The Cell and Cell Functions I

Chapter 3

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About how many cells are in the human body (approx) ?

1. 1 million

2. 10 million

3. 100 million

4. 1 billion

5. 10 billion

6. 100 billion

Outline for Today

I. Introduction

II. Major RegionsA. Plasma Membrane

1. Structure

2. Functions of membrane components

3. Structural modifications for adhesion/communication

4. Extensions of the plasma membrane

Introduction

Fig. 3.5

Lateral S

urface

Introduction

• Three basic regions

• Plasma Membrane• Cytoplasm

– Organelles in watery fluid

• Nucleus

Representative Animal Cell

Limiting border:separates cell from

extracellular materialNucleus

CytoplasmPlasma membrane

Introduction

• “Edges of the Cell” Apical Edge

Lateral Edge

Basal Edge(rests on anothercell or the basement membrane)

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Plasma (Cell) Membranes

Plasma Membrane Composition

• Phospholipids• Proteins• Cholesterol

Fig. 3.6

Plasma membrane comp

Proteinsintegral to bilayer

Proteins

peripheral to bilayer

water

water

Phospholipids

• Phospholipid bilayer• Hydrophobic region is

inside; hydrophilic region faces the extracellular fluid and the intracellular fluid– Therefore the

hydrophobic tails are sandwiched between the hydrophilic heads

• Some of these have sugars associated with them– Form the glycocalyx

Fig. 3.7

Plasma membrane structure

Plasma Membrane Structure Cholesterol

• Stiffens the membrane, making it less fluid – Up to a point

• Integrates with the lipid portion of the phospholipid– A lot in the typical cell

membrane Fig. 3.6

Proteins

• Transmembrane (integral) proteins go all the way through the membrane– Most are glycoproteins

• Peripheral proteins– At least partially

embedded in the membrane

– Usually associated with transmembrane proteins

Fig. 3.7

Plasma membrane structure

Functions of Membrane Proteins

1. Determines permeability

2. Transport across bilayer

3. Identification

4. Reception

5. Enzymatic Activity

6. Adhesion

Glycocalyx

• The outer component of a cell surface,

• usually contains strongly acidic sugars

• it carries a negative electric charge.

• Unique in everyone

Plasma membrane structure

Fig. 3.10a

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Adhesion

• Cells need to stick to each other along the lateral border and the basal border (to basement membrane)

• Specialized structures– Tight junctions– Desmosomes– Gap junctions

Plasma Membranes Adhesion

Fig. 5.28

Tight Junctions

• On the lateral surface• Want the cell fixed in

place• Tight binding between

integral proteins of one cell and another

• Space between the cells is stitched together so the space is occluded

• Compartmentalizes the space so nothing can pass between the apical and basal regions

• E.g. digestive tract

Plasma Membranes Adhesion

Fig. 5.28

Desmosomes

• Cell to cell (desmosome) or cell to matrix/basement membrane (hemidesmosome)

• Usually deep to tight junctions

• Button like area formed by thickening of membrane

• Sugar chains of glycoproteins hold cells together

• “spot welds”• e.g. in cardiac muscle and

skin

Plasma Membranes Adhesion

Fig. 5.28

Gap Junctions

• Function in communication– Sometimes called

communicating junctions

• Proteins in each cell align to form a pore– Direct cytoplasmic

communication results

• Only small stuff goes through– Monosaccharides and ions

• e.g. : heart

Plasma Membranes Adhesion

Fig. 5.28

Plasma Membrane: Extensions of the cell membrane

• Not all cells have them

• Epithelial cells have the most because they have a free edge

• Cilia, microvilli, and flagella

Fig. 3.10a

Extensions of the Plasma Membrane

i. Microvilli – “small fingers”

in epithelial cells on the free edge (apical border)

surface area of the membrane

High number in absorptive cells

Microvilli

• “little finger”• Increase surface area

by 15x to 40 x• Best in cells designed

for absorption– Kidney– Small intestines

• Anchored with actin

Fig. 3.10a

Extensions of cell membrane

Cilia• Hairlike structures 7-

10 µm long• Motile examples

include– In the inner ear for

balance– Trachea and Uterine

(Fallopian) tube have motile cilia• 50-200 on one cell

Fig. 3.11

Extensions of cell membrane

Cilia

Fig. 3.12

Movement is unidirectional

Extensions of cell membrane

Saline layer

Cilia

Photo: T. Chubb

Extensions of cell membrane

Flagella

• Structure is same as cilia but the flagellum is longer

• Only flagellum in humans in tail of sperm cell

Photo: T. Chubb

Extensions of cell membrane

We shall return

• We will return to the cell membrane when we talk about permeability and transport

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