A2.2 Cell structure

who was Robert Hooke?

first to use the word “cell” for structures in living organisms in 1665, this started cell theory

what is cell theory?

1. all organisms are composed of one or more cells

2. all cells come from preexisting cells

3. the cell is the structural and functional unit of all living things

what are structures common to cells in all living organisms?

• plasma membrane (cell membrane)

• genetic material

• cytoplasm

• ribosomes

plasma membrane (cell membrane)

made of lipids, separate the cell contents from everything else outside

genetic material

DNA; stores all instructions needed for the cell’s activities

cytoplasm

medium for chemical reactions, catalysed by enzymes produced inside the cell

ribosomes

particles that synthesise proteins

atypical cell structures in eukaryotes

• red blood cells

• muscle tissue

• fungal hyphae

• phloem

red blood cells

lack nucleus, mitochondria, other organelles to optimise haemoglobin storage, cannot repair itself —> lifespan of 4 months

how are red blood cells produced?

produced in bone marrow where they have a nucleus for the first 7 days until maturation

muscle tissue

individual muscle cells fuse together —> long striated muscle fibres (300mm), fibres surrounded by continuous membrane

multinucleated cells

more than one nucleus per cell —> giant cell with continuous cytoplasm (e.g. musle tissue, fungal hyphae)

fungal hyphae

fungi may have filamentous structures used for nutrient absorption and growth, no cell wall/ membrane

hyphal cells

typically separated by internal walls (septa) —> but some hyphae are not partitioned and have a continuous cytoplasm

what do striated muscle fibres challenge?

the idea that all living organisms are comprised of discrete cell units

what do aseptate fungal hyphae challenge?

the idea that living organisms are composed of autonomous cells + cell is a single unit

mycelium

very large tubular system of hyphae —> dense networks

phloem

found in plant stem

sieve elements

line the phloem interconnected by plasmodesmata into supracellular assemblies —> travers the length of a plant

sieve elements lack…

nuclei, mitochondria, and have few organelles —> relies on local companion cells for survival

what idea do phloem sieve tube elements challenge?

multicellular structures are composed of anatomically independent cells

phloem only has…

cekk membrane + cytoplasm

what do local companions cells provide?

ATP (energy) for survival

what are elements in phloem?

pores in cell walls of adjacent “cells”

what are the functions of life?

metabolism, response, homeostasis, growth, excretion, reproduction, nutrition

metabolism - functions of life

the web of all enzyme-catalysed reactions in a cell or organism (e.g. respiration)

response - functions of life

living things can respond to and interact with the environment

homeostasis - functions of life

the maintenance and regulation of internal cell conditions (e.g. water and pH)

growth - functions of life

living things can grow or change size/ shape

excretion - functions of life

the removal of metabolic waste

reproduction - functions of life

living things produce offspring (sexual + asexual)

nutrition - functions of life

feeding by either the synthesis of organic molecules or the absorption of organic matter (e.g. photosynthesis)

how are cells categorised?

prokaryote/ eukaryote cell structure

prokaryote cell structure

single-celled organisms of the domains: bacteria + archaea

eukaryote cell structure

animal/ plant/ fungi cells —> protists are eukaryotes

function of nucleus

DNA is replicated and transcribed to form mRNA, which is exported via the nuclear pores to the cytoplasm

function of nucleolus

constructs ribosomes

function of nuclear pore

regulating the passage of molecules (protein, RNAs) between the nucleus and cytoplasm + allow communication between nucleus and rest of cell

function of double nuclear membrane

acts as a physical barrier that protects the cell’s genetic material by separating the nucleus from the cytoplasm

function of rough endoplasmic reticulum (rER) + ribosomes

synthesises protein for secretion from the cell, the protein synthesised by ribosomes of rER passes into its cisternae —> carried by vesicles which bud off and move to golgi apparatus

function of smooth endoplasmic reticulum (sER)

used to synthesise lipids, phospholipids, and steroid. a special type of sER stores calcium ions in muscle when its relaxed

function of gogli apparatus

processes proteins brought in vesicles from rER —> most proteins are then carried in vesicles to the plasma membrane for secretion

function of lysosome

contain digestive enzymes, which can be used to break down ingested food in vesicles —> can also break down organelles or even whole cells

function of mitochondria

produce ATP for the cell by aerobic cell respiration, fat is digested here is its being used as an energy source in the cell

function of free ribosomes

synthesise proteins, releasing it to work in cytoplasm —> as enzymes or in other ways

function of chloroplast

produce glucose + wide variety of other organic compounds via photosynthesis —> if chloroplasts have been photosynthesising rapidly, they may contain starch grains

function of vacuoles and vesicles

vesicles are very small vacuoles used to transport materials inside the cell

function of microtubules and centrioles

centrioles form an anchor point for microtubules during cell division and also for microtubules inside cilia and flagella

function of cytoskeleton

• guide the movement of components within cell —> help plant cells to construct cell walls

• layer of microfilaments just inside the plasma membrane helps animal cells maintain their shape

function of cilia and flagella

cilia and flagella can be used for locomotion —> cilia can also be used to create a current in the fluid next to the cell

plastids

family of organelles with two outer membranes and internal membrane sacs

cell wall

a rigid layer outside the plasma membrane to strengthen and protect the cell

vacuole

flexible fluid-filled compartment surrounded by a single membrane

centrioles

cylindrical organelles that organise the assembly of structures composed of microtubules

undulipodia

cilia and flagella used to generate movement of a cell or movement of fluid adjacent to a cell

which eukaryotic cells have plastids?

plants —> plastids of varied types such as chloroplasts (photosynthesis) and amyloplasts (store starch)

what are the cell walls of plant cells made of?

cellulose

what are the cell walls of fungi made of?

chitlin

describe the vacuoles of animal cells

small temporary vacuoles expel excess water or digest food or pathogens taken by endocytosis

describe vacuoles of plant and fungi cells

often a large permanent vacuole, used for storage of substances and pressurising the cell

describe centrioles in animal cells

used to construct the spindle that moves chromosomes in mitosis and the 9+ 2 microtubules in cilia and flagella

which eukaryotic cells don’t have centrioles

plant and fungi (except with swimming)

which eukaryotic cells have undulipodia?

animal cells —> sperm

which eukaryotic cells have fon’t undulipodia?

plant and fungi cells

what ribosomes do prokaryotic cells have?

70S

what ribosomes do eukaryotic cells have?

80S

staphylo-

in clusters

streptobacilli

rod-shaped - bacillus

gram positive bacteria (staphylococcus aureus)

bacteria classified by the colour they turn in the Gram staining method —> due to different cell wall and membrane composition compared to Gram-negative bacteria

which of the following are unicellular?

prokaryotes and animal cells

which of the following are multicellular?

plant cells

size of prokaryote cells

small size —> usually less than 5μm

size of plant cells

larger size —> usually more than 5μm

size of animal cells

larger size —> usually more than 5μm

shape of prokaryote cells

often rod-shaped (bacilli), spheroidal (cocci), or helical (spirilli)

shape of plant cells

• usually regular flat sides

• cell junctions easily visible

shape of animal cells

• usually rounded

• junctions are between cells, hard to see in tissues

which of the following have cell walls?

prokaryote and plant cells

animal cells have no cell…

cell wall

prokaryotes - nucleus

no nucleus, nucleoid present in paler region of cytoplasm

plant cells - nucleus

• normally present

• not always visible

animal cells - nucleus

• normally present

• not always visible

plastids in prokaryote cells

simple internal structure —> no membrane-bound organelles

plastids in plant cells

present in chloroplasts/ amyloplasts

plastids in animal cells

• no chloroplasts/ stored starch

• cytoplasm contains many other organelles

vacuoles in prokaryote cells

none

vacuoles in plant cells

large ones often present

vacuoles in animal cells

only small ones present

endosymbiosis

one organism lives in another

example of endosymbiosis

chloroplasts and mitochondria evolved from small symbiotic prokaryotes that lived within larger host cells

symbiosis

interaction between two different organisms living in close physical association, typically to the advantage of both

mitochondria - ancestor cells

may have been aerobic bacteria —> able to use cellular respiration, ancestral host cell may have ingested these cells since they needed ocygen to survive

chloroplasts - ancestor

photosynthetic bacteria that used to live inside a larger host cells and remained alive —> continued to perform photosynthesis

did mitochondria or chloroplasts evolve first?

mitochondria - because almost all eukaryotes have them

evidence that supports endosymbiotic theory for mitochondria & chloroplasts

DNA, ribosomes, double membrane, size, transcription_protein synthesis, cell division

LUCA

Last Universal Common Ancestor

how did LUCA start

bacteria + archaea —> stemmed to eukaryotes

examples of differentiated cells

skin cells, red blood cells, liver cells, bone cells

where do differentiated cells originate from

stem cells

embryonic stem cells

entire genome is active and newly formed cells receive signals to deactivate (or more rarely activate) genes