6-1Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Chapter 6: Cells, tissues and signals

6-2Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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.)

6-3Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

6-4Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

6-5Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 6.7: Anchoring, gap and adherent junctions

6-6Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Tight (occluding) junctions

• Prevent free movement of molecules through extracellular fluid between cells

• Also restrict membrane proteins to specific area of plasma membrane

6-7Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

6-8Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

6-9Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

6-10Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

6-11Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

6-12Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

6-13Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

6-14Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

6-15Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

6-16Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

6-17Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

6-19Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Ground tissue

• Ground tissue includes storage and structural tissue

• Functions of ground tissue– photosynthesis– storage– aeration in low-oxygen environments– support and strength

6-20Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

6-21Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

6-22Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

6-23Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

6-25Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

6-26Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 6.18a: Chemical signals

6-27Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 6.18b: Chemical signals

6-28Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 6.18c: Chemical signals

6-29Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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 temperature-

sensitive channel proteins or active carriers

– electric and magnetic fields detected by modified ion channels in neuron membranes

6-30Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

6-31Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

6-32Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

6-33Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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+)

6-34Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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