Cell Transport and Nervous System

Selectively Permeable

Controls what enters/exits

Fluid & Dynamic

Always moving to maintain homeostasis

Mosaic

Proteins carbs lipids arranged like a patchwork

Phospholipid Bilayer

2 layers of phospholipids

Head

Phosphate + glycerol hydrophilic

Tails

2 fatty acid chains hydrophobic

Amphiphilic

Both hydrophilic and hydrophobic parts

Held by weak hydrophobic interactions

Membrane held together by weak forces

Head

Polar holds water

Tail

Nonpolar can bend if unsaturated

O₂ & CO₂

Enter easily

Water

Uses protein channel

Polar molecules/ions

Hard to pass

Nonpolar/hydrophobic molecules

Pass easily

Fluid

Lipids and proteins can move

Mosaic

Proteins and carbs randomly placed

Peripheral Proteins

Loosely bound to surface

Integral Proteins

Span or penetrate membrane

Transport

Large or hydrophilic molecules

Enzymatic activity

Membrane proteins catalyze reactions

Signal transduction

Sends signals to other cells

Intercellular joining

Joins cells together

Cell recognition

Glycoproteins identify cells

Attachment to cytoskeleton/ECM

Anchors membrane

Cell recognition

Membrane carbohydrates identify cells

Immune response

Basis of immune system

Usually oligosaccharides

Less than 15 sugar units

Aquaporin

H₂O channel

Integral Protein

Embedded in both layers

Channel Protein

Forms tunnel for molecules

Carrier Protein

Transports substances

Na⁺/K⁺ Pump

Active transport moving Na⁺ and K⁺

Peripheral Proteins

On one layer only weakly attached to integral proteins

Surface Proteins

Outside of cell includes glycoproteins and glycolipids

Functions

Signaling and recognition

Can be removed with soap/water

Easily washed away

↑ Temp → ↑ Fluidity

More movement less packing

↓ Temp → ↓ Fluidity

Tight packing becomes rigid

Double Bonds

More fluid better for cold

Longer Tails

Less fluid

Cholesterol

Stabilizes membrane prevents too fluid or too rigid

Concentration Gradient

Difference in concentration

Passive Transport

High to low no energy

Diffusion

Molecules move across membrane

Osmosis

Water moves toward higher solute

Active Transport

Low to high needs ATP

Facilitated Transport

Uses proteins to move large or polar molecules without ATP

Isotonic

Equal solute no net water movement

Hypotonic

Water enters cell swells

Hypertonic

Water leaves cell shrivels

Primary

Uses ATP directly example Na⁺/K⁺ pump

Secondary

Uses gradient created by primary example Na⁺/Glucose symporter

Endocytosis

Brings materials in

Phagocytosis

Engulfs solids

Pinocytosis

Engulfs liquids

Receptor-mediated

Specific molecules

Exocytosis

Releases materials out

CNS

Brain and spinal cord

PNS

Carries messages to and from CNS

Somatic

Voluntary control

Autonomic

Involuntary control sympathetic and parasympathetic

Cell body

Contains nucleus and organelles

Nucleus

Contains DNA

Dendrites

Receive signals

Axon

Sends signals

Axon terminal

Releases neurotransmitters

Glial Cells

Support nourish and protect neurons

Sensory Input

Detects stimulus afferent neurons

Integration

Interneurons process information

Motor Output

Sends response efferent neurons

Unipolar

One process spinal cord

Multipolar

Many dendrites brain and spinal cord

Bipolar

One dendrite and one axon sensory organs

Myelin Sheath

Increases signal speed made by Schwann and oligodendrocytes

Nodes of Ranvier

Gaps that speed impulse

Reflex Arc

Receptor sensory neuron interneuron motor neuron effector

Negative Feedback

Returns to normal example sweating or shivering

Positive Feedback

Enhances effect example blood clotting

Astrocytes

Nutrient and structural support

Microglial cells

Immune defense

Oligodendrocytes

Myelin in CNS

Schwann Cells

Myelin in PNS

Ganglia

Neuron cell body clusters outside CNS

Myelinated Neurons

Conduction type is saltatory conduction ("jumps"), action potential only at nodes of Ranvier, signal spreads quickly

Myelin

Has high resistance, making it hard for ions to leave or enter the membrane

Unmyelinated Neurons

Slower conduction because the action potential must propagate continuously along the axon membrane

Resting Potential

About -70 mV, maintained by sodium-potassium pump, membrane polarized and ready to fire

Stimulus

Trigger opens some voltage-gated Na⁺ channels, Na⁺ influx causes depolarization

Threshold Potential

About -55 mV, if reached action potential fires (all-or-none), all VG Na⁺ channels open

Depolarization

Inside becomes more positive, VG Na⁺ channels close at peak

Repolarization

At peak VG K⁺ channels open, K⁺ exits, membrane returns toward negative

Hyperpolarization

K⁺ channels close slowly, inside becomes more negative than resting, refractory period

Return to Resting Potential

Na⁺/K⁺ pump restores normal ion distribution and resting potential

Synapse

Gap between presynaptic and postsynaptic neurons

Neurotransmitters

Chemical messengers used by neurons to communicate

Synapse Step 1

Action potential arrives at axon terminal, causing depolarization

Synapse Step 2

Voltage-gated Ca²⁺ channels open, Ca²⁺ enters terminal

Synapse Step 3

Ca²⁺ binds to NT vesicles, vesicles fuse with membrane

Synapse Step 4

Neurotransmitters released into synapse via exocytosis

Synapse Step 5

NTs bind to ligand-gated channels on postsynaptic neuron, ions enter or exit