Understanding Eukaryotic Cells, Membranes, and Transport Mechanisms

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Sep 30, 2025

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Biology

Cell Membrane and Transport

NOT drawn to scale!!

Explain how eukaryotic cells are similar to and different from prokaryotic cells. (ON YOUR OWN)

Explain how each eukaryotic organelle/structure (cell membrane, nucleus, mitochondria, chloroplasts, endoplasmic reticulum and plant cell walls, and vacuoles) contribute to the overall function of the cell.

(ON YOUR OWN)

Review all of the cell parts

Cell Membrane and Cell Transport

Learning Outcomes

Understand the fluid mosaic model of membranes

Describe the functions of phospholipids, proteins, and carbohydrates in membranes

Explain what substances are permeable to the cell membrane and how they are transported

describes membranes as a fluid lipid bilayer with floating proteins and carbohydrates.

Fluid – the phospholipid bilayer is viscous and individual phospholipids can move position

Mosaic – the phospholipid bilayer is embedded with proteins, resulting in a mosaic of components

The fluid mosaic model of the plasma membrane structure describes the plasma membrane as a fluid combination of phospholipids, cholesterol, proteins, and carbohydrates

All membranes in and around a cell are made of a phospholipid bilayer and proteins

Membranes are assemblies of carbohydrates, proteins, and lipids held together by non-covalent forces. They regulate the transport of molecules, control information flow between cells, generate signals to alter cell behavior, contain molecules responsible for cell adhesion in the formation of tissues, and can separate charged molecules for cell signaling and energy generation.

Phospholipid Bilayer

The plasma membrane is composed mainly of phospholipids, which consist of fatty acids and alcohol. The phospholipids in the plasma membrane are arranged in two layers, called a phospholipid bilayer. As shown in the Figure, each phospholipid molecule has a head and two tails. The head “loves” water (hydrophilic) and the tails “hate” water (hydrophobic). The water-hating tails are on the interior of the membrane, whereas the water-loving heads point outwards, toward either the cytoplasm or the fluid that surrounds the cell.

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The plasma membrane is a phospholipid bilayer with embedded proteins. There are other components, such as cholesterol and carbohydrates, which can be found in the membrane in addition to phospholipids and protein.

Figure 3.8

Membrane proteins determine functions of cell membranes, including serving as pumps, gates, receptors, cell adhesion molecules, energy transducers, and enzymes.

Carbohydrates covalently linked to proteins (glycoproteins) or lipids (glycolipids) are also a part of cell membranes, and function as adhesion and address loci for cells.

HIV docks at and binds to the CD4 receptor, a glycoprotein on the surface of T cells, before entering, or infecting, the cell.

Figure 3.19

An example of a glycoprotein that is embedded in the cell membrane of immune cells such as helper T cells.

Found primarily on the surface of CD4 T lymphocytes (CD4 cells). To enter a host cell, HIV binds to a CD4 receptor and a coreceptor (either CCR5 or CXCR4) on the host cell.

There are many metabolic diseases caused by a recessive allele (version of a gene) that makes a defective protein. Cystic fibrosis requires constant treatments, and the life expectancy is about 35. It is caused by a defective gene for a chloride channel (a protein).

https://www.youtube.com/watch?v=itQns51NKOo (first 3 minutes)

Transport mechanisms

Mechanism	Good for…
Diffusion across bilayer	Small, nonpolar molecules
Facilitated diffusion by membrane proteins	Medium size polar molecules or ions
Endocytosis or exocytosis	Large molecules
Active transport by proteins	Going against concentration gradient

This table will be given to you on the exam

Oxygen, Carbon Dioxide, and water are some molecules that enter and exit the cells.

Figure 3.10 Passive transport.

(a) Diffusion of molecules across the plasma membrane occurs with the concentration gradient and does not require energy. Small hydrophobic molecules, carbon dioxide, and oxygen can diffuse across the membrane.

Transport of medium-sized molecules

Water always moves to “even” out the concentration of solutes on both sides of a membrane.

Figure 3.10 Passive transport.

(b) Facilitated diffusion is the diffusion of molecules through proteins. Molecules move with their concentration gradient, which does not require energy.

Figure 3.11 Active transport.

Active transport moves substances against their concentration gradient and requires energy (ATP) to do so.

Transport of LARGE molecules

Figure 3.12 Movement of large substances.

(a) Exocytosis is the movement of substances out of the cell. (b) Endocytosis is the movement of substances into the cell.

Transport mechanisms

Mechanism	Good for…
Diffusion across bilayer	Small, nonpolar molecules
Facilitated diffusion by membrane proteins	Medium size polar molecules or ions
Endocytosis or exocytosis	Large molecules
Active transport by proteins	Going against concentration gradient

This table will be given to you on the exam

Oxygen, Carbon Dioxide, and water are some molecules that enter and exit the cells.

In osmosis, water always moves from an area of higher concentration (of water) to one of lower concentration (of water). In this system, the solute cannot pass through the selectively permeable membrane.

Figure 3.21

Osmotic pressure changes the shape of red blood cells in hypertonic, isotonic, and hypotonic solutions. (credit: modification of work by Mariana Ruiz Villarreal)

Figure 3.22

1. throughout the cytoplasm 2. from an area with a high concentration of other solutes to a lower one 3. from an area with a low concentration of solutes to an area with a higher one 4. from an area with a low concentration of water to one of higher concentration

Water moves via osmosis ________.

Case study – 2 types of cells Bioluminescent algae and sperm cells

egg cell many sperm cells

Some living organisms require unique abiotic conditions! Laguna Grande in Fajardo, Puerto Rico

You can see them if you paddle a kayak to certain locations in the Caribbean

https://www.youtube.com/watch?v=VwfG9s1A8pI

3 min.

These are two different representations of the reaction that produces bioluminescence in dinoflagellates. What are the reactants? What are the products? What is the role of luciferase?

light

Sperm cell case study

Tom and Tami have been married for three years and are ready to start a family, but after a year of trying, Tami was still not pregnant. Their doctor told them that when a couple has trouble conceiving a child, the easiest thing to check first is to determine if the husband is producing semen with a normal number of healthy sperm cells per milliliter (approximately 20-60 million/ml). Semen consists of living sperm cells in a solution of sugars, buffer molecules to regulate the pH, and hormones.

http://www.youtube.com/watch?v=6rauO_M9tb4

📖 Struggling with where to start this assignment? Follow this guide to tackle your assignment easily!

Step 1: Compare Eukaryotic and Prokaryotic Cells

Identify similarities:
- Both have a plasma membrane, cytoplasm, ribosomes, and genetic material (DNA).
Identify differences:
- Eukaryotic cells have a nucleus and membrane-bound organelles, while prokaryotic cells do not.
- Size difference: Eukaryotic cells are generally larger.
- Complexity: Eukaryotic cells have compartmentalization; prokaryotes have a simpler structure.

Step 2: Explain the Function of Key Eukaryotic Organelles

Cell Membrane: Selectively permeable barrier controlling the entry and exit of substances; involved in communication and cell signaling.
Nucleus: Stores DNA and coordinates cell activities such as growth, metabolism, and reproduction.
Mitochondria: Produce ATP via cellular respiration; the “powerhouse” of the cell.
Chloroplasts (Plant Cells): Conduct photosynthesis to produce energy in the form of glucose.
Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids and detoxifies chemicals.
Plant Cell Walls: Provide structural support, protection, and shape to plant cells.
Vacuoles: Storage of water, nutrients, and waste; helps maintain turgor pressure in plant cells.

Step 3: Understand the Fluid Mosaic Model of Membranes

Plasma membranes consist of phospholipid bilayers, proteins, cholesterol, and carbohydrates.
Fluid: Phospholipids can move, allowing flexibility.
Mosaic: Embedded proteins form diverse functions (transport, adhesion, receptors).

Step 4: Membrane Components and Their Functions

Phospholipids: Form bilayer; hydrophilic heads face outward, hydrophobic tails face inward.
Proteins: Serve as channels, pumps, enzymes, receptors, and adhesion molecules.
Carbohydrates: Glycoproteins and glycolipids act as cell recognition and adhesion markers.
Cholesterol: Maintains membrane fluidity and stability.

Step 5: Cell Transport Mechanisms

Mechanism	Purpose	Example
Diffusion	Small, nonpolar molecules move along the concentration gradient	O₂, CO₂
Facilitated Diffusion	Medium-sized polar molecules or ions move via membrane proteins	Glucose
Active Transport	Moves substances against the concentration gradient using ATP	Na⁺/K⁺ pump
Endocytosis	Large molecules enter the cell	Nutrients, bacteria
Exocytosis	Large molecules exit the cell	Hormones, waste

Osmosis: Water moves from high to low concentration of water across a selectively permeable membrane.
Hypertonic/Isotonic/Hypotonic Solutions: Affect cell shape due to osmotic pressure.

Step 6: Case Studies

Bioluminescent Algae:
- Reactants: Luciferin + O₂
- Products: Light + Oxidized luciferin
- Role of luciferase: Enzyme that catalyzes the reaction producing light.
Sperm Cells and Fertility:
- Normal sperm count: 20–60 million/ml
- Components: Sperm cells, sugars, buffer molecules, and hormones

Step 7: Review and Resources

Rewatch videos:
Review OpenStax Biology resources for diagrams and explanations.
Summarize each organelle, transport mechanism, and case study in your notes for exam preparation.

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