Cell and Plasma Membrane

Cell Theory

Cell Theory

• Cells are the smallest structural and functional units of life

• All living organisms are composed of one or more cells

  • Bacteria vs. Humans

• Cells arise from other cells

Why do cells stay small?

TO MAXIMIZE SURFACE-TO-VOLUME RATIO!

• Cells must be tiny for materials to move in and out of them fast enough to meet the cell’s metabolic needs

Plasma Membrane

separates the living cell from its nonliving surroundings

Plasma Membrane functions

• Barrier – separates extracellular fluid from intracellular fluid

• Selective permeability – controls what enters and exits the cell

• Cell markers and receptors – cell recognition, binds hormones, cell communication

• Adhesion – between other cell membranes or extracellular materials

Structure of Plasma Membrane

Phospholipid Bilayer

Phospholipid Bilayer structure

• Hydrophilic heads orientate themselves towards the extracellular and intracellular fluid

• Hydrophobic tails orientate themselves inward, away from the fluid

Cholesterol

• stabilizes the membrane

Glycocalyx

• serve as biological markers

  • Self vs. non-self

Proteins

• Peripheral – attach to inner or outer surface

• Integral – embedded in the membrane

Functions of membrane proteins

Transport

Cell adhesion molecules

Receptors

Enzymes

2 types of transportation of membrane protein

• Channels – water filled pathways that allow select ions in/out of the cell

• Carrier proteins- transport select substances across the membrane

Cell adhesion molecules

play a role in anchoring cells to each other and the cytoskeleton, responsible for stickiness of cells

(Intercellular joining)

Receptors

cell recognition, cell signaling, binding of hormones

Cell-Cell recognition)

Enzymes

facilitate chemical reactions on inner and outer membrane surfaces

Plasma Membrane Transport

Passive transport

Active Transport

Passive transport

does not require energy to move solutes across a membrane

Types of passive transport

Diffusion (2 types)

Filtration

Diffusion

• Solutes move down their concentration gradient until evenly distributed throughout the solution

Simple diffusion

• solutes diffuse across the membrane unassisted

  • Small non-polar and lipid-soluble solutes (O2, CO2)

Facilitated diffusion

• proteins carry or assist solutes across the membrane

  • Charged ions move through protein channels

  • Large molecules such as glucose or amino acids are carried across

Carrier protein

(Facilitated diffusion)

-Transports a specific substance

-Can reach saturation when all binding sites are occupied (Transport maximum)

-Other closely related compounds can compete for the same binding site

Fick’s law of diffusion

• Factors that affect the rate of diffusion

What are the Factors that affect the rate of diffusion

• the magnitude of the concentration gradient

• the permeability of the plasma membrane to a substance.

• the surface area of the membrane across which diffusion takes place

• the molecular weight of a substance

• the distance through which diffusion takes place

• temperature

Osmosis

• diffusion of water across a membrane down its concentration gradient

Water moves by osmosis from the area of higher water concentration to the area of lower water concentration

Tonicity

(Osmosis)

ability of a solution to change the shape of a cell by altering its internal water volume, depends on concentration of non-penetrating solutes

Types of Tonicity

• Isotonic solution – contains equal concentration solutes as the cell

• Hypertonic solution – contains more solutes than the cell

• Hypotonic solution – contains less solutes than the cell

Filtration

• water and solutes are pushed across a membrane from an area of higher pressure to an area of lower pressure

  • Non-selective process, only large molecules cannot pass

Active transport

• movement of solutes across the membrane requires energy (ATP)

  • Occurs during movement of solutes against their concentration gradient

  • Or very, very  large molecules

Solute pumping

• proteins use ATP to transport solutes against concentration gradient

Primary active transport

• energy is provided directly by the hydrolysis of ATP

  • Sodium-Potassium pump – maintains a higher concentration of potassium inside the cell and a higher concentration of sodium outside the cell

  • Pumps 3 Na+ out and 2 K+ in

Cotransporters

(Solute Pumping)

carrier proteins that transport two or more substrates across a membrane

(2 TYPES)

Symport

moves two substrates in same direction

Antiport

moves two substrates in opposite directions

Secondary active transport

primary transport of one molecule creates an ion gradient used to drive another molecule against its concentration gradient

• EX: Na+ stays low in the cell so that we continue to absorb glucose

Exocytosis

(Active transport)

• moves material from cell interior to the extracellular space

  • Vesicles fuse with the plasma membrane expelling the contents out of the cell

Endocytosis

(Active Transport)

moves material from extracellular space into the interior of the cell

(2 Types)

Pinocytosis

(“cellular drinking”) a cell “gulps” droplets of fluid

Phagocytosis

• (“cellular eating”) a cell engulfs a particle

Receptor mediated endocytosis

• receptors bind specific substances and initiates endocytosis