Introduction to Cells The Cell Membrane Chapter 4

Introduction to Cells & The Cell Membrane Chapter 4

Overview • Cell structure & function • Cell membrane structure & function • How cells interact

Cell Theory 1. Organisms consist of 1 or more cells. 2. Cell = smallest unit of life 3. Continuity of life comes from growth & division of cells

What is a Cell? Made of C, H, O, N + trace elements Has all the properties of life: – Metabolism – Responsiveness – Growth – Reproduction Cells differ in size, structure, & function

Cell Function Performs all vital physiological functions Maintains homeostasis via processes happening within Building blocks of all organisms

Generalized Cell Structure Plasma membrane Region of DNA Cytoplasm

Plasma Membrane Lipid bilayer – Separates internal & external environments Semi-permeable – Controls passage of substances in/out of cell

Region of DNA Cellular control centre Holds genetic information Eukaryotic cells: – Inside membrane-bound nucleus Prokaryotic cells: – Free-floating in cytoplasm

Cytoplasm “Guts” of cell Semi-fluid matrix – Colloidal properties Contains all structural components (organelles, etc. )

Why Are Cells So Small? Surface area: volume ratio Volume increases with cube of diameter V = 4/3πr 3 SA increases with square of diameter SA = 4πr 2

Diameter (cm) 0. 5 1. 0 1. 5 SA (cm 2) 0. 79 3. 14 7. 07 Volume (cm 3) 0. 06 0. 52 1. 77 SA: volume ratio 13: 1 6: 1 4: 1

If cells were big: – Plasma membrane would have to work very hard to service all of cytoplasm – Materials would have a harder time moving through the cytoplasm – Movement of substances across membrane would not be fast enough to maintain cell activity Cells that aren’t tiny are usually long & thin or have folds to increase SA

Cell (Plasma) Membrane Semi-permeable barrier between interior of cell & external environment Fluid-mosaic model: Lipid bilayer (prevents movement of water-soluble substances) Dynamic pattern of proteins (some able to move through membrane) (involved in membrane function)

Lipid Bilayer Consists of: – Phospholipids – Glycolipids – Cholesterol – Lipid Rafts

Membrane Lipids: Phospholipids Hydrophilic (polar) head Hydrophobic (non-polar) fatty acid tails (unsaturated = kinks in tails ↑ membrane fluidity)

Biological membranes naturally form closed, spherical structures that can reseal quickly if torn

Membrane Lipids: Glycolipids 5% of membrane lipids Phospholipids with sugar groups Found only on outer membrane surface (cell signalling & recognition)

Membrane Lipids: Cholesterol 20% of membrane lipids Wedges between phospholipid tails Has both hydrophobic & hydrophilic regions Maintains integrity of membrane by keeping it firm & impermeable to some water-soluble molecules Increases fluidity of membrane by ensuring that fatty acid chains don’t crystallize

Membrane Lipids: Lipid Rafts 20% of membrane lipids Found only on outer membrane surface Variety of tightly packed saturated lipids (= stable, less fluid) Used as concentrating platforms for cell signalling molecules

Overview of Membrane Proteins 50% of membrane mass Carry out most membrane functions • Integral proteins • Peripheral proteins

Overview: Integral Proteins Most are transmembranal Have both hydrophobic & hydrophilic regions Used mainly for transport (channels, carriers, receptors)

Overview: Peripheral Proteins Usually located at one membrane surface Attach loosely to integral proteins or membrane lipids Functions include: structural support for cell, enzymatic action, joining cells, changing cell shape

Major Membrane Proteins Receptor proteins Recognition proteins Adhesion proteins Communication proteins Transport proteins (passive & active transporters)

1. Receptor Proteins Binding sites for hormones, etc. Allow changes in cell activities (protein synthesis, cell division, etc. ) Different cells have different receptor proteins

2. Recognition Proteins In multicellular organisms Identify cells as foreign or self Used in tissue defense, cell adhesion, etc.

3. Adhesion Proteins In multicellular organisms Allow cells of same type to find & stick to each other or to other substances (e. g. proteins in EC matrix)

4. Communication Proteins Multicellular organisms Form channels between cytoplasm of 2 cells Allow chemical & electrical signals to flow between cells

5. Transport Proteins Have interior channels Solute enters channel & binds weakly to protein Protein changes shape Channel closes behind solute & opens in front of solute Solute is released on other side Protein regains normal shape

5 a. Passive Transport Proteins Move solutes & water across membrane down concentration gradients No energy input required One-way or bi-directional Some are ion-selective channels: (have gates that open/close depending on molecular, chemical, etc. signal)

5 b. Active Transport Proteins Pump solutes across membrane against concentration gradients Require energy input One-way or bi-directional Some are co-transporters: (allow passive transport of some solutes while pumping others in the opposite direction)

So What is a Concentration Gradient? Different in concentration of ions or molecules between 2 adjacent areas With no energy input, molecules move down gradient from [high] to [low]

Diffusion Net movement of ions/molecules down concentration gradient Each ion/molecule has its own gradient

Factors Affecting Diffusion Rate 1. Steepness of concentration gradient 2. Temperature 3. Size of ions/molecules 4. Electric gradients 5. Pressure gradients

So … the cell membrane is semi-permeable = allows passage of some substances but not of others What controls what, how much, & when substances cross the membrane?

Membrane is mostly non-polar = allows small, non-polar molecules to cross (e. g. O 2, CO 2, etc. ) = impermeable to ions & large, polar molecules (e. g. glucose, Na+, K+, etc. ) Water (although polar) can slip through gaps caused by kinks in tails or can use aquaporin transporters

Movement Mechanisms Passive transport – Simple diffusion – Facilitated diffusion Active transport Exocytosis Endocytosis

1. Passive Transport Net movement down concentration gradient No ATP energy required

1 a. Simple Diffusion Direct diffusion through membrane Non-polar, lipid-soluble, & small molecules e. g. O 2, CO 2, fat-soluble vitamins e. g. [O 2] in blood is higher than in cell, so continuously diffuses in

1 b. Facilitated Diffusion Substances transported passively via channels/carriers (proteins) Some channels are always open; others open & close on cue Transport limited by # & activity of channels/carriers

2. Active Transport Pumps solutes against concentration gradient ATP energy required Substrate-specific transporters (activated by phosphate group from ATP)

e. g. sodium-potassium pump • Uses carrier enzyme Na+K+ATPase • [K+] ↑ inside cell, [Na+] ↑ outside cell • Both seep through leakage channels down concentration gradients • Na+K+ pump drives Na+ out & pumps K+ in

Now we know how ions & molecules cross the selectively permeable plasma membrane. What about water?

Osmoregulation = control of water balance Prevents excessive uptake / loss of water e. g. fish have gills & kidneys

Osmosis Diffusion of water across semi-permeable membrane from [high] to [low] How can water have a concentration? [H 2 O] ↑ as [dissolved solute ] ↓ In other words, the more dilute a solution is, the more concentrated the water is

Tonicity Relative solute concentrations of two fluids e. g. on opposite sides of a membrane Determines the direction & how much water movement will occur across a membrane

When comparing two fluids: Hypotonic solution: = the one with fewer solutes Hypertonic solution: = the one with more solutes Isotonic solutions: = have the same concentrations

Water diffuses from hypotonic fluids to hypertonic fluids

a. Hypotonic Animal Cell

b. Hypertonic Animal Cell

c. Isotonic Animal Cell

So Why Don’t Animal Cells Burst Easily? As H 2 O enters cell via osmosis, solutes are transported out = osmoregulation Hydrostatic pressure against the membrane also controls osmosis Note: there is a point where cells will lyse (burst)

Because of their rigid cell walls, plant cells face a different scenario

Plant Cells & Osmoregulation Isotonic solution = becomes flaccid & wilts

Hypertonic solution = shrivels = plasmolysis (plasma membrane separates from cell wall)

Plant cells are happiest in hypotonic environments Become turgid Net inflow of water Cell wall expands but does not burst

Other Transport Mechanisms Large particles & other substances can be moved between the external environment, the plasma membrane, & the interior of the cell via: • Exocytosis • Endocytosis

Remember: Membranes are self-sealing Hydrophobic interactions between phospholipid tails & water molecules creates spherical structures

1. Exocytosis Vesicle w/i cytoplasm moves to cell membrane Fuses with membrane Releases contents to exterior of cell

2. Endocytosis Outer membrane inpouches around particle outside of cell Contents released to cell interior (can then be moved to or stored in organelles) 3 types: – Receptor-mediated endocytosis – Phagocytosis – Bulk-phase endocytosis

2 a. Receptor-Mediated Endocytosis Substance (hormone, vitamin, mineral, etc. ) binds to receptors on membrane Pit forms beneath receptor Pit sinks into cytoplasm, forming vesicle

2 b. Phagocytosis “Cell eating” e. g. amoebas, macrophages, etc. Pseudopods extend around substance, forming vesicle Vesicle moves into cytoplasm & fuses with lysosome, which digests contents of vesicle

2 c. Bulk-Phase Endocytosis Non-selective process Vesicle forms around ECF & carries it into cytoplasm

Doesn’t the continuous formation of vesicles drastically change the surface area of the cell membrane?

Endocytosis & exocytosis occur at rates that maintain the total SA of the plasma membrane Losses via endocytosis ≈ replacements via exocytosis

Cell Junctions Some cells are free-floating Most knit together Molecular structures allow communication b/w cells: Plant cells: plasmodesmata Animal cells: tight junctions, desmosomes, gap junctions

a. Plasmodesmata Channels Connect cytoplasms of 2 adjacent cells Allow rapid exchange of materials

b. Tight Junction Integral proteins of adjacent cells fuse Impermeable to leakage between cells Join cells of most body tissues e. g. stomach lining

c. Desmosomes a. k. a. adhering junction Zipper-like seal Binds cells together internally & externally by protein filaments Distributes tension evenly to ↓ risk of tearing e. g. skin, heart muscle, neck of uterus

d. Gap Junctions Channels that connect cytoplasms of 2 adjacent cells Allow rapid exchange of materials Occur in electrically-excitable tissues e. g. heart muscle, smooth muscle (allows synchronicity of electrical activity & contraction)

Cell-Environment Interactions Membrane receptors Integral proteins & glycoproteins used as binding sites – Contact signalling – Chemical signalling

a. Contact Signalling Physical contact between cells Glycocalyx = glycoproteins with branching sugar sidechains = unique to each cell type Aids in cell recognition

b. Chemical Signalling Ligands = signalling chemicals that bind to membrane receptors (include neurotransmitters, hormones, etc. ) Receptors sense molecules outside cell & activate signal transduction pathways that lead to cellular responses Many receptors involved in diseases but also used as targets for drugs

e. g. G-protein linked receptors • Ligand (1 st messenger) binds to receptor • Receptor activates G-protein • G-protein stimulates effector protein (enzyme) • Effector protein produces 2 nd messenger inside cell • 2 nd messenger activates kinase enzymes • Kinases activate other enzymes → various cellular responses