Cell Structure and Function: An Overview

Cell Structure and Function: Overview

• Topic examines cells (eukaryotic and prokaryotic) as fundamental life units.
• Structure-function relationship is key to understanding cellular life processes.

Cell Theory

• Cells are the smallest, most basic unit of life.
• All cells originate from pre-existing cells.
• All living things are composed of one or more cells.

Basic Cell Structures and Types

• Basic structures apply to all cells: plasma membrane, genetic material (DNA), cytoplasm and ribosomes.
• Two main cell types: eukaryotic (with membrane-bound organelles) and prokaryotic (without membrane-bound organelles).
• Eukaryotic cells are typically larger than prokaryotic cells.

Cell Study Techniques

• Microscopes are tools for visualizing cell internal structure.
• Light microscopes and electron microscopes are the two main types.
• Electron microscopes offer higher resolution due to smaller wavelengths of electrons, over light microscopes.
• Micrometers (µm) are to measure whole cells or large cell structures.
• Nanometers (nm) are to measure cell ultrastructures and large organic molecules.

Cell Fractionation

• Process of cell lysis ('homogenisation') to release contents, and using ('ultracentrifugation') to separate organelles by density and size.
• 'Homogenate' is placed in a centrifuge and spun; largest organelles form a pellet, supernatant liquid is spun at higher speeds for further separation.
• Scientists can study both the structure and function of organelles and purified samples.

Eukaryotic Cells and Organelles

• The cytoplasm with a the cytosol which suspends organelles is highly organized.
• Eukaryotic organelles are membrane-bound or non-membrane bound.
• Examples are: nucleus, chloroplasts, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, ribosomes, centrioles, flagella and cilia, and cell surface membrane.

Nucleus

• Holds the genetic information as chromatin, made of DNA and histones.
• It is commonly spherical/ovoid structure bounded by a double membrane which is a nuclear envelope, separated by a perinuclear space.
• Outer nuclear membrane is studded with ribosomes and connects to the endoplasmic reticulum.
• Nucleoplasm is the matrix within the nucleus which contains enzymes, nucleotides, nucleoli, and chromatin
• Nucleolus contains DNA coding for ribosomal RNA (rRNA), assembled for ribosomal subunits, and prominent in non-dividing cells.
• Chromatin exists as heterochromatin (tightly packed DNA) or euchromatin (loosely packed DNA).
• The nucleus directs protein synthesis through mRNA transcription.
• Direct protein synthesis is by mRNA according to the nucleotide sequence in the DNA (called transcription)
• Followed by template mRNA for protein synthesis (translation).
• Ribosomal subunit synthesis and assembly occur in the nucleolus, with ribosomal proteins that are imported and assembled with rRNA molecules before exiting to the cytoplasm.

Endoplasmic Reticulum (ER)

• Complex interconnected membrane-bound network.
• Smooth ER and rough ER are the two types.
• Rough ER is abundant in cells secreting proteins and has ribosomes on its surface.
• Smooth ER abundant in cells involved in lipid metabolism.
• They form an intracellular transport system via the lumen.
• The cisternae provides suitable environmental conditions for polypeptides to fold into their native 3D conformation.
• Post-translational chemical modifications of proteins can occur like glycosylation of proteins to form glycoproteins.
• Inner smooth ER membranes are embedded with enzymes for synthesizing lipids and steroids.
• Provides increased reaction surface area.

Golgi Apparatus

• Cisternae, flattened membrane-bound sacs, make up structure plus cis (receiving) and trans (secreting) faces.
• Transport vesicles deliver contents from the ER to the cis face, adding the membrane to the Golgi apparatus.
• Material is processed as it moves from cisterna to cisterna.
• Modifying, sorting, and packaging from ER products takes place here.
• Proteins and lipids are processed further.
• Forms secretory vesicles and lysosomes.
• The Gogli is involved sugar/carbohydrate synthesis (pectin and hemicellulose in plants; glycosaminoglycans in animals).

Lysosomes

• Contains hydrolytic enzymes (acid hydrolases) that break down complex molecules (hydrolysing) into simpler ones.
• Digests ingested particles via endocytosis (material into cell), releases the hydrolytic enzymes into a secondary lysosome (new vesicles), and digestion takes place here.
• Acts for disposal of old organelles (autophagy).
• Lysosomal enzymes break down cellular soluble products into the surrounding cytoplasm where they may be reused to form new organelles.
• Lysosomal membrane ruptures, and liberates enzymes in cell autolysis which killing process in the cell.

Endomembrane System

• Transport proteins and lipids through a network of sacs and vesicles.

Chloroplast

• Photosynthetic plant cells have these organelles.
• Double membrane-bound.
• The interior stroma contains interconnected thylakoids which are stacked into grana.
• Light-dependent reactions which occurs at thylakoid membranes, increases rate by increasing pigments, electron carriers and ATP to embed in.
• The stroma contains enzymes to catalyzes light-independent reactions, like with the Calvin cycle.
• Contains 70S ribosomes to synthesise chloroplast proteins, circular DNA, and enzymes.
• Can store sugars synthesized during photosynthesis, broken down during no light.

Mitochondria

• Found in eukaryotic cells that aerobically respire.
• Double-membrane bound: smooth outer membrane and folded inner membrane called cristae in the gel-like matrix.
• Aerobic respiration enzymes are located on the cristae and in the matrix.
• Matrix also containing DNA molecules and ribosomes.
• Contains protein and electrons of the Krebs cycle for ATP (adenosine triphosphate) synthesis (cellular energy).

Membrane-Bound Organelles

• Separates chemical reactions in incompatible situations.
• Specialized cells that can perform different functions.
• Separating different reactions within the cell.
• High concentrations of enzymes and molecules can be collected to increase efficiency.
• Provides and maintains optimal environments for reactions.

Ribosomes

• Site of protein synthesis
• Found in eukaryotes and prokaryotes, and also occur freely attached to the rough ER and nucleus
• These made up of rRNA and ribosomal proteins which are the two subunits that form functional groups for protein synthesis.
• Proteins synthesized by free ribosomes function in the cytoplasm or transported to organelles.
• Proteins produced by ribosomes bound pass into the ER to for functional proteins.

Centrioles

• Contain in centrosomes of animal cells.
• They are found as right angles to each other.
• Hollow tubes contain in each centriole.
• Microtubules (tubulin is proteins) make up the tubule walls.
• Forms mitotic spindle and organtizes with a microtubule.

Prokaryotic Cells

• Always present structures include: peptidoglycan cell wall, the cytoplasmic or cell surface membrane, DNA, 70S ribosomes and cytoplasm.
• Smaller, lacks membrane-bound organelles (e.g., no nucleus).
• Consists of single cells, some with filaments.
• Cell wall, DNA (is circular), 70S ribosomes, and also cytoplasm.
• It is strong and rigid due to being peptidoglycan based.
• Protects the integrity, the meshwork (water, ions and molecules permeable) for anchoring external cell components.
• Infolds membrane (carry out metabolic functions via specialised membranes), and nucleid (region for DNA).
• It is single, circular and is dsDNA which is double stranded.
• Small circular DNA molecules of nonessential genes called the plasmids.

Endosymbiotic Theory

• This theory proposes that eukaryotic organelles (mitochondria and chloroplasts) originate the following.
• • Host cells take in endocytosis of free-living prokaryotes.
• • Two different cell species come to benefit one another, also known as symbiosis

Viruses

• It is small, ranging at 20nm size to 300nm.
• Very simple structures- infectious particles.
• DNA/RNA is contained by the viral chromosomes with genome (codes of component synthesis), and enzymes replication (function).
• This capsid (protein-shell), consists of subunits or capsomeres.
• Nucleocapsid forms genetic material by combining capsid/genome structure.
• Some are naked and lack envolepes.

Viral Envelope

• Composed of glycoproteins/phospholipids.
• Comes as protective coat/attaches from spikes glycoprotein and help bind to the host cell.

Bacteriophages

• Viruses only effect bacteria cells with tail, fibres, contractive sheath with capsid.
• Attach specific receptors of bacteria due to tail fibre, viral specificity and base Plate/tail fibres.
• Contain viral-specific enzymes, which play a role during the stages of replication to facilitate reactions
• These reactions normally will not happen.

Viral Replication Living Characteristics

• DNA/RNA material to instruct the cell synthesize new viruses to form all components.
• Can then synthesis very quickly.
• There is a change over time from the mutation undergoing, to evolve.
• Can reacts from environmental stimuli (chemicals/radiation/heat).

Viral Replication Non-Living Characteristics

• acellular/ no cytoplasm in organelles.
• Must dependently rely on host-cell to reproduce independently.
• Can have an extracellular environment that has no metabolism take an extended amount.
• can not do any of the metabolism functions from 1's own, but will have to gain source of enzyme (nucleotide, ATP, ribosome) with replication of DNA.

Cellular Transport

• Maintains internal environmental equilibrium, regulating the movement and synthesis for new minerals/molecules.

Membrane Structure & Functions

• Regulate (O2), energy with glucose and movement minerals into outside, create maintain for environmental external and internal cells.
• 2 Main components that consist membrane -- proteins and phospholipids.
• Cholesterol/carbohydrates- found within, protein glycosylation, proteins lipid is combined.
• Membranes can't perform barrier function then have the membrane to became easily selective for substances flow then create compartmentalisations.

Membrane Structure Overview

• Two components: lipids and proteins with carboydrates.
• Phospholipids are lipids that have phosphate groups/ hydrocarbon chains (two -- polar and non-polar region).
• Molecule Amphipathic (both hydrophobic, hydrophillic property).
• Are in bilayer(double layer membrane), hydrophilic heads interact with water, but hydrophobic (hydrocarbon chain) is isolated within.

Membrane Fluidity regulation

• held by weak, no covalent bonds - phospholipids/hydrophobic reactions can be easily flexible.
• Must have a fluid membranes to functional properly.
• For protein can be embed (function normally).

Cholesterol function

• Stabilizes with function to help with fluidity (inhibits phospholipic moment)
• Prevents membranes from solidifying at any high rate.
• Hydrophobic core is within nonpolar membranes (permeable for the molecules).
• The selective part is with hydrophobic (polar molecules do need that protein transport).

Proteins Structure and Function

• The function differs upon the different areas.
• Can also function as selective permeables.
• They assist with a variety of protein globular which are within membranes.
• These are positioned within membranes which interact (hydrophobic interactions).
• Polar, charged region are on their outside (aquatic environment).
• Glycoproteins - glycosylated membranes form.

Glycocalyx

• Distinguishes cell types (cell-cell recognition).
• Helps to forms the tissues (cell adhesion).
• In receptors - have glycoproteins respond receptors, function in receptors (chemical signals).
• Made of both proteins and sugars.

Molecules that can pass through membrane

• Lipid - can diffuse (no need of transport proteins).
• Can create protein channels, pores (small diameter; need the solute for transport) for some (appropriately and inorganic charged, sized) ones to pass through.
• Ungated all the time - pores stay open.
• Gated only open when present appropriate.
• Channels- has to move certain amount of voltage in the close conformation.

Osmosis

• Movement solvent for greater water. and for partially
• Doesnt need ATP that is passive with expenditure.
• H2O - ability move well. - Polar in a nature; water easily/permeable moves as H2O and can also move to water, low level (very rate for flow to go across easily).
• Needs a certain water protein (aquaporins) for molecules through aquaporins (hydrophilic can pores for water).

Types of solutions and affect on cell

• These can all can determine and have been solution to high level potential for different plants/animal species depend.
• Hypotonic/animal-cells-enter leading to the end to cell, and lysing in a cell wall is present.
• The hypertonic is then to shrivel where those components from the water for moving with that certain water concentration.

Active Transport of Membranes

• Uses transport transport, molecules enter with a great protein.
• Always active (not depending if is has anything like a concentration gradients or even not).
• Inhibit energy of cells (stop ATP from been form); prevent any of that source and use from is, transport will stop.
• Need ATP (protein carrier).

Protein Carriers and Pumps

• Sodium pump is on the two, with carrier to changes/ returns for when the pump needs release.
• High protons- transports these lower/greater region.
• ATP- is the lysosomes with acidic lumen by that proteins.

Transport of Molecules: Endocytosis

• Internal is within for larger transport components.
• Membrane can fusion (certain lipid bilayers to cause the cell and enter is great).
• Cell membrane will cause its membrane is with transport is great.
• Then is it to separate all (cell, cytosol all well).
• (membrane - lysosome - enzyme products and diffuse the component).

Endocytosis

• 3 forms with Pinocytosis: form channels by cell surface in take is. form - transports rest follows that pathway.
• Phagocytosis in engulf in some great transports. - In animals they work (specialized cells with work the micro).

Exocytosis

• The is cell (membrane) and then they can be used by the transport great proteins,
• Golgi with that transport of Micro (is on it to have all, it, transports by the Micro is on that energy and need).