Unit 1, Topic 1 Cells as the basis of life

Question 1

What are the main differences between prokaryotic and eukaryotic cells?

Answer 1

Prokaryotic cells, such as bacteria, are simpler and smaller, lacking membrane-bound organelles and a nucleus, with their DNA instead being located in a region called the nucleoid.

Eukaryotic cells, found in more complex organisms such as plants, animals and fungi, contain membrane-bound organelles like the nucleus, mitochondria and chloroplasts.

Question 2

What is the nucleus?

Answer 2

The nucleus is often referred to as the control centre of the cell, playing a crucial role in regulating cellular activities and storing genetic information. Enclosed by a double membrane called the nuclear envelope, the nucleus contains the cell’s DNA, which is organised into chromosomes. The nuclear envelope features pores that allow the exchange of materials between the nucleus and the cytoplasm, enabling communication and the transport of molecules such as RNA and proteins. Within the nucleus, the nucleolus is responsible for producing ribosomes, the structures that synthesise proteins.

Question 3

What is the mitochondria?

Answer 3

Mitochondria are organelles that produce energy for cellular activities through cellular respiration, converting glucose and oxygen into ATP, the cell’s main energy source. They have a unique structure with an outer membrane and a highly folded inner membrane (cristae) that houses ATP-producing enzymes, enhancing energy production efficiency. Mitochondria also contain their own DNA and can replicate independently within the cell.

Question 4

Main function of the rough endoplasmic reticulum

Answer 4

The rough endoplasmic reticulum (rough ER) is an organelle that synthesizes and processes proteins. It is studded with ribosomes, which produce proteins by translating mRNA into polypeptide chains. The rough ER ensures proteins are properly folded and functional before they are transported within the cell or secreted. Proteins are then packaged into vesicles and sent to the Golgi apparatus for further processing and sorting.

Question 5

Main function of Smooth Endoplasmic Reticulum

Answer 5

The smooth endoplasmic reticulum (smooth ER) is involved in lipid and steroid hormone synthesis, carbohydrate metabolism, and drug detoxification. Unlike the rough ER, it lacks ribosomes, giving it a smooth appearance. It is active in cells that produce steroid hormones, such as those in the adrenal glands, and in liver cells, where it detoxifies harmful substances. The smooth ER also synthesises phospholipids, vital for cell membranes, and assists in carbohydrate metabolism.

Question 6

main functions of the Golgi apparatus, or Golgi body?

Answer 6

The Golgi apparatus, or Golgi body, modifies, sorts, and packages proteins and lipids. It consists of flattened, membrane-bound sacs called cisternae and receives molecules from the endoplasmic reticulum in vesicles. Within the Golgi, proteins and lipids are modified, such as by adding carbohydrate groups to form glycoproteins. The Golgi then sorts and packages these molecules into vesicles for delivery to their final destinations, including secretion outside the cell or incorporation into the cell membrane, ensuring efficient cellular function.

Question 7

What are lysosomes?

Answer 7

Lysosomes are membrane-bound organelles that function as the cell’s digestive system, containing enzymes that break down macromolecules like proteins, lipids, and carbohydrates. They degrade cellular waste, damaged organelles, and foreign substances through processes such as endocytosis, recycling essential components back into the cell to maintain its health. Lysosomes also contribute to programmed cell death (apoptosis) by releasing enzymes that help dismantle the cell when it is no longer needed.

Question 8

Main functions of vacuoles? How do they differ in plant vs animal cells?

Answer 8

Vacuoles are membrane-bound sacs found in cells that serve various functions depending on the type of cell. In plant cells, a large central vacuole stores water, nutrients and waste products, contributing to turgor pressure, which helps maintain cell structure and rigidity. It can also contain pigments and defensive chemicals. In animal cells, vacuoles are smaller and typically involved in processes like nutrient storage, waste removal and maintaining cellular pH.

Question 9

What are chloroplasts and what is their role in photosynthesis?

Answer 9

Chloroplasts are specialized organelles in plant cells and some algae that perform photosynthesis, converting light energy into chemical energy. They contain chlorophyll, which captures light energy to convert carbon dioxide and water into glucose and oxygen. Chloroplasts have a double membrane, with the inner membrane forming thylakoid sacs stacked into grana, where light-dependent reactions occur. The surrounding fluid, called stroma, is where light-independent reactions (Calvin cycle) take place. Chloroplasts have their own DNA, supporting the endosymbiotic theory that they originated from free-living prokaryotes engulfed by early eukaryotic cells.

Question 10

How do plant and animal cells differ?

Answer 10

Animal and plant cells differ in several key ways. Plant cells have a rigid cell wall made of cellulose, providing structural support and a fixed shape, whereas animal cells only have a flexible plasma membrane and often exhibit a more varied shape. Plant cells contain chloroplasts for photosynthesis, enabling them to produce their own food from sunlight, while animal cells do not have chloroplasts and rely on external sources of nutrition. Additionally, plant cells usually feature a large central vacuole for storing water, nutrients and waste, which helps maintain cell rigidity, while animal cells have smaller vacuoles used mainly for storage and digestion. Animal cells also commonly contain lysosomes for breaking down waste and cellular debris, a feature less prominent in plant cells.

Question 11

Knowing that prokaryotic cells don’t contain membrane bound organelles, which organelles do only eukaryotic cells contain?

Answer 11

Eukaryotic cells contain several membrane-bound organelles that are absent in prokaryotic cells. These include:

Nucleus: Contains the cell’s genetic material (DNA) and controls cellular activities.

Mitochondria: Produce energy (ATP) through cellular respiration.

Endoplasmic Reticulum (ER):
Rough ER: Involved in protein synthesis and processing.

Smooth ER: Synthesizes lipids and detoxifies chemicals.

Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for transport.

Lysosomes: Contain digestive enzymes to break down waste and cellular debris.

Chloroplasts (in plants and some algae): Conduct photosynthesis to produce glucose and oxygen.

Vacuoles: Store materials and help maintain cellular pressure; large central vacuole in plant cells.

Peroxisomes: Break down fatty acids and detoxify harmful substances.

Question 12

Order the Hierarchical Organisation in Multicellular Organisms

Answer 12

Cells: The basic unit of life, cells are specialised to carry out specific functions. For example, muscle cells contract to enable movement, while nerve cells transmit electrical signals.

Tissues: Groups of similar cells that work together to perform a particular function. For example, muscle tissue is made up of muscle cells that work together to produce movement, while nervous tissue consists of neurons that transmit signals.

Organs: Structures made up of different tissues that work together to perform specific functions. For example, the heart is an organ composed of muscle tissue, connective tissue and nervous tissue, all of which work together to pump blood.

Systems: Groups of organs that work together to perform complex functions necessary for the survival of the organism. For example, the digestive system is responsible for obtaining nutrients, the respiratory system for exchanging gases, and the excretory system for removing wastes.

Question 13

How does the body obtain and distribute nutrients at different levels of organisation (cellular, tissue, organ levels)?

Answer 13

Cellular Level: Specialised cells in the digestive system, such as epithelial cells lining the intestines, are responsible for absorbing nutrients from digested food.

Tissue Level: These epithelial cells form the lining of the intestines, a tissue that increases surface area for maximum nutrient absorption.

Organ Level: The intestines, part of the digestive system, work with the stomach and other organs to break down food, absorb nutrients, and transfer them to the bloodstream. The circulatory system then distributes these nutrients to all cells in the body, ensuring that each cell receives the necessary energy and building blocks for growth and repair.

Question 14

How does the body exchange gases at different levels of organisation (cellular, tissue, organ levels)?

Answer 14

Cellular Level: In the respiratory system, alveolar cells in the lungs facilitate the exchange of oxygen and carbon dioxide between the air and the blood.

Tissue Level: These alveolar cells form the thin, permeable tissue of the alveoli, which are small air sacs in the lungs that allow for efficient gas exchange.

Organ Level: The lungs, as organs of the respiratory system, work with the circulatory system to bring oxygen into the body and expel carbon dioxide. Oxygenated blood is then transported to cells throughout the body, while carbon dioxide is carried back to the lungs for exhalation.

Question 15

How does the body remove waste at different levels of organisation (cellular, tissue, organ levels)?

Answer 15

Cellular Level: In the excretory system, kidney cells, specifically in the nephrons, filter waste products from the blood, including urea, excess salts, and water.

Tissue Level: These kidney cells form the tissue of the nephrons, which are the functional units of the kidneys responsible for filtering blood and forming urine.

Organ Level: The kidneys, as organs of the excretory system, work with the circulatory system to remove waste products from the blood and excrete them as urine. This urine is then stored in the bladder before being expelled from the body, ensuring that toxic substances do not accumulate and disrupt cellular function.

Question 16

Each body system contains specialised cells and tissues that are structurally suited to their functions.

Provide examples of specialised cells, their structure and function.

Answer 16

Neurons

Structure: Neurons have long, branching extensions called dendrites and axons. Dendrites receive signals from other neurons, while the axon transmits these signals over long distances.

Function: Neurons are responsible for transmitting electrical impulses throughout the nervous system, facilitating communication between the brain, spinal cord, and other parts of the body.

Red blood cells

Structure: Red blood cells are disc-shaped and lack a nucleus, allowing them to have a flexible, biconcave shape. This shape increases their surface area for oxygen absorption and transport.

Function: To carry oxygen from the lungs and deliver it throughout the body.

Muscle cells

Structure: Muscle cells, or myocytes, are elongated and contain numerous mitochondria to provide the energy needed for contraction. They have a striated appearance due to the arrangement of actin and myosin filaments.

Function: Muscle cells contract and relax to facilitate movement and maintain posture. There are three types: skeletal (voluntary movement), cardiac (heart contractions), and smooth (involuntary movements in organs).

Epithelial cells

Structure: Epithelial cells are tightly packed in layers and form continuous sheets. They may have specialised structures such as cilia or microvilli depending on their location.

Function: They cover and protect surfaces and cavities of organs and structures, such as the skin, the lining of the gut and the respiratory tract. They also facilitate absorption, secretion and filtration.

Sperm cells are another example however, the reproductive system was not studied.

Question 17

The size and shape of cells and tissues are often adapted to maximise their surface area relative to their volume, a concept known as the surface area to volume ratio (SA:V). This adaptation is particularly important in systems where the exchange of substances is critical. Provide examples in respiratory and digestive systems.

Answer 17

Example in the Respiratory System: The alveoli in the lungs are tiny, thin-walled sacs with a large surface area relative to their volume. This design is essential for gas exchange, as it maximises the surface area available for oxygen and carbon dioxide to diffuse across the alveolar and capillary membranes. The increased SA:V ensures that oxygen can efficiently enter the bloodstream and carbon dioxide can be expelled from the body.

Example in the Digestive System: The lining of the small intestine features finger-like projections called villi, which significantly increase the surface area for nutrient absorption. Each villus is further covered with microvilli, forming a brush border that maximises the absorption of nutrients into the bloodstream. The large surface area provided by the villi and microvilli enables efficient nutrient uptake, supporting the body’s energy and nutrient needs.

Question 18

The size and shape of cells and tissues are often adapted to maximise their surface area relative to their volume, a concept known as the surface area to volume ratio (SA:V). This adaptation is particularly important in systems where the exchange of substances is critical. Provide examples in respiratory and digestive systems.

Answer 18

Example in the Respiratory System: The alveoli in the lungs are tiny, thin-walled sacs with a large surface area relative to their volume. This design is essential for gas exchange, as it maximises the surface area available for oxygen and carbon dioxide to diffuse across the alveolar and capillary membranes. The increased SA:V ensures that oxygen can efficiently enter the bloodstream and carbon dioxide can be expelled from the body.

Example in the Digestive System: The lining of the small intestine features finger-like projections called villi, which significantly increase the surface area for nutrient absorption. Each villus is further covered with microvilli, forming a brush border that maximises the absorption of nutrients into the bloodstream. The large surface area provided by the villi and microvilli enables efficient nutrient uptake, supporting the body’s energy and nutrient needs.

Question 19

Explain the structure of the Cell Membrane.

Answer 19

The cell membrane is primarily composed of a phospholipid bilayer, where hydrophilic heads face outward and hydrophobic tails face inward, creating a semi-permeable barrier. Embedded within this bilayer are protein channels and carriers that facilitate selective transport of molecules like ions and glucose. Cholesterol molecules within the bilayer help regulate membrane fluidity and stability. Additionally, glycoproteins on the outer surface assist in cell recognition, communication, and immune responses.

Question 20

What is simple diffusion?

Answer 20

Simple diffusion is the passive movement of small, non-polar molecules, such as oxygen and carbon dioxide, from an area of higher concentration to an area of lower concentration through the phospholipid bilayer. This process does not require energy as molecules move down their concentration gradient until equilibrium is achieved.

Question 21

What is facilitated diffusion?

Answer 21

Facilitated diffusion involves the transport of larger or polar molecules, such as glucose and ions, across the cell membrane through specific protein channels or carriers. These membrane proteins assist the movement of these substances down their concentration gradient. Unlike simple diffusion, facilitated diffusion requires the presence of these transport proteins but still does not require energy.

Question 22

What is osmosis?

Answer 22

Osmosis is a specific type of diffusion that involves the movement of water across a semi-permeable membrane from an area of lower solute concentration to an area of higher solute concentration. This process is influenced by the tonicity of the surrounding solution, which determines the effect on the cell’s shape and volume.

Question 23

What happens to a cell in isotonic, hypertonic and hypotonic solutions?

Answer 23

Isotonic Solution: The concentration of solutes outside the cell is equal to the concentration inside the cell. Water moves in and out of the cell at equal rates, so the cell maintains its shape and volume.

Hypertonic Solution: The concentration of solutes outside the cell is higher than inside. Water moves out of the cell to balance the solute concentrations, causing the cell to shrink and potentially become crenated.

Hypotonic Solution: The concentration of solutes outside the cell is lower than inside. Water moves into the cell, causing it to swell and potentially burst (lysis) if the influx is excessive.

Question 24

Define active transport.

Answer 24

Active transport is the movement of substances against their concentration gradient, from an area of lower concentration to an area of higher concentration. This process requires energy in the form of ATP.

Question 25

What are endocytosis and exocytosis, and how do they differ?

Answer 25

Endocytosis is the process where the cell membrane engulfs extracellular material to form a vesicle that enters the cell. It includes phagocytosis (ingesting large particles) and pinocytosis (ingesting fluids and small particles).

Exocytosis is the process where cells expel substances by enclosing them in vesicles that fuse with the cell membrane, releasing their contents outside the cell.

The main difference is that endocytosis brings materials into the cell, while exocytosis expels materials out of the cell.

Question 26

What are the differences between active vs passive transport?

Answer 26

Passive Transport: Moves substances from higher to lower concentration without energy, relying on concentration gradients. Examples include simple diffusion, facilitated diffusion, and osmosis.

Active Transport: Requires energy (usually ATP) to move substances from lower to higher concentration against their gradient. Includes protein-mediated transport and bulk transport methods like endocytosis and exocytosis.

Question 27

How does the surface area-to-volume ratio affect cell efficiency and diffusion?

Answer 27

Cell size is constrained by the surface area-to-volume ratio, which affects transport efficiency. As cells grow, their volume increases faster than their surface area, lowering this ratio. A higher ratio, seen in smaller cells or those with specialized structures like microvilli, allows for more efficient material exchange. Larger cells with a low ratio may struggle with effective diffusion, so cells often stay small or adopt shapes that optimize this ratio for better nutrient uptake, waste removal, and gas exchange.