Biology - Cells and cell transport

What are the differences between electron microscopes and light microscopes? (7)

1. Electron microscopes use a beam of electrons focused by electromagnets, light microscopes use light focused by glass lenses

2. Electrons have a much shorter wavelength (~0.005 nm) than light (400-700 nm)

3. Electron microscopes have a much greater resolution (~0.05 nm) than light microscopes (~200 nm)

4. Electron microscopes have a higher maximum useful magnification (over 1 million ×) than light microscopes (~1500 ×)

5. Electron microscopes can reveal the ultrastructure of cells in much greater detail

6. Electron microscopy requires dead, dehydrated specimens in a vacuum, whereas light microscopy can be used with living specimens

7. Electron microscope images are black and white (often false-coloured), while light microscope images are colour

Compare transmission electron microscopes (TEM) and scanning electron microscopes (SEM) (7)

Transmission electron microscopes (TEM) - passes a beam of electrons through a thin specimen stained with heavy metal salts

Advantages of TEM - produces images of internal cell structures and has a higher resolution than SEM

Limitations of TEM - equires very thin sections, does not produce a 3D image, specimens may deteriorate in the electron beam

Scanning electron microscopes (SEM) - scans a specimen coated in a thin layer of heavy metal (e.g. gold) with an electron beam

• Electrons reflected from the surface produce an image of the specimen's surface features

Advantages of SEM - gives a 3D appearance and can examine thicker specimens

Limitation of SEM - lower resolution than TEM and only shows the surface of specimens

What are the equations used in magnification calculations and unit conversions?

Magnification = Image size ÷ Actual size

Actual size = Image size ÷ Magnification

Image size = Actual size × Magnification

Conversion of mm to µm - multiply by 1000

Conversion of µm to nm - multiply by 1000

mm —> nm = x10⁶

What are the features of prokaryotic cells? (6)

Prokayote - a small, single-celled organism that lacks a nucleus and membrane-bound organelles e.g bacteria

• A typical bacterial cell contains a cell wall (murein/peptidoglycan), cell membrane, circular DNA, ribosomes, cytoplasm and food reserve granules

• Plasmids - small circular DNA molecules carrying non-essential genes

• Pili - used for attachment and genetic exchange between cells

• Flagella - used for locomotion

• Mesosomes - associated with respiration and binary fission

What are the characteristics of eukaryotic cells? (6)

Eukaryotes - make up the vast majority of living organisms, including plants and animals

• Eukaryotic cells contain a nucleus surrounded by a nuclear membrane

• The cytoplasm contains numerous membrane-bound organelles

Examples of organelles includes: mitochondria, endoplasmic reticulum, Golgi apparatus and lysosomes

• Eukaryotic cells contain 80S ribosomes for protein synthesis

• Eukaryotic cells are generally larger and more complex than prokaryotic cells

What structures are found only in a plant cell? (4)

Cellulose cell wall - provides support and maintains cell shape

Starch grains - store carbohydrate reserves

Chloroplasts - contain chlorophyll and are the site of photosynthesis

Vacuole - large, permanent, contains cell sap (soluble sugars, salts and pigments)

What are the differences between prokaryotic and eukaryotic cells? (6)

1. Prokaryotes have no nucleus, whereas eukaryotes have a nucleus

2. Prokaryotes lack membrane-bound organelles such as mitochondria, Golgi apparatus and endoplasmic reticulum, which are present in eukaryotes

3. Prokaryotes contain 70S ribosomes, whereas eukaryotes contain 80S ribosomes

4. Prokaryotic DNA is circular, whereas eukaryotic DNA is linear

5. Prokaryotic cells are smaller (0.5-10 µm) than eukaryotic cells (10-100 µm)

6. Prokaryotic flagella are simple and lack microtubules, eukaryotic flagella contain a 9+2 arrangement of microtubules

What is differential centrifugation and how are cell organelles prepared for it? (5)

Differential centrifugation - separates cell structures based on their density by centrifuging at different speeds

1. Cells are first homogenised (broken open) in an ice-cold, isotonic buffer solution

2. The isotonic solution prevents organelles from gaining or losing water by osmosis so they don’t burst or shrivel up

3. The low temperature prevents enzyme activity and autolysis (self-digestion) of organelles

4. The homogenate is centrifuged at increasing speeds to separate and isolate organelles according to their density

What are the stages of differential centrifugation? (6)

1. The homogenate is filtered to remove debris, intact cells and tissues

2. The filtrate is centrifuged at a low speed, causing the heaviest particles to form a pellet (sediment)

3. The liquid above the pellet, called the supernatant, is removed and centrifuged at a higher speed for longer

4. Nuclei form the next pellet, while lighter organelles remain in the supernatant

5. The process is repeated using increasing speeds and centrifugation times to separate organelles by density

6. Organelles are typically isolated in the order: nuclei → chloroplasts → mitochondria → endoplasmic reticulum → ribosomes

What are the structures and functions of the nucleus? (5)

Nucleus - contains the cell's genetic material (DNA), which controls the development, structure and function of the cell

Nuclear envelope - a double membrane that surrounds the nucleus

Nuclear pores - allow communication between the nucleus and cytoplasm

Nucleoplasm - a substance in the nucleus which contains chromatin (DNA and proteins) and nucleoli

• The nucleus controls protein synthesis, cell division and the production of ribosomes

What are the functions and the structure of centrioles? (3)

Centrioles - occur as a pair within animal cells

Function: produce spindle fibres (microtubules) that separate chromosomes during mitosis and meiosis

• Each centriole is a small, hollow cylinder composed of 9 triplets of microtubules

What are the structures in mitochondria and its functions? (8)

Mitochontria - the site of aerobic respiration and ATP production

Structure:

Double membrane - surrounds the mitochondria and consists of an outer membrane and an inner membrane

Intermembrane space - the space between the membranes

Cristae - folding of the inner membrane, providing a large surface area for oxidative phosphorylation

• The cristae contain stalked particles with enzymes involved in the electron transport chain and ATP synthesis

Matrix - contains enzymes for the Krebs cycle, as well as mitochondrial DNA and ribosomes

Mitochondrial DNA - allows mitochondria to replicate and ribosomes enable protein synthesis

• Cells with high energy demands, such as muscle cells, liver cells and cells carrying out active transport, contain many mitochondria

What is a ribosome and its main function? (4)

Ribosomes - small organelles (about 20 nm in diameter) made of a large and small subunit

• Each subunit contains ribosomal RNA (rRNA) and protein

Location in the cell - free in the cytoplasm, arranged in polysomes, or attached to the rough endoplasmic reticulum (RER)

Main function - the site of protein synthesis, where amino acids are joined together to form polypeptides

What is the structure and functions of the endoplasmic reticulum? (6)

Endoplasmic reticulum (ER) - consists of flattened membrane sacs called cisternae, forming an internal transport system within the cell

Rough endoplasmic reticulum (RER) - has ribosomes attached to its surface

• The RER synthesises and transports proteins, particularly secretory proteins that are exported from the cell

• Proteins produced by the RER are often transported to the Golgi apparatus for modification and packaging

Smooth endoplasmic reticulum (SER) - lacks ribosomes

• The SER is involved in the synthesis and transport of lipids

Describe the structure and functions of the Golgi body (8)

Golgi body - consists of stacks of flattened membrane-bound sacs called cisternae

• The cisternae are continuously formed at one end and pinched off as vesicles at the other end

• Cells specialised for secretion contain large Golgi bodies

Functions:

• Produces glycoproteins e.g mucin

• Modifies, packages and secretes proteins e.g enzymes

• Formation of cell walls in plant cells

• Formation of lysosomes

• Lipid biosynthesis

Describe the structure and functions of lysosomes (7)

Lysosomes - small spherical organelles surrounded by a single membrane

• They contain hydrolytic (digestive) enzymes such as proteases, nucleases and lipases, which digest proteins, nucleic acids and lipids

• The enzymes are enclosed within lysosomes to prevent damage to the cell

• Enzymes are synthesised on the rough ER, processed by the Golgi apparatus, and packaged into lysosomes

Main functions:

Digestion of material - lysosomes fuse with vesicles formed by endocytosis and release enzymes to digest material, products are absorbed by the cytoplasm

Autophagy - lysosomes digest worn-out organelles and unwanted cell structures within autophagic vacuoles

Enzyme release - enzymes may be released outside the cell

What are the structures found in chloroplasts and their main functions? (5)

Chloroplasts - found in photosynthetic plant cells and are the site of photosynthesis

• They are surrounded by a double membrane envelope

• Contains flattened membrane sacs called thylakoids, which are stacked into grana

Grana - provide a large surface area for chlorophyll and are the site of the light-dependent reactions

Stroma - surrounds the membrane system, contains enzymes, sugars and starch grains and is the site of the light-independent reactions

Describe cell differentiation (5)

Cell differentiation - the process by which cells become specialised to perform specific functions

• Specialised cells are grouped together to form tissues

Tissue - a group of similar cells that perform a specific physiological function

Organ - consists of different tissues working together to perform a specific function

• Several organs work together in an organ system, e.g. the digestive system or plant vascular system

How are palisade mesophyll cells adapted for photosynthesis? (5)

Palisade mesophyll cells - located beneath the upper epidermis of the leaf

Adaptations:

• Contain numerous chloroplasts to maximise light absorption for photosynthesis

• Thin cell walls allow rapid diffusion of carbon dioxide

• The cells are long and columnar (cylindrical), providing a large surface area for gaseous exchange

• There are few air spaces between cells, allowing maximum light to be absorbed

How are columnar epithelial cells adapted for absorption? (4)

Columnar epithelial cells - found in the lining of the small intestine and proximal convoluted tubule of the kidney

• They are specialised for the absorption of small molecules, often by active transport

• The cell surface membrane is folded into microvilli, increasing the surface area for absorption

• They contain numerous mitochondria to provide ATP for active uptake of substances such as amino acids and glucose