CHAPTER 3: CELL BIOLOGY

cell

are the basic units of all living things, including humans. Cells vary in size, structure, and function but share common characteristics.

plasma membrane, nucleus, cytoplasm, organelles

Basic Cell Parts

plasma membrane

Outer boundary of the cell; separates internal environment from external environment

nucleus

Usually located centrally; directs cell activities

cytoplasm

Material between the nucleus and plasma membrane

organelles

Specialized structures within the cytoplasm that perform specific functions

cell metabolism and energy use, synthesis of molecules, communication, reproduction and inheritence

Four Characteristic Functions of Cells

Cell metabolism and energy use

chemical reactions occur within the cell, involving energy transfer

communication

cells send and receive chemical/electrical signals (e.g., nerve cells signal muscle cells).

synthesis of molecules

cells produce proteins, nucleic acids, and lipids.

reproduction and inheritance

most cells contain a complete copy of DNA; cells divide to produce new cells with the same genetic info; gametes transmit genetic info to the next generation

plasma membrane

the outermost component of the cell.

glycolipids

Carbohydrates combine with lipids

glycoproteins

Carbohydrates combine with lipids and proteins

glycocalyx

collection of glycolipids, glycoproteins, and absorbed carbohydrates on the outer cell surface

phospholipids and cholesterol

Two main lipids

Phospholipids

Form a lipid bilayer (double layer)

hydrophilic

A polar (charged) head → ——- (water-loving), faces outward toward water

hydrophobic

Nonpolar tails → —— (water-fearing), face each other in the interior

Fluid-Mosaic Model

The membrane is not rigid — it behaves like a flexible, dense fluid where components can move around.

Integral proteins and Peripheral proteins

Proteins are classified by location:

Peripheral proteins

Attached to the inner or outer surface, not embedded within

Integral proteins

Penetrate deeply into (or fully through) the lipid bilayer

Marker molecules, Attachment proteins, Transport proteins, Receptor proteins, Enzymes

Five Functions of Membrane Proteins

Cadherins and Integrins

Attachment Proteins

cadherins

attach cells to other cells

integrins

attach cells to extracellular molecules; work in pairs; also aid communication

specificity, competition, saturation

Transport Proteins — 3 Key Characteristics

Channel Proteins, Carrier Proteins (Transporters), ATP-Powered Pumps

Three Classes of Transport Proteins

CHANNEL PROTEINS

Form a tiny channel through the membrane

Leak (nongated) ion channels

always open

Gated ion channels

open/close in response to conditions

Ligand-gated

open/close when a ligand binds

Voltage-gated

open/close with change in membrane potential

1. molecule enters carrier from extracellular fluid 2. binds to a site 3. carrier changes shape and releases molecule on the other side

Carrier Proteins (Transporters) Steps:

Uniport

Moves one type of molecule/ion in one direction

Symport (cotransport)

Moves two different substances in the same direction

Antiport (countertransport)

Moves two different substances in opposite directions

ATP-Powered Pumps

se cellular energy (ATP) to move ions/molecules against their gradient

Receptor Proteins

Have an exposed receptor site on the outer cell surface

Receptor Proteins

Bind specific chemical signals (ligands)

Receptors linked to channel proteins and Receptors coupled to G protein complexes

Two major mechanisms of receptor proteins:

Membrane Enzymes

Can be on the inner or outer membrane surfac

Lipid-soluble molecules

diffuse directly through the lipid bilaye

Small, non-lipid-soluble molecules

diffuse between phospholipids

Large, non-lipid-soluble molecules or ions

need transport proteins

Large particles/whole cells

moved in a vesicle

Solution

solute(s) dissolved in a solvent

diffusion

movement of solutes from high → low concentration until equilibrium is reached (no net movement, though random motion continues)

Concentration gradient

difference in concentration between two points

Osmosis

diffusion of water across a selectively permeable membrane

Osmotic pressure

force required to prevent water movement across the membrane by osmosis; the more solutes, the greater the osmotic pressure and the greater the "pull" for water

Isosmotic

Same solute concentration in the two solution

Hyperosmotic

Solution has a greater solute concentration than the one it's compared t

Hyposmotic

Solution has a lower solute concentration than the one it's compared to

Mediated transport

uses membrane transport proteins (carrier or channel) to move substances

Active Transport

Mediated transport process requiring ATP

Active Transport

Moves substances against their concentration gradient (low → high)

Vesicular Transport

Movement of larger volumes of material via membrane-bound sacs (vesicles); requires ATP but does not show the same specificity/saturation as other membrane transport.

Secondary Active Transport

Active transport (e.g., Na⁺–K⁺ pump) creates an ion concentration gradient

Secondary Active Transport

That gradient's potential energy is then used to move a different substance across the membrane (via a carrier protein), without directly using ATP for that second substance

Endocytosis

vesicle brings material into the cell

Phagocytosis (cell-eating)

Solid particles ingested; forms phagocytic vesicles; done mainly by white blood cells to eliminate harmful substances

Pinocytosis (cell drinking)

Molecules dissolved in liquid taken in via small vesicles; common in intestines, kidney, liver cells, capillaries

receptor-mediated endocytosis

Specific molecules bind receptors on the membrane → vesicle forms and takes in molecules + receptors; increases rate of uptake for substances like cholesterol and growth factors

Exocytosis

vesicle releases material out of the cell

Transcytosis

Combination of endocytosis (one side of the cell) + exocytosis (opposite side)

Cytoplasm

cellular material outside the nucleus but inside the plasma membrane; about half cytosol and half organelles

Cytosol

The fluid portion of the cytoplasm; a colloid solution

Cytoskeleton

Supports the cell and holds organelles in place

microtubules, microfilaments (actin filaments), intermediate filaments

Three protein groups

Microtubules

Support/scaffolding; cell division; intracellular transport; form centrioles, spindle fibers, cilia, flagella

microfilaments

Structure; mechanical support for microvilli; changes in cell shape; muscle contraction

Intermediate filaments

Mechanical strength (e.g., support nerve cell extensions)

Cytoplasmic Inclusions

Chemicals either produced or taken in by the cell (not membrane-bound organelles)

Hemoglobin

transports O₂ in red blood cells

Melanin

pigment in skin, hair, eyes

Lipochromes

aging pigments

Organelle

a structure within a cell specialized for a particular function

Nucleus

Large, membrane-bound structure; usually near the cell's center

Histones

proteins around which DNA wraps to form nucleosomes

Nucleolus

A dense region within the nucleus (no membrane); usually 1–several per nucleus

Nucleolus

Site where ribosomal subunits are assembled

Ribosomes

Sites of protein synthesis

Rough ER

Synthesizes/modifies proteins for secretion or membrane use

Smooth ER

Synthesizes lipids (phospholipids, cholesterol), steroid hormones, carbohydrates; detoxification; stores Ca²⁺ (important in muscle contraction

Golgi Apparatus

Stack of flattened membranous sacs (cisternae), like stacked dinner plates

Cis face

receives material from the ER

Golgi Apparatus

modifies, packages, and distributes proteins/lipids (e.g., adds carbohydrates to form glycoproteins/glycolipids)

Secretory Vesicles

Pinch off the Golgi apparatus; move to the cell surface; release contents by exocytosis

Lysosomes

Membrane-bound vesicles from the Golgi apparatus

Peroxisomes

Smaller membrane-bound vesicles

Lysosomes

Contain hydrolytic (digestive) enzymes

Peroxisomes

Break down fatty acids and amino acids

Proteasomes

Large protein complexes (not membrane-bound) that break down and recycle unneeded/damaged proteins

Proteasomes

Barrel-shaped; ends regulate which proteins enter

Proteasomes

The "powerhouse of the cell" — major site of ATP production

Mitochondria

Structure: outer membrane (smooth) + inner membrane (folded into cristae, increasing surface area)

Centrosome

region near the nucleus; center of microtubule formation

Cilia

Move materials over the cell surface (e.g., clear mucus in the respiratory tract)