Surface Area Volume And Methods Of Movement Flashcards
Fluid Mosaic Model - 1972, Singer and Nicolson
• The fluid mosaic model describes the structure of the plasma membrane
as a mosaic of components in which the components are able to flow
and change position, while maintaining the basic integrity of the
membrane.
• Fluid because the individual phospholipid molecules can move relative to each
other. This provided a flexible structure that can change shape.
• Mosaic because the proteins that are embedded in the membrane vary in size,
pattern and shape in the same way as tiles in a mosaic.
• Membranes range from 5–10 nm thick. (0.000000005cm) or 5x10-9m
Key features
Structure= phospholipid
feature= Bi layer with hydrophobic tails inwards and hydrophilic heads outwards
Function= Allow lipid soluble substances to enter and leave the cell. Prevent water soluble substances form leaving the cells Make the membrane flexible and self sealing
Structure= Proteins
Feature= Surface proteins that act as cell receptors for hormones with attached glycolipids and provide structural support Full thickness proteins that act as protein channels and carrier proteins in order to move molecules across the membrane
Function= Structural support Channels for water soluble substances Active transport through carrier proteins Cell surface receptors for identification and adherence to other cells Act as receptors for hormones
Structure= Cholesterol
Feature= Add structural strength and prevent water and ion movement. Anchor tails of phospholipids to provide strength
Function= Prevent water leakage Reduce lateral movement of phospholipids
Structure= Glycolipids & Glycoproteins
Feature= Glycolipid (carbohydrate and lipid) + Glycoprotein (protein and lipid)
Function= Cell surface receptor, increase stability of the membrane and helps cells join together to form tissues Cell surface receptors, help cells join together to form tissues and allow cells to recognise each other
Structure= Cytoskeleton
Feature= Network of fibrous proteins
Function= Provide internal strength to the cell and help maintain its shape but also to change its shape as a result of moving material into and out of cells or ‘waves’ to movement such as in the cilia to remove mucus
Surface area
• Where the surface area is small compared to the volume, specialised
exchange and transport mechanisms are required to maximise the
rate of exchange.
• Examples of these mechanisms are the circulatory system where this
system allows nutrients and oxygen to be supplied to all cells and
waste products removed.
• This also includes the cell specialisations that increase surface area in
relation to volume and the cotransport mechanism in the ileum
Calculating surface area:volume
Step 1: measure the length and height of one side of the cube and multiply them together.
Step 2: multiply the value by the number of sides – will always be 6. This gives you surface area.
Step 3: Multiply the length, width and height of the cube. This value is the volume.
Step 4: Divide the surface area by the volume and this is your SA:V ratio
Impact
• As a cell increases in size, its surface area-to-volume ratio decreases.
• If the cell grows too large, the plasma membrane will not have
sufficient surface area to support the rate of diffusion required for the
increased volume. In other words, as a cell grows, it becomes less
efficient.
• One way to become more efficient is to divide; another way is to
develop organelles that perform specific tasks. These adaptations
lead to the development of more sophisticated cells called eukaryotic
cells
concentration gradient
A concentration gradient exists where there is a higher concentration of a substance on one side of a barrier than on the other.
• Going from high to low concentration is going down the concentration gradient – diffusion, facilitated
diffusion and osmosis
• Going from low to high concentrationis going against the concentration gradient –active transport
Types of Movement
Two ways in which transport occurs…
• Passive Transport-
• Diffusion
• Facilitated Diffusion
• Osmosis
Does not require energy
• Active Transport
• Endocytosis/Exocytosis
• Sodium potassium pump
• Co-transport
Passive Transport: Diffusion
• In diffusion molecules will move from a region of high
concentration to an area of low concentration.
• The higher the concentration the faster the rate of
diffusion
• The lower the concentration the slower the rate of
diffusion
• Where there is no concentration gradient there will
be no diffusion as there is a state of equilibrium
• Once the molecules even up the movement will slow and eventually
stop (remember- the random motion of particles continues)
• This is called the state of equilibrium
Diffusion within the lungs
• The structural specialisations of the
epithelial cells that make up the
alveoli support diffusion through
their thin membranes creating a
large surface area for gas exchange.
• Recall that diffusion is driven by the
size of the concentration gradient
on each side of a membrane. The
Higher the concentration gradient
the faster the rate of diffusion until
there is an equal amount on each
side
Diffusion in the alveoli in your lungs
The differences in partial pressure (the P
value) between the inside of the capillary
and the air in the alveoli create a
concentration gradient for diffusion to
occur.
This is the same as the rest of the body
where the tissues and the blood have a
concentration gradient that drives
diffusion.
Note the values in the alveolus and the
blood vessels –the different values are the
size of the concentration gradient
Passive Transport: Facilitated Diffusion
• To facilitate something means to make it easier
• This methods required the presence of specific proteins in the cell
membrane – channel proteins and carrier proteins.
• Channel proteins are a fixed shape but carrier proteins can
change shape (but do not need ATP to do this). They both are
highly specific to only one type of ion or molecule.
• This allows the movement of molecules that cannot diffuse
through the cell membrane but the cell requires to function
Examples of facilitated diffusion
• Large polar molecules such as glucose and amino acids from the ilium to
the cells lining the ilium
• Ions such as sodium ions (Na+) and chloride ions (Cl-) – examples of this
and other ions are found in the nervous system
• Remember – the movement of molecules from high concentration to low
concentration is diffusion. If this movement requires the aid of a protein
(for example because the molecule is charged and cannot pass directly
through the phospholipid bilayer) this is facilitated diffusion
Influences on Movement of Materials
• Concentration Gradient
• Temperature
• Distance
• Metabolic rate
• Surface Area and thickness of membrane
Concentration gradient
• The greater the difference in concentration, the more rapid the
diffusion.
• The closer the distribution of the material gets to equilibrium, the
slower the rate of diffusion becomes which is why cotransport is
needed for glucose absorption
• Example - If the partial pressure of O2 and CO2 in the lungs was the
same no diffusion would take place as there is no concentration
gradient to drive movement
Temperature
• Higher temperatures increase the kinetic energy and therefore the
movement of the molecules, increasing the rate of diffusion. Lower
temperatures decrease the energy of the molecules, thus decreasing
the rate of diffusion.
• Kinetic energy is gained as temperature increases but our biological
processes are designed for optimal efficiency at normal body
temperature only to prevent denaturing
Metabolic rate
• During high energy expenditure the metabolic rate will increase and lessen
when we are inactive.
• Your body has a minimum energy need to survive – Basal Metabolic Rate
(BMR) – mine is 1913 Calories as an example. Age, height, weight and
activity level can impact this.
• During exercise the body needs to be able to respond to the increased
need for energy increases heart rate, respiration rate and releases stored
glucose for cells. When the body is unable to meet the need lactic acid is
produced and activity decreases
Distance to travel
• The greater the distance that a substance must travel, the slower the rate
of diffusion. This places an upper limitation on cell size. A large, spherical
cell will die because nutrients or waste cannot reach or leave the centre of
the cell.
• Diffusion across multiple membranes takes longer than through simple
epithelial cells such as found in the lungs and digestive tract which is why
these are specialised and adapted to support their function.
• Fluid or mucus in the lungs as an example would increase the distance and
decrease the ability to diffuse
Surface area and thickness of the plasma membrane
• Increased surface area increases the rate of diffusion, whereas a
thicker membrane reduces it. This is why our villi have a brush border
of microvilli.
• This is similar to what you will have looked at with enzyme activity
and the increase in surface area increases the reaction speed in
digestion.
• The intestines, the lungs and the kidneys have very high surface area,
this adaptation means that a high level of absorption can take place in
a smaller area
Passive Transport: Osmosis
The ‘Diffusion’ of water molecules across a membrane
• The movement of water molecules from a region of high concentration
to a region of low concentration through a selectively permeable
membrane (that only allows the passage of water)
• All living cells are be surrounded by water (extracellular fluid) that
needs to be held at the correct tonicity to not impact the cell.
• An example of this is osmosis within the kidney in order to concentrate
urine and reclaim water from the filtrate that would otherwise be lost
Osmosis in your nephrons (kidneys)
• the body regulates water balance via the kidneys.
• Water is drawn back into the body through osmosis in order to maintain the correct amount needed – homeostasis
• Urine is darker when we are dehydrated as it is less diluted with water (just like a squash drink) and clearer or transparent when you are fully hydrated.
• The rate of osmosis is governed by the hormone ADH
Impact of incorrect water potential
• If the water potential of the blood is too high or too low then water will move into cells or from cells causing cells or shrink or swell as water is lost/gained (which can impact function).
• The correct function of osmoregulation is that the metabolic processes take place in the correct chemical environment and the balance of water and ions such as potassium and sodium is maintained for effective cellular function.
• Blood pressure can also be impacted as water can move from the plasma into cells and vice versa. Oedema can also be caused by excess water retention
Active Transport
• Active transport is the process by which dissolved molecules move across a cell membrane from a lower to a higher concentration. In active transport, particles move against the concentration gradient - and therefore require an input of energy from the cell in the form of ATP
• Sometimes dissolved molecules are at a higher concentration inside the cell than outside, but, because the organism needs these molecules, they still must absorb more.
• Active transport of carbohydrates (once they have reached equilibrium) allows the body to absorb more than diffusion alone provides
Energy for active transport
• Adenosine Triphosphate (ATP) is the primary energy source for cells.
• Energy is stored in the bond of the phosphate group and hydrolysing this bond releases the energy and a
phosphate group leaving the molecule Adenosine Diphosphate (ADP).
• The group of enzymes that hydrolyses this reaction are known as ATPases
Co-transport (active transport and facilitated diffusion)
• This occurs when the transport of one substance is coupled with the transport of another substance across a membrane.
• Glucose & sodium are co-transported via this method:
• The sodium ion in the ilium co-transports the glucose molecule through the cell wall of the microvilli.
• The glucose molecule is then transferred from the microvilli into the blood capillary by a carrier protein in facilitated diffusion
The stages of co-transport
- Na+ ions are actively transported out of epithelial cells through a protein carrier molecule. 3 Na+ ions are transported out of the epithelial cells into the bloodstream in exchange for 2 K+ ions.
- The movement gives a higher concentration of Na+ ions in the lumen of the intestine rather than inside of the cell. This maintains the concentration gradient.
- Na+ ions move down the concentration gradient using a co-transport protein. Both Na+ and glucose can bind to the protein (the binding of one makes the other more effective). 2 Na+ and a glucose molecule must bind before they can be transported across the membrane.
- The glucose moves into the blood plasma using facilitated diffusion
Link to Immunity
• Endocytosis and exocytosis are used by phagocytes (part of your innate immune system) to ingest, digest and excrete non-self matter.
• Cells also use exocytosis to secrete products they make into the body via the Golgi apparatus
• Cells also use endocytosis to ingest liquid (pinocytosis)
The sodium potassium pump
• This pump is found in the cell membrane and moves sodium and potassium ions against their concentration gradient so requires energy.
• It exchange three intracellular Na+ ions for two extracellular K+ ions.
