Cell: structure, functions, communication (wk 1/2)

Cell Theory

1. All living things are made up of cells

2. Cells are the basic units of life

3. Cells come from pre-existing cells that have multiplied

Prokaryotes vs Eukaryotes

Prokaryotes - lack membrane bound organelles

Eukaryotes - contain membrane bound organelles

What is cell diversity

many different types of cells → in multicellular organisms to keep it alive

• structure of cell depends on its function

• e.g. biconcave disk for RBCs

what is the Organisation of living things

Cells -> tissues -> organs -> organ systems -> organisms

What are cells

→ plus impact of certain cells on weight

Smallest unit of life

• Humans are made of cells and water

• The number of cells of one type have no correlation with out much they impact our weight

  • Most of our cells are red blood cells -> but they don’t contribute much to our weight

  • Muscle cells contribute the most to our weight

what are tissues + types

Tissue are made up of similar cells types that have a common function

• Epithelial Tissue - lines our skin and muscles

• Connective Tissue - connects and provides padding for other tissue

• Muscle tissue - for movement

• Nervous tissue - to send messages throughout the body

What are Organs

The many types of basic tissues types that go together to form organs

• Two, three, or four of the tissues described above

• E.g. Stomach -> made of nervous (nerves) tissue to trigger digestion, muscle cells to contract stomach, and epithelial cells (stomach cells release digestive enzymes)

What are Organ systems

Different organs work together to achieve a common purpose

• E.g. The digestive system: mouth, oesophagus, stomach, and intestines

• E.g. Integumentary system: skin, nails, glands, hair

• We have different organ systems within the body (e.g., nervous, cardiovascular and respiratory systems).

What are Organisms

When all structural levels work together to aid living

• Cells within organ systems provide the basis for body system interactions

Examples of body system interactions to sustain life:

• Energy supply → digestive system, liver metabolism, mitochondria at the cellular level

• Scaffold → skeletal system and connective tissue

• Propulsion system → muscular system

• Pumps, ventilation, plumbing → cardiovascular and respiratory systems

• Purification plant → kidneys and liver

• Protection → immune system and skin

• Control systems → nervous and endocrine systems

• Create, construct and repair → growth, stem cells, tissue regeneration

Nucleus

The control centre

• Nucleoplasm of nucleus holds most of genome

• Nucleolus - site of rRNA transcription in protein synthesis and makes ribosome

Rough endoplasmic reticulum (RER)

Contains ribosomes, which is where proteins are synthesised and folded in protein synthesis

Smooth endoplasmic reticulum (SER)

Lipid and steroid hormone production

Fun fact: its involved in breaking down alcohol, so the more you drink, the more SERs you have in your cells, but the byproduct of this breakdown is toxic to the liver

Golgi apparatus

Processes and packages the materials made by the SER and RER and sends them to other places in the cell

• Like the post office

• Seals these materials in vesicles

• Moves proteins and lipids around

Mitochondria

Makes ATP (turns ADP into ATP)

• Replicate by fission and contain their own DNA – maternal

• Has two membranes: outer and inner

• Scientists think that billions of years ago, mitochondria were bacteria that lived in cells and formed a symbiotic relationship with them

Lysosomes

Waste disposal of the cell

• Engulf wastes in the cell (endocytosis)

• Have strong digestive enzymes that break down the waste

• Throw it out into the cell cytosol after (exocytosis)

• These digested materials can either be used again by the cell (like if they are organic materials) or they can be removed from the body through other means systems (e.g. urea being urinated out)

Cytoskeleton

Provide structure, support and transport

• Filaments and tubules - holds the cell up

• Protein fibres - act as highways that move materials from one side of the cell to another

• Motor proteins help move materials around cell

Cytoplasm

Everything inside the cell membrane except for the nucleus -> includes organelles

- Cytosol = the jelly substance

Genome

contains all the DNA (genes) of an organism

• DNA provides the information that allows replication, growth, differentiation of the cell

• ~20,000 genes in humans

Proteome + example

all the proteins that an organism possesses

• Each cell type has its characteristic set of proteins that are expressed to allow it to perform its specific function

• 60-80,000 proteins in humans created by these 20,000 genes

Metabolome - a subset of proteins in the proteome that make up the metabolic activity of organisms

Fluid in the cellular environment + importance

Cells live in fluids

• 2/3 of fluid in body is in the cells -> intracellular fluids

• 1/3 of fluid is extracellular fluids -> outside cells

  • Blood (e.g. plasma)

  • Interstitial fluid -> between cells

Doctors look at blood and measure its contents to get an idea of how well cells are functioning

• Fluid content: tightly controlled, but quite a bit of variability around “normal”

• Glucose increase = diabetes

• Elevated electrolytes / CO2 = CV disease

• K+ - over 7 mM you’re in heaven

• Albumin drop = renal disease

• Acid base balance

  • Too much CO2 = Acidosis= blood too acidic (low pH)

  • Too little CO2 = Alkalosis= blood too basic (high pH)

Cells like to live in a constant environment (homeostasis) so they can function as optimally as possible

• Small deviations from normal pH can affect enzyme function, protein structures, ion channels and heart function

Structure of cell/plasma membrane

Made up of two layers of phospholipids -> called phospholipid bilayer

• Hydrophilic head = phosphate

• Hydrophobic tail = lipid

• Even distribution of charge between the head and tail since they are both polar molecules

  • Polar molecule - have an uneven distribution of charge

Qualities of cell membrane

Is semipermeable = only lets some substances through but not others

• Impermeable to most essential molecules and ions

  • E.g. Ions such as K+, Na+, Ca2+, Cl-, HCO3-'

  • E.g. Small water-soluble molecules like glucose -> dissolve in water but not lipids so they can't get through

  • E.g. Large molecules like proteins and RNA

• Highly permeable to small non-polar molecules

  • E.g. O₂, CO₂, anaesthetics

• Only slightly permeable to small uncharged polar molecules

  • E.g. water

Has proteins

• Categorised as integral or peripheral

  • All transport proteins are integral proteins

• Use these transport proteins to move substances in and out of cell (active transport) that are impermeable

Cell membranes are flexible (not rigid)

• They move depending on water concentration

Passive vs active transport

Active	Passive
Movement opposes concentration gradient	Movement follows concentration gradient → keeps occurring until equilibrium is reached (no more gradient)
Requires ATP	Does not require energy
Requires transport proteins	Transport proteins sometimes used
Examples: endocytosis / phagocytosis exocytosis sodium-potassium pump sodium-glucose pump	Examples simple diffusion facilitated diffusion osmosis

Simple diffusion

Movement (of solute) from areas of high concentration to low concentration

• When molecules move down the concentration gradient

• Occurs until you have equilibrium (same amount on both sides)

• When there is no gradient, no diffusion will occur

• NO transport protein

E.g. dropping dye in water

E.g. smell permeating a room

Osmosis

Diffusion of a solvent

• Move from high solvent concentration to low solvent concentration

• OR move from area of low solute concentration to high solute concentration

• Depends on tonicity

• Occurs through semipermeable membrane

e.g. water → through aquoporins if cell moves lots of water like kidneys

Osmotic pressure - force to stop osmosis across the membrane

Facilitated diffusion

Diffusion of substances through a transport protein

• Substance binds to protein

• Protein opens and allows it to diffuse through

• Protein acts as a pore that opens up the membrane to allow diffusion

• Slow because it moves one/two molecules at a time

e.g. Glucose, other nutrients, some ions

What is Tonicity

how a solution changes cell volume at equilibrium due to osmotic water movement across a semipermeable membrane

types of tonicity

Importance of Active Transport

Two types: primary and secondary

• Requires membrane proteins

Used to maintain ion concentration inside and outside the cell

• Ions carry charge = their movement changes the potential difference (voltage) between the cell and its environment

• Potential difference to communicate info between cells, maintaining muscle function, etc…

Ubiquitous Sodium-Potassium Pump

Primary Active Transport:

Creates and maintains an electrochemical gradient (concentration and electrical differences across the membrane)

• 3 positive charges (sodium) move in to cell but two leave (potassium)

• Net loss of one positive charge from outside environment per cycle

• Makes cell slightly more positive inside than outside

• Needed for transmitting electrical signals in nerve cells and muscle → through process of action potential

How it works

1. Phosphate binds to receptor in pump

2. Pump opens and allows three sodium ions to travel into cell

3. Then allows two potassium ions to travel out of cell

Sodium-Glucose Pump

Secondary Active Transport:

• Driven by the concentration gradient created by primary active transport

• Found in the gut

• Facilitates the reabsorption of glucose in the kidneys

How it works

1. Sodium binds to pump

2. This creates a binding site for glucose that allows it to bind to pump as well

3. Pump closes on outside of cell and opens on inside of cell

4. Moves sodium and glucose inside cell

Vesicular Transport types + function

Phagocytosis

When immune cells (phagocytes: neutrophils and macrophages) engulf pathogens to kill them

• Curves around and engulfs pathogens

• Digestive enzymes break down and kills pathogen

→ type of endocytosis

Homeostasis

and how it is mantained

maintaining stability in an internal environment regardless of external factors

• Keeping a constant state of balance

• Restoring thing back to normal -> however there is a slight seesaw/deviation from the normal/desired level

Maintained primarily through negative feedback loops

• Negative feedback tries to keep variables within certain limits (tolerance limits) to maintain optimal functioning

e.g. blood glucose levels, blood acidity, temp…

Negative feedback loops

Reduces deviations of controlled variables from their set points

• Restoring them toward equilibrium

• Encourages stability within a physiological range

• STOP STOP STOP

e.g. body temp, glucose regulation

Body temp regulation

What are the Systems to maintain homeostasis

Nervous system

• Set of neurons (peripheral), spine and brain (central)

• Sends fast electric signals to our body

• Electric signal travels through neurons

Endocrine system

• Group of glands that produce hormones

• Slower acting chemical based (hormone) messaging

• Hormones (chemical messengers) travel through blood stream and arrive at cells in our body

  • Hormones tell those cells how to behave and act

  • Hormone - a chemical/organic molecule messenger made by endocrine cells

endo vs exocrine

example of endocrine glands

function of hormones

The Pituitary Gland

In the brain

• Normally works alongside hypothalamus to maintain homeostasis

• Two parts: anterior and posterior

• Anterior -> releases hormones that cause other cells to release hormones

§ Growth Hormone/GH

§ FSH and LH (in ovaries/testes)

§ Thyroid stimulating hormone/TSH (in thyroid gland)

• Posterior -> releases hormones that act directly on targets

§ Oxytocin, antidiuretic hormone/ADH

Positive feedback loops

Amplifies deviations in a variable

• Drives variable it further away from equilibrium rather than returning it toward a set point

• Encourages instability and promotes the change

• MORE MORE MORE

e.g. childbirth, blood clotting

Childbirth feedback loop

How do cells signal info to eachother

Endocrine system

1. Endocrine cell gland releases hormones into the blood

2. Hormones travel through blood to specific target cells (make up target tissue)

3. These target cells have a specific receptor that binds to the hormone

4. The binding causes the cell to produce a response

a. No bind = no response

What is Signal transduction

How the cell translates that signal (hormone or electric signal) into a response

• Convert extracellular signals into intracellular responses

• Use receptors, transducers, and second messengers

• Amplify small signals into large cellular effects

• Activate specific target proteins to produce a response

Process of signal transduction

Via signalling cascade → step by step activation of molecules in a cell to procure a response

and signal amplification → where a small amount of signal to produces a large response (due to this signalling cascade)

• One activated receptor can activate multiple downstream signalling molecules

why do we need a signal transduction pathway for water soluble hormones

Water soluble hormones (e.g. glucagon) cannot cross cell membrane -> lipids are hydrophobic

• Need to activate response from outside the cell -> through transmembrane receptors

• Receptor then transduces signal

Glucose Regulation: Glucagon

1. Glucagon binds to G protein-coupled receptor on liver cells

   1. Glucagon is made by alpha cells in pancreas when blood sugar levels are to low

   2. Hypoglycaemia - large drop in blood sugar levels

2. G protein is activated

3. Causes ATP to be converted to cyclic AMP (cAMP)

4. cAMP activates protein kinase A

5. Causes phosphorylation to occur

   1. Can produce lots/little amounts of phosphorylated proteins depending on desired scale of response

6. Phosphorylated proteins act on their target to produce a response

   1. Convert glycogen stored in liver cells into glucose that is released into the blood stream

   2. Increases blood sugar levels

7. When hormone is not bound = no second messengers = no more response is produced

Why is Calcium a 2nd messenger

Amount of calcium within cells is very low -> too much calcium = activation of too many things = lead to cell death

• A slight increase in Calcium (Ca2+) above baseline can cause a powerful response

• Ca2+ binds proteins -> changes conformation and activity

• Rapid Ca2+ entry into cell or release of Ca2+ stored in ER release triggers cellular responses

E.g. Ca2+ triggers synaptic vesicle exocytosis.

Why are Steroid hormones different to water soluble hormones

1. Normally bound to proteins but when they aren't can diffuse through cell membrane -> i.e. are lipid soluble

2. Can enter cell and bind to a receptor without use of transport protein

   1. only proteinless hormones can enter through transport proteins

3. Changes gene expression (genes are activated or repressed)

4. Causes a change in protein produced (increase or decrease)

5. New proteins drive a response

6. Slow but sustained effects -> takes a long time to see result of change in gene expression

Clinical example: corticosteroids reduce inflammation by suppressing inflammatory gene expression