The Dynamic Cell

Units

3 cell theories

Robert Hooke - coined the term cell

Theodore Schwann - all living things are made of cells

Rudolf virchow - all cells come from cells

Characteristics of all cells

A surrounding membrane

cytoplasm - cell contents in thick fluid

Organelles

2 basic types of cells

prokaryotic cells:

All bacteria are prokaryotes

No membrane bound nucleus

Nucleoid = region of DNA concentration

organelles not bound by membranes

eg: Spherical (cocci), Rod - shaped (bacilli), spiral

Eukaryotic cells:

Nucleus bound by membrane

possess many organelles

include fungi, protists, plant, and animal cells

eg: blood cells, neurons, skin cells

why do cells have compartments

Eukaryotic cells are larger than prokaryotic cells

larger cells means greater diffusion distance which can make chemical reaction slower.

what is cell compartmentalisation

• All cells have a plasma membrane.

• Eukaryotic cells have membrane-bound organelles that create separate compartments.

• Each organelle has a specific function.

• Prokaryotes lack membrane-bound organelles and have one main compartment containing the enzymes needed for basic functions.

• Compartmentalisation allows different processes to happen separately and efficiently.

components of all cells

plasma membrane

cytoplasm

organelles

what are the 2 membranes

monolayer and bilayer

Plasma membrane features

composed of phospholipids, cholesterol, proteins and other components

Phospholipids drift within the bilayer

function: regulates what enters and leaves the cell

Cytoplasm components

cytoskeleton (filaments and fibres)

cytosol fluid

organelles (not nucleus)

storage of substances

The nucleus structure and function

Structure: Genetic info stored in nucleus as DNA

DNA packaged in the nucleus as chromosomes

Chromosomes 108 nucleotides in length

DNA has histone proteins that form chromatin

function: Site of cellular DNA storage, replication and transcription

Ribosomes structure and function

Structure: Composed of RNA and protein

Ribosomes carry out protein synthesis at two locations (in cytosol, free ribosomes or on the outside of endoplasmic reticulum which are bound ribosomes)

Function : protein biosynthesis

Endoplasmic Reticulum

Smooth ER

no ribosomes involved

function = Carbohydrate metabolism, drug metabolism, calcium ion movement

Rough ER

ribosomes involved

Function: Site of translation of proteins

Golgi apparatus function

prepares proteins for use or export

what happens in protein trafficking

molecules come in vesicles

vesicles fuse with golgi membrane

molecules may be modified by golgi

molecules go into separate vesicle

vesicle leaves golgi

vesicles combine with plasma membrane to secrete contents

Mitochondria structure and function

Structure - Have their own DNA, Bound by double membrane

Function = Break down organic compounds by aerobic respiration + produces ATP

Lysosomes structure and function

structure: Contains digestive enzymes

Function: Aid in cell renewal, break down old cell parts, digestive invaders

structure and function of cytoskeleton

structure = contains microfilaments (actin) , microtubules( tubulin) and intermediate fibres (neurofilament in neurons)

function: Provide the cell with a certain shape and mechanical stability

Cell movement

Distribution of chromosomes in cell division

acts as an anchor to cells

Microtubules have motor proteins which pull them

cells require communication for:

survival, adaptation and changing environmental conditions

prokaryotes respond to environmental changes (temperature, osmotic stress, nutrients) by:

Movement - moving towards or away from stimulus

protein synthesis: synthesising new proteins

how do unicellular organisms communicate

eg: Yeast cells exchange chemical signals called ‘mating factors’ to find and attract mating partners. These mating factors are short peptides that bind to specific receptors.

How do Multicellular organisms communicate

they respond to their physical environment and other cells. Cells need to know their correct location within the organism . Stimuli include:

growth factors

Hormones

Cell - matrix

Cell to cell contact

light Photons

Touch

Temperature

what is Signal Transduction

signal transduction is the mechanism by which a cell detects external stimuli and converts this detection into an internal cellular response usually involving a change in cellular activity.

3 phases of signal Transduction

Receptor - the target cell detects a signal from outside the cell. This occurs when a biochemical signal called a ligand, binds to a specific receptor protein, typically located on the cell surface.

Transduction - binding of ligand to receptor initiates intracellular events. This converts initial signal into a different signal

Response - The transduced signal triggers a specific cellular activity

key concepts of cell signalling

Specific Molecular Recognition- crucial for interactions between ligands and receptors or between signalling molecules

Molecular switches - proteins can be activated or inactivated through phosphorylation/ dephosphorylation

Signal Amplification - The process by which ligand - receptor interaction can lead to larger cellular response

Types of Cell Communication

Cell Junctions:

Gap Junctions: Channels connecting the cytoplasm of adjacent cells, allowing direct passage of small water soluble ions and molecules. This enables rapid communication between neighboring cells.

Cell - Cell recognition - cells communicate through direct contact between molecules on their surfaces.

Chemically Signalling:

Paracrine signalling - A secreting cell releases signalling molecules that diffuse through extracellular fluid to act on nearby target cells

Synaptic signalling - A specialised form of paracrine signalling occurring at synapses between nerve cells. An electrical signal triggers the release of neurotransmitters which diffuse across synaptic cleft to act on the taret cell

Autocrine signalling - A cell releases signalling molecules that bind to receptors on its own surface, essentially signalling to itself

Endocrine - Signalling cells release hormones into the bloodstream

Ligands and Types of receptors

ligands - external signal molecules that initiate cell signalling. They can be proteins, peptides, lipids, small molecules, or environmental stimuli.

Receptors - cellular proteins that binds to ligands. They are typical located on cell surface but can be intracellular

Types of receptors;

Transmembrane - proteins that span the plasma membrane having an extracellular

Ion channel linked receptors - ligand binding causes a change that open or closes an ion channel alternating the ion concentrating inside the cell

G protein linked receptors - These receptors are coupled to G proteins. Ligand binding activates the G protein, Which then interacts with other enzymes

Enzymes linked receptors - These receptors often have enzymatic activity themselves or are directly associated with enzymes. Ligand binding activates the enzyme activity.

Intracellular receptors - receptors located within the cytoplasm or nucleus.

Signal Transduction Pathways

Molecular Switches: Proteins that can be switched between active and inactive

Signal Amplification - A single ligand receptor interaction can lead to activation of many downstream molecules, amplifying the signal.

Second Messengers - Small non - protein water - soluble molecules that are produced or released in response to ligand binding

What is Phosphorylation

Phosphorylation is the addition of phosphate group to a protein

This process is catalysed by enzymes called kinases

Dephosphorylation the removal of a phosphate group is catalysed by phosphatases

Phosphorylation often acts a molecular switch altering protein activity

examples of signal amplification

ion channel - A ligand binding to an ion channel receptor

can cause the channel to open allowing large flux of ions around membrane

Enzymatic Generation of Second Messengers - An activated receptor can activate an enzyme that produces many molecules of a second messenger.

Phosphorylation - A series of kinase activating other kinases in sequence

three phases of cell signalling

reception (receives signal) - cell detects external signal, ligand binds to specific receptor protein, this activates receptor and starts signalling pathway

Transduction (relay and amplify) - signal is converted into other signals inside cell, series of molecular switches, signalling proteins and second messengers relay signal, signal amplification - occurs allowing ligand receptor to produce a large effect

response (produce effect) - signal triggers cellular response, activating or inhibiting enzymes, transcription factors (altering gene expression)

how is gene expression regulated

Transcription factors can bind to DNA and regulate the expression of specific genes

some transcription factors can activates genes that produce proteins that further amplify