phosphate groups) point out towards the membrane surface
Individual phospholipid molecules can move around within their own monolayers by diffusion

Cholesterol:Cholesterol molecules also have hydrophobic tails and hydrophilic heads
Fit between phospholipid molecules and orientated the same way (head out, tail in)
Are absent in prokaryotes membranes

Glycolipids:These are lipids with carbohydrate chains attached
These carbohydrate chains project out into whatever fluid is surrounding the cell (they are found on the outer phospholipid monolayer)

What proteins do cell membranes contain

Glycoproteins:These are proteins with carbohydrate chains attached
These carbohydrate chains also project out into whatever fluid is surrounding the cell (they are found on the outer phospholipid monolayer)

Proteins:The proteins embedded within the membrane are known as intrinsic proteins (or integral proteins). They can be located in the inner or outer phospholipid monolayer
Most commonly, they span the entire membrane - these are known as transmembrane proteins
Transport proteins are an example of transmembrane proteins as they cross the whole membrane
Proteins can also be found on the inner or outer surface of the membrane, these are known as extrinsic proteins (or peripheral proteins)

Function of phospholipids

Form the basic structure of the membrane (phospholipid bilayer)T
he tails form a hydrophobic core comprising the innermost part of both the outer and inner layer of the membrane
Act as a barrier to most water-soluble substances (the non-polar fatty acid tails prevent polar molecules or ions from passing across the membrane)T
his ensures water-soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell and unwanted water-soluble molecules cannot get in
Can be chemically modified to act as signalling molecules by:Moving within the bilayer to activate other molecules (eg. enzymes)
Being hydrolysed which releases smaller water-soluble molecules that bind to specific receptors in the cytoplasm

Function of cholesterol

Increases the fluidity of the membrane, stopping it from becoming too rigid at low temperatures (allowing cells to survive at lower temperatures), This occurs because cholesterol stops the phospholipid tails packing too closely together
Interaction between cholesterol and phospholipid tails also stabilises the cell membrane at higher temperatures by stopping the membrane from becoming too fluid
Cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them and causing phospholipids to pack more closely together
They also contribute to the impermeabilty of the membrane to ions

Increases mechanical strength and stability of membranes (without it membranes would break down and cells burst)

Function of glycoprotein and glycolipids

Glycolipids and glycoproteins contain carbohydrate chains that exist on the surface (the periphery/extrinsically), which enables them to act as receptor molecules, This allows glycolipids and glycoproteins to bind with certain substances at the cell's surface
There are three main receptor types:
signalling receptors for hormones and neurotransmittersreceptors involved in endocytosis
receptors involved in cell adhesion and stabilisation (as the carbohydrate part can form hydrogen bonds with water molecules surrounding the cell

Some act as cell markers or antigens, for cell-to-cell recognition (eg. the ABO blood group antigens are glycolipids and glycoproteins that differ slightly in their carbohydrate chains)

Function of proteins

Transport proteins create hydrophilic channels to allow ions and polar molecules to travel through the membrane. There are two types:channel (pore) proteinscarrier proteins

Each transport protein is specific to a particular ion or molecule,Transport proteins allow the cell to control which substances enter or leave

When do membranes become less fluid

Membranes become less fluid when there is:

An increased proportion of saturated fatty acid chains as the chains pack together tightly and therefore there is a high number of intermolecular forces between the chains
A lower temperature as the molecules have less energy and therefore are not moving as freely which causes the structure to be more closely packed

When do membranes become more fluid

Membranes become more fluid when there is:

An increased proportion of unsaturated fatty acid chains as these chains are bent, which means the chains are less tightly packed together and there are less intermolecular forces
At higher temperatures, the molecules have more energy and therefore move more freely, which increasing membrane fluidity

What is cell signalling

Cell signalling is the process by which messages are sent to cells
Cell signalling is very important as it allows multicellular organisms to control / coordinate their bodies and respond to their environments
Cell signalling pathways coordinate the activities of cells, even if they are large distances apart within the organism

What are the basic stages in a cell signalling pathway

A stimulus or signal is received by a receptor
The signal is converted to a 'message' that can be passed on - this process is known as transduction
The 'message' is transmitted to a target (effector)
An appropriate response is made

Transmission of messages in cell signalling pathways requires crossing barriers such as cell surface membranes. Cell surface membranes are therefore very important in signalling pathways as the membrane controls which molecules (including cell signalling molecules) can move between the internal and external environments of the cell. Signalling molecules are usually very small for easy transport across cell membranes, Typically in cell signalling pathways, signalling molecules need to cross or interact with cell membranes

What ar sinalling molecules called

Signalling molecules are often called ligands

How ligands are involved in the cell signalling pathway

Ligands are secreted from a cell (the sending cell) into the extracellular space
The ligands are then transported through the extracellular space to the target cell
The ligands bind to surface receptors (specific to that ligand) on the target cell
These receptors are formed from glycolipids and glycoproteins

The message carried by the ligand is relayed through a chain of chemical messengers inside the cell, triggering a response

What is diffusion

The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration.The molecules or ions move down a concentration gradient
The random movement is caused by the natural kinetic energy of the molecules or ions
As a result of diffusion, molecules or ions tend to reach an equilibrium situation (given sufficient time), where they are evenly spread within a given volume of space.The rate at which a substance diffuses across a membrane depends on several factors

Factors affecting diffusion

Steepness of concentration gradient
Temperature
Surface Area
Properties of molecules or ions

What substances cant diffude through the phospholipid bilayer

Large polar molecules such as glucose and amino acids
Ions such as sodium ions (Na+) and chloride ions (Cl-)

What is facilitated diffusion

Movement of specific molecules across cell membranes through protein channels

What transport proteins are used in facilitated diffusion

Channel proteins
Carrier proteins

They are highly specific (they only allow one type of molecule or ion to pass through)

Channel proteins

Channel proteins are water-filled pores
They allow charged substances (eg. ions) to diffuse through the cell membrane
The diffusion of these ions does not occur freely, most channel proteins are 'gated', meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore
This allows the channel protein to control the exchange of ions

Carrier Proteins

Unlike channel proteins which have a fixed shape, carrier proteins can switch between two shapes
This causes the binding site of the carrier protein to be open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape
The direction of movement of molecules diffusing across the membrane depends on their relative concentration on each side of the membrane
Net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient (from an area containing many of that specific molecule to an area containing less of that molecule)

Osmosis

Osmosis is the diffusion of water molecules from a dilute solution (high concentration of water) to a more concentrated solution (low concentration of water) across a partially permeable membrane. In doing this, water is moving down its concentration gradient. The cell membrane is partially permeable which means it allows small molecules (like water) through but not larger molecules (like solute molecules)

Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane

What is water potential

Water potential describes the tendency of water to move out of a solution
A dilute solution has a high water potential (the right-hand side of the diagram below) and a concentrated solution has a low water potential (the left-hand side of the diagram below)
The water potential of pure water (without any solutes) at atmospheric pressure is 0kPa, therefore any solution that has solutes will have a water potential lower than 0kPa (it will be a negative value)

Can water pass through the phospholipid bilayer

Water can pass through the phospholipid bilayer because water molecules are small molecules that can pass between phospholipids in the cell membrane. Although water molecules are polar, they can still pass through the bilayer because of their small size.When interpreting questions on water potential, remember - the more negative the water potential, the lower the water potential (the further it is away from pure water which has a water potential of 0 kPa).

What is active transport

Active transport is the movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration, Active transport requires carrier proteins (each carrier protein being specific for a particular type of molecule or ion)

How do facilitated dffusion and active transport differ

Although facilitated diffusion also uses carrier protein, active transport is different as it requires energy
The energy is required to make the carrier protein change shape, allowing it to transfer the molecules or ions across the cell membrane
The energy required is provided by ATP (adenosine triphosphate) produced during respiration

What is active transport important in

Reabsorption of useful molecules and ions into the blood after filtration into the kidney tubules
Absorption of some products of digestion from the digestive tract
Loading sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant
Loading inorganic ions from the soil into root hairs

Large quantities of materials that need to cross the cell surface membrane

Large molecules such as proteins or polysaccharides
Parts of cellsWhole cells eg. bacteria

Endocytosis and exocytosis

Bulk transport of large quantities into cells = endocytosis
Bulk transport of large quantities out of cells = exocytosis
These two processes require energy and are therefore forms of active transport

Endocytosis

Endocytosis is the process by which the cell surface membrane engulfs material, forming a small sac (or 'endocytic vacuole') around it, There are two forms of endocytosis:
Phagocytosis:This is the bulk intake of solid material by a cellCells that specialise in this process are called phagocytes, The vacuoles formed are called phagocytic vacuoles, An example is the engulfing of bacteria by phagocytic white blood cells

Pinocytosis:This is the bulk intake of liquidsIf the vacuole (or vesicle) that is formed is extremely small then the process is called micropinocytosis

Exocytosis

Exocytosis is the process by which materials are removed from, or transported out of, cells (the reverse of endocytosis), The substances to be released (such as enzymes, hormones or cell wall building materials) are packaged into secretory vesicles formed from the Golgi body, These vesicles then travel to the cell surface membrane, Here they fuse with the cell membrane and release their contents outside of the cell, An example is the secretion of digestive enzymes from pancreatic cells

What is the surface area

The surface area refers to the total area of the organism that is exposed to the external environment

What is the volume

The volume refers to the total internal volume of the organism (total amount of space inside the organism)

What happens when the size of organism

As the surface area and volume of an organism increase (and therefore the overall 'size' of the organism increases), the surface area : volume ratio decreases, this is because volume increases much more rapidly than surface area as size increases

What happens when a plant is placed in a dilute solution (high water potential)

If a plant cell is placed in pure water or a dilute solution, water will enter the plant cell through its partially permeable cell surface membrane by osmosis, as the pure water or dilute solution has a higher water potential than the plant cell
As water enters the vacuole of the plant cell, the volume of the plant cell increases
The expanding protoplast (living part of the cell inside the cell wall) pushes against the cell wall and pressure builds up inside the cell - the inelastic cell wall prevents the cell from bursting
The pressure created by the cell wall also stops too much water entering and this also helps to prevent the cell from bursting
When a plant cell is fully inflated with water and has become rigid and firm, it is described as fully turgid
This turgidity is important for plants as the effect of all the cells in a plant being firm is to provide support and strength for the plant - making the plant stand upright with its leaves held out to catch sunlight
If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant wilts

What happens when a plant is placed in a concentrated solution (low water potential)

If a plant cell is placed in a solution with a lower water potential than the plant cell (such as a concentrated sucrose solution), water will leave the plant cell through its partially permeable cell surface membrane by osmosis
As water leaves the vacuole of the plant cell, the volume of the plant cell decreases
The protoplast gradually shrinks and no longer exerts pressure on the cell wall
As the protoplast continues to shrink, it begins to pull away from the cell wall
This process is known as plasmolysis - the plant cell is plasmolysed

What happens if an animal cell is place in a concentrated solution (low water potential)

if an animal cell is placed in a solution with a lower water potential than the cell (such as a concentrated sucrose solution), water will leave the cell through its partially permeable cell surface membrane by osmosis and the cell will shrink and shrivel up
This occurs when the cell is in a hypertonic environment (the solution outside of the cell has a higher solute concentration than the inside of the cell)

What happens when an animal cell is placed in dilute solution (high water potential)

an animal cell is placed in pure water or a dilute solution, water will enter the cell through its partially permeable cell surface membrane by osmosis, as the pure water or dilute solution has a higher water potential. The cell will continue to gain water by osmosis until the cell membrane is stretched too far and the cell bursts (cytolysis), as it has no cell wall to withstand the increased pressure created
This occurs when the cell is in a hypotonic environment (the solution outside of the cell has a lower solute concentration than the inside of the cell)This is why a constant water potential must be maintained inside the bodies of animals

What happens when an animal cell is placed in an isotonic environment

If an animal cell is in an isotonic environment (the solution outside of the cell has the same solute concentration as the inside of the cell), the movement of water molecules into and out of the cell occurs at the same rate (no net movement of water) and there is no change to the cells

In what type of cell does plasmolysis occur

Be careful with your scientific terminology - animal cells do not plasmolyse because they do not have a cell wall. In a solution with a lower water potential than the cell itself, animal cells will shrink. Plasmolysis only occurs in plant cells.