Biology Chapter 5 review

Membranes

Plasma membrane functions

• Defines the outer border of cells and organelles

• Controls what enters and exits the cell

• Receives external signals and initiates cellular responses

• Adheres to neighboring cells

Fluid Mosaic model

A mosaic of components (phospholipids, cholesterol, proteins, and
carbohydrates) that give the membrane a fluid character

• describes the cell membrane as a flexible phospholipid bilayer studded with a dynamic patchwork of proteins, cholesterol, and carbohydrates that drift laterally

• describes the plasma membrane as a flexible, dynamic phospholipid bilayer with proteins, cholesterol, and carbohydrates drifting laterally throughout it

Phospholipid

The plasma membrane is composed of this lipid

• Hydrophillic head: a glycerol molecule and a phosphate group (polar)

• Hydrophobic tails: 2 fatty acid chains (nonpolar)

Phospholipid tails

The PM is composed of a lipid called phospholipid

• Each fatty acid can be either saturated or unsaturated

• Saturated = no carbon-carbon double bonds

• Unsaturated = one or more double bonds

Phospholipids in water rearrange into bilayer

Phospholipids automatically rearrange into a bilayer in water because they are amphipathic, meaning they have both a hydrophilic (water-loving) region and a hydrophobic (water-fearing) region

Proteins

The second major component of membranes

• function as transporters, receptors, enzymes, or in binding

• functional proteins embedded within or loosely attached to the phospholipid bilayer of a cell membrane, responsible for transport, cell signaling, and enzymatic catalysis

Types of membrane proteins

• Integral proteins - cross completely into the bilayer. a protein permanently embedded within the phospholipid bilayer of a cell membrane

• Peripheral proteins - found only on the surfaces

• loosely attached to the inner or outer surface of the plasma membrane rather than embedded in its hydrophobic core

Integral membrane protein

A protein permanently embedded within the phospholipid bilayer of a cell membrane

• cross completely into the bilayer

• contain both hydrophobic (nonpolar) amino acid regions embedded in the membrane's interior and hydrophilic (polar) regions exposed to the aqueous environments

• penetrate through membrane and parts of the protein exposed to outside and cytoplasm. Can act as transporters, signaling molecules, receptors

Peripheral membrane protein

A protein that temporarily binds to the surface of a cell membrane or to integral proteins without embedding into the hydrophobic core of the phospholipid bilayer

• found only on the surfaces

• on the border (outside or cytoplasmic side) of cell membrane but does not penetrate the membrane

Integral protein regions

Integral membrane proteins have regions that are hydrophobic (made of hydrophobic amino acids) and hydrophilic regions

• Amphipathic Structure: These proteins contain both hydrophobic (nonpolar) amino acid regions embedded in the membrane's interior and hydrophilic (polar) regions exposed to the aqueous environments inside and outside the cell

Membrane protein functions

• Transporter

• Enzyme

• Cell-surface receptor (signal transduction)

• Cell-surface identity marker (cell-cell recognition)

• Cell-to-cell adhesior (intercellular joining)

• Attachment to the cytoskeleton

Cell membrane carbohydrates

The third major component of PM are carbohydrates

• On the exterior surface of the plasma membrane, bound to either proteins (glycoproteins) or to lipids (glycolipids)

• Function in cell-cell recognition & attachment

• short, branched chains of sugars attached to lipids or proteins on the exterior surface of the plasma membrane, functioning primarily in cell-to-cell recognition and immune signaling.

• Immune Response: Allow the immune system to distinguish between "self" cells and "non-self" invaders (such as pathogens or foreign tissue).

Glycoprotein

A membrane protein covalently bonded to a short carbohydrate chain (oligosaccharide) that always faces the extracellular (outside) environment

Functions:

• Cell-to-Cell Recognition

• Cell Signaling & Communication

• Immune Response: Helps the immune system distinguish self cells from foreign invaders or pathogens

• Intercellular adhesion

• Self-cell recognition

Glycolipid

A lipid molecule covalently bound to a carbohydrate chain, located exclusively on the outer surface of the plasma membrane

• Cell-to-cell recognition & attachment

• lipids with a carbohydrate chain attached, located on the extracellular surface of the plasma membrane

Self-cell recognition

An organism's ability—primarily through its immune system and cell membranes—to distinguish its own healthy body cells ("self") from foreign cells or molecules ("non-self")

• Surface Molecules (Markers): Body cells display specific identifying molecules on their outer plasma membranes, often consisting of glycoproteins and glycolipids

• No receptor, no recognition

• Glycoproteins: self-cell recognition

• An autoimmune disease occurs when this recognition fails. The immune system mistakenly identifies self-antigens as foreign, triggering an adaptive immune response that attacks healthy tissues.

Receptor proteins

Specialized proteins located on the plasma membrane or inside a target cell that bind to specific signaling molecules (ligands) to trigger an intracellular response, marking the crucial first step of cell communication and signal transduction

• Our immune system’s T cells have CD4 receptor glycoproteins that recognize HIV as “self”

• The shape of the receptor's binding site matches only one specific ligand

Viruses can infect cells by binding to receptor proteins

Viruses infect host cells by using specific viral surface proteins (like capsids or spike proteins) to bind to complementary receptor proteins on the host cell's plasma membrane

• Lock-and-Key Fit: The interaction relies on a highly specific structural match between the virus's outer proteins and the host cell's surface molecules

• a specific lock-and-key recognition where viral surface proteins (or glycoproteins) physically bind to specific receptor molecules on the outer surface of the host cell membrane

Cell membrane cholesterol

Stabilizes the phospholipid bilayer

Functions as a fluidity buffer in the cell membrane, stabilizing membrane fluidity across a range of temperatures

• Prevents membrane from being too fluid at high
  temperatures by restraining phospholipid movement

• At low temperatures cholesterol wedges between phospholipid tails, preventing them from packing too closely together and being rigid, which keeps the membrane flexible.

What are the major components (molecules) of PM?

• Phospholipids

• Proteins

• Small carbohydrates

• Cholesterol

Membrane fluidity

The dynamic, flexible nature of the fluid mosaic model, where individual phospholipids and proteins can move sideways (laterally) within the lipid bilayer

The membrane is fluid but maintains its structure
Fluidity is affected by:

• Phospholipid type

• Temperature

• Cholesterol

Phospholipid type

A factor that affects membrane fluidity

• Phospholipids with saturated fatty acids pack together more closely than those with unsaturated FA; the more saturated, the more rigid

• Unsaturated Fatty Acids: Contain double bonds that introduce bends or kinks in the hydrocarbon tails, preventing the phospholipids from packing too closely together and maintaining fluidity at lower temperatures.

• Saturated Fatty Acids: Lack double bonds, allowing the straight tails to pack tightly together, which decreases fluidity and makes the membrane more rigid

Temperature

A factor that affects membrane fluidity

• Cold temperatures compress molecules making
  membranes more rigid

• At high temperatures molecules gain kinetic energy and move faster, increasing the distance between phospholipids and making the membrane more fluid/ permeable

• Cholesterol functions as a fluidity buffer in the cell membrane, stabilizing membrane fluidity across different temperatures

Cholesterol

Affects membrane fluidity

• Within the fatty acid layer, keeps membranes fluid when
  cold and not too fluid when hot

• At high temperatures restricts the movement of phospholipid tails, packing them closer together to decrease fluidity

• At low temperatures, cholesterol wedges between phospholipid tails and disrupts tight packing, increasing fluidity to prevent membrane from freezing

Plasma membranes are asymmetric

Because the inner (cytoplasmic) and outer (extracellular) surface of the phospholipid bilayer are different

• Interior proteins anchor fibers of the cytoskeleton to the membrane

• Exterior proteins bind to the extracellular matrix

• the outer layer (exoplasmic leaflet) and the inner layer (cytoplasmic leaflet) of the phospholipid bilayer have completely different chemical compositions, structures, and functions

Selectively permeable

The plasma membrane allows some molecules to pass through, but not others

Allows cytosol to differ from extracellular fluids.

• Example: cells keep different concentrations of sodium and potassium ions between the inside and outside

• The Cell membrane (plasma membrane) is selectively permeable. This means that some molecules can go into the cell but not others: the cell regulates (controls) what can go into the cell

Types of membrane transport

• Passive transport: no energy

• Active transport: requires energy

Diffusion

The simplest type of passive transport is diffusion

• Diffusion is when a substance from an area of high
  concentration moves to area of low concentration

• the passive movement of molecules from an area of high concentration to an area of low concentration down a concentration gradient

• No energy required

Passive transport

Movement stops when equilibrium is reached

• The movement of molecules across a cell membrane down a concentration gradient without using cellular energy (ATP)

• Only small nonpolar molecules (gases such O2, CO2, & hormones diffuse through membranes)

What molecules can pass through the plasma membrane?

Small, nonpolar molecules like O2, CO2, and N2 pass freely through the plasma membrane via simple diffusion

What passes freely

• Gases: Oxygen (O2), carbon dioxide (CO2), and nitrogen gas (N2)

• Hydrophobic/Lipid-soluble molecules: small hydrocarbons and lipid

Factors that affect diffusion rates

• Concentration gradients - Greater difference, faster diffusion

• Mass of the molecules - smaller molecules diffuse more quickly

• Temperature - Molecules move faster when temperatures are higher

• Solubility – more nonpolar (lipid-soluble) materials diffuse faster

Concentration gradient

Affects diffusion rates

• A higher difference in concentration between two areas means a faster net movement of particles

• Greater difference, faster diffusion

Mass of the molecules

Affects diffusion rates

• Smaller molecules diffuse more quickly

• The greater the molecular weight of a molecule, the slower its rate of diffusion

• The smaller the molecules, the faster the rate of diffusion

Temperature

Affects diffusion rates

• Molecules move faster when temperatures are higher

• Higher temperatures increase the rate of diffusion because thermal energy raises the kinetic energy (energy of motion) of particles, causing them to move faster and spread out more quick

Solubility

Affects diffusion rates

• More nonpolar (lipid-soluble) materials diffuse faster

• Substances that are lipid-soluble (hydrophobic, nonpolar molecules like oxygen, carbon dioxide, or small lipids) pass through quickly

Facilitated transport

Moves substances from high to low concentration through integral membrane proteins

• A type of Passive transport where molecules move from High to low concentration with the help of a transport protein. It is for ions (charge elements, such as K+) and polar molecules (charged molecules)

• Ions and small polar molecules diffuse this way

Types of facilitated transport proteins

• Channel proteins

• Carrier proteins

Channel proteins

The exposed parts composed of hydrophilic AA: attract ions and/or polar molecules

• Some open all the time

• Others are gated, open when a signal is received

• an integral membrane protein that forms a hydrophilic (water-loving) pore or tunnel through the plasma membrane, allowing specific polar molecules and charged ions to pass by passive transport down their concentration gradient without using ATP

Channel protein examples

An integral membrane protein that forms a hydrophilic pore or tunnel through the cell membrane, allowing specific polar molecules and ions to pass through via passive transport (without ATP)

• Aquaporins – only for H2O

• Muscle cells have gated ion channels allowing muscle contraction when opened

• Integral protein with a cavity (channel) where molecules transported fit

Aquaporins

Specialized channel proteins embedded in the plasma membrane that allow water molecules to cross the cell membrane quickly via facilitated diffusion

• specialized integral channel proteins embedded in the plasma membrane that allow water molecules to cross the cell membrane rapidly via facilitated diffusion

Carrier proteins

Carrier proteins are specific to a single substance

• Bind substance, changes shape & “carry it” to the other side

• Many allow movement in either direction

• Example: glucose transport proteins

• A transmembrane transport protein that binds to specific molecules on one side of the cell membrane and undergoes a shape change to shuttle them to the other side

• bind to molecules being transported across the membrane

Osmosis

The diffusion of water across a membrane

• Water always moves from an area of higher water concentration to lower water concentration

• the passive diffusion of water across a selectively permeable membrane from an area of higher water potential (lower solute concentration) to an area of lower water potential (higher solute concentration)

Tonicity

The ability of an extracellular solution to make water move into or out of a cell by osmosis, based on the relative concentration of non-penetrating solutes

the ability of an extracellular (surrounding) solution to cause a cell to gain or lose water through osmosis

Solutions outside cells are:

• Hypertonic

• Isotonic

• Hypotonic

NaCI concentration red blood cell

The concentration of dissolved salts inside the cell is equal to a 0.9% saltwater solution

• The internal solute concentration of a red blood cell is equivalent to an 0.9% mass/volume NaCl solution

Hypertonic solution

There is more solute (molecules) outside the cell than inside the cell and the solution is said to be Hypertonic. Water leaves the cell and the cell shrinks

There is more solute outside than inside the cell

• Water leaves the cell and cells become shriveled (crenated)

• Causes plasmolysis and crenation

Hypotonic solutions

An extracellular solution that has a lower solute concentration (and therefore a higher water potential) compared to the cytoplasm inside a cell

• The outside has less molecules than inside the cells

• Water enters the cell

• Cells w/o cell wall burst (hemolysis)

• Increase turgor pressure

Isotonic solution

A solution in which the salt concentration outside the cell (saline solution) equals the salt concentration inside the cell

• No not movement of water across the membrane

• Cells are at equilibrium

Osmoregulation

The active process by which organisms control the balance of water and solutes (such as salts and ions) across cell membranes to maintain internal homeostasis

• Ex; Without enough water, the plant on the left has lost turgor pressure (it wilts); the turgor pressure is restored by watering it (right)

• The active process by which cells and organisms regulate internal water and solute concentrations to maintain homeostasis

Turgor pressure

The outward hydrostatic pressure caused by water pushing the plasma membrane against the rigid cell wall inside a plant cell

• Occurs in hypotonic solutions

• Cells with cell walls (plants, fungi, bacteria) prefer hypotonic extracellular solutions

• The pressure exerted by the PM against the CW is
  critical to organismal growth & functions

Plasmolysis

The process where a plant cell's cytoplasm and plasma membrane shrink and pull away from the rigid cell wall because the cell loses water in a hypertonic environment

• Hypertonic solutions cause this (PM detaches from the CW)

• Plant leaves wilt

Osmoregulation by other organisms

Freshwater protists, like paramecia and amoebas have vacuoles that pump water out of their cells, so they do not burst

• Fish excrete diluted urine to get rid of excess H2O or salts

• Osmoreceptors of brain cells monitor solute concentrations in our blood, releasing hormones that affect kidney function

Active transport

The movement of ions or molecules across a cell membrane against their concentration or electrochemical gradient (from low to high concentration), which requires the input of metabolic energy, typically in the form of ATP

• Energy is always required

Active transport details

Ion or molecule (like glucose) is transported through a membrane protein:

• against its concentration gradient (from low to high concentration) or

• against its electrochemical gradient (ex. H+ ions to a solution that is positive)

• Energy is always required for active transport

Types of active transport

• Primary: ATP provides energy

• Secondary: electrochemical gradient provides energy

Primary active transport

A cellular process that uses chemical energy from ATP hydrolysis directly to move ions or molecules across a cell membrane against their concentration or electrochemical gradient

• a cellular process that directly uses chemical energy—usually from ATP hydrolysis—to move ions or molecules across a cell membrane against their concentration or electrochemical gradient

• Low → High concentration

Secondary active transport

The transport of a molecule or ion against its electrochemical gradient using the energy stored in an existing ion gradient, rather than directly breaking down ATP

• It does not use ATP directly. Instead, it taps into the potential energy of an electrochemical gradient

• Many amino acids and glucose enter the cell this way

Electrochemical gradients

An electrical gradient, where the cytoplasm contains more negative charged molecules (more neg ions) than the extracellular fluid, is critical for proper cell function

• Arise from the effects of concentration gradients and electrical gradients

• Chemical gradient: The difference in the concentration of a specific ion (Na+ or K+) between the inside and outside of a cell; ions tend to move from high to low concentration

• Electrical gradient: The difference in total electrical charge across the plasma membrane; the interior of a resting cell is typically negative relative to the exterior, attracting positive ions inward

Electrochemical gradients explained

The combined force of a chemical concentration difference and an electrical charge difference across a cell membrane that dictates the net direction an ion will move

• The Chemical Gradient (Concentration Gradient): The difference in the concentration of the ion across the membrane. Ions naturally tend to diffuse from an area of high concentration to an area of low concentration.

• The Electrical Gradient (Membrane Potential): The difference in charge or voltage across the membrane. Because ions carry charges, they are physically attracted to the side of the membrane with the opposite charge and repelled by the side with the same charge

Carrier proteins

Active transport occurs through transmembrane, integral carrier proteins called pumps

There are 3 types of pumps:

• Uniporter

• Symporter

• Antiporter

Uniporter

An integral membrane transport protein that moves a single type of molecule or ion across a biological membrane in one direction

• It operates via facilitated diffusion (passive transport)

• Not used in active transport because it moves that single substance down its concentration gradient (from high to low concentration).

Symporter

A membrane protein that moves two or more different molecules or ions across a cell membrane in the same direction at the same time

• A type of cotransporter protein used in secondary active transport.

• Carries two different molecules or ions, in the same
  direction

Antiporter

A membrane protein that moves two or more different molecules or ions in opposite directions across a cell membrane

• Carries two different molecules or ions, in different
  directions

• an integral membrane protein that moves two or more different molecules or ions in opposite directions across a cell membrane using secondary active transport

Primary active transport

Moves an ion or molecule against its concentration gradient using energy from ATP

• Example: Na+-K+ pump

• Moves 3 Na+ out and 2 K+ in using 1 ATP

• Sodium-potassium pump is an antiporter

• The sodium-potassium pump is classified as primary active transport because it directly uses energy from ATP hydrolysis to move sodium (Na+) and potassium (K+) ions against their respective concentration gradients

Secondary active transport

Uses electrochemical gradient created by primary active transport to move a substance against its concentration gradient

• Primary active transport creates electrochemical gradients by using energy from ATP to move ions across the cell membrane against their natural concentration and electrical directions

• Many amino acids and glucose enter the cell this way

How It Works

• Energy Source: It relies on an ion gradient (usually Na⁺) that was previously created by primary active transport using ATP.

• Coupled Transport: The driving ion moves down its electrochemical gradient (high to low concentration), releasing stored potential energy.

• Uphill Movement: That released energy immediately powers a second molecule to move against its gradient (low to high concentration)

How can secondary active transport be called active transport when ATP is not used? Why doesn’t it use ATP instead?

• Definition of Active Transport: Any transport process that moves a substance against its gradient (from low to high concentration) requires an input of cellular energy

• Energy Source: Instead of using ATP directly, secondary active transport uses an electrochemical gradient (stored potential energy) that was previously built by primary active transport

• Coupled Movement: A driving ion (Na+) moves down its gradient (releasing energy), and that coupled energy forces a second molecule up its gradient. Because at least one molecule is actively pumped against its gradient using stored cellular energy, the entire linked process is classified as active transport

Why do you think a K+ solution injection is lethal?

A potassium (K+) solution injection is lethal because it floods the extracellular fluid with potassium, which dissipates the vital electrochemical gradient across cell membranes

Bulk transport

When cells import or export molecules/particles that are too large to
pass through a transport protein

• An energy-requiring process by which cells move large molecules, macromolecules, or large quantities of smaller particles across the plasma membrane using membrane-bound vesicles

• A type of active transport

• Energy is required

• Importing by bulk transport is called endocytosis and exporting is called exocytosis
  
  

Endocytosis

An active transport process where a cell engulfs external substances by folding its plasma membrane inward to form a vesicle that brings materials into the cell

• Importing by bulk transport is called endocytosis

• an active transport process where a cell brings large molecules, fluids, or whole cells inside by folding its plasma membrane inward to form a vesicle

Three types of endocytosis

• Phagocytosis

• Pinocytosis

• Receptor mediated endocytosis

Phagocytosis

A type of bulk transport and endocytosis where a cell engulfs large, solid particles or whole cells using its plasma membrane

• (cellular eating), the cell membrane surrounds a particle and engulfs it

• a type of active transport (bulk transport) known as "cell eating," where a cell engulfs large, solid particles or whole cells into a vesicle

Pinocytosis

A type of active transport and endocytosis where a cell engulfs extracellular fluid and dissolved solutes by folding inward to form small, fluid-filled vesicles

• (cellular drinking), the cell membrane surrounds a small volume of fluid, and pinches off

Receptor-mediated endocytosis

A targeted cellular process where specific extracellular molecules (ligands) bind to specialized receptor proteins on the cell membrane, triggering the membrane to fold inward and pinch off into a vesicle

• A specific substance binds to receptors on the external surface of the membrane

• an active, highly specific type of bulk transport where a cell internalizes specific extracellular macromolecules (ligands) by infolding its plasma membrane

• substance binds to a receptor on the surface of the membrane. Binding of receptor triggers the membrane to engulf the particles and brings them
  inside the membrane through a vesicle

Exocytosis

Vesicles containing substances fuse with the plasma
membrane. The contents are then released to the exterior of the cell

• Exporting by bulk transport is called exocytosis

• Discharge of materials out of the cell

• Used in plants to export cell wall material

• Used in animals to secrete hormones, neurotransmitters, digestive enzymes

Ion channel

A transmembrane protein that forms a hydrophilic pore, allowing specific charged ions to pass passively through the hydrophobic plasma membrane down their electrochemical gradient

• Passive Transport: Ions move via facilitated diffusion, requiring no metabolic energy (ATP)

• Channels are specific, letting only particular ions (like Na⁺, K⁺, Ca²⁺, or Cl⁻) pass through based on size and charge

Coupled transport

Also known as secondary active transport or cotransport, is the movement of one substance across a cell membrane against its concentration gradient, powered by the simultaneous downhill movement of another substance down its electrochemical gradient

• Energy Source: It does not use ATP directly. Instead, it uses stored energy from an electrochemical gradient created earlier by primary active transport (like the sodium-potassium pump)

Cotransport

A type of secondary active transport where a membrane protein couples the "downhill" diffusion of one molecule down its electrochemical gradient to the "uphill" transport of another molecule against its concentration gradient

Facilitated diffusion

The passive transport of polar molecules and ions across a cell membrane via specialized transport proteins down their concentration gradient, requiring no metabolic energy (ATP)

• Channel Proteins: Form hydrophilic tunnels or pores that let specific ions or small polar molecules pass through. (Example: Aquaporins for water, ion channels for K+ or Na+)

• Carrier Proteins: Bind to a specific molecule, change shape, and release it on the other side of the membrane. (Example: GLUT transporters for glucose)

Cell membrane carbohydrates function in

Cell-to-cell recognition & attachment

• Carbohydrates on the cell membrane primarily function as cell-to-cell recognition markers and attachment sites

• Glycoproteins: self-cell recognition