Bio 1113.02 | CHAPTER 11 CELL-CELL INTERACTIONS

The cell surface – role in communication

• It is rare for cells to be bounded simply by a plasma membrane

• Most cells possess a protective layer or wall that forms just

beyond the membrane

• This layer generally consists of a cross-linked network of long

filaments surrounded by a stiff ground substance

Cell wall in Plants

Consists of long strands of cellulose and other gelatinous

polysaccharides such as pectins

Connections and communication between plant cells

Plant cells are glued together by middle lamella:

– Continuous with adjacent plant cell walls

Plasmodesmata

Plant cells are connected by gaps in their cell walls called

plasmodesmata

• Plasma membranes, cytoplasm, and smooth ER of two

cells connect

ECM in Animals

Most animal cells secrete the extracellular matrix (ECM)

• Fibrous component is mostly collagen (protein )

• Ground substance is made of proteoglycans

Connections and communication between

Animal Cells

• Animal cells have a variety of membrane proteins that attach

cells together

How do distant cells communicate?

Distant cells in multicellular organism can communicate

through signaling molecules

– Neurotransmitters may open or close channels in distant

cells

– Hormones are information-carrying molecules

• Secreted from a cell

• Small molecules—peptides, steroids, or even gases

• Circulates in the body

• Acts on target cells far from the signaling cell

Signal reception- Hormones

Hormones bind to signal receptor molecules

• Receptor change shape and activity after binding

• Only cells with appropriate signal receptors will respond to

a particular signaling molecule

Signal reception- Hormones

• Lipid-soluble signaling molecules diffuse across plasma

membrane:

• Receptors are in target cells’ cytoplasm

• Lipid-insoluble do not cross plasma membrane:

• Have to be recognized at cell surface

• Receptors are on cell’s plasma membrane

Processing Lipid-Soluble Signaling

Molecules

Examples: estrogen and

cortisol diffuse across

plasma membrane and

bind to receptors inside

the cell

• The hormone–receptor

complex

– Is transported to the

nucleus

– Where it alters gene expression

Processing Lipid-insoluble Signaling

Molecules

• Hormones that cannot

diffuse across the plasma

membrane bind to

membrane receptors, signal

that arrives has to produce

intracellular signal

• It triggers a complex series

of events collectively called

signal transduction

• Message transmitted may

be amplified as signal

changes from one form to

another

Signal transduction and amplification

• Two types of signal transduction systems:

– G-protein-coupled receptors:

• Initiate production of intracellular second messengers

• Amplify and diversity signal

– Enzyme-linked receptors phosphorylate proteins inside

target cell

Signaling via G-Protein-Coupled Receptors

• G protein-coupled receptors are trans-membrane receptors

that work with the help of a G protein

Signaling via G-Protein-Coupled Receptors

• G proteins are intracellular peripheral membrane proteins

– When activated by a signal receptor they trigger

production of a second messenger

• G proteins are regulated by guanine nucleotides

Second messengers

• Are small molecules which diffuse rapidly throughout the cell

• Can be produced quickly in large quantities

• They are not restricted to a single role

• The same second messenger can have different roles in different

cells while receiving the same signal

Some second messengers activate ……

• Kinases add a phosphate group to other proteins

Some second messengers activate protein kinases

• Kinases add a phosphate group to other proteins

Signaling via Enzyme-Linked Receptors

The receptor tyrosine kinases (RTKs) are the best-known

group of enzyme-linked receptors

– Are trans-membrane proteins

– Directly catalyze a reaction inside the cell

Signal Response

The ultimate response to a cell–cell signal varies from signal

to signal and from cell to cell

• Falls into two general categories:

1. A change in which genes are being expressed in the

target cell

2. A change in the activation or deactivation of a particular

target protein that already exists in the cell

Signal Deactivation

• Turning off cell signals is just as important as turning them on

• Cells have automatic and rapid mechanisms for signal

deactivation

– E.g., phosphatases remove phosphate groups from

proteins in the phosphorylation cascade

• These mechanisms allow the cell to remain sensitive to small

changes in signaling