Cell structure student notes

Defining characteristics of life

Growth

Reproduction

Responsiveness

Metabolism

Who developed cell theory?

Schleiden and Schwann

Cell theory

The cell is the basic unit of life

What do prokaryotes lack?

a membrane around their genetic material (DNA) (lack)

internal membrane-bound structures (lack)

What are the two types of prokaryotes?

bacteria and archaea

What do eukaryotes have?

membrane around their genetic material (DNA) (have)

membrane bound internal structures (have)

What do membrane bound internal structures do in eukaryotes?

compartmentalize cell functions

External structures in the prokaryotic cell

Glycocalyces, Flagella, Axial filaments & Endoflagella, Pili/Fimbriae, The Cell Wall (Peptidoglycan & Teichoic acid), The Outer Membrane, The periplasmic space, and Plasma membrane

Glycocalyces (“sugar cup”):

sticky/gelatinous, outermost covering of some cells

capsule vs slime layer

Function of Glycocalyces:

protection from environment/prevent desiccation

adherence to surfaces

protect from immune system

biofilm formation

Flagella:

Hair-like structure (long, thin, helical) made of flagellin subunits (protein monomers)

used for motility

What are the three parts of the flagella

The filament

The hook

The basal body (anchor)

What do photo/chemoreceptors in the cytoplasmic membrane (CM) allow bacteria to do?

detect environmental signals

it taxis = motion/movement

Axial filaments and endoflagella

flagella that spiral around the cell = endoflagella which turn axial filaments

Pili/fimbriae

Hollow hair-like structures composed of pilin protein subunits (=monomer)

Can be involved in motility

What do conjugation pili allow bacteria to do?

transfer DNA (e.g., antibiotic resistance genes) from cell to cell

What do fimbriae allow bacteria to do?

to attach to various surfaces (e.g., host cell membranes) by means of adhesins

Involved in the formation of biofilms

The cell wall

semi-rigid coating generally composed of peptidoglycan (in bacteria)

determines the cell shape and maintains cellular integrity

Who has peptidoglycan

only eubacteria

What is the function of peptidoglycan

preventing osmotic lysis (in-rush of liquids → cell bursts), and maintaining cell shape

Structure of peptidoglycan

alternating covalently linked N-acetylglucosamine (NAG or G or gluNAc)

alternating covalently linked N-acetylmuramic acid (NAM or M or murNAc)

the tetrapeptide “tails” of NAM form cross-links

What are peptide cross-links essential for

the strength of PG (chain-link fence) and are formed by bacterial enzymes = transpeptidases

Teichoic acid

Gram-positive cell walls contain this ( = glycerol + phosphates + ribitol (sugar alcohol) which help anchor the PG to the cell membrane (lipoteichoic acid)

Outer membrane

Found in gram-negative bacteria

Lipid bilayer with embedded lipoproteins that attach it to the cell wall

What does the outer membrane act as

a coarse sieve and controls transport of certain proteins through porins (channel proteins; protects cells from penicillin, lysozyme, bile salts)

Components of the outer membrane

LPS (endotoxin) = polysaccharide (core + repeat units) + lipid A (glucosamine-pyrophosphate polymer + long-chain fatty acids)

Periplasmic space

the area between the cell membrane and outer membrane of gram-negatives

very active area of cell metabolism (digestive enzymes + transport proteins)

Cell membrane/cytoplasmic membrane/plasma membrane function:

boundary between cell and environment

semi or selectively permeable → control of substances into and out of cell

site for electron transport chain and photosystems (photosynthesis) in some bacteria

flagellar base in flagellated bacteria

site of cell wall synthesis

assists with DNA replication, etc.

What is the cell membrane/cytoplasmic membrane/plasma membrane made of

made of phospholipids + integral proteins

What do integral proteins act as in the cell membrane

act as pores, channels, and carriers to import and export nutrients and other important substances

act as transmitters to allow the cell to sense the environment e.g., rhodopsin (light-sensing retinal protein) and bacteriorhopsin (photosynthetic protein)

Internal structures of the prokaryotic cell

Cytosol, Ribosomes, Nuclear region, Cytoskeleton, Inclusions, Endospores

Cytosol:

semi-fluid (~Jello), 80-90% water with dissolved substances (proteins, carbohydrates, lipids, inorganic ions)

Ribosomes:

rRNA + protein

spherical

protein synthesis machinery

contain a large and a small subunit [50 S (Svedberg unit) + 30 S] = 70S (sedimentation depends on mass, size, AND shape)

Nuclear region (nucleoid):

location of the bacterial chromosome (DNA molecule) + associated RNA and protein

Cytoskeleton:

internal network of fibers (protein) in rod-shaped bacteria that works to form the cell’s shape

Inclusions:

small bodies in the cytoplasm → granules and vesicles

Granules:

very compacted (almost solid);

contain specific substances e.g., glycogen, polyphosphate, sulfur

vesicles/vacuoles

membrane-enclosed structures

can be filled with gas, iron (magnetosomes)

Endospores:

resistant, dormant, survival structures that protect a bacterium’s DNA from harsh environmental conditions

Eukaryotic cell structure characteristics:

tend to be larger and more complex than prokaryotic cells; eukaryotic cells have a lower surface-to-volume ratio

External structures of the Eukaryotic cell

glycocalyces, appendages, cell walls,

Glycocalyces (eukaryotic cell)

made of sticky carbohydrates anchored to the cell membrane by membrane proteins and lipids;

helps to anchor cells to each other;

strengthen the cell surface;

protect somewhat against drying;

function in cell-to-cell recognition and communication in multicellular organisms

Appendages in eukaryotic cell:

flagella - shaft/filament composed of paired microtubules arranged in the “9+2” pattern;

covered with cell membrane and contains cytosol

What anchors the flagellum in the cell membrane, what is it composed of (eukaryotic cell structure)

The basal body of the flagella;

microtubule triplets (“9 + 0”)

True or false:

Eukaryotic cells have positive and negative chemo-/phototaxis but do not move in runs and tumbles

How do flagella move like in eukaryotic cells?

like whips due to cross-bridging b/t dynein and other proteins in the flagella

Do prokaryotes have cilia

No

Cilia:

shorter and generally more numerous than flagella (100’s/1000’s)

Same composition and microtubule basic arrangement (9+2 for hair; 9 + 0 for basal body) as flagella

move in unison

Pseudopodia (false feet)

Extrusion of cell membrane and cytoplasm

flow of cytoplasm into pseudopod moves cell

Cell walls (eukaryotic cell)

present in fungi, algae, and plants

provide protection from the environment;

provide cell shape and support against osmotic pressure

What do plant cell walls contain?

cellulose (paper, dietary fiber)

what do fungal cell walls contain

chitin and/or glucomannan (can have cellulose, too)

what do algae/algal cell walls have

can contain cellulose, agar, carrageenan, silicates, algin, or calcium carbonate

Interior structure contain what? (eukaryotic cells)

cytoplasm (semi-liquid) + organelles + cytoskeleton

Cytoskeleton (eukarya)

composed of protein filaments;

forms support network

anchors organelles

is active in cytoplasmic streaming

moves cell membrane

centrioles and centrosome

plays roles in mitosis, cytokinesis, and flagella/cilia division

what are centrioles composed of

nine microtubule triplets (~ “9 + 0 arrangement of flagella/cilia)

two centrioles arranged at right angles = centrosome;

located at nuclear poles (centrosome portion of cytoplasm) of animal and some fungal cells

Ribosomes

site of protein synthesis

40S + 60S subunits + rRNA → 80S ribosomes

can be attached to ER or free in cytoplasm

Nucleus

Spherical/Oval organelle

nucleoplasm (~cytoplasm) + nucleoli + chromatin + nuclear envelope

Nuclear envelope =

double membrane i.e., 4 phospholipid bilayers

nuclear pores

nucleoli - sites for RNA synthesis

Chromatin/chromosomes = DNA + histones

Endoplasmic reticulum

netlike arrangement of hollow tubules in the cytoplasm → increased working surface area

functions as a transport system

What are the two types of endoplasmic reticulum

Smooth, Rough

Rough endoplasmic reticulum

Studded w/ ribosomes: SYNTHESIZES PROTEINS for export, incorporation into membranes, and delivery (via tubules) to other organelles

Smooth endoplasmic reticulum

SYNTHESIZES LIPIDS, aids in detoxification

Golgi Body/Apparatus

Shipping department of the cell: receives products from the ER via transport vesicles, stores the products packaged in secretory vesicles, which export the products to their destinations

Lysosome =

Small membrane-bound organelles

filled with digestive enzymes

fuse with vacuoles to hydrolyze nutrient molecules or destroy invading microorganisms

Peroxisomes

contain enzymes (peroxidase, catalase) that degrade oxygen radicals/peroxide produced during metabolism

Endocytosis/phagocytosis:

Phagosomes (vesicles) form around debris (microbes/tissue scraps) and carry it into the cell → fuse with lysosomes → digestion (phagolysosomes) → nutrients released into Golgi body → transported via secretory vesicles as is waste

Exocytosis =

residue/waste from phagocytosis released from the cell via vesicles

Mitochondria

powerhouse of the cell

performs oxidative reactions to produce ATP

complex structure

outer membrane + inner membrane folded to form cristae + matrix

Chloroplasts

Light-harvesting structures

complex structures

outer and inner membranes + stroma (= fluid between thylakoids and inner membrane) + thykaloids containing chlorophyll.

Theory of endosymbiosis

A primitive, larger anaerobic prokaryote phagocytizes a primitive aerobic respiring prokaryote/ photosynthesizing prokaryote

The bacterium becomes an endosymbiont, living within the host cell, generating ATP/photosynthesizing for the host; the host provides protection for the endosymbiont

The endosymbiont eventually evolves into a mitochondrion/chloroplast

Supporting evidence for endosymbiosis:

a. Mitochondria and chloroplasts are self replicating

b. M/C have their own self-replicating, circular chromosomes similar to prokaryotic chromosomes

c. M/C are prokaryote-sized

d. M/C membranes are arranged like those of aerobic/photosynthetic bacteria

e. M/C have ribosomes similar to prokaryotic 70S ribosomes and are inhibited by antibiotics targetting 70S ribosomes

f. M/C ribosomal RNA sequences are similar to bacterial rRNA sequences

g. M/C have double membranes

Passive processes: What is simple diffusion?

Movement of molecules from high → low concentration without energy

Molecules that diffuse easily?

O2, CO2, ethanol, medium fatty acids

What is facilitated diffusion?

Movement down gradient via channel/carrier proteins

Non-specific vs specific permeases?

non-specific = many substances

specific = one substance

What is osmosis

Diffusion of water across a selectively permeable membrane

Isotonic solution effect?

no net water movement; cell stays same

Hypertonic solution effect?

water leaves cell → cell bursts

What is active transport? (active processes)

movement against gradient (low→ high) using ATP

Uniport definition?

one substance transported at a time

Antiport definition?

two substances move in opposite directions

Symport definition?

Two substances move together in same direction

How can gradients power transport?

Energy from one molecule’s gradient drives another’s movement

What is group translocation? (prokaryotes only)

substance is chemically modified during transport (e.g., glucose → G6P)

Why is group translocation efficient?

modified product cannot diffuse back out

works at very low concentrations

Energy source for group translocation?

PEP (phosphoenolpyruvate)

What is endocytosis?

cell membrane invaginates to bring substances into cell

receptor-mediated endocytosis

uses specific receptors to bind and internalize substances

What is phagocytosis?

cell engulfs debris/microbes → forms phagosome → fuses with lysosome

What is exocytosis?

vesicles release waste or secretions outside the cell