Cell Structure and Function

Cells

the basic structural and functional unit of all living organisms.

Surface area-to-volume ratio

the relationship between a cell's outer surface and its internal volume; limits how large a cell can grow before diffusion becomes too slow to support it.

Light microscopes

microscopes that use visible light and lenses to magnify specimens; lower resolution than electron microscopes but can view living cells.

Electron microscopes

microscopes that use beams of electrons instead of light, giving much higher magnification and resolution; used on non-living, specially prepared samples.

Prokaryotic cells

simple cells lacking a membrane-bound nucleus or organelles (e.g., bacteria).

Eukaryotic cells

complex cells containing a true, membrane-bound nucleus and membrane-bound organelles.

Cytoplasm

the gel-like fluid inside a cell, outside the nucleus, containing the organelles.

Nucleus

the membrane-bound organelle that houses the cell's DNA and controls cell activities.

Cell wall

a rigid outer layer outside the plasma membrane (in plants, fungi, bacteria) that provides structural support.

Plasma membrane

the phospholipid bilayer enclosing the cell, controlling what enters and exits.

Flagella

long, whip-like structures used for cell locomotion.

Capsule

a protective, often sticky outer layer surrounding some bacterial cells.

Organelles

specialized structures within a cell that perform specific functions.

Phospholipid bilayer

the two-layered arrangement of phospholipids (hydrophilic heads out, hydrophobic tails in) that forms the basic structure of the plasma membrane.

Peripheral proteins

proteins loosely attached to the surface of the membrane, not embedded within it.

Integral proteins

proteins embedded within the membrane, often spanning it partially or fully.

Transmembrane proteins

integral proteins that span the entire width of the membrane, contacting both the inside and outside of the cell.

Fluid-mosaic model

the model describing the cell membrane as a fluid (flexible) layer with a "mosaic" of different proteins embedded in it.

Adhesion proteins

membrane proteins that help cells stick to each other or to surrounding structures.

Receptor proteins

membrane proteins that bind specific signaling molecules and trigger a response inside the cell.

Transport proteins

membrane proteins that help move substances across the membrane.

Channel proteins

transport proteins that form pores allowing specific substances (like ions) to pass through the membrane.

Cell surface markers

molecules on the membrane surface (often glycoproteins) that identify a cell to other cells (e.g., immune recognition).

Carbohydrate side chains

sugar chains attached to membrane proteins or lipids, contributing to cell identity and recognition.

Chromosomes

tightly coiled structures of DNA and protein that carry genetic information.

Nucleolus

a dense region within the nucleus where ribosomal RNA is synthesized and ribosomes are assembled.

Ribosomes

small structures (free or attached to ER) that synthesize proteins.

Endoplasmic reticulum (ER)

a network of membranes involved in synthesis and transport of proteins and lipids.

Rough ER

ER studded with ribosomes; synthesizes and processes proteins.

Smooth ER

ER without ribosomes; synthesizes lipids and detoxifies chemicals.

Golgi bodies (apparatus)

stacked membrane structures that modify, sort, and package proteins and lipids for transport.

Vesicles

small membrane-bound sacs used to transport materials within or out of the cell.

Mitochondria

organelles that produce ATP through cellular respiration; the "powerhouse" of the cell.

Adenosine triphosphate (ATP)

the molecule that stores and provides usable energy for cellular processes.

Cristae

the folded inner membranes of mitochondria that increase surface area for ATP production.

Chloroplasts

organelles in plant/algal cells that conduct photosynthesis, converting light energy into chemical energy.

Lysosomes

membrane-bound organelles containing digestive enzymes that break down waste, debris, and worn-out organelles.

Centrioles

cylindrical structures that help organize microtubules during cell division.

Microtubule organizing centers (MTOC)

regions (like the centrosome) from which microtubules originate and grow.

Peroxisomes

organelles that break down fatty acids and detoxify harmful substances, producing hydrogen peroxide as a byproduct.

Central vacuole

a large, fluid-filled organelle in plant cells that stores water, nutrients, and waste, and maintains turgor pressure.

Cell sap

the fluid contents of the central vacuole in plant cells.

Cytoskeleton

the internal network of protein fibers that gives a cell shape, support, and enables movement.

Microtubules

hollow, tube-shaped cytoskeletal fibers made of tubulin; involved in structural support and movement (e.g., cilia, flagella).

Microfilaments

thin, solid cytoskeletal fibers made of actin; involved in cell shape and movement.

Tubulin

the protein subunit that makes up microtubules.

Cilia

short, hair-like structures used for movement or moving substances across a cell's surface.

Euglena

a single-celled eukaryotic organism with flagella, often capable of photosynthesis.

Paramecium

a single-celled eukaryotic organism covered in cilia, known for its slipper-like shape.

Chitin

a tough, structural polysaccharide found in the cell walls of fungi (and exoskeletons of arthropods).

Facilitated transport

movement of substances across a membrane with the help of transport proteins, without using cellular energy.

Aquaporins

channel proteins that specifically allow water to move quickly across the membrane.

Simple diffusion

the passive movement of small or nonpolar molecules directly across the membrane, from high to low concentration.

Facilitated diffusion

passive movement of substances across the membrane through transport proteins, from high to low concentration.

Passive transport

movement of substances across the membrane that does not require energy (includes diffusion, facilitated diffusion, osmosis).

Osmosis

the diffusion of water across a selectively permeable membrane, from high to low water concentration.

Tonicity

the ability of a solution to cause a cell to gain or lose water.

Isotonic

a solution with equal solute concentration to the cell, causing no net water movement.

Hypertonic

a solution with higher solute concentration than the cell, causing the cell to lose water.

Hypotonic

a solution with lower solute concentration than the cell, causing the cell to gain water.

Water potential

the measure of the tendency of water to move from one area to another, based on solute concentration and pressure.

Solute (osmotic) potential

the component of water potential caused by dissolved solutes; more solute lowers water potential.

Solute

a substance dissolved in a solution.

Active transport

movement of substances across the membrane against their concentration gradient, requiring energy (ATP).

Sodium-potassium pump

an active transport protein that moves sodium out of and potassium into the cell, using ATP.

Endocytosis

the process of a cell taking in materials by engulfing them with the membrane.

Pinocytosis

"cell drinking"; endocytosis of liquids or small dissolved particles.

Phagocytosis

"cell eating"; endocytosis of large particles or whole cells.

Receptor-mediated endocytosis

endocytosis triggered by specific molecules binding to receptor proteins on the membrane.

Clathrin

a protein that coats the membrane during receptor-mediated endocytosis, helping form the vesicle.

Bulk flow

the movement of fluids or materials in bulk, driven by pressure differences rather than diffusion.

Dialysis

the diffusion of solutes across a selectively permeable membrane.

Exocytosis

the process of a cell expelling materials by fusing a vesicle with the plasma membrane.