cell membranes

amphiphatic molecule

has both hydrophilic head and hydrophobic tail

function of membranes

controls the passage of substances

what are the types of membranes

1. plasma membrane

2. internal membranes in eucaryotic cell

plasma membrane function

creates a border between a cell ad its environment

internal membranes in eukaryotic cell 

creates compartments inside the cell 

hydrophobic hydrocarbon chains

form the core of the membrane have low permeability to large molecules and hydrophilic particles

factors that influence on permeability of particular substance

1. size→ smaller can pass

2. polarity→ non-polar can pass (water)

diffusion

passive movement of particles from a region of higher concentration to a region of lower concentration, it tends to equalize the concentrations, depends on the concentration not the membrane 

down concentration gradient

from higher concentration to lower concentration as a result of random motion of particles.

what molecules can passively diffuse?

oxygen, carbon dioxide, urea, ethanol

example of simple diffusion

cornea because it has no blood so its cells obtain oxygen by simple diffusion from the air 

What is not possible because of the centre of membrane being hydrophobic?

ions with positive or negative charges cannot easilly diffuse through

what is in the extracellular fluid?

sugars and carbohydrates

peripheral protein

hydrophilic molecules that are attached to the membrane surface, located on either cytoplasmic or extracellular side

integral protein

hydrophobic molecules that span the entire membrane (transmembrane proteins) or partially embedded in the hydrocarbon chains

myelin sheath around nerve fibers

insulation of the nerve 18% of protein content

plasma membrane 

transport, exchange of materials 50% protein content

membrane of chroloplasts and mitochondria

electron transport, synthesis of ATP 75% protein content

function of integral protein

binding sites for signaling molecules, channels and carriers for molecules

osmosis

passive movement of water molecules from a region of lower solute concentration to a region of higher solute concentration, across a partially permeable membrane

selectively permeable membrane

sugar molecules cannot pass through pores, but water molecules can

intermolecular bonds

restrict the movement of the water molecules

aquaporins

water channels which greatly increase membrane permeability to water

plasmolysis

cytoplasm shrinks due to osmosis of water, the cell membrane pulls away from the cell wall in plant cells

What is the basic structure thet forms the foundation of all cell membranes?

the lipid bilayer, composed of phospholipids arranged in a double layer

the lipid bilayer

serves as a barrier that separates the internal environment of the cell form the external environment, while also allowing selective exchange of substances, it can repair itself if disrupted

What does the phospholipid molecule consist of?

hydrophilic head and hydrophobic tail→ this arrangement minimizes energy and creates a stable structure

How does the lipid bilayer form?

spontaneously due to the interactions between water and amphipathic phospholipids

What property allows the lipid bilayer to control what substances can pass through it?

selective permeability, which allows small nonpolar molecules to pass while blocking large or polar molecules

fluid nature of phospholipids

can move laterally within the bilayer, it is essential for membrane functions like cell signaling and transport

What is the primary function of the lipid bilayer in cell membranes? 

forming a selective barrier that separates the cell from its environment

cholesterol

helps modulate membrane fluidity and provides stability by fitting between phospholipids

glycolipids

a molecule consisting of a lipid with attached carbohydrate groups, involved in cell recognition and signaling

three main components od phospholipids 

• a phosphate group(hydrophilic head) 

• the fatty acid tails(hydrophobic) 

• the glycerol backbone connecting them 

properties od the lipid bilayer

• fluidity

• self-sealing

• selective permeability

factors that influence membrane fluidity

• temperature

• fatty acid composition

• cholesterol content

permeability of the lipid bilayer

• easily permeable to small non-polar molecules

• slightly permeable to small polar molecules

• impermeable to ions and large polar molecules

Why ions can’t pass through the lipid bilayer easily? 

because they are charged and interact strongly with water, making it energetically unfavourable to cross the hydrophobic core

homeostasis

the maintenance of stable internal environment despite external changes

functions od the lipid bilayer as a barrier

1. maintaining distinct internal and external environments

2. regulating the passage of substances

3. protecting the cell from harmful substances

lipid bilayer is found in 

1. cell membranes 

2. vesicles 

3. lysosomes 

4. endoplasmic reticulum

Which component of the phospholipid is responsible for its hydrophilic properties?

phosphate group

transport mechanisms

• passive transport- does not require energy ATP

• active transport -requires energy ATP

simple diffusion 

movement of molecules from an area of high concentration to an area of low concentration, occurs naturally due to the random motion of particles 

facilitated diffusion

requires transport proteins to help molecules cross the membrane

proteins involved in facilitated diffusion

• channel proteins- form pores for specific ions

• carrier proteins- change shape to transport specific molecules

active transport 

moves molecules against their concentration gradient and requires energy in the form of ATP 

types of active transport

• endocytosis- taking substances into the cell(phagocytosis ,pinocytosis)

• exocytosis- releasing substances out of the cell

phagocytosis

engulfing solid particles

pinocytosis 

engulfing liquid 

factors affection diffusion rate

• concentration gradient- greater difference=faster

• temperature- higher=faster

• surface area- larger=faster

• molecule size- smaller=faster

importance of membrane transport

• maintaining cellular homeostasis

• obtaining nutrients

• removing waste products

• communication between cells

fluid mosaic model 

describes the structure of cell membranes

types of integral proteins

• transmembrane proteins- span the entire membrane, with parts exposed on both sides

• monotopic proteins- embedded in only one layer of the bilayer

functions of membrane proteins

• transport

• signal transduction

• cell adhesion

• enzymatic activity

function of carbohydrate chains 

• cell recognition 

• protection

• adhesion 

membrane asymmetry

the composition and arrangement of molecules differ between the inner and outer layers

examples of membrane asymmetry

• glycoproteins and glycolipids are found only on the outer surface

• specific phospholipids are more concentrated on one side

• peripheral proteins have distinct locations and functions on each side

water potential

the potential energy of water in a system, determining the direction of water movement

water potential is affected by

1. solute potential(more solutes=lower potential)

2. pressure potential(physical pressure increases potential)

hypotonic solution

lower solute concentration outside the cell (water enters the cell, cell may swell or burst)

hypertonic solution

higher solute concentration outside the cell (water leaves the cell, cell shrinks)

isotonic solution

equal solute concentration (no net water movement, cell maintains normal shape)

osmosis in kidneys

• concentrate urine

• reabsorb water

mechanisms regulating osmosis

• controlling the number of aquaporins

• adjusting solute concentrations

• changing membrane permeability

function of channel proteins 

facilitation of the passive transport of specific molecules and ions across the membrane 

principles of channel proteins

• from high to low concentration

• no energy

channel proteins play crucial role in 

• nerve signaling 

• muscle contraction 

• water balance 

pump proteins 

• are embedded in the membrane 

• are specific to the substances they transport 

• require energy to function 

difference between pump proteins and channel proteins

channel proteins allow passive movement of substances while pump proteins facilitate active transport

active transport using pump proteins

1. target molecule or ion binds to a specific site on the protein pump

2. ATP is broken down, releasing energy

3. the pump protein changes shape, moving the molecule across the membrane

4. the molecule is released on the other side of the membrane

5. the pump returns to its original shape, ready for another cycle

symporters 

protein that transports molecules in the same direction simultaneously 

functions od active transport 

• nutrient uptake 

• waste removal 

• maintaining electrical gradients 

antiporters

protein that transports two different molecules in the opposite directions 

types of passive transport

• simple diffusion

• facilitated diffusion

• osmosis

difference between channel proteins and carrier proteins 

channel proteins open or gate to allow specific molecules while carrier proteins bind to specific molecules and change shape to transport  

glycoprotein

a molecule consisting of protein with attached carbohydrate chains,

function of glycoproteins and glycolipids

they recognise each other, distinguish self and foreign cells

glycocalyx 

a carbohydrate-rich layer on the cell surface formed by glycoproteins and glycolipids. It functions in cell adhesion, protection and communication. 

functions of glycoproteins and glycolipids

• cell recognition

• cell adhesion

• protection and support

• cell communication

features of the fluid mosaic model

• fluidity

• mosaic

• dynamic

• semi-permeable

functions of peripheral proteins

• cell signaling

• structural support

• enzyme activity