Plasma Membrane Structure

Question 1

What Is the Plasma Membrane?

Answer 1

The plasma membrane, also called the cell membrane, surrounds all cells.

The same membrane structure is found around membrane-bound organelles such as:

Smooth and rough endoplasmic reticulum

Golgi apparatus

Lysosomes and vesicles

Double-membrane organelles like the nucleus, mitochondria, and chloroplasts

Question 2

Why Are Membranes Important?

Answer 2

communication & Signalling
Enable communication between cells and with the external environment

Allow cells to respond to hormones and signals

🚧 Compartmentalisation
Create separate internal environments (e.g. nucleus vs cytoplasm)

Allow specialised conditions and concentration gradients

🚪 Selective Permeability
Partially permeable: controls the entry and exit of substances

Maintains homeostasis by regulating what enters and exits the cell

Question 3

The Fluid Mosaic Model
https://tse1.mm.bing.net/th/id/OIP.mCGr1_l_NTJakfABx0QvGwHaD5?cb=ucfimg2&ucfimg=1&rs=1&pid=ImgDetMain&o=7&rm=3

Answer 3

The fluid mosaic model describes the structure of the cell membrane:

Fluid: Phospholipids in the bilayer are constantly moving

Mosaic: Proteins are randomly embedded like tiles in a mosaic

Question 4

The main component of the membrane is the phospholipid bilayer.

🧪 Structure:
Each phospholipid has a:
https://tse4.mm.bing.net/th/id/OIP.DXsdmjmysb-9bWwiqnxrbQHaF6?cb=ucfimg2&ucfimg=1&rs=1&pid=ImgDetMain&o=7&rm=3

Answer 4

Hydrophilic (water-attracting) head

Hydrophobic (water-repelling) tail

Phospholipids arrange themselves into a bilayer:

Heads face outwards towards water

Tails face inwards, away from water

Question 5

What Can & Can’t Pass Through:
Cannot pass through:

Answer 5

cannot - Water-soluble substances

Charged ions (e.g. Na⁺, Cl⁻)

Polar molecules

Can pass through:

Small, non-polar molecules (e.g. O₂, CO₂)

Lipid-soluble substances (e.g. steroid hormones)

Question 6

Proteins in the Membrane
Membrane proteins play key roles in transport, signalling, and recognition. name the 3 proteins

Answer 6

1) channel / carrier proteins
2) peripheral proteins
3) glycoproteins

Question 7

1️⃣ Channel & Carrier Proteins

Answer 7

Help charged or large polar molecules cross the membrane

Involved in:

Facilitated diffusion

Active transport

Question 8

2️⃣ Peripheral Proteins

Answer 8

Sit on the inner or outer surface of the membrane

Receptor proteins can bind to:

Hormones

Signalling molecules

Trigger responses inside the cell

Question 9

3️⃣ Glycoproteins

Answer 9

Proteins with carbohydrate chains attached

Functions:

Antigens for cell recognition (e.g. immune response)

Cell signalling

Cell adhesion (help cells stick together to form tissues)

Question 10

2 Lipids in the Membrane

Answer 10

1️⃣ Glycolipids
2️⃣ Cholesterol

Question 11

1️⃣ Glycolipids

Answer 11

Phospholipids with carbohydrate groups

Similar in function to glycoproteins:

Cell recognition

Signalling

Adhesion

Question 12

2️⃣ Cholesterol

Answer 12

A type of lipid that fits between phospholipids

Functions:

Restricts movement of phospholipids, making the membrane less fluid

Increases membrane stability, especially in animal cells (which lack cell walls)

Vital for free-floating cells like:

Red blood cells

White blood cells

Helps them maintain shape while moving through the bloodstream