Transcript Chapter 6

Chapter 6: Cells, tissues and
signals
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
6-1
Extracellular matrix
•
In animal cells, plasma membranes of adjacent
cells may be separated by extracellular matrix
– fluid lattice network of proteins in hydrated
polysaccharide gel
•
Proteins
– structural (collagen, elastin)
– adhesive (fibronectin, laminin, others)
•
Polysaccharide
– glycosaminoglycans
(cont.)
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Extracellular matrix (cont.)
•
•
Proteins in extracellular matrix integrate cell
activities and guide cell movement
Types of extracellular matrix
– interstitial matrix

found in connective tissues
– basal lamina

underlies epithelial cell layers
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Intracellular matrix
•
Plasma membranes of adjacent cells connected by
junctions
– molecular complexes
•
Tight (occluding) junctions
– prevent passage of molecules through extracellular space
•
Anchoring junctions
– sites of attachment for mechanical support of tissues
•
Communicating junctions
– specialised for communication between cells
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6-4
Fig. 6.7: Anchoring, gap and adherent
junctions
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Tight (occluding) junctions
•
•
Prevent free movement of molecules through
extracellular fluid between cells
Also restrict membrane proteins to specific area of
plasma membrane
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6-6
Anchoring junctions
•
Anchoring junctions provide mechanical support
– desmosomes

plaques connected to intermediate filaments of cytoskeleton
 linked by glycoproteins (cadherins)
– hemidesmosomes

anchor cells to the extracellular matrix
– adherens junctions

adhesion belts of actin filaments that run parallel with and
are connected to plasma membrane by plaques
 focal junctions of integrins connect actin filaments to
extracellular matrix
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Communicating (gap) junctions
•
Communicating junctions are specialised for
electrical and chemical communication between
cells
– provide pathway of low electrical resistance
– permit rapid current spread between cells
•
Communicating junctions are highly regular protein
channels
– each links to a similar unit in adjacent cell plasma
membrane
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6-8
Epithelia
•
Epithelial tissues form continuous layers as
surfaces
– cells bound together by tight and anchoring junctions
– protection, regulation of exchange of materials, secretion
•
Epithelia categorised by
– number of layers


simple (one layer)
stratified (more than one layer)
– shape of cells

squamous (flattened)
 cuboidal
 columnar
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Connective tissue
•
Connective tissues provide basic structural,
metabolic and defensive support
– bone, cartilage, blood, adipose tissue, fibroblasts
•
Components of connective tissue
– extracellular materials (usually more abundant than cells)

matrix of polysaccharides and proteins
– fibres

collagen
 reticulin
 elastin
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Muscle
•
Muscle cells are capable of contraction
– composed of actin and myosin filaments
– movement of animal
– movement of internal organs
•
Striated muscle
– highly organised
– skeletal and cardiac muscle
•
Smooth muscle
– less regularly arranged than striated muscle
– internal (visceral) organs
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Nervous tissue
•
Nerve cells (neurons) carry information
– interconnecting network for transmitting information
•
Structure of neuron
– information received by branching dendrites
– signal transmitted along elongate axon
•
•
Neurons are unable to divide
Neurons are supported by glial cells
– maintain composition of extracellular environment
– form myelin sheaths
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Plant cells
•
Plants have rigid cell walls
– limits size and shape of cells
•
Rigid cell wall allows plant cells to be surrounded
by a hypotonic solution
– without wall, inflow of water by osmosis would cause
plant cells to burst
•
Cell walls are important in plant metabolism
– contain enzymes
– act as pathway for transport, absorption and secretion
– act as a barrier against pathogens
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Cell wall structure
•
Cell walls of plants are composed of cellulose
– chains of (1→4)-linked β-D-glucose molecules organised
into microfibrils
– microfibrils embedded in and cross-linked to matrix of
non-cellulosic polysaccharides, pectin and proteins
•
Cell walls of fungi are composed of chitin
– polymer of (1→4)-linked β-N-acetylglucosamine
•
Cell walls of bacteria are composed
polysaccharides cross-linked by amino acids
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Development of cell walls
•
Cell division
– plasma membrane develops between the two new cells
– wall material is added between the two membranes
– middle lamella rich in pectin molecules
•
Primary cell walls
– glucose precursor molecules transported across plasma
membrane by carrier molecules
– cellulose chains assembled on external face of plasma
membrane
•
Secondary walls contain lignin
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Plasmodesmata
•
Rigid cell walls limit contact between adjacent cells
• Plasmodesmata are channels that link plasma
membranes and cytosol of adjacent cells
– bounded by plasma membrane
•
Narrow desmotubule runs through most
plasmodesmata
– desmotubule continuous with endoplasmic reticulum
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Plant tissue
•
Apical meristem is specialised region at growing
tip of root or stem
– continually dividing cells
•
In each pair of daughter cells
– one remains part of meristem
– other differentiates as part of mature body of plant
•
Types of plant tissues derived from meristem
– dermal tissue
– ground tissue
– vascular tissue
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Dermal tissue
•
Dermal tissue forms the outer covering of a plant
• Epidermis is major type of dermal tissue
– one or several closely-packed cells
– secretes waterproof cuticle of cutin (lipid)
•
Epidermis contains stomata (pores) that perforate
cuticle
– stomata allow exchange of gases
– guard cells control size of stomatal opening
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Ground tissue
•
•
Ground tissue includes storage and structural
tissue
Functions of ground tissue
–
–
–
–
photosynthesis
storage
aeration in low-oxygen environments
support and strength
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Vascular tissue
•
•
Vascular tissue transports water, minerals and
synthesised carbohydrates
Xylem
– transports water and minerals from roots to leaves
•
Phloem
– transports photosynthetic products from leaves to place
of use or storage
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Cell signals
•
•
Cells gather information about their surroundings
and use it to control their activities
Cells also produce signals that influence behaviour
of other cells
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Types of signals
•
Stimuli that act as signals for cells may be
– physical

light, heat
– chemical

•
•
food, hormones
Cells may be specialised to receive, transmit or
respond to signals
Chemical messengers used to transmit information
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Responses to signals
•
•
Signal detection involves receptor proteins that
respond to specific stimuli
Specialised nerve cells contain receptors
– photoreceptors

light
– thermoreceptors

heat
– mechanoreceptors

pressure or stretch
– chemoreceptors

chemicals
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Fig. 6.17: Cell responds to signals
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Chemical stimuli
•
•
Cells receive chemical signals by direct interaction
between signal molecule (ligand) and a specific
receptor molecule of plasma membrane of
responding cell
Types of signals
– lipid-soluble chemical signals

enter cells freely and interact with intracellular receptors
– water-soluble chemical signals

bind to cell surface and signal is relayed into cell
– surface-bound chemical signals

located on one cell and binds to receptor on surface of
responding cell
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PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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Fig. 6.18a: Chemical signals
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Fig. 6.18b: Chemical signals
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Fig. 6.18c: Chemical signals
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Physical stimuli
•
Cells respond to a variety of physical stimuli
• Types of signals
– light

photoreceptors contain light-absorbing chromophores that
respond to different wavelengths
– mechanical stimuli


mechanoreceptors for pressure, stretch and hearing detect
stimuli by distortion of cell
heat detected by thermoreceptors with temperaturesensitive channel proteins or active carriers
– electric and magnetic fields

detected by modified ion channels in neuron membranes
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Signal processing
•
•
Signal received by receptor must be processed
into information that produces an appropriate
cellular response
Signal processing may
– be direct
– involve one or more intracellular molecular steps
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Direct responses
•
•
Activated receptor acts directly to produce a
cellular response
Steroids
– generate local responses
– hormone-receptor complex binds to specific region of
DNA
– alters rate of synthesis of particular protein
•
Membrane permeability
– regulation of cytoskeleton by cell adhesion receptors
– regulation of membrane permeability by channel-linked
receptors
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G-protein-linked receptors
•
Many signalling pathways involve intermediate
proteins linked to receptors
– G-proteins

•
guanosine triphosphate (GTP) binding regulatory proteins
G-protein-linked receptors act through G-proteins
to indirectly alter the activity of an ion channel or
intracellular enzyme
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Second messengers
•
G-proteins usually act by altering concentration of
second messenger molecules
– second messengers amplify the signal following receptor
activation by the stimulus
•
Second messengers
–
–
–
–
cyclic AMP (cAMP)
cyclic GMP (cGMP)
inositol trisphosphate (IP3)
Calcium (Ca2+)
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Protein phosphorylation
•
In many signalling pathways, second messengers
trigger protein phosphorylation
• Second messengers activate protein kinases
• Protein kinases attach phosphate groups to
proteins, changing the activity of the protein
• Phosphatases remove phosphates from proteins
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