BIOL126 Lecture Notes - Active Transport, Lipid Bilayer, Fluid Mosaic Model
10 Jun 2018
School
Department
Course
Professor
Cells
2.1 What is a cell?
• Basic unit of life: self-sustaining and self-replicating
• Outer boundary of cell is a thin, phospholipid bilayer membrane
•Body cells contain fluid (cytoplasm) & surrounded by extracellular fluid (interstitial fluid, blood,
lymph)
2.2 The cell membrane
•Phospholipid bilayer
•Fluid mosaic model
•Contains phosphate, lipids, proteins and carbohydrates.
2.2.1 its structure
•Integral/transmembrane proteins span bilayer; peripheral proteins attached to surface (inner or
outer)
•Glycolipids and glycoproteins have range of functions in cluding acting as enzymes, receptors,
self recognition molecules
•Major function to lubricate and protect membrane surface
•Conditions inside cell very different to external environment; membrane important in maintaining
cell homeostasis
2.2.2 its functions
1. Physically isolates cell from external environment
2. Regulates exchange with environment
3. Sensitivity to environment
4. Structural support
2.2.3 different methods of transport across the cell membrane, with examples:
2.2.3.1 diffusion and osmosis
Diffusion:
•Molecules move (have kinetic energy) so bump into and bounce off other molecules = even
distribution of solutes through solution
•Net movement from areas of high concentration across a membrane to areas of low
concentration i.e. “down” the concentration gradient; process known as diffusion (or osmosis with
movement of water)
Osmosis:
•diffusion by water molecules across a membrane through water channels (aquaporins), to the
side with the higher concentration of dissolved solutes (solutes diffuse in an opposite direction to
water)
•Osmotic pressure and hydrostatic pressure determine which way water moves; hydrostatic
pressure opposes osmotic pressure
•Osmotic pressure: force with which water moves into a solution due to it’s dissolved solute
concentration
• Hydrostatic pressure: force generated by pushing against a fluid; created by heart
2.2.3.2 carrier mediated: facilitated diffusion, active transport, secondary active
transport
Carrier mediated transport:
•Requires specialised integral/transmembrane proteins
•Passive or active process depending on substance transported, and nature of transport
mechanism
•Integral proteins bind specific ions or organic molecules, and transport them across the
membrane
•Receptor for an integral protein is typically a glycoprotein
•All carrier-mediated transport mechanisms have 3 things in common:
1. Specificity: each integral protein only binds and transports certain molecules (eg. glucose
transporter will not transport other sugars)
2. Saturation limit: when all relevant carrier proteins are utilised, they are saturated
3. Regulation: Binding of other molecules such as hormones can affect activity
find more resources at oneclass.com
find more resources at oneclass.com
•In cotransport/symport two molecules moved the same way simultaneously
•In countertransport/antiport the two molecules are moved in opposite ways simultaneously
Facilitated diffusion:
•Some important nutrients eg. glucose and amino acids, are too large to diffuse and not lipid-
soluble
•Passively transported across membrane by carrier protein: molecule to be transported binds to
receptor site on carrier protein, which now changes shape = moves molecule across
•Molecules moved “down” concentration gradient until carrier protein receptors are saturated
Active transport:
•ATP needed to move molecules across membrane
•Disadvantage: requires cellular energy
•Advantage: cell can import or export molecules regardless of their
concentration gradient
•All cell membranes have ion pumps (carrier proteins) to actively
transport Na+, K+, (calcium) Ca2+ and magnesium (Mg2+) ions
across the membrane
•Specialised cell membranes also transport chloride (Cl-), iodide (I-)
and iron (Fe3+) ions
•May move one ion in one direction only, or two ions at once. If an ion
inside is swapped for an ion outside, it is an exchange pump eg.
Sodium-potassium pump
•(picture) sodium potassium exchange pump- For each ATP converted to ADP, the pump
transports 3 Na+ out and 2 K+ in.
Secondary active transport:
•(picture) Here glucose will only be transported
once 2 Na+ ions have also bound to the carrier
protein.
2.2.3.3 vesicular; endocytosis and
exocytosis
Vesicle transport:
•Molecules move into or out of the cell packaged
in membrane-bound compartments (vesicles), so
contents separate from cytoplasm
•Two types: Endocytosis (importing into cell); and Exocytosis (exporting from cell)
•Endocytosis: 3 types, all needing ATP;
1. Receptor-mediated endocytosis (using endosomes)
2. Pinocytosis (using pinosomes)
3. Phagocytosis (using phagosomes)
•Exocytosis: wastes, secretory products eg. mucins, hormones, enzymes packaged into vesicles
by Golgi apparatus to be released outside cell
•
2.3 Cytoplasm and the cellular organelles
2.3.1 does each do?
•Cytoplasm = everything between cell membrane and nuclear membrane; consistency of a colloid
•30% of cell weight is protein
•Cytosol = intracellular fluid
•Substances dissolved in cytosol include organelles, proteins, ions, dissolved nutrients, waste
products and gases
•Cytosol conditions:
-greater protein and K+ concentration inside cell than outside
-less Na+ concentration inside cell than outside
-many enzymes to catalyse reactions (some also om membrane)
find more resources at oneclass.com
find more resources at oneclass.com
-energy molecules and amino acids may be stored eg. liver & muscle cells.
•Cell organelles = internal structures with specific tasks within the cell
2.3.2 how does its structure assist this?
Cell organelles:
•Membranous: nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, peroxisome,
lysosome
•Non-membranous: cytoskeleton, microvilli, centrioles, cilia, ribosomes, proteasomes
•Cytoskeleton mostly actin protein: anchors cytoskeleton to cell membrane; interacts with myosin
to cause movement or change of cell shape
2.4 Cell specialisation
2.4.1 why cells vary in structure and function
•Microvilli: extension of membrane; support from internal cytoskeleton of microfilaments (mostly
actin)
•Cilia: array of microtubules that contract to create movement; mostly to sweep substances
across cell surface eg. ciliary escalator in airways
•Keratin: epithelial cells, hair and nails contain the waterproofing protein keratin; multiple layers of
dead epithelial cells outer layer of skin; protects skin against abrasion and water loss
2.5 ‘ATP’ and its role as energy ‘currency’ in our cells
•Adenosine triphosphate (ATP) is energy molecule of cells
•Mitochondria (singular mitochondrion): energy factories of cell (produce 95% of cell energy i.e.
ATP)
•The more active (hard working) cell is, the greater the mitochondria/cell eg. liver, kidney and
muscle cells all have lots of mitochondria
2.6 Overview of the processes involved in ATP production:
2.6.1 lycolysis, the Krebs cycle and the electron transport system
2.6.2 Difference between aerobic and anaerobic ATP production; where and why they
occur and the results in
energy yield
-Adenosine Triphosphate, recycled
from ADP (adenosine
diphosphate), recycled from AMP (adenosine monophosphate)
-High energy molecule for cell: energy stored in PO43- (phosphate) group bond (attached to
adenosine via phosphorylation)
-Energy needed to add phosphate to AMP ! ADP ! ATP
-Enzyme ATPase breaks ATP ! ADP + energy
-Energy currency of cell: ATP stores energy or releases energy to do “work” for cell
-In cytoplasm (glycolysis/anaerobic respiration) and in mitochondria (aerobic/cellular respiration).
-Most ATP produced in mitochondria (95%) through series of steps in two separate processes:
Krebs/Citric acid cycle and electron transport system (ETS)
find more resources at oneclass.com
find more resources at oneclass.com
