cell membrane

Peripheral Proteins

loosely attached on the inner or outer surface

Functions (H.E.L.P.):

• Handle cell shape changes (during division or muscle contraction)

• Enzymes – speed up chemical reactions

• Linking cells together

• Plasma membrane support

Integral Proteins

embedded through the lipid bilayer

Have both hydrophilic (touches heads) and hydrophobic (touches tails) parts.

Functions (I R.E.A.C.T.):

• Intercellular joining

• Receptors for signal transduction (receive hormones, signals)

• Enzymatic activity

• Attachment to cytoskeleton & extracellular matrix

• Cell-cell recognition (like ID tags)

• Transport (channels, carriers, or pumps using ATP)

Glycoproteins

proteins with carbohydrate chains attached

• Help with cell recognition and communication (like ID badges).

Passive Transport

❌ No energy needed

Moves from high → low concentration

Diffusion, Osmosis, Facilitated Diffusion

✅ Uses energy (ATP)

Moves from low → high concentration

Sodium-potassium pump, Endocytosis, Exocytosis

Diffusion

• Molecules move from high concentration → low concentration until evenly spread.

• This happens because of constant motion and collisions between particles.

• Faster if:

  • The concentration difference is larger

  • The temperature is higher

  • The molecule is smaller

Simple Diffusion

• Substances pass directly through the lipid bilayer (no help needed).

• Works for small, nonpolar, or lipid-soluble molecules.

  • Examples: O₂ (oxygen), CO₂ (carbon dioxide), alcohol, fat-soluble vitamins

• Water-soluble particles can’t pass through the lipid part but can use protein channels (tiny pores).

Facilitated Diffusion

• For large or polar molecules that can’t pass through on their own.

• They need help from transport/carrier proteins.

• The carrier protein changes shape (conformational change) to carry the molecule across.

  • Examples: Glucose, amino acids, simple sugars

Osmosis

• Definition: Movement of water across a semipermeable membrane from high water concentration → low water concentration (or from low solute → high solute).

• Type: Passive transport (no ATP needed).

• There are aquaporins (protein channels) that let water move in/out easily.

Isotonic

Cell stays the same size

Hypertonic

cell shrinks, water moves out 

hypotonic

cell swells up, water into cell

Filtration

• Definition: Movement of water and small solutes through a membrane due to pressure differences (not concentration).

• Type: Passive process.

• Movement: From high pressure → low pressure.

• Example: In the kidneys, blood

Dialysis

• Definition: Diffusion of solutes (not water) through a semipermeable membrane.

• Purpose: Used in medical treatments to remove wastes from the blood (as in dialysis machines).

• Type: Passive process.

Primary Active Transport

• Main example: Sodium–Potassium (Na⁺–K⁺) Pump

• What it does:

  • Pumps 3 sodium ions (Na⁺) out of the cell

  • Pumps 2 potassium ions (K⁺) into the cell

• Why: Keeps Na⁺ high outside and K⁺ high inside — essential for muscle and nerve cell function.

• Uses: An enzyme called sodium–potassium ATPase (uses ATP energy).

Secondary Active Transport

• Definition: Uses energy indirectly from another ATP-powered pump (like the sodium-potassium pump).

• The main pump creates a concentration gradient, and that stored energy is then used to move other substances against their own gradient.

• Example:

  • The Na⁺/K⁺ pump first moves sodium out of the cell (using ATP).

  • Later, sodium “falls back in” through a carrier protein — and as it moves, it drags another molecule (like glucose or amino acid) along with it.

• Type: Active transport (but indirectly uses ATP).

👉 Think of it like this:
Primary = directly uses ATP
Secondary = uses the leftover energy from primary transport

Vesicular Transport

Definition: Moves large particles, fluids, or whole molecules in or out of the cell using membrane vesicles.

• Energy used: ATP or GTP (another energy-rich molecule).

• Two main types: Endocytosis (into the cell) and Exocytosis (out of the cell).

Endocytosis

The plasma membrane folds inward to form a vesicle that brings materials INTO the cell. three different types,

Type	Nickname	What It Does	Example
Phagocytosis	“Cell eating”	Engulfs large particles (like bacteria or debris) by extending pseudopods (false arms)	White blood cells
Pinocytosis (Bulk-phase)	“Cell drinking”	Takes in fluids and dissolved solutes	Absorption by cells lining intestines
Receptor-Mediated Endocytosis	“Targeted intake”	Uses receptors on the membrane to bind and bring in specific molecules	Uptake of hormones, enzymes, or cholesterol

Exocytosis

(Out of the Cell), The reverse of endocytosis.

• The cell packages substances in a vesicle, moves it to the plasma membrane, the vesicle fuses, and releases its contents outside the cell.

Examples of substances released:

• Neurotransmitters (from nerve cells)

• Hormones

• Mucus

• Wastes